Why quantum mechanics might be wrong?
15 May 2008
Observations of the
cosmic microwave background might deal blow to theory.
The question of whether
quantum mechanics is correct could soon be settled by observing
the sky — and there are already tantalizing hints that the
theory could be wrong.
Antony Valentini, a
physicist at Imperial College, London, wanted to devise a test
that could separate quantum mechanics from one of its closest
rivals — a theory called bohmian mechanics. Despite being one of
the most successful theories of physics, quantum mechanics
creates several paradoxes that still make some physicists
uncomfortable, says Valentini.
background patterns of space could help us focus on quantum
ESA / Hubble Heritage Team
For instance, quantum
theory uses probability to describe the properties of a
particle. These properties obtain definitive values only when
they are measured, which means that you cannot predict a
particle's position or momentum, for instance, with certainty.
These premises troubled
Albert Einstein. He believed that particles contain extra
properties — or 'hidden variables' — that determine their
behaviour completely. If only we knew what these hidden
variables were, we could predict the fate of particles and the
outcome of measurements with certainty. Bohmian mechanics is one
of a suite of 'hidden variables' theories — many now discredited
— formulated to tackle this problem
Neck and neck
So far it’s been impossible to pick apart quantum mechanics from
bohmian mechanics — both predict the same outcomes for
experiments with quantum particles in the lab.
But Valentini thinks that the stalemate could be broken by
analysing the cosmic microwave background — the relic radiation
left behind after the Big Bang. The cosmic microwave background
contains hot and cold temperature spots that were generated by
quantum fluctuations in the early Universe and then amplified
when the Universe expanded.
Using the principles of quantum mechanics, cosmologists have
calculated how these spots should be distributed.
“It’s far too early to
say that this is definite evidence of a breakdown in
quantum mechanics – but it is a possibility.”
Imperial College, London
However, Valentini’s calculations show that the hidden-variables
theory might give a different answer. “Any violation of quantum
mechanics in the early Universe would have a knock-on effect
that we could see today,” says Valentini.
Almost all measurements of the cosmic microwave
background seem to fit well with the predictions of quantum
mechanics, says Valentini. But intriguingly, a distortion that
fits one of Valentini’s proposed signatures for a failure of
quantum mechanics was recently detected by Amit Yadav and Ben
Wandelt at the University of Illinois at Urbana-Champaign (see 'Deflating
inflation?'). That result has
yet to be confirmed by independent analyses, but it is
tantalizing, Valentini adds.
“It’s far too early to say that this is definite evidence of a
breakdown in quantum mechanics — but it is a possibility,” he
Hiranya Peiris, an expert on the cosmic microwave background at
the University of Cambridge, UK, is impressed by the new work.
“This is a pretty cool new idea,” she says. “Nobody has ever
thought of using the cosmic microwave background to look into
really fundamental quantum questions — cosmologists just assume
that quantum mechanics is correct,” she says.
But Peiris adds that Valentini must now come up with more
detailed predictions about the types of distortion that will
arise in the cosmic microwave background to convince
cosmologists that they are really caused by a breakdown of
quantum mechanics. “He has thrown some really exciting ideas out
there, but now he needs to do the nitty-gritty calculations,”
Vlatko Vedral, a quantum physicist at the University of Leeds,
UK, agrees that the cosmic microwave background will be a useful
way to test quantum mechanics. But he adds that even if quantum
mechanics is shown to break down in the early Universe, that
doesn’t necessarily mean that the hidden-variables theory is
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