Theoretically, Photons don't have rest mass but they behave as material particles in many cases.
But is it possible to stop a photon without destroying it ie. Can we slow it down so much to make it almost at rest and if not Why?
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$\begingroup$ @johnrennie They are different and just look apparently same $\endgroup$– Abhishek ChoudharyCommented May 27, 2018 at 10:02
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2$\begingroup$ They look like duplicates to me. If you can clarify exactly what you're asking and why the linked question doesn't answer it then I'd be happy to reopen. Note that if you're asking about the photon speed in vacuum then that is always $c$ for all observers. Photon speed only varies in a medium. $\endgroup$– John RennieCommented May 27, 2018 at 10:12
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$\begingroup$ @JohnRennie I am asking why they can't be stopped is there any law violation and the question is genuine and the other one is not the same $\endgroup$– Abhishek ChoudharyCommented May 27, 2018 at 10:17
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$\begingroup$ Are you asking why the speed of photons is always $c$ in a vacuum? As the answers to the other question show photons can be stopped in a medium. In that case it would be a duplicate of Why and how is the speed of light in vacuum constant, i.e., independent of reference frame? $\endgroup$– John RennieCommented May 27, 2018 at 10:27
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$\begingroup$ @johnrennie physics.stackexchange.com/q/394116 This is also a duplicate why don't you bother about this $\endgroup$– Abhishek ChoudharyCommented May 27, 2018 at 10:29
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1 Answer
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No, we cannot make photons travel at any value other than the speed of light. It is a postulate of the theory of relativity and it works because solutions to the equations of electromagnetism indicate that light/photons must travel at a constant speed regardless of observer. A way to think of it is like, because photons have zero rest mass that is why they can travel at the only other speed available that is less than the speed of light, that is: light speed.
One can presumably slow down light in optically denser media but in reality the light does travel at $c$ while bouncing and interacting with atoms in the medium so it only seems that light is slowed down by refraction.
answered May 27, 2018 at 9:56
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$\begingroup$ So photons don't slow in a medium do they keep colliding which makes them apparently slow? $\endgroup$ Commented May 27, 2018 at 9:57
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2$\begingroup$ Yes, indeed. In between interactions of absorption and re-emissions the photons always travel at $c$ but it's the extra time due to these interactions that makes it seem that light is slowed down. $\endgroup$ Commented May 27, 2018 at 10:01