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May 1[edit]

Changing the speed of light, when the medium is not a perfect vacuum.[edit]

Is it possible to exert forces, on light moving in a medium that is not a perfect vacuum, for example in water, in order to change the speed of light in that medium?

If it's impossible, then why ? After all, as we know, it is possible to apply forces that slow down - or accelerate - massive bodies, so why is it impossible to apply such forces also to the light when it moves in water etc., so that the forces will make the light move (in water) at a slower speed, for example at the speed at which light moves when it's in glass? Or so that the forces will make the light move (in water) at a faster speed, for example at the speed at which light moves when it's in the air? 2A06:C701:747E:800:D855:A0F7:59BA:AFAB (talk) 18:35, 1 May 2023 (UTC)[reply]

So, the thing about all of the physics involved is that the speed of light is invariable, full stop. That's part of what general relativity says; any "acceleration" supposedly experienced by light (whether changing direction or speeding up/slowing down) is not an acceleration, it's spacetime curvature. Light being "slowed" in a material is due to interactions with the photons with particles of the medium, see Speed of light, to wit "This type of behaviour is generally microscopically true of all transparent media which "slow" the speed of light..." Light still travels in the space between, say, atoms and molecules, at the speed of light, however interactions between photons and the atoms themselves results in the apparent speed of light to slow, photons move in straight lines at the speed of light; however the apparent speed at which light waves move through a medium is slowed due to quantum interactions between the photons and the particles of the medium. When you say "the speed of light in water" what you are saying is "the speed light appears to move in water due to quantum interactions between photons and atoms" or some such, the light itself, insofar as it is actually traveling, still moves at "c". --Jayron 32 18:54, 1 May 2023 (UTC)[reply]

It is not completely true as speed of light in matter can exceed the speed of light in vacuum. Ruslik_Zero 20:13, 1 May 2023 (UTC)[reply]

What? it seems you confuse photons with electrons (in Cherenkov radiation). 2A06:C701:747E:800:D855:A0F7:59BA:AFAB (talk) 20:50, 1 May 2023 (UTC)[reply]

Well, that depends on your definition of "speed" and "light". At the most basic level, the photon can only travel through the quantum field at c. No faster, no slower. What we call light is, yes, photons, but it is also a more complex phenomenon that has several definitions of "velocity", such as phase velocity or group velocity. As a quantum particle (a disturbance in a quantum field), it can only move at the speed of c. --Jayron 32 11:08, 2 May 2023 (UTC)[reply]

Right, if you adopt the "path integral" or "sum-over-histories" interpretation of QM, you can think of photons being absorbed, emitted, and in between always traveling at exactly c. I think this is mostly of philosophical importance in this particular case; you would never actually calculate anything that way.

Alternatively, you can take the view that light in matter is not composed of photons but of polaritons, which do travel at sub-c speeds. --Trovatore (talk) 17:37, 2 May 2023 (UTC)[reply]

Fizeau experiment may be relevant. catslash (talk) 21:34, 1 May 2023 (UTC)[reply]

My point was only that you should be very careful when claiming that "the speed of light is invariable, full stop". It is only true in a sense that there is a fundamental constant called "speed of light in vacuum", which is of course is invariable. As to the real speed of electromagnetic waves in a medium - everything depends on the definition. Ruslik_Zero 20:23, 2 May 2023 (UTC)[reply]

Yes, but the OP is asking about exerting a force on light. What thing are we exerting a force on? A photon? Photons don't accelerate in any meaningful sense, they just move at c in a straight line. --Jayron 32 11:17, 3 May 2023 (UTC)[reply]

The speed of light (viewed as a wave phenomenon) in a medium (or in a vacuum) is set by the electric permittivity and magnetic permeability of the medium. To change the speed of light in water, you have to change those properties. It doesn't work by pushing it (although a change in pressure could change them, so that the index of refraction of ice is different from that of liquid water). You can push stuff which has mass and you could say that wet water has mass, but the matter is complicated as the mass of wet water depends on the energy. This speed of light in a medium, or more exactly the phase velocity of light, can be larger than the speed of light in a vacuum. The group velocity can't be larger than the speed of light in a vacuum.

How would you push light? When you push something to make it move faster, you perform work to add kinetic energy. We can do that with light by hitting it with charged particles; it's called inverse Compton scattering. Not easy to do in a dense medium. It leads to a change of frequency for the light, which may, in a medium, result in a change of velocity. For visible light in water, increasing the energy leads to a lower velocity. PiusImpavidus (talk) 09:33, 2 May 2023 (UTC)[reply]

However, see Superluminal group velocities and single waves wherever v=c carry energy and information. Modocc (talk) 11:54, 2 May 2023 (UTC)[reply]