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Question about the a relativistic rocket

  1. Oct 10, 2012 #1
    Question about the a "relativistic rocket"

    If one was to measure the redshifts and distances of all galaxies around earth, they would come to a conclusion that they were in the center of the universe because all the galaxies are "moving away from them" faster as the distance to them increases. However, this would be true for anyone on the universe because the universe is homogeneous and doesn't have a "center". It's like being a surface of a balloon being inflated, and the balloon surface not having a center.

    If I was in a rocket moving at .9c and moving away from earth would I:
    1) If I measure distances/redshifts towards all galaxies around me, will I have the same observation (about universe expanding equally in all directions).
    2) If my event horizon behind me reduces, does it mean that it shifts forward also and that I can "receive new events" from something ahead of me?
    3) If I was to take Hubble with me and observe some star that it in direction perpendicular to my direction of motion, would I see that the star is round or "squished" in the direction of my motion?
  2. jcsd
  3. Oct 10, 2012 #2


    Staff: Mentor

    Re: Question about the a "relativistic rocket"


    What "event horizon" are you talking about?

    If you're asking if the star would be Lorentz-contracted along your direction of motion, yes, it would. However, its actual visual appearance would be somewhat more complicated because of Penrose-Terrell rotation:

  4. Oct 11, 2012 #3


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    Science Advisor

    Re: Question about the a "relativistic rocket"

    Already for the motion of the earth relative to the cosmological background radiation the small velocity leads to an anisotropy in the radiation (a kind of dipole moment) which has to be subtracted.

    What you mean is the so-called particle horizon or cosmological horizon. Yes, the horizon increases. But please be careful, this depends on the expansion rate of the universe; there are cosmological scenarios where the particle horizon shrinks for a stationary observer and where local relativistic motion can't compensate this effect. An example is inflation where the expansion rate exceeds the speed of light.
  5. Oct 11, 2012 #4
    Re: Question about the a "relativistic rocket"

    I actually meant the Rindler Horizon, but that may occur only if the rocket is accelerating. So the question I guess still stands - if the rocket is accelerating, will there be a rindler horizon and/or is there an inverse effect in forward direction.

    the cosmological horizon is interesting though. If we could "hurl" hubble at high velocity, we could see more. Though we'd probably have better optics technology by the time we have energy to hurl an object into space at such high speed.

    Now as far as the velocity with respect to the universe expansion "rest point" being "observable", doesn't that mean that there is such thing as observable/measurable absolute velocity and velocity "at rest"? Or does just not have any significance even if there is.
  6. Oct 11, 2012 #5


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    Science Advisor

    Re: Question about the a "relativistic rocket"

    There are always observable/measurable velocities w.r.t. "something". Yes, the cmb defines a "global" reference frame and the motion w.r.t. this frame can be measured. But this does not contradict relativity.
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