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Is Time Synchronous Throughout the Universe?

  1. Sep 21, 2009 #1
    Meaning just like it says. Are there any time 'shifts' within this very huge Universe? Is Time a constant same-pace dimension throughout?
     
  2. jcsd
  3. Sep 21, 2009 #2

    chroot

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    The General Theory of Relativity indicates that time is an essentially local phenomenon. There is no consistent way of defining a specific moment in time -- "now" -- that applies everywhere in the universe.

    - Warren
     
  4. Sep 21, 2009 #3

    marcus

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    Warren is right, there no one single officially-approved way to slice the 4D loaf into slices of "now".

    But practically speaking, astronomers do have slicing that they tend to use a lot. It is only defined approximately and we don't know the ultimate extent of applicability, nevertheless it's quite handy, almost indispensable. Sometimes called "universe time" or "cosmic time".

    It depends on the standard cosmo model, the Friedman model, that virtually everybody uses. Also can be called FLWR (Friedman, Lemaitre, Robertson, Walker). Derives from General Relativity after some simplifying assumptions are made like matter is approximately uniformly distributed. Or FWRL, whatever, or FRW. The standard expansion model universe.

    The point is that the universe is full of microwave radiation which keeps getting cooler as the universe expands and so the Background temperature can be taken as a clock.

    Our "now" consists of all the observers who see the same temperature we do. Each observer sitting on his own rock somewhere in his own galaxy, holding a thermometer to the sky. (Or a microwave antenna measuring wavelengths, same thing.)

    All the observers who measure 2.728 Kelvin, like we do, are part of our "now". They belong to our "slice". That is our timelike hypersurface---a 3D slice of the 4D loaf that gives a geometrical meaning to simultaneity.

    Or maybe it is 2.726 Kelvin. The trouble is measurements are always fuzzy and approximate anyway.

    And we have a criterion of rest. An observer is at rest if he measures the approx. same Background temperature in all directions. If he is moving at some substantial speed he will see a Doppler hotspot ahead of him, where the microwave background temperature is hotter, or the wavelengths shorter, because of his motion relative to the Background.

    The Hubble Law which is basic to standard cosmo uses these ideas of Now and Stationary Observer. It says that distances between stationary observers are now increasing at a rate which proportional to what the distance is now.
    v = H d

    d is the distance now. v is the current rate that the distance is increasing----in kilometers per second or whatever units are convenient. H is a proportionality factor (which is the same all over the universe now, but which changes gradually with time.)

    Occasionally cosmic time or universe time is called "Friedman time" because it is the time according to which the standard universe model runs. And most often astronomers simply say "time" without clarifying----they just assume you know what they mean.

    Like, "the light from that galaxy was emitted when the universe was 3 billion years old and has been traveling for 10 billion years, and it got here to our telescope yesterday". Statements like that typically assume we are using a Friedman clock.

    So in pure General Relativity, with no simplifying assumptions and no nice Background radiation, there truly is no preferred time. Each observer has his own personal, or "proper" time, which is his own "property" so to speak. Which is great. Total anarchy. But for practical purposes, working cosmologists cheat and keep this informally preferred time around, and the corresponding idea of being at rest (with respect to the ancient matter and the ancient light of the universe) because it's so useful.

    If you like simple differential equations and want to see one that governs the growth of largescale distance, the keyword would be "Friedman equations" or Friedmann with two Ns.
     
    Last edited: Sep 21, 2009
  5. Sep 21, 2009 #4
    I always thought that time was synchronized throughout the entire universe. No point in the universe is specialm this would include time would it not? So if we had some grand 'god' like vantage of the universe from an outside perspective all points in the universe would have precisely the same time... just because we don't OBSERVE that doesn't mean it's not true? Or did I misunderstand everything in this thread. loll
     
  6. Sep 21, 2009 #5

    chroot

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    The theory of general relativity precludes any such 'god-like vantage point' from which you can observe everything in the universe at once.

    - Warren
     
  7. Sep 21, 2009 #6
    Just look at two people moving close to the speed of light with respect to each other. Both see there clock moving at a normal rate but see the others clock moving slowly.
     
  8. Sep 23, 2009 #7
    captains log.. stardate 2.726 Kelvin...

    now imagine if you could surf on a ray of light. what would things look like? from the light's point of view, is everything perfectly still because time is moving at an infinitely small rate (or maybe even zero)? we might interpret light to take 8 minutes to travel from the sun to the earth, but does the light view itself as traveling that distance instantaneously?
     
  9. Sep 23, 2009 #8
    I would assume since the speed of light actually has the said set speed then no it wouldn't view i as travelling the distance instantaneously. Light still travels at the same speed even if you yourself are travelling at the speed of light. (Like einstien though of, would you be able to see yourself in the mirror still?) So for one to say that light still has a value of speed shows that time must still be occuring regardless of the how 'fast/slow' it appears...
     
  10. Sep 24, 2009 #9

    chroot

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    The theory of relativity explicitly prohibits this, so it is a meaningless premise.

    - Warren
     
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