Darwinisme Quantique

[Punu]

http://www.nature.com/news/2004/041220/full/041220-12.html

  Citation

Natural selection acts on the quantum world

by Philip Ball

Objective  reality may owe its existence to a 'darwinian' process that

advertises certain quantum states.

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, hich

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 Mexico

Because,  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."

Top

References

1. Ollivier  H.,  Poulin D. & Zurek W. H. Phys. Rev. Lett., 93. 220401

  (2004). | Article | PubMed | ChemPort |

2. Zurek          W.          H.          Arxiv,          Preprint

  http://www.arxiv.org/abs/quant-ph/0105127 (2004).