AtD: The history of the world is the story of light

Tue Mar 10 20:42:15 CDT 2009

I'M NOT LOOKING, HONEST!
Mar 5th 2009  

The good news is reality exists. The bad is it's even stranger than
people thought

"HOW wonderful that we have met with a paradox. Now we have some hope
of making progress." So said Niels Bohr, one of the founders of quantum
mechanics. Since its birth in the 1920s, physicists and philosophers
have grappled with the bizarre consequences that his theory has for
reality, including the fundamental truth that it is impossible to know
everything about the world and, in fact, whether it really exists at
all when it is not being observed. Now two groups of physicists,
working independently, have demonstrated that nature is indeed real
when unobserved. When no one is peeking, however, it acts in a really
odd way. 

In the 1990s a physicist called Lucien Hardy proposed a thought
experiment that makes nonsense of the famous interaction between matter
and antimatter--that when a particle meets its antiparticle, the pair
always annihilate one another in a burst of energy. Dr Hardy's scheme
left open the possibility that in some cases when their interaction is
not observed a particle and an antiparticle could interact with one
another and survive. Of course, since the interaction has to remain
unseen, no one should ever notice this happening, which is why the
result is known as Hardy's paradox.

This week Kazuhiro Yokota of Osaka University in Japan and his
colleagues demonstrated that Hardy's paradox is, in fact, correct. They
report their work in the NEW JOURNAL OF PHYSICS. The experiment
represents independent confirmation of a similar demonstration by Jeff
Lundeen and Aephraim Steinberg of the University of Toronto, which was
published seven weeks ago in PHYSICAL REVIEW LETTERS.

The two teams used the same technique in their experiments. They
managed to do what had previously been thought impossible: they probed
reality without disturbing it. Not disturbing it is the
quantum-mechanical equivalent of not really looking. So they were able
to show that the universe does indeed exist when it is not being
observed.

The reality in question--admittedly rather a small part of the
universe--was the polarisation of pairs of photons, the particles of
which light is made. The state of one of these photons was inextricably
linked with that of the other through a process known as quantum
entanglement. 

The polarised photons were able to take the place of the particle and
the antiparticle in Dr Hardy's thought experiment because they obey the
same quantum-mechanical rules. Dr Yokota (and also Drs Lundeen and
Steinberg) managed to observe them without looking, as it were, by not
gathering enough information from any one interaction to draw a
conclusion, and then pooling these partial results so that the total
became meaningful. 

What the several researchers found was that there were more photons in
some places than there should have been and fewer in others. The
stunning result, though, was that in some places the number of photons
was actually less than zero. Fewer than zero particles being present
usually means that you have antiparticles instead. But there is no such
thing as an antiphoton (photons are their own antiparticles, and are
pure energy in any case), so that cannot apply here. 

The only mathematically consistent explanation known for this result is
therefore Hardy's. The weird things he predicted are real and they can,
indeed, only be seen by people who are not looking. Dr Yokota and his
colleagues went so far as to call their results "preposterous". Niels
Bohr, no doubt, would have been delighted. 

See this article with graphics and related items at http://www.economist.com/science/displaystory.cfm?story_id=13226725

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