Scientists say that Albert Einstein's principle of the constancy of the speed of light holds up under extremely tight scrutiny, a finding that rules out certain theories predicting extra dimensions and a "frothy" fabric of space.The finding also demonstrates that basic ground- and space-based observations of the highest-energy gamma-rays, a form of electromagnetic energy like light, can provide insight into the very nature of time, matter, energy and space at scales extremely far below the subatomic level -- something that few scientists thought possible.Dr. Floyd Stecker of NASA's Goddard Space Flight Center in Greenbelt, Md., discusses the implications of these findings in a recent issue of Astroparticle Physics. His work is based partly on an earlier collaboration with Nobel laureate Sheldon Glashow of Boston University."What Einstein worked out with pencil and paper nearly a century ago continues to hold up to scientific scrutiny," said Stecker. "High-energy observations of cosmic gamma rays don't rule out the possibility of extra dimensions and the concept of quantum gravity, but they do place some strict constraints on how scientists can go about finding such phenomena."

Einstein stated that space and time were actually two aspects of a single entity called spacetime, a four-dimensional concept. This is the foundation to his theories of special and general relativity. For example, general relativity posits that the force of gravity is the result of mass distorting spacetime, like a bowling ball on a mattress.

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