Skip to comments.Natural selection acts on the quantum world
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.
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?
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."
Gibberish. On three levels.
Ok, if its gibberish, can you demonstrate a time event that is NOT a kinetic energy event?
Evolution (of all things) is evidence of God's existence. St Thomas Acquinas
The power available through kinetic energy is vastly underrated. Time, however, is a percept.
"I donno about this one. But it's a slow day for new threads."
L O L!
Here come the Quantum Creationists.
Well, this is interesting. Darwinism continues to show general utility. How would Creationism possibly be used here?
I always said it was turtles all the way down.
Sounds like a joke thread to me. All life is macroscopic in nature - even viruses and bacteria. No living thing exists on the Plank scale. An observation, in the sense of quantum mechanics occurs on the size scale were quantum mechanics dominates. To influence a quantum state, you must observe on the quantum scale. At our large scale, quantum effects blurr into the classical laws of physics. This is the decoherence mentioned in the article.
This is the part of the articel I have trouble with: 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.
I always thought that the quantum states were properties associated with materials invovled. For example, the green in the leaves of a tree arise from the absorption of light of a frequency whose energy matches the quantum transition from a ground state to an excited state in chlorophyll. One molecule of chlorophyll is like every other and has the same transition state. The only change would be slight variations in its chemical environent that can casue the energy for the transition to shift slightly higher or lower, hence casuing a broadening of the wavelength window responsible for the observed color. I don't see where the authors see a continuum of states and some are selected on a macroscopic scale when the quantum effects are entirely macroscopic and the possible states are determined before even reaching the classical domain.
It is physically impossible even for two macroscopic observers to see the exact same thing. For example, if two people were looking at a flower, each person sees something different. Different photons of light reach the different people. No two people detect the same photon or the same transition.
The environment as a whole is always declaring itself to itself (more or less clearly and distinctly, as Descartes might have put it), and the states of the environment which are able to successfully (and consistently) declare themselves to their surroundings are the states which come to be regarded as objective by observers (should there be any).
From one part to all of the others: makes sense to me.
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