Can photon be at rest or does it have to travel at the speed of light?

!Jon Snow!
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Can you explain please!


thank you.
 
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Light is defined to travel at c in any Inertial Reference Frame (IRF). Therefore it cannot be at rest. Unlike other particles that travel at less than c or that are at rest in one IRF, when you use the Lorentz Transformation process to see what the speeds of those particles are in another IRF traveling at any speed short of c with respect to the first IRF, photons continue to travel at c while slower particles can end up traveling at other speeds including going in the other direction or stopping altogether. You can't stop a photon except by ending its existence.
 
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I was thinking of saying something like this, but I was concerned that we might get photons and "light" mixed up. When we say the speed of light in a vacuum is c, we're saying the group velocity of the EM wave in vacuum is c. This is not trivially related to the speed of a photon.
 
!Jon Snow! said:
I was thinking of saying something like this, but I was concerned that we might get photons and "light" mixed up. When we say the speed of light in a vacuum is c, we're saying the group velocity of the EM wave in vacuum is c. This is not trivially related to the speed of a photon.

"Photon" is one of the more misunderstood words floating around...

It sounds as if you're thinking that a photon is a "particle of light" and that a beam of light consists of a stream of photons flowing by, rather like the way that water coming out of a firehose is a stream of water molecules flowing by. That picture is hopelessly misleading, as a photon is not a particle in the sense that a bullet or a grain of sand would be.

Because relativity is not a quantum mechanical theory, it has no photons and light is electromagnetic waves as described by the classical electromagnetic theory of Maxwell. Photons only come into the picture when you consider the quantum mechanical behavior in which a light wave, no matter how spread out in space it is, delivers energy to a single points when it interacts with matter. Thus, when someone says "this is where the photon is", they are being careless with the English language; it would be more accurate to say "right here is where the light wave delivered a packet of energy". The photon doesn't even have a position, let alone a velocity, except at the moment that it is interacting with matter.
 
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Massless particles must travel at c. A 'stationary' massless particle is not an option in any reference frame.
 
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