# Photons and redshift

The energy of a photon is kinetic and like all kinetic energy, is completely observer dependent.

But it’s impossible to have a relative speed differential between a photon and an observer so what causes redshift? Is it caused by the relationship between the observer and the object that emitted the light? If so how can that be, isn’t a photon once it’s been emitted completely divorced from the object that emitted it?

Don’t think of it as “relative motion.” Photon energy is simply something that depends on your reference frame.

But in my reference frame the photon will alway have a velocity of c. If my motion isn’t related to the source of the photon then where does the redshift come from?

It’s the number of times the “wave” passes your position that determines frequency. If you are moving towards the source, you will see more waves/sec, so the frequency and energy will appear higher. Conversely, if something is moving away, you will see fewer waves/sec, and the frequency and energy will appear lower. The speed is the same though.

Yes, a photons enrgy is hv, it’s the frequency that changes not the speed. Think of a phton as an oscillation in an electromagnetic field, a different speed would cause the apparent frequency/wavelength of these oscillations to change.

Be careful, people, this question is a trap.

Let’s say that the only things in the universe are a pencil of light and myself. Since light will always travel towards or away from me at c it would seem there can be only one reference frame, and therefore no redshift or blueshift would be possible.

The only difference between the above and the real universe is the fact that in the real universe there must be an emitter. So it would seem that redshift must be due to my relative motion with respect to the emitter, not with respect to the light itself. And yet I thought all the properties of light were independent of the emitter.

The above post by Ring was actually by me. I used the wrong browser.

No, it’s still possible to have multiple reference frames. Indeed, you can tell because the light would look different from different reference frames. It’s impossible to say which reference frame is “true” or “stationary relative to the light source,” but if you fire a rocket for a few seconds in the same direction as the light beam, you can see the light beam change to a shorter wavelength.

I think you’re still stuck with the conception that speed is related to kinetic energy. In case of photons, the speed is a constant but photons can have different kinetic energy or momentum. If you are moving fast and have a “head-on collision” with a photon, the photon has the same relative speed but more momentum. If the same photon hits in your butt it’ll do so with a have smaller momentum but still the same speed.