[tt] Quantum untanglement: Is spookiness under threat? - fundamentals - 02 November 2007 - New Scientist

[Brian Atkins]

http://www.newscientist.com/channel/fundamentals/mg19626281.400-quantum-untanglement-is-spookiness-under-threat.html

Quantum untanglement: Is spookiness under threat?

     * 02 November 2007
     * NewScientist.com news service
     * Mark Buchanan

POOR old Einstein. He didn't much like quantum theory, and who can blame him? 
It's just so... well, peculiar. Nothing in the quantum world exists until you 
measure it. Certainty melts away. And then there's what Einstein famously called 
"spooky action at a distance". He didn't like any of it. Yet experiments show 
quantum mechanics to be the most accurate physics theory in history. Not only 
does quantum theory make all the right predictions, physicists largely agree 
that modern experiments, combined with quantum theory's mathematics, leave no 
room for alternatives. There is no competing theory that banishes the weirdness 
and embraces a reality that exists independent of our observations of it. The 
spookiness, it seems, is here to stay.

Or is it? Listen to Joy Christian at the University of Oxford and you may wonder 
if these grandiose quantum conclusions are really necessary at all. He claims 
that physicists' supposed proofs of the impossibility of more "realistic" 
theories rest on false assumptions and so don't prove much at all.

"Contrary to the received wisdom," he says, "quantum theory doesn't rule out the 
possibility of a deeper theory, even one that might be fully deterministic." 
Christian's conclusion follows from a relatively simple calculation using 
alternative mathematics, described in a paper now under review at the journal 
Physical Review Letters.

It is a controversial finding, and many theorists disagree with his result, but 
if Christian is right, perhaps the angst felt by Einstein and others will have 
been for naught. Whatever the answer, Christian is by no means alone in 
questioning whether quantum theory is the "final theory" it is often cracked up 
to be. "A theory that yields 'maybe' as an answer should be recognised as an 
inaccurate theory," says Gerard 't Hooft of the University of Utrecht in the 
Netherlands.

For decades, physicists have struggled to unite quantum theory with Einstein's 
theory of gravity. This difficulty, and lingering inconsistencies at the heart 
of quantum theory itself, have convinced a growing number of researchers that 
something deeper is going on behind the scenes - a "pre-quantum" world of 
certainties and objective realities which, once understood, might reveal how the 
strange rules of quantum physics emerge from something less strange. A few even 
think they are starting to see its tantalising outlines.

The proofs of unavoidable quantum weirdness centre on entanglement, the spooky 
quantum link that Einstein found so distasteful. Entangled pairs of particles 
such as photons are routinely created in the lab for all kinds of experiments. 
Send a photon into a "non-linear" crystal and a pair of entangled photons emerge 
whose characteristics are mysteriously linked. According to quantum theory, it 
makes no sense to talk about the properties of just one of the entangled photons 
that appears from the crystal, since all of the information about the photons - 
such as their "up" or "down" spin states - lies only in their joint properties. 
Such photons remain connected, even over vast distances.

Quantum theory also asserts that particles have no particular spin before they 
are observed. Instead, the spin is an indefinite superposition, pointing up and 
down simultaneously. Only when you measure the spin, do you "force" it into the 
up or down state.

Do this to an entangled photon and its counterpart responds instantly, even if 
they are light years apart. If you measure one photon and find its spin pointing 
"up", you'll find that the other has spin "down". Consequently, quantum theory 
appears to dictate that what happens in one part of the universe can have 
instantaneous "non-local" effects in another part, which seems to threaten the 
basic assumptions of Einstein's special theory of relativity.

Physicists since Einstein have wondered if this peculiarity might only 
demonstrate a problem with quantum theory itself. Could a better theory account 
for the linked spins without resorting to this crazy non-locality? One obvious 
idea is that the particles really do have designated spin all along, but that 
this is somehow hidden until measured: maybe the spins are created equal and 
opposite at the moment particles are entangled, and maybe quantum theory just 
isn't enough to capture these details.

As enticing as this "hidden variables" explanation is, it doesn't seem to work. 
It was in 1964 that physicist John Bell at Europe's CERN laboratory for particle 
physics near Geneva, Switzerland, apparently proved that an appeal to such a 
hypothesis would never bear fruit. Bell imagined an experiment that would send 
particles from millions of entangled pairs to distant places around the globe, 
where experimenters would measure their spins. He assumed that some "real", 
pre-existing properties of the two particles would determine the measurement 
outcomes. He also assumed that relativity remains intact, so if measurements of 
entangled particles were made at the same moment, the properties of one particle 
could not possibly affect its entangled twin quickly enough. From these 
assumptions, Bell predicted what such experimenters would find.

Remarkably, his results revealed that if his simple assumptions held true then 
the experimental outcomes would conflict with quantum theory. Many delicate 
experiments have been carried out since then to test Bell's scenario and, in 
every case, they have supported quantum theory. Most physicists take these 
experiments as definitive proof that hidden variables are simply not an option: 
you must accept that distant, instant influences between particles are possible 
or give up on any deeper, underlying reality.

Recent experiments have gone further and tried to establish which of the two 
ideas has to go: locality or realism. They concluded that we have to abandon the 
idea of an objective reality (New Scientist, 23 June, p 30). All of this rests 
on the fundamental assumption that Bell's original argument was sound, and most 
physicists have accepted his conclusions for 40 years. But if Christian is 
right, they've been overlooking an alternative all this time.

Bell assumed the hidden variables in his argument would be familiar numbers, 
akin to the value of a velocity or a mass. Such numbers obey the ordinary rules 
of algebra, including a law that says that the order of multiplication doesn't 
matter - so that, for example, 2 × 5 equals 5 × 2. This property of 
multiplication is called commutation. The idea that hidden variables are 
commuting numbers might seem so basic as to be beyond question, but Christian 
argues it is important to question this point because mathematicians know that 
different kinds of variables needn't obey commutative algebra. Take rotations in 
space, for example. They differ fundamentally from ordinary numbers in one 
important respect: the order of rotations matters (see Diagram). Rotations do 
not commute.

In 1843, the Irish mathematician William Rowan Hamilton found a way to capture 
this non-commuting property in a set of number-like quantities called 
quaternions. Later, the English mathematician William Clifford generalised 
Hamilton's quaternions into what modern mathematicians call Clifford algebra, 
widely considered the best mathematics for representing rotations. So convenient 
are quaternions that they are commonly used in computer graphics and aviation.

So why is all this important? Christian argues that the existence of this other 
algebra reveals a weakness at the core of Bell's proof: the only hidden 
variables Bell considered were ordinary numbers. But ordinary numbers are not 
the be all and end all. "Why should theorists be obliged to remain unimaginative 
and consider only commuting numbers in their theories?" Christian says.
Was Bell wrong?

He claims that Bell's argument no longer leads to its impressive conclusion if 
you allow that hidden variables can have other algebraic properties. Following 
the logic through, Christian shows that a local, realistic model can actually 
reproduce everything that quantum theory can. Christian concludes that Bell's 
theorem is simply not equipped to say whether or not hidden variables are a 
possible explanation for non-local quantum effects. "When I started out looking 
at this, it never occurred to me that Bell's theorem might turn out to be 
wrong," says Christian. "But that's what I found."

Einstein might have been relieved, and it's a shot in the arm for those seeking 
a deeper reality beyond quantum theory that might be more "reasonable" and akin 
to classical physics. At this early stage, however, many physicists consider it 
too good to be true.

Philippe Grangier at the Institute of Optics in Orsay, France, is one of those 
who believe Christian's claim is unwarranted. "The argument is just too remote 
from Bell's hypothesis to have anything relevant to say about his theorem," he 
says. "It might be worth considering as an alternate formulation of quantum 
mechanics, maybe with a more 'realistic' flavour, but the 'disproof' argument 
simply makes no sense."

The debate seems likely to continue for some time while researchers puzzle over 
details. However it turns out, Christian's work reflects a growing willingness 
among physicists to question whether quantum theory is really the ultimate 
foundation for theoretical physics. Even those who doubt Christian's conclusion 
suggest that there's a long way to go to before we truly understand quantum 
mechanics. "I have no problem thinking that quantum theory is incomplete," says 
Nicolas Gisin of the University of Geneva.

Twenty years ago, it was heretical even to raise such an idea, but physicists 
are now questioning quantum theory for a range of reasons. Lee Smolin of the 
Perimeter Institute in Waterloo, Canada, for one, doubts that physicists can 
really make headway building a true theory of quantum gravity and space-time 
before making some serious revisions to quantum theory itself. The inability of 
theorists to extend quantum theory to the entire universe, he suggests, may 
imply that it only works for parts of the universe, as an approximation of some 
deeper reality.

Smolin and Fotini Markopoulou, also at the Perimeter Institute, have been 
exploring how hints of that deeper theory might emerge from primitive notions of 
geometry. Their research centres on the concept of loop quantum gravity, in 
which the smooth picture of space-time is replaced at the Planck scale of around 
10-35 metres by a network of abstract links and nodes. In 2004, the pair found 
that quantum theory tumbles out as an approximate set of physical laws when you 
zoom out to the far larger scales of the subatomic world (Physical Review D, vol 
70, p 124029). The results are promising, although they stress it's only a 
start. "There's a lot to fill in before we can say it's a complete model," says 
Smolin.

't Hooft has been pursuing similar ideas and has proposed that the universe is 
deterministic at some fundamental level. He suggests that quantum theory may be 
akin to thermodynamics in the sense that it describes physical systems on 
average, rather than at a deeper, more detailed level. 't Hooft has constructed 
various determinsitic theories in which the vacuum of empty space holds the key. 
To him, the vacuum consists of an enormous class of distinct states that evolve 
in a deterministic way, details that are ignored by quantum theory. If 't Hooft 
is right, his idea could explain how randomness arises in quantum theory and why 
it fails to make specific predictions.

Other researchers, stimulated by long-standing paradoxes of quantum theory, are 
pursuing experiments that may reveal chinks in it. Markus Aspelmeyer and 
colleagues at the University of Vienna in Austria have created fragile entangled 
states between photons and far larger objects, such as small mirrors. Their aim 
is to explore why we see quantum behaviour on the very small scales, but never 
seem to find it in everyday objects.

A photon in a quantum superposition can, for example, pass through two narrow 
slits in a screen at the same time, yet we cannot walk through two doors 
simultaneously. In practice, researchers assume that superpositions somehow 
collapse into one specific state or another whenever large objects are involved. 
For decades, theorists unsatisfied with the vagueness of that explanation have 
tried to construct a more specific theory. Mathematician Roger Penrose at the 
University of Oxford suggests that the collapse may be linked to gravity. His 
idea is that massive objects in a superposition, such as a heavy particle 
passing through a double slit, would stir up unknown forces that would drive the 
superposition to collapse rapidly leaving the object in a well-defined place.

Aspelmeyer says that experimental techniques are becoming sensitive enough to 
test such proposals. His group hopes soon to entangle a mirror weighing less 
than a microgram with a beam of light in a superposition of states containing 
different numbers of photons. Upon reflection, the beam would give the mirror a 
momentum kick and send the mirror into a superposition of positions. This might 
trigger the kind of collapse predicted by Penrose. Other teams are pursuing 
similar experiments. "I'd be surprised if one of these groups didn't report some 
results pretty soon," Aspelmeyer says.

For Stephen Adler at the Institute for Advanced Studies in Princeton, New 
Jersey, such renewed interest in the deeper foundations of quantum theory is 
long overdue. For more than two decades, Adler has been quietly developing what 
he calls "emergent quantum theory" - an idea that builds quantum physics from 
the bottom up, starting from a hypothetical lower level that obeys classical 
physics.

Like Smolin and Markopoulou, Adler's scheme assumes a pre-quantum level of 
physical fields currently unknown to physics. He assumes these have certain 
basic features and then explores how something like quantum theory might emerge 
at higher levels. Intriguingly, he's found that it works if his pre-quantum 
fields have a mixture of both ordinary algebraic properties and non-commuting 
properties, much like those considered by Christian. What emerges from this 
hypothetical foundation are basic quantities of quantum field theory, providing 
a basis for all of quantum theory.

Even more exciting, says Adler, is that fluctuations in his pre-quantum fields 
lead naturally to the collapse of quantum superpositions. His theory predicts 
that this will happen all the more rapidly in large objects, just as in the 
theories of Penrose and others. It is not yet clear whether the assumptions in 
Adler's research stand up, but the work has been turning heads. "This work is 
truly ingenious," says physicist Philip Pearle, now retired from Hamilton 
College in New York. "Is it the long-sought formulation that makes quantum 
theory understandable? I'd say a definite maybe."

So after decades of physicists bending their minds over the weirdness of the 
quantum world, it is just possible that its uncertainties and paradoxes may give 
way to something a little less weird and more definite. Suddenly it's more 
acceptable to challenge the dogma and to look for a more fundamental, simpler story.

"When I started 30 years ago," Pearle recalls, "almost no one was doing this 
kind of work. Now lots of people are looking at the standard ideas of physics 
with new eyes." Surely Einstein would have approved.

Quantum World - Learn more about a weird world in our comprehensive special report.
 From issue 2628 of New Scientist magazine, 02 November 2007, page 36-39

-- 
Brian Atkins
Singularity Institute for Artificial Intelligence
http://www.singinst.org/