Perusing the current "Proceedings of the Royal Society," one may come
across a scientific tribute to John Cage and, specifically, his composition
"4' 33"" - the one in which the pianist sits in silence for that amount of
time.

Computer scientists Graeme Mitchison, of the University of Cambridge,
and Richard Jozsa, of the University of Bristol, were inspired by the idea,
especially by the audience response of applauding after four minutes and 33
seconds of NOTHING.  What if, they asked, there could be a computer giving
a response without ever being switched on? Their proposed "counterfactual
computation" taps into worlds in which the computer did run in order to
extract the result into a world in which it did not.

Like Cage's piece, this feat is not about doing nothing, but rather about
doing nothing in the time normally allotted for doing something.  "Due time
must be allowed for the machine not to run," say the study's authors.  To
determine the outcome of a computation while the machine stays off, they
employ a quantum computer, something far faster than existing computers.
Using the principles of quantum mechanics to achieve massively parallel
processing, quantum computers are expected to perform many logic operations
at the same time.  (After decades of study and effort, the actual building
of a quantum computer is still in the future.)

But imagine, say Mitchison and Jozsa, that such a computer exists.  Quite
apart from streamlining information technology, this hypothetical machine
would highlight, "in a particularly poignant way," the counter-intuitive
nature of quantum physics, creating a parallel to Cage's feat of making
the audience applaud (or hiss) in response to nothing.

Quantum systems can exist in two incompatible states at once, a condition
known as "superposition." The most famous example is Schrodinger's cat,
which can be both alive and dead if its fate is determined by a quantum
superposition of two possible outcomes.  A quantum computer uses such
superpositions to enlarge its computational power.  A superposition
generally collapses into one state or the other if measured - we can never
actually see a superposition, even as we cannot hear Cage's work.  except
in another dimension.

Janos Gereben/SF, CA
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