THIS is a story about God—about His Intentions, and His Limitations.
But it is not about religion. When Sandu Popescu talks about God, he is not
referring to a deity, and there is no thought of spiritual transcendence in what
he says. “God does not play dice,” Albert Einstein once said, expressing
contempt for the notion that randomness might be an inherent part of whatever
spirit, urge or process is behind our Universe. Now Popescu, a physicist at the
University of Cambridge, wants to turn Einstein’s phrase on its head, and to ask
some questions that probe even deeper.
Nearly a century after we first glimpsed the quantum nature of our world, the
details of quantum events remain utterly unpredictable. So we may as well admit
it: from atomic transitions to nuclear decays, the world really does seem to be
random. God really does play dice. What Popescu wants to know is—why? Why
is the Universe quantum mechanical? What’s the point?
Physicists usually ask “how” questions. How do photons and electrons pull off
the quantum trick of being in many places at once? How does measuring them
mysteriously cause them to make up their minds? By transforming “how” into
“why”, Popescu aims to sidestep this impenetrable forest of quantum weirdness.
He’s more interested in the cosmic plan behind it. “Why does God play dice?” he
asks. “What does He get out of it?” Or, in more down-to-earth terms, why is the
world as it is and not otherwise?
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It’s an audacious question. Yet he and other physicists have some clues and
leads—and perhaps even the embryo of an answer. The randomness in the
quantum world, they suspect, has a purpose. If they are right, the effect of
quantum uncertainty is not to build chaos and disorder into our world. Quite the
opposite. God uses it to ensure that all of the Universe’s far-flung regions
remain a coherent part of His overall plan.
That paradoxical conclusion follows from studies of one of the weirdest of
all quantum happenings, a phenomenon known as “quantum entanglement”.
Entanglement is an unnerving kind of link that can develop between two or more
photons, electrons or atoms, even if they inhabit distant parts of the Universe.
Consider, for example, a pion, a subatomic particle which can decay into an
electron and its antiparticle, a positron. When this happens, the particles fly
off in opposite directions. But according to quantum theory, no matter how far
apart the particles get, they remain mysteriously connected.
Up and down
One of the oddities of quantum particles is that their properties only take
on definite values when measured. The electron and positron, for instance, are
both effectively spinning. Either particle’s spin is equally likely to be
clockwise (known as “up”) or anticlockwise (“down”)—but you won’t know
which unless you measure it. Until that measurement is made the particle is in a
weird indefinite state, a “superposition” of both spins. What is definite,
however, is that in an entangled state, the spins of the two particles are
intimately linked. Since the original pion had no spin, the positron and
electron must always spin in opposite senses so that their net spin remains
zero. If you find the electron’s spin to be “up”, you’ll find the positron’s to
be “down”, and vice versa.
So it is as if the two entangled particles, no matter how far they are apart,
are not really separate at all. Measure one, and as its spin becomes definite
this triggers the other to respond. Its indeterminate spin also becomes
definite, in the opposite direction to that of its partner. What is astonishing
and disturbing is that this response happens instantaneously— even if the
particles are separated by huge distances.
Consequently, quantum theory requires action at a distance. What happens in
one part of the Universe can have instantaneous “nonlocal” consequences in other
parts, no matter how far away they might be. And this poses a problem, because
instantaneous action at a distance is a punch in the nose for Einstein. His
theory of relativity—the cornerstone of physics —claims that our
Universe has an absolute speed limit. Nothing, according to Einstein, can travel
faster than light.
So you might wonder—do we really need to swallow this nonlocal quantum
weirdness? Perhaps there is a better theory that accounts for these
entanglements without action at a distance?
Think of this: if someone separated a pair of your shoes by a great distance
and then weighed one, they would immediately have a good estimate of the weight
of the other. There’s no mystery here. Nothing nonlocal. Shoes have weight. And
if they come from a pair, their weights are correlated from the outset. Could
something similar be true for entangled particle pairs? Despite what quantum
theory says, perhaps the particles do have definite spins, arranged oppositely
at all times, and measurements merely reflect this pre-existing situation.
This is an obvious possibility. It might even be true. The trouble is, it
doesn’t cushion the blow for relativity. In 1964, physicist John Bell of CERN,
the European Laboratory for Particle Physics, examined this line of argument in
detail and proved a famous theorem which fellow physicist Henry Stapp of the
Lawrence Berkeley Laboratory in California calls “the greatest discovery of all
science”. Bell first supposed that quantum theory doesn’t say all there is to
say about quantum particles. He then proved that if any more complete
theory—any theory imaginable—were to give predictions in agreement
with quantum theory, it would necessarily still contain the same kind of
nonlocal influences as ordinary quantum theory. “What Bell gave us,” says
philosopher David Albert of Columbia University in New York, “is a proof that
there is a genuine nonlocality in the workings of nature, however we attempt to
describe it, period.” Every conceivable story about entangled states has to be
nonlocal. There is no escape. Unless, of course, entangled states don’t really
exist, and quantum theory is wrong.
Farewell, isolation
But we can be pretty sure it isn’t wrong because there are experiments to
prove it. In 1981, Alain Aspect of the University of Paris at Orsay showed using
pairs of photons that entanglement works just as quantum theory says it does.
Other researchers have since improved on Aspect’s results. Last year, Nicolas
Gisin and his colleagues at the University of Geneva used photon pairs that
travelled inside fibre-optic cables to separate cities in Switzerland to show
that entanglement can persist even for particles separated by 30 kilometres.
Distance is irrelevant. Despite what a few diehards say (See “Sceptics”),
it looks as if entanglement and nonlocality are real.
What’s more, entanglement does not only apply to pairs of particles. At
Cambridge, mathematician Noah Linden has been working with Popescu to understand
entanglement between larger numbers of particles. They have found that in the
typical quantum state occupied by any group of particles the links between the
particles are mostly of a nonlocal character. Quantum theory isn’t just a tiny
bit nonlocal. It’s overwhelmingly nonlocal. Nonlocality is the rule for our
Universe.
That is an unsettling conclusion. Nonlocality cuts into the idea of the
separateness of things, and threatens to ruin the very notion of isolation. To
isolate an object we ordinarily move it a long way away from everything else, or
build impenetrable walls around it. But the link of entanglement knows no
boundaries. It isn’t a cord running through space, but lives somehow outside
space. It goes through walls, and pays no attention to distance.
Does this mean the idea of separateness is doomed? And if faster-than-light
connections are possible, is relativity—despite its huge
successes—doomed too?
Uncontrollable outcomes
This is where God’s dice-playing comes in. Popescu believes that the
randomness at the heart of quantum mechanics is God’s safeguard against such
grotesque consequences. It ensures what physicist Abner Shimony of Boston
University calls the “peaceful coexistence” of quantum theory and relativity.
Sure, the outcome, up or down, at one end of an entangled link instantaneously
alters what happens at the other end. But the outcomes themselves are completely
uncontrollable. No matter which particle you measure, you find the results up or
down randomly, in equal measure. So you can’t control the outcome at the other
end. You can’t use the link to send any kind of message.
And whatever tricks you try, this block on sending information
instantaneously seems to remain unbreachable. Suppose you chose two separate
axes, say A and B, on which to measure the spin of your particles. If you
measured the spin of one particle on axis A, then its partner’s spin on axis A
would immediately be defined.
Likewise for spins on axis B. The fact that you can’t control whether the
spin is up or down would no longer matter. As long as you had some kind of
device to tell you on which axis the spin had been defined you would have a way
of sending a binary code: ABBABBAB, for example, would convey the same
information as the conventional digital byte 01101101.
But it turns out that any conceivable detector capable of doing this is also
prohibited by the mathematics of quantum theory. An experimenter at the other
end can’t possibly learn from individual outcomes, from the statistics of the
outcomes, or from anything else, what was the sequence of your measurements.
Quantum randomness prevents it.
So a stream of entangled particles is something like a combination of the
most perfect telephone link and the most useless handsets you could ever
imagine. The link can carry influences instantaneously across the Universe. But
the handsets at either end have the property that when you talk into them, they
randomise your speech. “Hello, it’s me” you say, and into the line goes “Nbsl
Cvdibobo”. You can send a message faster than light, all right. You just can’t
extract any meaning from it when it arrives. Whatever goes from one particle to
the other, as Asher Peres of Technion, Israel Institute of Technology in Haifa
puts it, is “information without information”.
According to Popescu, this answers the “why” question. For despite the raw
nonlocality in the links of entanglement, randomness ensures that quantum theory
doesn’t transgress the letter of Einstein’s law. At the core of Einstein’s
theory is the “no-signalling” criterion: you cannot send energy or information
from one place to another faster than light. This protects the chain of cause
and effect, and ensures that effects never happen before their causes. In a
deterministic world, any action at a distance would violate no-signalling. But
quantum theory allows what Shimony calls “passion-at-a-distance”, a weaker
linking up of distant things which stops just short of upsetting the principle
of causality.
So the picture of God’s world is this: through relativity, He ensures a
degree of separateness and individuality for distant pieces of His Universe.
Through quantum entanglement, he maintains links between distant regions, and
keeps the whole Universe coherently connected. It’s the randomness that makes it
possible for God to tie distant parts of the Universe together more tightly than
He otherwise could, while ensuring that cause and effect stay distinct. This is
what He gets by playing dice.
“It is wondrous”, says Popescu, “that quantum mechanics combines nonlocality
and causality”. But he hopes to wring from his questions a bit more insight than
that. In playing dice, does God have no other choice but to use quantum rules?
“Is quantum mechanics the only theory that can reconcile nonlocality with
relativity?” asks Popescu. If so, this might explain not only why the Universe
contains randomness, but why it enters the world in quantum mechanical
clothing.
In the early 1990s, Gisin explored the question of whether quantum theory
could be modified in any way and still be consistent with what we know
experimentally about the world. He found that fiddling with the edifice is an
extraordinarily sensitive business. “If you try to alter the theory very
slightly by adding some nonlinearity to the Schrodinger equation,” he says,
“then quantum nonlocality immediately becomes malignant: it can be used for
faster-than-light signalling.” But what if you alter it wildly? Is there any
theory whatsoever besides the quantum theory in which nonlocality and causality
can coexist?
To find out, Popescu and his colleague Daniel Rohrlich of Tel Aviv University
in Israel have been playing some odd intellectual games. Their idea is to probe
the realm of possible theories, and to consider alternative theories that go
beyond quantum theory.
This isn’t so much an exercise in physics as in mathematics. It’s not hard to
dream up nonlocal theories. You can make up any number of them at will just by
inventing forces that act at a distance between particles. However, most of
these theories violate relativity by allowing faster-than-light signalling. It’s
also not hard to invent theories that respect no-signalling. Any theory with
strictly local causes, for example, will do it. But the interesting theories are
those that achieve both nonlocality and no-signalling at once. Are there many
theories like that? Or is quantum theory the only one?
Simple and fundamental
Popescu and Rohrlich haven’t had to go far to find an answer: quantum theory,
they think, is not the only nonlocal theory with no-signalling. Their proof
comes in the form of a model world they have constructed, in which particles can
be entangled even more strongly than they are in the quantum world. This
super-entanglement leads to “supercorrelations” between spin measurements. And
yet the physics still doesn’t violate no-signalling. So this hypothetical world
provides a proof of principle: there are other inhabitants of the weird
theoretical terrain where nonlocality and causality can coexist.
This doesn’t mean that quantum theory is about to be ousted by one of these
alternatives: in our world, quantum theory unquestionably rules. But the mere
existence of these theories means that the need to have both nonlocality and
causality is not enough to tie God’s hands and fix the laws of physics. There
has to be something else.
“Our models raise a question,” says Popescu. “What is the minimal set of
principles—nonlocality plus no-signalling plus something else, simple and
fundamental, from which we could derive quantum mechanics?” Is there something
we don’t yet know about, some other principle as deep and pervasive as both
causality and nonlocality?
So while we may know why God plays dice, we don’t yet know why he throws them
as he does. Why quantum mechanical dice? What else constrains His hands? More
bold questions for Popescu and his colleagues to get their teeth into.
Does the world really allow weird “nonlocal” connections between very distant
objects? Some physicists refuse to believe it. “There are two kinds of
physicists still believing in locality,” says Nicolas Gisin of the University of
Geneva. “Some who simply cannot believe in nonlocality, and others searching for
logical loopholes in the experiments.” And there is at least one loophole.
True, there are famous experiments which seem to prove that quantum particles
can communicate instantaneously over large distances. But the particle detectors
that do the measuring are still not very good. They detect only a small fraction
of the particles that fly through them. So to draw conclusions, physicists have
to assume that the detected particles are a good sample. In 1970, physicist
Philip Pearle of Hamilton College in Clinton, New York, showed that in all these
experiments, entanglement might seem to be occurring not because it is real, but
because of sampling errors. Pearle pointed out the loophole merely as a logical
possibility, not as a serious issue. But a few physicists are clinging to it and
hoping that further experiments will disprove nonlocality.
Emilio Santos of the University of Cantabria in Santander, Spain, is one of
them. “Experiments such as these,” he says, “cannot discriminate between quantum
mechanics and theories based on local influences.” He bases this claim on models
that he and others have developed in which certain properties of the particles
not currently known to physicists—called “hidden
variables”—determine not only what the spins of particles are when they
are detected, but also whether they are detected at all. If these hidden
variables conspire in just the right way, then such models lead to results that
make it seem as if nonlocality is real.
On strict logic, these models can’t be ruled out. But most physicists don’t
give them much weight. “These models are ad hoc,” says Gisin. “It would be
amazing if these hidden variables were found to exist.” Asher Peres of Technion,
Israel Institute of Technology in Haifa, puts it more bluntly: “People who
dislike quantum theory will never be convinced. They will find all kinds of
reasons not to believe in experimental evidence.”