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Galaxies velocity - I'm puzzled

  1. Dec 4, 2009 #1
    Hi, my knowledge in astronomy is very little. So pardon me if my thinking becomes stupid.
    All the galaxies are moving away from one another with acceleration. The more distant they are, the more acceleration they have. Acceleration implies increase of velocity. So my point is that if the galaxies have accelerated motion, then very far away there may be some galaxies which have already crossed the velocity of light, c. I mean they may be moving faster than light right now! Is it correct?
    My second point is our milky way has also accelerated motion as all the galaxies are moving away form one another. So, in future some day, its velocity will also cross the velocity of light.
    I don’t know whether those predictions stated above are correct or wrong. Any help will be highly appreciated.
  2. jcsd
  3. Dec 4, 2009 #2
    i think that the speed of light is the maximum speed of any thing in the universe and nothing can cross this barrior. in my view as the things approach the speed of light the time dilates (compared to the other frame of reference where the speed is not that high) making the speed of light constant and still the objects not apporaching its speed. you better seek the general theory of relativity for it.
  4. Dec 4, 2009 #3


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    There ARE galaxies that are moving away faster than the speed of light. That's not a problem: their light can never reach us, so we can't ever see them.
  5. Dec 4, 2009 #4
    Actually I believe we can see many galaxies that are currently moving away faster than the speed of light. Certainly this is true for the ones that emitted the light we see billions of years ago before their relative speed increased past the speed of light, even though they are CURRENTLY receding faster than light.
  6. Dec 4, 2009 #5
    I agree 100% with u.
    But alas! none of you addressed my second point where I stated about the velocity of milky way. Can any expert tell me when milky way's velocity will cross the velocity of light?
  7. Dec 4, 2009 #6

    D H

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    With respect to what? There is no master reference frame against which the position and velocity of everything is referenced.
  8. Dec 4, 2009 #7
    thanks for my correction.
  9. Dec 4, 2009 #8


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    Yes, we can certainly see galaxies that are currently moving at the speed of light; we just won't be able to see them as they appear now.
  10. Dec 4, 2009 #9


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    The galaxies don't actually "accelerate to faster than the speed of light", since matter can never attain the speed of light, rather, it's the space between our galaxies which is expanding (the expansion of which is speeding up). All space expands, and because these galaxies are so far away, with so much space between us, they appear to move away faster than the speed of light.

    The analogy is imagine we had a loaf of bread baking. If we think of the galaxies as raisins in the loaf of bread, every second each raisin gets, say, 1 cm away from all the adjacent raisins. A raisin 1 unit away, would move 1 cm away each second. A raisin which is 2 units away, would move 2 cm away in 1 second. A raisin which is 3 trillion trillion units away would move three trillion trillion cm away in 1 second, thus appearing to move faster than the speed of light.

    This analogy is of course flawed. It's a 2-D analogy for a 4-D phenomenon, and it's medium of expansion is also matter (i.e. the bread) rather than space-time. But it's useful for visualizing these things.
  11. Dec 5, 2009 #10
    Thank you all for responding. I want to specially thank D H and Matterwave who made the whole scenario clear to me.
    That's the thing I was missing. With respect to our milky way it appears that those distant galaxies are moving faster than light. But with respect to any neighboring galaxy of those distant galaxies, they no longer move faster than light. It's all about the distance.
    And regarding to milky way's velocity D H wrote:

    Yes, now I understand the scenario. It's all about relative motion. May be milky way is right now moving faster than light with respect to very distant galaxy! But with respect to any neighboring galaxy, our milky way's current velocity would be less than the velocity of light. Like D H said there's no master reference frame. If we change our reference frame galaxy, the corresponding motion of milky way will also change.
    Thank you guys.
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