91av

ONE of the most troubling features of quantum mechanics, the theory that
describes the world of atoms, is that it seems to allow communication at faster
than light speed, thereby apparently violating another of our most cherished
theories, relativity. Now, however, an American physicist is claiming the effect
is an illusion.

“Experiments which seem to be confirming the effect are in fact confirming
something else,” says Frank Tipler of Tulane University in New Orleans. In place
of instantaneous communication, Tipler favours something seemingly much more
bizarre: “The existence of an infinity of other universes in which all possible
histories are played out.”

Faster-than-light communication, or “non-locality”, is best demonstrated by
the case of two electrons prepared in a state where their combined angular
momentum is zero—the spin of one is pointing “up” and the other “down”.
According to quantum theory, the spins of the electrons making up this
“entangled pair” do not initially have definite directions. It is only when one
is measured that it adopts a definite direction: its “wave function” is said to
“collapse” into a particular value. Because the wave functions of an entangled
pair are connected, the collapse of one prompts an instantaneous collapse in the
other.

However, Tipler points out that a collapsing wave function is not a part of
quantum theory but a classical add-on. A theory that treated observers as
quantum objects would not require collapsing wave functions. He claims the only
framework that treats everything as quantum is the “many worlds” interpretation,
formulated in 1957 by American physicist Hugh Everett.

Tipler has shown that if observers far apart fortuitously measure the spins
of the two electrons of an entangled pair and get an identical result—such
as up-up—the many worlds framework ensures that both observers split into
two universes: one in which the two observers measure up-down and one in which
they get down-up.

In reality, researchers look at large numbers of pairs of particles in search
of a statistical correlation between all the particles at two locations.
According to a mathematical theory known as Bell’s inequalities, a local system
produces a different correlation from a nonlocal one. Experiments with pairs of
photons have supported the nonlocal hypothesis.

But Tipler says these experiments are misleading. He says that if you only
make local measurements of spins and then compare them without faster-than-light
signalling to “confirm the split into two worlds”, then you get the same
correlations seen in the photon pair experiments.

Max Tegmark of the University of Pennsylvania is ambivalent about these
assertions. “I think I agree with Tipler’s claims, but don’t see anything really
new in them,” he says. But John Cramer of the University of Washington in
Seattle sees a problem with Tipler’s third measurement—the communication
and comparison of the spins. “The problem is there is no `first’ comparison
measurement that can uniquely join the split ends of Tipler’s multiple
universes,” he says.