speed of light slowed down?

Question

We all know that light travels at about 300,000kmph in vacuum.

But say light enters a madium like glass, water or sodium..... then the speed of light is slowed down (for example in the Bose-Einstein condensate light has been slowed down to a few mph !). So does this mean photons can still exist at speeds below speed c ?

I have heard of something like the atoms in the medium absorbs the light and then emits it again so thats what slows it down but im a bit unclear about how this happens?

note: a small typo above c should be 300000kmps.

Answer 1

Slowing the speed of light within a medium is a well known fact, and there is no violation of Relativity or any other known physical laws.

This is how light is slowed down:

When light is subject to physical conditions other than a vacuum, the effective velocity of the electromagnetic waves that make up the light can change. The most well-known instance of this is when light is transmitted through a refractive material that has an electric permittivity and a magnetic permeability different from a vacuum. When this occurs the index of refraction can be determined for a material as follows:

n = c / v

where n is the index of refraction, c is the speed of light in a vacuum, and v is the apparent speed of light through the material. As the index of refraction increases, the observed speed of light in the medium decreases. Theoretically, there is no limit to the index of refraction, and it is possible to obtain values for n that approach infinity. When this happens, the speed of light in the material becomes effectively equal to zero, and light has "stopped moving".

Microphysically, in the quantum mechanical view, photons of light are interacting through a lattice arrangement of atoms in a material either through repeating processes of absorption and re-emission, or by many different scattering processes. In the case of absorption and re-emission, there is a finite amount of time for a material to absorb and re-emit a photon, this lag time will cause an effective "slowing" of the observed photon speed, even though when between absorptions and re-emissions the photon is traveling at c. For scattering processes, the photon takes a path that is a longer distance than the metric displacement along its trajectory. As the photon is scattered, it follows a circuitous path which is constantly changing direction but on average is propagating in a straight-line direction. This also has the effect of appearing to slow down the speed of light since the photons are traveling the speed of light along a longer distance.

Now, there are many many mechanisms which light can be slowed down. But usually these ALL involved putting the light within some form of medium, such as Bose-Einstein condensates of rubidium atoms.

So unless you heard that people have slowed down light in the vacuum of space, it's really NO big news.

Answer 2

You have asked a complicated question. But just take glass: it has long been known that glass slows light; all optics are based on this reality.

If the atoms of the medium really absorbed the photons, re-emission -- if it happened at all -- would be randomized more (in direction and frequency) than is observed, I think.

A space probe's path is altered by its trajectory traveling near a planet. It's not that the planet absorbs then emits the space probe. It's that they share the property of having mass, and their gravitational fields interact to alter the trajectory.

Likewise the photon and the particles of the optical medium (e.g. the constituent electrons and protons) share the property of being electromagnetic. In this case the e-m fields interact to slow the photon's trajectory.

c is defined as "the speed of light in vacuum," right? If photons couldn't exist at speeds below c, why do you think they add "in vacuum?"

Answer 3

Check out the first link below. The article describes the effort of two teams at Harvard actually stopping light. They observed that they could "pause" or "freeze" a photon inside a medium (one team did it in a rubidium vapor, the other in a crystal) and could later restart the pulse. I won't recap what the article says, since they describe the experiments better than I can.

It's caused a stir, though I think it's mostly caused a stir because of the implications for the field of "quantum communication" and possible application areas like areas like quantum computing. The implications of this research are especially interesting for quantum computing since the information encoded in photons isn't much use in a computer unless it can be controlled and manipulated, which the light stopping experiments suggest not only can be done but also suggest avenues of research on finding ways to actually do it.

The second link ponders the implications for physics at a more fundamental level. It doesn't necessarily represent a complete overturning of special relativity, but some extensions of the theory might be possible. Particularly of note was that there were was no indication that the speed of light in a vacuum needs to be revised in any substantial way. As for what causes light to slow down in a medium in general, I don't know the answer to this but it looks like the best place to look for an answer is in quantum physics in general, and quantum optics in particular.

Answer 4

yes, surprisingly they have slowed the speed of light........

this is extremely chaotic because it has thrown the entire scientific community into a craze... the one constant we believed we could count on is the speed of light, however we discovered that it can change, and that it quite posssibly has changed and warped throughout the earth's history

Answer 5

FYI, it is 300,000 km per second, not per hour