Wobegon. Axial spin is another example of conjugate variables—it is im-
possible to simultaneously determine the spin of a photon around more
than one axis. This parallels the situation in Lower Wobegon as well—
each pollster only asked a single question.
Bell devised this as a thought experiment to test whether the hypothesis
that the photons had a hidden program imprinted on both, as suggested
by Einstein, Podolsky, and Rosen, was valid. If such were the case, the
two photons would have the same spin more than^5 / 9 of the time. Within
a few years, thousands of trials of Bell’s experiment were performed—
and the detectors did not record the same direction of spin for the photons more
than half the time. This constituted undeniable proof that there was no
hidden program with which the photons were imprinted.
Was there another possible solution? With the hidden variables explana-
tion ruled out, the next most likely possibility was that somehow the pho-
tons could signal each other. The instant the first photon’s spin was
recorded, it sent a message to the other photon along the lines of “Some-
one just measured my spin around axis 1 and I spun to the left.”
The theory of relativity places no restrictions on the existence of signal-
ing mechanisms, but it does require that no signals can be sent faster
than the speed of light. By the early 1980s, improvements in technology
enabled a more sophisticated version of the above experiment to be per-
formed. In this experiment, the detector equipment for the two photons
were placed a significant distance apart, and a randomizing device in-
stalled that would select the axis for the second photon after the spin of
the first photon had already been measured. What made this experiment
so interesting was that a new wrinkle had been added: the technology
was now so good that the axis for the second photon could be selected in
a shorter period of time than it would take a light beam to go from the
first detector to the second. The first photon to be measured could there-
fore send a signal to the other photon, but it could not be received in time
for the second photon to act upon it—the spin of the second photon was
measured before a signal traveling at the speed of light from the first pho-
ton could reach it. The results of this experiment were obtained in a labo-
ratory by Alain Aspect^11 in 1982 with a detector separation distance
measured in the meters. The separation distance was increased to eleven
kilometers in the late 1990s, but the results were still the same. If the same
axis were chosen, the photons always spun in the same direction—but
they spun in the same direction no more than half the time.
This is one of the great mysteries of quantum mechanics, unresolved
more than a century after Max Planck first hatched the idea of the quan-
tum. Although special relativity forbids either matter, energy, or informa-
62 How Math Explains the World