Reversing wavefunction collapse

In summary, the question of whether the observational process from quantum to classical can ever reverse is highly debated and controversial. It depends on the interpretation of quantum mechanics and the definition of a measurement. Those who believe that quantum mechanics applies to everything argue that in principle, the transition can be reversed, but it is practically impossible to control all the quantum degrees of freedom. However, those who follow the Copenhagen interpretation believe that measurements involve a special, irreversible process and thus cannot be reversed. Currently, there is no experimental evidence to support either view, but as technology advances, it may become possible to challenge the Copenhagen interpretation.
  • #1
Loren Booda
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Does the observational process quantum-->classical ever reverse?
 
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  • #2
That's a very controversial question because it depends on exactly what you mean by quantum-->classical.

If you take the view that quantum mechanics applies to everything then there really is no quantum-->classical transition and there is no fundamental reason why the transition that occurs in a measurement could not be reversed. However, there is a practical problem in that you would have to be able to control the quantum mechanical degrees of freedom of very complicated objects, including the measuring device, the environment of the measuring device and possibly the observer as well.

However, if you take the view that wavefunction collapse represents some real physical process, then it would not be possible to reverse the transition that occurs in a measurement even in principle.

If you take a Copenhagenish view, then a measurement is simply a 'thing' that casues an irreversible quantum-->classical transition and hence it cannot be reversed by definition. If you found something that you say is a reversible quantum-->classical transition then a copenhagenist would simply deny that such a thing could be regarded as a measurement in the first place. It is a common copenhagen strategy is to frame things in a way such that seemingly interesting questions actually have no meaning. Depending on your view, this is either an extremely useful way to think about quantum mechanics or simply plain nonsense (I opt for the latter).

At present there is no clear-cut experimental way to resolve this issue and clearly no way to rule out the Copenhagen view in any case. However, as the technology for coherent control of larger and larger systems is developed then it should be possible to at least make one of the other positions seem incredibly unlikely.
 
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  • #3
Even in the Copenhagen interpretation, it seems possible to "reverse" wave function collapse in a sense. An isolated system once measured can't be "unmeasured," but nothing is isolated. Let it interact with something, and you'll have a mixed state again. Now separate everything out again, and you can repeat your experiment. The environment will be slightly changed though...
 
  • #4
From what I remember, "environment induced decoherence" would prevent the "undoing" of a measurement. However, I have seen articles about a "quantum eraser". I think one of the authors of this article was a guy named Scully. You can do a search on this. I think there may be other articles that include references to a "quantum eraser" or "quantum erasure".
 
  • #5
"Environment induced decoherence" prevents the undoing of a measurement in a practical sense, as I was trying to explain in my previous post without introducing the jargon. Actually, it is the same sense in which one cannot reverse a transition to thermal equilibrium. For example, suppose you have a box with heat-proof partition in the middle with cold gas on one side and hot gas on the other. When you remove the partition, the gasses will mix together and you will end up with a box full of warm gas. It is theoretically possible to reverse this transition, if you could control all the microscopic degrees of freedom of the gas particles, but in practice it is almost impossible.

Even in the Copenhagen interpretation, it seems possible to "reverse" wave function collapse in a sense. An isolated system once measured can't be "unmeasured," but nothing is isolated. Let it interact with something, and you'll have a mixed state again. Now separate everything out again, and you can repeat your experiment. The environment will be slightly changed though...

This is true in an interpretation that says everything can be described by quantum physics, which is assumed by most modern physicists. On the other hand, Copenhagen assumes that the measurement process cannot be described by the unitary dynamics of quantum physics and so a copenhagenist would simply deny that what you have described constitutes a measurement. Simply put, if you have control of all the quantum degrees of freedom of a system, then that system is not a measuring device according to Copenhagen.
 

1. What is wavefunction collapse?

Wavefunction collapse is a phenomenon in quantum mechanics where a quantum system's state is determined upon observation or measurement. Prior to measurement, the system is described by a wavefunction that contains all possible states the system could be in. However, upon measurement, the system is found to be in only one of those possible states, and the wavefunction "collapses" to that specific state.

2. Can wavefunction collapse be reversed?

Currently, there is no scientific evidence or theory that suggests that wavefunction collapse can be reversed. The collapse of the wavefunction is considered to be a fundamental aspect of quantum mechanics and is widely accepted by the scientific community.

3. Why would we want to reverse wavefunction collapse?

The idea of reversing wavefunction collapse is often explored in science fiction and theoretical discussions, but it has no practical applications in reality. The collapse of the wavefunction is necessary for the functioning of quantum systems, and reversing it would go against the laws of quantum mechanics.

4. Is there any research being done on reversing wavefunction collapse?

While there may be some theoretical discussions on the concept, there is currently no active research being conducted on reversing wavefunction collapse. The scientific community generally agrees that it is not possible.

5. How does wavefunction collapse relate to the Schrödinger's cat thought experiment?

Schrödinger's cat is a thought experiment that illustrates the concept of superposition, where a cat in a sealed box can be both alive and dead at the same time. Upon opening the box and observing the cat, the wavefunction collapses, and the cat is found to be either alive or dead. This thought experiment highlights the role of observation and measurement in wavefunction collapse.

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