"Actually, I'm a Quantum Presbyterian..."
Posted on 12/23/2004 8:31:39 AM PST by PatrickHenry
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, 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 Letters [Ollivier H., Poulin D. & Zurek W. H. Phys. Rev. Lett., 93. 220401], 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."
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 einselection [Zurek W. H. Arxiv, Preprint, link at footnote 2 in original article], 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."
Decoherence is caused by the interaction with the environment. Environment monitors certain observables of the system, destroying interference between the pointer states corresponding to their eigenvalues. This leads to environment-induced superselection or einselection, a quantum process associated with selective loss of information. Einselected pointer states are stable. They can retain correlations with the rest of the Universe in spite of the environment. Einselection enforces classicality by imposing an effective ban on the vast majority of the Hilbert space, eliminating especially the flagrantly non-local "Schrödinger cat" states. Classical structure of phase space emerges from the quantum Hilbert space in the appropriate macroscopic limit: Combination of einselection with dynamics leads to the idealizations of a point and of a classical trajectory. In measurements, einselection replaces quantum entanglement between the apparatus and the measured system with the classical correlation.
And here's a link to the PDF version of Zurek's pre-print article:
Decoherence, einselection, and the quantum origins of the classical
Ladder operators, perhaps. :-)
All right, I'll bite, I don't mind.
What about when the light is acting like a wave?
Rayleigh, Jeans
had not the means
Einstein didn't want 'em
It took Neils Bohr
and several more
to figure out the quantum...
(Hazy quote from memory, from (I believe) an old article in Physics Today...)
Cheers and Merry Christmas!
Merry Christmas, too!
"Actually, I'm a Quantum Presbyterian..."
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.
If you assemble enough people, those in the back won't be able to see the tree, because others are in the way. The people in front used up those photons, you see (or not, as the case may be).
...einselection, a quantum process associated with selective loss of information.Zurek speaks of 'environmental monitoring', seeming to reserve 'observing' for the sort of monitoring that we do. What might monitoring mean if there are no observers like us around? Something along these lines, I'd surmise: exchanges of energy/momentum. A source of photons, for example, is announcing itself to its surroundings, and any absorption event of any of the emitted photons constitutes a 'monitoring' of the emitter by the absorber. If the source is constant and prolific, there will be many opportunities for many different absorbers to 'monitor' that source. It's precisely such sources that observers (whenever and wherever they come to be) come to recognize as 'objective'.
If I can find some time, I'm going to read selected portions of Zurek's pre-print (skipping over the parts that would require too much work to understand). If I find reason to modify the previous paragraph, I'll post a correction on this thread. However, if I don't post a correction, don't take that as evidence that the previous paragraph is correct. Maybe I just didn't get around to reading the pre-print, or, if I did, I didn't properly understand it!
Yup. "Charm"ing.
If you assemble enough people, those in the back won't be able to see the tree ...
And if everyone goes home, the decoherence goes with them, the forest reverts to quantum mush, and the elves come out to play.
 |
[sighs] These kind of threads always make me wonder why if the researchers have a deep insight into randomness, they don't live in Las Vegas . . . |
The 'pointer states' are the 'robust' states, which are able to survive and continue to announce themselves to their surroundings. The analogy with Darwinism is a bit strained, I'll agree. Zurek wouldn't be the first physicist to co-opt some aspect of evolution into physics. Lee Smolin comes to mind.
I read it. It hurt. I'm going to go lie down for a while.
Those with a deep understanding of randomness either avoid Las Vegas (and other gambling estblishments) or the own one (or at least they own a floating crap game, on a riverboat.)
I dunno. When I was a teenager, and just learning about quantum, I misread the term "hadron" as "hardon."
I was giggling for days until I realized my mistake.
Ah, the wonders of selective dyslexia...
Color me poor, but
I bet some day, someone with
stochastic rachets
("Parrondo games") makes
roulette a "game" of the past.
Now, lottery games . . .
Decoherence is caused by the interaction with the environment. Environment monitors certain observables of the system, destroying interference between the pointer states corresponding to their eigenvalues. This leads to environment-induced superselection or einselection, a quantum process associated with selective loss of information. Einselected pointer states are stable. They can retain correlations with the rest of the Universe in spite of the environment. Einselection enforces classicality by imposing an effective ban on the vast majority of the Hilbert space, eliminating especially the flagrantly non-local "Schrödinger cat" states...Thank you, modern science, for providing yet more evidence of the truth of creationism! All you scientist scaled the mountaintops only to find the priests already there. Here's how it works...
This loss of information happened at The Fall. When G-D kicked Adam & Eve out of the garden, He removed H~s protection of all the majickal, non-localized quantum states, eventually leaving only cold, cruel, Darwinian objectivity to survive.
In short: Objectivity is all Satan's fault.
But now, if we let G*D into our hearts, He'll extend H-s protection back to all those fragile quantum states while you pray to H^m. This is why prayer produces miracles.
(This argument, or something like it, coming soon to an AiG or Creation/Evolution Headlines website near you. :-)
What's the superiority of Zurek's approach over Bohmian mechanics?
Just wondering.
Ideas based on the immersion of the system in the environment have recently gained enough support to be described (by sceptics!) as “the new orthodoxy” (Bub, 1997). This is a dangerous characterization, as it suggests that the interpretation based on the recognition of the role of the environment is both complete and widely accepted. Neither is certainly the case.In other words, Zurek's approach is not known to work, whereas Bohmian mechanics is known to be equivalent to the Copenhagen interpretation where the latter is unambiguous.
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