Quantum theory is a scientific masterpiece - but physicists
still aren't sure what to make of it
It is a question physicists are still wrestling with today.
Quantum particles such as atoms and molecules have an uncanny ability to appear
in two places at once, spin clockwise and anticlockwise at the same time, or
instantaneously influence each other when they are half a universe apart. The
thing is, we are made of atoms and molecules, and we can't do any of that. Why?
"At what point does quantum mechanics cease to apply?" asks Harvey
Brown, a philosopher of science at the University of Oxford.
Although an answer has yet to emerge, the struggle to come up
with one is proving to be its own reward. It has, for instance, given birth to
the new field of quantum information that has gained the attention of high-tech
industries and government spies. It is giving us a new angle of attack on the
problem of finding the ultimate theory of physics, and it might even tell us
where the universe came from. Not bad for a pursuit that a quantum cynic - one
Albert Einstein - dismissed as a "gentle pillow" that lulls good
physicists to sleep.
Unfortunately for Einstein quantum theory has turned out to be a
masterpiece. No experiment has ever disagreed with its predictions, and we can
be confident that it is a good way to describe how the universe works on the
smallest scales. Which leaves us with only one problem: what does it mean?
Physicists try to answer this with "interpretations" -
philosophical speculations, fully compliant with experiments, of what lies
beneath quantum theory. "There is a zoo of interpretations," says
Vlatko Vedral, who divides his time between the University of Oxford and the
Centre for Quantum Technologies in Singapore.
No other theory in science has so many different ways of looking
at it. How so? And will any one win out over the others?
Take what is now known as the Copenhagen interpretation, for
example, introduced by the Danish physicist Niels Bohr. It says that any
attempt to talk about an electron's location within an atom, for instance, is
meaningless without making a measurement of it. Only when we interact with an
electron by trying to observe it with a non-quantum, or "classical",
device does it take on any attribute that we would call a physical property and
therefore become part of reality.
Then there is the "many worlds" interpretation, where
quantum strangeness is explained by everything having multiple existences in
myriad parallel universes. Or you might prefer the de Broglie-Bohm
interpretation, where quantum theory is considered incomplete: we are lacking
some hidden properties that, if we knew them, would make sense of everything.
There are plenty more, such as the Ghirardi-Rimini-Weber
interpretation, the transactional interpretation (which has particles
travelling backwards in time), Roger Penrose's gravity-induced collapse
interpretation, the modal interpretation... in the last 100 years, the quantum
zoo has become a crowded and noisy place (see diagram).
For all the hustle and bustle, though, there are only a few
interpretations that seem to matter to most physicists.
Wonderful Copenhagen
The most popular of all is Bohr's Copenhagen interpretation. Its
popularity is largely due to the fact that physicists don't, by and large, want
to trouble themselves with philosophy. Questions over what, exactly,
constitutes a measurement, or why it might induce a change in the fabric of
reality, can be ignored in favour of simply getting a useful answer from
quantum theory.
That is why unquestioning use of the Copenhagen interpretation
is sometimes known as the "shut up and calculate" interpretation.
"Given that most physicists just want to do calculations and apply their
results, the majority of them are in the shut up and calculate group,"
Vedral says.
This approach has a couple of downsides, though. First, it is
never going to teach us anything about the fundamental nature of reality. That
requires a willingness to look for places where quantum theory might fail,
rather than where it succeeds (New Scientist, 26 June 2010, p 34). "If
there is going to be some new theory, I don't think it's going to come from
solid state physics, where the majority of physicists work," says Vedral.
Second, working in a self-imposed box also means that new
applications of quantum theory are unlikely to emerge. The many perspectives we
can take on quantum mechanics can be the catalyst for new ideas. "If
you're solving different problems, it's useful to be able to think in terms of
different interpretations," Vedral says.
Nowhere is this more evident than in the field of quantum
information. "This field wouldn't even exist if people hadn't worried
about the foundations of quantum physics," says Anton Zeilinger of the
University of Vienna in Austria.
At the heart of this field is the phenomenon of entanglement,
where the information about the properties of a set of quantum particles
becomes shared between all of them. The result is what Einstein famously termed
"spooky action at a distance": measuring a property of one particle
will instantaneously affect the properties of its entangled partners, no matter
how far apart they are.
When first spotted in the equations of quantum theory,
entanglement seemed such a weird idea that the Irish physicist John Bell
created a thought experiment to show that it couldn't possibly manifest itself
in the real world. When it became possible to do the experiment, it proved Bell
wrong and told physicists a great deal about the subtleties of quantum
measurement. It also created the foundations of quantum computing, where a
single measurement could give you the answer to thousands, perhaps millions, of
calculations done in parallel by quantum particles, and quantum cryptography,
which protects information by exploiting the very nature of quantum
measurement.
Both of these technologies have, understandably, attracted the
attention of governments and industry keen to possess the best technologies -
and to prevent them falling into the wrong hands.
Physicists, however, are actually more interested in what these
phenomena tell us about the nature of reality. One implication of quantum
information experiments seems to be that information held in quantum particles
lies at the root of reality.
Adherents of the Copenhagen interpretation, such as Zeilinger,
see quantum systems as carriers of information, and measurement using classical
apparatus as nothing special: it's just a way of registering change in the
information content of the system. "Measurement updates the
information," Zeilinger says. This new focus on information as a
fundamental component of reality has also led some to suggest that the universe
itself is a vast quantum computer.
However, for all the strides taken as a result of the Copenhagen
interpretation, there are plenty of physicists who would like to see the back
of it. That is largely because it requires what seems like an artificial
distinction between tiny quantum systems and the classical apparatus or
observers that perform the measurement on them.
Vedral, for instance, has been probing the role of quantum
mechanics in biology: various processes and mechanisms in the cell are quantum
at heart, as are photosynthesis and radiation-sensing systems (New Scientist,
27 November, p 42). "We are discovering that more and more of the world
can be described quantum mechanically - I don't think there is a hard boundary
between quantum and classical," he says.
Considering the nature of things on the scale of the universe
has also provided Copenhagen's critics with ammunition. If the process of
measurement by a classical observer is fundamental to creating the reality we
observe, what performed the observations that brought the contents of the
universe into existence? "You really need to have an observer outside the
system to make sense - but there's nothing outside the universe by
definition," says Brown.
That's why, Brown says, cosmologists now tend to be more
sympathetic to an interpretation created in the late 1950s by Princeton
University physicist Hugh Everett. His "many worlds" interpretation
of quantum mechanics says that reality is not bound to a concept of
measurement.
Instead, the myriad different possibilities inherent in a
quantum system each manifest in their own universe. David Deutsch, a physicist
at the University of Oxford and the person who drew up the blueprint for the
first quantum computer, says he can now only think of the computer's operation
in terms of these multiple universes. To him, no other interpretation makes sense.
Not that many worlds is without its critics - far from it. Tim
Maudlin, a philosopher of science based at Rutgers University in New Jersey,
applauds its attempt to demote measurement from the status of a special
process. At the same time, though, he is not convinced that many worlds
provides a good framework for explaining why some quantum outcomes are more
probable than others.
When quantum theory predicts that one outcome of a measurement
is 10 times more probable than another, repeated experiments have always borne
that out. According to Maudlin, many worlds says all possible outcomes will
occur, given the multiplicity of worlds, but doesn't explain why observers
still see the most probable outcome. "There's a very deep problem
here," he says.
Deutsch says these issues have been resolved in the last year or
so. "The way that Everett dealt with probabilities was deficient, but over
the years many-worlds theorists have been picking away at this - and we have
solved it," he says.
However Deutsch's argument is abstruse and his claim has yet to
convince everyone. Even more difficult to answer is what proponents of many
worlds call the "incredulous stare objection". The obvious
implication of many worlds is that there are multiple copies of you, for
instance - and that Elvis is still performing in Vegas in another universe. Few
people can stomach this idea.
Persistence will be the only solution here, Brown reckons.
"There is a widespread reluctance to accept the multiplicity of yourself
and others," he says. "But it's just a question of getting used to
it."
Deutsch thinks this will happen when technology starts to use
the quantum world's stranger sides. Once we have quantum computers that perform
tasks by being in many states at the same time, we will not be able to think of
these worlds as anything other than physically real. "It will be very
difficult to maintain the idea that this is just a manner of speaking,"
Deutsch says.
He and Brown both claim that many worlds is already gaining
traction among cosmologists. Arguments from string theory, cosmology and
observational astronomy have led some cosmologists to suggest we live in one of
many universes. Last year, Anthony Aguirre of the University of California,
Santa Cruz, Max Tegmark of the Massachusetts Institute of Technology, and David
Layzer of Harvard University laid out a scheme that ties together ideas from
cosmology and many worlds (New Scientist, 28 August 2010, p 6).
But many worlds is not the only interpretation laying claim to
cosmologists' attention. In 2008, Anthony Valentini of Imperial College London
suggested that the cosmic microwave background radiation (CMB) that has filled
space since just after the big bang might support the de Broglie-Bohm
interpretation. In this scheme, quantum particles possess as yet undiscovered
properties dubbed hidden variables.
The idea behind this interpretation is that taking these hidden
variables into account would explain the strange behaviours of the quantum
world, which would leave an imprint on detailed maps of the CMB. Valentini says
that hidden variables could provide a closer match with the observed CMB
structure than standard quantum mechanics does.
Though it is a nice idea, as yet there is no conclusive evidence
that he might be onto something. What's more, if something unexpected does turn
up in the CMB, it won't be proof of Valentini's hypothesis, Vedral reckons: any
of the interpretations could claim that the conditions of the early universe
would lead to unexpected results.
"We're stuck in a situation where we probably won't ever be
able to decide experimentally between Everett and de Broglie-Bohm," Brown
admits. But, he adds, that is no reason for pessimism. "I think there has
been significant progress. A lot of people say we can't do anything because of
a lack of a crucial differentiating experiment but it is definitely the case
that some interpretations are better than others."
For now, Brown, Deutsch and Zeilinger are refusing to relinquish
their favourite views of quantum mechanics. Zeilinger is happy, though, that
the debate about what quantum theory means shows no sign of going away.
Vedral agrees. Although he puts himself "in the many worlds
club", which interpretation you choose to follow is largely a matter of
taste, he reckons. "In most of these cases you can't discriminate
experimentally, so you really just have to follow your instincts."
The idea that physicists wander round the quantum zoo, choosing
a favourite creature on a whim might seem rather unscientific, but it hasn't
done us any harm so far.
Quantum theory has transformed the world through its spin-offs -
the transistor and the laser, for example - and there may be more to come.
Having different interpretations to follow gives physicists ideas for doing
experiments in different ways. If history is anything to go by, keeping an open
mind about what quantum theory means might yet open up another new field of
physics, Vedral says. "Now that really would be exciting."
