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Author Topic:   Quantum physics: Copenhagen vs decoherence interpretations
Son Goku
Inactive Member


Message 31 of 57 (473305)
06-28-2008 7:40 AM
Reply to: Message 30 by PaulK
06-22-2008 3:43 PM


Re: Foundational QM
For anybody unfamiliar with delayed choice there is a description of the experiment: here
Randman holds that arbitrary actions can - somehow radically change the past based on the "delayed-choice" experiments. My understanding is that these experiments would only narrow down the possible past states.
Well delayed choice experiments certainly cannot change the past. There is actually a theorem to that effect. That is QM, in any given interpretation, does not allow influence of the past.
Could you please explain how delayed choice experiments fit into the various interpretations? (I think that I could work it out, but I'd rather it was done by someone more qualified).
Take the standard double slit experiment. Once I have said what each interpretation makes of this it is easy to know what to make of the delayed choice experiment. In the double slit experiment we have two slits for the photon to pass through with a screen in front. The weirdness of this experiment is that the photon will appear to pass through both screens and give an interference pattern. However attempting to determine which one it went through will cause it to have no interference pattern and hence appear to go through only one slit. Only three interpretations have something to say on the issue, in the others its resolution is fairly simple. (E.g. in the Bayesian interpretation we’re only improving our knowledge. In theories which replace QM, this experiment is nothing special.) Here is what the three most relevant interpretations say about this experiment:
Modern Copenhagen:
I'll start with this because it is the simplest. If you don't have a detector at either of the slits, then the particle's wavefunction will spread out and pass through both slits. The peaks and troughs coming out of the two slits will interfere constructively and destructively with each other (very much like an actual wave) and give the interference pattern when it meets the screen.
If you put detectors on the two slits, the particle will be measured to be at one of them. At which point the wavefunction will collapse to be concentrated at that slit only. It will then propagate out from only that slit and hence cannot interfere with itself. So there is no interference pattern.
Many Worlds:
Very similar to the above except instead of collapse, you have splitting into different worlds where the photon hits different parts of the screen. Or in the case of having detectors on the slits, we have splitting into two different worlds where the photon went through two different slits.
Historical Copenhagen:
In this interpretation QM is all about atomic systems and their interactions with experimental apparatus. Since it is meaningless to talk about the photon on its own in this interpretation, we can’t say it’s doing anything. All we can say is not putting detectors on the slits and putting detectors on the slits are two different sets of experimental apparatus. Hence we are dealing with separate scenarios for QM. One where we see particle behaviour and one where we see wave behaviour.
Wheeler’s delayed choice
Now Wheeler’s delayed choice is simply a scenario where we don’t have detectors on the slits and rather have detectors in the form of telescopes (or what ever) behind the screen. We choose to lift the screen or not, after such a time when the photon would have passed the original slits. For each of the three above interpretations this is no real problem. Modern Copenhagen just says that now the telescopes can cause collapse, in which case the photon has collapsed to the state of |Located in telescope>, nothing that incredible. Many-Worlds just replaces this collapse by world splitting. Finally Historical Copenhagen says by choosing to lift or not lift the screen we are choosing two different experiments and hence two different scenarios for QM and we can’t say anything about the photon anyway.
The delayed choice experiment is a problem in what might be called a cut down version of Historical Copenhagen. Rather than saying that we can’t speak about a photon at all and only speak of the photon + apparatus system, this cut down interpretation says that experiments “decide” or actualise the photon’s behaviour. It holds that the act of using detectors or measurement itself decides if the photon is particle-like and uses one slit or wave-like and uses both slits. This interpretation is basically Historical Copenhagen without the central creed that we can’t speak of the photon itself. If you hold to such an interpretation (and one should know that there are other problems for this interpretation), then you must conclude that detecting the photon with a telescope decided its particle-like behaviour. Since the photon is particle like, it went through just one slit, something which occured in the past. So it seems that the future can determine the past. However even in this interpretation delayed choice can’t be used for actual acausal influence of the past. Rather the future measurment determines what the past "was" for the photon only. However since the photon didn't interacting with anything until the telescope, this doesn't effect the external world.
Unfortunately, yet again, this cut down interpretation is also called “The Copenhagen Interpretation” by many people. It’s rarely held by modern physicists, but was once relatively common (1930s and 40s)*. For some reason this interpretation is heavily used in popular science texts. It can usually be spotted by phrases such as “Experiments decide . .”, “The observer’s central role . ” or “The photon doesn’t exist until measurement”.
Watch out for this interpretation (which doesn’t really have a name, since it’s rarely discussed in physics circles) being mixed with the two other Copenhagen interpretations.
*It wasn’t helped along by Bohr himself who said “Nothing exists until it is measured”, by which he meant “Nothing exists as far as we are concerned until it is measured” That is only the marks on the screen or the blip on the telescope caused by the photon/apparatus interaction have operational meaning for us. However one can easily see how this can be misread as a statement about the photons actual existence. A good example of why, in this area especially, people's words can't be taken out of context.

This message is a reply to:
 Message 30 by PaulK, posted 06-22-2008 3:43 PM PaulK has replied

Replies to this message:
 Message 34 by PaulK, posted 06-28-2008 1:55 PM Son Goku has not replied
 Message 35 by randman, posted 06-28-2008 4:01 PM Son Goku has replied

  
Son Goku
Inactive Member


Message 32 of 57 (473306)
06-28-2008 7:45 AM


Bayesian vs Historical Copenhagen and opinions
Straggler or anybody else who was reading, I was wondering if I could ask two questions.
1. Was my explanation of the difference between Bayesian and Historical Copenhagen in message 29 sufficient?
2. What do you make of the interpretations? I'd be interested in hearing what people think of the various interpretations. Preferences or dislikes, e.t.c.

Replies to this message:
 Message 33 by Straggler, posted 06-28-2008 1:06 PM Son Goku has replied

  
Straggler
Member
Posts: 10333
From: London England
Joined: 09-30-2006


Message 33 of 57 (473348)
06-28-2008 1:06 PM
Reply to: Message 32 by Son Goku
06-28-2008 7:45 AM


Re: Bayesian vs Historical Copenhagen and opinions
1. Was my explanation of the difference between Bayesian and Historical Copenhagen in message 29 sufficient?
Hmmm. I supect that the only person who can answer that is you. Based on my/our understanding of what you have said. I will try and describe my understanding such that you can assess whether or not I have "got it".
Beyesian - There is an as yet undiscovered deeper theory that more definitely describes quantum phenomenon. Our current theories are statistical approximations that give practical results which will eventually be better explained by a true understanding of subatomic scales and the 'real' laws that govern these scales. A post-quantum theory is the ultimate answer.
Historical Copenhagen - Our current theories cannot fundamentally be improved upon. They are as close to reality as we can get regarding the subatomic world given our macroscopic observational limitations and the need for this to interact with that which we are studying. QM is a macroscopic theory of microscopic phenomenon and as macroscopic beings it is the only sort of theory we can hope to obtain.
2. What do you make of the interpretations? I'd be interested in hearing what people think of the various interpretations. Preferences or dislikes, e.t.c.
I don't claim to have sufficient knowledge on which to make fully informed declarations but my instinctive and vaguely rational answer are a follows -
1) Bayesian (as I understand it) - If a deeper theory of QM provided an effective quantum theory of gravity that would seem to be the measure of any such theory. There is certainly a gap to fill in this respect. But the inherent probabalistic nature of QM and the evidence for this suggest otherwise.
2) Many worlds - Seems to solve most practical objections to QM but intuitively seems like madness in any conceptual sense. That however may well be the best subjective reason to have any belief in it's veracity!! Many worlds is probably the underlying interpretation of QM as I was taught it as an undergraduate in the early 90s.
Happy to be corrected so feel free to comment on my (mis)interpretations!!

This message is a reply to:
 Message 32 by Son Goku, posted 06-28-2008 7:45 AM Son Goku has replied

Replies to this message:
 Message 37 by Son Goku, posted 06-29-2008 6:52 AM Straggler has replied

  
PaulK
Member
Posts: 17822
Joined: 01-10-2003
Member Rating: 2.2


Message 34 of 57 (473353)
06-28-2008 1:55 PM
Reply to: Message 31 by Son Goku
06-28-2008 7:40 AM


Re: Foundational QM
Thanks a lot, that's very helpful.

This message is a reply to:
 Message 31 by Son Goku, posted 06-28-2008 7:40 AM Son Goku has not replied

  
randman 
Suspended Member (Idle past 4899 days)
Posts: 6367
Joined: 05-26-2005


Message 35 of 57 (473374)
06-28-2008 4:01 PM
Reply to: Message 31 by Son Goku
06-28-2008 7:40 AM


Re: Foundational QM
Son Goku, if the Copehenhagen interpretation was just the mechanistic theory you just described, then there would be no Many-Worlds interpretation. There is a reason for positing the Many-Worlds, a rather fantastic scenario, and it has to do with the uncomfortable aspects which are not mentioned in your post, with the Copenhagen interpretation and indeed with standard QM as a whole.
In terms of the past, you know what Wheeler was getting at with his delayed-choice experiment. He was getting at the idea that the particle is not a definite state in the past (until observation) as a discrete form at all. That's how he describes it, and so when it is observed today, that indeed causes it to collapse and so become discrete, and from a layman's perspective, that occurs in the past, in the prior trajectory (which did not really exist according to this interpretation except as a potential).
So the past, present, and future in some sense is all being formed together.
Edit to add: just finished your last paragraph and see where you distinquish between various ideas call called the Copenhagen interpretation. I decided not to rewrite my post since I do think that if perfectly suitable mechanistic interpretations explained QM, no one would be looking at fantastic scenarios such as the Many-Worlds explanation to explain what's observed.
Edited by randman, : No reason given.

This message is a reply to:
 Message 31 by Son Goku, posted 06-28-2008 7:40 AM Son Goku has replied

Replies to this message:
 Message 36 by Son Goku, posted 06-28-2008 6:43 PM randman has not replied

  
Son Goku
Inactive Member


Message 36 of 57 (473392)
06-28-2008 6:43 PM
Reply to: Message 35 by randman
06-28-2008 4:01 PM


Re: Foundational QM
Son Goku, if the Copehenhagen interpretation was just the mechanistic theory you just describedp.
I'm confused as to the use of the word mechanistic. If you are talking about historical copenhagen there is no mechanisms in the interpretation, only macroscopic descriptions of microscopic phenomena. Even in Modern Copenhagen collapse isn't given a mechanism. So with no "mechanisms" in anything called Copenhagen, do they truly deserve the adjective "mechanistic"?
then there would be no Many-Worlds interpretation
This could simply be my misunderstanding, but I don't see how this follows or what it means. There is a Many-Worlds interpretation because it's what you get when you let QM run free on all levels, microscopic and macroscopic. It preserves all the standard mathematics of QM and interprets everything in a totally realist sense. So Many-Worlds is just:
1. QM applies on all levels.
2. The mathematics is literally true in a realist sense. That is the wavefunction is real, not just a macroscopic description or something similar.
The justification of the Many-Worlds interpretation is independant of any qualities the Historical or Modern Copenhagen interpretations may have.
There is a reason for positing the Many-Worlds, a rather fantastic scenario, and it has to do with the uncomfortable aspects which are not mentioned in your post, with the Copenhagen interpretation and indeed with standard QM as a whole.
What are these uncomfortable aspects?
In terms of the past, you know what Wheeler was getting at with his delayed-choice experiment.
Yes, proposing a paradox or confusing situation for the "collapse acutalises things" interpretation. (To give it some name)
He was getting at the idea that the particle is not a definite state in the past (until observation) as a discrete form at all. That's how he describes it, and so when it is observed today, that indeed causes it to collapse and so become discrete, and from a layman's perspective, that occurs in the past, in the prior trajectory (which did not really exist according to this interpretation except as a potential).
So the past, present, and future in some sense is all being formed together.
Yes, exactly. In this interpretation, the past of the photon (with no influence to the external world) is determined by future measurement. So in this interpretation in certain circumstances we have future macroscopic/microscopic interaction affects the microcopic past.
This is exactly what I stated, so I'm not sure I understand your objection.
I decided not to rewrite my post since I do think that if perfectly suitable mechanistic interpretations explained QM, no one would be looking at fantastic scenarios such as the Many-Worlds explanation to explain what's observed.
This is confusing for two reasons as:
(A) Nowhere, in any of my posts, did I claim that mechanistic interpretations explained QM. I have simply given accounts of what each interpretation says. First in general and then to the double-slit and delayed choice experiments.
(B) The Many-Worlds is more mechanistic than any of the Copenhagen interpretations (Modern, Historical and "collapse acutalises things"), which aren't mechanistic at all*. This is because it gives an explicit account of what goes on during supposed collapse.
*In fact their lack of a mechanism is perhaps why they all have the title Copenhagen. Their lack of a mechanism is the biggest criticism put against them.

This message is a reply to:
 Message 35 by randman, posted 06-28-2008 4:01 PM randman has not replied

  
Son Goku
Inactive Member


Message 37 of 57 (473431)
06-29-2008 6:52 AM
Reply to: Message 33 by Straggler
06-28-2008 1:06 PM


Re: Bayesian vs Historical Copenhagen and opinions
Hmmm. I supect that the only person who can answer that is you. Based on my/our understanding of what you have said. I will try and describe my understanding such that you can assess whether or not I have "got it".
......
Your descriptions are perfect. I'm glad, the difference between the two can sometimes be hard to get across.
1) Bayesian (as I understand it) - If a deeper theory of QM provided an effective quantum theory of gravity that would seem to be the measure of any such theory. There is certainly a gap to fill in this respect. But the inherent probabalistic nature of QM and the evidence for this suggest otherwise.
Many people have this opinion. That is the theory which underlies QM is also the theory which explains quantum gravity. In fact there has been a big debate on whether there is any point in attempting quantum gravity if we don't have the right interpretation of quantum mechanics or have replaced it with the correct theory.
2) Many worlds - Seems to solve most practical objections to QM but intuitively seems like madness in any conceptual sense.
Yes, that's what I think. Since it's the only theory where QM is totally right and the collapse mechanism is explained, it solves everything. However it sounds totally mad. I don't know what to make of it at the end of the day. Technically it makes the least assumptions of any interpretation. The funny thing is that I've met others who think:
1. QM is the fundamental theory
2. Applies at all scales
but disagree with Many-Worlds, not knowing it's the only thing consistent with (1) and (2). People find it difficult to learn that (1) and (2) lead to Many-Worlds. I think there is something about it that "feels" false to a human being.
Many worlds is probably the underlying interpretation of QM as I was taught it as an undergraduate in the early 90s.
Exactly when Many-Worlds became the popular interpretation. I know it was taught to undergraduates in Imperial College around that time.

This message is a reply to:
 Message 33 by Straggler, posted 06-28-2008 1:06 PM Straggler has replied

Replies to this message:
 Message 38 by Straggler, posted 06-29-2008 10:25 AM Son Goku has replied

  
Straggler
Member
Posts: 10333
From: London England
Joined: 09-30-2006


Message 38 of 57 (473449)
06-29-2008 10:25 AM
Reply to: Message 37 by Son Goku
06-29-2008 6:52 AM


Re: Bayesian vs Historical Copenhagen and opinions
Your descriptions are perfect. I'm glad, the difference between the two can sometimes be hard to get across.
Well any demonstration of understanding on my part says more about the clarity of your explanation than anything else. Percy was even talking about a new 'Hall of Fame' forum in relation to your post......
Many people have this opinion. That is the theory which underlies QM is also the theory which explains quantum gravity. In fact there has been a big debate on whether there is any point in attempting quantum gravity if we don't have the right interpretation of quantum mechanics or have replaced it with the correct theory.
What are the leading candidates for such a theory? String theory?
2) Many worlds - Seems to solve most practical objections to QM but intuitively seems like madness in any conceptual sense.
Yes, that's what I think. Since it's the only theory where QM is totally right and the collapse mechanism is explained, it solves everything. However it sounds totally mad. I don't know what to make of it at the end of the day. Technically it makes the least assumptions of any interpretation. The funny thing is that I've met others who think:
1. QM is the fundamental theory
2. Applies at all scales
but disagree with Many-Worlds, not knowing it's the only thing consistent with (1) and (2). People find it difficult to learn that (1) and (2) lead to Many-Worlds. I think there is something about it that "feels" false to a human being.
A fair amount of modern physics is pretty counter-intuitive. Many of the practical conclusions of relativity "feel" crazy. The difference might be that in many cases we can actually experimentally verify that the apparently "crazy" implications of the maths does indeed describe reality.
Is it even remotely theoretically possible to verify or refute a many worlds interpretation of QM in any way at all?
However I still think 'many worlds' takes counter-intuitiveness to new levels surpassing even relativity.
It seems appropriate to quote Niels Bohr at this point.
"We are all agreed that your theory is crazy. The question that divides us is whether it is crazy enough to have a chance of being correct."
Exactly when Many-Worlds became the popular interpretation. I know it was taught to undergraduates in Imperial College around that time.
Ah yes. Professor Chris Isham was the QM lecturer and quite a big name in the quantum gravity field at the time I think. I loved the semi-philosophical tangents he used to go off at during his lecture but struggled a bit with the maths content of the course.
BTW - I have just got round to ordering the book you recommended by Crease and Mann. Should receive sometime soon and will let you know how I get on with it.

This message is a reply to:
 Message 37 by Son Goku, posted 06-29-2008 6:52 AM Son Goku has replied

Replies to this message:
 Message 40 by Son Goku, posted 07-06-2008 10:26 AM Straggler has not replied

  
Agobot
Member (Idle past 5530 days)
Posts: 786
Joined: 12-16-2007


Message 39 of 57 (473471)
06-29-2008 6:20 PM


The absurdity of the Universe vs Many Worlds Scenario
The absurdity of the scales of the universe, the seemingly pointless life, the cold indifference of this indescribably vast universe all point to an absurdity. I don't think for a moment this absurdity will end beyond the limit of our universe. Just 2000 years ago people thought the world(presently universe) was not much bigger than our Solar System. Now we know the "world" is more than 30 billion light years in diameter, with all the absurd implications arising from this. I am not prepared for this absurdity to end. IMHO, the "madness" is just beginning and the many world scenario is beginning to settle down in my mind along with the idea that what we perceive as 'world' is much much bigger than we currently think it is. It just makes sense, among a dozen other theories that so far fail to. One day, I believe, we'll be talking about it as a simple fact of life.
On the other hand, if everything in the universe made sense to us, wouldn't this point us to some super artificial intelligence standing behind all this? Then some scientist might come up with a theory that nothing in the universe was made to make sense to us - we are just super adaptive to the laws of the universe and that they don't make sense. We just think they do, but they don't. And that would start another never-ending cycle of confusion. For the record, the Universe makes much less sense now with all our knowledge, science and discoveries, than 2000 years ago. But Einstein put it best - "The universe is not stranger than we imagine, it's stranger than we can imagine"(sometimes i get the feeling Einstein is trying to tell us something that's not conveyed with the first reading of this thought). Just my 2 cents worth
Edited by Agobot, : No reason given.
Edited by Agobot, : No reason given.
Edited by Agobot, : No reason given.

  
Son Goku
Inactive Member


Message 40 of 57 (474176)
07-06-2008 10:26 AM
Reply to: Message 38 by Straggler
06-29-2008 10:25 AM


Re: Bayesian vs Historical Copenhagen and opinions
What are the leading candidates for such a theory? String theory?
Yes, String Theory would still be the leading candidate. Just as a piece of information (and one of the few things I know with my limited knowledge of String Theory), String Theory is still a quantum theory. That is it takes the rules of quantum theory as they currently are with no changes. Perhaps cavediver could tell what the predominante interpretation of quantum theory was/is in the String community.
Is it even remotely theoretically possible to verify or refute a many worlds interpretation of QM in any way at all?
Many-Worlds can be easily falsified by finding some length scale where Quantum Mechanics stopped working. For example if we found that large molecules where just totally classical or just could not be put in superposition. Basically we find some size at which superposition stops happening for some reason. We know Buckyballs can be put in superposition, but that's a long way off the macroscopic world.
BTW - I have just got round to ordering the book you recommended by Crease and Mann. Should receive sometime soon and will let you know how I get on with it.
I hope it is useful. Very good read regardless.

This message is a reply to:
 Message 38 by Straggler, posted 06-29-2008 10:25 AM Straggler has not replied

  
duanbei12 
Suspended Member (Idle past 5737 days)
Posts: 2
Joined: 07-13-2008


Message 41 of 57 (475029)
07-13-2008 3:46 AM


spam removed
spam removed
Edited by AdminNosy, : spam

  
Son Goku
Inactive Member


Message 42 of 57 (479111)
08-24-2008 5:23 PM


Quantum Field Theory
Just noticed something I didn't mention. All of these interpretational viewpoints carry over exactly into quantum field theory, with very little change from quantum mechanics.
So for instance, if one were to be an adherent of the Historical Copenhagen interpretation, then you can still say Quantum Field Theory is a theory of how objects in the macroscopic world interact with the microscopic world, only now the macroscopic world obeys special relativity instead of Newton's laws.

Replies to this message:
 Message 43 by Straggler, posted 09-15-2008 5:54 PM Son Goku has replied

  
Straggler
Member
Posts: 10333
From: London England
Joined: 09-30-2006


Message 43 of 57 (482273)
09-15-2008 5:54 PM
Reply to: Message 42 by Son Goku
08-24-2008 5:23 PM


Freewill
What is the many worlds interpretation take on the notion of freewill? Does it exist? Or are my "choices" just statistical branches along an ever branching timeline?
Do I really choose which path in life to take or are there multiple "me's" taking all possible paths with the one I find myself existing in little more than a statistical inevitability?
Does QM have anything to say at all regarding freewill as far as you are concerned? What is the common view of freewill as far the main advocates of MW advocates are concerned?

This message is a reply to:
 Message 42 by Son Goku, posted 08-24-2008 5:23 PM Son Goku has replied

Replies to this message:
 Message 44 by Son Goku, posted 09-15-2008 6:11 PM Straggler has replied

  
Son Goku
Inactive Member


Message 44 of 57 (482281)
09-15-2008 6:11 PM
Reply to: Message 43 by Straggler
09-15-2008 5:54 PM


Re: Freewill
Does QM have anything to say at all regarding freewill as far as you are concerned?
I'll answer this first. To answer directly, I think QM has nothing to say about free will. The brain is too hot and damp, to use crude language, for quantum mechanics to have any real influence on it. It is basically a classical object. Obviously QM controls the atoms of the brain, but I don't think any "quantum" ideas need be considered.
What is the common view of freewill as far the main advocates of MW advocates are concerned?
From speaking to them the common view is, to use your words, "choices are just statistical branches along an ever branching timeline".
Do I really choose which path in life to take or are there multiple "me's" taking all possible paths with the one I find myself existing in little more than a statistical inevitability?
The latter.
Hopefully my answers aren't too brief.

This message is a reply to:
 Message 43 by Straggler, posted 09-15-2008 5:54 PM Straggler has replied

Replies to this message:
 Message 45 by Straggler, posted 09-15-2008 7:45 PM Son Goku has replied

  
Straggler
Member
Posts: 10333
From: London England
Joined: 09-30-2006


Message 45 of 57 (482312)
09-15-2008 7:45 PM
Reply to: Message 44 by Son Goku
09-15-2008 6:11 PM


Re: Freewill
What is the common view of freewill as far the main advocates of MW advocates are concerned?
From speaking to them the common view is, to use your words, "choices are just statistical branches along an ever branching timeline".
Do I really choose which path in life to take or are there multiple "me's" taking all possible paths with the one I find myself existing in little more than a statistical inevitability?
The latter.
How depressing and nihilistic!! Not that this in itself necessarily points towards it being wrong.
Do the views of Roger Penrose regarding QM and it's relationship with consciousness, freewill etc. etc. have much support within the community of scientists working in QM and other areas of modern theoretical physics?
BTW - The Second Creation: Finally got round to reading it. Not yet finished but a great read so far. The copy I have is quite old (1988 I think) but the principles it explains and the historical narrative approach are just what I was looking for.
Edited by Straggler, : No reason given.

This message is a reply to:
 Message 44 by Son Goku, posted 09-15-2008 6:11 PM Son Goku has replied

Replies to this message:
 Message 46 by Son Goku, posted 09-16-2008 5:09 AM Straggler has replied
 Message 47 by Syamsu, posted 09-16-2008 8:17 AM Straggler has replied

  
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