In my studies, I learned that one of the great debates of the nineteenth
century had been about how light travels through a vacuum.

(Light from the stars, in fact, can effortlessly travel trillions upon trillions
of miles through the vacuum of outer space.) Experiments also showed
beyond question that light is a wave. But if light were a wave, then it
would require something to be "waving." Sound waves require air, water
waves require water, but since there is nothing to wave in a vacuum, we
have a paradox. How can light be a wave if there is nothing to wave? So
physicists conjured up a substance called the aether, which filled the
vacuum and acted as the medium for light. However, experiments conclusively
showed that the "aether" does not exist.


Finally, when I became a graduate student in physics at the University
of California at Berkeley, I learned quite by accident that there was an
alternative, albeit controversial, explanation of how light can travel
through a vacuum. This alternative theory was so outlandish that I
received quite a jolt when I stumbled across it.

We physicists, too, receive quite a shock when we first stumble across Kaluza-Klein theory for the first time. Since the theory was considered to be a wild speculation, it was never taught in graduate school; so young physicists are left to discover it quite by accident in their casual readings.

This alternative theory gave the simplest explanation of light: that it
was really a vibration of the fifth dimension, or what used to called the
fourth dimension by the mystics. If light could travel through a vacuum,
it was because the vacuum itself was vibrating, because the "vacuum"
really existed in four dimensions of space and one of time. By adding
the fifth dimension, the force of gravity and light could be unified in a
startlingly simple way.