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Speed of light?

  1. May 12, 2009 #1
    how can it be that the distant galaxies moove away from us faster than the speed of light and thus are not observable. now dont get me rong i know its not the objects mooving but space itself, any relevant theories on this ? thanks
  2. jcsd
  3. May 12, 2009 #2


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    Staff: Mentor

    Objects moving at/above the speed of light are not necessarily unobservable just as objects moving faster than the speed of sound are not necessarily inaudible. You just have to be inside the light or sound cone to see/hear them. Ie, if an object passes you at twice the speed of sound at a distance of 1100 feet, you hear it 1 second after it passes since the speed of sound is 1100 fps and the sound only has to travel 1100 feet to get to you (draw a picture based on that and you can construct the shape and angle of the mach cone).

    An object moving faster than the speed of sound isn't dragging its sound with it and an object moving faster than light (ie, via the expansion of space) is not dragging its light with it, though the expansion of space makes the math a little more difficult for light than for sound as there is a visible horizon to the observable universe.
  4. May 14, 2009 #3


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    Science Advisor

    We can see (receive light from) objects with Hubble recession velocities greater than the speed of light (c). Objects at the particle horizon (redshift = infinity) have Hubble recession velocity = 2c in the flat, matter-dominated model, for example. In that model, objects with redshift equal to 3 are receding at c today and were moving away from us faster when the light was emitted long ago. If you want a popular explanation, see

    "Misconceptions about the Big Bang," Charles H. Lineweaver & Tamara M. Davis, Sci. Am. 292, #3, Mar 05, pp. 36-45.

    For an explanation for undergrad physics students, see

    "Can galaxies exist within our particle horizon with Hubble recessional velocities greater than c?" W.M. Stuckey, Am. J. Phys. 60, #2, Feb 92, pp. 142-146.
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