杏吧原创

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.

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

Faster-than-light communication, or 鈥渘on-locality鈥�, is best demonstrated by
the case of two electrons prepared in a state where their combined angular
momentum is zero鈥攖he spin of one is pointing 鈥渦p鈥� and the other 鈥渄own鈥�.
According to quantum theory, the spins of the electrons making up this
鈥渆ntangled pair鈥� do not initially have definite directions. It is only when one
is measured that it adopts a definite direction: its 鈥渨ave function鈥� is said to
鈥渃ollapse鈥� 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 鈥渕any worlds鈥� 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鈥攕uch
as up-up鈥攖he 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鈥檚 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 鈥渃onfirm the split into two worlds鈥�, then you get the same
correlations seen in the photon pair experiments.

Max Tegmark of the University of Pennsylvania is ambivalent about these
assertions. 鈥淚 think I agree with Tipler鈥檚 claims, but don鈥檛 see anything really
new in them,鈥� he says. But John Cramer of the University of Washington in
Seattle sees a problem with Tipler鈥檚 third measurement鈥攖he communication
and comparison of the spins. 鈥淭he problem is there is no `first鈥� comparison
measurement that can uniquely join the split ends of Tipler鈥檚 multiple
universes,鈥� he says.