# Rest frame of a photon question

#### timmdeeg

Gold Member
I've read that in relativity the concept of the rest frame of a photon doesn't make sense. Why is that?
Consider a mass in its restframe relative to which another mass can have an arbitrary velocity $v < c$.

Now consider a photon in its hypothetical restframe. Can a mass have an arbitrary velocity $v < c$ relative to the photon?

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#### PeterDonis

Mentor
Now consider a photon in its hypothetical restframe.
There isn't one. That's the whole point of the FAQ.

#### Mister T

Gold Member
Now consider a photon in its hypothetical restframe.
There is no rest frame for a photon, hypothetical or otherwise.

It is possible to conjecture a scenario where photons have rest frames, but such a conjecture would be science fiction, not science. And it definitely would not be an allowed topic for discussion on PF. Thank goodness,

#### sweet springs

I think of another scenario to deny rest frame of a photon. The rest frame is defined as momentum of system, say a photon, is zero there. By the relation E=pc energy is also zero. A photon vanishes in its rest frame. When we move against this photon vacuum at the rest frame, photon would get momentum and energy, and revive. But we have not observed such generated photons from vacuum by Lorentz transformation.

As a digression of the question a photon may be not massless perfectly but has a tiny tiny mass following the line as once thought massless neutrionos are now believed to have mass. Neutrino and photon with mass have rest frame. Even in this digression massless particle, if it exists, has no rest frame.

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#### timmdeeg

Gold Member
There isn't one. That's the whole point of the FAQ.
Sure. To recognize this by himself I was hoping the question "Can a mass have an arbitrary velocity $v < c$ relative to the photon? " should be helpful.

#### PeterDonis

Mentor
To recognize this by himself I was hoping the question "Can a mass have an arbitrary velocity v<cv < c relative to the photon? " should be helpful.
But you posted in the original FAQ thread, not the thread where I moved his post. So I thought you were responding to the original FAQ.

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