1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Special Relativity Question

  1. Oct 29, 2011 #1


    User Avatar

    1. The problem statement, all variables and given/known data

    Two photons travel along the x-axis of S, with a constant distance L between them.
    Find the distance between them as observed in S' (standard config - i.e. moving with speed v in x direction wrt S)
    How is this result connected to the Doppler effect?

    2. Relevant equations

    3. The attempt at a solution

    So normally the distance would be lorentz contracted - right? So it would be L/gamma

    but i guess it is not here? and how is it related to doppler? any hints would be great ! thanks
  2. jcsd
  3. Oct 31, 2011 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    What you see is a pair of photons pass S at speed c, S determines the distance to be L.

    S', at speed v wrt to S also sees the photons pass at speed c - what length will S' measure?

    Presumably S has timed them, S' gets the same time dilated, so the distance between the photons that S' measures is expanded.

    Reality check:
    imagine S and S' are both at x'=x=0 when the first photon arrives.
    in the time it takes for the second photon to arrive at S (still at the origin) S' has moved on and has yet to see the photon pass. He has to wait a bit longer for the second photon to pass... so he measures a longer time. Longer time, same speed, means longer distance.

    This also means that S' and S will disagree about the color of the photons - S' will decide the photons are a bit redder... in terms of the relativistic doppler shift, this is because the wavelength has expanded.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook