Question

I have read the explanation of the "red shift" for photons and I understand that gravity affects the energy of the photon, esp as it is escaping, (leaving), a gravitational field. However, the red shift is used to determine if stars are approaching or leaving an observer. That would imply that photons lose energy preferentially based upon the observer's position with respect to the star's direction. Is that true?

Answer 1

No, I don't think position matters.

Answer 2

Would there be a reason to expect that the photons emitted from a star which is moving toward an observer would have more/less energy than those emitted from a star which is moving away from an observer? Would it matter if the observer were moving and not the star?

Answer 3

All motion is relative. Whether you want to say that the star is moving or the observer, it depends on your frame of reference. Moving towards yields blue shift. Photon energy does depend on frequency, so yes, there would be more energy from the blue shifted light than from the red-shifted.

Answer 4

Thank you, CliffSedge. Do you know where the extra energy comes from?

Answer 5

What extra energy?
Blue shifted light has more energy than red because there is relative motion towards. This increases the frequency of the incident light.
The 'extra' energy doesn't come from anywhere. Any apparent local increase of energy is balanced by a local decrease of energy somewhere else.

Answer 6

Ok, I understand that the energy difference (blue vs red) is offset by energy difference somewhere else. Obviously, it would be interesting to know more about the "local decrease of energy somewhere else", but there's an even more perplexing question to me. Would it matter that the source was moving toward one observer at half the speed of light and away from the other observer similarly (ignoring that it isn't possible to have such a large mass travel that fast)? In that case, would the energy difference exceed the initial energy of both photons? Somewhere in this I'm certain I've displayed a gaping hole in my understanding. Would you mind helping me out here?

Answer 7

"Would it matter that the source was moving toward one observer at half the speed of light and away from the other observer similarly?"

The only difference is that one observer sees a blue shift and the other observer sees a red shift. Those two essentially cancel out in the sense that from the source's frame of reference, there is no color shift.

"(ignoring that it isn't possible to have such a large mass travel that fast)"

Why is that not possible?

"In that case, would the energy difference exceed the initial energy of both photons?"

No.

"Somewhere in this I'm certain I've displayed a gaping hole in my understanding."

Not so gaping, but yes, a hole. ;-)

Answer 8

Ok, from the source's frame of reference, there is no distinction between the two photons other than that they are moving opposite each other. Their initial energy (wavelength or color) are the same and their velocities are the same. When measured simultaneously, anywhere along their paths, these facts will remain true, with respect to the source's frame of reference. Also, the velocities would be equal for all 3 frames of reference and at any points. I'm not sure how to measure that, but I've been told it is true. Therefore, from your explanation, since the velocity of the photon at the approaching observer's frame of reference is the same as the velocity of the photon at the receding observer's frame of reference then is it the difference in energy between the source's frame of reference and the observer's frame of reference that is being transferred (+ or -) to the detected energy of the photon?