Does "Decoherence" really explain anything?

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I just read an excerpt from Philip Ball's new book and was disappointed by its hyperbolic, boosterish tone. He adopts the pose that physicists have made great strides in understanding what happens during measurement. Wave functions decohere, etc. He concludes by saying something like "now we just need to understand what causes this...."
Is this any better than saying "my new theory explains everything--superposition occurs when magic unicorns play with each other and then we observe only one state at measurement when the magic unicorns become angry with each other and are by themselves!"
Concretely, has there been contemporary advance in the mathematics that allows better predictions than what Heisenberg / Schroedinger et. al had available to them? Is this just a matter of Heisenberg's uncertainty principle with a new interpretation (that, so far, doesn't seem to offer anything except hope...)?
 

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  • #2
stevendaryl
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I just read an excerpt from Philip Ball's new book and was disappointed by its hyperbolic, boosterish tone. He adopts the pose that physicists have made great strides in understanding what happens during measurement. Wave functions decohere, etc. He concludes by saying something like "now we just need to understand what causes this...."
Is this any better than saying "my new theory explains everything--superposition occurs when magic unicorns play with each other and then we observe only one state at measurement when the magic unicorns become angry with each other and are by themselves!"
Concretely, has there been contemporary advance in the mathematics that allows better predictions than what Heisenberg / Schroedinger et. al had available to them? Is this just a matter of Heisenberg's uncertainty principle with a new interpretation (that, so far, doesn't seem to offer anything except hope...)?
Decoherence explains why it is so hard (or impossible) to observe superpositions of macroscopically different states. So what that means is that a lot of the subtleties and controversies of interpreting quantum mechanics cannot be easily resolved by experiment (particularly questions of "wave function collapse").
 
  • #3
PeroK
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I just read an excerpt from Philip Ball's new book and was disappointed by its hyperbolic, boosterish tone. He adopts the pose that physicists have made great strides in understanding what happens during measurement. Wave functions decohere, etc. He concludes by saying something like "now we just need to understand what causes this...."
Is this any better than saying "my new theory explains everything--superposition occurs when magic unicorns play with each other and then we observe only one state at measurement when the magic unicorns become angry with each other and are by themselves!"
Concretely, has there been contemporary advance in the mathematics that allows better predictions than what Heisenberg / Schroedinger et. al had available to them? Is this just a matter of Heisenberg's uncertainty principle with a new interpretation (that, so far, doesn't seem to offer anything except hope...)?
I guess the book is: "Why everything you thought you knew ..."

https://www.philipball.co.uk/

Yes, well, that book is more entertainment than science. One reviewer said:

"Easily the best book I’ve read on the subject." - Margaret Wertheim, Washington Post

I doubt, however, that she has read this one:

https://www.goodreads.com/book/show/211439.Modern_Quantum_Mechanics

Which is science, rather than entertainment.
 
  • #4
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Concretely, has there been contemporary advance in the mathematics that allows better predictions than what Heisenberg / Schroedinger et. al had available to them?
Well, there's Schrödinger's cat in a box. The 1930-vintage quantum mechanical prediction for this situation was so bad that no one believed it (which was, of course, the point that Schrodinger was making when he suggested the thought experiment). Incorporating decoherence into the theory leads to a better prediction consistent with everything else we know about feline mortality.

So to answer the question in the thread title: yes, decoherence explains something. It explains how the macroscopic world can behave the way we know it does, yet still be consistent with quantum mechanics.
 
  • #5
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Decoherence explains why it is so hard (or impossible) to observe superpositions of macroscopically different states.
Is there any way to experimentally confirm the very short time scales that decoherence predicts? (E.g., are there certain interactions for which it predicts a measurable time that no other theory predicts?)

(Short version: Is there any empirical evidence for this supposition of "Decoherence"?)
 
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  • #6
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Is there any way to experimentally confirm the very short time scales that decoherence predicts? (E.g., are there certain interactions for which it predicts a measurable time that no other theory predicts?)

(Short version: Is there any empirical evidence for this supposition of "Decoherence"?)
Decoherence predicts that superposition/interference effects will disappear from a macroscopic system very quickly. Without decoherence, the prediction is that these effects will not disappear.
They do disappear, so the alternatives are all falsified.

Another example comes from the many experiments that demonstrate superposition of small but still macroscopic objects. All of these work by controlling decoherence,and the better it is controlled the longer-lived the superposition is. So we can say that the theory works for all experimentally accessible time scales.

Generally when we've reached this point, it's sterile to reject a theory unless either there are serious nternal inconsistencies or there is a plausible candidate for an alternative theory.
 
  • #7
stevendaryl
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Is there any way to experimentally confirm the very short time scales that decoherence predicts? (E.g., are there certain interactions for which it predicts a measurable time that no other theory predicts?)

(Short version: Is there any empirical evidence for this supposition of "Decoherence"?)
Decoherence is an inevitable feature of quantum mechanics, once you have a large number of particles involved. It's not an additional hypothesis, it's a consequence of Schrodinger's equation, applied to systems involving many degrees of freedom.
 
  • #8
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Decoherence predicts
Doesn't the mathematics underlying Heisenberg's Uncertainty Principle also essentially make these same predictions?
 
  • #9
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Doesn't the mathematics underlying Heisenberg's Uncertainty Principle also essentially make these same predictions?
No, and in fact I'm having some trouble even seeing a connection. Decoherence is about the time evolution of coherent superpositions interacting with the environment, and the uncertainty principle is a statement about the eigenfunctions of non-commuting observables.
 
  • #10
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Decoherence happens whenever a small quantum system interacts with a big environment.

Measurement devices are neccessarily macroscopic and therefore somehow big in relation to the quantum systems which we use in real experiments. So if we want to describe the interaction between a system of interest and a measurement device quantum mechanically, the resulting quantum dynamics of the system of interest shows the effect of decoherence.

But the quantum dynamics of open systems is much more general than the special case of trying to describe what happens during a measurement. Whenever you follow the dynamics of a subsystem (like an oscillating molecule) which isn't perfectly isolated (like when it is oscillating in air or water), you get open quantum dynamical effects like decoherence of varying strength.

So decoherence is primarily an observable physical phenomenon and only secondly a part in the puzzle of the measurement problem.
 
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  • #11
zonde
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There is decoherence (1) and there is (environment induced) decoherence hypothesis (2).
(1) is observable physical phenomenon. For example here: https://arxiv.org/abs/quant-ph/0402146
(2) is a patch for interpretation of QM that rejects any hidden variables (Copenhagen and similar interpretations).
In Copenhagen interpretation any system within ensemble of identically prepared systems is completely described by wavefunction. As a result in experimental setup where there is "single particle" interference between two spatially separate paths all detection events that contribute to interference pattern should result from identical physical processes. As a result we have physical process where "particle travels by both paths". Such process is meaningless in "classical" world and so we have to imagine that at this microscopic level physical mechanisms are unlike "classical" world i.e. it is counterintuitive quantum world.
Environment induced decoherence hypothesis is supposed to make counterintuitive quantum world appear like classical world. Well, that's the motivation. Result however is nothing more than smoke and mirrors.
 
  • #12
stevendaryl
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There is decoherence (1) and there is (environment induced) decoherence hypothesis (2).
(1) is observable physical phenomenon. For example here: https://arxiv.org/abs/quant-ph/0402146
(2) is a patch for interpretation of QM that rejects any hidden variables (Copenhagen and similar interpretations).
It's not just a hypothesis that interaction with the environment leads to decoherence. The sleight-of-hand that people sometimes use that is something like this:
  1. We start off with some system in superposition of several eigenstates of some observable. This system is described by a pure state density matrix.
  2. The system becomes entangled with the environment. Now the system no longer has its own state, but the system+environment is describable by a composite state.
  3. Because the environmental degrees of freedom are unobservable (and unknown), we trace over the environmental degrees of freedom to get a density matrix for the system alone. The result is described by a mixed-state density matrix.
  4. A mixed state density matrix can be reinterpreted as describing a situation where the system has a definite, though unknown value for the observable.
Steps 1 & 2 are just quantum mechanics. Step 3 is just pragmatic, we're replacing an unknown composite density matrix by an effective mixed density matrix, which is easier to reason about. Step 4 is the one that seems like a sleight-of-hand move. There are two ways to get a mixed state density matrix: (1) By incorporating classical uncertainty, or (2) by performing a trace over some degrees of freedom. In Steps 1-3, we're clearly dealing with the second way, and then in Step 4, we pretend we're dealing with the first way. This gives you collapse without collapse.
 
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  • #13
zonde
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It's not just a hypothesis that interaction with the environment leads to decoherence. The sleight-of-hand that people sometimes use that is something like this:
  1. We start off with some system in superposition of several eigenstates of some observable. This system is described by a pure state density matrix.
  2. The system becomes entangled with the environment. Now the system no longer has its own state, but the system+environment is describable by a composite state.
  3. Because the environmental degrees of freedom are unobservable (and unknown), we trace over the environmental degrees of freedom to get a density matrix for the system alone. The result is described by a mixed-state density matrix.
  4. A mixed state density matrix can be reinterpreted as describing a situation where the system has a definite, though unknown value for the observable.
Steps 1 & 2 are just quantum mechanics. Step 3 is just pragmatic, we're replacing an unknown composite density matrix by an effective mixed density matrix, which is easier to reason about.
Certainly, there is no handwaving in steps 1 to 3.

Step 4 is the one that seems like a sleight-of-hand move. There are two ways to get a mixed state density matrix: (1) By incorporating classical uncertainty, or (2) by performing a trace over some degrees of freedom. In Steps 1-3, we're clearly dealing with the second way, and then in Step 4, we pretend we're dealing with the second way. This gives you collapse without collapse.
Interpretation part in step 4 is, well, interpretation dependent. If we assume hidden variables i.e. then we have mixture of hidden variables already in steps 1-3. So disappearance of coherence does not make system to assume some definite state because it had it already before decoherence. But that of course does not mean that interpretation in step 4 is wrong, just unnecessary.
However interpretation in step 4 is problematic when considering Wheeler's delayed-choice experiment. In closed state beams can interfere at second beamsplitter and show interference. So there should be no decoherence. In open state we take away second beamsplitter so there is no reason to introduce decoherence anywhere and yet the photons have to have definite state right after first beamsplitter (without any decoherence taking place).
Alternatively you can say that decoherence takes place when photon encounters detector. Well, this could work given you have no objections toward non-local decoherence as replacement for non-local collapse.
 
  • #14
f95toli
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Is there any way to experimentally confirm the very short time scales that decoherence predicts? (E.g., are there certain interactions for which it predicts a measurable time that no other theory predicts?)

(Short version: Is there any empirical evidence for this supposition of "Decoherence"?)
This is slightly bizarre -but understandable- statement. "Decoherence" is not just some philosophical add-on to QM as some books would like you to think. It is an integral part of every single real-life experiment and application of quantum mechanics.
The coherence time (the time before the system decoherence) is a performance metric (In the form of T1, T2 and T2*) of atomic clocks, qubits for quantum computers, many sensors etc.
Not to mention NMR/MRI which is to some extent just a measurement of decoherence (an MRI image is essentially just a spatial map of the T1 time -the time it takes for the atoms to relax- as function of position).

There are lots of people who have spent a good part of their working life trying to come up with methods for avoiding decoherence in a specific system (I am one of them:cool:)
 

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