“This would say that any reasonable laws of physics that
someone could write down, they can all reasonably simulate
any other laws,” Aaronson said.
But Shor’s factoring algorithm contends that quantum
computers could do things a Turing machine couldn’t. So
either quantum computing is actually impossible (not likely,
as that would imply some flaw in quantum mechanics itself ),
or the Church-Turing thesis is incorrect as a statement
about the physical world. Unless somehow
there exists an unknown way for an ordinary computer to simulate quantum physics. “No one has proved that there isn’t
one,” said Aaronson. “But this would be
an astounding mathematical discovery.”
Quantum roots
An equally or more profound discovery
would be identifying the physical principle that requires reality to obey the rules
of quantum mechanics in the first place.
In the beginning, the quantum pioneers
simply figured out the math that works — a
math that requires the weirdness of multi-
ple possible realities (SN: 11/20/10, p. 15). Inquiring into the
underlying reason why such weird math worked so well was
long considered foolish. A standard response to junior physi-
cists who raised such questions was “shut up and calculate.”
But in recent decades, the quest to find a physical prin-
ciple from which quantum mechanics can be built has
become more popular, and quantum information has been
at the heart of many such efforts. Much of the work along
these lines was inspired by the late physicist John Archibald
Wheeler, who believed that quantum physics — and existence
itself — might have its roots in aspects of information theory.
His slogan “it from bit” summarized the view that reality
somehow emerges from cosmic information processing.
Several efforts to derive quantum math from basic principles have echoed the it-from-bit philosophy. Last year, for
instance, Giulio Chiribella of the Perimeter Institute for
Theoretical Physics in Waterloo, Canada, and collaborators
from Italy showed a way to derive quantum mechanics from
a set of five axioms plus one postulate, all rooted in information theory terms (SN: 8/13/11, p. 12).
“In this approach the rules by which information can be
processed determine the physical theory, in accordance with
Wheeler’s program ‘it from bit,’ ” Chiribella and Italian colleagues Giacomo Mauro D’Ariano and Paolo Perinotti wrote
in Physical Review A.
Their system is built on axioms such as “causality” — in
essence, the notion that signals from the future cannot affect
the present. In other words, the odds of an experiment turn-
ing out one way or the other do not depend on a future choice
of which measurements to perform. Another axiom, called
“ideal compression,” asserts that the information in a system
always can be condensed into an encoded form that could
then be decoded to reproduce all of the original information.
Some physicists
believe that the
physical principle
underlying
quantum
physics should
be crisp and
clear and even
in retrospect,
perhaps, obvious.
Relatively simple
Chiribella and colleagues’ axiomatic system is not the
only one to reproduce the formulas of quantum physics,
though (but they claim theirs is the first to do so using only
principles that can be expressed in terms of actual physical operations). And none of the various approaches satisfy
everybody, anyway. Some physicists ( Wheeler was among
them) believe that the physical principle underlying quantum physics should be crisp and clear and even in retrospect,
perhaps, obvious.
Christopher Fuchs, a quantum physicist at the Perimeter
Institute, argues that the secret principle (or principles) at
the core of quantum mechanics ought to be similarly simple
to the two pillars of Einstein’s theory of relativity: The speed
of light is constant, and the laws of physics do not depend
on how you are moving. Axioms that need paragraphs of
explanation do not meet this crisp-and-clear test. Fuchs still
appreciates information’s role in quantum theory, but he
does not believe that Wheeler’s “it from bit” is the right story.
“I suspect quantum theory is mostly about information,”
Fuchs said in a recent interview. “But there is going to be
some piece remaining behind that you really can’t pin down
as being a statement about information. Instead I expect
it to be a statement about the character of the world.… The
distillate that remains, the part that can’t be given an infor-mation-theoretic reason, will be our first glimpse into what
quantum reality is all about.” s
