Is there another layer of reality beyond quantum physics?
29 November 2001
PHILIP BALL
Albert Einstein never liked some of the counter-intuitive predictions of
quantum theory, arguing instead that there was a further, hidden layer to
reality it failed to describe. But since the 1980s, Einstein's objections
have been largely ruled out.
Now Karl Hess and Walter Philipp of the University of Illinois at
Urbana-Champaign provide evidence that Einstein may have been right to be
sceptical - there may indeed be another set of rules underlying quantum
theory 1.
Quantum theory describes the behaviour of atoms and subatomic particles
and says that their energy is 'quantized': it can be altered only in
discrete jumps. Although Einstein himself made seminal contributions to
quantum theory, he famously disagreed with the Danish physicist Niels Bohr
about how it should be interpreted.
In 1935 Einstein, together with the physicists Boris Podolsky and Nathan
Rosen, concocted a 'thought experiment' in which quantum theory seemed to
permit 'spooky' action at a distance, whereby a measurement made on one
particle instantaneously determines the properties of another particle, no
matter how great the distance between the two particles.
Uncomfortable with this bizarre outcome, Einstein suspected that a still
more fundamental theory underlies quantum mechanics (just as quantum
mechanics underlies the older 'classical mechanics' of Isaac Newton). He
invoked 'hidden variables' - quantities that do away with things like
quantum uncertainty, but which cannot be measured directly. Bohr
disagreed, arguing instead that we simply have to resign ourselves to the
fact that quantum theory is counterintuitive.
There the argument floundered until the 1960s when Irish physicist John
Bell showed that hidden variables could have observable consequences. He
demonstrated that the outcome of the Einstein-Podolsky-Rosen (EPR)
experiment differs if hidden variables do or don't exist.
When it became possible to perform EPR experiments for real in the 1980s,
the results showed that, provided Bell was right, there were no hidden
variables. They seemed to show that Einstein was wrong and Bohr was right.
Hess and Philipp find that EPR experiments don't necessarily rule out
hidden variables at all, and there may, indeed, be another layer to
reality. They argue that Bell overlooked a large class of possible hidden
variables whose behaviour is consistent with the existing experimental
findings.
They find that if hidden variables have properties that change over time,
yet are related to each other, the predictions change. For example, the
hands of a clock in London and a clock in New York circulate periodically
and do not directly influence one another, but nevertheless the different
times shown by each are correlated with one another.
If hidden variables are 'time-correlated' in such a manner, Bell's theory
breaks down. The researchers show that in such a case, the results of EPR
experiments can be explained without needing to invoke the spooky action
at a distance that Einstein considered so unlikely. This does not mean
that hidden variables exist, just that they cannot be completely ruled out.
References
Hess, K. & Philipp, W. Bell's theorem and the problem of decidability
between the views of Einstein and Bohr. Proceedings of the National
Academy of Sciences USA, advanced online publication,
DOI:10.1073/pnas.251525098 (2001).
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