How Does Motion Affect Photon Spacing and Relate to the Doppler Effect?

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SUMMARY

The discussion focuses on the relationship between photon spacing and the Doppler effect as observed from different reference frames. When two photons travel a constant distance L along the x-axis in frame S, an observer in frame S', moving at speed v relative to S, measures the distance between the photons as L/gamma, where gamma is the Lorentz factor. The key insight is that while S measures the distance as L, S' perceives an expanded distance due to time dilation, leading to a redshift in the observed color of the photons. This phenomenon illustrates the relativistic Doppler shift, where the wavelength of light appears longer to a moving observer.

PREREQUISITES
  • Understanding of Lorentz contraction and the Lorentz factor (gamma)
  • Familiarity with the principles of special relativity
  • Knowledge of the Doppler effect in the context of light
  • Basic concepts of photon behavior and speed of light
NEXT STEPS
  • Study the derivation of the Lorentz transformation equations
  • Explore the mathematical formulation of the relativistic Doppler effect
  • Investigate the implications of time dilation on measurements in special relativity
  • Examine case studies involving photon behavior in different inertial frames
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This discussion benefits physicists, students of relativity, and anyone interested in the implications of motion on light behavior and the Doppler effect.

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Homework Statement




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?


Homework Equations





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
 
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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.
 

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