Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Red shift/blue shift help

  1. Jun 28, 2012 #1
    What I want to know is.... if objects apparently travelling away from us at high speed, shift the light towards the red end of the spectrum ( and towards us, the blue )... surely the same effect would be experienced if the distant object was in fact stationary (hypothetically) and it was us travelling away/towards from the object at high speed? so when we calculate the distance and speed of distant bodies in the universe with redshift.... how do we know that the entire spectrum shift can be attributed to the velocity of the receding body alone? surely it cant be so.. so we must be measuring the speed of it, in relation to us...a combination of both objects speeds, and the resultant spectral shift...a culmination of these too. would this not create the illusion that it had moved further/faster than it actually has? I am clearly not a physicist and hope I am not just chatting boll*ks, but another example to help me...if there were 2 objects on a frictionless surface which were placed near to each other.. and we had a point of view shot from one of these objects.. . If they were both pushed in different directions at different speeds...it would appear to us that we were still stationary and the other was moving away at a misinterpreting speed..would it not?! but surely if we could measure the objects in relation to another central object/s then we could resolve the components and work out, not how far or fast we are moving away from each other, in relation to each other..but how fast we are actually moving from each other from a point in space/time, and therefore actual velocities/rather than apparent.
    Anyway, im just a business student and likely gone horribly wrong somewhere (probably the start) but wouldn't pester you serious folk if I hadn't tryed looking it up already!
    Thanks ....Ollie
  2. jcsd
  3. Jun 28, 2012 #2
    Velocity is relative. This means that in the situation you have described, both viewpoints are equally correct. Saying the distant galaxy is moving away from us at 100 km/s is the same as saying that we are moving away from the distant galaxy at 100 km/s. There is an arbitrary choice of where you set the 0 point of the velocity. You can only judge relative velocities between objects. There is no such thing as an absolute velocity or a velocity relative to empty space. Space doesn't have a velocity.
  4. Jun 29, 2012 #3


    User Avatar
    Gold Member
    2016 Award

    As Khashishi has already explained, speed is only meaningful in a relative sense and you are right about both viewpoints being correct.

    BUT ... if you are thinking about cosmological objects (distant galaxies), keep in mind that they are ALL red-shifted. How could that be if we were moving away from some and towards others?
  5. Jun 29, 2012 #4


    User Avatar
    Science Advisor
    Gold Member

    It might help you to look up the difference between the normal Doppler Effect which applies to waves that propagate in a medium like sound in air and for which the speed of both the source and the receiver with respect to the medium are important and the Relativistic Doppler Effect for light where only one speed is important, the relative speed between the source and the receiver.
  6. Jun 29, 2012 #5
    If you are talking about distant galaxies, then the answer is that we must reject your premise. All the distant galaxies are red-shifted because we are moving away from all of the distant galaxies. The only galaxies we are moving towards are (relatively) close to us. IIRC, we're on a collision course with the Andromeda galaxy.
  7. Jul 2, 2012 #6
    Thanks Khashishi, really appreciated! and cheers George, very useful, was pondering the differences between them.
    This is a bit irrelevant but don't want to start a new thread for a quick question. I was thinking.... light is obviously effected by the curves in space time, evident by gravitational lensing. how is it though that light can be bent or effected in this respect, but the curves can not effect the speed of the light... surely if it can bend its direction in one direction,then in could bend the light towards, or speed the light as it approached and "dipped" into this curve and slowed as it exited?! Or is the idea similar to projectile motion, in that the horizontal and vertical components are independent of each other? so the light would maintain the same speed whatever was happening to its direction? Cheers!
  8. Jul 2, 2012 #7


    User Avatar
    Science Advisor
    Gold Member

    You need to first learn the concepts of Special Relativity which excludes gravity and the bending of light. Do you understand that when we measure the speed of light, we can only do it locally, that is, we have to first measure a distance using a rigid ruler (not a laser distance measuring device or anything else based on light) and then we have to put a mirror at the far end of the measured distance and then we have to measure the time it takes for a flash of light to go from the near end of the measured distance to the far end, reflect off the mirror and get back to the starting point. Then we divide double the distance by the time interval. When we get done, we cannot know if the time it took for the light to go to the mirror is the same as it took for the light to get back. Do you understand why this is so?
  9. Jul 4, 2012 #8
    George...I have started reading on the subject and feel a very basic understanding of the concepts forming. I cant however provide a satisfactory answer with what I have learnt... please could you elucidate.
  10. Jul 4, 2012 #9


    User Avatar
    Gold Member

    It's Dr. Seuss.

    Red shift, blue shift. One shift, two shift.

    I'm sorry, I couldn't resist.
  11. Jul 4, 2012 #10
    This IS an effect that local observers do not measure while distant observers do.

    It's useful to keep in mind that all observers see the LOCAL speed of light as 'c'..meaning right where they are, no curvature. But over distances curvature does affect the observed speed of light. The same comments hold for gravitational potential near and far.

    One discussion of the observed speed of light in curved spacetime is here:

  12. Jul 5, 2012 #11


    User Avatar
    Science Advisor
    Gold Member

    I like to but I don't know what you have been reading nor what your question is.

    I think the best place to start with the issue I brought up in post #7 is Einstein's 1905 paper introducing Special Realtivity to the world. Just read the first section, it's only a page or so long and then let me know if you have any questions about it.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Red shift/blue shift help
  1. Red shift Blue shift (Replies: 6)

  2. Red shift or blue shift? (Replies: 16)