why is the 'rest mass' of any photon=0

Because photons are massless. They do not possess any inherent mass.

They have to be, to move at c. That's a known because of relativity.

... and according to the theory of evolution, photons that moved slower than the speed of light became extinct.

Is there any reason why photons should have no rest mass and move at c, instead of having a tiny one and move imperceptibly slower? I ignore it.

 

Exactly the same question remained open long enough for the neutrino.

 

Please don't answer "because of equations" or "because of model". If they're massive, adjust said models a little bit and keep c for the asymptotic speed.

 

What is well observed is that photons from very remote sources arrive at the same time, so their energy doesn't influence perceptibly the propagation speed. But this holds for neutrinos as well...

 

ngosh@india, could you choose more detailed thread titles?

A better answer would be "because that's what we've observed."

In what kind of observation, please?

 

What is the experiment that makes a difference between no rest mass and a mass too small to distinguish?

 

For the neutrino, the doubt lasted for very long. What should make the photon different? Is there some reason that necessitates a photon without a rest mass, say for the consistency of a theory, or is it just the usual photon's model within the theory?

 

For the neutrino, "no rest mass" and "too small to observe" was not the same to people's taste, and after several decades efforts, an answer exists.

sorry,mr.enthalpy,i will try to choose more detailed title next time.

In what kind of observation, please?

 

What is the experiment that makes a difference between no rest mass and a mass too small to distinguish?

 

For the neutrino, the doubt lasted for very long. What should make the photon different? Is there some reason that necessitates a photon without a rest mass, say for the consistency of a theory, or is it just the usual photon's model within the theory?

 

For the neutrino, "no rest mass" and "too small to observe" was not the same to people's taste, and after several decades efforts, an answer exists.

 

Neutrino masses are predicted to be of the order of about ~1 eV, with the lowest possible value being ~0.05 eV. The current upper limit on the photon mass is of the order ~10^-17 eV.