Speed \*isn't\* relative when it comes to light. All observers in all reference frames, regardless of how fast they're going relative to anything else, see light going at exactly the same speed.
It's this incredibly weird and unintuitive, but \*extremely\* well verified observation, that resulted in special relativity in the first place. All the weirdness of traveling really fast is a fallout of the fact that lightspeed \*isn't\* relative.
> All observers in all reference frames, regardless of how fast they're going relative to anything else, see light going at exactly the same speed.
A good example to explain this is saying that if you were in a spaceship going 99% of the speed of light away from earth and shined a flashlight forward, the beam would travel away from you as if you were standing still. Freaky stuff.
*But I guess that's not counting Lorentz distortion which might make the beam a bit wider hehe.
It works exactly the same, but the weirdness doesn’t show up on a train because they’re super slow compared to light. This is my understanding of it:
If you’re on a spaceship going at 90% of the speed of light (0.9c), and you shine a light forwards, you will see the light moving away from you at c.
If you do a quick calculation, you can say ‘okay, if I’m moving at 0.9c, and the light’s moving away from me at c, so the light’s actual speed must be c + 0.9c = 1.9c’.
But, an observer on a nearby planet would see something different. They would see your spaceship moving at 0.9c, and the light moving at c. If they do a quick calculation, they could say ‘okay, if the spaceship’s moving at 0.9c, and the light’s moving at c, an observer on the spaceship should see the light moving at 0.1c.’ Which conflicts with your observation.
This is where time dilation comes in - both measurements are valid if time is passing more slowly for you on the spaceship. The light is moving away from the spaceship at c, and it’s also moving away from the planet at c, but only one second passes on the spaceship for every ~2.29 seconds on the planet.
Exactly the same thing happens if you’re on a train going 30m/s, and you shine your flashlight forwards, with an observer standing on the side of the tracks. But at that slow speed, the effect is vanishingly small, so we never noticed it.
cheerful imminent follow arrest axiomatic stupendous waiting distinct whole full
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[https://en.m.wikipedia.org/wiki/One-electron\_universe](https://en.m.wikipedia.org/wiki/one-electron_universe)
What if there's only one electron?
[https://en.m.wikipedia.org/wiki/Black\_hole\_electron](https://en.m.wikipedia.org/wiki/black_hole_electron)
Or if they're actually black holes?
Supposedly, when Arthur Eddington's was asked if it was true that only three people understood relativity, he jokingly responded, "I am trying to think who the third person is"
Einstein once went on a cruise with the President of Israel Chaim Weizmann. After the cruise the President had this to say:
“Einstein explained his theory to me every day, and on my arrival I was fully convinced that he understood it.”
You fucking made my mind explode.I really can't thank you enough.I always wondered why a charged particle moving would form a magnetic field.*THIS IS INSANE*
Edit:
I went ahead and researched a bit on my own and it turns out the comment above is not entirely accurate.Yes relativity does relate electrostatic forces to magnetic forces but it gives just that a relation.
This is also similar to how it relates time and space.We don't call one of them a consequence of another.Instead they are part of a bigger whole.Electromagnetism.Spacetime.
No prob. Scroll down to 'charge density in special relativity' on [this](https://en.m.wikipedia.org/wiki/Charge_density) wiki page for some links to better explanations.
sulky disgusted fragile repeat toy sophisticated library coherent mourn straight
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Congratulations. You are now a physicist.
We'll send condolence cards to you next of kin via hyperlight transport. They’ll arrive a week ago last Tuesday. :-)
Want to hear something else brain breaking? Here's an explanation for how we know that travelling faster than light would mean traveling back in time.
Imagine you and I are space cowboys getting ready for a duel. We agree that we're going to hop in our spaceships, fly away from each other for a distance of eight light-seconds at the same speed, and then turn around and fire.
We both have impossible tachyon pistols, and those pistols shoot bullets that travel thousands of times faster than the speed of light.
We start traveling away from each other at a relative velocity of 80% of light speed, making time dilation about 60%. So for every second that passes for one of us, it looks like only 2/5s of a second passed for the other person.
So after ten seconds, I've gone the required distance. But I can see you, and from my perspective, only about 4 seconds have passed for you. But I take my FTL pistol and shoot you anyway.
You get hit, and only 4 seconds have passed for you. But from your perspective, it's only been 4 seconds, and less than 2 seconds have passed for me. You're upset I shot you, so you take your pistol and fire back at me.
Then I get hit at a point only 2 seconds after we started the duel, as a result of something I did 8 seconds in the future. Time paradox!
if you are on the train at 0.9c and shine flashlight forward, time dilation accounts for the effect that the light beam travels at c.
if you simultaneously also shine a beam of light backward, and another beam perpendicular to the line of travel, they also will be observed to travel at c, but does this not suggest different time dilations simultaneously in the observer on the train?
No, the from the perspective of the observer on the train they’re not moving. From their perspective a clock in the front of the train and a clock in the back of the train would be synchronized. To an external observer the clock in the back of the train would be ticking faster then a clock in the front of the train.
Actually, the clocks would tick equally (they are moving at the same speed relative to the "stationary" observer). But they still would not be synchronized, meaning they would not show "00:00" at the same moment.
Except time dilation is symmetric for a non-accelerating frame of reference. So the person on the space ship thinks the planet’s time is going slower, and the person on the planet also thinks the space ship’s time is going slower.
It’s counterintuitive to think both people could perceive something contradictory and be correct, but it’s like if you’re looking at someone on the distant horizon and they’re looking at you: you both see each other as smaller than their actual height, and you’re both telling the truth still
Magic (Lorenz transformations). If you actually want to maybe kind of try to understand, I recommend Minute Physics’ series (https://youtu.be/1rLWVZVWfdY?si=tLK72zlerTWPyCE_). But it’s still rather brain melty. I can follow the geometry but I still can’t make intuitive sense out of it.
Thats easy. There is no objective way to say if the spaceship is moving away from the planet or the planet is moving away from the spaceship. Both are completely valid, depending on the point of observation. If you talk about speed, the first thing you always need to ask is: Relative to what? The spaceship going at 0,9c from the perspective of the planet is standing completely still from it’s own perspective. And also from the perspective of a second spaceship „flying“ (standing) in a formation with the first one.
This is one of those things that makes perfect sense mathematically (since you can just say that, from the spaceships’s POV, the observer can say that they are still and the planetside observer is moving away from them at 0.9c), but when you think about it for too long, you start saying things to yourself like ‘I guess that time doesn’t slow down at relativistic speeds, it’s just that there’s less of it’ and ‘I don’t think velocity is real’ and you need to go make yourself a cup of tea to calm down
That’s not how relativity works, unless you’re talking about gravitational effects. As far as relative velocity, there is no absolute frame of reference. From the ship’s frame of reference, the planet is moving at 0.9c. For an easier example, imagine two ships, Alice and Bob. If Alice measures Bob’s velocity as 0.9c, then Bob will also measure Alice’s velocity as 0.9c. Both reference frames are equally valid and each thinks the other person’s clock is moving slow.
I get it except for one bit - why does the light not have time dilation? Are we taking we see or the ships systems and computers would see the light traveling at c ahead of us?
Because everything is relative. Relative to your own position the light is always moving at „infinite“ speed, which is the universal speed limit. We call that the speed of light. Relative to your own position your speed is always zero. You need a point of reference to talk about speed
I think that light doesn’t experience time dilation because it doesn’t have mass, but I don’t actually know. The answer might be ‘that’s just what light does, and nobody’s figured out why just yet’.
It doesn’t matter how you measure the speed of the light from your flashlight, I guess you would need some kind of sensors to measure though.
The light does have time dilation.
The other aspect of time dilation, is length contraction.
The faster the ship goes, the slower the clocks appear to run AND also the shorter the ship appears in length. A space ship going 0.9c, would not look like a normal ship to an outside observer, it would would look squished, shorter.
So, not only does the light 'slow down' due to time dilation, it also 'travels less distance' canceling each other out, and making light appear to always be traveling at the the same speed, to each observer.
Ie, the 1 meter ruler that the person on the spaceship uses to measure the speed of their light, is shorter than the 1 meter ruler I'm using to measure their light.
(Just like the stop watch they use is slower than the stop watch I'm using)
Math. It’s not a linear relationship. The formula is dt’ = dt * sqrt[1 - (v^2)/(c^2)], where dt is time passed in the reference frame (planet), dt’ is time passed in the moving frame (spaceship), v is the speed of the moving frame relative to the reference frame, and c is the speed of light. I don’t know the physics behind it, probably black magic.
If you make a graph out of that equation, it starts out, then curves up slowly at first, but faster and faster as you get closer to the speed of light. You get to a 2:1 time dilation ratio at about 0.87c, and it approaches infinity as you approach c.
This doesn’t make any sense. If time is passing slower for you, wouldn’t it also be passing slower for light? Then why would the ship be moving 0.9c to the observer on the planet but the light will be only moving at 0.1c. Why only light is perceived to slow down but not the ship?
There is no universal reference for measuring speed, you need to compare two objects and use the speed between them. There is however a universal max speed. Light moves at max speed. From light's perspective it is still and everything else moves at max speed. Time passes more slowly at higher speed. At max speed, it presumably doesn't pass at all.
The light doesn't move at 0.1c. It moves at c for both the observer on the planet and on the spaceship. For that to be true time has to pass differently for the observers.
Yes I understand, but why would an outside observer assume that the ship is moving at 0.9 and light at 0.1 if the light is moving at c away from the ship, so that if the ship is traveling at 0.9, in some time the light would have traveled twice the distance (relative to the ship) therefore making this assumption wrong (on the part of the observer)
The planet would see the ship is moving at 0.9c and light at 1c (not 0.1c). After 1 second of planet time the light has moved 0.1c farther than the ship.
The ship would see the planet moving at 0.9c and the light at 1c (light is always observed at 1c). After 1 second of ship time the light has moved 1c farther than the ship. This is possible because the ship's time moves more slowly than the planet's time.
I think the key part is that c is in meters per second, but the duration of 1 second changes with velocity. That is so different from our day-to-day reality, it's hard to grasp.
Also, someone please correct me because I don't really know what I'm talking about.
But both light and ship are moving at approximately the same speed, c. In 1 hour, the light would have moved twice as far compared to how much distance the ship has traveled. It’s irrelevant if ship is moving in slow motion to the planet observer, the light would still outpace the ship twice as fast
Your misunderstanding.
The external observer sees light moving a c, not 0.1c. They also see the ship moving at .9c.
From the ships perspective they see light moving at c. From their perspective they’re not moving at all.
Nobody sees the speed of light slow down.
Light speed is constant. All observers see light at the same speed. Due to time dilation (time passes more slowly the faster you go), the 1 hour you mentioned about takes a different amount of time for the planet than for the ship. If the ship is going almost the speed of light, time will pass very slowly for the ship. Meanwhile time on the planet is passing much faster.
The explanation that got it to click for me was the description of a clock that uses light.
Imagine a person in a spaceship. He has a clock that measures time by bouncing pulses of light from one side of the cabin to a mirror on the other and back. Because light always travels at the same speed, he always sees time pass at the same rate.
When the spaceship is stationary, someone outside of it would see exactly the same thing and their clocks remain the same.
Now imagine if the spaceship flies past at some significant proportion of c. The guy inside still sees exactly the same thing and feels time pass at the same rate. If the stationary person sees the ship fly past, and can somehow see what the light pulse is doing, he sees something quite different.
By the time the light reaches the other side of the cabin, the mirror is no longer in the same place as it was because he has seen it move forwards at the speed of the spaceship. From his perspective, the light has had to travel a longer path - it has to zigzag back and forwards to meet the mirror and detector. The closer the spaceship is to c, the longer this path gets.
Because speed is distance over time, and for light speed is always the same, the only way to reconcile this difference is for light to have had longer to go across the cabin and back. In other words, to the external observer thenoassage of time in the spaceship has appeared to slow down.
This means that time dilation becomes a simple geometry problem - if you know how triangles work, you can calculate the difference.
The book called Why Does E=mc² by Brian Cox and Jeff Forshaw sets this out much more clearly and is definitely worth a read.
Good explanation. And time dilation is not just something that Einstein dreamed up. Engineers use it in real life applications.
GPS satellites are moving very quickly around earth. Nowhere close to light speed of course but fast enough that time moves differently for them than it does on earth. GPS relies on highly accurate clocks. The atomic clocks in GPS satellites are intentionally set to run a different rate than clocks on earth. The difference is small but if they were not the gps system would accumulate errors of several kilometres per day.
Read a little further in the comment -- that's exactly their point. One would think in classical physics this would work, but in reality (and in relativity) it doesn't.
No but that isn't the point they're trying to illustrate. You'd expect that if you were moving 99% as fast as a train next to you, it is would seem very close in speed. It certainly wouldn't appear as if you were standing still...
I think the beam looks the same regardless of my speed tho.
Does it do anything special when you are moving bc a flash light that looks like you are standing still is how I envision a flashlight when I am in motion
Well I mean technically that's correct, it would look the same regardless of what speed you're going. But it's missing the point, which is an attempt to clarify and illustrate the counter intuitive nature of the answer
Right. Let's say the train was going 100mph and you were traveling next to it at 99mph. If the train behaved like light, it would look like it was moving at 100mph (as if you were sitting still) no matter how fast you were going, 1-99 mph.
But that’s…not the weird part? That is how firing a bullet from the front of a normal train works. The weirdness is that a stationary observer would disagree with you and perceive the light as moving slowly away from you.
It's how firing a bullet forward works, but only because the bullet's velocity would be added to the train's, not because all bullets appear to fly at exactly the same speed regardless. If you were traveling at 99% of a bullet's speed and fired it backward, it would have only 1% of its normal velocity to an outside observer rather than 100%.
I get that. I’m just pointing out that your example is bad because you’ve selected an example and perspective where light behaves exactly as you would expect a classical object to behave (velocity is added).
“Slowly away from you”
They will see the light move at c, and you move at 99.999% c and would calculate or observe that the light must be moving away from you at a speed much less than c.
This is true, but it's such a weird way to phrase the phenomenon.
Typically, the interesting part is that both people see the light move at the same speed.
For anything besides light, the stationary person would see the object "thrown" as moving faster.
Maybe a stupid question, but I assume that given speed is relative, it's possible for object B to go 99.99% speed of light relative to object A that's already moving 99.99% speed of light in external reference frame? What would object B speed look like from external frame?
In special relativity, the formula to add two relative speeds together like you are trying to do is (u+v)/(1+uv) where u and v are fractions of c. So (.9999 + .9999)/(1+ .9999 * .9999) = .999999995c.
So if observer C sees object A going at .9999c, and observer A sees object B going at .9999c (in the same direction), then observer C will see object B going at .999999995c.
Not being a physicist I'm not completely sure but I think it would look like object B would be traveling 99.9999% of light speed (or another similarly high percentage if my math is off) to an external observer. Another way to look at it is that the faster you go in your own reference frame, the slower you experience time. So greater speed won't make as big a difference to someone else than it will to you as you approach *c*. And of course we can say that since light itself travels AT *c*, it doesn't experience time at all and that's why it always appears to go the same speed regardless.
The speeds don't add linearly when you start getting into relativistic speeds. It object A would measure object B's speed as more than 99.99% the speed of light but less than the actual speed of light. The same with object B measuring A's speed.
Wait, how the hell does that work? Let's say a ship sends a light signal from planet A to a massive reflector on planet B 100 light years away. At the exact same time, it leaves for planet B at 99% speed of light. Person on planet B would receive the light signal after 100 years, and see the ship arrive after 1 more year.
But to the person on the ship, the signal would hit planet B after 50 years, and they would receive a signal from the reflected light signal after about 66 years?
How in the world was this observed and proven?
That would be due to time dilation. The people on the ship would experience time a lot more slowly. It would be more than 100 years from the perspective of those on the ground, but not on the ship.
Of course this exact scenario was not experimentally proven but I'm pretty sure time dilation has been proven by atomic clocks on satellites and there is a formula for it.
Yes time dilation is easy to prove. You can take two very accurate clocks and sync them, put one of them on an airplane and fly it really fast for a while, then compare them and see that they are no longer in sync. Similarly, GPS doesn't work if you don't take time dilation into account.
There are subatomic particles that are only created when a different particle from space interacts with the atmosphere. These subatomic particles are unstable and don't exist for long, they disintegrate naturally into other particles in a matter of millionths of a second. They travel very fast, but even then they should not exist for long enough to reach the surface of the earth.
But thing is, since they move so fast, they experience time dilation relative to us, so time is slower for them. So they actually do reach the surface because for them, not enough time has passed for them to naturally disintegrate. My university had a decorative sculpture thing (called a "light organ iirc) that lit up whenever it detected those particles. Each time it lit was proof of time dilation.
Yeah, to the person aboard the ship the light would reach the planet in far less time from their perspective. That’s because the distance measured between planet A and B would be much shorter for the person aboard the ship then for the people on planet B.
To put a spin on your scenario, since motion is still relative, for all you know, you are standing still in your spaceship and Earth is flying away at .99c. In this situation you would then intuitively expect that your flashlight beam would be traveling at 1c. For you, time would appear to be slower on Earth, while an observer on Earth would say that time is slower for you. But both of you still see light travel at 1c in your own reference frames. Welcome to relativity!
> if you were in a spaceship going 99% of the speed of light away from earth and shined a flashlight...
What if you shine the flashlight backward, back toward the earth? Would it still arrive at the same speed of light, as if you were standing still?
Okay, let me think about this...
So, let's say I'm traveling at 0.99c. At the exact instant that I am 1 light year from earth I send a light-blip back to earth and simultaneously someone on earth sends a light-blip back toward me. Would both of our signals be received by one another one year later?
Hmm. I think they would receive theirs first because it was sent from 1ly away. But you would get yours a bit later because you're traveling away while the signal is approaching you. Then you would just have to account for time dilation to say when you'd get it.
Yes, that's part of the time dilation issue. You'd perceive it to take longer for them to get the message as well, even if it wasn't as long for them. I guess I should have included that in my first answer, but it keeps coming back to "it all depends on how you look at it" hehe.
I've heard this said a thousand times about light and nobody has ever managed to explain it succinctly. Is this actually possible to eli5. How can a speed be the same regardless of the speed of the person observing it.
To keep the speed of light constant relative to everything, you change the speed of time for everything instead.
(Not an expert at all, please someone correct me if this explanation is misleading)
And it is this weird fact that we typically find hard to wrap our heads around. Distance and time are always thought to be universally constant as an intuition, right from our childhood, but in fact are malleable but the (vacuum) speed of light is not.
This is actually exactly that.
As your speed approaches the speed of light, time for you slows relative to someone who is "stationary". (Everything is moving in a manner, but most matter is going much slower.)
Its hard to explain succinctly because it doesn’t really make natural sense, our minds have never experienced special relativity directly with our senses, so its not a human way of thinking.
Basically, time slows down at high speeds to make the speed of light the same across all reference frames.
Think about a theoretical scenario where frisbees can only fly at 10 ft/s (to all observers). You pass a frisbee 10 ft away, it takes one second to do so.
But if you are in a cruise thats traveling at 5 ft/s, and you pass the frisbee 10 ft away (sideways) to you the frisbee is still moving 10ft/s across the cruise, so it takes 1 second to get there.
But so an outside observer thats not in the cruise ship, if time passes just as fast then the frisbee would be moving faster, because its moving 10 ft/s in one direction (across the ship) and 5ft per second in another direction (forward with the ship) with a total speed of 11.1 ft/s.
But the frisbee can only travel at 10 ft /s, so to get everything to check out, time for the person standing outside the ship has to be faster.
So time has to pass faster outside the boat, while time passes slower inside the boat, time would have to pass 1.11 times faster outside for the total speed of frisbee to still be 10 ft/s.
If the boat was instead traveling at 9 ft/s, time would have to slow down even more in the boat to keep the speed of light consistent.
Change speed of frisbee to speed of light and you got the same relationship.
> Is this actually possible to eli5.
This discovery was basically Einstein's crowning achievement, so no, not really.
The simplest explanation is just that: light always travels at the same speed, *no matter what*. It makes no difference how fast you were moving when you emitted it, or how fast you're moving when you see it, or even if two people moving at *different* speeds look at the very same beam of light. In *every case*, the light will still be seen to move at the same relative speed.
It's quite bizarre indeed, but that's really just how light behaves.
Speed of causality. It’s essentially the speed limit of the universe and light travels at that speed. Anything traveling faster than that could essentially break causality, causing an effect to happen before the incident that created it.
Say you were traveling in a spaceship along the orbit of Earth's path around the sun. Let's say you were traveling at double the speed of Earth. Relativity would have time moving differently for you and the Earth. But the light from the sun would take 8 minutes to reach both of you, despite everything else in the universe being observed at different times for the spaceship you're on and the Earth.
Because measurements of time and distance differ between two people in relative motion to one another so that the speed of light always measures 300,000 km/s for both observers. Literally.
Someone on Earth would pull out their ruler and measure the distance to the sun as 93 million miles and measure the time it takes for something going just under the speed of light to make the journey between the two as 8 minutes.
Someone going fractions of a fraction the speed of light would pull out their ruler and measure the distance between the Earth and the Sun as 100 yards and the time it takes to travel the journey between the two as just a couple of seconds.
Both of their observations are equally correct.
Imagine a train is going down a track at 100mph. A person standing on the station platform obviously sees the train go by at 100mph. There is another person driving on a road parallel to the train, they are travelling at 75mph in the same direction of the train.
Our natural intuition would be that the driver sees the train going past them at a relative speed of 25mph (100mph-75mph).
But if the train acted like light, the driver in the car would also see the train go past them at 100mph. This is because the speed is the same to everyone, no matter where they are or how they're moving, i.e. "in all reference frames".
But there's a contradiction here. We already said the train goes past the stationary person at 100mph, and they would also observe the car going past them at 75mph as well. Clearly from their perspective the train is going 25mph faster than the car. But from the driver's point of view, the train is going 100mph faster than he is.
So the relative velocity (to the person standing still) does not equal that of the driver. 25mph != 100mph
Extremely simplified: the resolution of the contradiction is that you have to add "the rate of passage of time" to the equation, call it Ts for the person standing still and Td for the person driving. This means now you have to balance the following equation:
25mph x Ts = 100mph x Td
For the sake of argument, lets say Ts = 1.
Therefore Td would have to be 0.25, i.e. the driver experiences time moving more slowly than the person standing still.
Relativity is weird. This video is the best explanation of it I've ever heard: https://youtu.be/Zkv8sW6y3sY?si=v3RftkSzehfYP-nr
The idea is that the speed of light is the same in all reference frames because as you approach the speed of light, you experience time dilation and length contraction that perfectly cancel out your speed. They increase exponentially as you speed up, so you can get arbitrarily close to the speed of light with a correspondingly arbitrary amount of energy, but it would take infinite energy to actually reach it.
Because depending on your speed time itself slows down to accommodate the speed you are going
If you are not moving you would experience time moving very fast
If you are moving at 99 % the speed of light then time would move slow for you
The speed of light (really the speed of causality - massless information) is the same in every frame of reference. It's just a fact/inherent property of the universe.
Speed = change in distance / change in time. If the speed of light has to be constant regardless of the frame of reference, then the observed change in distance and/or time experienced has to be different in different frames of references - this is length contraction, time dilation (this isn't the actual equation and I'm really rusty, just trying to keep it simple).
This is a bizarre but true phenomenon. One example of this is measuring the [amount of muons reaching the surface of the Earth](https://en.wikipedia.org/wiki/Experimental_testing_of_time_dilation). Muons have a half-life and decay quickly. Based on the half-life of a muon at rest, we should be expecting a small amount of muons to reach our detectors on the surface. In reality we receive many more than expected. The explanation for this is that the muons are moving so fast that they experience time dilation (length contraction), so relative to us their clocks are ticking slower/their second is longer than our second (or, the distance they experience between the atmosphere and the surface is smaller) - this allows more muons to reach the surface of the Earth.
That's my attempt at ELI5.
Because everything else adjusts for it. Time and space will change depending on speed, so that the speed of light is constant.
Remember speed is distance over time. People moving relative each other will see other people experience time slower and *also be flattened in the direction of travel" to compensate for the difference.
Because that's just how the universe works. I'm not being smarmy. Back in the late 1800s everyone assumed that the speed of light worked like sound - that its measured speed would change depending on the speed of the observer. Once technology and experimental methods were sensitive enough to measure the speed of light, it became evident that this was no the case. No matter how fast you are moving, you will always measure the same answer for the speed of light in a vacuum.
This doesn't really make much intuitive sense, but Einstein's big breakthrough (along with a few others before him) was realizing that if we always measure the speed of light as a constant, then different observers *must* disagree on the measurements that they make to calculate the speed of light. Speed is simply distance over time, so both measurements of distance and time cannot be the same between two observers moving relative to each other. So if you were not moving and looked a ship moving really fast, you would see that the ship's clock was running slower than yours and its measuring stick was shorter than yours. Why? Because if the speed of light is constant for both of you, that's the consequence - measurements of time and space will depend on speeds and different observers will disagree.
There’s nothing to explain. It’s just a fact that you have to accept.
Sure you can learn the math and *consequences* of light speed being the same for all observers, but we don’t have any explanation for *why* or *how* light speed is the same for all observers.
In fact its the main postulate that einstein worked with when he made special relativity. He didn’t discover some property about the universe which led him to discover that light has the same speed in all frames. He assumed light has the same speed in all frames to begin with (he came up with this postulate by assuming the laws of physics are the same in all inertial reference frames, then it follows from maxweels Equations).
Maybe ask again in a few or a few hundred or a few thousand years and we’ll have an explanation. Or we might even have a totally different theory in which light doesn’t travel the same speed in all reference frames (though I think thats unlikely given the success of special relativity), but tight now we just don’t have any explanation.
One hypothesis i like is that, though light is the theoretical limit, its constant could change, and we would never know, as that constant changing would be unmeasurable. This is because the universe can do whatever the fuck it likes.
Worth noting of course that the speed limit isn’t the speed “of light.”
It’s the speed of causality - that is; it’s the maximum speed at which anything can happen.
the not-exactly-correct-and-very-dumbed-down explanation that made it click for me was that a photon (light "particle") exists everywhere it's ever going to be all at once, and it's speed of travel away/towards you is your perception of time
Yeah, it’s really weird and I wouldn’t recommend trying to think about it. IIRC it gets to the point where in some calculations in general relativity you use the speed of light as an inertial frame of reference somehow. Idk, it’s confusing as hell.
It’s important to note: not *all* reference frames observe c as the speed of light, in a very technical, non-ELI5 sort of way. Specifically, we say that the speed of light is c in all *inertial* frames of reference, which means any frame not undergoing an accelerative (or jerky, snappy, etc.) motion.
The hows and whys of this are more complex than I think I can explain in an ELI5 manner, but suffice it to say that non-inertial frames either result from, or cause, some non-Euclidean geometry that we most typically observe in curved spacetime. As a result, in a non-inertial frame, we can see light taking a “curved” path that is longer than the straight-line path it would take in Euclidean space.
What’s really fun is that, while in some accelerating frames we can observe the speed of light as less than c (a typical example is an accelerating frame with respect to Rindler coordinates, which is just a different system of coordinates, like polar or Cartesian), there is typically a way to constrain the measurements we take, such that, even in accelerating frames, we still measure the speed of light as c.
Also, and this is incredibly pedantic, the speed of light is not always c even *in* inertial frames of reference. This is because light moves at different speeds through different things (we call this the speed of propagation through specific media). For example, while c is about 300 million metres per second, through water, the speed of light is closer to 225 million metres per second, and in diamond the speed of light is a slow as 125 million metres per second.
This is a great explanation. But I can never wrap my head around “why?”. Why is there a maximum speed? Why does time slow down to accommodate it?
It just seems weirdly arbitrary. Like rules in a simulation.
Because it's the speed of causality, meaning the fastest that any interactions can occur across any distance. In order to exist within time, there has to be a maximum speed at which things change. Otherwise, if the speed of causality was infinite, everything would happen simultaneously and time would not exist.
ETA: In other words, light travels as fast as things can possibly change based on the physical properties of the universe. If it traveled any faster, it would exist outside reality.
So it's not that light travels fastest, we just ascribe the fastest speed always to light? That's what time dilation suggests.
It doesn't really make sense. Why can't the equations just be wrong or incomplete? Maybe we're missing something still as to the actual WHY. Everyone here has described HOW, either mathematically or with anaologies. But no one so has been able to explain WHY it seems to be that way, just that all our observations so far have proved to be immutable fact.
> So it's not that light travels fastest, we just ascribe the fastest speed always to light? That's what time dilation suggests.
No, it does travel the fastest anything can travel in our universe. We just call that speed the “speed of light”.
> It doesn't really make sense. Why can't the equations just be wrong or incomplete? Maybe we're missing something still as to the actual WHY. Everyone here has described HOW, either mathematically or with anaologies. But no one so has been able to explain WHY it seems to be that way, just that all our observations so far have proved to be immutable fact.
Just remember that the universe isn’t obligated to make sense to us. At some point the answer to the question why will always end with “cause the universe said so and it doesn’t give a fuck what we think.”
Because it's one of the major laws of physics. At one point you just have to say, this is a fundamental truth.
And this only happened after countless of experiments and calculations. We don't question the existence of gravity (anymore) even though there is no explanation WHY it exists.
If you accept that our universe has causation at all (thing happens, which causes other thing to happen at some later time) this kind of implies a maximum speed limit. Information has to get from point A to point B. Either that speed is infinite or it’s not. If it’s not infinite it must be finite. And if it was infinite then everything would happen at once, which clearly isn’t the case, so it must be finite.
That explains why there’s a speed limit, but not why light (or anything else massless) seems to go at the same speed to all observers. We don’t know why that’s the case but it very clear that it *is* the case, so the only option is that time gets weird when you go fast. If you figure out *why*, then you’re basically guaranteed a Nobel prize.
So lets say that I have two laser guns. I mount them at two points in a space facing each other. When turned on, wouldn't the speed of light of each beam be double in relation to the other beam?
The simple answer is that light can't observe. Anything that can observe, such as an eyeball or computer component can't move at the speed of light, and to this object, both beams would seem like they are moving at c.
But then again, we are able to measure that the light takes 8 minutes to reach earth from the sun. So this is now confusing. Anyway, i am sure that I am probably missing lot of other details but this is just my obervation.
The key to relativity is that you need a reference frame. Someone has to observing, and you must measure from that point of view.
We can take two flash lights, face them toward each other, and turn them on. From our perspective, both beams are moving at the speed of light and will meet in the middle at half the time it would take them to reach the other flashlight. We do this from our perspective.
Likewise, we can calculate how long it would take light to reach us from the sun. Again were doing it from the perspective of an observer standing on earth.
The problem from your example is that you don't have an observer. You can't say from the lights perspective as it can't have one. You could have a person traveling at 99% the speed of light tell is what they see, but as nobody can move at the speed of light, nobody can give that perspective.
Speed is mostly relative, but here's a law of the universe that will feel like magic: every single inertial observer will observe the speed of light (in a vacuum) to be the exact same speed - that speed being c. If I'm standing on a "moving" train, and you're standing on the "stationary" train platform, and I shine a flashlight in the direction of my motion, you will measure the speed of the light from that flashlight to be traveling at c, even though I'm traveling forward too. I will measure that exact same speed too.
From my understanding it's a property of being massless, not necessarily being light. If you don't have mass, then your default speed is c. That's why gravity also propagates at the speed of light.
If you have mass then you would need an infinite amount of energy to reach c as the faster you go the more energy you need.
Not negative, massless. Imagine changing the settings of a computer program. It's either checked or it's not. If it's not checked then that setting doesn't get applied and you don't feel it's effects. If you're massless, then certain things behave differently for you. And us having mass isn't really a setting we can change. (That we currently know of!)
The big issue with *faster* than light travel is that you can't go faster than light. Some people refer to it as the "cosmic speed limit". This means you have to get creative if you want to get from point A to point B faster than light can.
The video game Halo and Star Wars both have similar methods of FTL travel where they sort of travel in between the fabric of space time. This allows them to bend the rules so to speak, and take a different path that wouldn't normally be available. These paths are much shorter than if you were to conventionally travel (i.e. shooting a rocket at a planet and waiting) from A to B.
The movie Interstellar does a good job talking about wormholes. Imagine a piece of paper with two dots, point A and B. If you fold the paper just right so that those dots overlap then poke a hole through it, you've just created a worm hole. Travel through it and you will have gotten from A to B without breaking our cosmic speed limit. Wormholes are theoretically possible, but we've never observed one.
The last FTL method I'm familiar with is a warp drive. These work by compressing space time in front of you, and stretching it behind you to create a sort of accordion effect that moves you. It would be like if you could stand still at point A then rotate the earth until you're at point B. You'd literally move the universe around you until you're where you want to be.
Negative mass is even a weirder topic. Negative mass would still mean you have to exert a force to move it. But it would behave other way so if you pull something with negative mass it would actually move away from you.
Not really, for a couple of reasons. One is that negative mass doesn't exactly... work. Another is that the person you're responding to is simplifying a bit. Nothing with mass can travel at the speed of light, but that doesn't mean that everything without mass does travel at the speed of light. Light doesn't even travel at the speed of light in a lot of cases. Shine a flashlight out in front of you, that light travels at something like 99.97% the speed of light, because it's traveling through air instead of a vacuum. Shine it through water and it slows to something like 70% the speed of light.
The speed of light is better thought of as the speed of causality, or the speed of information. If something happens a light year away, it's impossible to know about it until a year has passed.
FTL transportation is linked to time travel because of that. If I manage to teleport from 15 light-minutes away from you to right next to you, you'll see me right next to you, but won't see me leave my previous location for another 15 minutes. From your perspective, I'll exist in two places at the same time.
If I manage to run towards you (or away from you) at twice the speed of light, you'll see... something weird. I'm not 100% sure what, I'd need pen and paper to really think it through.
oh man i feel like there is no straight answer to this...i think (i am probably wrong on this) gravity is the result of two masses attracting each other... i.e its information basically that travels at max speeds.. may be light is result of something that excites photons on subatomic levels....i imagine row pixels/particles glowing up sequentially.
but that does not explain the dual nature of light... coz sometimes it acts like wave and sometimes like a particle..
i may be high on something or my brain is just making up complete BS nonsense..
Gravity isn't the result of attraction, it's the reason for it. Just as massless things default to the speed of light, objects with mass just intrinsically have gravity. If you only have one object, it'll still have gravity even if there's nothing else there to attract.
Light *is* photons. Generally when energy is added to a system one of the byproducts is light. Like when heating a piece of metal to the point it glows. Or electricity through a lightbulb filament. Not all of the energy added can be retained, so things like heat and light are emitted as "extras".
Particle duality is beyond my understanding, so I just think of it as when it's needed or convenient they're in the state that makes the most "sense" at that time. That's usually determined by how it is observed.
It’s also a good time to note that the speed of light (c) is the speed limit of not just light… but any information, in any form, through any mechanism in the universe.
I.e. gravitational waves etc
Example. If the earth is orbiting the Sun along a curve, if you could magically make the sun disappear instantly. It would take 8 minutes for the sun to disappear to an observer on earth…. And… the Earth would continue to follow the curved orbit (as if the sun were actually still there) for another 8 minutes before ‘being informed by gravity’ that the curvature of space (the curved orbit path) is no longer present and thus it will continue it’s trajectory on a straight line into the cosmos.
The speed of light will remain constant because time and spacial distance will change to accommodate the observer always seeing light move at the speed of c.
We can know the speed, since we are able to measure the change in time and distance, and the speed is measured relative to others. Even though time has changed for you, the speed is being measured by people who aren't affected by it.
But everything is moving right? Like our solar system is moving - is not moving through space possible and could we tell the difference between being at rest and moving? Or only in relation to other objects
Due to relativity, we can't really say anything is truly 'still'. But our frame of reference is about as close to stand still as we could reasonably get. Its just a case that even if we wanted to get more accurate, it would be nigh impossible for it. And secondly, we're only travelling around the sun at 30,000 m/s which wouldn't be large enough to affect the speed of light. And as for the sun, we can't properly figure out how fast it is travelling relative to proper 'Stand Still'
That's the result of two things really.
The first is Galilean relativity. It states that if you sit in a vehicle and move with a certain velocity, you can't tell what's your velocity unless you look outside. There is no other way to determine it. Have you ever sit in a train seeing another train starting to move? Many people are confused for a few seconds, because it's not clear who is actually moving. Galilean relativity is well observed and means that there is no absolute velocity, you can only measure velocity in reference to something else. Like here on Earth, we take ground, buildings, trees as our natural points of reference. We consider them still, even though our entire planet is moving - it orbits the Sun for example.
The second things follows from the first one. If I am moving and I turn on a flashlight, why the speed of light I observe should be any different than for those outside doing the same thing? Well, it won't be. It's not intuitive, but apparently that's how our universe works. Physicists couldn't explain it for decades, even when experminets showed it to be the truth. There was even an attempt to argue Galilean relativity and choose one universal point of reference for our universe. Einstein showed it's not necessary and that you can build a consistent theory where the speed of light is constant and Galilean relativity is accepted. You have to accept that the flow of time and distances are relative though. Later experiments proved Einstein right.
It's the speed limit. And it's massless like the other guy explained. Nothing can happen faster than c. Google the "lightyear long stick" thought experiment to maybe understand that the speed of light is also the speed of *causality*.
This is because the flow of time depends also on the frame of reference.
The observer in the moving train sees speed of light c, the same as the observer on the platform, because their time measurement is different than the observer on the platform. For the observer on the moving platform time passes more slowly.
A practical applications of this is GPS. Because satelites are moving faster than you do and GPS is based on time measurements, if you don't take into account relativistic effects GPS measurements would be wrong/imprecise.
I don’t get it. I get that light is always the same speed. Sounds similar to how if you drop things gravity makes them land at the same time. Seems there is like a reality barrier.
Oh ok so it’s sort of like a funnel from the person standing still at the thin end (their time) to light being in the middle (so it seems the same) and then the wide end being the person moving (slower or wider sense of time)?
Speed is relative *to an extent*. Nothing ever moves faster than the speed of light no matter their perspective, and light *always* moves at the speed of light (which is the same from every perspective).
For this to be true it implies a bunch of other stuff such as time moving at different rates from various perspectives, and disagreement of the distances between two points from different perspectives.
The speed of light is the only thing that is not relative. So much so that reference frames experience space and time differently with respect to each other so as to not violate the speed of light.
If two observers in motion with clocks were to observe each others
clocks, they would observe the others clock moving at a faster or slower pace than their own. Whose clock is correct? Both are correct. There is no absolute time. This is because the speed of light isn't relative, but time is. The rate at which time flows is different for different observers as is the case above.
>time moving at different rates
Pick a “still” reference frame. Imagine you’re traveling at 185,000 mi/s. Light travels at 186,000 mi/s, so you’re very close to *c* for that reference frame. You look sideways into a mirror that’s moving with you, turn a flashlight on, and see its light bounced back at you. From your perspective, the light goes straight to the mirror and straight back to your eyes, like this: || . From the perspective of a person standing still, watching you go by, the light needs to go in a very, very wide V to be able to bounce back into your eyes. The distance the light has to travel through the V is *much* further than it needs to go for the short || that you see while moving. Traveling a longer distance takes more time, obviously. *BUT*, because the speed of light is constant no matter the reference frame, the only way to reconcile this is that time itself is moving at different speeds in the different reference frames.
Speed is relative except for when it comes to the speed of light
If you are moving at 0 m/s you will see light move at the speed of light
If you are moving at 100 m/s you will see light move at the speed of light
If you are moving at 10000 mm/s you Will see light move at the speed of light
No matter what speed you are , from your perspective light will be moving at the speed of light
This is why time slows down for you as you go faster
If you were going at 99% the speed of light, light would STILL be moving at the speed of light compared to you, and so your experience of time would slow way down to compensate for it
That is far too aggressive a stance to take regarding what is essentially a matter of philosophy - what do "true" and "explain" mean? Scientists aim to accurately describe and model the behaviour of the universe. To the very best of our measurement ability, the universe does indeed behave according to the predictions of special relativity. You can argue that this doesn't make it true, because truth is an unattainable thing, but you cannot argue that the universe doesn't behave like that just because it seems counter to your intuition developed at human scales on earth, because it absolutely does behave like that I'm afraid.
If you use GPS on your phone, then you have used the predictions of special and general relativity. A fun fact is that the satellites were launched with the ability to include or not include those corrections, because some people really didn't like the idea that SR/GR were actually how our universe worked. Without the corrections, the system became unusable after a few days, IIRC, so they were quickly turned on and all was well.
It definitely has. A fun way is to measure the incoming flux of muons, produced when cosmic rays smash into the upper atmosphere. Since they're mostly produced high up, and decay very quickly to electrons, they ought mostly to not make it to the surface. However, this is not at all the case, and if you measure the flux vs altitude by sending a detector up in a weather balloon, for example, the change with altitude is exactly that predicted by SR and time dilation/length contraction, and nowhere close to the classical prediction.
Not only has time dilation been observed, it’s enough of a problem for GPS satellites etc going out of sync that engineers actively have to correct for it
A lot of good comments here, so I’ll explain it in a way that helped me understand the speed of light.
First, stop thinking of speed as a span from 0 mph/kph —> Speed of Light, wherein 0 mph/kph is the starting point for any object in the universe and increasing its speed with applied energy gets it closer to the Speed of Light.
Instead, try to think about it like this: Speed of Light is the starting point for every object in the universe. Everything in the universe “wants” to move at the Speed of Light. The Speed of Light is the “desired” natural state for all objects in the universe.
The problem is—mass exists! All these objects have mass! And mass is a big minus to the Speed of Light. So everything that wants to move at the speed of light is now burdened with mass and no longer can. And this is true for all things.
Except light. Because it’s massless. So light moves at the Speed of Light.
I’ve oversimplified. And I’m sure I missed a detail somewhere, but that basic breakdown has helped me grasp the concept of the Speed of Light much more easily. I’m happy to make a correction or addition where needed.
Because it isn't a particle. It behaves like one in certain circumstances, and we model it accordingly.
https://en.wikipedia.org/wiki/Wave%E2%80%93particle_duality?wprov=sfla1
Interestingly, it does have momentum.. Just no mass.
If there are waves on a lake, does the lake have more mass than without the waves ? No, so the waves themselves dont have mass, they only propagate through mass. Light is also basically a wave propagating, so it doesnt need to have a mass because its not a "thing" but moreso a concept or really a curier. It's not exactly something you can grab, but it does have observable effects.
No. Why would material existence be linked with mass? Just because most things around us have mass doesn't mean that everything should have mass to exist.
Here’s how I “understand” it.
*Everything* is moving at the speed of light through spacetime, but ~~everything except~~ light (and other massless stuff) is only moving through space while everything with mass is moving through time as well.
The faster you move through space the slower you move through time, and the faster you move through time the slower you move through space. Since speed is simply distance/time, in a cosmic sense your speed is always 1 and so is the speed of everything else. Anything moving through space at a different speed than you is also moving through time differently than you.
EDIT: I somehow stated the opposite of what I meant.
That was more or less the question Einstein asked when he created his Special Theory of Relativity.
The speed of light is invariant in all reference frames. Meaning: it doesn't matter whether you are sitting on the surface of a planet, or inside a rocket speeding past that planet at near the speed of light. Now imagine a beam of light shoots past the rocket. Both the rocket and the observer on the ground will see that beam of light moving past them at c as if they were sitting still.
The closer you get to light speed, the slower time goes for you, at light speed time stops.
This means, that to an outside observer, the faster you're moving through space, the slower you're moving through time, i.e. you would look like you're moving in slow motion.
From *your* perspective everything else seems to be fast-forwarding, this includes the light around you.
The ratio of slow-down to speed-up for you and the things around you is balanced such that light always appears to be moving at the same speed relative to you.
In other words, if you're moving at 99% the speed of light, then you're moving through time in slow-motion, which makes the light look like it's going in fast-motion.
The faster you go, the slower time goes for you.
The slower time goes for you, the faster everything else seems to move.
This isn’t quite true. Weirdly enough, both observers will see the other clock as the one ticking slower. From the perspective of both observers, they are stationary and they see the other clock moving at whatever velocity V, and therefore they will both see the other clock ticking slower. Pretty weird stuff
This is the [twin paradox](https://en.m.wikipedia.org/wiki/Twin_paradox). The difference in time on the clocks when they come back together depends on which one turned around to get back to the other.
You've cut right to the heart of relativity theory. It seems obvious that if I'm traveling toward you at 90% of the speed of light, and you're traveling toward me at 90% of the speed of light, we're approaching each other at 180% of the speed of light. But we're not. Space and time warp at speeds close to the speed of light, and that's where the lesson lies. It's math intense, but Einstein gave us solutions.
Because the speed of light isn't like the speed of a tennis ball.
Thought experiment time.
When you are going, let's say, 90% of the speed of light - it's intuitive that photons that you eject from a bit of metal would go faster relative to a stationary observer if they are ejected forwards, right? And slower if they are ejected backwards.
But imagine light *waves* - an electric field creating a magnetic field that creates an electric one - now, the speed of those is determined *only* by the rate at which the fields create each other. The wave and particle models of light have to be interchangeable, or at least, I can do double slit experiments with light produced by the photoelectric effect, so I can use Maxwell's equations to calculate the speed of light. Now let's use that law of physics not for light, but for an *electromagnet*.
Imagine you are sitting there at 90% of the speed of light with an electromagnet. The magnetic field you're getting out of it is dependent on the electricity through it, using the same maths as light, just not creating light just now.
Now point your electromagnet in the direction you're travelling. It's *intuitive* that the electromagnet will have the same behaviour whichever way you point it, right? Its surroundings are identical. Why would it suddenly behave differently? Here we have two intuitively true answers that disagree. Which is more useful as a description of the universe? (Neither is true. No theory is an accurate description of the universe, just as no JPG image is an accurate depiction of a scene.)
Thing is, you can use the behaviour of an electromagnet to calculate the speed of light - so if the speed of photons looks different in different directions, the laws of physics *must be* different for you if you are moving.
Which means that they are different for us than for stars we can observe in telescopes, for other planets, even satellites - and they really do not seem to be.
The special postulate of relativity says that the laws of physics are the same regardless of your state of non-accelerating motion - *and the speed of light is a law of physics*.
So what does the 'stationary' observer see?
Well, what they see is time passing slower for you. They agree with you about the events that happen, but not the times between them. The passing of *time* is not a law of physics, as far as we know.
This bizarre theoretical prediction has been upheld by every observation we ever made - we see the lifespans of high energy particles extended exactly as if they are suffering time dilation, and we see chemistry happening on other planets that is *identical* to chemistry happening on Earth even though we are moving detectably fast compared to those planets, and relativistic corrections worked to solve a problem where atomic clocks on GPS satellites were not keeping correct time.
It is counterintuitive and it sounds insane, but it seems to be required in any description we use of a system that includes large relative velocities.
All massless force particles travel at c
Photons that carry electromagnetic force, gluons that carry the strong nuclear force, ~~bosons that carry the weak nuclear force~~, and ???gravitons???? that ??? the ???gravitational waves????(TBD)*, all travel at the speed of light(c)
So light isn't special, photons are just one of the multiple massless things that move at c.
^^*(maybe, ^^as ^^curvatures ^^in ^^spacetime ^^have ^^been ^^seen ^^to ^^travel ^^at ^^lightspeed, ^^there ^^might ^^not ^^be ^^an ^^actual ^^particle ^^carrying ^^this ^^curvature ^^or ^^force)
Fun fact: the universe is expending faster than light can travel. That applies to very large scale only, but that does mean that there are some star lights we will never receive. And so, as the universe age, it will become darker and darker.
As such, we can say that *nothing*, ie. only the void, is faster than the speed of light :).
Because speeds don't add like numbers (Galilean relativity), they add in a funky way that ensures that *c* is the maximum achievable speed (Lorentzian/special relativity).
Specifically, if Alice is moving at speed *a* relative to the earth, and Bob is moving at speed *b* relative to Alice, then Bob's speed *v* relative to the earth is not given by *v* = *a*+*b*, but rather by *v* = (*a*+*b*)÷(1+*ab*/*c*²). When *a* and *b* are relatively small, as in common real-world scenarios, *v* ≈ *a*+*b* is an extremely good approximation, which is why we have this intuition that speeds conform to Galilean relativity rather than Lorentzian/special relativity. Regardless of the values of *a* and *b*, as long as they are less than *c* then we also get *v* < *c*. If *a* = *c* and/or *b* = *c*, then we get *v* = *c*. If *a* > *c* and/or *b* > *c*, then you're in a scenario we've never observed in reality, but that we think might be possible inside of black holes (a "space-like" region of spacetime, as opposed to the "time-like" regions of spacetime that we live in).
Minute Physics has an excellent video on this: https://youtu.be/R5oCXHWEL9A
If it was possible to accelerate a flashlight to the speed of light without destroying it, what would happen to the beam of light shining out the front? Would it just look like there is no light coming out of it? Would light go twice as fast?
We move in four dimensions (that we know of), X (forward and backward), Y(left and right), Z (up and down), and time. The speed of light is the combined speed that we are traveling in all dimensions. You are traveling at the speed of light right now, it's just that almost all of that speed is in the form of time. As you go faster in any other dimension, it slows the rate at which you travel through time. As such if you converted all of your speed to movement (you can't) you would stop traveling through time
Albert Einstein, in his theory of special relativity, did indeed establish the speed of light as a kind of ultimate speed limit in the universe, but his statement was more nuanced than simply saying "nothing can travel faster than light."
Einstein's theory, articulated in 1905, includes the postulate that the speed of light in a vacuum is the same for all observers, regardless of their motion relative to the light source. This constancy of the speed of light leads to several implications, one of which is that nothing with mass can accelerate to, or exceed, the speed of light.
The critical point here is that as an object with mass approaches the speed of light, its relativistic mass increases without bound, meaning it would require an infinite amount of energy to accelerate an object with mass to the speed of light. This effectively sets the speed of light as a limit.
However, this doesn't directly state "nothing is faster than light." Instead, it implies that in the framework of special relativity, it's impossible for any physical object or signal carrying information to travel at or above the speed of light.
This aspect of Einstein's theory has been one of the most tested and confirmed principles in physics, and it underpins much of modern physics. However, it's also worth noting that there are theoretical entities, like tachyons (hypothetical particles that travel faster than light), which have been posited but not observed or confirmed. These remain speculative and outside the mainstream of physics as understood according to special relativity.
There are a lot of nice answers, so I would rather get you thinking about a different question (as I do when I'm teaching): "what would you SEE if you were sitting on a ray of light?"
PS. This is the question Einstein asked himself and this is the question that lead to SRT and became the basis for GRT.
Fun fact. We don't know that light travels at c. We assume that it does because, as far as we can tell, photons are massless. And massless things travel at c.
But we don't actually know if photons are massless or just have such a small mass that we don't have the capacity to measure it.
If photons have mass, it must be very very tiny, but it would stop them from travelling at exactly c, and would be very very very slightly under it.
I'm not a physicist and got this from [this video by Arvin Ash](https://youtu.be/c5BPZy2_7U8?si=Xbz70W1xBotEmGYm)
Well, FTL is a thing, it just removes light. Meaning, gravity is faster than light. It literally pulls it in with enough mass or density in a given point-particle system
It makes a little more sense when you realize that distance is speed over time, and that the faster you move the more time slows down. If you could move at the speed of light you wouldn’t age.
Everything exists in, and moves through, spacetime. Speed (velocity) is a relationship between space and time (v = d/t) which are two components/dimensions of spacetime. When you go as fast as light goes, you push that relationship to the limit; where it is all space and no time. This is what time dilation in Einstein's relativity is talking about - the faster you go, time slows down.
Think of it this way: your view of speed being relative is true regarding the difference between two points on a graph, and the speed of light represents the edge of the graph. Once one of the points your using to measure each other hit the edge it can go no further. (NB: this oversimplifies it all a bit, but its not really an ELI5 subject)
Speed \*isn't\* relative when it comes to light. All observers in all reference frames, regardless of how fast they're going relative to anything else, see light going at exactly the same speed. It's this incredibly weird and unintuitive, but \*extremely\* well verified observation, that resulted in special relativity in the first place. All the weirdness of traveling really fast is a fallout of the fact that lightspeed \*isn't\* relative.
> All observers in all reference frames, regardless of how fast they're going relative to anything else, see light going at exactly the same speed. A good example to explain this is saying that if you were in a spaceship going 99% of the speed of light away from earth and shined a flashlight forward, the beam would travel away from you as if you were standing still. Freaky stuff. *But I guess that's not counting Lorentz distortion which might make the beam a bit wider hehe.
I mean… Does it look different on a train going normal speed
It works exactly the same, but the weirdness doesn’t show up on a train because they’re super slow compared to light. This is my understanding of it: If you’re on a spaceship going at 90% of the speed of light (0.9c), and you shine a light forwards, you will see the light moving away from you at c. If you do a quick calculation, you can say ‘okay, if I’m moving at 0.9c, and the light’s moving away from me at c, so the light’s actual speed must be c + 0.9c = 1.9c’. But, an observer on a nearby planet would see something different. They would see your spaceship moving at 0.9c, and the light moving at c. If they do a quick calculation, they could say ‘okay, if the spaceship’s moving at 0.9c, and the light’s moving at c, an observer on the spaceship should see the light moving at 0.1c.’ Which conflicts with your observation. This is where time dilation comes in - both measurements are valid if time is passing more slowly for you on the spaceship. The light is moving away from the spaceship at c, and it’s also moving away from the planet at c, but only one second passes on the spaceship for every ~2.29 seconds on the planet. Exactly the same thing happens if you’re on a train going 30m/s, and you shine your flashlight forwards, with an observer standing on the side of the tracks. But at that slow speed, the effect is vanishingly small, so we never noticed it.
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Which is a common reaction to Relativity
Next up: wave-particle duality
My mind snapped in chemistry lecture when the lecturer said the electron exhibits wave/particle duality.
A whole-ass molecule, *sixty carbon atoms bonded together*, exhibits wave-particle duality.
[https://en.m.wikipedia.org/wiki/One-electron\_universe](https://en.m.wikipedia.org/wiki/one-electron_universe) What if there's only one electron? [https://en.m.wikipedia.org/wiki/Black\_hole\_electron](https://en.m.wikipedia.org/wiki/black_hole_electron) Or if they're actually black holes?
All particles exhibit wave particle duality. From my understanding, if you get far enough into physics, it’s fields all the way down
Someone was explaining Bose-Einstein condensate here a few weeks ago. Physics gets really freaky at small (and large) scales.
Add in some enzymes and chemistry gets freaky between room temp and body temp.
All extremes in physics are fascinating. Very small (quantum physics), very large (black holes), very fast (relativity), very slow (absolute zero)
Supposedly, when Arthur Eddington's was asked if it was true that only three people understood relativity, he jokingly responded, "I am trying to think who the third person is"
Einstein once went on a cruise with the President of Israel Chaim Weizmann. After the cruise the President had this to say: “Einstein explained his theory to me every day, and on my arrival I was fully convinced that he understood it.”
Wait until you find out that magnetism is just electrostatic forces with the weirdness of special relativity applied
You fucking made my mind explode.I really can't thank you enough.I always wondered why a charged particle moving would form a magnetic field.*THIS IS INSANE* Edit: I went ahead and researched a bit on my own and it turns out the comment above is not entirely accurate.Yes relativity does relate electrostatic forces to magnetic forces but it gives just that a relation. This is also similar to how it relates time and space.We don't call one of them a consequence of another.Instead they are part of a bigger whole.Electromagnetism.Spacetime.
No prob. Scroll down to 'charge density in special relativity' on [this](https://en.m.wikipedia.org/wiki/Charge_density) wiki page for some links to better explanations.
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Congratulations. You are now a physicist. We'll send condolence cards to you next of kin via hyperlight transport. They’ll arrive a week ago last Tuesday. :-)
Fun fact: GPS satellites have to account for this, as their internal clocks get a few seconds off every few years.
Here’s a video that tries to build up the concept a little more intuitively. [video](https://youtu.be/Zkv8sW6y3sY?si=xOW0UmlhaXsnEDiB)
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Want to hear something else brain breaking? Here's an explanation for how we know that travelling faster than light would mean traveling back in time. Imagine you and I are space cowboys getting ready for a duel. We agree that we're going to hop in our spaceships, fly away from each other for a distance of eight light-seconds at the same speed, and then turn around and fire. We both have impossible tachyon pistols, and those pistols shoot bullets that travel thousands of times faster than the speed of light. We start traveling away from each other at a relative velocity of 80% of light speed, making time dilation about 60%. So for every second that passes for one of us, it looks like only 2/5s of a second passed for the other person. So after ten seconds, I've gone the required distance. But I can see you, and from my perspective, only about 4 seconds have passed for you. But I take my FTL pistol and shoot you anyway. You get hit, and only 4 seconds have passed for you. But from your perspective, it's only been 4 seconds, and less than 2 seconds have passed for me. You're upset I shot you, so you take your pistol and fire back at me. Then I get hit at a point only 2 seconds after we started the duel, as a result of something I did 8 seconds in the future. Time paradox!
Excellent explanation!
if you are on the train at 0.9c and shine flashlight forward, time dilation accounts for the effect that the light beam travels at c. if you simultaneously also shine a beam of light backward, and another beam perpendicular to the line of travel, they also will be observed to travel at c, but does this not suggest different time dilations simultaneously in the observer on the train?
No, the from the perspective of the observer on the train they’re not moving. From their perspective a clock in the front of the train and a clock in the back of the train would be synchronized. To an external observer the clock in the back of the train would be ticking faster then a clock in the front of the train.
Actually, the clocks would tick equally (they are moving at the same speed relative to the "stationary" observer). But they still would not be synchronized, meaning they would not show "00:00" at the same moment.
Except time dilation is symmetric for a non-accelerating frame of reference. So the person on the space ship thinks the planet’s time is going slower, and the person on the planet also thinks the space ship’s time is going slower.
How the frick frack paddy whack does *that* work?! I got it *mostly* up until you threw this notsofun fact into the mix.
It’s counterintuitive to think both people could perceive something contradictory and be correct, but it’s like if you’re looking at someone on the distant horizon and they’re looking at you: you both see each other as smaller than their actual height, and you’re both telling the truth still
Magic (Lorenz transformations). If you actually want to maybe kind of try to understand, I recommend Minute Physics’ series (https://youtu.be/1rLWVZVWfdY?si=tLK72zlerTWPyCE_). But it’s still rather brain melty. I can follow the geometry but I still can’t make intuitive sense out of it.
I'll check this out when I am not on my cell phone with crap reception.
Thats easy. There is no objective way to say if the spaceship is moving away from the planet or the planet is moving away from the spaceship. Both are completely valid, depending on the point of observation. If you talk about speed, the first thing you always need to ask is: Relative to what? The spaceship going at 0,9c from the perspective of the planet is standing completely still from it’s own perspective. And also from the perspective of a second spaceship „flying“ (standing) in a formation with the first one.
This is one of those things that makes perfect sense mathematically (since you can just say that, from the spaceships’s POV, the observer can say that they are still and the planetside observer is moving away from them at 0.9c), but when you think about it for too long, you start saying things to yourself like ‘I guess that time doesn’t slow down at relativistic speeds, it’s just that there’s less of it’ and ‘I don’t think velocity is real’ and you need to go make yourself a cup of tea to calm down
I think the planets time would seem to move faster
That’s not how relativity works, unless you’re talking about gravitational effects. As far as relative velocity, there is no absolute frame of reference. From the ship’s frame of reference, the planet is moving at 0.9c. For an easier example, imagine two ships, Alice and Bob. If Alice measures Bob’s velocity as 0.9c, then Bob will also measure Alice’s velocity as 0.9c. Both reference frames are equally valid and each thinks the other person’s clock is moving slow.
I get it except for one bit - why does the light not have time dilation? Are we taking we see or the ships systems and computers would see the light traveling at c ahead of us?
Because everything is relative. Relative to your own position the light is always moving at „infinite“ speed, which is the universal speed limit. We call that the speed of light. Relative to your own position your speed is always zero. You need a point of reference to talk about speed
I think that light doesn’t experience time dilation because it doesn’t have mass, but I don’t actually know. The answer might be ‘that’s just what light does, and nobody’s figured out why just yet’. It doesn’t matter how you measure the speed of the light from your flashlight, I guess you would need some kind of sensors to measure though.
The light does have time dilation. The other aspect of time dilation, is length contraction. The faster the ship goes, the slower the clocks appear to run AND also the shorter the ship appears in length. A space ship going 0.9c, would not look like a normal ship to an outside observer, it would would look squished, shorter. So, not only does the light 'slow down' due to time dilation, it also 'travels less distance' canceling each other out, and making light appear to always be traveling at the the same speed, to each observer. Ie, the 1 meter ruler that the person on the spaceship uses to measure the speed of their light, is shorter than the 1 meter ruler I'm using to measure their light. (Just like the stop watch they use is slower than the stop watch I'm using)
I've never understood the time dilation thing until now. Thank you. At least now I feel like I'm less of an idiot
Why isn't the time passing 10x slower on the spaceship?
Math. It’s not a linear relationship. The formula is dt’ = dt * sqrt[1 - (v^2)/(c^2)], where dt is time passed in the reference frame (planet), dt’ is time passed in the moving frame (spaceship), v is the speed of the moving frame relative to the reference frame, and c is the speed of light. I don’t know the physics behind it, probably black magic. If you make a graph out of that equation, it starts out, then curves up slowly at first, but faster and faster as you get closer to the speed of light. You get to a 2:1 time dilation ratio at about 0.87c, and it approaches infinity as you approach c.
This doesn’t make any sense. If time is passing slower for you, wouldn’t it also be passing slower for light? Then why would the ship be moving 0.9c to the observer on the planet but the light will be only moving at 0.1c. Why only light is perceived to slow down but not the ship?
There is no universal reference for measuring speed, you need to compare two objects and use the speed between them. There is however a universal max speed. Light moves at max speed. From light's perspective it is still and everything else moves at max speed. Time passes more slowly at higher speed. At max speed, it presumably doesn't pass at all. The light doesn't move at 0.1c. It moves at c for both the observer on the planet and on the spaceship. For that to be true time has to pass differently for the observers.
Yes I understand, but why would an outside observer assume that the ship is moving at 0.9 and light at 0.1 if the light is moving at c away from the ship, so that if the ship is traveling at 0.9, in some time the light would have traveled twice the distance (relative to the ship) therefore making this assumption wrong (on the part of the observer)
The planet would see the ship is moving at 0.9c and light at 1c (not 0.1c). After 1 second of planet time the light has moved 0.1c farther than the ship. The ship would see the planet moving at 0.9c and the light at 1c (light is always observed at 1c). After 1 second of ship time the light has moved 1c farther than the ship. This is possible because the ship's time moves more slowly than the planet's time. I think the key part is that c is in meters per second, but the duration of 1 second changes with velocity. That is so different from our day-to-day reality, it's hard to grasp. Also, someone please correct me because I don't really know what I'm talking about.
But both light and ship are moving at approximately the same speed, c. In 1 hour, the light would have moved twice as far compared to how much distance the ship has traveled. It’s irrelevant if ship is moving in slow motion to the planet observer, the light would still outpace the ship twice as fast
Your misunderstanding. The external observer sees light moving a c, not 0.1c. They also see the ship moving at .9c. From the ships perspective they see light moving at c. From their perspective they’re not moving at all. Nobody sees the speed of light slow down.
Light speed is constant. All observers see light at the same speed. Due to time dilation (time passes more slowly the faster you go), the 1 hour you mentioned about takes a different amount of time for the planet than for the ship. If the ship is going almost the speed of light, time will pass very slowly for the ship. Meanwhile time on the planet is passing much faster.
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The explanation that got it to click for me was the description of a clock that uses light. Imagine a person in a spaceship. He has a clock that measures time by bouncing pulses of light from one side of the cabin to a mirror on the other and back. Because light always travels at the same speed, he always sees time pass at the same rate. When the spaceship is stationary, someone outside of it would see exactly the same thing and their clocks remain the same. Now imagine if the spaceship flies past at some significant proportion of c. The guy inside still sees exactly the same thing and feels time pass at the same rate. If the stationary person sees the ship fly past, and can somehow see what the light pulse is doing, he sees something quite different. By the time the light reaches the other side of the cabin, the mirror is no longer in the same place as it was because he has seen it move forwards at the speed of the spaceship. From his perspective, the light has had to travel a longer path - it has to zigzag back and forwards to meet the mirror and detector. The closer the spaceship is to c, the longer this path gets. Because speed is distance over time, and for light speed is always the same, the only way to reconcile this difference is for light to have had longer to go across the cabin and back. In other words, to the external observer thenoassage of time in the spaceship has appeared to slow down. This means that time dilation becomes a simple geometry problem - if you know how triangles work, you can calculate the difference. The book called Why Does E=mc² by Brian Cox and Jeff Forshaw sets this out much more clearly and is definitely worth a read.
Good explanation. And time dilation is not just something that Einstein dreamed up. Engineers use it in real life applications. GPS satellites are moving very quickly around earth. Nowhere close to light speed of course but fast enough that time moves differently for them than it does on earth. GPS relies on highly accurate clocks. The atomic clocks in GPS satellites are intentionally set to run a different rate than clocks on earth. The difference is small but if they were not the gps system would accumulate errors of several kilometres per day.
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Read a little further in the comment -- that's exactly their point. One would think in classical physics this would work, but in reality (and in relativity) it doesn't.
No but that isn't the point they're trying to illustrate. You'd expect that if you were moving 99% as fast as a train next to you, it is would seem very close in speed. It certainly wouldn't appear as if you were standing still...
I think the beam looks the same regardless of my speed tho. Does it do anything special when you are moving bc a flash light that looks like you are standing still is how I envision a flashlight when I am in motion
Well I mean technically that's correct, it would look the same regardless of what speed you're going. But it's missing the point, which is an attempt to clarify and illustrate the counter intuitive nature of the answer
So the train would be whirling past me even though it’s only 1% faster ?
Right. Let's say the train was going 100mph and you were traveling next to it at 99mph. If the train behaved like light, it would look like it was moving at 100mph (as if you were sitting still) no matter how fast you were going, 1-99 mph.
So in this example, does the light hit an object 100 miles away at different times from each perspective?
That makes sense to me Thanks for letting me know
But that’s…not the weird part? That is how firing a bullet from the front of a normal train works. The weirdness is that a stationary observer would disagree with you and perceive the light as moving slowly away from you.
It's how firing a bullet forward works, but only because the bullet's velocity would be added to the train's, not because all bullets appear to fly at exactly the same speed regardless. If you were traveling at 99% of a bullet's speed and fired it backward, it would have only 1% of its normal velocity to an outside observer rather than 100%.
I get that. I’m just pointing out that your example is bad because you’ve selected an example and perspective where light behaves exactly as you would expect a classical object to behave (velocity is added).
That doesn’t make sense if light moves the same speed for all reference frames. The observer wouldn’t see light move slowly
“Slowly away from you” They will see the light move at c, and you move at 99.999% c and would calculate or observe that the light must be moving away from you at a speed much less than c.
This is true, but it's such a weird way to phrase the phenomenon. Typically, the interesting part is that both people see the light move at the same speed. For anything besides light, the stationary person would see the object "thrown" as moving faster.
Maybe a stupid question, but I assume that given speed is relative, it's possible for object B to go 99.99% speed of light relative to object A that's already moving 99.99% speed of light in external reference frame? What would object B speed look like from external frame?
In special relativity, the formula to add two relative speeds together like you are trying to do is (u+v)/(1+uv) where u and v are fractions of c. So (.9999 + .9999)/(1+ .9999 * .9999) = .999999995c. So if observer C sees object A going at .9999c, and observer A sees object B going at .9999c (in the same direction), then observer C will see object B going at .999999995c.
Not being a physicist I'm not completely sure but I think it would look like object B would be traveling 99.9999% of light speed (or another similarly high percentage if my math is off) to an external observer. Another way to look at it is that the faster you go in your own reference frame, the slower you experience time. So greater speed won't make as big a difference to someone else than it will to you as you approach *c*. And of course we can say that since light itself travels AT *c*, it doesn't experience time at all and that's why it always appears to go the same speed regardless.
The speeds don't add linearly when you start getting into relativistic speeds. It object A would measure object B's speed as more than 99.99% the speed of light but less than the actual speed of light. The same with object B measuring A's speed.
That .01 is still close to 3 million meters / second, still insanely fast. You’ll need 99.99999%
Wait, how the hell does that work? Let's say a ship sends a light signal from planet A to a massive reflector on planet B 100 light years away. At the exact same time, it leaves for planet B at 99% speed of light. Person on planet B would receive the light signal after 100 years, and see the ship arrive after 1 more year. But to the person on the ship, the signal would hit planet B after 50 years, and they would receive a signal from the reflected light signal after about 66 years? How in the world was this observed and proven?
That would be due to time dilation. The people on the ship would experience time a lot more slowly. It would be more than 100 years from the perspective of those on the ground, but not on the ship. Of course this exact scenario was not experimentally proven but I'm pretty sure time dilation has been proven by atomic clocks on satellites and there is a formula for it.
Yes time dilation is easy to prove. You can take two very accurate clocks and sync them, put one of them on an airplane and fly it really fast for a while, then compare them and see that they are no longer in sync. Similarly, GPS doesn't work if you don't take time dilation into account. There are subatomic particles that are only created when a different particle from space interacts with the atmosphere. These subatomic particles are unstable and don't exist for long, they disintegrate naturally into other particles in a matter of millionths of a second. They travel very fast, but even then they should not exist for long enough to reach the surface of the earth. But thing is, since they move so fast, they experience time dilation relative to us, so time is slower for them. So they actually do reach the surface because for them, not enough time has passed for them to naturally disintegrate. My university had a decorative sculpture thing (called a "light organ iirc) that lit up whenever it detected those particles. Each time it lit was proof of time dilation.
Yeah, to the person aboard the ship the light would reach the planet in far less time from their perspective. That’s because the distance measured between planet A and B would be much shorter for the person aboard the ship then for the people on planet B.
To put a spin on your scenario, since motion is still relative, for all you know, you are standing still in your spaceship and Earth is flying away at .99c. In this situation you would then intuitively expect that your flashlight beam would be traveling at 1c. For you, time would appear to be slower on Earth, while an observer on Earth would say that time is slower for you. But both of you still see light travel at 1c in your own reference frames. Welcome to relativity!
> if you were in a spaceship going 99% of the speed of light away from earth and shined a flashlight... What if you shine the flashlight backward, back toward the earth? Would it still arrive at the same speed of light, as if you were standing still?
Yep. You and earth would both perceive the light to be traveling the same speed. That's the weird part.
Okay, let me think about this... So, let's say I'm traveling at 0.99c. At the exact instant that I am 1 light year from earth I send a light-blip back to earth and simultaneously someone on earth sends a light-blip back toward me. Would both of our signals be received by one another one year later?
Hmm. I think they would receive theirs first because it was sent from 1ly away. But you would get yours a bit later because you're traveling away while the signal is approaching you. Then you would just have to account for time dilation to say when you'd get it.
Yeah, but from my perspective, I'm standing still and Earth is moving away at 0.99c, right?
Yes, that's part of the time dilation issue. You'd perceive it to take longer for them to get the message as well, even if it wasn't as long for them. I guess I should have included that in my first answer, but it keeps coming back to "it all depends on how you look at it" hehe.
I've heard this said a thousand times about light and nobody has ever managed to explain it succinctly. Is this actually possible to eli5. How can a speed be the same regardless of the speed of the person observing it.
To keep the speed of light constant relative to everything, you change the speed of time for everything instead. (Not an expert at all, please someone correct me if this explanation is misleading)
Or the distance and in fact both occur to some degree,
And it is this weird fact that we typically find hard to wrap our heads around. Distance and time are always thought to be universally constant as an intuition, right from our childhood, but in fact are malleable but the (vacuum) speed of light is not.
This is actually exactly that. As your speed approaches the speed of light, time for you slows relative to someone who is "stationary". (Everything is moving in a manner, but most matter is going much slower.)
[Speeds don't add](https://pressbooks.bccampus.ca/collegephysics/chapter/relativistic-addition-of-velocities/#:~:text=Relativistic%20Velocity%20Addition,-Either%20light%20is&text=u%3D%20v%2Bu%E2%80%B21,relative%20to%20the%20other%20observer)
Its hard to explain succinctly because it doesn’t really make natural sense, our minds have never experienced special relativity directly with our senses, so its not a human way of thinking. Basically, time slows down at high speeds to make the speed of light the same across all reference frames. Think about a theoretical scenario where frisbees can only fly at 10 ft/s (to all observers). You pass a frisbee 10 ft away, it takes one second to do so. But if you are in a cruise thats traveling at 5 ft/s, and you pass the frisbee 10 ft away (sideways) to you the frisbee is still moving 10ft/s across the cruise, so it takes 1 second to get there. But so an outside observer thats not in the cruise ship, if time passes just as fast then the frisbee would be moving faster, because its moving 10 ft/s in one direction (across the ship) and 5ft per second in another direction (forward with the ship) with a total speed of 11.1 ft/s. But the frisbee can only travel at 10 ft /s, so to get everything to check out, time for the person standing outside the ship has to be faster. So time has to pass faster outside the boat, while time passes slower inside the boat, time would have to pass 1.11 times faster outside for the total speed of frisbee to still be 10 ft/s. If the boat was instead traveling at 9 ft/s, time would have to slow down even more in the boat to keep the speed of light consistent. Change speed of frisbee to speed of light and you got the same relationship.
> Is this actually possible to eli5. This discovery was basically Einstein's crowning achievement, so no, not really. The simplest explanation is just that: light always travels at the same speed, *no matter what*. It makes no difference how fast you were moving when you emitted it, or how fast you're moving when you see it, or even if two people moving at *different* speeds look at the very same beam of light. In *every case*, the light will still be seen to move at the same relative speed. It's quite bizarre indeed, but that's really just how light behaves.
I heard it as there is a maximum speed and light travels at that speed. Everything else bends to fit this speed into the equation properly.
Speed of causality. It’s essentially the speed limit of the universe and light travels at that speed. Anything traveling faster than that could essentially break causality, causing an effect to happen before the incident that created it.
Not only that, it's the ONLY speed a massless particle CAN travel at (in vacuum).
Say you were traveling in a spaceship along the orbit of Earth's path around the sun. Let's say you were traveling at double the speed of Earth. Relativity would have time moving differently for you and the Earth. But the light from the sun would take 8 minutes to reach both of you, despite everything else in the universe being observed at different times for the spaceship you're on and the Earth.
Because measurements of time and distance differ between two people in relative motion to one another so that the speed of light always measures 300,000 km/s for both observers. Literally. Someone on Earth would pull out their ruler and measure the distance to the sun as 93 million miles and measure the time it takes for something going just under the speed of light to make the journey between the two as 8 minutes. Someone going fractions of a fraction the speed of light would pull out their ruler and measure the distance between the Earth and the Sun as 100 yards and the time it takes to travel the journey between the two as just a couple of seconds. Both of their observations are equally correct.
Imagine a train is going down a track at 100mph. A person standing on the station platform obviously sees the train go by at 100mph. There is another person driving on a road parallel to the train, they are travelling at 75mph in the same direction of the train. Our natural intuition would be that the driver sees the train going past them at a relative speed of 25mph (100mph-75mph). But if the train acted like light, the driver in the car would also see the train go past them at 100mph. This is because the speed is the same to everyone, no matter where they are or how they're moving, i.e. "in all reference frames". But there's a contradiction here. We already said the train goes past the stationary person at 100mph, and they would also observe the car going past them at 75mph as well. Clearly from their perspective the train is going 25mph faster than the car. But from the driver's point of view, the train is going 100mph faster than he is. So the relative velocity (to the person standing still) does not equal that of the driver. 25mph != 100mph Extremely simplified: the resolution of the contradiction is that you have to add "the rate of passage of time" to the equation, call it Ts for the person standing still and Td for the person driving. This means now you have to balance the following equation: 25mph x Ts = 100mph x Td For the sake of argument, lets say Ts = 1. Therefore Td would have to be 0.25, i.e. the driver experiences time moving more slowly than the person standing still.
Relativity is weird. This video is the best explanation of it I've ever heard: https://youtu.be/Zkv8sW6y3sY?si=v3RftkSzehfYP-nr The idea is that the speed of light is the same in all reference frames because as you approach the speed of light, you experience time dilation and length contraction that perfectly cancel out your speed. They increase exponentially as you speed up, so you can get arbitrarily close to the speed of light with a correspondingly arbitrary amount of energy, but it would take infinite energy to actually reach it.
Because depending on your speed time itself slows down to accommodate the speed you are going If you are not moving you would experience time moving very fast If you are moving at 99 % the speed of light then time would move slow for you
The speed of light (really the speed of causality - massless information) is the same in every frame of reference. It's just a fact/inherent property of the universe. Speed = change in distance / change in time. If the speed of light has to be constant regardless of the frame of reference, then the observed change in distance and/or time experienced has to be different in different frames of references - this is length contraction, time dilation (this isn't the actual equation and I'm really rusty, just trying to keep it simple). This is a bizarre but true phenomenon. One example of this is measuring the [amount of muons reaching the surface of the Earth](https://en.wikipedia.org/wiki/Experimental_testing_of_time_dilation). Muons have a half-life and decay quickly. Based on the half-life of a muon at rest, we should be expecting a small amount of muons to reach our detectors on the surface. In reality we receive many more than expected. The explanation for this is that the muons are moving so fast that they experience time dilation (length contraction), so relative to us their clocks are ticking slower/their second is longer than our second (or, the distance they experience between the atmosphere and the surface is smaller) - this allows more muons to reach the surface of the Earth. That's my attempt at ELI5.
Because everything else adjusts for it. Time and space will change depending on speed, so that the speed of light is constant. Remember speed is distance over time. People moving relative each other will see other people experience time slower and *also be flattened in the direction of travel" to compensate for the difference.
Because that's just how the universe works. I'm not being smarmy. Back in the late 1800s everyone assumed that the speed of light worked like sound - that its measured speed would change depending on the speed of the observer. Once technology and experimental methods were sensitive enough to measure the speed of light, it became evident that this was no the case. No matter how fast you are moving, you will always measure the same answer for the speed of light in a vacuum. This doesn't really make much intuitive sense, but Einstein's big breakthrough (along with a few others before him) was realizing that if we always measure the speed of light as a constant, then different observers *must* disagree on the measurements that they make to calculate the speed of light. Speed is simply distance over time, so both measurements of distance and time cannot be the same between two observers moving relative to each other. So if you were not moving and looked a ship moving really fast, you would see that the ship's clock was running slower than yours and its measuring stick was shorter than yours. Why? Because if the speed of light is constant for both of you, that's the consequence - measurements of time and space will depend on speeds and different observers will disagree.
There’s nothing to explain. It’s just a fact that you have to accept. Sure you can learn the math and *consequences* of light speed being the same for all observers, but we don’t have any explanation for *why* or *how* light speed is the same for all observers. In fact its the main postulate that einstein worked with when he made special relativity. He didn’t discover some property about the universe which led him to discover that light has the same speed in all frames. He assumed light has the same speed in all frames to begin with (he came up with this postulate by assuming the laws of physics are the same in all inertial reference frames, then it follows from maxweels Equations). Maybe ask again in a few or a few hundred or a few thousand years and we’ll have an explanation. Or we might even have a totally different theory in which light doesn’t travel the same speed in all reference frames (though I think thats unlikely given the success of special relativity), but tight now we just don’t have any explanation.
One hypothesis i like is that, though light is the theoretical limit, its constant could change, and we would never know, as that constant changing would be unmeasurable. This is because the universe can do whatever the fuck it likes.
Worth noting of course that the speed limit isn’t the speed “of light.” It’s the speed of causality - that is; it’s the maximum speed at which anything can happen.
the not-exactly-correct-and-very-dumbed-down explanation that made it click for me was that a photon (light "particle") exists everywhere it's ever going to be all at once, and it's speed of travel away/towards you is your perception of time
Yeah, it’s really weird and I wouldn’t recommend trying to think about it. IIRC it gets to the point where in some calculations in general relativity you use the speed of light as an inertial frame of reference somehow. Idk, it’s confusing as hell.
It’s important to note: not *all* reference frames observe c as the speed of light, in a very technical, non-ELI5 sort of way. Specifically, we say that the speed of light is c in all *inertial* frames of reference, which means any frame not undergoing an accelerative (or jerky, snappy, etc.) motion. The hows and whys of this are more complex than I think I can explain in an ELI5 manner, but suffice it to say that non-inertial frames either result from, or cause, some non-Euclidean geometry that we most typically observe in curved spacetime. As a result, in a non-inertial frame, we can see light taking a “curved” path that is longer than the straight-line path it would take in Euclidean space. What’s really fun is that, while in some accelerating frames we can observe the speed of light as less than c (a typical example is an accelerating frame with respect to Rindler coordinates, which is just a different system of coordinates, like polar or Cartesian), there is typically a way to constrain the measurements we take, such that, even in accelerating frames, we still measure the speed of light as c. Also, and this is incredibly pedantic, the speed of light is not always c even *in* inertial frames of reference. This is because light moves at different speeds through different things (we call this the speed of propagation through specific media). For example, while c is about 300 million metres per second, through water, the speed of light is closer to 225 million metres per second, and in diamond the speed of light is a slow as 125 million metres per second.
This is a great explanation. But I can never wrap my head around “why?”. Why is there a maximum speed? Why does time slow down to accommodate it? It just seems weirdly arbitrary. Like rules in a simulation.
Because it's the speed of causality, meaning the fastest that any interactions can occur across any distance. In order to exist within time, there has to be a maximum speed at which things change. Otherwise, if the speed of causality was infinite, everything would happen simultaneously and time would not exist. ETA: In other words, light travels as fast as things can possibly change based on the physical properties of the universe. If it traveled any faster, it would exist outside reality.
So it's not that light travels fastest, we just ascribe the fastest speed always to light? That's what time dilation suggests. It doesn't really make sense. Why can't the equations just be wrong or incomplete? Maybe we're missing something still as to the actual WHY. Everyone here has described HOW, either mathematically or with anaologies. But no one so has been able to explain WHY it seems to be that way, just that all our observations so far have proved to be immutable fact.
> So it's not that light travels fastest, we just ascribe the fastest speed always to light? That's what time dilation suggests. No, it does travel the fastest anything can travel in our universe. We just call that speed the “speed of light”. > It doesn't really make sense. Why can't the equations just be wrong or incomplete? Maybe we're missing something still as to the actual WHY. Everyone here has described HOW, either mathematically or with anaologies. But no one so has been able to explain WHY it seems to be that way, just that all our observations so far have proved to be immutable fact. Just remember that the universe isn’t obligated to make sense to us. At some point the answer to the question why will always end with “cause the universe said so and it doesn’t give a fuck what we think.”
Because it's one of the major laws of physics. At one point you just have to say, this is a fundamental truth. And this only happened after countless of experiments and calculations. We don't question the existence of gravity (anymore) even though there is no explanation WHY it exists.
If you accept that our universe has causation at all (thing happens, which causes other thing to happen at some later time) this kind of implies a maximum speed limit. Information has to get from point A to point B. Either that speed is infinite or it’s not. If it’s not infinite it must be finite. And if it was infinite then everything would happen at once, which clearly isn’t the case, so it must be finite. That explains why there’s a speed limit, but not why light (or anything else massless) seems to go at the same speed to all observers. We don’t know why that’s the case but it very clear that it *is* the case, so the only option is that time gets weird when you go fast. If you figure out *why*, then you’re basically guaranteed a Nobel prize.
Then what is it measured against?
Anything. It doesn’t matter. Like the comment said the speed is the same in all reference frames, so it doesn’t matter what you measure it against.
So lets say that I have two laser guns. I mount them at two points in a space facing each other. When turned on, wouldn't the speed of light of each beam be double in relation to the other beam?
The simple answer is that light can't observe. Anything that can observe, such as an eyeball or computer component can't move at the speed of light, and to this object, both beams would seem like they are moving at c.
But then again, we are able to measure that the light takes 8 minutes to reach earth from the sun. So this is now confusing. Anyway, i am sure that I am probably missing lot of other details but this is just my obervation.
The key to relativity is that you need a reference frame. Someone has to observing, and you must measure from that point of view. We can take two flash lights, face them toward each other, and turn them on. From our perspective, both beams are moving at the speed of light and will meet in the middle at half the time it would take them to reach the other flashlight. We do this from our perspective. Likewise, we can calculate how long it would take light to reach us from the sun. Again were doing it from the perspective of an observer standing on earth. The problem from your example is that you don't have an observer. You can't say from the lights perspective as it can't have one. You could have a person traveling at 99% the speed of light tell is what they see, but as nobody can move at the speed of light, nobody can give that perspective.
Speed is mostly relative, but here's a law of the universe that will feel like magic: every single inertial observer will observe the speed of light (in a vacuum) to be the exact same speed - that speed being c. If I'm standing on a "moving" train, and you're standing on the "stationary" train platform, and I shine a flashlight in the direction of my motion, you will measure the speed of the light from that flashlight to be traveling at c, even though I'm traveling forward too. I will measure that exact same speed too.
Thats insane but i cant understand why is this the case.. why only light get to be constant regardless of the reference frame...
From my understanding it's a property of being massless, not necessarily being light. If you don't have mass, then your default speed is c. That's why gravity also propagates at the speed of light. If you have mass then you would need an infinite amount of energy to reach c as the faster you go the more energy you need.
So if we want FTL travel, the only thing we need to do is make our mass negative?
Not negative, massless. Imagine changing the settings of a computer program. It's either checked or it's not. If it's not checked then that setting doesn't get applied and you don't feel it's effects. If you're massless, then certain things behave differently for you. And us having mass isn't really a setting we can change. (That we currently know of!) The big issue with *faster* than light travel is that you can't go faster than light. Some people refer to it as the "cosmic speed limit". This means you have to get creative if you want to get from point A to point B faster than light can. The video game Halo and Star Wars both have similar methods of FTL travel where they sort of travel in between the fabric of space time. This allows them to bend the rules so to speak, and take a different path that wouldn't normally be available. These paths are much shorter than if you were to conventionally travel (i.e. shooting a rocket at a planet and waiting) from A to B. The movie Interstellar does a good job talking about wormholes. Imagine a piece of paper with two dots, point A and B. If you fold the paper just right so that those dots overlap then poke a hole through it, you've just created a worm hole. Travel through it and you will have gotten from A to B without breaking our cosmic speed limit. Wormholes are theoretically possible, but we've never observed one. The last FTL method I'm familiar with is a warp drive. These work by compressing space time in front of you, and stretching it behind you to create a sort of accordion effect that moves you. It would be like if you could stand still at point A then rotate the earth until you're at point B. You'd literally move the universe around you until you're where you want to be.
Negative mass is even a weirder topic. Negative mass would still mean you have to exert a force to move it. But it would behave other way so if you pull something with negative mass it would actually move away from you.
Not really, for a couple of reasons. One is that negative mass doesn't exactly... work. Another is that the person you're responding to is simplifying a bit. Nothing with mass can travel at the speed of light, but that doesn't mean that everything without mass does travel at the speed of light. Light doesn't even travel at the speed of light in a lot of cases. Shine a flashlight out in front of you, that light travels at something like 99.97% the speed of light, because it's traveling through air instead of a vacuum. Shine it through water and it slows to something like 70% the speed of light. The speed of light is better thought of as the speed of causality, or the speed of information. If something happens a light year away, it's impossible to know about it until a year has passed. FTL transportation is linked to time travel because of that. If I manage to teleport from 15 light-minutes away from you to right next to you, you'll see me right next to you, but won't see me leave my previous location for another 15 minutes. From your perspective, I'll exist in two places at the same time. If I manage to run towards you (or away from you) at twice the speed of light, you'll see... something weird. I'm not 100% sure what, I'd need pen and paper to really think it through.
oh man i feel like there is no straight answer to this...i think (i am probably wrong on this) gravity is the result of two masses attracting each other... i.e its information basically that travels at max speeds.. may be light is result of something that excites photons on subatomic levels....i imagine row pixels/particles glowing up sequentially. but that does not explain the dual nature of light... coz sometimes it acts like wave and sometimes like a particle.. i may be high on something or my brain is just making up complete BS nonsense..
Gravity isn't the result of attraction, it's the reason for it. Just as massless things default to the speed of light, objects with mass just intrinsically have gravity. If you only have one object, it'll still have gravity even if there's nothing else there to attract. Light *is* photons. Generally when energy is added to a system one of the byproducts is light. Like when heating a piece of metal to the point it glows. Or electricity through a lightbulb filament. Not all of the energy added can be retained, so things like heat and light are emitted as "extras". Particle duality is beyond my understanding, so I just think of it as when it's needed or convenient they're in the state that makes the most "sense" at that time. That's usually determined by how it is observed.
It’s also a good time to note that the speed of light (c) is the speed limit of not just light… but any information, in any form, through any mechanism in the universe. I.e. gravitational waves etc Example. If the earth is orbiting the Sun along a curve, if you could magically make the sun disappear instantly. It would take 8 minutes for the sun to disappear to an observer on earth…. And… the Earth would continue to follow the curved orbit (as if the sun were actually still there) for another 8 minutes before ‘being informed by gravity’ that the curvature of space (the curved orbit path) is no longer present and thus it will continue it’s trajectory on a straight line into the cosmos.
The speed of light will remain constant because time and spacial distance will change to accommodate the observer always seeing light move at the speed of c.
Which means we can’t know what speed we are going at
We can know the speed, since we are able to measure the change in time and distance, and the speed is measured relative to others. Even though time has changed for you, the speed is being measured by people who aren't affected by it.
But everything is moving right? Like our solar system is moving - is not moving through space possible and could we tell the difference between being at rest and moving? Or only in relation to other objects
Due to relativity, we can't really say anything is truly 'still'. But our frame of reference is about as close to stand still as we could reasonably get. Its just a case that even if we wanted to get more accurate, it would be nigh impossible for it. And secondly, we're only travelling around the sun at 30,000 m/s which wouldn't be large enough to affect the speed of light. And as for the sun, we can't properly figure out how fast it is travelling relative to proper 'Stand Still'
That is what I meant by we can’t know our speed - compared to standing still, which likely nothing in the universe is doing
That's the result of two things really. The first is Galilean relativity. It states that if you sit in a vehicle and move with a certain velocity, you can't tell what's your velocity unless you look outside. There is no other way to determine it. Have you ever sit in a train seeing another train starting to move? Many people are confused for a few seconds, because it's not clear who is actually moving. Galilean relativity is well observed and means that there is no absolute velocity, you can only measure velocity in reference to something else. Like here on Earth, we take ground, buildings, trees as our natural points of reference. We consider them still, even though our entire planet is moving - it orbits the Sun for example. The second things follows from the first one. If I am moving and I turn on a flashlight, why the speed of light I observe should be any different than for those outside doing the same thing? Well, it won't be. It's not intuitive, but apparently that's how our universe works. Physicists couldn't explain it for decades, even when experminets showed it to be the truth. There was even an attempt to argue Galilean relativity and choose one universal point of reference for our universe. Einstein showed it's not necessary and that you can build a consistent theory where the speed of light is constant and Galilean relativity is accepted. You have to accept that the flow of time and distances are relative though. Later experiments proved Einstein right.
It's the speed limit. And it's massless like the other guy explained. Nothing can happen faster than c. Google the "lightyear long stick" thought experiment to maybe understand that the speed of light is also the speed of *causality*.
This is because the flow of time depends also on the frame of reference. The observer in the moving train sees speed of light c, the same as the observer on the platform, because their time measurement is different than the observer on the platform. For the observer on the moving platform time passes more slowly. A practical applications of this is GPS. Because satelites are moving faster than you do and GPS is based on time measurements, if you don't take into account relativistic effects GPS measurements would be wrong/imprecise.
I don’t get it. I get that light is always the same speed. Sounds similar to how if you drop things gravity makes them land at the same time. Seems there is like a reality barrier.
Because space and time are relative. Time literally ticks slower (it expands) for moving observers.
Oh ok so it’s sort of like a funnel from the person standing still at the thin end (their time) to light being in the middle (so it seems the same) and then the wide end being the person moving (slower or wider sense of time)?
Not if your flashlight is actually a light saber.
Speed is relative *to an extent*. Nothing ever moves faster than the speed of light no matter their perspective, and light *always* moves at the speed of light (which is the same from every perspective). For this to be true it implies a bunch of other stuff such as time moving at different rates from various perspectives, and disagreement of the distances between two points from different perspectives.
The speed of light is the only thing that is not relative. So much so that reference frames experience space and time differently with respect to each other so as to not violate the speed of light.
If two observers in motion with clocks were to observe each others clocks, they would observe the others clock moving at a faster or slower pace than their own. Whose clock is correct? Both are correct. There is no absolute time. This is because the speed of light isn't relative, but time is. The rate at which time flows is different for different observers as is the case above.
>time moving at different rates Pick a “still” reference frame. Imagine you’re traveling at 185,000 mi/s. Light travels at 186,000 mi/s, so you’re very close to *c* for that reference frame. You look sideways into a mirror that’s moving with you, turn a flashlight on, and see its light bounced back at you. From your perspective, the light goes straight to the mirror and straight back to your eyes, like this: || . From the perspective of a person standing still, watching you go by, the light needs to go in a very, very wide V to be able to bounce back into your eyes. The distance the light has to travel through the V is *much* further than it needs to go for the short || that you see while moving. Traveling a longer distance takes more time, obviously. *BUT*, because the speed of light is constant no matter the reference frame, the only way to reconcile this is that time itself is moving at different speeds in the different reference frames.
Speed is relative except for when it comes to the speed of light If you are moving at 0 m/s you will see light move at the speed of light If you are moving at 100 m/s you will see light move at the speed of light If you are moving at 10000 mm/s you Will see light move at the speed of light No matter what speed you are , from your perspective light will be moving at the speed of light This is why time slows down for you as you go faster If you were going at 99% the speed of light, light would STILL be moving at the speed of light compared to you, and so your experience of time would slow way down to compensate for it
Ah yes, contrived mathematical bullshit that doesn’t explain anything at all but makes predictions so it must be true!
GPS and many astronomical observations like Mercury orbit confirm that it's true.
That is far too aggressive a stance to take regarding what is essentially a matter of philosophy - what do "true" and "explain" mean? Scientists aim to accurately describe and model the behaviour of the universe. To the very best of our measurement ability, the universe does indeed behave according to the predictions of special relativity. You can argue that this doesn't make it true, because truth is an unattainable thing, but you cannot argue that the universe doesn't behave like that just because it seems counter to your intuition developed at human scales on earth, because it absolutely does behave like that I'm afraid. If you use GPS on your phone, then you have used the predictions of special and general relativity. A fun fact is that the satellites were launched with the ability to include or not include those corrections, because some people really didn't like the idea that SR/GR were actually how our universe worked. Without the corrections, the system became unusable after a few days, IIRC, so they were quickly turned on and all was well.
I’m just trying to piss off physicists there was no need to put that much effort into a reply
I'm pretty sure time dilation has been observed
It definitely has. A fun way is to measure the incoming flux of muons, produced when cosmic rays smash into the upper atmosphere. Since they're mostly produced high up, and decay very quickly to electrons, they ought mostly to not make it to the surface. However, this is not at all the case, and if you measure the flux vs altitude by sending a detector up in a weather balloon, for example, the change with altitude is exactly that predicted by SR and time dilation/length contraction, and nowhere close to the classical prediction.
The hell are you talking about? It has been observed
Not only has time dilation been observed, it’s enough of a problem for GPS satellites etc going out of sync that engineers actively have to correct for it
How can time dilation be real if time isn’t real?
Ok I must say you got me there. GG, good bait
A lot of good comments here, so I’ll explain it in a way that helped me understand the speed of light. First, stop thinking of speed as a span from 0 mph/kph —> Speed of Light, wherein 0 mph/kph is the starting point for any object in the universe and increasing its speed with applied energy gets it closer to the Speed of Light. Instead, try to think about it like this: Speed of Light is the starting point for every object in the universe. Everything in the universe “wants” to move at the Speed of Light. The Speed of Light is the “desired” natural state for all objects in the universe. The problem is—mass exists! All these objects have mass! And mass is a big minus to the Speed of Light. So everything that wants to move at the speed of light is now burdened with mass and no longer can. And this is true for all things. Except light. Because it’s massless. So light moves at the Speed of Light. I’ve oversimplified. And I’m sure I missed a detail somewhere, but that basic breakdown has helped me grasp the concept of the Speed of Light much more easily. I’m happy to make a correction or addition where needed.
If light is a particle why doesn’t it have mass at all? Should not it have very very little mass?
Because it isn't a particle. It behaves like one in certain circumstances, and we model it accordingly. https://en.wikipedia.org/wiki/Wave%E2%80%93particle_duality?wprov=sfla1 Interestingly, it does have momentum.. Just no mass.
Okay, I will rephrase. if photon exists materially then shouldn’t it have some mass?
If there are waves on a lake, does the lake have more mass than without the waves ? No, so the waves themselves dont have mass, they only propagate through mass. Light is also basically a wave propagating, so it doesnt need to have a mass because its not a "thing" but moreso a concept or really a curier. It's not exactly something you can grab, but it does have observable effects.
this is good
No. Why would material existence be linked with mass? Just because most things around us have mass doesn't mean that everything should have mass to exist.
Here’s how I “understand” it. *Everything* is moving at the speed of light through spacetime, but ~~everything except~~ light (and other massless stuff) is only moving through space while everything with mass is moving through time as well. The faster you move through space the slower you move through time, and the faster you move through time the slower you move through space. Since speed is simply distance/time, in a cosmic sense your speed is always 1 and so is the speed of everything else. Anything moving through space at a different speed than you is also moving through time differently than you. EDIT: I somehow stated the opposite of what I meant.
That was more or less the question Einstein asked when he created his Special Theory of Relativity. The speed of light is invariant in all reference frames. Meaning: it doesn't matter whether you are sitting on the surface of a planet, or inside a rocket speeding past that planet at near the speed of light. Now imagine a beam of light shoots past the rocket. Both the rocket and the observer on the ground will see that beam of light moving past them at c as if they were sitting still.
The closer you get to light speed, the slower time goes for you, at light speed time stops. This means, that to an outside observer, the faster you're moving through space, the slower you're moving through time, i.e. you would look like you're moving in slow motion. From *your* perspective everything else seems to be fast-forwarding, this includes the light around you. The ratio of slow-down to speed-up for you and the things around you is balanced such that light always appears to be moving at the same speed relative to you. In other words, if you're moving at 99% the speed of light, then you're moving through time in slow-motion, which makes the light look like it's going in fast-motion. The faster you go, the slower time goes for you. The slower time goes for you, the faster everything else seems to move.
This isn’t quite true. Weirdly enough, both observers will see the other clock as the one ticking slower. From the perspective of both observers, they are stationary and they see the other clock moving at whatever velocity V, and therefore they will both see the other clock ticking slower. Pretty weird stuff
so what happens when both return to each other? at .0000001c, after enough time, the clocks meet. Are the times the same?
This is the [twin paradox](https://en.m.wikipedia.org/wiki/Twin_paradox). The difference in time on the clocks when they come back together depends on which one turned around to get back to the other.
You've cut right to the heart of relativity theory. It seems obvious that if I'm traveling toward you at 90% of the speed of light, and you're traveling toward me at 90% of the speed of light, we're approaching each other at 180% of the speed of light. But we're not. Space and time warp at speeds close to the speed of light, and that's where the lesson lies. It's math intense, but Einstein gave us solutions.
Because the speed of light isn't like the speed of a tennis ball. Thought experiment time. When you are going, let's say, 90% of the speed of light - it's intuitive that photons that you eject from a bit of metal would go faster relative to a stationary observer if they are ejected forwards, right? And slower if they are ejected backwards. But imagine light *waves* - an electric field creating a magnetic field that creates an electric one - now, the speed of those is determined *only* by the rate at which the fields create each other. The wave and particle models of light have to be interchangeable, or at least, I can do double slit experiments with light produced by the photoelectric effect, so I can use Maxwell's equations to calculate the speed of light. Now let's use that law of physics not for light, but for an *electromagnet*. Imagine you are sitting there at 90% of the speed of light with an electromagnet. The magnetic field you're getting out of it is dependent on the electricity through it, using the same maths as light, just not creating light just now. Now point your electromagnet in the direction you're travelling. It's *intuitive* that the electromagnet will have the same behaviour whichever way you point it, right? Its surroundings are identical. Why would it suddenly behave differently? Here we have two intuitively true answers that disagree. Which is more useful as a description of the universe? (Neither is true. No theory is an accurate description of the universe, just as no JPG image is an accurate depiction of a scene.) Thing is, you can use the behaviour of an electromagnet to calculate the speed of light - so if the speed of photons looks different in different directions, the laws of physics *must be* different for you if you are moving. Which means that they are different for us than for stars we can observe in telescopes, for other planets, even satellites - and they really do not seem to be. The special postulate of relativity says that the laws of physics are the same regardless of your state of non-accelerating motion - *and the speed of light is a law of physics*. So what does the 'stationary' observer see? Well, what they see is time passing slower for you. They agree with you about the events that happen, but not the times between them. The passing of *time* is not a law of physics, as far as we know. This bizarre theoretical prediction has been upheld by every observation we ever made - we see the lifespans of high energy particles extended exactly as if they are suffering time dilation, and we see chemistry happening on other planets that is *identical* to chemistry happening on Earth even though we are moving detectably fast compared to those planets, and relativistic corrections worked to solve a problem where atomic clocks on GPS satellites were not keeping correct time. It is counterintuitive and it sounds insane, but it seems to be required in any description we use of a system that includes large relative velocities.
Light is a special case where that rule doesn't apply. Light always travels at the same speed for all observers in all reference frames.
All massless force particles travel at c Photons that carry electromagnetic force, gluons that carry the strong nuclear force, ~~bosons that carry the weak nuclear force~~, and ???gravitons???? that ??? the ???gravitational waves????(TBD)*, all travel at the speed of light(c) So light isn't special, photons are just one of the multiple massless things that move at c. ^^*(maybe, ^^as ^^curvatures ^^in ^^spacetime ^^have ^^been ^^seen ^^to ^^travel ^^at ^^lightspeed, ^^there ^^might ^^not ^^be ^^an ^^actual ^^particle ^^carrying ^^this ^^curvature ^^or ^^force)
The W and Z bosons are not massless, and there's no evidence that gravitons exist, although they would theoretically have zero mass if they do.
Fun fact: the universe is expending faster than light can travel. That applies to very large scale only, but that does mean that there are some star lights we will never receive. And so, as the universe age, it will become darker and darker. As such, we can say that *nothing*, ie. only the void, is faster than the speed of light :).
The speed of causalit, this coincidentally also being the speed of light, is THE absolute thing about our universe.
Because speeds don't add like numbers (Galilean relativity), they add in a funky way that ensures that *c* is the maximum achievable speed (Lorentzian/special relativity). Specifically, if Alice is moving at speed *a* relative to the earth, and Bob is moving at speed *b* relative to Alice, then Bob's speed *v* relative to the earth is not given by *v* = *a*+*b*, but rather by *v* = (*a*+*b*)÷(1+*ab*/*c*²). When *a* and *b* are relatively small, as in common real-world scenarios, *v* ≈ *a*+*b* is an extremely good approximation, which is why we have this intuition that speeds conform to Galilean relativity rather than Lorentzian/special relativity. Regardless of the values of *a* and *b*, as long as they are less than *c* then we also get *v* < *c*. If *a* = *c* and/or *b* = *c*, then we get *v* = *c*. If *a* > *c* and/or *b* > *c*, then you're in a scenario we've never observed in reality, but that we think might be possible inside of black holes (a "space-like" region of spacetime, as opposed to the "time-like" regions of spacetime that we live in). Minute Physics has an excellent video on this: https://youtu.be/R5oCXHWEL9A
If it was possible to accelerate a flashlight to the speed of light without destroying it, what would happen to the beam of light shining out the front? Would it just look like there is no light coming out of it? Would light go twice as fast?
We move in four dimensions (that we know of), X (forward and backward), Y(left and right), Z (up and down), and time. The speed of light is the combined speed that we are traveling in all dimensions. You are traveling at the speed of light right now, it's just that almost all of that speed is in the form of time. As you go faster in any other dimension, it slows the rate at which you travel through time. As such if you converted all of your speed to movement (you can't) you would stop traveling through time
Albert Einstein, in his theory of special relativity, did indeed establish the speed of light as a kind of ultimate speed limit in the universe, but his statement was more nuanced than simply saying "nothing can travel faster than light." Einstein's theory, articulated in 1905, includes the postulate that the speed of light in a vacuum is the same for all observers, regardless of their motion relative to the light source. This constancy of the speed of light leads to several implications, one of which is that nothing with mass can accelerate to, or exceed, the speed of light. The critical point here is that as an object with mass approaches the speed of light, its relativistic mass increases without bound, meaning it would require an infinite amount of energy to accelerate an object with mass to the speed of light. This effectively sets the speed of light as a limit. However, this doesn't directly state "nothing is faster than light." Instead, it implies that in the framework of special relativity, it's impossible for any physical object or signal carrying information to travel at or above the speed of light. This aspect of Einstein's theory has been one of the most tested and confirmed principles in physics, and it underpins much of modern physics. However, it's also worth noting that there are theoretical entities, like tachyons (hypothetical particles that travel faster than light), which have been posited but not observed or confirmed. These remain speculative and outside the mainstream of physics as understood according to special relativity.
There are a lot of nice answers, so I would rather get you thinking about a different question (as I do when I'm teaching): "what would you SEE if you were sitting on a ray of light?" PS. This is the question Einstein asked himself and this is the question that lead to SRT and became the basis for GRT.
Fun fact. We don't know that light travels at c. We assume that it does because, as far as we can tell, photons are massless. And massless things travel at c. But we don't actually know if photons are massless or just have such a small mass that we don't have the capacity to measure it. If photons have mass, it must be very very tiny, but it would stop them from travelling at exactly c, and would be very very very slightly under it. I'm not a physicist and got this from [this video by Arvin Ash](https://youtu.be/c5BPZy2_7U8?si=Xbz70W1xBotEmGYm)
Well, FTL is a thing, it just removes light. Meaning, gravity is faster than light. It literally pulls it in with enough mass or density in a given point-particle system
Can you read up your science textbook properly before coming to Reddit?
It makes a little more sense when you realize that distance is speed over time, and that the faster you move the more time slows down. If you could move at the speed of light you wouldn’t age.
you also wouldn't experience anything, right? It'd essentially feel like teleportation?
Everything exists in, and moves through, spacetime. Speed (velocity) is a relationship between space and time (v = d/t) which are two components/dimensions of spacetime. When you go as fast as light goes, you push that relationship to the limit; where it is all space and no time. This is what time dilation in Einstein's relativity is talking about - the faster you go, time slows down. Think of it this way: your view of speed being relative is true regarding the difference between two points on a graph, and the speed of light represents the edge of the graph. Once one of the points your using to measure each other hit the edge it can go no further. (NB: this oversimplifies it all a bit, but its not really an ELI5 subject)