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Hello. I am a freelance researcher and fact checker working on a story for Forbes ASAP Magazine. They are working on an upcoming issue that will include a trivia game of sorts. The trivia questions cover a wide range of topics I wonder if you might be able to give me a brief answer to one of them. The question is this:

If light can be slowed down, why can't it be speeded up?

From what I've found, I believe it has something to do with light observed in a vacuum but I'm not able to find the precise answer to the question. I'd like to thank you in advance for any assistance you may be able to provide.

Light can't really be slowed down or speeded up. People talk about the speed of light in a material being less than the speed of light in vacuum, and this view does describe an overall effect in conveniently measurable terms, but it doesn't say anything essential about light. What is happening as light traverses a material is a lot of scattering, absorption and re-emission -- something analogous to pinball.

Tim Mooney

In the strictest sense, light doesn't actually get slowed down. Its AVERAGE speed within a material, such as glass, is less than its speed in a vacuum, but not really because the light slows down. The light gets "delayed" by atoms.

A ray of light is composed of "photons", individual energy bundles that interact with individual particles and individual atoms. A photon of light hits an atom, and gets absorbed. It is not an energy level the atom can hang on to, so it gets released and continues on its way to another atom. The same process happens again. As a result of these tiny delays, the light takes longer to pass through one meter of the material than through one meter of empty space.


Your question is far from trivial. Without getting too theoretical, and painting with a rather broad brush, here's a common-sense answer that may satisfy your supervisor.

Indeed, it appears that the maximum speed of light as quoted is its speed in a vacuum -- about 186,000 miles per second. However, a vacuum is just empty space with nothing there to offer any resistance to light's passage. Think of it as an empty hallway and the maximum speed at which you could run through it. It would be easy to reduce your speed by cluttering the hallway with objects with which you might collide, or around which you must run. Would not that reduce your speed?

Likewise for light. For purposes of this discussion, in empty space (the vacuum) light's maximum speed is 186,000 miles per second. Any medium through which light must pass, absorbs some of the beam, diminishes its intensity, and reduces the rate of passage because of an interaction between the light waves and the medium itself.

Thus, light's speed can be reduced, but not increased to any speed faster than that achieved in a medium offering no resistance to its passage -- namely, the vacuum. There are a few notable exceptions to this generalization. Nevertheless, it is at least in the ball-park


Lets try this for a an answer: Any known medium other than vacuum slows light down. In the absence of any retarding medium, light travels at its maximum possible speed, which is the speed of light in vacuum.

Ali Khounsary

Two tenets of General Relativity are the invariability of the speed of light in a vacuum ( this value is a constant for all observers, in all frames of reference), and the fact that this speed is the highest at which information can travel. It is also true that light can be slowed down from its value in vacuum : by passage through a media having an index of refraction greater than unity. In fact, if you were to look down into a cooling pond, where spent nuclear fuel is stored, you'ld see a blue glow. This is Cherenkov radiation, and it is due to electrons moving faster than of light (in water.).

Jim Rubin
Pit Certification Project Leader
NMT-16 (Nuclear Materials Science)
Nuclear Materials Technology Division
Los Alamos National Laboratory


You ask a simple question that does not have a quite-so-simple answer:

1. The speed of light TRAVELING IN A VACUUM is about 186,000 miles/sec [300,000 km/sec]. That is an experimental observation that has been tested directly or indirectly hundreds, even thousands of times, and not once has the speed of light ever been observed to exceed that speed. Why the speed of light has that particular value, and not some other value, no one knows.

2. Electricity, magnetism, and light are all interrelated aspects of the same phenomenon. The laws of physics, the equations, that describe this interrelation were derived by James Clerk Maxwell in the 19th century, prior to Einstein and his theory of relativity. Maxwell's laws, which are classical physics, also forbid the propogation of electricity, magnetism, or light faster than the speed of light. If that propogation were to be faster than the speed of light in a vacuum, Maxwell's laws would require that infinite forces exist, which they do not. The point is this: while the constancy of the speed of light in a vacuum is required by the theory of relativity, that requirement pre-dates Einstein and relativity.

3. The laws of physics do not forbid light traveling through a transparent material to go slower than its speed in a vacuum. In fact it always does. Recently scientists have even been able to stop light dead in its tracks! This result of very elegant, difficult experiments not only do not violate any laws of physics, but are predicted by the laws of physics, provided the proper experimental conditions are met.

So the short answer is that the speed of light has never been observed to exceed its speed in a vacuum, and if it were observed to do so, all of the laws of both classical theory and modern theory of light would have to be scrapped.

I don't know how this explanation can be honed into a trivia game, but go for it!!

Vince Calder

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