Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

What is the age of the universe for a high velocity planet?

  1. Jul 7, 2015 #1
    I think it was Kurt Gödel who asked this while taking about time. The age of the universe and its size, is whatever it is for us because we are moving at a specific speed relative to light? If that is true, then a consciousness living on a planet moving at triple the speed of earth relative to light will observe a different age of the universe? Please help.
  2. jcsd
  3. Jul 7, 2015 #2


    User Avatar
    Science Advisor

    Welcome to the Physics Forums.

    There is no such thing as a "speed relative to light". Speed relative to an object is shorthand for "speed as determined in a reference frame where the object is at rest". There are no reference frames in which light is motionless. More generally, in both special and general relativity, there is no frame of reference which is picked out by the laws of physics as being in any way "special".

    However it is possible to pick out a reference frame in which the large scale features of the universe are "isotropic". Roughly speaking, if you are at rest in this frame, the universe looks the same no matter which direction you look. For instance, there is no systematic blue shift of the light from stars off in one direction and no corresponding red shift of the light from stars off in the opposite direction. If you were in motion relative to this frame, there would be systematic blue shifts and red shifts of this sort.

    The age of the universe -- the elapsed proper time of a hypothetical observer who is moving with respect to an isotropic frame would indeed be lower than that for an observer who is stationary with respect to that frame.

    It is standard practice in cosmology to report the age of the universe using coordinates in which the large scale structure of the universe is isotropic.

    [What I write above is basically a regurgitation of what I have learned on these forums]
    Last edited: Jul 7, 2015
  4. Jul 7, 2015 #3
    I dont think age of universe depend velocity of a planet.It depends Hubble constant and some parameter( Which they are ##Ω_r,Ω_m,Ω_Λ...##).Further info look this https://en.wikipedia.org/wiki/Age_of_the_universe cosmological parameters section.Or you can watch this video.The important thing is Hubble constant.

    D=a(t).x then differantiate it respect to time you get V=a'(t)x now divide a(t) the equation V=a'(t)x you get V/a(t)=a'(t)/a(t)x then V=a'(t)/a(t)xa(t) the black font is equal D and italic font equal H so hubble constant is a'(t)/a(t) so its depends time of universe not another thing.
    I want to say I might be wrong

    Last edited: Jul 7, 2015
  5. Jul 7, 2015 #4


    User Avatar
    Science Advisor
    Gold Member

    Age of the universe per Hubble constant is simply age per clocks moving with the Hubble flow, that is clocks for which isotropy and homogeneity are observed. It is no more intrinsic than any other coordinate dependent notion of time, e.g. the time coordinate of the exterior Schwarzschild metric (which is time as measured by static clocks at 'infinity').

    What jbriggs444 wrote is a perfectly good answer.

    To provide a hypothetical realization: If, early in the life of universe, a super-massive BH formed (not plausible, but this is a thought experiment), with a planet orbiting at the closest stable circular orbit, the time elapsed since the big bang would indeed be substantially less than for a typical planet. Using the conventional factorization possible for a spherically symmetric field, part of this would be due to 'gravitational time dilation' and part would be due to very high orbital speed of the planet relative to static observers.
    Last edited: Jul 7, 2015
  6. Jul 7, 2015 #5
    This seems such a fascinating thing from a philosophical point of view.

    “Matter in motion determines the shape of space-time. The possibility arises that some reference frames might be privileged, namely those that follow, as Gödel put it, the mean motion of matter in the universe. Time relative to those frames of reference bears the designation “cosmic time,” and this opens up the possibility that time in something like the pretheoretical sense might after all be consistent with relativity, in particular with general relativity. It is time in this sense that is (or should be) invoked when cosmologists speak of the age of the universe.”

    Excerpt From: Yourgrau, Palle. “A World Without Time.” Basic Books, 2011-12-01. iBooks.
    This material may be protected by copyright.

    What are your thoughts on this "mean motion of matter"?
  7. Jul 7, 2015 #6


    User Avatar
    Science Advisor
    Gold Member

    That's a valid point of view, but note, it describes this point of view as 'possibly consistent with general relativity', not required by it. One can choose to imagine there is preferred frame in SR, and get universally correct answers. A cosmological preferred coordinate system has much more utility and motivation than an SR preferred frame, but it can never be considered required or intrinsic in GR (per se). However, it is simply a matter of definition that when you say "age of the universe" without any description of a special point of view, you mean age per standard cosmological coordinates (or equivalently, more physically, age per any clock that 'sees' isotropy and homogeneity).
  8. Jul 10, 2015 #7
    Is that Air in D? Wow, this looks like Phinds' balloon analogy only backwards :smile:
  9. Jul 10, 2015 #8
    Yeah thats right
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: What is the age of the universe for a high velocity planet?
  1. Age of the universe (Replies: 2)

  2. Age of the universe (Replies: 12)

  3. Age of universe? (Replies: 4)