Update 01/01/2004:The paper referred to this in this article can be found here.
Lorentz invariance, the theory that space-time is uniform, places important constraints not only on quantum gravity theory but also on variability of the speed of light. Varying speed of light theories are all the rage. But, we have seen earlier this year hard limits to possible Lorentz invariance violations as I commented on here. Now even harder limits on Lorentz invariance violations and thus limits on the variability of the speed of light have been reported by NASA:
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."
2003 has been a very good year for Einstein.
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