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I Why was time dilation under GR a new concept?

  1. Dec 19, 2016 #1
    Edit: I mean a new concept as in new with regards to the century before. I'm aware that lorentz and maybe others had some idea even before GR

    So time slows down for the individual under a stronger gravitational field than for an individual under a weaker gravitational field.

    But why does this nessicarily have to go against common sense?

    Can't it just be thought of as the gravitational field slowing down all the physical processes of an object?

    Say Alice is under heavy gravity compared to Bob. Well, Alice's physiological processes would be slower. This doesn't contradict common sense, as sometimes, things move slower in general( something moving through viscous fluid for example). It's just that somehow under higher gravity, everything for Alice, her molecules, the molecules of the immediate environment etc, would move slower in unison compared to everything in Bobs enviornment. Well, if her physiological processes are slower due to the field, then of course Bob would perceive her to be aging more slowly.

    So then why do so many people consider this time dialation to be almost "mystic". Why would we need a correspondence principle? It doesn't really go against our a priori intuitive understanding of reality if thought in that way.
     
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  3. Dec 19, 2016 #2

    Dale

    Staff: Mentor

    First, it isn't the gravitational field that causes time dilation, it is the gravitational potential. Second, I have never met anyone whose "common sense" told them that the gravitational potential should slow down all the physical processes of an object.
     
  4. Dec 19, 2016 #3
    Because Newton's theories assumed time to be something distinct and separate from space.
     
  5. Dec 19, 2016 #4
    Oh got it. That makes sense, it's the potential because the field has to differ to have a time effect.

    I meant that if we assume that the gravitational potential is the cause of it all, then maybe some of the "mysticism" can be taken away by viewing the result as the slowing down of all physical processes. The result can work according to common sense. It's just not the cause.
     
  6. Dec 19, 2016 #5

    Dale

    Staff: Mentor

    Even in a uniform field there can be time dilation.
     
  7. Dec 19, 2016 #6
    Oh right. Special relativity. Both observers would preceive the same dialtion. This goes against common sense.
     
  8. Dec 19, 2016 #7

    Nugatory

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    Staff: Mentor

    No, that's not what's going on here. In a uniform gravitational field the potential still will not be uniform and there will be gravitational time dilation that is unrelated to the time dilation of SR and in which both observers agree about which clock is the slow one.
     
  9. Dec 19, 2016 #8

    Nugatory

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    Staff: Mentor

    Not easily. There are many very different physical processes that measure the passage of time. It wouldn't be totally amazing to find that some of these are slowed by gravitational fields... but all of them? by exactly the same amount? That really does defy common sense. For example:

    It makes sense that gravity would affect the behavior of a pendulum clock and the behavior of a sandglass. But it's hard to devise any mechanism that would have the same effect on both devices, especially when you consider the wide range of variables (grain size, grain density, depth of sand in upper chamber, diameter and shape of orifice, ....) which lead to different interactions with the gravitational field in a sandglass. And after you've managed that, you have to propose mechanisms by which the gravitational field affects the reproductive rate of the bacteria growing in my petri dish, and the rate at which an ice cube on my kitchen counter melts, and the oscillation rate of the quartz crystal in my laptop computer, and the decay time of radioactive nuclei (for which the gravitational forces are well and thoroughly negligible), and the rate at which my heart beats, and the rate at which the grass in the lawn grows.... And all of these mechanisms have to have exactly the same slowing effect at all possible gravitational field strengths.
     
    Last edited: Dec 20, 2016
  10. Dec 19, 2016 #9
    Oh ok. I see the error. At different heights, the potential will differ classically.( the usual PE=mgh). The same happens in GR

    Hmm actually, I thought of something interesting. So what if, in a thought experiment, the gravitational field lines have no beginning and no end. That is, a universe with only a uniform g field, no earth, stars etc, only the uniform g field and the two observers. Then we can't say who is at a higher elevation since they are both the same distance to infinity. So we can't decide in this case who the time dialation occurs for. Is this what is going on classically? That the earth is so large that we can consider the field for observers separated by a reasonable distance to be within an uniform infinite gravitational field?
     
    Last edited: Dec 19, 2016
  11. Dec 19, 2016 #10
    When you put it that way, yeah I see why it's amazing.

    I think looking at it classically, the field would have to behave in crazy ways that would have to be going in all sorts of directions and strengths and differ in how this works just to make everything slow in unison for every object. I don't think this could even work in classical mechanics.
     
  12. Dec 20, 2016 #11

    Nugatory

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    Staff: Mentor

    The mathematical definition of the gravitational potential would still work just fine; it's defined in terms of the forces experienced by a particle moving from one point to the other without regard to any distance to infinity and provides an unambiguous notion of higher and lower. However, this thought experiment is irrelevant in general relativity; an infinite uniform gravitational field is not a solution to the Einstein field equations so is disallowed.
     
  13. Dec 20, 2016 #12
    I know that higher and lower is unambiguous. But isn't that for the same particle? If a particle drops from A to B, then it would have the higher potential at A compared to B.


    But if I have two observers in a uniform gravitational field, both starting at rest, they experience the same force, and the same final velocities. It doesn't matter if one is higher, lower, or at the same elevation. Classically, the dynamics only depend on the change in height for a particle. So if two particles are at different locations, if the change in heights are the same, the dynamics are the same.

    So in GR it's different? Is it because in the above, it's still like an infinite field? So the Einstein field equations doesn't provide the solution?
     
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