Einstein wins again - for now

Einstein's theory of special relativity stands as one of the pillars of modern physics. That's why physicists - ever wary of the foundation - keep looking for cracks.

Some thought they had found one with "fast-light" materials. When light pulses travel through the materials, they seem to travel faster than light travels in a vacuum. In some tests, the leading edge of the pulse appeared to exit even before it had entered. That would violate a key postulate of relativity.

It turns out fast-light concerns were unfounded, new experiments show. But poking at relativity theory, nearly 100 years old, remains a profound quest. Einstein's theory says the vacuum speed of light, designated by "C," is an absolute limit that nothing meaningful can exceed. In the fast-light debate, the key word is "meaningful." Fast-light pulses travel measurably quicker than C but carry no information and aren't meaningful in themselves. If they did carry information, skeptics said, they would travel at speeds slower than C.

So three physicists added information to see what would happen. They explained in the journal Nature how they impressed distinctive discontinuities (bumps) on otherwise smooth fast-light pulses. One bump represented number 1. The other represented 0. These are the ones and zeros of binary arithmetic that encode computer data.

When experimenters shot encoded light pulses through a fast-light medium, in this case potassium vapor, the ones and zeros traveled at speeds slightly less than C.

Voila! Einstein wins again.

The supposed crack in relativity theory turns out to be a kind of optical illusion. Light passes through fast-light materials in ways that can make it appear that the leading edge of a pulse exits an experiment before it has entered. That would be an effect preceding its cause.

But a light pulse without information can't cause any effect. When data are detected, it is found to travel more slowly than C. Causality and standard relativity theory is preserved. At least for now.

The challenge is that scientists remain unable to marry relativity theory, gravity, and quantum theory in one overarching scheme. Some physicists think relativity will break down when dealing with space and time on the smallest scale. Scientists increasingly are testing special relativity's most subtle aspects.

For example, standard relativity conceives of space and time as a continuum. But if, instead, they operate like discrete and infinitesimally small granules - imagine a microscopic space-time foam - then relativity wouldn't stand up.

University of Maryland researchers looked for evidence in gamma rays coming from the Crab Nebula. If the foaminess exists, they said, the energetic electrons that generate the rays would be limited to a top speed significantly less than the universal limit C. They weren't. Gamma ray energies showed the electrons reached speeds just under C. So far, special relativity remains intact.

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