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Watch it around 2:00 minutes. https://youtu.be/gcvq1DAM-DE

Do objects move closer to Earth because they experience time at different rates, really? Does it make sense? The video also represents the object kind of rotating, when in reality they would just fall straight to Earth, right? What is the best way to explain gravity in layman’s terms? In this sense, the rubber sheet analogy makes much more sense...

Edit: trying to explain more what’s in the video... the uploader argues that objects move closer to Earth because they experience time differently, so they kind of rotate. It’s difficult to explain more because most of my problems with the video are with the graphical representations and not so much with what’s being said.

Edit 2: changed the title of the question as well.

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  • $\begingroup$ But then does time dilation cause gravity or gravity cause time dilation? $\endgroup$
    – Wouter
    Jun 13, 2021 at 10:54

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There is much to dislike about the popularized video, but its basic idea isn’t far from the truth. If you work through the equations of weak-field GR, you will find that ${{g}_{00}}$, which measures the rate of passage of proper time (i.e., aging) along a world-line relative to coordinate time, can be identified with $1+2\Phi $, where $\Phi $ is Newton’s gravitational potential. The geodesic equation predicts the same deflection of trajectories as Newton’s equation of gravitational force.

How would a change in the passage of time cause deflection? It’s hard to understand for a point particle, and that’s why the video showed a dumbbell. There is an analogous problem in QM about the deflection of a wave packet moving through a potential gradient. It will have wave-fronts of constant phase orthogonal to the direction of motion. The potential affects the rate of phase change, so if the left side of the packet undergoes more rapid phase change than the right side, the trajectory will veer left.

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  • $\begingroup$ "There is much to dislike about the popularized video". What's wrong with it? $\endgroup$
    – Jon
    Jan 4 at 19:32
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The video just poses that time is curved around Earth, without an explanation.

Richard Feynman has given a nice explanation of why a mass falls down in a gravity field in which time is curved. The space part of spacetime, which only affects fast-moving objects (wrt the gravity producing mass), doesn't have to be taken into consideration.

Imagine two freely floating rockets (side by side) of height $L$ in outer space. The rockets have a light on top that gives a flash of light every second. A person who is attached to the floor of both rockets. They see the subsequent flashes one second after another too but a bit ($\frac{L}{c}$) later than the light that emits them. So the time at the top flows with the same rate as the time on the floor, according to both persons. Now one rocket starts to accelerate, parallel to the other, and at the same time, the lights on the top give a flash of light. The person on the floor of the accelerating rocket receives this flash of light a bit earlier than the person in the freely floating rocket. The rocket keeps accelerating and the lights give off the second flash of light. The person in the accelerating rocket receives this second flash not the same amount earlier as the first flash (otherwise the second person would receive the flashes one second after each other too), but a higher amount earlier as the first flash because the rocket is accelerating towards the second flash.
This means the person in the accelerating rocket sees that there is less than a second passed between receiving the two flashes. The same holds for all subsequent flashes. So for the person in the accelerated rocket time goes slower.
Now, according to the equivalence principle, there is no difference (locally) between a person sitting in an accelerated rocket (usually accelerated elevators are used to illustrate the principle, at the core of general relativity) and a person sitting on a mass (say, Earth) which gives objects the same acceleration as the acceleration of the rocket ($9,8\frac{m}{{sec}^2}$).
So time slows down in a gravitational field while objects fall down. The higher above the surface of the Earth you are, the slower the rocket has to accelerate to obtain the same acceleration caused by gravity, so time moves faster the higher you are (time is curved, which is also the case in a field with constant acceleration for every object, i.e. in a constant gravity field, though in this case of a constant gravitational field there are some difficulties see this article; it's quite complicated). Both the slowing of time and acceleration occur simultaneously and don't cause each other: it's just the Natural state of gravity. The slower pace of time, when going up in the field, and acceleration go hand in hand (in a parallel way, so to speak).
So (for non-relativistic speeds of a mass) it's not the curvature of time that causes an object to accelerate in the gravitational field of the Earth.

If instead, we would send a photon perpendicular to the accelerating rocket, a person in the rocket sees the photon bend down. Time is not involved because photons do not age. In this case, it's only the curvature of the space part of spacetime (if time is curved space is also curved and vice-versa) that goes along with a deflected photon in the gravitational field of a big mass.

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  • $\begingroup$ I am not an expert, but this sounds like an amazing answer upon very first reading it. Thank you very much $\endgroup$
    – user137288
    Sep 21, 2020 at 19:19
  • $\begingroup$ @RobertoValente Mile grazie! $\endgroup$ Sep 21, 2020 at 20:06
  • $\begingroup$ To give credit to the video creator, it is not the purpose of that particular video to give an explanation why time is curved around Earth. That is explained in another video of the same creator, which is linked in the video itself. At the same time, although a good description, this answer does not actually answer the question of the OP $\endgroup$
    – fishinear
    Jul 10, 2021 at 13:20
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I have heard it explained as the clock-rate gradient across a particle will cause the the particle to move toward the portion that has a slower clock because the particle effectively spends more time in the slower time region. Not sure that is any better or worse than the 'drag' analogy of the video, but it makes sense to me.

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No, time dilation does NOT cause gravity. The simplest way to understand this is recognize that time dilation is a relative phenomenon, dependent on an observer's state of motion and their position relative to masses, while gravity is an objective phenomenon all observers will agree upon.

The issue with these videos is that they conflate two aspects of gravity: 1) the apparent gravitational attraction which is the result of inhabiting an accelerating frame whilst believing you are stationary, and 2) real gravity, which are the tidal forces about a mass / the curvature of spacetime.

On a small patch at the surface of the earth, for instance, we perceive a "gravitational field" wherein things fall down. According to the equivalence principle, however, this field is a fictitious one: we merely are accelerating upwards while mistaking our motion for stationary. Meanwhile, tidal forces/space-time curvature (real gravity) ensures that we can remain stationary with respect to the rest of the earth, even though different parts of its surface are accelerating outwards in different directions.

Now, what these videos are trying to explain is the cause of apparent gravitational attraction, i.e. the observed gravitational fields. But since these fields are fictitious, they don't actually require any explanations whatsoever. If you want to know why objects come together in perceived gravitational fields, well it's simply because one (or both) of those objects is literally accelerating towards the other.

If you want to know why these perceived fields have different strengths at different locations, this is because spacetime curvature (gravity) causes the effects of acceleration to differ at different locations.

Lastly, we can know that time dilation has nothing to do with spacetime curvature because we can construct a theory of curved spacetime without any time or spatial dilation/contraction whatsoever. It's called Newton-Cartan physics. Essentially, it signifies that space-time curvature is solely a consequence of the equivalence principle, which is as true in classical mechanics as it is in relativity.

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In the gravitational time dilation equation, t=t'(√(1-2GM/Rc^2)). If mass were to induce higher time dilation, the value of R has to decrease and spatial velocity has to be increased, such that speed in spacetime remains c. That is how I'd understand the spatial motion in a curved spacetime.

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  • $\begingroup$ But what about after the object hits the ground? It still has time dilation but no speed through space (unless space falls through it), so how does its speed in spacetime remain at $c$? $\endgroup$ Mar 21, 2022 at 4:31

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