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Concerning Rest Frame Wikipedia states:

For example, in the rest frame of a neutrino particle travelling from the Crab Nebula supernova to Earth the supernova occurred in the 11th Century AD only a short while before the light reached Earth, but in Earth's rest frame the event occurred about 6300 years earlier.

If a neutrino has a rest frame, why can't a photon have one as well?

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2 Answers 2

Neutrinos have mass and travel slightly below light speed, therefore an inertial frame for the neutrino exists, while it doesn't exist for a massless photon which travels exactly at $c$. We don't know the masses of the neutrinos, but neutrino oscillations tell us that the three neutrino families must have a mass difference. For all we know, one of the three neutrinos could still be massless, but it is assumed that all neutrino families have a non-vanishing mass.

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There are 3 types of neutrino: electron neutrino $\nu_e$, muon neutrino $\nu_\mu$ and tau neutrino $\nu_\tau$, here's image that shows masses and other details: click here (Note that Electron neutrino has very low mass $<2.2$ $eV$ but it's not zero) –  Gigi Butbaia May 2 at 18:28
It could still be $0$. Moreover, the flavor eigenstate of the neutrino does not coincide with the mass eigenstate, therefore the mass of an $\nu_e$ is ill defined. To be precise, one'd need to write $\nu_1 < 2.2$ eV, where $1$ denotes the lightest mass eigenstate of the neutrino. –  pfnuesel May 2 at 18:46
The mass limit exhibited by @GigiButbaia is very much obsolete. That limit is from direct observation of beta decay kinematics, but cosmological observation put strict limits on the sum of the masses of the neutrino species that are much less than 1 eV. –  dmckee May 3 at 4:14

Photons travel at exactly light speed, rather than just close to light speed. That means that for photons, the time and space axes collapse to a single axis, reducing the number of dimensions the photon "sees" to one. Unfortunately, a single dimension is not sufficient for a frame of reference for four dimensional space time, so the photon cannot have a rest frame.

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I am confused here :"Unfortunately, a single dimension is not sufficient for a frame of reference for four dimensional space time" . Can you please explain it elaborately? –  user22180 Aug 5 at 13:48
The photon exists in a one dimensional world, so it can't have a four dimensional rest frame. –  Warren Dew Aug 5 at 23:51

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