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In The Universe in a Nutshell chapter 4, Hawking explains the warping of spacetime according to general relativity.

Near a massive but ordinary star, spacetime is warped such that the light emitted from the star appears to move slower when near the star, and appears to "speed up" to the constant speed of light c when away from the star, from the viewpoint of an outside observer. Of course, we can't directly measure this experimentally because there is no way to measure the speed of light near a star from the reference point far away from a star. (Do I have this right?)

Hawking writes that this "slower" light is due to the warping of spacetime near the star, and this leads into the discussion of how spacetime and light behaves near a black hole.

I've always heard that light cannot escape a black hole due to the extreme gravity from the super-dense black hole (star). And so it make sense that near an ordinary massive star, light can escape but "struggle" to do so, going "slower" near the star before "breaking free" and "speeding up" to what we see as the speed of light farther away from it. (I use scare quotes as I realize these idioms are shortcuts for understanding relativity and not literally true. Feel free to correct me if I am using them incorrectly.)

So which is it? Is the light escaping from a massive star held back a bit by the pull of gravity, or by the warping of spacetime near it? Or is it both, additively? Or are both concepts two sides of the same coin in a way I am missing?

Patrick Szalapski
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2 Answers2

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I believe it is the warping of spacetime. Because gravity described by general relativity is't like a normal force you would think of it is the bending of spacetime that is gravity. And light moves constant in a vacuum. PLEASE PLEASE PLEASE correct me if I am wrong because I love information.

T. Fisher
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I don't have enough physics knowledge, so please correct me as you see fit.

Short answer: both.

Long answer: the "actual" reason can be said to be the warping of spacetime as Hawking describes. But what is this warping for? Isn't it because of the gravity? The gravity affects particles as it warps the spacetime, and these two are just different ways of saying one thing. Even earth bends spacetime and so attracts other objects. This is this warped spacetime that let's earth to attract you to it and this warping is so negligible that gravity is the smallest force of nature. This means that the two scenarios you have mentioned as two distinct answers are both the same, and warped spacetime is the reason of the other answer based on common experience of gravity.

seyed sepehr mousavi
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  • So the "gravity" of earth pulls me toward it solely due to the warping of spacetime? If so, how do gravitons fit in? – Patrick Szalapski Dec 13 '19 at 17:17
  • If that's so, let's then consider a particle at rest near the massive star. Now I think of gravity from the star as a force that acts on this particle. But if gravity is nothing but the warping of spacetime, how could a particle at rest then be accelerated by gravity? I think I understand that the warping of spacetime can "alter" the trajectory of a speeding particle, but if spacetime is merely warped but still locally normal around the particle, what force is acting on the particle? – Patrick Szalapski Dec 13 '19 at 17:23
  • Gravitons are just theoretical particles in analogy with photons. Why? Because gravity has its own force, field and so particle. This is exactly like electromagnetics when we have electromagnetic forces, field and particles (called photons). Therefore, electromagnetic force changes you as its waves touch you, which is when the electromagnetic field is disturbed. The same is true for gravity: gravitational force acts on you as its waves touch you which is when the gravitational field is disturbed. The gravitational field is actually spacetime which changes as gravitational waves pass. – seyed sepehr mousavi Dec 13 '19 at 17:27
  • So the answer to your commented question is analogue to the answer to this question: where are photons if the electromagnetic force acts on me by changing the electromagnetic field? – seyed sepehr mousavi Dec 13 '19 at 17:28
  • About your second commented question, envision a water wave. People can ride on these waves. One might argue that this force is not from the wave but the earth, but a wave has indeed a momentum and a force that can change the speed. You just need to think about warping of spacetime as a wave is the medium of spacetime (called gravitational wave). – seyed sepehr mousavi Dec 13 '19 at 17:36
  • By the way, if you agree that warping affects the direction of the velocity, you have implicitly agreed that it is exerting a force on the particle as changing the trajectory changes the velocity which is an acceleration that needs force. This force is that force that you asked what that is! – seyed sepehr mousavi Dec 13 '19 at 17:37