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Galaxies travelling faster than the speed of light?

  1. Jan 11, 2010 #1
    Galaxies travelling faster than the speed of light??

    1st off, Hello to everyone on the board. There are some very interesting ideas/posts.

    I watched a Youtube video titled "The Hubble Ultra Deep Field in 3D" (beautiful video by the way)
    It states that some galaxies are moving away from us faster than the speed of light.
    I would guess the COMBINED speed of the 2 galaxies could be faster than the speed of light but it isn't stated as such in the video.

    If this is the case, let me ask a hypothetical question....

    If galaxy "A" is moving away from galaxy "B" faster than the speed of light, then wouldn't the 2 galaxies be moving back in time relative to each other?
    Just pondering some ideas:-)

    Thanks for any ideas or replies.
    Rod
     
  2. jcsd
  3. Jan 11, 2010 #2

    bcrowell

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    Re: Galaxies travelling faster than the speed of light??

    Hi, RoDnSuE -- welcome to Physics Forums!

    Relativity doesn't forbid distant objects from moving faster than c relative to one another. It only forbids one object from whizzing right by another at >c. There is a very good discussion of this sort of thing in Martin Gardner's wonderful popular-level book Relativity Simply Explained.

    According to current cosmological models, there are other galaxies that are in some sense receding from ours at >c ("in some sense" because it's not trivial to define relative velocity for distant objects in an unambiguous way). However, light from these galaxies hasn't reached us yet, so we can't observe them. If a galaxy is receding from us at 0.999999c, then its light is extremely red-shifted as seen by us, which makes it difficult, but not impossible, for us to observe it.
     
  4. Jan 12, 2010 #3
    Re: Galaxies travelling faster than the speed of light??

    Thanks Bcrowell. I ordered a copy of the book you suggested.
    Your response made me realize something more clearly (I hope)...
    When scientists state the speed of an object (galaxy, star, whatever) they are measuring it from their perspective.
    I guess what I'm trying to say is, speed (or even motion in general) is relative to the person making the measurement. That being said, we can never ascertain that anything in the universe is in a "static", non moving state, since all measurments are relative to our state of motion. Am I thinking wrong here?

    Thanks,
    Rod
     
    Last edited: Jan 12, 2010
  5. Jan 12, 2010 #4

    DrChinese

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    Re: Galaxies travelling faster than the speed of light??

    To add to the puzzle: There are galaxies that are receding from us at >2c. In fact, I think the fastest recession speed observed is currently >3c. This may seem impossible, but it is a consequence of the expansion of spacetime itself. It may seem as if that light could never reach us, but in fact there is a mechanism. Best to see the work of Lineweaver and Davis (the link is a Scientific American article but there is scholarly work of theirs available if you want more specifics):

    http://www.astro.princeton.edu/~aes/AST105/Readings/misconceptionsBigBang.pdf [Broken]
     
    Last edited by a moderator: May 4, 2017
  6. Jan 12, 2010 #5

    bcrowell

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    Re: Galaxies travelling faster than the speed of light??

    Ah, thanks for pointing out my mistake (and doing it so diplomatically :-). I can think of at least two methods of defining recessional velocity: (1) by solving the SR expression for the Doppler shift for v, or (2) by taking dL/dt, where t is proper time for a clock in the cosmological model's preferred rest frame, and L is the distance as measured across a surface of simultaneity as defined by the same time coordinate. It sounds like they're using #2, and that does seem like the natural choice, since the model has a preferred time coordinate. In cases where there's no preferred time coordinate, of course there's no unambiguous answer to the question of how fast a distant object is moving "right now."
     
    Last edited by a moderator: May 4, 2017
  7. Jan 12, 2010 #6

    bcrowell

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    Re: Galaxies travelling faster than the speed of light??

    Yep, that's absolutely right :-)
     
  8. Jan 15, 2010 #7
    Re: Galaxies travelling faster than the speed of light??

    Thank you very much for the replies. It's funny how one question and even the answers to those questions, can bring up so many **seemingly** unrelated questions :-)
    Example... While reading the article, the words "two-dimensional" came up and I found myself remembering the late Carl Segan's description of a 2 dimensional world called
    'Flatland". That in turn made me search for the video on youtube and sending me down another track than I had started.

    I had to delay finishing the article drchinese recommended because my step-daughter had a minor auto accident and my priorities had to shift for a day or two. Anyway, back into the physics fray now.

    When I finish, I'm sure I will have some more questions. I'm down to the "Hubble constant" section and it looks VERY interesting/intriguing/baffling:-).
    Thanks again
    Rod
     
  9. Jan 15, 2010 #8

    bcrowell

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    Re: Galaxies travelling faster than the speed of light??

    Hi, Rod,

    I think Sagan was echoing a 19th-century book called Flatland, by Abbott. There is also a modern sequel called Flatterland, which I enjoyed a lot. There's a kids' book series these days called Flat Stanley, which my daughters enjoyed a while back, but I haven't read any of them.

    Glad to see that word "minor" there -- best wishes for getting that all worked out. Luckily my kids are not quite old enough to drive yet :-)

    -Ben
     
  10. Jan 16, 2010 #9
    Re: Galaxies travelling faster than the speed of light??

    When they do start driving, make sure they understand that somone waving them on that the way is safe is RELATIVE to that person. Lack of pun intended:-)
    That was my daughters mistake.

    Rod
     
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