system
Given the following post, respond as if you're commenting on Reddit:
human
Can light, be it wave or particle, ever be motionless? or do they have to have some speed at all times?
gpt
A full and precise explanation would require more math than is appropriate here.  However, the short answer is no.  A photon (which is massless) cannot stand still; it must always travel at c as a consequence of relativity.  In fact, the existence of a massless particle at all is a consequence of relativity, so it should be no surprise that we need relativity to deal with it.

However, this discussion really only holds in vacuum.  In matter, we can have many photons that interfere with each other in such a way that it *appears* that light is moving slower than c, an effect known as index of refraction.  The index of refraction of a material can be (in principal) arbitrarily large, so light in the material can travel arbitrarily slowly.

Moreover, if you take two mirrors and put a photon between them, then the photon bounces back and forth.  The effect is a standing wave which does not propagate through space.  You can sort of think of this as a photon standing still, but probably a better way of thinking about it is as a superposition of photons traveling in opposite directions.  It's worth noting that this phenomenon is as exotic as it sounds: it happens in a microwave, water waves in a bathtub, etc.