Question about doppler shift in relativity

In summary, the conversation discussed the concept of Doppler shift and how it relates to the speed of light. The participants also touched on the conservation of energy and how it may not be applicable on cosmological scales. Finally, they discussed a homework problem involving the movement of an observer and the change in frequency of photons. The use of four-vectors was also mentioned as a potential solution to the problem.
  • #1
skate_nerd
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I've got a somewhat simple question, and I think its from me overlooking something stupid...but how does using doppler shift work if in any moving rest frame the speed of light is supposed to stay at 3(10^8) m/s? I know that E=hc/λ, so for the wavelength of the light λ to change, the only thing that could change is the energy right? So does moving towards or away from a light source change the energy of the photons coming at you, but only in your frame of reference?
 
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  • #2
Well, the wavelength is also related to the frequency, independently of the energy.
 
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  • #3
Okay so yeah I was overlooking something stupid haha. Well I'm just a little confused about this homework problem, its about pair annihilation. A positron-electron pair annihilate into two photons, and I need to find the speed an observer would have to move to observe one of the photons wavelengths as twice that of the other photon's wavelength. So is this saying that if the observer moves at a certain speed, only the frequency of the photons change?
 
  • #4
This is an excellent question and one that I had a problem with at first because it seems to violate the principle of conservation of energy.

As space expands, or as some prefer to say, as things get farther apart (on cosmological scales), light loses energy and thus changes frequency. The energy just disappears. BUT WAIT, I said ... how can that be. Energy can't just disappear.

Well, turns out that conservation of energy is not a valid concept on cosmological scales and the energy DOES just disappear. Weird, huh?

Now, keep in mind, this happens over BILLIONS of years. The cosmic microwave background, for example, is radiation that started out as visible light (see "surface of last scattering") and is now in the microwave part of the spectrum and is VERY weak but then it has been traveling towards us for almost 15 billion years.
 
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  • #5
skate_nerd said:
Okay so yeah I was overlooking something stupid haha. Well I'm just a little confused about this homework problem, its about pair annihilation. A positron-electron pair annihilate into two photons, and I need to find the speed an observer would have to move to observe one of the photons wavelengths as twice that of the other photon's wavelength. So is this saying that if the observer moves at a certain speed, only the frequency of the photons change?
Have you been taught about four-vectors?
 
  • #6
skate_nerd said:
Okay so yeah I was overlooking something stupid haha. Well I'm just a little confused about this homework problem, its about pair annihilation. A positron-electron pair annihilate into two photons, and I need to find the speed an observer would have to move to observe one of the photons wavelengths as twice that of the other photon's wavelength. So is this saying that if the observer moves at a certain speed, only the frequency of the photons change?

Ah, that's a different problem than what my previous post addressed. I thought you were asking about cosmological Doppler shift.
 
  • #7
@phinds, I was actually trying to think of this problem by relating it to cosmological doppler shift, because I'm a little more familiar with that than doppler shift on a quantum scale...@DaleSpam I just recently started learning about four-vectors, yes.
 
  • #8
Actually, I just found a page in my text containing the derivation for the Longitudinal Doppler Shift Formula of Relativity Theory. Pretty cool stuff. So apparently one photon will have a higher frequency than the "rest frequency", and the other will have a lower frequency than the "rest frequency".
 
  • #9
skate_nerd said:
@DaleSpam I just recently started learning about four-vectors, yes.
OK, so this problem is fairly easy with four-vectors. Start with the four-momentum of the photons in the COM frame, ##P=(E/c,\mathbf p )##. Then use the deBroglie relationship ##P=\hbar K## to get the four-wavevector ##K=(\omega/c,\mathbf k )##. Then you can just transform that to the frame where ω of one is twice that of the other.

EDIT: you can make this really easy by using units where c=1 and where ω=1.
 
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1. What is the Doppler effect in relation to relativity?

The Doppler effect is a phenomenon in which the frequency of waves, such as sound or light, appears to change depending on the relative motion between the source of the waves and the observer. In the context of relativity, the Doppler effect applies to both sound and light waves and is affected by the relative velocities of the source and observer.

2. How does the Doppler effect change in the presence of high velocities?

In the theory of relativity, the Doppler effect changes significantly at high velocities. As an object's velocity approaches the speed of light, the observed frequency of waves emitted by the object increases or decreases depending on whether the object is moving towards or away from the observer. This phenomenon is known as the relativistic Doppler effect.

3. Can the Doppler effect be used to measure an object's velocity in relativity?

Yes, the Doppler effect can be used to measure an object's velocity in relativity. By measuring the change in frequency of waves emitted by an object, the observer can determine the object's velocity relative to their own frame of reference. This is commonly used in fields such as astronomy to measure the velocities of distant objects.

4. How does the Doppler effect factor into the theory of special relativity?

The Doppler effect is a key component of the theory of special relativity, which explains how the laws of physics remain the same for all observers in uniform motion. In special relativity, the observed frequency of waves can be affected by the relative velocities of the source and observer, and this must be taken into account when making measurements in different frames of reference.

5. Can the Doppler effect be used to detect the expansion of the universe?

Yes, the Doppler effect is commonly used in cosmology to detect the expansion of the universe. By analyzing the redshift or blueshift of light from distant galaxies, scientists can determine how fast those galaxies are moving away from or towards us, which provides evidence for the expansion of the universe.

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