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Redshift/blueshift question

  1. Aug 4, 2006 #1
    I've recently read that when a galaxy or star(if I'm right) comes closer to earth, its spectrum shifts to blue. Correspondingly, when it travels away from earth its spectrum shifts to red. The cause of the shift is Doppler effect.

    My question: Why don't we observe doppler effect when an ambulance drives away? If the light on top of the ambulance was blue, shouldn't we expect any redshift? Using the same thoughts, why don't we observe blueshift on the front lights of a truck which comes close to us?

  2. jcsd
  3. Aug 4, 2006 #2


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    They are going much too slow wrt the speed of light to notice the difference. You do notice the difference in their sirens, though.
    Last edited: Aug 16, 2006
  4. Aug 4, 2006 #3
    Does the speed of galaxies or stars approach the speed of light?
  5. Aug 4, 2006 #4


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    Very distant galaxies move (relative to us) at a significant fraction of the speed of light.
  6. Aug 4, 2006 #5


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    The motion of galaxies through space seldom exceeds a few percent of the speed of light, and even then, only in dense clusters. However, the expansion of space itself causes very distant galaxies to recede from us at speeds near or even greater than the speed of light.
  7. Aug 5, 2006 #6
    Thank you very much
  8. Aug 7, 2006 #7
    Does Doppler effect occur after crossing a specific barrier?
  9. Aug 7, 2006 #8

    Is it possible anything can travel faster than the speed of light - that too matter? Or is it with respect to us? Even if it is with respect to us, is it possible for something to move faster than the speed of light or equal to it?
  10. Aug 7, 2006 #9


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    Not really, but every detection device, including your eyes, has a certan sensitivity associated with it. If the effect is weaker than the sensitivity of your device, you won't detect it.
  11. Aug 7, 2006 #10

    Its not so much that they are travelling faster than the speed of light, rather that space is stretching. Special relativity and its conclusions only make sense in flat, static space-time. When space starts stretching, things get more complicated.
  12. Aug 7, 2006 #11
    I can't understand how it is possible for an object to exceed or at least approach the speed of light. We can't even prove that space expands. Anyway, we should open another topic for this conversation.
  13. Aug 7, 2006 #12
    We can't 'prove' anything about the universe.

    We can show that the data is quite well explained if we accept that space is stretching. Or at least, well enough explained to accept it as a provisional hypothesis. I reiterate:

  14. Aug 17, 2006 #13
    Franznietsche is right. From Wikipedia:


    ... v > c is possible for objects dominated by cosmological redshift because the spacetime which separates the objects (eg a quasar from the Earth) is expanding and spacetime is described by general relativity...

    HereĀ“s some more on this topic from the UCLA:


    ...Thus for the largest known redshift of z=6.3, the recession velocity is not 6.3*c = 1,890,000 km/sec. It is also not the 285,254 km/sec given by the special relativistic Doppler formula 1+z = sqrt((1+v/c)/(1-v/c)). The actual recession velocity for this object depends on the cosmological parameters, but for an OmegaM=0.3 vacuum-dominated flat model the velocity is 585,611 km/sec. This is faster than light...
    Last edited: Aug 17, 2006
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