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Question:
Why does light go 300,000 km/s and no another velocity?



Replies:
Light does travel at other speeds. It depends upon the media through which the light is traveling. The constancy of the speed of light applies to the speed of light in a vacuum, that is not interacting with other forms of matter.

Vince Calder


Light does go slower than this, when it goes thru something other than empty space. For example it goes about 2/3 this speed in water, which has an index of refraction of about 3/2. So it is actually possible for something to go faster than light, in say, water. But not in free space. This value is the ultimate speed limit.

In the end it is based on the experimental observation that light seems to go this speed and no faster. If you are riding on a train going fast, and shine a light, it will go not at the summed speed of the train and the stationary light, but at the speed of the stationary light. You cannot make light go faster by making the flashlight that emits it go faster, and add their speeds. Experiment says so. Theory takes this as a given, and creates math rules that match this reality, and goes on. But it is amazing how many things pop out, from just this assumption. A good book, written at the level of algebra in high school, and quite clear ---

*It's About Time : Understanding Einstein's Relativity (Hardcover) * by N. David Mermin

Steve Ross


This is a question that we do not really have a fundamental or elementary answer for.

Ultimately the speed of light in vacuum is just what it is and we are none the wiser as to exactly why the value of "c" is 300,000 km/s. We can measure it just fine. We can measure it in different mediums too. And we have got nice theories that explain why the speed of light changes in different mediums from the value in vacuum.

At the end of the day the speed of light is, at least at present time, just a fundamental constant of nature. If someone could actually predict the value of the speed of light from some fundamental theory, that would be a huge discovery!

From modern theories of cosmology all the way to Maxwell's equations of electrodynamics or to special relativity, the speed of light is treated just as an input to the theory (well, in Maxwell's case it is an input from both the inverse-square-root of the permeability and susceptibility of free space).

Now that I have told you that the speed of light just "IS" as far as current theories are concerned, it is possible to speculate about various things. For instance... we know what the value for the speed of light is. We know that it has not changed since man first measured it. In fact, we know that it has been a constant value for a REALLY long time because of the beautiful predictions of cosmology and astrophysics. However, it might be possible that in the very, very early universe, that the speed of light was not equal to its present value. So people that study the early universe have been trying to come up with various predictions as to what would happen if the speed of light changed in the early universe. Can they make a prediction? Even better, can they make a prediction that solves one of the current problems in this kind of physics? Maybe. Maybe not. Make a prediction and compare it to the evidence. That is science! (they do in fact try many things, such as has the charge of the electron changed with time? Has the mass of an electron changed with time? )

Michael S. Pierce
Materials Science Division
Argonne National Laboratory



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