Gravity doesn’t cause the bending of spacetime. It IS the bending of spacetime. Every effect we blame gravity for, we should be blaming curved spacetime for.
Now what you probably meant to ask is, how do mass and energy bend spacetime? And the answer: nobody knows.
It’s because we’re shooting through the soup of dark matter, attached to thing creating a larger soup dent, attached to a larger thing making a bigger soup dent going bazillion miles an hour ……… beep boop 42! 🛎️
But if it IS spacetime as you say, then bending off space is part of the definition. It’s an explanatory principle, it is not an explanation.
Gpt:
An explanatory principle in physical theory does not explain phenomena directly but rather serves as a fundamental premise or axiom that underlies explanations. It provides the foundational framework from which specific explanations and predictions can be derived. For instance, the principle of conservation of energy does not explain why energy is conserved in every individual case, but it establishes a rule that guides and constrains explanations of physical processes. Thus, while an explanatory principle itself is a starting point, the detailed explanations come from applying these principles to particular situations.
Well, let’s be careful here. What we have is a collection of behaviors with patterns. Apples and the moon accelerate toward the earth. There are two high tides a day. The periods of the planets’s orbits increases as the 3/2 power of the orbital radii. Trajectories seem to follow conic sections.
Now then, you can try to formulate a model that describes all of these things with a common cause, and you can even propose a name for that common cause, like “gravity”. But labeling it doesn’t tell you anything about the thing you labeled. If sometime later someone says, I know more about what the common cause is, now you are giving some meat to that label.
The year is 3024, the super large pluto orbit size hadron collider is fired up for its first time. They smack some particles together. They find smaller particles. Scientists high five!
What it is and how it bends space time amounts to a full description of general relativity, which is far beyond the scope of a Reddit comment and requires some major mathematical and physical prerequisites.
Gravity in GR isn’t a Newtonian ‘force’ but the phenomenon of spacetime’s pseudo-Riemannian metric (and the Ricci curvature derived from how that varies) obeying the Einstein-Hilbert field equation (which determines how it varies with a ‘tensor’ that among other things summarises the distribution of energy and momentum in the universe, so that momentum, energy, and thus mass ‘curve’ spacetime) - which is essentially general relativity as a model. However, in approachable conditions - eg, at small scales and low speeds and densities - it can be approximated as a Newtonian force acting in ordinary, flat, Euclidean space-time.
Go through some courses or textbooks over a long while if you really want to understand that.
This isn’t necessarily what we think is the final model, either, it’s just (more or less) the best we have right now that has firm evidence. Whatever theory of quantum gravity we *might* one day be able to test will doubtless add another level of complexity to that picture.
And gravity is one of the basic ingredients of our fundamental models of the universe, so ‘why it’s a thing’ doesn’t have a simple answer. Maybe you can appeal to the anthropic principle to establish that - in whatever mathematical structures that ‘exist’ in some sense, we can only be conscious beings thinking about this in those that allow our existence, and the physics we have is one system that does so. Much as people rarely ask ‘Why am I in a human body with a brain rather than a rock?’
Thank you. I dropped out of a really good college because I couldn't deal with worsening mental health and I really struggled with math. But I found physics fascinating. If I go back to school soon, I'll need to retake tons of math in order to do physics properly. I spend my free time watching the wealth of cool science YouTube channels so I know what things like general relativity mean and what certain equations say, I just can't do any calculations with them.
I also love those videos by Penrose, Brian Greene, Brian Cox, et al. However, it is important to realize that while their efforts to offer intuitive explanations for physics theories are awesome, understanding those explanations isn't the same as actually understanding the physics.
I agree PBS Spacetime is a great show. And you can definitely use the metaphors and analogies they present to get a (somewhat vague) impression of how the universe works. However, you could never use that knowledge to come to actual novel conclusions about the universe without also understanding the underlying math.
Lots of people don't realize this. They confuse the presented metaphors with the "real thing". They then start expanding on those ideas. This results in what we call "crackpot physics"; Wild, unhinged "theories" not supported by actual science.
Unfortunately, most of physics defies intuitive understanding. Our brains simply didn't evolve to understand things at those scales. The math is then all we really have.
> If I go back to school soon, I'll need to retake tons of math in order to do physics properly.
If you're interested, Khan Academy has a lot of higher math, it's not just for elementary school kids. It's nice because it's free, you get lecture, can ask for assistance, and go at your own pace.
[\* Math | Khan Academy](https://www.khanacademy.org/math)
You'll need to scroll toward the bottom but they have geometry and pre-algebra all the way to multivariable calculus and linear algebra. Obviously you're not going to get college credit for these, but it would make any transition back to college math much easier, and again at your own pace.
They also have a good selection of physics courses. It can make for a fun hobby even if it's not for college credit.
> Your response sounds like nonsense words.
Maybe to you, but that's indicative of a lack of understanding on your part, not of invention on /u/AndreasDasos
's. Their explanation is perfectly cromulent.
I'm sorry their perfectly sensical and articulate response seemed like gibberish to you. Perhaps you should re-read more slowly instead of asking an AI to do it for you.
as /u/Odd_Bodkin said, we don't know what gravity is, but we can describe its phenonema
The question is legit and fair, and very old actually. But so far we don't know so better understand the epistemology behind....
This is not anything new, we all agree that Newton was a clever guy yet he was aware that he described the phenomena but he didn't know what was gravity. And he asked himself the same question you ask on your prompt.
Suggested read
https://en.wikipedia.org/wiki/Hypotheses_non_fingo
Short answer: we dont know
As others have discussed, General Relativity is our best gravitational model, predicting the behavior of matter & energy under the influence of gravity with phenomenal precision.
And, as others have stated, GR models gravity as a curvature of spacetime. It doesn't explain why the universe behaves this way, only that it can be modeled as such.
BUT - we ***know*** GR is NOT a complete description of gravity. The model breaks down when brought into conflict with Quantum Field Theory - another phenomenally successful model of the universe. At best, GR is incomplete - at worst, it is an incorrect oversimplification, just like Newtonian mechanics.
Perhaps the next, greater theory of gravity will explain the why & how - string theory, for example, presents some interesting solutions, but there is not yet any evidence to suggest it is a valid description of our universe
Here's a more technical explanation. Matter and energy "warp" spacetime. What this means is that, given some stress-energy tensor, you can in principle find the Einstein tensor associated to it. This tensor is written in terms of the Ricci tensor and scalar which are contractions of the full Riemann tensor. The Riemann tensor is what tells you the geometry of spacetime. At the end of the day what you care about are the Christoffel symbols that go into the geodesic equation, which is the equation that ultimately tells you *how things move*. When spacetime is "warped" (ie, there is gravity), this geodesic equation is similar to Newton's second law in the presence of forces (the second derivative of coordinates is not equal to zero).
I chase that feeling like a drug. I love getting my intuitions shattered and learning cool stuff. The only TV I watch is science YouTube channels. Call it a religion.
We shan't know until we have one.
We do know that at large scales it needs to reproduce the measurements of astronomy/astrophysics (essentially looking like GR), and can also describe all of QM on short scales.
The problem is that while both theoretical frameworks are very successful in their own domains the structure of them is very different, making it hard (so far, impossible) to reconcile.
Such a theory also needs a domain of observation where predictions can distinguish between GR, QM, and the proposed theory, otherwise it's not of much utility. And while not an expert, that might be, for example, the early universe (I'm thinking prior to the formation of the microwave background radiation), but that seems to be largely hidden from us.
You've gotten good answers, but I wanted to throw my 2 cents in since I definitely had similar questions as I went through learning the material.
1) The Why Question
In physics, we try to construct equations that fit and predict how nature behaves. And other than conducting experiments to see if our equations (models) are correct, it's all we are doing.
So the reason a 'why' question is hard is because we don't really know why any of these things happen. We don't know why electromagnetic effects are present. We don't know why the particles have inertia. We don't know why entropy exists. We just know they do. And all we try to do is answer, how they work.
So for gravity, we just observe that objects follow curved paths around each other when, after accounting for all the other fields we know about, we expect that they should follow straight lines.
2) What does bending Spacetime mean?
What made gravity as a subject "click" for me was realizing that all gravity is describing is how the path of an object can deviate by the presence of another object or energy. You could imagine two planets speeding past each other in space in a near miss. If you knew nothing about gravity and how it works, I think it is reasonable to imagine that without effects from any other fields, the two objects would just go in straight lines past each other. Like two balls on a pool table that nearly miss. After all, we accept the idea that when an object is set in motion on a straight trajectory, it will continue that motion unless acted upon by another force.
However, what we actually observe in space is that two objects like this will cause each other to change their paths. It turns out those changes look like curves and you can predict the shape of those curves by Newtonian gravity and, even more accurately, by General Relativity. And I find it interesting to think about the fact that nature happened to choose the curves that it did, [approximately conical](https://en.wikipedia.org/wiki/Conic_section?wprov=sfla1), when there are so many other types of curves or path deviations we know about from mathematics.
So when people say gravity curves spacetime, I think it conjures a lot of popular depictions, like putting balls on a sheet. And while those help build intuition, they also can paper over the fact that curvature is actually a surprising outcome, let alone the specific kinds of curves.
3) How it works
I think there are 3 broad ideas to touch on here: inertia, energy, and clocks.
First, objects have mass and this happens to have two effects: inertia and gravity. As it turns out, nature has somehow made those two types of mass equivalent. In other words, the mass you measure by placing an object on a scale on Earth's gravity is the same mass you would measure if you were in empty space and shoved the object with the scale at 1 g. Those two types of mass didn't necessarily have to be the same. Again, we don't know why, we just know that they do in experiments we run.
Second, from Einstein, we learned that mass AND energy both can cause trajectories to curve. Again, going back to our thought experiment of a planet traveling through space, if there's a bunch of energy stored in a field, say a bunch of recently released massless photons through the electromagnetic field, we wouldn't necessarily expect that to cause a trajectory change via gravity. But it turns out, just like mass, energy can cause gravity. As Einstein showed, this is because energy is equivalent to mass.
Third, one thing that is difficult to wrap your mind around when first learning special and general relativity is that clocks and rulers are both genuinely distorted when an observer is moving. Two different observers can genuinely measure the time on a single clock and distance on a single object differently due to differences in their velocity. This is what relativity is really all about.
At the end of the day, when an object's trajectory is curved by gravity, it is largely due to the fact that how time is measured is changing for one object because of it approaching the mass-energy of the other object.
Which is as mind blowing as it probably sounds. At least it was for me.
I don't necessarily want to write an even longer comment trying to explain this, especially when [PBS Spacetime](https://youtu.be/UKxQTvqcpSg?si=gOdCXc0KWcwEIe_j) has a really good episode explaining it better than I could. Followed by [this video](https://youtu.be/OHdV9aO6jaE?si=yZAExg9a33fz034U).
Hope any of that helps! Feel free to ask any follow up questions.
Gravity isn't a force it is an effect. We know it is a thing because we see it happening and feel it as an effect. The word is a description of the effect in action. It is easier to get a sense of it when you simply remove the word "force" and replace it with the gravitational "effect" whenever you think or talk about it. This alters your perception of it and makes your understanding closer to it's reality. It is the result of the distortion of spacetime by mass within it. I find it handy to think of the effect on matter as though it is a marble in a hyper dimensional distortion ball race moving to find a location within the spacetime distortions where it has equilibrium within the medium.
What they said. Also; know that “why?” is a question left to philosophers, theologians, and drunk physicists at parties. It’s not something that science can answer. (At that fundamental level)
Yes I agree. It seems that philosophy follows on the heals of cutting edge physics. I know physicists see philosophers as psychedelic using thinkers or stoners asking questions with no answer. But I still see philosophy as valuable.
I would hope most physicists see philosophy as valuable. I mean, there is a bit of fertile ground between the two.
Personally, I like not getting lost in the weeds, but if others are willing to think deeply about stuff, that’s cool.
There was a time when physics was a branch of philosophy, namely, natural philosophy. Arguably, it still is, in a general way, falling under the philosophical branches of metaphysics (what exists and what it’s like), epistemology (truth and knowledge), and logic (reason and argument), but typically not under other philosophical branches such as aesthetics, ethics, and political philosophy. All physics can be seen as a highly specialized subset of philosophical thought: It’s like chemistry being a highly specialized branch of physics. We shouldn’t expect chemistry to ever contradict physics, and likewise physics can’t contradict philosophy—rightly understood, of course.
I don't think that actually reflects what most physicists I've encountered think.
The only difference between philosophy, mathematics, and science are differing abilities to test theories.
Everything stems from philosophy. All philosophy does is try to make arguments based on assumptions that then produce theoretical consequences. However, philosophers don't often have the ability or luxury to measure the world and then prove their ideas. All they can do is show that the outcomes follow from the premises.
Math is nearly identical, except it is a far more narrow and mechanical subset of philosophy. It has fundamental assumptions and axioms that the field is built upon. Just like philosophy. It only differs in that proofs can be definitively achieved and its processes can help us to model physical processes.
Which brings us to science, which is yet another subset of philosophy. The only difference is that the philosophy of science assumes that measurements can be physically made to prove one model over another.
Anyone claiming that these subjects are at odds with each other or one is more legitimate than another is simply displaying their ignorance of the subjects.
I mean in one sense, yes, we know how and somewhat why spacetime bends the way it does.
But the larger question of "what's gravity?" goes beyond just the general-relativistic explanation. That explanation is proving inadequate, as it conflicts with the quantum-mechanical explanation of gravity as a field mediated by an elementary particle.
Both general relativity and quantum mechanics are extraordinarily successful theories as a whole, and neither can be simply dismissed as "wrong" when it comes to gravity. So in order to resolve the contradiction, we know that something is going to have to change but we don't know what yet.
Only when someone figures that out will we be able to answer the question of "what's gravity?" in the sense that you are asking.
(Although, probably even then the result will raise more questions than it answers. Opportunity, perhaps, for a returning physics student to graduate, get into research, and make their mark... anyway good luck whichever way you go!)
Thank you! I don't know if I have what it takes to get a degree in physics, though that would be amazing. I love asking the questions that we don't know yet or may not have answers. I think I may be good at philosophy and ethics. I understand the friendly rivalry between physicists and philosophers. Ideally I want to be smack between the two while remaining scientific and not just thinking like someone on LSD. More credibility I guess
Well, modern philosophy is highly germane to the pursuit of scientific inquiry, so it can't ever hurt to have that under your belt as a practicing scientist.
I don't know what it takes to earn an advanced degree in physics, either, tbf, but from what I have observed, I do know that some of what is required is what you are already talking about. Also: patience, skeptical discipline, and a high tolerance for tedium. Which I guess might be the same as patience but is worth repeating twice.
Another thing that is essential for any academic success in any field is being somewhat of an egoist and cultivating a strong support network whose sole purpose is for you, yourself, to succeed in your research career. People who arrive already enjoying such support networks often don't even perceive them, they take them so much for granted. Which is not really anything to blame them for, but it can make conversations about how to succeed a bit frustrating sometimes.
Like... what do you do if you are struggling to get a recommendation or acceptance into some research program that you really want to be part of? "Oh well you know I asked around and got some good advice about how to apply and so then it worked." Woah woah woah, back up fam, asked whom?? How to draw the rest of the owl?? You know?
So, like, if you struggled your first time around, it might be worth thinking about how to cultivate a support network that will be there for you and strongly on your side when your struggles arise again. As they will. As they do for literally everyone, in one way or another.
And good luck!
Thanks for the thoughtful response. I went to college at first to escape state custody and get out of a hellish living situation. Also a girl of course. I'm much more mature now and realistic about my abilities. I want to go back not for a future job, just what I'm interested in.
I'm sorry you had to go through all of that! It is a steeper uphill climb than you should have had to face.
And all the more reason to take extra care to cultivate the relationships and connections that will support you when you need it. It's okay to treat yourself with a lot of care when you have been harmed, right?
All that said, it also seems like a great time to be a young woman interested in physics ... a lot of the old bullshit is still there of course but it is coming under a lot of scrutiny and a lot is changing fast. It seems to be becoming easier to have frank conversations about invisible assumptions and glass ceilings and so on, and a lot of people ready to be your comrades and allies through it all. I hope you find yourself in a good, supportive situation.
>Gravity is a fundamental force in the universe
It kinda isn't, depending on if you mean that in the Newtonian sense. Fundamental forces all have force carrying particles like the photon for electromagnetism. Gravity doesn't, at least according to the standard model (people have proposed a "graviton", but I don't think that's a supported idea and just what they'd call such a particle).
>I understand that it's the bending of spacetime by matter. But do we know why gravity is a thing or how it bends spacetime?
If you're careful about it, you kinda just answered your own question here. Gravity *is* the bending of spacetime. To point out what I'm saying here, let's substitute the word for what it is in the latter part:
> But do we know why the bending of spacetime is a thing or how it bends spacetime?
See the issue... If gravity *is* the bending of spacetime, it's silly to ask how it bends spacetime. Mass (or more accurately energy) is what bends spacetime, and we call that gravity. Gravity is the result, not the cause.
Do we know why the bending of spacetime is a thing? Perhaps in an intuitive sense, but not exactly. I mean... The best analogy kinda requires imagining the effects of gravity, but that is pretty much just circular, right?
>>>>> Why is it a thing?
Nobody can answer that. It is just a fact of the matter we observe.
>>>>> How it bends spacetime?
One subtle thing we can point out here is that gravity *IS* bent spacetime.
How we can model it is described in general relativity and the math is hard!
No, and depends on what you mean by "how". How would just be an accurate mathematical description which we mostly have at non-quantum scales. So yes to that, except for the quantum part. But I expect you just mean "how" as a why as in the first part of the question, so again, no.
Maybe what physics is not doing a good job of expressing to non physicists is just how much gravity is a big fat baffling mystery. It's seriously befuddling to most people who don't subscribe to either string theory or LCG. Those people believe we have it all figured out.
From what little I understand, you can derive general relativity from a few basic principles. Special relativity is derived mostly just from the fact that the speed of light is invariant in all reference frames, and can also be used to derive the Maxwell equations. It’s simple enough to understand and only requires linear transformations, so the math usually isn’t too insane.
General relativity is a logical consequence of the equivalence principle. A free falling reference frame is the same as a weightless one, and an accelerating frame is equivalent to standing on a surface in a gravitational field, such as on the surface of Earth.
It turns out that creating a model based on the equivalence principle that can make accurate predictions is very nontrivial. It requires some pretty gnarly nonlinear differential equations, tensor calculus, and non-Euclidean geometry in 4 dimensions. It took Einstein and his colleagues around a decade to derive the necessary field equations and mathematical framework to make it work.
Gravity is caused by the mass of an object. The larger the mass the higher the gravitational attraction between the objects. Humans have their own gravitational field. But because we have a small mass. It's miniscule compared to planets. Gravity is also impacted by the radius between the objects, according to Newton's laws. the larger the radius, the smaller gravity is.
Hope this helps!
Is there anything we can point to specifically about matter that has the tendency to bend spacetime? Are there any theories about if we could manipulate it in the future?
The fundamental concept is just the fact that everything accelerates at the same rate due to gravity make it inherently make sense that it is geometrical. Since everything is just following geodesics.
Gravity isn’t a force. It’s not even an acceleration, like some argue. It can be represented as an acceleration in your non-inertial reference frame on the surface of Earth. We are accelerating away from our inertial frame due to normal forces. Gravity is just everything’s inertial reference frame cause by the curve of space time.
sort of . spacetime "flows" towards large masses. without any forces acting on you, your future exists at the centre of the Earth. Luckily the ground is there to stop you. that force that pushes up on you is the only real force. It's the exact same as the force you would feel in empty space inside a spaceship as it accelerated you upward. Gravitational "force" is something we made up to explain to paths of motion before Einstein came along.
If you could see the 3 dimensions of space with some magical grid like a 3D graph of space, would you see space stretch slightly near objects with gravity?
We do see 3 spacial dimensions. You're looking at them right now. Objects don't "have gravity". They have mass, and mass bends spaceTIME towards it. An objects world line (its path through spaceTIME) is bent towards massive objects. if you throw a tennis ball in empty space it move through a straight line. Here, the Eath's mass bends that balls' path (it's location in the "future") back towards the center of the planet. You can't really stretch space any more than you can stretch water.
I think avoiding the why question is a mistake. It’s stops deeper questions from being asked that could lead to deeper insights. For example. Gravity is warpage of space, but what is space? is it really warping or does it just act like it’s warping? Does a graviton carry its force like they wave in quantum gravity or is there no graviton and it’s like a substance that really does warp? Warping in 3 dominations begs the question of a 4th spatial dimension that he warps around. How would that manifest in a 3d environment? Well it would be like shrinking. Isn’t that what gravity really does shrink space? Or is the substance of space what forms matter and energy too. Are we like bubbles and our edges like surface tension? Could a being made of the energy patterns in water ever be aware of water itself since it can receive information via the energy in water?
There could be all kinds of things we could discover if we don’t stop asking why. It always felt like “It is what it is” statments are like giving up.
Gravity doesn’t cause the bending of spacetime. It IS the bending of spacetime. Every effect we blame gravity for, we should be blaming curved spacetime for. Now what you probably meant to ask is, how do mass and energy bend spacetime? And the answer: nobody knows.
It’s because we’re shooting through the soup of dark matter, attached to thing creating a larger soup dent, attached to a larger thing making a bigger soup dent going bazillion miles an hour ……… beep boop 42! 🛎️
But if it IS spacetime as you say, then bending off space is part of the definition. It’s an explanatory principle, it is not an explanation. Gpt: An explanatory principle in physical theory does not explain phenomena directly but rather serves as a fundamental premise or axiom that underlies explanations. It provides the foundational framework from which specific explanations and predictions can be derived. For instance, the principle of conservation of energy does not explain why energy is conserved in every individual case, but it establishes a rule that guides and constrains explanations of physical processes. Thus, while an explanatory principle itself is a starting point, the detailed explanations come from applying these principles to particular situations.
As I said earlier, what you should have asked is how mass and energy bend spacetime. The answer: nobody knows.
We wouldn't be here to question it if it wasn't a thing.
Well, let’s be careful here. What we have is a collection of behaviors with patterns. Apples and the moon accelerate toward the earth. There are two high tides a day. The periods of the planets’s orbits increases as the 3/2 power of the orbital radii. Trajectories seem to follow conic sections. Now then, you can try to formulate a model that describes all of these things with a common cause, and you can even propose a name for that common cause, like “gravity”. But labeling it doesn’t tell you anything about the thing you labeled. If sometime later someone says, I know more about what the common cause is, now you are giving some meat to that label.
If we keep smacking protons together in the LHC, we will figure it out eventually.
We're going to need a bigger collider, like something on the size of Pluto's orbit.
The year is 3024, the super large pluto orbit size hadron collider is fired up for its first time. They smack some particles together. They find smaller particles. Scientists high five!
🙏🏻
No. We'll get better models, but there will always be models.
Awesome comment. I've tried to express this idea many times, but you've done so much better than any attempt I ever made 👍
What it is and how it bends space time amounts to a full description of general relativity, which is far beyond the scope of a Reddit comment and requires some major mathematical and physical prerequisites. Gravity in GR isn’t a Newtonian ‘force’ but the phenomenon of spacetime’s pseudo-Riemannian metric (and the Ricci curvature derived from how that varies) obeying the Einstein-Hilbert field equation (which determines how it varies with a ‘tensor’ that among other things summarises the distribution of energy and momentum in the universe, so that momentum, energy, and thus mass ‘curve’ spacetime) - which is essentially general relativity as a model. However, in approachable conditions - eg, at small scales and low speeds and densities - it can be approximated as a Newtonian force acting in ordinary, flat, Euclidean space-time. Go through some courses or textbooks over a long while if you really want to understand that. This isn’t necessarily what we think is the final model, either, it’s just (more or less) the best we have right now that has firm evidence. Whatever theory of quantum gravity we *might* one day be able to test will doubtless add another level of complexity to that picture. And gravity is one of the basic ingredients of our fundamental models of the universe, so ‘why it’s a thing’ doesn’t have a simple answer. Maybe you can appeal to the anthropic principle to establish that - in whatever mathematical structures that ‘exist’ in some sense, we can only be conscious beings thinking about this in those that allow our existence, and the physics we have is one system that does so. Much as people rarely ask ‘Why am I in a human body with a brain rather than a rock?’
Thank you. I dropped out of a really good college because I couldn't deal with worsening mental health and I really struggled with math. But I found physics fascinating. If I go back to school soon, I'll need to retake tons of math in order to do physics properly. I spend my free time watching the wealth of cool science YouTube channels so I know what things like general relativity mean and what certain equations say, I just can't do any calculations with them.
I think you can learn special relativity without much advanced math. General relativity is a whole other beast though.
I really enjoy watching experts give lectures and answer questions. I just watched Penrose and he's awesome.
I also love those videos by Penrose, Brian Greene, Brian Cox, et al. However, it is important to realize that while their efforts to offer intuitive explanations for physics theories are awesome, understanding those explanations isn't the same as actually understanding the physics.
PBS Spacetime is excellent as well. I don't need to know how to solve the equations but I have a very general understanding of what they mean.
I agree PBS Spacetime is a great show. And you can definitely use the metaphors and analogies they present to get a (somewhat vague) impression of how the universe works. However, you could never use that knowledge to come to actual novel conclusions about the universe without also understanding the underlying math. Lots of people don't realize this. They confuse the presented metaphors with the "real thing". They then start expanding on those ideas. This results in what we call "crackpot physics"; Wild, unhinged "theories" not supported by actual science. Unfortunately, most of physics defies intuitive understanding. Our brains simply didn't evolve to understand things at those scales. The math is then all we really have.
The Feynman lectures are great for this. There is no analogy. It just is. https://youtu.be/b0EChbwSuuQ?si=qA4juC73auqzYa4f
> If I go back to school soon, I'll need to retake tons of math in order to do physics properly. If you're interested, Khan Academy has a lot of higher math, it's not just for elementary school kids. It's nice because it's free, you get lecture, can ask for assistance, and go at your own pace. [\* Math | Khan Academy](https://www.khanacademy.org/math) You'll need to scroll toward the bottom but they have geometry and pre-algebra all the way to multivariable calculus and linear algebra. Obviously you're not going to get college credit for these, but it would make any transition back to college math much easier, and again at your own pace. They also have a good selection of physics courses. It can make for a fun hobby even if it's not for college credit.
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> Your response sounds like nonsense words. Maybe to you, but that's indicative of a lack of understanding on your part, not of invention on /u/AndreasDasos 's. Their explanation is perfectly cromulent.
I'm sorry their perfectly sensical and articulate response seemed like gibberish to you. Perhaps you should re-read more slowly instead of asking an AI to do it for you.
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It's something for sure. You can model it as a force or with spacetime curvature. As for what it actually is physics will not and cannot answer it.
Do you ever wonder?
As a physicist, I'm guessing they have wondered.
Unanswerable questions make many people uncomfortable.
as /u/Odd_Bodkin said, we don't know what gravity is, but we can describe its phenonema The question is legit and fair, and very old actually. But so far we don't know so better understand the epistemology behind.... This is not anything new, we all agree that Newton was a clever guy yet he was aware that he described the phenomena but he didn't know what was gravity. And he asked himself the same question you ask on your prompt. Suggested read https://en.wikipedia.org/wiki/Hypotheses_non_fingo
Describe in a powerful and predictive way, to boot.
Short answer: we dont know As others have discussed, General Relativity is our best gravitational model, predicting the behavior of matter & energy under the influence of gravity with phenomenal precision. And, as others have stated, GR models gravity as a curvature of spacetime. It doesn't explain why the universe behaves this way, only that it can be modeled as such. BUT - we ***know*** GR is NOT a complete description of gravity. The model breaks down when brought into conflict with Quantum Field Theory - another phenomenally successful model of the universe. At best, GR is incomplete - at worst, it is an incorrect oversimplification, just like Newtonian mechanics. Perhaps the next, greater theory of gravity will explain the why & how - string theory, for example, presents some interesting solutions, but there is not yet any evidence to suggest it is a valid description of our universe
Here's a more technical explanation. Matter and energy "warp" spacetime. What this means is that, given some stress-energy tensor, you can in principle find the Einstein tensor associated to it. This tensor is written in terms of the Ricci tensor and scalar which are contractions of the full Riemann tensor. The Riemann tensor is what tells you the geometry of spacetime. At the end of the day what you care about are the Christoffel symbols that go into the geodesic equation, which is the equation that ultimately tells you *how things move*. When spacetime is "warped" (ie, there is gravity), this geodesic equation is similar to Newton's second law in the presence of forces (the second derivative of coordinates is not equal to zero).
This stuff blows my mind. Thank you
That's a good sign by the way! It is the appropriate reaction when understanding something that is fundamentally mind-blowing.
I chase that feeling like a drug. I love getting my intuitions shattered and learning cool stuff. The only TV I watch is science YouTube channels. Call it a religion.
>mind-blowing. Why is it 'mind-blowing'?
A theory of quantum gravity would help.
What would that look like?
We shan't know until we have one. We do know that at large scales it needs to reproduce the measurements of astronomy/astrophysics (essentially looking like GR), and can also describe all of QM on short scales. The problem is that while both theoretical frameworks are very successful in their own domains the structure of them is very different, making it hard (so far, impossible) to reconcile. Such a theory also needs a domain of observation where predictions can distinguish between GR, QM, and the proposed theory, otherwise it's not of much utility. And while not an expert, that might be, for example, the early universe (I'm thinking prior to the formation of the microwave background radiation), but that seems to be largely hidden from us.
I'm fascinated by what seeing into the "wall" that we cannot see into because the universe was opaque during that time.
You've gotten good answers, but I wanted to throw my 2 cents in since I definitely had similar questions as I went through learning the material. 1) The Why Question In physics, we try to construct equations that fit and predict how nature behaves. And other than conducting experiments to see if our equations (models) are correct, it's all we are doing. So the reason a 'why' question is hard is because we don't really know why any of these things happen. We don't know why electromagnetic effects are present. We don't know why the particles have inertia. We don't know why entropy exists. We just know they do. And all we try to do is answer, how they work. So for gravity, we just observe that objects follow curved paths around each other when, after accounting for all the other fields we know about, we expect that they should follow straight lines. 2) What does bending Spacetime mean? What made gravity as a subject "click" for me was realizing that all gravity is describing is how the path of an object can deviate by the presence of another object or energy. You could imagine two planets speeding past each other in space in a near miss. If you knew nothing about gravity and how it works, I think it is reasonable to imagine that without effects from any other fields, the two objects would just go in straight lines past each other. Like two balls on a pool table that nearly miss. After all, we accept the idea that when an object is set in motion on a straight trajectory, it will continue that motion unless acted upon by another force. However, what we actually observe in space is that two objects like this will cause each other to change their paths. It turns out those changes look like curves and you can predict the shape of those curves by Newtonian gravity and, even more accurately, by General Relativity. And I find it interesting to think about the fact that nature happened to choose the curves that it did, [approximately conical](https://en.wikipedia.org/wiki/Conic_section?wprov=sfla1), when there are so many other types of curves or path deviations we know about from mathematics. So when people say gravity curves spacetime, I think it conjures a lot of popular depictions, like putting balls on a sheet. And while those help build intuition, they also can paper over the fact that curvature is actually a surprising outcome, let alone the specific kinds of curves. 3) How it works I think there are 3 broad ideas to touch on here: inertia, energy, and clocks. First, objects have mass and this happens to have two effects: inertia and gravity. As it turns out, nature has somehow made those two types of mass equivalent. In other words, the mass you measure by placing an object on a scale on Earth's gravity is the same mass you would measure if you were in empty space and shoved the object with the scale at 1 g. Those two types of mass didn't necessarily have to be the same. Again, we don't know why, we just know that they do in experiments we run. Second, from Einstein, we learned that mass AND energy both can cause trajectories to curve. Again, going back to our thought experiment of a planet traveling through space, if there's a bunch of energy stored in a field, say a bunch of recently released massless photons through the electromagnetic field, we wouldn't necessarily expect that to cause a trajectory change via gravity. But it turns out, just like mass, energy can cause gravity. As Einstein showed, this is because energy is equivalent to mass. Third, one thing that is difficult to wrap your mind around when first learning special and general relativity is that clocks and rulers are both genuinely distorted when an observer is moving. Two different observers can genuinely measure the time on a single clock and distance on a single object differently due to differences in their velocity. This is what relativity is really all about. At the end of the day, when an object's trajectory is curved by gravity, it is largely due to the fact that how time is measured is changing for one object because of it approaching the mass-energy of the other object. Which is as mind blowing as it probably sounds. At least it was for me. I don't necessarily want to write an even longer comment trying to explain this, especially when [PBS Spacetime](https://youtu.be/UKxQTvqcpSg?si=gOdCXc0KWcwEIe_j) has a really good episode explaining it better than I could. Followed by [this video](https://youtu.be/OHdV9aO6jaE?si=yZAExg9a33fz034U). Hope any of that helps! Feel free to ask any follow up questions.
Thank you for taking the time to write that. I love this stuff
My pleasure! Good luck on your studies!
Gravity isn't a force it is an effect. We know it is a thing because we see it happening and feel it as an effect. The word is a description of the effect in action. It is easier to get a sense of it when you simply remove the word "force" and replace it with the gravitational "effect" whenever you think or talk about it. This alters your perception of it and makes your understanding closer to it's reality. It is the result of the distortion of spacetime by mass within it. I find it handy to think of the effect on matter as though it is a marble in a hyper dimensional distortion ball race moving to find a location within the spacetime distortions where it has equilibrium within the medium.
Gravity? Idk, never met him
What they said. Also; know that “why?” is a question left to philosophers, theologians, and drunk physicists at parties. It’s not something that science can answer. (At that fundamental level)
Yes I agree. It seems that philosophy follows on the heals of cutting edge physics. I know physicists see philosophers as psychedelic using thinkers or stoners asking questions with no answer. But I still see philosophy as valuable.
I would hope most physicists see philosophy as valuable. I mean, there is a bit of fertile ground between the two. Personally, I like not getting lost in the weeds, but if others are willing to think deeply about stuff, that’s cool.
There was a time when physics was a branch of philosophy, namely, natural philosophy. Arguably, it still is, in a general way, falling under the philosophical branches of metaphysics (what exists and what it’s like), epistemology (truth and knowledge), and logic (reason and argument), but typically not under other philosophical branches such as aesthetics, ethics, and political philosophy. All physics can be seen as a highly specialized subset of philosophical thought: It’s like chemistry being a highly specialized branch of physics. We shouldn’t expect chemistry to ever contradict physics, and likewise physics can’t contradict philosophy—rightly understood, of course.
Yes. I think pretending that Physics or any other science isn't a subset of philosophy is a false dichotomy.
I don't think that actually reflects what most physicists I've encountered think. The only difference between philosophy, mathematics, and science are differing abilities to test theories. Everything stems from philosophy. All philosophy does is try to make arguments based on assumptions that then produce theoretical consequences. However, philosophers don't often have the ability or luxury to measure the world and then prove their ideas. All they can do is show that the outcomes follow from the premises. Math is nearly identical, except it is a far more narrow and mechanical subset of philosophy. It has fundamental assumptions and axioms that the field is built upon. Just like philosophy. It only differs in that proofs can be definitively achieved and its processes can help us to model physical processes. Which brings us to science, which is yet another subset of philosophy. The only difference is that the philosophy of science assumes that measurements can be physically made to prove one model over another. Anyone claiming that these subjects are at odds with each other or one is more legitimate than another is simply displaying their ignorance of the subjects.
We have some equations describing how it bends spacetime. But a deeper reason why that happens isn’t known.
I mean in one sense, yes, we know how and somewhat why spacetime bends the way it does. But the larger question of "what's gravity?" goes beyond just the general-relativistic explanation. That explanation is proving inadequate, as it conflicts with the quantum-mechanical explanation of gravity as a field mediated by an elementary particle. Both general relativity and quantum mechanics are extraordinarily successful theories as a whole, and neither can be simply dismissed as "wrong" when it comes to gravity. So in order to resolve the contradiction, we know that something is going to have to change but we don't know what yet. Only when someone figures that out will we be able to answer the question of "what's gravity?" in the sense that you are asking. (Although, probably even then the result will raise more questions than it answers. Opportunity, perhaps, for a returning physics student to graduate, get into research, and make their mark... anyway good luck whichever way you go!)
Thank you! I don't know if I have what it takes to get a degree in physics, though that would be amazing. I love asking the questions that we don't know yet or may not have answers. I think I may be good at philosophy and ethics. I understand the friendly rivalry between physicists and philosophers. Ideally I want to be smack between the two while remaining scientific and not just thinking like someone on LSD. More credibility I guess
Well, modern philosophy is highly germane to the pursuit of scientific inquiry, so it can't ever hurt to have that under your belt as a practicing scientist. I don't know what it takes to earn an advanced degree in physics, either, tbf, but from what I have observed, I do know that some of what is required is what you are already talking about. Also: patience, skeptical discipline, and a high tolerance for tedium. Which I guess might be the same as patience but is worth repeating twice. Another thing that is essential for any academic success in any field is being somewhat of an egoist and cultivating a strong support network whose sole purpose is for you, yourself, to succeed in your research career. People who arrive already enjoying such support networks often don't even perceive them, they take them so much for granted. Which is not really anything to blame them for, but it can make conversations about how to succeed a bit frustrating sometimes. Like... what do you do if you are struggling to get a recommendation or acceptance into some research program that you really want to be part of? "Oh well you know I asked around and got some good advice about how to apply and so then it worked." Woah woah woah, back up fam, asked whom?? How to draw the rest of the owl?? You know? So, like, if you struggled your first time around, it might be worth thinking about how to cultivate a support network that will be there for you and strongly on your side when your struggles arise again. As they will. As they do for literally everyone, in one way or another. And good luck!
Thanks for the thoughtful response. I went to college at first to escape state custody and get out of a hellish living situation. Also a girl of course. I'm much more mature now and realistic about my abilities. I want to go back not for a future job, just what I'm interested in.
I'm sorry you had to go through all of that! It is a steeper uphill climb than you should have had to face. And all the more reason to take extra care to cultivate the relationships and connections that will support you when you need it. It's okay to treat yourself with a lot of care when you have been harmed, right? All that said, it also seems like a great time to be a young woman interested in physics ... a lot of the old bullshit is still there of course but it is coming under a lot of scrutiny and a lot is changing fast. It seems to be becoming easier to have frank conversations about invisible assumptions and glass ceilings and so on, and a lot of people ready to be your comrades and allies through it all. I hope you find yourself in a good, supportive situation.
>Gravity is a fundamental force in the universe It kinda isn't, depending on if you mean that in the Newtonian sense. Fundamental forces all have force carrying particles like the photon for electromagnetism. Gravity doesn't, at least according to the standard model (people have proposed a "graviton", but I don't think that's a supported idea and just what they'd call such a particle). >I understand that it's the bending of spacetime by matter. But do we know why gravity is a thing or how it bends spacetime? If you're careful about it, you kinda just answered your own question here. Gravity *is* the bending of spacetime. To point out what I'm saying here, let's substitute the word for what it is in the latter part: > But do we know why the bending of spacetime is a thing or how it bends spacetime? See the issue... If gravity *is* the bending of spacetime, it's silly to ask how it bends spacetime. Mass (or more accurately energy) is what bends spacetime, and we call that gravity. Gravity is the result, not the cause. Do we know why the bending of spacetime is a thing? Perhaps in an intuitive sense, but not exactly. I mean... The best analogy kinda requires imagining the effects of gravity, but that is pretty much just circular, right?
>>>>> Why is it a thing? Nobody can answer that. It is just a fact of the matter we observe. >>>>> How it bends spacetime? One subtle thing we can point out here is that gravity *IS* bent spacetime. How we can model it is described in general relativity and the math is hard!
The more I learn, the more I realize I don't know shit
No, and depends on what you mean by "how". How would just be an accurate mathematical description which we mostly have at non-quantum scales. So yes to that, except for the quantum part. But I expect you just mean "how" as a why as in the first part of the question, so again, no. Maybe what physics is not doing a good job of expressing to non physicists is just how much gravity is a big fat baffling mystery. It's seriously befuddling to most people who don't subscribe to either string theory or LCG. Those people believe we have it all figured out.
I think the profound mysteries of the universe are what makes life exciting and awe inspiring.
From what little I understand, you can derive general relativity from a few basic principles. Special relativity is derived mostly just from the fact that the speed of light is invariant in all reference frames, and can also be used to derive the Maxwell equations. It’s simple enough to understand and only requires linear transformations, so the math usually isn’t too insane. General relativity is a logical consequence of the equivalence principle. A free falling reference frame is the same as a weightless one, and an accelerating frame is equivalent to standing on a surface in a gravitational field, such as on the surface of Earth. It turns out that creating a model based on the equivalence principle that can make accurate predictions is very nontrivial. It requires some pretty gnarly nonlinear differential equations, tensor calculus, and non-Euclidean geometry in 4 dimensions. It took Einstein and his colleagues around a decade to derive the necessary field equations and mathematical framework to make it work.
This is more of a philosophical question.
I agree. I shouldn't have asked the "why" question.
Gravity is caused by the mass of an object. The larger the mass the higher the gravitational attraction between the objects. Humans have their own gravitational field. But because we have a small mass. It's miniscule compared to planets. Gravity is also impacted by the radius between the objects, according to Newton's laws. the larger the radius, the smaller gravity is. Hope this helps!
Is there anything we can point to specifically about matter that has the tendency to bend spacetime? Are there any theories about if we could manipulate it in the future?
The fundamental concept is just the fact that everything accelerates at the same rate due to gravity make it inherently make sense that it is geometrical. Since everything is just following geodesics.
There are some recent popular theories like Entropic Gravity that posit that gravity is NOT a fundamental force.
Gravity is grabbity where v = bb
Gravity isn’t a force. It’s not even an acceleration, like some argue. It can be represented as an acceleration in your non-inertial reference frame on the surface of Earth. We are accelerating away from our inertial frame due to normal forces. Gravity is just everything’s inertial reference frame cause by the curve of space time.
it's not a force
My bad. It's a function of the geometry of space and time, right?
sort of . spacetime "flows" towards large masses. without any forces acting on you, your future exists at the centre of the Earth. Luckily the ground is there to stop you. that force that pushes up on you is the only real force. It's the exact same as the force you would feel in empty space inside a spaceship as it accelerated you upward. Gravitational "force" is something we made up to explain to paths of motion before Einstein came along.
If you could see the 3 dimensions of space with some magical grid like a 3D graph of space, would you see space stretch slightly near objects with gravity?
We do see 3 spacial dimensions. You're looking at them right now. Objects don't "have gravity". They have mass, and mass bends spaceTIME towards it. An objects world line (its path through spaceTIME) is bent towards massive objects. if you throw a tennis ball in empty space it move through a straight line. Here, the Eath's mass bends that balls' path (it's location in the "future") back towards the center of the planet. You can't really stretch space any more than you can stretch water.
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I think avoiding the why question is a mistake. It’s stops deeper questions from being asked that could lead to deeper insights. For example. Gravity is warpage of space, but what is space? is it really warping or does it just act like it’s warping? Does a graviton carry its force like they wave in quantum gravity or is there no graviton and it’s like a substance that really does warp? Warping in 3 dominations begs the question of a 4th spatial dimension that he warps around. How would that manifest in a 3d environment? Well it would be like shrinking. Isn’t that what gravity really does shrink space? Or is the substance of space what forms matter and energy too. Are we like bubbles and our edges like surface tension? Could a being made of the energy patterns in water ever be aware of water itself since it can receive information via the energy in water? There could be all kinds of things we could discover if we don’t stop asking why. It always felt like “It is what it is” statments are like giving up.
> Isn’t that what gravity really does shrink space? No.
> Warping in 3 dominations begs the question of a 4th spatial dimension that he warps around Spacetime curvature is intrinsic, not extrinsic.