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How do we know space and time are related?

  1. May 9, 2013 #1
    I know the general theory as proposed by Einstein but how has it been proved beyond a reasonable doubt?
  2. jcsd
  3. May 9, 2013 #2


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    There's a sticky thread at the top of the relativity sub-forum on experimental confirmation of relativity.
  4. May 9, 2013 #3


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    Actually, that wording is incorrect. It should be "how do we know space and time are just facets of a single entity called spacetime?"

    That is, they are not "related" they are parts of a single thing.
  5. May 9, 2013 #4


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    Also, you don't prove a theory beyond reasonable doubt. All you can do is find out how accurate the theory's predictions are. This way you can prove beyond a reasonable doubt that a theory's predictions are much better than the predictions of some older theory.
  6. May 10, 2013 #5
    Indeed. GR doesn't play nice with QFT, so we know that it must, in fact, be wrong!
    But it has been tested to ridiculous accuracy - as has QFT - and that makes things tricky.
  7. May 10, 2013 #6
    phinds, that would seem to imply a block universe (the universe is a 4-D space-time that is "all there at once" as a single entity). It has been emphasized over and over here that it is not possible to know that there is a single entity called space-time. Forum members have been given demerits for asserting this, and not long ago Vandam was removed from the forum for persisting in maintaining that the universe exists as a 4-dimensional entity.
    Last edited by a moderator: May 12, 2013
  8. May 10, 2013 #7


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    I don't think it necessarily implies that. But more importantly, since neither you nor anyone else has been able to state any experimental results that would be different on the block universe interpretation vs. another interpretation, the question of whether the block universe interpretation is correct is a philosophical question, not a scientific question.

    No, what people have been given "demerits" for is insisting that the question of whether the block universe interpretation is correct is a scientific question, without being able, as I said above, to give any way of resolving it by experiment. That doesn't mean it's a meaningless question, but it does mean there's no point in discussing it in a forum that's supposed to be for discussing scientific questions.
  9. May 11, 2013 #8
    The statement was: ..."That is, they are not "related" they are parts of a single thing."

    What other meaning are you ascribing to this statement? Do you think that phinds should have qualified that statement as a philosophical statement in order to avoid misleading ryanvb03? Should the "single thing" be understood as a philosophical thing--or perhaps a mathematical thing?

    I'm not pushing block universe one way or the other right now. I was just trying to get clarification about the "single thing" for ryanvb03. However, in other posts I have presented a summary of a proof related by Paul Davies in his book, "About Time." Neither you nor anyone else has presented an argument that directly refutes that proof.
  10. May 11, 2013 #9


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    I think you are making the mistake of assuming that "4D spacetime" is the same thing as, or implies, a "block universe". I've never fully understood what the "block universe" is supposed to be, but it seems to be more that just 4D spacetime; it seems to have some extra interpretation in terms of eternalism or determinism or something. I don't think anyone here would argue against the mathematical model of spacetime, without any "block" philosophy.

    The entity "spacetime" certainly does exist as a valid abstract mathematical concept that accurately models our Universe according to all experimental evidence so far.
  11. May 11, 2013 #10
    Well said phinds!
  12. May 11, 2013 #11


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    Yes, and that's not the same as saying the block universe interpretation is true, which is what you were claiming. One can give scientific meaning to the statement "space and time are parts of a single thing": for example, one can point to all the phenomena associated with time dilation, length contraction, relativistic Doppler, relativity of simultaneity, etc., which are all observable phenomena. But none of those observations require the block universe interpretation.

    That seems off topic for this thread, and indeed for these forums. The issue as far as PF is concerned is not whether the arguments you presented are valid or not, but that they are philosophy, not science. (I happen to think they are not valid, but this is not the place to argue the point, because it's a matter of philosophy, not science. If you really want to get into that in detail, PM me or email me--my email address is in my PF profile.)
  13. May 12, 2013 #12


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    bobc2, I would like to "second" this comment, this is the key point wrt the block universe discussions on this forum.
  14. May 23, 2013 #13
    I couldn't find the "sticky thread at the top of the relativity sub-forum on experimental confirmation of relativity". Does someone have a link to this forum? Sorry, I am new here. I just got to thinking about the tests that I have heard about dealing with the measurements of atomic clocks on earth and in space and how there is a difference between them due to the force of gravity. Is this the test that was described?
  15. May 23, 2013 #14
  16. May 28, 2013 #15
    Maybe so; however I find Bob's reaction normal. Phinds made the strange claim that they (space and time) are not "related". However, the LT express just how those two things are related.
    So if phinds was referring to the equations then it was just wrong; more likely he made an (unprovable) metaphysical claim, and that was what Bob was getting at. Phinds what exactly did you mean?
  17. May 28, 2013 #16


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    Not exactly. Here's how I read phinds' statement (he's welcome to correct me if I'm wrong): space and time aren't "related" because "related" implies that they are two separate things with a relationship between them. But the whole point of the math of LT's is that there is no one single unique way to split up spacetime into "space" and "time"; saying "space and time are related" implies that there is. It's better to say "space and time are parts of a single thing, spacetime".

    But even the latter way of stating it can still be misleading. I would phrase it like this: "space" and "time" are observer-dependent ways of splitting up "spacetime". We feel an intuitive desire to do this splitting because our minds are structured to perceive "space" and "time" as two separate things. But our minds are also structured to assume that there is one, unique, absolute way to do the splitting, and there isn't. So in our physical theories, we don't treat the splitting as fundamental; the fundamental thing is spacetime, a single thing, not "space" and "time". We only put in the splitting if it helps to match up the theory with our observations, and when we do, we have to do it in a way that recognizes the observer-dependence of the splitting. Hence the math of LTs.

    (Note, btw, that if you only deal with spacetime and geometric entities in spacetime, you never have to use LTs at all. You can express all of the physics without ever doing a Lorentz transformation.)
  18. May 28, 2013 #17
    Maybe so, but that still smells like philosophy. For example the LT permit to calculate the time on a clock and the proper length of a ruler, which are certainly separate and very different things that no physicist will confound. Of course, endless philosophical discussions have been held and continue to be held about such things as the nature of space and time. I think that such things are counterproductive for the topic here, which is about the predictions of the theory.
  19. May 28, 2013 #18


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    Part of it probably is, yes. But I think a scientific part can be separated out fairly easily from what I said. Note, for example, that I said you can express all of the physics in terms of spacetime and geometric objects in spacetime; that includes predictions of all experimental results. For example:

    You don't need LTs to do that. If you know the worldline of the clock, you can compute the proper time between any two events on that worldline without doing a LT. If you know the worldlines of the ruler's endpoints, you can compute the proper distance between those endpoints in any spacelike hypersurface you choose without doing a LT.

    Sure, but "proper time along the worldline of a particular clock" is not the same thing as "time", unqualified; and "proper distance between the two worldlines of a ruler's endpoints, in a particular spacelike hypersurface" is not the same thing as "space", unqualified. The proper time and proper distance are geometric objects in spacetime, defined in a particular way that is independent of coordinates and reference frames and Lorentz transformations; in other words, they are perfectly good scientific things that can be discussed scientifically. "Space" and "time" are much vaguer concepts that can be given multiple meanings, many (if not most) of which are not really scientific but philosophical, as you say.

    I agree that the philosophical discussions are counterproductive; but as I said, one can separate out particular scientific concepts that are useful to discuss. I've given examples of that above.
  20. May 28, 2013 #19


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    Yes, that is exactly what I was saying (or at least intending to convey) and I do not see that as being at all a "philosophical" point of view but rather a purely scientific one.
  21. May 28, 2013 #20
    I also commented on my likeness to Phinds comment which was well said, though perhaps confusing if not taken literally.

    "Single entity called spacetime" could not have been more clear imo though.
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