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diogenes0
2009-Nov-27, 10:57 AM
I'm trying to understand roughly what quantum decoherence is about. I tried to explain it to myself in the following way, so if anyone can tell me if this makes any sense I'd appreciate it!

As you get more and more particles' wave functions interacting with one another, you begin to greatly limit the plausible values for their various variables.

In other words, a single particle's superpositions might contain a varied range of possible values for its variables (many with significant probabilities) -- so no decoherence. But, since even small classical objects are made up of so many billions upon billions of particles (and their interacting wave functions), the range of possible values for those particles' respective variables become restricted to that which will result in the classical object. The restriction to certain values would still be probabilistic, but with the probabilities for the classical outcome increasingly approaching 100% with every additional particle. And since there is a lot of radiation everywhere interacting with everything, decoherence is the norm. Was that anywhere near correct?

Ken G
2009-Nov-27, 04:40 PM
It's not far off, I'd say you have a useful kernel there from which to think about decoherence. To take it to the next level of understanding, you need to recognize that the issue is not so much the range of values for any subset of the system (that doesn't necessarily hone in on one result if you add more subsystems, though the average over all the subsystems will approach a central limit), it is the correlations between the behavior of that subsystem and any other subsystem, or even between that subsystem and itself. Take the classic two-slit experiment. If all you have is a single noninteracting quantum, there are correlations between the two slits that cause a two-slit diffraction pattern, not a sum of two one-slit patterns. But if you embed that quantum into a complex macro system and it undergoes decoherence, then you'll get two one-slit patterns added up. The range of behaviors for the quantum is not all that different-- it still presents a wide diffraction pattern, but the details are quite different because of the presence or absence of those correlations. In a nutshell, decoherence replaces a situation of "a little of both" with a situation of "either one or the other and we just don't know which", so that's all about internal correlations.

nokton
2009-Nov-27, 05:23 PM
Thanx diogenes for asking the question, and Ken Gs response.
Quantum decoherence is a property determined by size and mass.
The relationship between the quantum world and the classical
world has a point where one can be either or, but never the same.
The quantum world has different rules, and physics we are not aware of.
Quantum decoherence occurs when quantum physics breaks down in the
face of increased mass, then classical physics rule.
I would think, if we could determine the point of decoherence, we could
understand much more.
Nokton

dhd40
2009-Nov-28, 07:54 PM
Quantum decoherence is a property determined by size and mass.
What do you mean by "size" and "mass"? Is it simply the number of individual particles (what I think it is) in an "agglomerate" of particles?


Quantum decoherence occurs when quantum physics breaks down in the face of increased mass, then classical physics rule.
I would think, if we could determine the point of decoherence, we could
understand much more.
Nokton

That point of decoherence: Would you expect it to be a fixed number, say, 17 H-atoms, or would you expect it to be smeared out over a certain range of H-atoms, e.g. gradually starting at 14 H-atoms and ending at 20 H-atoms? In other words, would the transition from coherence to decoherence look like a step function?

(I wonder whether my questons make sense at all)