Input on a basic Copenhagen question

[Mr Book]

TL;DR Summary

Am I wrong about the Copenhagen Interpretation mandating causation into the past?

As if you couldn't guess from the question, I'm very much a layman wrt physics, but here it is anyway:

I've read a number of (non-technical) books on quantum physics but I can't seem to find much - indeed anything at all - on the role of time in collapse of the wave function when considering the Copenhagen interpretation. Specifically, how observation must influence past events if the Copenhagen interpretation is to be believed.

I recall reading that a group of scientists proposed an argument against the CI, questioning how observation could cause wave collapse prior to any observers existing, i.e. pre-life on Earth. On the face of it, this seemed reasonable, as how could someone alive today influence what happened billions of years in the past. But isn't that what happens with any observation? The time-span is of course shorter, with an experimental lag being the time it takes the photons to reach the eye of the observer and be processed in the brain, but the concept is the same - that the wave collapse must occur in the past, irrespective of whether 'the past' refers to a billionth of a second or ten billion years.

Am I wrong about this? If not, does anyone know where I can read about it (in laymen's terms!)

 

[PeroK]

Summary:: Am I wrong about the Copenhagen Interpretation mandating causation into the past?

Yes, that's wrong. If you are going to consider the interaction of a distant galaxy with something on Earth, then you have a physically extended system that evolves over millions or billions of years. You can't consider it as a pair of isolated systems with subsequent retro-causation.

 

[Mr Book]

I'm not sure I understand that, but retrocausality seems to give some avenues of research.

As for my confusion, I'm not clear what the timescale has to do with it. If the past can be affected by the present, what does the length of time have to do with it? If observing the double-slit experiment directly in the lab causes waveform collapse, as some believe, then so surely must performing the experiment and, say, waiting a year to view the results. And if a year, why not ten, or ten billion? How is an unobserved experiment in a lab different to an unobserved creation of a star in terms of waveform collapse?

 

[PeroK]

I'm not sure I understand that, but retrocausality seems to give some avenues of research.

As for my confusion, I'm not clear what the timescale has to do with it. If the past can be affected by the present, what does the length of time have to do with it? If observing the double-slit experiment directly in the lab causes waveform collapse, as some believe, then so surely must performing the experiment and, say, waiting a year to view the results. And if a year, why not ten, or ten billion? How is an unobserved experiment in a lab different to an unobserved creation of a star in terms of waveform collapse?

The length of time, in a sense, has nothing to do with it. You may have a system with a scale of several metres (double-slit) which evolves over a few seconds or a system of billions of light years (observing distant galaxy) which evolves over billions of years. There's no need for retrocausaility in either case.

You have the measurement problem, of course.

I suspect you've been misled by popular science sources trying to sensionalise QM with tales of retrocausality at the heart of the CI. It ain't so.

That said retrocausality is one of the possible explanations for entanglement. See:

https://www.physicsforums.com/threads/question-about-an-entanglement-paper.966466/#post-6135121

But that's not the CI.

 

[StevieTNZ]

You've placed this thread into the 'Intermediate' grade - which I suspect is in error.

A good book to learn about quantum theory for the layman is https://www.bookdepository.com/Sneaking-a-Look-at-God-s-Cards/9780691130378

 

[StevieTNZ]

Another scholarly published book (by Oxford University Press) is https://quantumenigma.com/book-description/ That has some pages dedicated to Wheeler's delayed choice experiment.

 

[PeroK]

John Bell thought it hard to believe that a conscious human being with detailed PhD-knowledge about QM, makes the universal (or Earthly) wavefunction collapse. A strict follower of the Copenhagen interpretation and the axioms following, can always maintain this though. Only if non-local hidden variables are shown to exist, this "irrefutable" stand can be proven false.

There is nothing in the CI that requires "consiciousness to cause collapse". Technically, collapse requires the interaction of a QM system with a macroscopic measuring device - hence the measurement problem.

It may also be worth learning about the concept of decoherence.

 

[DrChinese]

I'm not sure I understand that, but retrocausality seems to give some avenues of research.

As for my confusion, I'm not clear what the timescale has to do with it. If the past can be affected by the present, what does the length of time have to do with it? If observing the double-slit experiment directly in the lab causes waveform collapse, as some believe, then so surely must performing the experiment and, say, waiting a year to view the results. And if a year, why not ten, or ten billion? How is an unobserved experiment in a lab different to an unobserved creation of a star in terms of waveform collapse?

As this is the Quantum Interpretations section, there is some latitude to discuss causality with respect to various types of collapse. In your example regarding wave collapse, a couple of points should be considered.

1. Wave collapse may or may not be "physical". It is not clear that anything special happens at a specific time and place. I.e. for entangled particle pairs: is there is an actual effect that goes from A to B (or B to A)? There is no evidence either way. If there were such an effect: note that in one direction (say A to B) there would be causality; or in the other direction (B to A) there would be retrocausality. There is no preferred descriptive direction in standard QM.

2. There does not need to be an actual measurement to trigger collapse, and certainly there is no requirement that human consciousness is required. Perhaps a better way to visualize the situation is to examine the overall "context". QM is contextual. The context of a quantum mechanical setup can involve multiple (many, infinite?) spots in spacetime, and they need not share a traditional light cone. Once this is clear, you realize that trying to apply classical causation to the setup can never be possible.

In virtually any interpretation of QM, there is no true element of causality. I would say QM is acausal.

 

[Mattergauge]

Another scholarly published book (by Oxford University Press) is https://quantumenigma.com/book-description/ That has some pages dedicated to Wheeler's delayed choice experiment.

Suppose I perform a double slit experiment with electrons. Very low electron intensity. Every time an electron has passed the two slits (shortly after) I close one slit. Will the total wavefunction notice this? I.e, will an interference pattern arise or two particle like regions, on the screen?

 

[PeroK]

Suppose I perform a double slit experiment with electrons. Very low electron intensity. Every time an electron has passed the two slits (shortly after) I close one slit. Will the total wavefunction notice this? I.e, will an interference pattern arise or two particle like regions, on the screen?

How do you know when the electron has passed the slits?

 

[StevieTNZ]

Suppose I perform a double slit experiment with electrons. Very low electron intensity. Every time an electron has passed the two slits (shortly after) I close one slit. Will the total wavefunction notice this? I.e, will an interference pattern arise or two particle like regions, on the screen?

To have known the electrons have passed the slits, you need to place detectors there. Therefore you'll get no interference. If you decide to close the slit after the system reaches the end screen, the interference pattern doesn't change.

 

[Mattergauge]

To have known the electrons have passed the slits, you need to place detectors there. Therefore you'll get no interference. If you decide to close the slit after the system reaches the end screen, the interference pattern doesn't change.

I mean closing after a one-electron wave has passed the two slits, but before it hits the screen. Will the closing affect the the superposition?

 

[DrChinese]

Suppose I perform a double slit experiment with electrons. Very low electron intensity. Every time an electron has passed the two slits (shortly after) I close one slit. Will the total wavefunction notice this? I.e, will an interference pattern arise or two particle like regions, on the screen?

For a double slit setup: if you close a slit when you *know* (by whatever means) that the particle has already gone by the 2 slits, then no... of course there is no change in the interference pattern.

Further: It is possible with photons to place polarizers over each of the 2 slits. When the polarizers are parallel, there WILL be interference. When the polarizers are crossed (perpendicular), there is NO interference pattern. In the crossed case, you will know which slit the photon traversed even without looking at polarization of each individual particle. So effectively you can learn which slit information without blocking a slit.

 

[PeterDonis]

I mean closing after a one-electron wave has passed the two slits

That doesn't change the answer @StevieTNZ gave you; in order to know that the "one-electron wave" has passed the slits, you need to place a detector there, and doing that destroys the interference.

 

[StevieTNZ]

I mean closing after a one-electron wave has passed the two slits, but before it hits the screen. Will the closing affect the the superposition?

I suggest reading 'Sneaking a Look at God's Cards'.

 

[PeterDonis]

How is an unobserved experiment in a lab different to an unobserved creation of a star in terms of waveform collapse?

The original Copenhagen interpretation, with its focus on "observers" is outdated now that we have a considerable body of theory on decoherence. Decoherence theory makes it clear that, for example, when a star forms, it decoheres almost instantly, and that is sufficient to cause "wave function collapse" in any QM interpretation where collapse is a thing, including any modern version of Copenhagen. The fact that it might be ten billion years before a human observes light from the star is irrelevant.