Three Roads To Quantum Gravity

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THREE ROADS TO QUANTUM GRAVITY
collisions among neutron stars or black holes. The most energetic
of these are subject to a threshold for a similar reason,
because they may interact with a background of diffuse
starlight coming from all the stars in the universe. As in the
case of the cosmic rays, photons have been seen with energies
that exceed that threshold, coming from an object called
Markarian 501.
Thus, all of a sudden, there is a real possibility that quantum
gravity has become an experimental science. This is the
most important thing that could have happened. It means
that experimental relevance, rather than individual taste or
peer pressure, must now become the determining factor for
the correctness of an idea about quantum gravity.
Moreover, in the last several months, a startling implication
of the theory of quantum gravity has emerged. This is
the possibility that the speed of light may depend on the energy
carried by a photon. This effect appears to come about
as a result of the interaction of light with the atomic structure
of space. These effects are tiny and so do not contradict
the fact that so far all observations have concluded that the
speed of light is constant. But for photons that travel very
long distances across the universe, they add up to a significant
effect, which can be observed with current technology.
The effect is very simple. If higher frequency light travels
slightly faster than lower frequency light, then if we observe a
very short burst of light coming from very far away, the higher
energy photons should arrive slightly before those of lower energy.
This could be observed in the gamma ray busts. The effect
has not yet been seen, but if it is indeed there, it may be
observed in experiments planned for the near future.
At first I was completely shocked by this idea. How could
it be right? Relativity, based on the postulate of the constancy
of the speed of light, is the foundation of all our understanding
of space and time.
But as some wiser people explained to me, these new developments
do not necessarily contradict Einstein. The basic
principles enunciated by Einstein, such as the relativity of
motion, may remain true. There still is a universal speed of