# Faster than light in a medium [duplicate]

It is a remarkable well-stablished fact that the light velocity in vacuum, $c \simeq 3.10^8 m/s$ is the upper limit for the velocity of ordinary matter, never to be achieved.

It is also well-stablished that in different media, the light velocity 'varies': in a higher density medium, for propagating from point A to B, light passes trough electrons and multiple particles, being absorbed and re-emited by some of these particles, than 'slowing down' the process in average, being the net velocity smaller than $c$.

My question is: would there exist known 'materials' (particles or of any kind) to propagate faster than light in such (any) kind of medium (in which light velocity is less than $c$)? It is not against GR and SR, as long as this particles do not exceed $c$.

This is a similar question to this one,

Could a human beat light in a footrace?

but not considering a man as subject (any material), and mirrors.

This point out to an 'intuitive view' of mine, that in terms of interaction of such particle/materials with the medium it should be considered 'lighter' than light (the interaction)...

## marked as duplicate by John Rennie, Kyle Kanos, Mo_, Cosmas Zachos, Chris♦Jan 21 '18 at 22:09

• – Cosmas Zachos Jan 12 '18 at 19:46
• It's another question, although part of the answers given to that question apply to mine. Anyway, I think the discussion here does not apply there. – Rodrigo Fontana Jan 13 '18 at 2:45

Yes, and when it happens to charged particles in a dielectric medium you see Cherenkov Radiation.

Intuitively, I think of it this way. Normal materials are held together by the electromagnetic field, which propagates at the speed of light. In a material, this speed might be lower than $c$ for the reasons you specified. However, if you have a particle which interacts "weakly" with the electromagnetic field, the field will not slow the particle down substantially, and it can travel faster than light in the medium.

So this can happen when electrons move through various kinds of media, and produce Cherenkov radiation, but also particles with weak interactions can exceed the speed of light in a medium, like safesphere suggested (although you don't see the Cherenkov radiation in that case).

EDIT: I see you wanted specific examples. This actually happens all the time, but one source of extremely high-energy particle is from cosmic rays, striking the atmosphere. These break the local speed of light, emit Chernenkov radiation, and can be detected via things like the Auger observatory.

• Good answer, but you need to be careful with phrasing on this forum. Cherenkov Radiation is produced by particles interacting electromagnetically (not "weakly"). For example, neutrinos do not produce this radiation. Your use of the term "weakly" may lead to a confusion, because neutrinos do interact weakly, but not with the gelectrimagnetic field. Furthermore, the "electromagnetic field" is a classical concept that does not translate well into the quantum world. So speaking of an interaction of a quantum particle with a classical field also may lead to a confusion, unless limits are clarified. – safesphere Jan 12 '18 at 19:12
• I've changed the wording, but since I modified my answer with "intuitively", I think I'm safe from an overly detailed critique (which is contextual "on this forum"). For instance, you could easily replace "electromagnetic field" with "quantum electromagnetic field" in my answer and it would be just as correct. – levitopher Jan 12 '18 at 19:47

Yes, for example, solar neutrinos pass in the glass of your windows much faster than light does, but not faster than the speed of light in vacuum.

• Comments are not for extended discussion; this conversation has been moved to chat. If you havre information you want to preserve in your answer, please edit it in instead of using comments. – ACuriousMind Jan 13 '18 at 10:11