Scientists say 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 few scientists thought possible.

Dr. Floyd Stecker of NASA's Goddard Space Flight Center in Greenbelt, Md., discussed the implications of these findings in a recent issue of Astroparticle Physics. His work was 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 space-time, 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 space-time, like a bowling ball on a mattress.

General relativity is the theory of gravity on a large scale; quantum mechanics is the theory of the atom and subatomic particles on a very small scale. Quantum mechanics does not describe gravity but rather the other three fundamental forces: electromagnetism (light), strong forces (binding atomic nuclei), and weak forces (seen in radioactivity).

Scientists have long hoped to meld these theories into one "theory of everything" to describe all aspects of nature. These unifying theories -- such as quantum gravity or string theory -- may involve the invocation of extra dimensions of space and also violations of Einstein's special relativity, such as the speed of light being the maximum attainable velocity for all objects.

Stecker's work involves concepts called the uncertainty principle and Lorentz invariance. The uncertainty principle, derived from quantum mechanics, implies that at the subatomic level virtual particles, also called quantum fluctuations, pop in and out of existence. Many scientists say space-time itself is made up of quantum fluctuations which, when viewed up close, resemble a froth or "quantum foam." Some scientists think a quantum foam of space-time can slow the passage of light. The foam would slow higher-energy light particles, or photons -- such as X-rays and gamma-rays -- more than lower- energy photons of visible light or radio waves.

A fundamental variation in light speed would violate Lorentz invariance, the basic principle of special relativity. Such a violation could be a clue to unification theories. Scientists have hoped to find Lorentz invariance violations by studying gamma-rays coming from the farthest reaches of the visible universe, where the quantum foam of space may act to slow light traveling to us for billions of years.

Stecker looked much closer to home to find that Lorentz invariance is not being violated. He analyzed gamma-rays from two relatively nearby galaxies about half a billion light- years away with supermassive black holes at their centers, named Mkn 421 and Mkn 501. Some of these galaxies' gamma-rays collide with infrared photons in the universe. These collisions result in the destruction of the gamma-ray and infrared photons, as their energy is converted into mass in the form of electrons and positively charged antimatter- electrons (called positrons), according to Einstein's famous formula E=mc 2.

Stecker and Glashow have pointed out that evidence of the annihilation of the highest-energy gamma-rays, obtained from direct observations of Mkn 421 and Mkn 501, demonstrates clearly that Lorentz invariance is alive and well and not being violated. If Lorentz invariance were violated, the gamma-rays would pass right through the extragalactic infrared fog with insufficient energy to cause annihilation.

"The implication is if Lorentz invariance is violated, it is at such a small level -- less than one part in a thousand trillion -- it is beyond the ability of our present technology to find," Stecker said. "These results may also be telling us the correct form of string theory or quantum gravity must obey the principle of Lorentz invariance."