"Goodby to Reality"

Quote:

This is why many have proposed more complete
theories, which suggest that events are at least partially governed by extra
"hidden variables". Now physicists from Austria claim to have performed an
experiment that rules out a broad class of hidden-variables theories that
focus on realism -- giving the uneasy consequence that reality does not
exist when we are not observing it (Nature 446 871).

Some 40 years ago the physicist John Bell predicted that many
hidden-variables theories would be ruled out if a certain experimental
inequality were violated - known as "Bell's inequality". In his thought
experiment, a source fires entangled pairs of linearly-polarized photons in
opposite directions towards two polarizers, which can be changed in
orientation. Quantum mechanics says that there should be a high correlation
between results at the polarizers because the photons instantaneously
"decide" together which polarization to assume at the moment of measurement,
even though they are separated in space. Hidden variables, however, says
that such instantaneous decisions are not necessary, because the same strong
correlation could be achieved if the photons were somehow informed of the
orientation of the polarizers beforehand.

Bell's trick, therefore, was to decide how to orient the polarizers only
after the photons have left the source. If hidden variables did exist, they
would be unable to know the orientation, and so the results would only be
correlated half of the time. On the other hand, if quantum mechanics was
right, the results would be much more correlated - in other words, Bell's
inequality would be violated.

Many realizations of the thought experiment have indeed verified the
violation of Bell's inequality. These have ruled out all hidden-variables
theories based on joint assumptions of realism, meaning that reality exists
when we are not observing it; and locality, meaning that separated events
cannot influence one another instantaneously. But a violation of Bell's
inequality does not tell specifically which assumption - realism, locality
or both - is discordant with quantum mechanics.

Markus Aspelmeyer, Anton Zeilinger and colleagues from the University of
Vienna, however, have now shown that realism is more of a problem than
locality in the quantum world. They devised an experiment that violates a
different inequality proposed by physicist Anthony Leggett in 2003 that
relies only on realism, and relaxes the reliance on locality. To do this,
rather than taking measurements along just one plane of polarization, the
Austrian team took measurements in additional, perpendicular planes to check
for elliptical polarization.

They found that, just as in the realizations of Bell's thought experiment,
Leggett's inequality is violated - thus stressing the quantum-mechanical
assertion that reality does not exist when we're not observing it. "Our
study shows that 'just' giving up the concept of locality would not be
enough to obtain a more complete description of quantum mechanics,"
Aspelmeyer told Physics Web. "You would also have to give up certain
intuitive features of realism."

However, Alain Aspect, a physicist who performed the first Bell-type
experiment in the 1980s, thinks the team's philosophical conclusions are
subjective. "There are other types of non-local models that are not
addressed by either Leggett's inequalities or the experiment," he said. "But
I rather share the view that such debates, and accompanying experiments such
as those by [the Austrian team], allow us to look deeper into the mysteries
of quantum mechanics."

About the author
Jon Cartwright is a reporter for Physics Web