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Name: Michael
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I have heard from what I believe to be a reliable source that the of light is variable. This person quoted the April issue of the New Scientist Magazine of 1995 pages 26-29 among others. I have not checked this, but a similar article can be found here:

Is it possible that Einstein was wrong? and why or why not?

The speed of light in a perfect vacuum is constant, but not the speed of light when it interacts with matter. When light interacts with matter (e.g. it's no longer in a perfect vacuum), the speed of the light can be affected in lots of ways. Look up "index of refraction" to learn more.

Hope this helps,

Burr Zimmerman


When comparing relativity to quantum mechanics, realize that they do NOT agree with each other. Relativity is a good model of objects on a large scale moving extremely fast. Quantum mechanics is a good model of reality at the scale of individual particles and atoms. Physics has not yet developed a tested theory that handles both. Superstring theory is the best we have, but we have not yet figured out how to test it.

On a large scale, where objects are definitely objects and waves are definitely waves, where every position is well defined and an object can be at only one position at a time, relativity seems to work. On a tiny scale, where whether something is a particle or a wave depends on how you measure it, where nothing has a clearly defined position and something can reach its destination before it leaves its original location, relativity does not work. A useful set of three one-hour videos that discuss this 'problem' and are free to watch can be found on the PBS Nova site:

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College

Hi Michael,

Einstein did not claim the speed of light was a constant. The claim is that the speed of light in a vacuum is constant, independent of the observer's frame of reference.

We have known for a long time that the speed of light can be changed. For example, the index of refraction of a medium is the ratio of the speed of light in a vacuum to the speed of light in the medium.

I think it is a good thing that you question the "laws" that we know. The laws that we have today explain the world as we understand it. As we learn more about the world the "laws" -- actually theories -- must be modified to fit the new information. This is a part of the scientific method.

Greg Bradburn

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