Natural selection acts on the quantum world

[R.A. Hettinga]

<http://www.nature.com/news/2004/041220/pf/041220-12_pf.html> Close window??

Published online: 23 December 2004


 Natural selection acts on the quantum world

Philip Ball

Objective reality may owe its existence to a 'darwinian' process that
advertises certain quantum states.

If observing the world tends to change it, how come we all see the same
butterfly?


A team of US physicists has proved a theorem that explains how our
objective, common reality emerges from the subtle and sensitive quantum
world.

If, as quantum mechanics says, observing the world tends to change it, how
is it that we can agree on anything at all? Why doesn't each person leave a
slightly different version of the world for the next person to find?

Because, say the researchers, certain special states of a system are
promoted above others by a quantum form of natural selection, which they
call quantum darwinism. Information about these states proliferates and
gets imprinted on the environment. So observers coming along and looking at
the environment in order to get a picture of the world tend to see the same
'preferred' states.

If it wasn't for quantum darwinism, the researchers suggest in Physical
Review Letters1, the world would be very unpredictable: different people
might see very different versions of it. Life itself would then be hard to
conduct, because we would not be able to obtain reliable information about
our surroundings... it would typically conflict with what others were
experiencing.

Taking stock

The difficulty arises because directly finding out something about a
quantum system by making a measurement inevitably disturbs it. "After a
measurement," say Wojciech Zurek and his colleagues at Los Alamos National
Laboratory in New Mexico, "the state will be what the observer finds out it
is, but not, in general, what it was before."

 They survive monitoring by the environment to leave 'descendants' that
inherit their properties. ?

Wojciech Zure
Physicist, Los Alamos National Laboratory in New MexicoBecause, as Zurek
says, "the Universe is quantum to the core," this property seems to
undermine the notion of an objective reality. In this type of situation,
every tourist who gazed at Buckingham Palace would change the arrangement
of the building's windows, say, merely by the act of looking, so that
subsequent tourists would see something slightly different.

Yet that clearly isn't what happens. This sensitivity to observation at the
quantum level (which Albert Einstein famously compared to God constructing
the quantum world by throwing dice to decide its state) seems to go away at
the everyday, macroscopic level. "God plays dice on a quantum level quite
willingly," says Zurek, "but, somehow, when the bets become macroscopic he
is more reluctant to gamble." How does that happen?

Quantum mush

The Los Alamos team define a property of a system as 'objective', if that
property is simultaneously evident to many observers who can find out about
it without knowing exactly what they are looking for and without agreeing
in advance how they'll look for it.

Physicists agree that the macroscopic or classical world (which seems to
have a single, 'objective' state) emerges from the quantum world of many
possible states through a phenomenon called decoherence, according to which
interactions between the quantum states of the system of interest and its
environment serve to 'collapse' those states into a single outcome. But
this process of decoherence still isn't fully understood.

"Decoherence selects out of the quantum 'mush' states that are stable, that
can withstand the scrutiny of the environment without getting perturbed,"
says Zurek. These special states are called 'pointer states', and although
they are still quantum states, they turn out to look like classical ones.
For example, objects in pointer states seem to occupy a well-defined
position, rather than being smeared out in space.

The traditional approach to decoherence, says Zurek, was based on the idea
that the perturbation of a quantum system by the environment eliminates all
but the stable pointer states, which an observer can then probe directly.
But he and his colleagues point out that we typically find out about a
system indirectly, that is, we look at the system's effect on some small
part of its environment. For example, when we look at a tree, in effect we
measure the effect of the leaves and branches on the visible sunlight that
is bouncing off them.

But it was not obvious that this kind of indirect measurement would reveal
the robust, decoherence-resistant pointer states. If it does not, the
robustness of these states won't help you to construct an objective reality.

Now, Zurek and colleagues have proved a mathematical theorem that shows the
pointer states do actually coincide with the states probed by indirect
measurements of a system's environment. "The environment is modified so
that it contains an imprint of the pointer state," he says.

All together now

Yet this process alone, which the researchers call 'environment-induced
superselection' or einselection2, isn't enough to guarantee an objective
reality. It is not sufficient for a pointer state merely to make its
imprint on the environment: there must be many such imprints, so that many
different observers can see the same thing.


Happily, this tends to happen automatically, because each individual's
observation is based on only a tiny part of the environmental imprint. For
example, we're never in danger of 'using up' all the photons bouncing off a
tree, no matter how many people we assemble to look at it.

This multiplicity of imprints of the pointer states happens precisely
because those states are robust: making one imprint does not preclude
making another. This is a Darwin-like selection process. "One might say
that pointer states are most 'fit'," says Zurek. "They survive monitoring
by the environment to leave 'descendants' that inherit their properties."

"Our work shows that the environment is not just finding out the state of
the system and keeping it to itself", he adds. "Rather, it is advertising
it throughout the environment, so that many observers can find it out
simultaneously and independently."

-- 
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R. A. Hettinga <mailto: rah at ibuc.com>
The Internet Bearer Underwriting Corporation <http://www.ibuc.com/>
44 Farquhar Street, Boston, MA 02131 USA
"... however it may deserve respect for its usefulness and antiquity,
[predicting the end of the world] has not been found agreeable to
experience." -- Edward Gibbon, 'Decline and Fall of the Roman Empire'