The speed of light, one of the constants scientists rely on to study the universe, appears to have held its ground under some tight NASA scrutiny despite other theories that it may slow as it moves through space.

By comparing gamma ray observations of two nearby galaxies, a NASA researcher found evidence that the speed of light is still traveling as fast as it ever has. The finding reinforces the relevance of Albert Einstein's special theory of relativity, which depends on a constant speed of light as the maximum speed attainable by any object.

"Special relativity is, of course, one of the backbones of modern physics," said theoretical physicist Floyd Stecker, of NASA's Goddard Space Flight Center in Greenbelt, Maryland, who conducted the study. "It's affirmation of violation, even in small amounts, would be fundamentally significant."

Stecker studied two galaxies, Markarian (Mkn) 421 and Mkn 501, with supermassive black holes at their cores, each sitting about a half a billion light-years away. The black holes in both galaxies spew intense gamma ray photons at Earth that can be seen colliding with infrared photons and annihilating one another.

If the particles were moving slower than the accepted speed of light - 186,000 miles (300,000 kilometers) per second - they wouldn't have enough energy to annihilate each other. The research is based in part on his past collaboration with Sheldon Glashow of Boston University and appears in the November issue of the journal Astroparticle Physics.

The constancy of the speed of light depends on the uncertainty principle and what is known as the Lorentz invariance, a fundamental principle of Einstein's special theory of relativity that states the laws of physics - including the speed of light - are same everywhere. The uncertainty principle, part of quantum physics, allows for virtual particles called quantum fluctuations to pop in and out of existence at the subatomic level, creating what some researcher believe is a "quantum foam" that makes up the fabric of spacetime.

Some researchers believe that quantum foam slows passing light in much the same way as air or water does, since light travels fastest in a vacuum. The foam would slow higher-energy particles, like X rays or gamma rays, more than lower ones like visible photons of light or radio waves. This variable in light speed would contradict the Lorentz invariance and deflate the foundation of Einstein's special theory of relativity.

It would also allow researchers more freedom use extra dimensions of space, whose existence also violate Einstein's special theory, in their pursuit to find a unifying "theory of everything" to describe the entire universe. But Stecker's research puts a damper on the search for extra dimensions.

"It rules out some of the suggested models that explained gravity by involving added dimensions," Stecker said. The findings, however, don't preclude the existence of those added dimensions, just lowers their energy levels down a bit, he added.

In fact, Stecker said it's possible that Lorentz invariance violations do occur in the universe, but at levels so tiny - less than one part in a thousand trillion - that they can't be detected by today's technology.