Undergrad Double Slit Experiment: Questions Raised & Explanations Needed

[.Scott]

That is why I picked it [the delayed erasure experiment] as a starting point.

Then why was it couched in terms of the concept of a wave-function and not decoherence?

Ahhh. I may be using inexpert and incorrect terminology. But I'm not sure if my point is lost.
Are you saying that a wave function collapse is not required for decoherence?
If that is true, what is the ultimate measure of wave function persistence or collapse?

If there is no way to determine when a wave function collapses, then I would think that the null hypothesis is that they never collapse.

 

[bhobba]

Are you saying that a wave function collapse is not required for decoherence? If that is true, what is the ultimate measure of wave function persistence or collapse?.

Collapse is not part of the QM formalism - just some interpretations eg MW doesn't have it.

I have zero idea what you mean by wave-function persistence.

When an observation happens the thing being observed is either destroyed or subject to a state preparation procedure. Obviously its state will change if you prepare a system differently. Decoherence explains how a preparation procedure results in a new state - with one exception - the so called problem of outcomes, which is simply why do we get an outcome at all ie why does a state result. By definition states and state preparation procedures are equivalent. So the question is - why do state preparation procedures exist -.its a basic assumption o QM they do. Every theory - every single one - has primitives unexplained by the theory - this is QM's fundamental one. Of course some interpretations attempt to explain it.

Thanks
Bill

 

[.Scott]

Collapse is not part of the QM formalism - just some interpretations eg MW doesn't have it.

OK. But since I was asking about a post from Nugatory that used the term, I will stick with that interpretation for this discussion.

I have zero idea what you mean by wave-function persistence.

It's just what I called what happens when the wave function does not collapse.

When an observation happens the thing being observed is either destroyed or subject to a state preparation procedure. Obviously its state will change if you prepare a system differently. Decoherence explains how a preparation procedure results in a new state - with one exception - the so called problem of outcomes, which is simply why do we get an outcome at all ie why does a state result. By definition states and state preparation procedures are equivalent. So the question is - why do state preparation procedures exist -.its a basic assumption o QM they do. Every theory - every single one - has primitives unexplained by the theory - this is QM's fundamental one. Of course some interpretations attempt to explain it.

I was asking about this post from Nugatory:

Experiments of this sort do not disprove the consciousness-causes-collapse hypothesis. The problem is that you cannot distinguish between whether the video camera collapsed the wave function or whether the video camera itself entered a superposition of "recorded this/recorded that" and the collapse happened when you, the conscious observer, looked at the image recorded by the video camera. Either way, you get the same experimental outcome: the camera recorded the image you saw.

As you can see, he was using the "wave function collapse" interpretation.
I questioned his assertion that "you cannot distinguish between whether the video camera collapsed the wave function or whether the video camera entered a superposition". It seems to me that we can devise experiments to make this distinction - so, bearing in mind that Nugatory can ace me on any QM discussion, I asked some questions.

I cannot tell from your responses whether you (bhobba) agree with Nugatory or not.

From what I can tell, you have dropped into a level of detail that is not relevant to Nugatory's assertion about distinctions. If there is a connection, I am completely missing it.

 

[bhobba]

I cannot tell from your responses whether you (bhobba) agree with Nugatory or not.

Of course I agree with him.

You are getting a bit too caught up in semantics. Physicists, like people in ordinary conversation, can be loose in terminology. Here, by collapse, Nugatory meant an actual outcome occurred. The modern incarnation of collapse, with our current understanding of decoherence, is why do we get any outcomes at all.

There is no way to tell where an actual outcome occurs. This is the famous Von-Neumann regress. The modern solution is to say it occurs just after decoherence because it solves many issues - but there is no way of proving it.

Thanks
Bill

 

[.Scott]

Of course I agree with him.

You are getting a bit too caught up in semantics. Physicists, like people in ordinary conversation, can be loose in terminology. Here, by collapse, Nugatory meant an actual outcome occurred. The modern incarnation of collapse, with our current understanding of decoherence, is why do we get any outcomes at all.

There is no way to tell where an actual outcome occurs. This is the famous Von-Neumann regress. The modern solution is to say it occurs just after decoherence because it solves many issues - but there is no way of proving it.

Thanks
Bill

So decoherence is no more evidence of collapse than any other outcome.
In that case, even human consciousness can be viewed as insufficient to trigger a collapse. Even without MWI, our conscious experience could be part of the wave function until collapsed by some further condition. Then, once collapsed, we have the outcome of a conscious experience.

This would seem to position wave function collapse precariously close to the domain of philosophy.

Am I on the right track?

 

[bhobba]

So decoherence is no more evidence of collapse than any other outcome.

I have zero idea what you mean by that.

Technically decoherence explains how a superposition is converted into a mixed state. The mixed state gives the probability of each of its parts occurring. We can say that happened just after decoherence or at any point in the process until it reaches a conscious observer. We can even say it occurs at the conscious observer.

Thanks
Bill

 

[sophiecentaur]

Is there something you can give as an example of possible physics many millions years ago, compared to physics now?

The light arriving here from a very distant star could well have been produced by a 'Physical Process' that took place a million years ago. If Physics had been that different in those days, the spectrum of the light would hardly be expected to be the same as light we observe that's produced in a lab. Fact is that the spectral patterns of all the light we see from distant objects are exactly the same as from local sources. The only difference is the Red Shift and the observed red shift seems to tie in with predictions pretty well.
So we can be pretty happy that Physics has not changed since humans have been 'observing' it.

But people are a bit too hung up on the term "observer" as if it only implies a guy in a white coat with a microscope. The atom that gets changed -or not- by the effect of an incident electron or photon is just as much an 'observer'. I'd bet the originator of the term 'observer' would be turning in his / her grave if they realized that people had failed to spot what they were getting at whilst using the term. But anthropomorphism is alive and well at a place near where you live.

 

[sophiecentaur]

until it reaches a conscious observer

Why does an 'observer' need to be conscious for an effect to take place? I could ask just how 'conscious' an observer needs to be, in order to be rated as one. Is my Aunt, who knows no Physics, qualified to identify a wave function collapsing? Would someone need to have a PhD to qualify? Could my dog do it? I could train him to bark when he sees it happen and, that way, he could make me conscious of the event.

 

[Nugatory]

This would seem to position wave function collapse precariously close to the domain of philosophy.
Am I on the right track?

That's as good of a track as any, and better than most.

 

[.Scott]

I have zero idea what you mean by that.

Let me rephrase it more explicitly: decoherence, as an experimental outcome, is no more an indication of wave function collapse as any other experimental outcome.

Technically decoherence explains how a superposition is converted into a mixed state. The mixed state gives the probability of each of its parts occurring. We can say that happened just after decoherence or at any point in the process until it reaches a conscious observer. We can even say it occurs at the conscious observer.

In my last post, a point I was making is that we can even say that collapse happens well beyond the point of the conscious observer.

 

[bhobba]

Why does an 'observer' need to be conscious for an effect to take place?

Of course it doesn't.

It can occur anywhere in the Von Neumann chain.

Technically its when an improper mixed state becomes a proper one which can be anywhere. There is no way to tell because an improper mixed state and a proper one are observationally the same.

In modern times its placed just after decoherence - that's where I place it in my interpretation because it resolves many issues - but you don't have to place it there.

Thanks
Bill

 

[bhobba]

In my last post, a point I was making is that we can even say that collapse happens well beyond the point of the conscious observer.

Well there you face a real problem because once a conscious observer is involved by the definition of conscious they experience an outcome as the result of the act of observation.

Thanks
Bill

 

[bhobba]

I could ask just how 'conscious' an observer needs to be, in order to be rated as one. Is my Aunt, who knows no Physics, qualified to identify a wave function collapsing? Would someone need to have a PhD to qualify? Could my dog do it? I could train him to bark when he sees it happen and, that way, he could make me conscious of the event.

Of course.

The reason its mentioned in this context is historical dating back to Von Neumann's original analysis. The above is just one of the many issues it faces and why most, including me, reject it.

Thanks
Bill

 

[.Scott]

Well there you face a real problem because once a conscious observer is involved by the definition of conscious they experience an outcome as the result of the act of observation.

It's not a real problem at all. Part of some outcomes is the conscious experiencing of the outcome. If it's not a problem for a video signal, it shouldn't be a problem for brain activity. If you allow video signals to be superimposed, why not the conscious experience of a human observer?

 

[abrogard]

It was developed by Schroedinger as an intuitive leap using rather dubious mathematics:
http://arxiv.org/abs/1204.0653

Ever since Dirac came up with his transformation theory the wave-function was recognised as no longer fundamental:
http://www.lajpe.org/may08/09_Carlos_Madrid.pdf

What's going on in the delayed choice experiment is well known and has been discussed in many threads on the forum:
https://www.physicsforums.com/threa...and-the-delayed-choice-quantum-eraser.623648/
'Decoherence is irreversible only when caused by a LARGE number of degrees of freedom. A quantum eraser involves a small number of degrees of freedom, which is why it is reversible.'

Thanks
Bill

I can't get that Carlos Madrid .pdf. Would you happen to have another source for it, please?

 

[bhobba]

I can't get that Carlos Madrid .pdf. Would you happen to have another source for it, please?

See attached

Thanks
Bill

 

[bhobba]

It's not a real problem at all. Part of some outcomes is the conscious experiencing of the outcome. If it's not a problem for a video signal, it shouldn't be a problem for brain activity. If you allow video signals to be superimposed, why not the conscious experience of a human observer?

By conscious observer is meant, and rather obviously so, an observer like us. We never experience superpositions. But even aside from that decoherence converts superpositions into mixed states so there is never a superposition to experience anyway.

As explained previously the entire issue is when does an improper mixed state become a proper one.

Thanks
Bill

 

[abrogard]

See attached

Thanks
Bill

Got it. Thank you, Bill.

:)

 

[zonde]

We never experience superpositions.

This is faulty argument. Superposition isn't something you could experience.

P.S. I like the way stevendaryl put it: https://www.physicsforums.com/threads/decoherence-question.808491/page-2#post-5079618

 

[Nugatory]

Many off-topic digressions and pieces of random misinformation have been removed from this thread in an attempt to flog it into a fair summary of the current understanding and issues around the role of consciousness in QM, and into a sensible response to @nnope's original (and quite reasonable) question. At this point I think that we've covered most of the ground, especially in an I level thread, so I am leaving the thread closed.

I may have overreached and removed some posts that raise interesting points around this discussion. If so, I apologize and would encourage you to either start another thread or PM me or one of the other mentors asking them to reopen this thread so you can add your post back in. The posts have been hidden, not deleted, so if you want a copy of one of the deleted posts, you can ask for it via PM as well.