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Space and Time

  1. Mar 20, 2009 #1
    After reading a few books on relativity, I came to the conclusion that space and time are "somethings". And that if you removed all matter and energy from the universe space and time, or spacetime, would still remain.

    Perhaps I missed something, but in my history book at school I found in a few brief paragraphs that claimed that in relativity thoery space and time dissapear along with the material objects. I was just hoping that you guys could help clear this up. Im thinking the book is not accurate because it also claims that relativity brought uncertainty into our veiw of the universe, which, in my opinion, doesn't seem to at all.
  2. jcsd
  3. Mar 20, 2009 #2


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    Does it really matter what the laws of physics would be like in such an empty universe? All we are really concerned with is how they work in this one.
  4. Mar 21, 2009 #3


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    If you remove the mathematics that describes matter from the theory, there is still some mathematics left. It describes spacetime. But the theory has now been crippled to the point where it isn't capable of making any predictions, so it isn't really a theory anymore.

    It doesn't really make sense to ask what would happen if you remove the actual matter from the actual universe, since this isn't possible even in principle.
  5. Mar 21, 2009 #4


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  6. Mar 21, 2009 #5


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    My impression is that Einstein said that the material objects which participate in defining events are real, and there are geometric relations between events, but he would say that space and time have no physical existence.

    The book is correct in saying that in GR space and time dematerialize, have no objective physical reality. But if it says that material events, like the collision between two objects, are unreal then it is wrong. GR does not say that. GR affirms the physical reality of such events.

    Here are some quotes from Einstein about this

    “Dadurch verlieren Zeit & Raum den letzter Rest von physikalischer Realität. ..."

    “Thereby time and space lose the last vestige of physical reality”.

    (Possible paraphrase: space does not have physical existence, but is more like a bunch of relationships between events)

    He was talking about the principle of general covariance, which is central to GR. It underlies what he is saying.

    In case anyone wants an online source, see page 43 of this pdf at a University of Minnesota website

    ==quote from the source material==
    ...In the introduction of the paper on the perihelion motion presented on 18 November 1915, Einstein wrote about the assumption of [U]general covariance[/U] “[b]by which time and space are robbed of the last trace of objective reality[/b]” (“[color=blue]durch welche Zeit und Raum der letzten Spur objektiver Realität beraubt werden,[/color]” Einstein 1915b, 831). In a letter to Schlick, he again wrote about general covariance that
    “[b]thereby time and space lose the last vestige of physical reality[/b]” (“[color=blue]Dadurch verlieren Zeit & Raum den letzter Rest von physikalischer Realität.[/color]” Einstein to Moritz Schlick, 14 December 1915 [CPAE 8, Doc. 165]).

    Both quotes are from Nov-Dec 1915, one being from a paper on perihelion motion. and the other from a letter to Moritz Schlick a few weeks later.
    Last edited by a moderator: Apr 24, 2017
  7. Mar 21, 2009 #6
    I think you well understood. Space ande time are something called a continuum in original Einstein theory. Mechanics of continuum is well described. For example, fluids are mathematically described through navier-stocks equations; today modern space-time theories described it as a continuum having a 4 dimension geometry and Einstein general relativity equation being the equivallent of navier-stocks equation and finally the equivallent of dynamics fundamental law.
  8. Mar 21, 2009 #7
    Special Relativity should be called Simple Relativity since it is is linear and assumes no mass in the area including the source and the observer. It thus is reduced to doppler effect ie. red-shift and blue-shift; the calculations are childs play.
  9. Mar 21, 2009 #8
    I am interested in how this interpretation explains length contraction and time dilation.

  10. Mar 21, 2009 #9
    I'm afraid you're not making a lot of sense. :uhh:
    How is it relevant to the thread that you know about the doppler effect and think the calculations are easy?
  11. Mar 21, 2009 #10
    ok then, if we accept the wave-particle duality of a photon then mCC=E=hn; and we can then calculate the mass of the particle as m=hn/CC based on the frequency for a stationary observer relative to the source; but when the observer is moving away relative to the source at a velocity of v then the sum of the energy should be Esum=mCC-mvv/2=m(cc-0.5vv); then take Esum=hnsum where n sum is decreased accordingly. Velocity is a vector so change to a plus sign for moving towards the source. For space-time changes become the photon and look around: length dilatation orthogonal to path of travel, contraction along the path, and the clock keeps ticking, so there is conservation of space-time, that is the sum of the space-time is the same for both the photon and the stationary observer, but if the observer is moving then the time change is evident as the change in frequecy.I must go and rest now.
  12. Mar 21, 2009 #11
    Back to the original question, if you have a bucket in each hand and spin yourself around the buckets swing outwards due to inertia. If you remove all matter from the universe and do the same experiment, do the buckets swing still outwards? If so, explain the difference between spinning the buckets around and not spinning them around in an empty universe.
  13. Mar 22, 2009 #12
    Could you explain the phrase "conservation of space-time". Is it something like invariance of the interval?

  14. Mar 22, 2009 #13
    I asked Jim Purcell something like that 30 years ago; was Fred the Cosmic Egg spinning? Jim's only reply was " no, no, no"; a few months later I handed him the formulae for relativity in rotational coordinates and he said it hurts his brain. Conservation of space-time is just that the sum of space-time is constant even when the velocity changes; think of a 4 dimentional box with volume x^4 at rest and then moving at C; the dimesions of the box may change but the volume is constant; the formula is Gh/CC for the Universal Flow Constant. A 4 dimensional box is ok with me, that is why I am called Sammy k-space.
  15. Mar 22, 2009 #14


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    Personally, I always find these Machian discussions about empty universes rather distasteful. Who cares what would happen in an empty universe? We have no way to verify or falsify such assertions, and since this universe is not empty even if you could verify or falsify them there is no guarantee that the results would have any bearing on this universe. All we know is that in a matter filled universe the buckets swing out.
  16. Mar 22, 2009 #15
    Well the implications if Fred was spinning come from conservation of angular momentum; if so, say in one axis then dkappa/dt would result in a ring of photons at a distance of rsub1; if Fred was spinning on 3 axis with the same velocity for all three we get a shell of photons. Both the ring and shell would form at some time dt after the big bang. Do you equate inertia with anti-gravity? That is do you need mass around you for the buckets to move?
  17. Mar 22, 2009 #16
    Charlie: good questions!!!

    I've posted here several times giving a list of examples why I also like to think of space and time as something...others disagree explaining they are merely mathematical representations. But if matter and energy are something, why not space and time?? And if space and time are "nothing" how does nothing "curve"? (a somewhat unfair question, perhaps.)

    Regarding the second part of your question...matter energy removed....I like to think in terms of such extreme questions as they can shed insight regarding less extreme variations....if Einstein could do thought experiments about "catching up with light", leading ultimately to SR and GR then such "crazy ideas" and questions ARE worthwhile.....
    I ignore comments about it not being worthwhile to consider such extremes.....possible universes without matter and energy. After all, maybe there are an infinite number of parallel universe, hence it could be very possible such universes exist...

    Which bring ups a closely related question....which is whether a universe can "appear" from nothing (bang, or bounce, or whatever) without matter and energy???...with only time and space....or are all four components joint manifestations of quantum foam that always appear together??? nobody knows of course.
    Last edited: Mar 22, 2009
  18. Mar 22, 2009 #17
    Is the observed rotational patterns in cosmic objects evidence of some type of initial rotation? Food for thought!
  19. Mar 22, 2009 #18
    We now have a formula to describe space/time: Gh/CC=3.95e-27 m^3/s if i did the math right, if some one could punch it in for me please.
  20. Mar 22, 2009 #19
    Gh/CC is the UNIVERSAL SPACE/TIME CONSTANT and there is indeed conservation of space/time!!!!!
  21. Mar 22, 2009 #20
    Regarding the conservation of space-time, are you saying that the "volume" formed by multiplying x,y,z and t is invariant with respect to velocity?

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