Natural selection acts on the quantum world

[indigobusiness]

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.

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

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

Wojciech Zure
Physicist, Los Alamos National Laboratory in New Mexico

[Orsino]

1. Are They Missing the Point?

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

Because we don't look at the trillions of individual quantum events that make up the butterfly. We can't see the quantum level at all, and even if we could, we would be likely to see the same "average" picture, anyway.

Despite decades of quantum theory, we are, by and large, not observers of the world on that level. What effects were these physicists expecting to find, really?

[indigobusiness]

2. I see your point. I think they were sharpening the point on this aspect:

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.

It isn't easy to wrap a mind around completely, but they do make sense in their questioning of the nature of decoherence.

But, like Richard Feynman said: "If you think you understand quantum physics, then you don't."

[On the Road]

3. This Part I Have Difficulty Understanding:

"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."

It says that "perturbation" from the environment eliminates all quantum states except the robust pointer states.

THAT's what I want to understand better. What are these other quantum "mush" states that get eliminated and how does perturbation eliminate them. And how common are pointer states versus other ones? And would the reason for pointer states occupying in a stable position be that their velocity is greater (position being inversely related to velocity)?

I thought decoherence was still pretty much of a mystery. Maybe it is, but they're chipping around the edges.

[On the Road]

4. For You Quantum Buffs on DU,

Here's a quote from one of the source articles:

"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 "Schrodinger 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."

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

According to my extremely limited understanding, it seems like the issue is one of interference. The pointer states are the only ones that don't cancel each other out due to interference.

It also seems to imply that the pointer states we observe are only a small fraction of all quantum states. The part we can't observe is vastly greater than what we can.

[phantom power]

5. "science" reporters really ought to cut it out.

When physicists say that "observing the world changes it", they are referring to some very specific, and very non-mystical, things.

For instance, if you attempt to measure a particle, you can only accomplish the measurement by bouncing some other particle off of it (broadly speaking). Well, bouncing one particle off another affects the properties of both. Observing the world changes it.

What it *doesn't* mean, is some kind of neo-magic effect of being able to "dictate" reality by some act of will. But "science" reporters can never seem to resist allusions to some notion that quantum physics allows us to dictate reality by some kind of pseudo scientific psychic power.

This also applies to collapsing quantum states. Measurement may collapse them, but even so, that doesn't mean we in any way *choose* how they collapse.

[indigobusiness]

6. It is the nature of the collapse that is being discussed

(if I'm not mistaken)

and broadly speaking only adds to the difficulty of this topic, it doesn't help clarify it.

If there is any realm where specifics matter, it is this.

[Ready4Change]

7. From reading this thread...

I like to get an impression of things when I read about them, then re-cast them, throw that at the more knowledgable parties, and see if what I say makes sense or not. Here goes:

This sounds like erosion. All the quantum states and reactions are sand, and "reality", or observation, is rain. It washes away the less stable, softer bits and leaves behind the harder, more stable bits. The softer, less likely bits are still very much real, but they just don't have as strong an effect on the final big picture.

Viewed up close there are just peaks and washed out areas. But viewed from further away patterns emerge. It is at this point that we leave the quantum realm and enter more classical physics.

Yah?