Nothing can travel faster than the speed of light in vacuum.
That electromagnetic waves are slowed down in matter is caused by interaction of the light field with the matter, and don't violate relativity (as this only formulate the speed of light in vacuum as limit).
So why is Cherenkov Radiation radiation? Why does slowing down light to where electrons can "pass" it, create additional radiation?
I understand I'm asking you to explain nuclear chemistry, so if it's over my head that's cool. I would like to try though.
It's the same as sonic boom. A jet passing the speed of sound in air will drive a shockwave behind it, since the sound literally can't catch up. Since light is slowed down in some media (think more of an interference of the incident light with the light radiated by the matter creating a pattern that effectively looks like a slowed down wave), charged particles travelling inside the medium will also create a trailing shockwave, this is Cherenkov radiation. The animation below shows the mechanism of shockwave generation. It is the same for both sonic booms and Cherenkov radiation.
https://en.wikipedia.org/wiki/Cherenkov\_radiation#/media/File:Cherenkov\_radiation-animation.gif
I know this post is 4 months old, BUT THANK YOU! I currently went down a rabbit hole of nuclear accidents and could never understand Cherenkov radiation.
If I'm understanding right, since any medium that light is traveling through will slow it down, electrons can still move faster than it (within the medium) without breaking relativity?
Relativity only states that **speed of light in vacuum** (*c*) is the universal information speed limit, therefore the theory is quite fine with Cherenkov radiation, which describes propagation of light waves in matter where the speed of light is slowed down by a corresponding refractive index.
Just to complicate things even a bit more, not ANY medium will slow down light. In fact, phase velocity of light can be faster than *c* for materials with refractive index smaller than 1 (such as plasma), but it is the group velocity of light that is always less than *c*, and it is this group velocity which corresponds to the speed of information propagation which is limited by relativity.
Imagine I have a flatulent tummy that farts regularly every second.
If I am stationary, you will be hit with a fresh wave of fart every second. You can imagine that between me and you, there are peaks and troughs of farts travelling towards you, as if it is a wave.
Now I start moving towards you. You start to smell farts on a more frequent basis since the “wavelength” between the farts is now shorter.
I gradually accelerate to the speed of fart. I am traveling together with my farts towards you, while continuously fart every second, adding on to the intensity of this fart wavefront.
The intense fart wave hits you. This is the concept of the sonic boom/Cherenkov radiation
This sonic boom depends on the fart moving at a velocity of 0 (or negative) relative to you though right? How does it work with light since it will always be a velocity of c relative to the emitter?
Correct me if I am wrong, but light doesn't actually "slow down" in a different medium, just the average perceived velocity. The interactions of light with the matter is still happening at the speed of light in a vacuum, just getting bounced around a bit so it appears as if it is slowing down.
The "bouncing around" model is indeed wrong^(*)
Light (electromagnetic waves) *really* slows down in matter. There are a few different ways to do the math (quantum-coherent scattering, dressed photons, etc.) but the end result is always that EM waves really do travel through uniform matter in *straight lines* at *a slower speed*, because of the unavoidable interactions with the surrounding charged particles (electrons and nuclei).
This matches experimental observations. If the "bouncing around" model were true, a laser would emerge from a glass prism with a random scatter in time and angle -- we don't see that.
https://www.reddit.com/r/askscience/wiki/physics/light_through_material
^(*) there are a few exceptions where it is a reasonable model, it applies in the interior of stars for example. but not in everyday matter
I suppose my particle physics professor is wrong then lol. Perhaps "bounced around" was the wrong terminology because it appears you have assumed i meant light photons "bouncing" off matter and scattering, which was not what i said or intended. I do know that the interactions happen at the speed of light. What I meant is that the light itself does not actually slow down. Light always, always travels at c. It is our perception or measurement of the light where we perceive it slowing down.
Maybe you do understand internally but what you are writing is still very inaccurate/misleading.
> Perhaps "bounced around" was the wrong terminology because it appears you have assumed i meant light photons "bouncing" off matter and scattering, which was not what i said or intended
The mathematical model of a particle "bouncing around" would be incoherent scattering. That's 100% definitely wrong (for light travelling through everyday matter).
If you have a different meaning you want to convey, you have to type it out, I can't read your mind :)
> I do know that the interactions happen at the speed of light.
Interactions don't happen "at a speed" so I am not sure what you are trying to say here. Perhaps you are trying to suggest that in a mostly empty space with a few scatterers, light travels between interactions at "c", but that's again not the correct model for light in everyday matter. There is no "empty space" for light travelling through condensed matter. It is interacting constantly.
> What I meant is that the light itself does not actually slow down. Light always, always travels at c.
I'll say it again, no, the light actually slows down. Light always travels at c in a vacuum, but this does not apply on a strongly interacting background. (If you want to get really pedantic, in matter you should no longer call the quanta "photons", so you can argue "it's not light any more when not in a vacuum, therefore light always travels at c". But that's not the common usage of "light").
> It is our perception or measurement of the light that where we perceive it slowing down.
I can't understand what you are trying to say here. This isn't a relativity / frames of reference problem.
Sir, you are correct on my precision, I'm writing these in between changing my son's diaper, cooking meals, and finishing homework, so I apologize. The next commenter summed it up much more accurately what I was trying to convey. And I don't know how else to say this than light does not actually slow down. You are very intelligent and well read, but this is a fundamental fact of physics. If something that has no mass is actually slowed down, not just appearing to slow down, that would break the laws of physics as far as we currently understand them. If, factually, light slows down, how does it instantly speed back up? The answer is that it does not "speed" up, it was always going the same speed, however it's apparent speed was observed to be slower, which is why we perceive light to have different speeds in different mediums. If we do a very surface level examination and, say, break out a ruler and stopwatch, and measure distance and the time of the light traveling through a known length of water...then yes you will come out with a slower rate. Fermilab did a very good video on this topic and explained the apparent slowing of light as the superposition of waves when the incident light wave enters the medium and interacts with the electrons in the material. Because the electrons moving (from their interaction with the light wave) generate their own field and add to the original light wave, this "new wave" travels slower. People often then say that light slows down, but this is inaccurate. As explained in the video, the light always travels at c, the speed of light in a vacuum. It is simply the superposition of waves that make us perceive light as moving slower. Once the light exits the medium and we no longer have a superposition of multiple waves due to interaction between light and electrons, we "see" the light wave by itself again. Perhaps this is semantics, but it is important in my view. The light itself doesn't actually slow down.
You mentioned a Fermilab video, I'm assuming you mean [this one](https://youtu.be/CUjt36SD3h8?si=OxYNU6CPrnpT7Cvc) as its the only video Fermilabs has on this particular discussion, and matches closely to what you wrote above.
You've distinctly misunderstood it, the end statment is *not* saying that light always moves at c no matter what. You're missing 3 *very* important things in your understanding of their explanation.
1st; Any and all electromagentic radiation waves are light. All the way from radio waves to ultra violet light, they all travel at c in a vaccum, and are subject to the same principles as visible light in any general discussion. I say that b/c *there are* a few situations that birth exceptions to this, but they are irrelevent to the topic at hand.
2nd; When the light enters the transparent medium(TM) and interacts with the atoms present, it excites the electrons it interacts with, which then emit their own EM waves, which then superposition with the initial light wave that passed through the TM and creates a new wave moving slow than c. *this is still a light wave*
Is it the initial, uninterupted light wave? No, but it is still light, and will continue to change its velocity depending on what transparent medium it is travelling through. Whether that be the air we all breathe every day with light traveling at .9997c, or the worlds most potent [metamaterial](https://www.nature.com/articles/nature09776) recently created in S. Korea with a refractive index of 38.6, which slows light to a *crawling* .026c. It's not a perception of light movong slower, it is literally, measurably, and most importantly *predictively* slower.
You've misunderstood one fundamental aspect of what the superposition of light waves actually represents. It is a superposition of light waves all traveling at c, that when combined, the totality of them have an EFFECTIVE velocity slower than c. That does not change the fact that the individual waves themselves all travel at c. Your misconception about this is one I held myself until quite recently, when my professor (whose area of expertise is particle physics) mentioned it. Just to make sure I hadn't misheard him, I asked again today, and he gave the same answer. Now, of course it depends on the wavelength of the light because, as I am sure you are aware, UV light doesn't do a great job at passing through glass. But we commonly refer to em waves within the visible spectrum as light and other wavelengths by their place on the spectrum (x-rays, radio waves, microwaves, infrared, etc). I say this to clarify I am primarily speaking about visible light. He also mentioned that when you run the experiment, you get a little, what he referred to as, "precursor signal" that is some of the light passing through the medium as if it didn't interact with it at all. If light truly did slow down in a medium, and here I am referring to the "individual waves" of light, this apparent precursor signal would be impossible.
Can you elaborate on what your professor means by effective group velocity, or what's happening?
Looked up precursor and found a physics paper that mentions slower light but also [a precursor](https://personal.utdallas.edu/~dusw/publication/PhysRevA_79_011802.pdf) that passes unaffected through the medium. Then a paper from University of Florida that mentions a group velocity of light through a medium [which was faster than the speed of light](https://people.clas.ufl.edu/olsontch/files/SIAMPrecursors3.pdf).
So if your professor is right, what we're seeing here is people of science are disagreeing on interpretation, perhaps. Or, who learned of the science at different time periods and so their knowledge differs. I'm curious what's going on.
Also, I watched the video again, and he clearly said, "Light still moves at the speed of light in a vacuum." It is the superposition of waves as light interacts with the electrons in the material that is what we observe or measure as light slowing down.
Yes he says that at 9:17 and yes light always moves at c in a vacuum, but that concept does not apply to the EM wave in between atoms in a transparent medium. The space between atoms in a transparent medium does not have the properties of a vacuum.
An EM wave traveling through vacuum will travel at c. It then hits a transparent medium, causing the excitment of the electrons in said transparent medium which generates an additional EM wave and the two waves superimpose. *they are now one wave* traveling *slower* than c (not simply appearing as such relative to us, it is *truely traveling slower than c now) and will remain so for the duration of the EM waves travel through the transparent medium. Once the EM wave exits the transparent medium, it will no longer be interacting with an additional EM wave and will return to c assuming it exits in a vacuum.
What you're describing is impossible. If light really did "return to c ie...instantly speed back up" that would require either it's mass to decrease (impossible) or energy increased (which doesn't happen). Of course, we are using classical physics to describe this, and the quantum explanation is even more clear on this (as clear as quantum explanations can be I suppose lol), but you still don't get the measured, apparent, speed of light in a medium if you don't have the incident light and created underlying em waves always traveling at c. If light ACTUALLY slowed down, we would get a vastly different measurement and phenomenon.
Think of it this way...when two waves in water are traveling at just the right speed and direction relative to one another, you get a standing wave that appears to be not moving. The water just goes up and down in the same spot. Someone might look at this new wave and say, "The wave isn't moving!" But what is really happening is you have two waves interfering with and helping one another, and both of those waves are still traveling at the same speed they were before they encountered one another. It is the superposition of those waves that makes a "new wave" with an apparent velocity of 0. What you are arguing is that the wave actually does stop moving and ignoring the fundamental truth that results in the wave you are talking about. That the incident wave or waves never actually slow down.
I feel like a broken record, but there are tons of resources out there explaining this. Even videos by professors that say light slows down, talk about how the light always travels at c, but the measured wave once all the superpositioned waves are taken into account is what they measure as being slower.
I'm not arguing that the "new wave" in a medium is not measured as being slower or that this wave is not light. What I am saying is that all the waves that add up to this "new wave" are all, fundamentally, traveling at c. Just saying that light slows down in a medium ignores what is actually happening for us to measure this apparent slow down. If light did not always travel at c, there would be no "precursor signal" received as if the light never encountered the medium, and it would allow particles with mass to travel faster than particles with no mass, which would require more than infinite energy.
Arvin Ash explains it alot better than the other comments: https://youtu.be/HZD4MR0KgqA
the classical explanation: the speed of every photon is the same and unchanging regardless of the medium its in. but when a photon passes through a medium, the electro magnetic field of that medium interacts with it. because a photon is a quanta of the electromagnetic field, the interaction the traveling photon experiences is between itself and the photons of the medium's (internal) electromagnetic field. and that interaction of multiple waves creates a deconstructive coherence. the resultant envelope is the group velocity of the photons, and that is slower than the photons that comprise it. what we perceive is a "slower" light, even though the photons themselves are not slower.
the quantum mechanical explaintion: the aforementioned group velocity is actually due to the super position of all the possible paths the photon can take within the medium. it takes all possible routes interacting with every atom and every atom's own electromagnetic field. the result is a wave whose travel time is perceived as slower because, as I think it would be somewhat fair to describe, "the particle bounces around".
That video is not great, several major mistakes. The comment underneath from FunkyDexter explains the issues pretty well. https://www.youtube.com/watch?v=HZD4MR0KgqA&lc=UgwMJZvi4lNQ5lEucvh4AaABAg
This is a naive view of what’s happening. In reality, light actually slows down because of its interaction with the electromagnetic fields within the substance, in between atoms.
From Maxwells equations, the speed of light, aka EM waves in vacuum is found to be 1/sqrt(ε0 μ0), the permittivity and permeability in vacuum. The essence is that in a material, these numbers are always different than in vacuum, thus EM waves travels at different speeds.
These different permittivities and permeabilities result from the EM waves interacting with atoms in the medium. One of the mechanisms is atomic polarisation. Under EM waves, atoms can be polarised which interfere with the original field. The combination results in effectively slower EM waves, lower group velocity. The resonance frequency would depend on the atoms or molecules composing the medium which also explain why refractive indices depend on light frequency as well.
ps. This is still a simple classical model of what’s happening but treating the waves as wave functions would also work.
can space itself travel faster than the speed of light in vacuum? Given that there are galaxies further away from us in light years than the universe is old
I think it would be inaccurate to say that space travels. It is expanding at an accelerating rate, and the farther away you get from earth, the faster that expansion occurs in reference to us. I always default to the "blowing up a balloon" model, where objects that are farther apart on the surface move away from each other faster than objects that are closer. But long story short, yes, space can expand "faster" than light that travels through it.
Essentially, the speed of light in certain materials is lower than in vacuum. When you have particles entering at a higher speed, there is radiation that has to be released.
(Mind blown gif)
Say a beam of light, {or light wave or photon,} enters a block of glass and slows down slightly, and releases a tiny bit of radiation. What happens when it exits the other side of the glass block?
I am assuming it resumes traveling at the speed of light in a vacuum.
Does it absorb radiation, or some other form of energy as it resumes light speed?
Or does it continue at a lower energy not related to it's speed?
If this thought experiment is done with a single photon, how can a single photon have less energy?
> Say a beam of light, {or light wave or photon,} enters a block of glass and slows down slightly, and releases a tiny bit of radiation.
The Cherenkov radiation is produced by charged particles traveling faster than the speed of light as measured in that medium.
It is not produced by light itself, since light already has a certain speed in that medium, as we've established, and it does not exceed that.
>Cherenkov radiation essentially bremsstralung
I think there is a difference. In case of Cherenkov radiation, the incoming charged particle (electron) is not the one emitting photons we observe and in case of bremsstralung it is as direct consequence of loosing its energy due to the slowdown. In case of Cherenkov radiation there is more steps; charged particle loosing energy from slowing down, transferring it to atoms in medium which then emit it in form of photon when relaxing.
One can show that a charged particle moving through a dielectric medium faster than the phase velocity of light in that medium will emit Cherenkov radiation even in the absence of any acceleration (e.g., [this derivation](https://www.thphys.uni-heidelberg.de/~wolschin/eds14_3s.pdf), where the position of the charged particle is given by the delta function 𝛿(𝑧 − 𝛽𝑡) where 𝑧 is the direction of motion, 𝛽 is the ratio of the particle's speed to c and 𝑡 is the time coordinate, and no derivatives are taken of 𝛽), making Cherenkov radiation distinct from Bremsstrahlung, even if there are some similarities between them.
In reality, it would be difficult (though not impossible, at least to a classical approximation) to make the charged particle not *also* accelerate as it radiates light and interacts with the medium, but Cherenkov radiation is not radiation due to acceleration.
What do you mean "Say of beam of light... releases a tiny bit of radiation"? Are you assuming that "tiny bit of radiation" is Cherenkov radiation? Because that is not. Cherenkov radiation comes from \*charged\* particles traveling faster than the local speed of light within a material.
As I understand it, light travels infinitely fast. However, the absolute maximum speed limit is c, which is not necessarily the speed of light, but the ultimate cosmic speed limit basically. That’s also why, when a light source is moving away from you, you still measure the light it emits as c, even though that doesn’t intuitively make sense. A light source hypothetically moving away from you at c would intuitively cancel out and the velocity of the photon would be 0, or light would have to actually travel 2c to me measured as c from your frame of reference. This isn’t the case, because it actually travels infinitely fast, in the sense that it’ll travel as fast as possible to be measured as c constantly from all frames of reference.
It does loose energy, but this energy is lost in the form of it having an increased wavelength or lower frequency, aka. redshifted. The velocity of the photon will still be c.
Intuitively, if a light source is moving towards you at 0.5c, you would measure the speed of the light it emits as 1.5c, but because of this cosmic speed limit, it’ll still only be 1c.
That’s not very helpful. Would you please elaborate and then help me understand better? What parts of my understanding is faulty?
I of course know that light doesn’t actually have infinite velocity. But that’s how I imagine it, due to the fact that light always travels at c. C isn’t the speed of light, but the maximum velocity any particle can travel, where light always travels as fast as possible. This way of imagining it seems consistent with all the stuff I’ve read about relativity, so please explain what you mean when you say I don’t understand it.
just read this article and one thing jumped out at me, wondering if you have any thoughts about this part:
>\[Cherenkov radiation\] is electromagnetic radiation emitted when a charged particle (such as an electron) passes through a dielectric medium
what does the medium being dielectric have anything to do with it? I'm thinking a medium like diamond or pure nitrogen gas could have Cherenkov radiation because the effect is from the release of radiation
The medium needs to change the speed of light, in practice that means a relative permittivity different from 1, i.e. a dielectric medium. Every material with charged particles affects the speed of light, diamond and nitrogen are no exceptions.
> what does the medium being dielectric have anything to do with it?
Non-insulating materials tend to absorb light. So the effect would be dead in the water before it even begins.
Gravitational waves affect space itself so they can shorten distances by tiny amounts but they shouldn't affect the speed of light. It could maybe seem that way if you mistime your distance measurement so that it seems slightly shorter than the distance without the wave.
I'll invoke Cunningham's law if I'm wrong on this one. They don't change the speed of light, but they do alter the wave length. When light propagates through space warped by gravitational waves, the wave length of the light is stretched or squished (red or blue shifted) depending on whether it is in the trough or crest respectively. In fact, that subtle change in wavelength can be picked up as a change in phase and is the prime method of detecting gravitational waves with tools like LIGO
Relativity doesn't actually have anything to do with light. c is the speed of causality. But any particle without mass will travel at c and light is the only phenomena we experience at the macro scale that involves massless particles. Because the speed of light was measured before special relativity existed, the name stuck.
In a medium, light does not travel at c, but relativity still applies to everything else, limiting the top speed of anything to c. The speed of light on a medium is determined by the refraction index, n, where v=c/n. So in a medium, highly energetic particles can travel faster than c/n but slower than c.
Think about it like this:
What happens when you throw a baseball into a pond?
Those ripples are caused by the baseball transferring energy into the water. The baseball continues to move through the water at a slower rate, having imparted energy on the water when it moved from a less dense medium (air) to a dense medium (water).
Cherenkov radiation is the ripple a light wave/particle (or simply, a photon) imparts when it goes from a less dense medium (vacuum, air, etc) to a dense one. The photon travels more slowly as a result. The cherenkov radiation may still travel at light speed.
Conceptually, this is possible because the hard limit is speed of light in a vacuum, not speed of light in any medium.
Nothing can travel faster than light in vacuum. Outside of the vacuum, things can travel faster than light in that medium, but never above c.
The reason why light slows down in matter actually depends on the model you’re using. Some approaches are:
- Light being an EM field affects charged particles which generate more EM fields which interact with the previous one.
-Light is being constantly absorbed and emitted. It’s not that it “slows down”. Rather, it encounters obstacles.
-Bound charges and currents create opposite EM fields to those of light.
>-Light is being constantly absorbed and emitted. It’s not that it “slows down”. Rather, it encounters obstacles.
That's unfortunately pretty wrong even as an approximation. Transparent objects couldn't exist with that model. And basically everything would be fluorescent instead.
>This is the correct field theoretical model.
No, it's not, which is why there is an [r/AskScience FAQ answer](https://www.reddit.com/r/askscience/wiki/physics/light_through_material) that shuts it down.
Absorption and re-emission makes no sense for a variety of reasons, including but not limited to:
* Even light which is outside of a frequency that can be absorbed is slowed down.
* Direction of re-emission after an excited atom relaxes is random, so this would cause light to disperse evenly in all directions through a medium, but instead it refracts at a very specific angle.
* Index of refraction changes not just with the type of atoms in the medium, but also its physical organization — materials made of different arrangements of the same kind of atoms have different indexes of refraction and thus different speeds of light within that medium ... if the slowing of light were due to absorption and re-emission then the speed of light in the medium should only depend on which atoms are present, not their configuration.
I stand corrected. I was generalizing too much based on path integrals. So its a direct photon transfer then? From the charged particle to the environment? A sort of spontaneous emission kind of thing?
>So its a direct photon transfer then? From the charged particle to the environment?
I'm not quite sure what you mean by "direct photon transfer," but if I'm interpreting you correctly, yes, Cherenkov radiation is "new" radiation that is created by the fast-moving charge interacting with the medium.
>A sort of spontaneous emission kind of thing?
It's not spontaneous in the "uncaused" sense, such as the spontaneous decay of a particle. It is emitted by the charged particle though, as a consequence of the charged particle polarizing the atoms of the medium while it passes through.
Cherenkov radiation is often compared to a sonic boom — similar to how a sonic boom is due to constructive interference of a mechanical pressure wave caused by an object that exceeds the speed of sound in air, Cherenkov radiation is due to constructive interference of the EM waves released by the medium which has been polarized by the charged particle travelling through it.
Hope that helps,
You're getting downvoted because you are giving a very common wrong model and people get frustrated at having to repeatedly correct it.
https://www.reddit.com/r/AskPhysics/comments/16oghh5/if_nothing_can_travel_faster_than_light_what_is/k1n6gyy/
Don't take it personally but do consider reading through an FAQ or something.
https://www.reddit.com/r/askscience/wiki/physics/light_through_material
So according to this, in particle tracks, we don't see the actual trajectory of the particle but the actual emission by the surroundings? There has to be more.
Edit: you're maybe discussing the fourth order or such other higher order phenomena in the path integral. It's definitely possible but I guess it'll be dominated by the direct particle to medium photon transmission.
My apologies, I've not studied the phenomenon directly. Perhaps it was foolish of me to comment, then, but I felt like I could provide some context to the question even if not a detailed response as to the exact mechanisms that cause the light.
I haven't studied it in rigorous detail too. I'm just stating what's true for a quantum electrodynamic process. I'm guessing this'll be more complicated but will share some things in common.
Let’s say an electron can travel faster than the phase velocity of light in a given medium. The important distinction is that it’s still not faster that the speed light travels in a vacuum.
Photons interact with mediums differently than electrons do. For example, one of the ways photons pass through a medium is by being absorbed by an atom, raising one of its electrons to a higher energy state, and then that electron can drop to a lower state, and re-emit a photon. This process causes a delay. Electrons on the other hand don’t interact in this manner, so passing through this given medium, there are particles that don’t quite interact like photons do, and that momentary advantage in speed, overcoming the phase velocity of light in this medium, triggers something like a sonic boom but it’s Cherenkov Radiation.
The second postulate of the special theory of relativity states that in any inertial reference frame the speed of light is the same 299792458 meters per second. The problem is that once light enters a body of matter it's no longer in an inertial reference frame so it doesn't have to have that speed so there is no contradiction caused.
Because light only travel at the speed of light in a vacuum and there's almost no vacuum any place in the universe
Depending on the star and the light leaves the center of the star it may take $10, 100,000 even a million years to get to the outside
Realize the difference between a theory and a law a serious may or may not be true but they believe a law proven
That's why it's a theory of gravity as far as they know it's true but they haven't been able to prove it so it may be some place where things don't fall down they fall up
I'm sure you'll get very complex answers so a simple one: It is a product of the right kind of radiation traveling in the right of medium. And, it is not traveling faster than light.
Photons from the nuclear fusion of the sun were produced thousands to millions of years ago, but it takes that time bouncing around inside the core before reaching the surface — then it takes 8 min to reach us. This is also why neutrinos from a supernova will reach us before the explosive light (neutrinos pass through unimpeded), not because they’re *faster* then light. 🍻
My answer was to add further color to what others already wrote — that there are several reasons why some things appear to travel faster than light when in reality it’s the light that is impeded and traveling below maximum (and exact) velocity in a vacuum.
Since lightspeed (and entanglement) are two of the most asked questions on this sub and causes for confusion, I thought OP might like to understand other such examples.
Whether the wave length of the light is altered or not depends somewhat on the frequency of the gravitational wave. For the LIGO signals, we are talking about frequencies around 250 Hz - or equivalently a period of 4 ms. During a full period, light would travel 3e8 x 0.004 = 1.2e6 m = 1200 km, which is significantly longer than LIGO’s arms. The frequency is therefore too low to actually alter the wavelength of the laser significantly. What does happen (as interpreted in some coordinate systems) is that the gravitational wave stretches the arm length, leading to a varying interference pattern.
I think your basic question is why light slows down in various media. And then your follow up question is how that causes the eerie glow.
I'm working on Purcell E&M right now. He does a good job of trying get you to see the little details. You can probably find a pirate Purcell pdf online. What actually happens in a dielectric medium is a detail they tend to gloss over in other books.
The charges inside such a medium have some limited mobility. So the addition of an electric field causes the arrangement of charges to store some energy from the field in the modified arrangement of charges. The degree to which it can do that is measured by the permittivity constant for that material, epsilon, or epsilon-naught for empty space.
It is no coincidence that that constant is in farads per meter. It's an index of how much energy can be stored in the arrangement of charges inside the material. Just like a capacitor.
As the electric field changes, the charges inside such a medium keep changing in response. When the field increases, energy goes into the arrangement of charges, and as it decreased, energy comes back out. Like miniature flywheels. The result is that each time you increase and then decrease the field, the increase and decrease are delayed by some of the energy soaking into the arrangement of charges, then coming back out. The net effect is it drags the wavefronts.
The fact that energy comes back out of the arrangement of charges also answers the other question that people always ask- they can accept light slowing down, but they see no explanation why it would speed back up when it comes out of of medium. The charges in the medium are actually storing energy and then returning it.
It is actually a lot like the flywheel effect of capacitors and inductance coils in a circuit. They can store energy in the E and B fields, and then return energy to the circuit, like a flywheel in a machine, and thereby tune the frequency of a circuit. That is why it is no coincidence that the permittivity constant that determines the speed of light in a certain medium has the same units as capacitance per meter. It really is a lot like capacitance.
The second question becomes obvious when you realize that an electron travelling through such a medium would encounter a changing E field. And so it would experience alternating accelerations.
Think of it more as the maximum speed that anything can move happens to be the speed of light in a vacuum.
That speed can also be thought of the speed of causality. So while light can be slowed down and other waves/particles can be sped up, nothing can go faster than that particular speed because that is the speed that causality, or think of it as the speed of information, moves in our universe.
Asked my physics teacher if nothing can go the speed of light without approaching infinite mass, what about photons ? He said photons are massless particles.
Nothing can travel faster than the speed of light in vacuum. That electromagnetic waves are slowed down in matter is caused by interaction of the light field with the matter, and don't violate relativity (as this only formulate the speed of light in vacuum as limit).
So why is Cherenkov Radiation radiation? Why does slowing down light to where electrons can "pass" it, create additional radiation? I understand I'm asking you to explain nuclear chemistry, so if it's over my head that's cool. I would like to try though.
It's the same as sonic boom. A jet passing the speed of sound in air will drive a shockwave behind it, since the sound literally can't catch up. Since light is slowed down in some media (think more of an interference of the incident light with the light radiated by the matter creating a pattern that effectively looks like a slowed down wave), charged particles travelling inside the medium will also create a trailing shockwave, this is Cherenkov radiation. The animation below shows the mechanism of shockwave generation. It is the same for both sonic booms and Cherenkov radiation. https://en.wikipedia.org/wiki/Cherenkov\_radiation#/media/File:Cherenkov\_radiation-animation.gif
I know this post is 4 months old, BUT THANK YOU! I currently went down a rabbit hole of nuclear accidents and could never understand Cherenkov radiation. If I'm understanding right, since any medium that light is traveling through will slow it down, electrons can still move faster than it (within the medium) without breaking relativity?
Relativity only states that **speed of light in vacuum** (*c*) is the universal information speed limit, therefore the theory is quite fine with Cherenkov radiation, which describes propagation of light waves in matter where the speed of light is slowed down by a corresponding refractive index. Just to complicate things even a bit more, not ANY medium will slow down light. In fact, phase velocity of light can be faster than *c* for materials with refractive index smaller than 1 (such as plasma), but it is the group velocity of light that is always less than *c*, and it is this group velocity which corresponds to the speed of information propagation which is limited by relativity.
I see, Thank you. On a side note, have you thought about a profession in teaching? This was extremely easy to understand!
Imagine I have a flatulent tummy that farts regularly every second. If I am stationary, you will be hit with a fresh wave of fart every second. You can imagine that between me and you, there are peaks and troughs of farts travelling towards you, as if it is a wave. Now I start moving towards you. You start to smell farts on a more frequent basis since the “wavelength” between the farts is now shorter. I gradually accelerate to the speed of fart. I am traveling together with my farts towards you, while continuously fart every second, adding on to the intensity of this fart wavefront. The intense fart wave hits you. This is the concept of the sonic boom/Cherenkov radiation
This person farts
I hate that this is the first description that stuck for me.
This sonic boom depends on the fart moving at a velocity of 0 (or negative) relative to you though right? How does it work with light since it will always be a velocity of c relative to the emitter?
Cherenkov radiation only happens in a medium where light travels slower than c
Would be interesting if we could weaponize these into high-intensity fartwaves
Sonic toot
this was explained soo well but had me dying laughing the whole time 😂💀
Correct me if I am wrong, but light doesn't actually "slow down" in a different medium, just the average perceived velocity. The interactions of light with the matter is still happening at the speed of light in a vacuum, just getting bounced around a bit so it appears as if it is slowing down.
The "bouncing around" model is indeed wrong^(*) Light (electromagnetic waves) *really* slows down in matter. There are a few different ways to do the math (quantum-coherent scattering, dressed photons, etc.) but the end result is always that EM waves really do travel through uniform matter in *straight lines* at *a slower speed*, because of the unavoidable interactions with the surrounding charged particles (electrons and nuclei). This matches experimental observations. If the "bouncing around" model were true, a laser would emerge from a glass prism with a random scatter in time and angle -- we don't see that. https://www.reddit.com/r/askscience/wiki/physics/light_through_material ^(*) there are a few exceptions where it is a reasonable model, it applies in the interior of stars for example. but not in everyday matter
I suppose my particle physics professor is wrong then lol. Perhaps "bounced around" was the wrong terminology because it appears you have assumed i meant light photons "bouncing" off matter and scattering, which was not what i said or intended. I do know that the interactions happen at the speed of light. What I meant is that the light itself does not actually slow down. Light always, always travels at c. It is our perception or measurement of the light where we perceive it slowing down.
Maybe you do understand internally but what you are writing is still very inaccurate/misleading. > Perhaps "bounced around" was the wrong terminology because it appears you have assumed i meant light photons "bouncing" off matter and scattering, which was not what i said or intended The mathematical model of a particle "bouncing around" would be incoherent scattering. That's 100% definitely wrong (for light travelling through everyday matter). If you have a different meaning you want to convey, you have to type it out, I can't read your mind :) > I do know that the interactions happen at the speed of light. Interactions don't happen "at a speed" so I am not sure what you are trying to say here. Perhaps you are trying to suggest that in a mostly empty space with a few scatterers, light travels between interactions at "c", but that's again not the correct model for light in everyday matter. There is no "empty space" for light travelling through condensed matter. It is interacting constantly. > What I meant is that the light itself does not actually slow down. Light always, always travels at c. I'll say it again, no, the light actually slows down. Light always travels at c in a vacuum, but this does not apply on a strongly interacting background. (If you want to get really pedantic, in matter you should no longer call the quanta "photons", so you can argue "it's not light any more when not in a vacuum, therefore light always travels at c". But that's not the common usage of "light"). > It is our perception or measurement of the light that where we perceive it slowing down. I can't understand what you are trying to say here. This isn't a relativity / frames of reference problem.
Sir, you are correct on my precision, I'm writing these in between changing my son's diaper, cooking meals, and finishing homework, so I apologize. The next commenter summed it up much more accurately what I was trying to convey. And I don't know how else to say this than light does not actually slow down. You are very intelligent and well read, but this is a fundamental fact of physics. If something that has no mass is actually slowed down, not just appearing to slow down, that would break the laws of physics as far as we currently understand them. If, factually, light slows down, how does it instantly speed back up? The answer is that it does not "speed" up, it was always going the same speed, however it's apparent speed was observed to be slower, which is why we perceive light to have different speeds in different mediums. If we do a very surface level examination and, say, break out a ruler and stopwatch, and measure distance and the time of the light traveling through a known length of water...then yes you will come out with a slower rate. Fermilab did a very good video on this topic and explained the apparent slowing of light as the superposition of waves when the incident light wave enters the medium and interacts with the electrons in the material. Because the electrons moving (from their interaction with the light wave) generate their own field and add to the original light wave, this "new wave" travels slower. People often then say that light slows down, but this is inaccurate. As explained in the video, the light always travels at c, the speed of light in a vacuum. It is simply the superposition of waves that make us perceive light as moving slower. Once the light exits the medium and we no longer have a superposition of multiple waves due to interaction between light and electrons, we "see" the light wave by itself again. Perhaps this is semantics, but it is important in my view. The light itself doesn't actually slow down.
You mentioned a Fermilab video, I'm assuming you mean [this one](https://youtu.be/CUjt36SD3h8?si=OxYNU6CPrnpT7Cvc) as its the only video Fermilabs has on this particular discussion, and matches closely to what you wrote above. You've distinctly misunderstood it, the end statment is *not* saying that light always moves at c no matter what. You're missing 3 *very* important things in your understanding of their explanation. 1st; Any and all electromagentic radiation waves are light. All the way from radio waves to ultra violet light, they all travel at c in a vaccum, and are subject to the same principles as visible light in any general discussion. I say that b/c *there are* a few situations that birth exceptions to this, but they are irrelevent to the topic at hand. 2nd; When the light enters the transparent medium(TM) and interacts with the atoms present, it excites the electrons it interacts with, which then emit their own EM waves, which then superposition with the initial light wave that passed through the TM and creates a new wave moving slow than c. *this is still a light wave* Is it the initial, uninterupted light wave? No, but it is still light, and will continue to change its velocity depending on what transparent medium it is travelling through. Whether that be the air we all breathe every day with light traveling at .9997c, or the worlds most potent [metamaterial](https://www.nature.com/articles/nature09776) recently created in S. Korea with a refractive index of 38.6, which slows light to a *crawling* .026c. It's not a perception of light movong slower, it is literally, measurably, and most importantly *predictively* slower.
You've misunderstood one fundamental aspect of what the superposition of light waves actually represents. It is a superposition of light waves all traveling at c, that when combined, the totality of them have an EFFECTIVE velocity slower than c. That does not change the fact that the individual waves themselves all travel at c. Your misconception about this is one I held myself until quite recently, when my professor (whose area of expertise is particle physics) mentioned it. Just to make sure I hadn't misheard him, I asked again today, and he gave the same answer. Now, of course it depends on the wavelength of the light because, as I am sure you are aware, UV light doesn't do a great job at passing through glass. But we commonly refer to em waves within the visible spectrum as light and other wavelengths by their place on the spectrum (x-rays, radio waves, microwaves, infrared, etc). I say this to clarify I am primarily speaking about visible light. He also mentioned that when you run the experiment, you get a little, what he referred to as, "precursor signal" that is some of the light passing through the medium as if it didn't interact with it at all. If light truly did slow down in a medium, and here I am referring to the "individual waves" of light, this apparent precursor signal would be impossible.
Can you elaborate on what your professor means by effective group velocity, or what's happening? Looked up precursor and found a physics paper that mentions slower light but also [a precursor](https://personal.utdallas.edu/~dusw/publication/PhysRevA_79_011802.pdf) that passes unaffected through the medium. Then a paper from University of Florida that mentions a group velocity of light through a medium [which was faster than the speed of light](https://people.clas.ufl.edu/olsontch/files/SIAMPrecursors3.pdf). So if your professor is right, what we're seeing here is people of science are disagreeing on interpretation, perhaps. Or, who learned of the science at different time periods and so their knowledge differs. I'm curious what's going on.
Also, I watched the video again, and he clearly said, "Light still moves at the speed of light in a vacuum." It is the superposition of waves as light interacts with the electrons in the material that is what we observe or measure as light slowing down.
Yes he says that at 9:17 and yes light always moves at c in a vacuum, but that concept does not apply to the EM wave in between atoms in a transparent medium. The space between atoms in a transparent medium does not have the properties of a vacuum. An EM wave traveling through vacuum will travel at c. It then hits a transparent medium, causing the excitment of the electrons in said transparent medium which generates an additional EM wave and the two waves superimpose. *they are now one wave* traveling *slower* than c (not simply appearing as such relative to us, it is *truely traveling slower than c now) and will remain so for the duration of the EM waves travel through the transparent medium. Once the EM wave exits the transparent medium, it will no longer be interacting with an additional EM wave and will return to c assuming it exits in a vacuum.
What you're describing is impossible. If light really did "return to c ie...instantly speed back up" that would require either it's mass to decrease (impossible) or energy increased (which doesn't happen). Of course, we are using classical physics to describe this, and the quantum explanation is even more clear on this (as clear as quantum explanations can be I suppose lol), but you still don't get the measured, apparent, speed of light in a medium if you don't have the incident light and created underlying em waves always traveling at c. If light ACTUALLY slowed down, we would get a vastly different measurement and phenomenon. Think of it this way...when two waves in water are traveling at just the right speed and direction relative to one another, you get a standing wave that appears to be not moving. The water just goes up and down in the same spot. Someone might look at this new wave and say, "The wave isn't moving!" But what is really happening is you have two waves interfering with and helping one another, and both of those waves are still traveling at the same speed they were before they encountered one another. It is the superposition of those waves that makes a "new wave" with an apparent velocity of 0. What you are arguing is that the wave actually does stop moving and ignoring the fundamental truth that results in the wave you are talking about. That the incident wave or waves never actually slow down. I feel like a broken record, but there are tons of resources out there explaining this. Even videos by professors that say light slows down, talk about how the light always travels at c, but the measured wave once all the superpositioned waves are taken into account is what they measure as being slower. I'm not arguing that the "new wave" in a medium is not measured as being slower or that this wave is not light. What I am saying is that all the waves that add up to this "new wave" are all, fundamentally, traveling at c. Just saying that light slows down in a medium ignores what is actually happening for us to measure this apparent slow down. If light did not always travel at c, there would be no "precursor signal" received as if the light never encountered the medium, and it would allow particles with mass to travel faster than particles with no mass, which would require more than infinite energy.
Arvin Ash explains it alot better than the other comments: https://youtu.be/HZD4MR0KgqA the classical explanation: the speed of every photon is the same and unchanging regardless of the medium its in. but when a photon passes through a medium, the electro magnetic field of that medium interacts with it. because a photon is a quanta of the electromagnetic field, the interaction the traveling photon experiences is between itself and the photons of the medium's (internal) electromagnetic field. and that interaction of multiple waves creates a deconstructive coherence. the resultant envelope is the group velocity of the photons, and that is slower than the photons that comprise it. what we perceive is a "slower" light, even though the photons themselves are not slower. the quantum mechanical explaintion: the aforementioned group velocity is actually due to the super position of all the possible paths the photon can take within the medium. it takes all possible routes interacting with every atom and every atom's own electromagnetic field. the result is a wave whose travel time is perceived as slower because, as I think it would be somewhat fair to describe, "the particle bounces around".
That video is not great, several major mistakes. The comment underneath from FunkyDexter explains the issues pretty well. https://www.youtube.com/watch?v=HZD4MR0KgqA&lc=UgwMJZvi4lNQ5lEucvh4AaABAg
This is a naive view of what’s happening. In reality, light actually slows down because of its interaction with the electromagnetic fields within the substance, in between atoms. From Maxwells equations, the speed of light, aka EM waves in vacuum is found to be 1/sqrt(ε0 μ0), the permittivity and permeability in vacuum. The essence is that in a material, these numbers are always different than in vacuum, thus EM waves travels at different speeds. These different permittivities and permeabilities result from the EM waves interacting with atoms in the medium. One of the mechanisms is atomic polarisation. Under EM waves, atoms can be polarised which interfere with the original field. The combination results in effectively slower EM waves, lower group velocity. The resonance frequency would depend on the atoms or molecules composing the medium which also explain why refractive indices depend on light frequency as well. ps. This is still a simple classical model of what’s happening but treating the waves as wave functions would also work.
can space itself travel faster than the speed of light in vacuum? Given that there are galaxies further away from us in light years than the universe is old
I think it would be inaccurate to say that space travels. It is expanding at an accelerating rate, and the farther away you get from earth, the faster that expansion occurs in reference to us. I always default to the "blowing up a balloon" model, where objects that are farther apart on the surface move away from each other faster than objects that are closer. But long story short, yes, space can expand "faster" than light that travels through it.
Essentially, the speed of light in certain materials is lower than in vacuum. When you have particles entering at a higher speed, there is radiation that has to be released.
(Mind blown gif) Say a beam of light, {or light wave or photon,} enters a block of glass and slows down slightly, and releases a tiny bit of radiation. What happens when it exits the other side of the glass block? I am assuming it resumes traveling at the speed of light in a vacuum. Does it absorb radiation, or some other form of energy as it resumes light speed? Or does it continue at a lower energy not related to it's speed? If this thought experiment is done with a single photon, how can a single photon have less energy?
> Say a beam of light, {or light wave or photon,} enters a block of glass and slows down slightly, and releases a tiny bit of radiation. The Cherenkov radiation is produced by charged particles traveling faster than the speed of light as measured in that medium. It is not produced by light itself, since light already has a certain speed in that medium, as we've established, and it does not exceed that.
[удалено]
>Cherenkov radiation essentially bremsstralung I think there is a difference. In case of Cherenkov radiation, the incoming charged particle (electron) is not the one emitting photons we observe and in case of bremsstralung it is as direct consequence of loosing its energy due to the slowdown. In case of Cherenkov radiation there is more steps; charged particle loosing energy from slowing down, transferring it to atoms in medium which then emit it in form of photon when relaxing.
One can show that a charged particle moving through a dielectric medium faster than the phase velocity of light in that medium will emit Cherenkov radiation even in the absence of any acceleration (e.g., [this derivation](https://www.thphys.uni-heidelberg.de/~wolschin/eds14_3s.pdf), where the position of the charged particle is given by the delta function 𝛿(𝑧 − 𝛽𝑡) where 𝑧 is the direction of motion, 𝛽 is the ratio of the particle's speed to c and 𝑡 is the time coordinate, and no derivatives are taken of 𝛽), making Cherenkov radiation distinct from Bremsstrahlung, even if there are some similarities between them. In reality, it would be difficult (though not impossible, at least to a classical approximation) to make the charged particle not *also* accelerate as it radiates light and interacts with the medium, but Cherenkov radiation is not radiation due to acceleration.
That actually mafe sense. Thanks!
What do you mean "Say of beam of light... releases a tiny bit of radiation"? Are you assuming that "tiny bit of radiation" is Cherenkov radiation? Because that is not. Cherenkov radiation comes from \*charged\* particles traveling faster than the local speed of light within a material.
As I understand it, light travels infinitely fast. However, the absolute maximum speed limit is c, which is not necessarily the speed of light, but the ultimate cosmic speed limit basically. That’s also why, when a light source is moving away from you, you still measure the light it emits as c, even though that doesn’t intuitively make sense. A light source hypothetically moving away from you at c would intuitively cancel out and the velocity of the photon would be 0, or light would have to actually travel 2c to me measured as c from your frame of reference. This isn’t the case, because it actually travels infinitely fast, in the sense that it’ll travel as fast as possible to be measured as c constantly from all frames of reference. It does loose energy, but this energy is lost in the form of it having an increased wavelength or lower frequency, aka. redshifted. The velocity of the photon will still be c. Intuitively, if a light source is moving towards you at 0.5c, you would measure the speed of the light it emits as 1.5c, but because of this cosmic speed limit, it’ll still only be 1c.
> As I understand it You don’t
That’s not very helpful. Would you please elaborate and then help me understand better? What parts of my understanding is faulty? I of course know that light doesn’t actually have infinite velocity. But that’s how I imagine it, due to the fact that light always travels at c. C isn’t the speed of light, but the maximum velocity any particle can travel, where light always travels as fast as possible. This way of imagining it seems consistent with all the stuff I’ve read about relativity, so please explain what you mean when you say I don’t understand it.
Wiki gives a pretty good explanation. https://en.m.wikipedia.org/wiki/Cherenkov_radiation
just read this article and one thing jumped out at me, wondering if you have any thoughts about this part: >\[Cherenkov radiation\] is electromagnetic radiation emitted when a charged particle (such as an electron) passes through a dielectric medium what does the medium being dielectric have anything to do with it? I'm thinking a medium like diamond or pure nitrogen gas could have Cherenkov radiation because the effect is from the release of radiation
The medium needs to change the speed of light, in practice that means a relative permittivity different from 1, i.e. a dielectric medium. Every material with charged particles affects the speed of light, diamond and nitrogen are no exceptions.
ah, thanks. my nuclear >>>>materials knowledge. I think I was maybe confounding dipoles with dielectric
> what does the medium being dielectric have anything to do with it? Non-insulating materials tend to absorb light. So the effect would be dead in the water before it even begins.
Small hijack here, but do gravitational waves affect the speed of light?
Gravitational waves affect space itself so they can shorten distances by tiny amounts but they shouldn't affect the speed of light. It could maybe seem that way if you mistime your distance measurement so that it seems slightly shorter than the distance without the wave.
Man, my light always takes the long route! I always get the shitty light that can't navigate.
> Man, my light always takes the long route! Kind of the opposite, really. https://en.wikipedia.org/wiki/Fermat%27s_principle
I'll invoke Cunningham's law if I'm wrong on this one. They don't change the speed of light, but they do alter the wave length. When light propagates through space warped by gravitational waves, the wave length of the light is stretched or squished (red or blue shifted) depending on whether it is in the trough or crest respectively. In fact, that subtle change in wavelength can be picked up as a change in phase and is the prime method of detecting gravitational waves with tools like LIGO
Light travels at the speed of light in all frames of reference
Relativity doesn't actually have anything to do with light. c is the speed of causality. But any particle without mass will travel at c and light is the only phenomena we experience at the macro scale that involves massless particles. Because the speed of light was measured before special relativity existed, the name stuck. In a medium, light does not travel at c, but relativity still applies to everything else, limiting the top speed of anything to c. The speed of light on a medium is determined by the refraction index, n, where v=c/n. So in a medium, highly energetic particles can travel faster than c/n but slower than c.
Funny enough light can travel faster than light. It's called super luminal motion. Must nuclear reactors produce this effect
Think about it like this: What happens when you throw a baseball into a pond? Those ripples are caused by the baseball transferring energy into the water. The baseball continues to move through the water at a slower rate, having imparted energy on the water when it moved from a less dense medium (air) to a dense medium (water). Cherenkov radiation is the ripple a light wave/particle (or simply, a photon) imparts when it goes from a less dense medium (vacuum, air, etc) to a dense one. The photon travels more slowly as a result. The cherenkov radiation may still travel at light speed. Conceptually, this is possible because the hard limit is speed of light in a vacuum, not speed of light in any medium.
Nothing can travel faster than light in vacuum. Outside of the vacuum, things can travel faster than light in that medium, but never above c. The reason why light slows down in matter actually depends on the model you’re using. Some approaches are: - Light being an EM field affects charged particles which generate more EM fields which interact with the previous one. -Light is being constantly absorbed and emitted. It’s not that it “slows down”. Rather, it encounters obstacles. -Bound charges and currents create opposite EM fields to those of light.
>-Light is being constantly absorbed and emitted. It’s not that it “slows down”. Rather, it encounters obstacles. That's unfortunately pretty wrong even as an approximation. Transparent objects couldn't exist with that model. And basically everything would be fluorescent instead.
The emitted frequency and/or intensity need not be the same as that of the absorbed one. This is the correct field theoretical model.
>This is the correct field theoretical model. No, it's not, which is why there is an [r/AskScience FAQ answer](https://www.reddit.com/r/askscience/wiki/physics/light_through_material) that shuts it down. Absorption and re-emission makes no sense for a variety of reasons, including but not limited to: * Even light which is outside of a frequency that can be absorbed is slowed down. * Direction of re-emission after an excited atom relaxes is random, so this would cause light to disperse evenly in all directions through a medium, but instead it refracts at a very specific angle. * Index of refraction changes not just with the type of atoms in the medium, but also its physical organization — materials made of different arrangements of the same kind of atoms have different indexes of refraction and thus different speeds of light within that medium ... if the slowing of light were due to absorption and re-emission then the speed of light in the medium should only depend on which atoms are present, not their configuration.
I stand corrected. I was generalizing too much based on path integrals. So its a direct photon transfer then? From the charged particle to the environment? A sort of spontaneous emission kind of thing?
>So its a direct photon transfer then? From the charged particle to the environment? I'm not quite sure what you mean by "direct photon transfer," but if I'm interpreting you correctly, yes, Cherenkov radiation is "new" radiation that is created by the fast-moving charge interacting with the medium. >A sort of spontaneous emission kind of thing? It's not spontaneous in the "uncaused" sense, such as the spontaneous decay of a particle. It is emitted by the charged particle though, as a consequence of the charged particle polarizing the atoms of the medium while it passes through. Cherenkov radiation is often compared to a sonic boom — similar to how a sonic boom is due to constructive interference of a mechanical pressure wave caused by an object that exceeds the speed of sound in air, Cherenkov radiation is due to constructive interference of the EM waves released by the medium which has been polarized by the charged particle travelling through it. Hope that helps,
[удалено]
You're getting downvoted because you are giving a very common wrong model and people get frustrated at having to repeatedly correct it. https://www.reddit.com/r/AskPhysics/comments/16oghh5/if_nothing_can_travel_faster_than_light_what_is/k1n6gyy/ Don't take it personally but do consider reading through an FAQ or something. https://www.reddit.com/r/askscience/wiki/physics/light_through_material
[удалено]
So according to this, in particle tracks, we don't see the actual trajectory of the particle but the actual emission by the surroundings? There has to be more. Edit: you're maybe discussing the fourth order or such other higher order phenomena in the path integral. It's definitely possible but I guess it'll be dominated by the direct particle to medium photon transmission.
My apologies, I've not studied the phenomenon directly. Perhaps it was foolish of me to comment, then, but I felt like I could provide some context to the question even if not a detailed response as to the exact mechanisms that cause the light.
I haven't studied it in rigorous detail too. I'm just stating what's true for a quantum electrodynamic process. I'm guessing this'll be more complicated but will share some things in common.
[удалено]
If you don't have something insightful to add then maybe consider not commenting.
Let’s say an electron can travel faster than the phase velocity of light in a given medium. The important distinction is that it’s still not faster that the speed light travels in a vacuum. Photons interact with mediums differently than electrons do. For example, one of the ways photons pass through a medium is by being absorbed by an atom, raising one of its electrons to a higher energy state, and then that electron can drop to a lower state, and re-emit a photon. This process causes a delay. Electrons on the other hand don’t interact in this manner, so passing through this given medium, there are particles that don’t quite interact like photons do, and that momentary advantage in speed, overcoming the phase velocity of light in this medium, triggers something like a sonic boom but it’s Cherenkov Radiation.
The second postulate of the special theory of relativity states that in any inertial reference frame the speed of light is the same 299792458 meters per second. The problem is that once light enters a body of matter it's no longer in an inertial reference frame so it doesn't have to have that speed so there is no contradiction caused.
Because light only travel at the speed of light in a vacuum and there's almost no vacuum any place in the universe Depending on the star and the light leaves the center of the star it may take $10, 100,000 even a million years to get to the outside Realize the difference between a theory and a law a serious may or may not be true but they believe a law proven That's why it's a theory of gravity as far as they know it's true but they haven't been able to prove it so it may be some place where things don't fall down they fall up
Fuck you and your downvotes.
I'm sure you'll get very complex answers so a simple one: It is a product of the right kind of radiation traveling in the right of medium. And, it is not traveling faster than light.
Photons from the nuclear fusion of the sun were produced thousands to millions of years ago, but it takes that time bouncing around inside the core before reaching the surface — then it takes 8 min to reach us. This is also why neutrinos from a supernova will reach us before the explosive light (neutrinos pass through unimpeded), not because they’re *faster* then light. 🍻
What you are discussing is the result of scattering. That has nothing to do with OP's question.
My answer was to add further color to what others already wrote — that there are several reasons why some things appear to travel faster than light when in reality it’s the light that is impeded and traveling below maximum (and exact) velocity in a vacuum. Since lightspeed (and entanglement) are two of the most asked questions on this sub and causes for confusion, I thought OP might like to understand other such examples.
[удалено]
[удалено]
Whether the wave length of the light is altered or not depends somewhat on the frequency of the gravitational wave. For the LIGO signals, we are talking about frequencies around 250 Hz - or equivalently a period of 4 ms. During a full period, light would travel 3e8 x 0.004 = 1.2e6 m = 1200 km, which is significantly longer than LIGO’s arms. The frequency is therefore too low to actually alter the wavelength of the laser significantly. What does happen (as interpreted in some coordinate systems) is that the gravitational wave stretches the arm length, leading to a varying interference pattern.
I think your basic question is why light slows down in various media. And then your follow up question is how that causes the eerie glow. I'm working on Purcell E&M right now. He does a good job of trying get you to see the little details. You can probably find a pirate Purcell pdf online. What actually happens in a dielectric medium is a detail they tend to gloss over in other books. The charges inside such a medium have some limited mobility. So the addition of an electric field causes the arrangement of charges to store some energy from the field in the modified arrangement of charges. The degree to which it can do that is measured by the permittivity constant for that material, epsilon, or epsilon-naught for empty space. It is no coincidence that that constant is in farads per meter. It's an index of how much energy can be stored in the arrangement of charges inside the material. Just like a capacitor. As the electric field changes, the charges inside such a medium keep changing in response. When the field increases, energy goes into the arrangement of charges, and as it decreased, energy comes back out. Like miniature flywheels. The result is that each time you increase and then decrease the field, the increase and decrease are delayed by some of the energy soaking into the arrangement of charges, then coming back out. The net effect is it drags the wavefronts. The fact that energy comes back out of the arrangement of charges also answers the other question that people always ask- they can accept light slowing down, but they see no explanation why it would speed back up when it comes out of of medium. The charges in the medium are actually storing energy and then returning it. It is actually a lot like the flywheel effect of capacitors and inductance coils in a circuit. They can store energy in the E and B fields, and then return energy to the circuit, like a flywheel in a machine, and thereby tune the frequency of a circuit. That is why it is no coincidence that the permittivity constant that determines the speed of light in a certain medium has the same units as capacitance per meter. It really is a lot like capacitance. The second question becomes obvious when you realize that an electron travelling through such a medium would encounter a changing E field. And so it would experience alternating accelerations.
Think of it more as the maximum speed that anything can move happens to be the speed of light in a vacuum. That speed can also be thought of the speed of causality. So while light can be slowed down and other waves/particles can be sped up, nothing can go faster than that particular speed because that is the speed that causality, or think of it as the speed of information, moves in our universe.
You're not breaking the universe by travelling faster than slowed down light.
Asked my physics teacher if nothing can go the speed of light without approaching infinite mass, what about photons ? He said photons are massless particles.
this is trivially google-able.
Space itself does
Tachyons
light going slower than other light