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This video on The True Cause of Gravity in General Relativity states that all other videos claiming that time dilation is the cause of gravity are "100% PERCENT UNEQUIVOCALLY COMPLETELY WRONG" (his capitalization.) Then at 22:25 he gives "the true cause of gravity" as the fact that every portion of a mass is accelerating outwards but spacetime curvature steps in to ensure the spacetime manifold compensates in just the right way and thus produces a stable configuration.

This seems to go against much of what is discussed here, including here. Can a GR expert make a comment on this theory?

foolishmuse
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    Debunking YouTube videos could be a full-time activity for the entire science community, and would have no lasting value since one could not keep up with them all. The concept of 'true cause' is philosophical at best. – Jon Custer Jun 23 '22 at 16:40
  • @JonCuster I have changed my question to "Can a GR expert make a comment on this theory?" I am more interested in understanding if his concept is correct than in debunking the video. – foolishmuse Jun 23 '22 at 16:51
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    There are as many ways to describe what's "really" going on as there are possible choices of spacetime coordinates. The time dilation picture is a nice crutch that works in a few special cases. The "spacetime river" picture advocated by the video is another nice crutch, which also works in a few special cases. It is nonsense to say that one is unambiguously wrong. If you're being really strict, they're both wrong in general, but useful because they provide slightly different intuition. – knzhou Jun 23 '22 at 16:53
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    In my opinion this website isn't really made for these kinds of discussions. You should consider other popular physics forums for your query. – Maximal Ideal Jun 23 '22 at 19:54
  • In any case, I never actually bought any of the popular video explanations exactly for the reasons that Dialect points out. – Maximal Ideal Jun 23 '22 at 19:58
  • I think it's a relevant question. It will be relevant to the understanding of science for the public. Even if it is beyond their comprehension, it is always better than the public are led to the reason why one theory or idea maybe correct than the other @MaximalIdeal – tryst with freedom Jun 23 '22 at 23:26
  • I haven't reviewed the linked video, but it's definitely true at least almost all pop-sci descriptions of "real" gravity are partly or completely nonsense, including some published by reputable institutions that ought to know better. – g s Jun 24 '22 at 02:28

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I wouldn't say that "time dilation is the cause of gravity", but it's certainly related to gravity and I can see why a pop-sci description of gravity might come out like that. But really gravity is "caused" by two things: (1) objects follow geodesics in spacetime, and (2) spacetime is curved by matter, energy, stress, and pressure, in a way described by the Einstein field equations. Geodesics in spacetime roughly correspond to lines which maximize elapsed time on clocks, which is where time dilation comes in: in order to maximize their elapsed time objects will naturally move in the direction of higher time dilation.

Youtube videos are really not a good place to learn about gravity. A relatively beginner friendly book is the free online text Exploring Black Holes. Chapter 2 covers most of the framework of how gravity works in GR, with Chapter 3 covering the details of the Schwarzschild metric (the one that describes spacetime near stars, planets, and black holes).

Eric Smith
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  • You said "in order to maximize their elapsed time objects will naturally move in the direction of higher time dilation." Why? Would objects give up energy and increase entropy by doing so? – foolishmuse Jun 24 '22 at 15:37
  • In your referenced book Exploring Black Holes, section 2.5, it says "if the stone is free, then the Principle of Maximal Aging tells us that the stone moves so that its summed wristwatch time is maximum across every pair of adjoining spacetime patches." My questions above stand: Why? Would objects give up energy and increase entropy by doing so? – foolishmuse Jun 24 '22 at 19:55
  • Energy and entropy don't come into it. The stone moves along a geodesic in spacetime. This is just the extension of Newton's first law (and in fact is identical to Newton's law in the case where spacetime is flat). – Eric Smith Jun 24 '22 at 21:11
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Disclaimer: I'm not a GR expert, but I know enough about the topic to answer.

Well certainly, it wasn't derived like so. In other words, Einstein didn't think (at least in the beginning) like this, and it's not how you introduce someone to gravity.

The video you linked has mostly correct answers, even though I disliked the rhetoric. Gravity is due to geodesics deviating. Imagine you and your friend are standing on the equator looking north. When you move forward, you move parallel to each other, yet meet the pole. This is gravity. Things get closer in space because space is curved.

Nevertheless, when calculating their motion when the studied object is stationary, you look at how the time component curves, which is what is meant in the videos. Why? Well similar to how you look at latitude circles. The reason you met you and your friend is that the latitude circles are shrinking. This is time dilation (or latitude contraction precisely)

So time dilation causes things to come closer together, but they're a result of a bigger picture, curvature, and geodesic deviation. I won't say gravity is caused by time dilation. But it's not totally wrong if I said this.

Habouz
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    "Nevertheless, when calculating their motion when the studied object is stationary, you look at how the time component curves, which is what is meant in the videos. Why? Well similar to how you look at latitude circles. The reason you met you and your friend is that the latitude circles are shrinking. This is time dilation (or latitude contraction precisely)" This is the best explanation of time dilation geodesics that I've ever seen. Thanks. – foolishmuse Jun 23 '22 at 20:11
  • "This is the best explanation of time dilation geodesics that I've ever seen." Glad I could help! – Habouz Jun 23 '22 at 20:20
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It is a good video in my opinion because it tries to separate relativity from gravity. As it notes somewhere, the Newton-Cartan approach uses the machinery of differential geometry (spacetime curvature, geodesics, and connections) to build the Newtonian gravity. We can wonder that if Newton was born in the late XIX century, he could present his law of gravity as an example of geodesic equation. The components of the proper acceleration at each point are the non-zero connections. From the connections we get the components of the curvature tensor, bringing the notion of a curved spacetime.

All this without any reference to electromagnetism, Maxwell equations and constancy of the light speed, that led to special relativity. In that N-C theory there is no metric, and so no time dilation. That way, time dilation can not be the key to our everyday understanding of gravity, that is after all Newtonian.

The metric is a novelty of the general theory of relativity, and it requires a correction of the curvature of an already curved Newtonian spacetime. But it's importance is related to star deviation on eclipses, or the fine correction of Mercury orbit.

Claudio Saspinski
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