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?


marked as duplicate by John Rennie quantum-mechanics May 27 '18 at 9:59

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • $\begingroup$ @johnrennie They are different and just look apparently same $\endgroup$ – Abhishek May 27 '18 at 10:02
  • 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 Rennie May 27 '18 at 10:12
  • $\begingroup$ @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 May 27 '18 at 10:17
  • $\begingroup$ 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 Rennie May 27 '18 at 10:27
  • $\begingroup$ @johnrennie physics.stackexchange.com/q/394116 This is also a duplicate why don't you bother about this $\endgroup$ – Abhishek May 27 '18 at 10:29

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.

  • $\begingroup$ So photons don't slow in a medium do they keep colliding which makes them apparently slow? $\endgroup$ – Abhishek May 27 '18 at 9:57
  • 1
    $\begingroup$ 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. $\endgroup$ – Tausif Hossain May 27 '18 at 10:01

Not the answer you're looking for? Browse other questions tagged or ask your own question.