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B What is the physics of motion through space?

  1. Dec 12, 2017 #26

    Mister T

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    If you're interested in understanding this I suggest you go back and read again the post you were responding to (my Post #11). It was a response to your Post #3 in which you were talking about motion of objects. What I see in your posts is a lingering notion that space is something physical that objects move through, and that it's possible to make a distinction between objects moving through space and objects not moving through space.
     
  2. Dec 13, 2017 #27
    The missing effect that used to puzzle me was that the expansion seemed to be operating without regard to inertia of the objects accelerating. It helped to consider objects in gravitational free fall (the changing distance between two objects dropped from the same height).

    In the hierarchy of position, velocity, acceleration, change in acceleration, change of change in acceleration, etc..., it seems curious that all have proper values except the first two; inertial frame of reference allows setting to either.

    Verbal mistake is to take a relative motion between two objects and deduce that therefore at least one of the pair is in absolute motion.
     
  3. Dec 13, 2017 #28

    Grinkle

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    I think the curious thing is that the speed of light is invariant across inertial frames. The other curious consequences follow.
     
  4. Dec 13, 2017 #29
    I don't know where the postulate of a minimal observable interval of space and time stands with respect to the standard model, so I won't post the link. Shan Gao makes an argument that this postulate leads directly to a maximum signal speed and its invariance, because c is the ratio of the minimum observable length to the minimum observable time interval... same in all inertial reference frames.
     
  5. Dec 13, 2017 #30

    PeterDonis

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    It's a common speculation, but so far no evidence has been found to support it. The usual view of that fact, by proponents of the speculation, is that the minimum scale is something like the Planck scale, which is about 20 orders of magnitude smaller than the smallest scales we can currently probe, so we have no way of actually seeing any evidence of the minimum scale with our current technology. We won't know whether this is valid unless and until we get some evidence one way or the other.
     
  6. Dec 14, 2017 #31

    Grinkle

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    Is Shan Gao making an argument that the experimental observation of an invariant maximum signal speed implies spacetime is discrete?
     
  7. Dec 14, 2017 #32
    I don't think so, better to judge for yourself; paper is titled On the invariance of the speed of light
     
  8. Dec 14, 2017 #33

    Grinkle

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    @bahamagreen Thanks. I also don't read him to be arguing in that direction.
     
  9. Dec 16, 2017 #34
    I'd like to thank everyone who contributed to the thread. I have read all your replies, but I cannot respond to every one without turning my post into an unreadable mess.

    @bahamagreen is right about some of the things that puzzle me.

    One is the mentioned disconnect between the coordinates plus their first derivative (which seem relative and therefore suspicious - like those projections of the same fish on the different walls of the aquarium - an effect sometimes used to speculate about the nature of quantum entanglement) and higher-order derivatives like acceleration, change in acceleration, etc. (which seem objective and linked to physical forces). For example, it feels non-intuitive how photons, traveling at the speed of light, allegedly "experience zero time" and, from their perspective, "instantly" connect the emitter (cause) with the absorber (effect) - as if both were directly adjacent on some deeper dimension (unlike their observer-specific spacetime "projections").

    The other puzzle is whether the Universe is an emergent phenomenon so the properties of spacetime, such as the speed of light limit, have local, quantum origins (such as the abovementioned relationship between the Planck units). I admit I find geometric explanations of motion to be utterly boring, since they only seem to describe how objects move, not explain why they do - which is very practical, but seems like a problem already solved 100 years ago, with not much potential for surprising discoveries.

    (Peter is right - let me read up on QFT).
    Thanks again everyone!
     
  10. Dec 16, 2017 #35

    Nugatory

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    That's easy - just look at the word "allegedly". The allegation is false and a pretty good rule of thumb is that if whatever you're learning from says things like that, you're wasting your time with that source.
    However, that's all that empirical science ever does. On close scrutiny, all scientific explanations of why something works the way it does turn out to be statements of how the universe behaves, not why.
     
  11. Dec 16, 2017 #36

    Mister T

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    If you look at the definition of proper time, you see that the notion of proper time doesn't exist for a particle moving at speed ##c##. That is not the same thing as saying the proper time is zero. Although a phrase like "there is no time experienced" doesn't make that distinction clear (if at all) and is therefore open to misinterpretation. Some authors seem to propagate that misunderstanding, either on purpose or because they are unaware of the distinction.
     
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