Can a gravitational field be represented as an acceleration? (2nd try)

Discussion in 'Alternative Theories' started by Richard777, Dec 21, 2018.

  1. Neddy Bate Valued Senior Member

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    This sounds like the opposite of what Q-reeus just said. He said the rotating and gravitational cases produce different measurements. I don't know which is correct, but it seems like Einstein had the opportunity to say the measurements would be the same in both cases, but I don't recall him doing so specifically.

    Yes, that is what I think as well. SR is built up from its own postulates, in flat spacetime, and should therefore stand on its own in flat spacetime. GR is built up from SR, for the cases where there is not flat spacetime. GR should therefore reduce to SR in the case of flat spacetime.
     
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  3. Q-reeus Valued Senior Member

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    The Equivalence Principle only strictly applies locally i.e. 'at a point'. Owing to the very different origins of 'g-forces' in the self-gravitating vs rotating disk, you can't even rely on integrating proper 'g-forces' over a radial path, to determine the global non-Euclidean geometry. If you did, the conclusion would be that in both cases it's only the radial metric that differs from the 'zero g-forces' limit. The global metric needs to be evaluated from either a non-rotating inertial frame (for rotating disk), or 'at infinity' (for gravitating disk).
    Keep in mind too that even if the vastly larger centripetal or gravitationally induced stresses & strains could be exactly cancelled, the disk shape itself would not reflect the global metric distortions. The overall shape will adjust to produce the least net elastic strain energy possible (D'Alembert's principle I think covers it).
     
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  5. Michael 345 New year. PRESENT is 69 years old Valued Senior Member

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  7. Mike_Fontenot Registered Member

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    I inadvertently left out one VERY important trait that Einstein had: although he could think as abstractly as anyone (and often more so than most), he also kept himself grounded in the practical. In particular, from the beginning, before he talked about time and space, he insisted on being clear about how he was prepared to measure those quantities.
     
  8. James R Just this guy, you know? Staff Member

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    Q-reeus:

    Maybe it would be a good idea for you to dial back the passive aggressiveness in 2019? What do you think?

    You may have noticed that we've just had Christmas and the New Year period. Moderators are not on call 24 hours a day, and do not read every thread.

    I note that you took no steps at all to draw moderator attention to either of the two threads you are concerned about. Nevertheless, you wasted no opportunity to criticise the administration.

    If you have some kind of personal issue with me or another moderator, perhaps we can discuss it by private messaging. That might be more productive than your regular whines.
     
  9. Q-reeus Valued Senior Member

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    Looks like my so-called 'whining' - what I would term apt if unflattering observations - have finally had an effect re the two threads formerly parked in Physics & Math. One could hope that's a good sign for SF starting into 2019.

    As for dealing with any personal issues via 'conversations' (what used to be laughably labelled 'private' messages), such has from past experiences never been worth the effort. Always win-win for admin/mod, always lose-lose for yours truly. Forum rules and guidelines can say one thing, but we both know interpretation and 'discretionary application' is what really matters.
     
  10. James R Just this guy, you know? Staff Member

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    No. I moved the threads because I happened to stumble across them when reviewing my unread threads list. Nothing to do with you.
     
  11. Q-reeus Valued Senior Member

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    At face value imo an amazing admission. Given just how modest is the typical posting rate, aggregated across the entire SF site, let alone in a Science sub-forum. Where presumably you alone among the current staff would have any notional competency and/or interest in monitoring/evaluating . On auto-pilot here for the most part it seems.
     
  12. James R Just this guy, you know? Staff Member

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    It sounds like you imagine I occupy a paid position here, wherein one of my responsibilities is to jump whenever you call. Both notions are mistaken.
     
  13. Write4U Valued Senior Member

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    Is a curvature the same as acceleration? Or is acceleration a product of curvature?
     
  14. Mike_Fontenot Registered Member

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    Neither is true. I think it is a BIG mistake to think of them as the same thing. They are distinct concepts. When there are no gravitational fields present, special relativity is ALL that is needed. The usefulness of the equivalence principle is to allow us to sometimes use the solution to a special relativity scenario to tell us some things about what general relativity must be like, or to give us the solution to a general relativity scenario, without having to use the more cumbersome general relativity machinery.
     
  15. origin In a democracy you deserve the leaders you elect. Valued Senior Member

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    Why do you say that?
    Write4U's second question was correct, what we sense as gravitational acceleration is due to the curvature of space.
    Ask an Astronomer.
     
    Write4U likes this.
  16. Mike_Fontenot Registered Member

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    Acceleration in the presence of a gravitational field (and no other forces) is indeed caused by curvature of space (or spacetime) ... that's general relativity. What I was objecting to is that some people view acceleration in the absence of any significant masses as itself producing a gravitational field, and I think that's wrong-headed.
     
  17. origin In a democracy you deserve the leaders you elect. Valued Senior Member

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    I agree.
     
  18. Write4U Valued Senior Member

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    Thanks for that additional info. I was not aware of that factoid.......

    Please Register or Log in to view the hidden image!

     
  19. Write4U Valued Senior Member

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    But IMO, gravity and acceleration are very closely related.

    I am thinking of Einstein's man-in-the-box thought experiment.
    https://en.wikipedia.org/wiki/Equivalence_principle
    Inside Einstein's Mind | Gravity Is Acceleration
    https://idahoptv.pbslearningmedia.o...nside-einsteins-mind-gravity-is-acceleration/
     
    Last edited: Jan 10, 2019

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