Quantum physics: Copenhagen vs decoherence interpretations

In that context, cavediver was discussing nonrelativistic quantum mechanics, which is certainly not causal because it doesn't take relativity into account. Nonrelativistic QM lives in Newton's space and time, not the spacetime of Einstein's special theory of relativity.However quantum field theory, which is basically quantum mechanics made relativistic, does obey causality. This isn't anything to do with quantum mechanics in particular, any theory which doesn't take relativity into account is "acausal".It's not that local realism or causality or both is violated by QM. It's locality or realism or both. Causality being violated in a theory with no relativistic content is a very vague statement.

It may be more useful to have a read of this:
The Kochen-Specker Theorem
At least the non-mathematical parts.The Kochen-Specker Theorem is one of the deepest results in the foundations of QM. It'll give you a good idea of the wierdness Qm requires.As a side note nonlocality, e.t.c. don't have anything to do with the Copenhagen interpretation (where by Copenhagen interpretation, I mean what Bohr and co. actually thought) and the Many-Worlds interpretation. They're related to a seperate issue.Edited by Son Goku, : Forgot the link

There are few issues here.
Firstly any theory which aims to match the same set of experimental evidence as QM must first, due to Bell's Theorem, incorporate nonlocality.However due to the Kochen-Specker theorem any theory must also drop one of the following three:
(1)Value realism: Any measurable constructed out of things I've already measured should be measurable. Example, if energy is observable, then energy squared is observable.
(2)Value definiteness: All observables defined for a QM system have definite values at all times.
(3)Noncontextuality: A system possesses a property independently of how it is measured. Example, measurement of energy doesn't depend on you measuring it with position based equipment or momentum based equipment.So even if we've incorporated nonlocality and dropped one of (1),(2),(3) we still don't have to use orthodox QM, those are simply the conditions a theory must obey to match orthodox QM.However if you take orthodox QM at face value you are then lead into the interpretational issues, which is separate from nonlocality. You are then asking what does collapse mean. Is it real?
This is what the Copenhagen and Many-Worlds interpretations are about. Although in both the Many-Worlds and the historical Copenhagen interpretation collapse isn't real. It's only modern Copenhagen (which is vastly different from the original) that it is "real".In short:
Nonlocality is an issue you see when discussing alternatives to orthodox QM. The interpretations involve taking orthodox QM at face-value.

I just wanted to add something about one of the more misunderstood interpretations of Quantum Mechanics. The Many Worlds scenerio comes from assuming that quantum mechanics as it is currently formulated (or any quantum theory) applies at all levels. In case people think it sounds like some off the wall idea, it is actually the direct prediction of QM if you allow it to be applicable even at our macroscopic scale.

This question is much easier to answer than the others so I'll deal with it first. Basically, the answer is no. Most would think this is not a question of consciousness. In fact the view that consciousness is involved was always a very a minor one, proposed originally by Wigner. However even he lost interest in it very quickly. Big question, so if there is anything unclear in what follows don't hesitate to ask.First let's get the pragmatists out of the way, as you have said. Although I should say that this position has gone from being very common in past decades to uncommon today. This is due to a post 2000 revival in interest in the foundations of QM. For the purpose of what follows I will call the "I don't care" interpretation the Dirac interpretation.Now I'll explain my notation. If a cat is definitely dead I will denote it: |Dead>. If it is definitely alive then: |Alive>. In Schrodinger’s cat experiment we have that the state is |Dead> + |Alive>. Now my symbols above can contain more information. For instance the state of the Cat being dead and me off in Budapest is:
|Cat is dead and I’m off in Budapest>
Any of these |Insert phrase> things are what may be called a wavefunction.Here is low down of what the most common interpretations say:
Many Worlds:
Quantum Mechanics applies at all levels. This means even large scale objects like you and I can be in superposition. So what happens in this model is that the state of the universe goes from:|State of World> =
|I haven’t looked at the cat yet and the cat is alive> + | I haven’t looked at the cat yet and the cat is dead>To|State of World> =
|The cat is alive and I see him alive> + |The cat is dead and I see him dead>There is no collapse here because the first state can evolve smoothly into the second in the standard formalism of quantum mechanics. One should also note that Many-Worlds is totally deterministic.Bayesian interpretation:
The wavefunction only represents our knowledge of the system. Hence collapse is simply when we learn more. That is upon seeing the cat alive, we know the cat is alive and hence there is no probability for anything else. Collapse is therefore not a real event.Historical Copenhagen/ Bohr Interpretation:
We are classical thinking things. Hence the quantum world makes no sense to us. The formalism of QM is the closest we will get to understanding it. It is a formalism for explaining our big classical world’s interaction with the small quantum world. Collapse is a part of that formalism. Like all of QM though we must not try to understand what it really is/means. That is all.Modern Copenhagen:
This is actually the only model with collapse in it. Basically we have:|State of World> =
|I haven’t looked at the cat yet and the cat is alive> + | I haven’t looked at the cat yet and the cat is dead>Goes to:
|The cat is alive and I see him alive>Or|The cat is dead and I see him dead>With a 50:50 chance of each. This is collapse, because there is no way to go from the first state to either of the second two, in the standard formalism, without an instant jump.Dynamical collapse:
Basically Modern Copenhagen with attempts at modelling what process causes collapse.QM is incomplete, there is another theory underlying it:
Fairly obvious. Not so much an interpretation really.Now as to which interpretation is the standard/dominant one, it depends on the area of physics and the period of history you’re talking about. In the 30s it was definitely either the Dirac Interpretation or Historical Copenhagen. After the 40s until 90s-00s it was Modern Copenhagen or Dirac. Recently all the interpretations have been kind of levelling off to have even numbers of proponents. However in certain areas different interpretations dominate. For instance in quantum computing many-worlds dominates. In particle physics it’s still Dirac which dominates. The Many-Worlds is definitely widely accepted. For a brief period in the 90s, due to the arrival of quantum computation, it may have been the most widely held. Today it’s probably roughly even with others. Usually one interpretation is preferred in a given circumstance because it allows easier intuition of the physics. For quantum computation many-worlds definitely seems to make things easier. For Measurement Theory, which is the applied science of what goes on when several hundreds of quantum mechanical measurements are made and how we may use them in engineering, Modern Copenhagen is significantly more useful.
Currently the preferences are rooted in reasons like these, however that’s not to say the interpretations differ only in a philosophical sense. For instance if QM is incomplete the new theory will probably make new predictions. If dynamical collapse is true we should see evidence for the mechanism of collapse. To falsify Many Worlds we’d just need to find some scale where QM does not apply.Just to mention something of interest. A lot of the "QM is incomplete" people have attempted to make theories which replace it and are deterministic. The funny thing is in a lot of these theories when you try to model what the subatomic world looks like when viewed by a large classical observer, you get an effective theory of the universe from their point of view and this effective theory is basically QM. So in these "QM is incomplete" models, historical Copenhagen ends up being partially correct in the sense that QM is (in these models) a formalism for explaining a big classical objects interaction with the small quantum world.Edited by Son Goku, : Spelling error.

I should also mention something about the meaning of states such as |Dead> + |Alive>. The exact meaning of it depends on the interpretation.
In almost all of the interpretations mention aboved it means being simultaneously dead and alive. However in the Bayesian Interpretation |Dead> + |Alive> means "could be dead or alive, but we don't know".Also a warning if anybody decides to read up on this stuff, the word "Copenhagen Interpretation" is desperately lacking a standardisation. Several texts or articles will say it and they could be referring to either (what I have called) Historical Copenhagen or Modern Copenhagen. Even worse some people pick and mix between Historical and Modern, switching back and forth between them or fusing them together. In fact sometimes people don't realise they disagree on their interpretation of QM since they both just use "Copenhagen" for their opinion.Wikipedia, sensibly, calls Historical Copenhagen, the Copenhagen interpretation and calls Modern Copenhagen, Objective collapse.Also the Bayesian interpretation is occasionally called the ensemble/statistical interpretation.Most of the interpretations I haven't mentioned are extensions/modifications of ones I have. (For instance Many-Minds is a modifcation of Many worlds and Consistent Histories is a extension of/attempt to clarify Historical Copenhagen). Although quite often the more advanced interpretations stray into modifying things so much that they may be a new theory rather than an interpretation of QM.Finally, obviously a lot of people who hold to Modern Copenhagen end up drifting to Dynamical Collapse since they want to actually have a description of collapse.

If I'm being totally correct I should write:(1/2)|Alive> + (1/2)|Dead>.The square of the number in front of the state is the chance of that state eventually being observed. If the squares of these numbers are summed it should equal 1.
(If anybody is wondering, in case the html style formatting for square root is unclear, 1/2 is 1 over the square root of 2.) They would be a term in Schrodinger's equation. Specifically they are what you solve Schrodinger's equation to obtain. However, looking back it may have been better to refer to (1/2)|Decayed> + (1/2)|Undecayed>, refering to the atom whose decay determines the death of the cat. This is because only the Many-Worlds accepts that something macroscopic like a Cat can be in a superposition like (1/2)|Alive> + (1/2)|Dead>. The answer to all these questions is yes. Superposition occurs at both levels, there are as many worlds as there are possible subatomic states and the number of worlds is massive. Yes they are quite similar. The funny thing is that Einstein was a Bayesian and Bohr was a historical Copenhagener. The great debates concerning QM in the 20s and 30s where actually between these two, seemingly similar, interpretations.
Usually a Bayesian person is implying that QM is a statistical approximation to something underneath. However a Historical Copenhagener (from here on a HCer) thinks there is absolutely nothing underneath and the quantum world is fundamentally unknowable since all we can ever have are our experimental equipment and its interactions. The best way to phrase the difference between them is this:
Bayesian: QM is an approximate theory of atomic systems themselves.
HCer: QM is a totally accurate theory of the interaction of our equipment with atomic systems. There can not be a theory of atomic systems themselves, even an approximate one.I hope this kind of makes the distinction clearer, since the way I've phrased it previously makes it sound like they're both just sound like "I don't what atomic systems are doing". The big difference is that a HCer says that it makes no sense to talk about an atom itself, only an atom and some equipment which will measure it.To take specific state:
(1/2)|Alive> + (1/2)|Dead>
A Bayesian would say this means (apologies for repetition) "could be alive or dead, but I don't know". A HCer however interprets this as some fundamentally incomprehensible statement about what statistical spread of results our equipment will obtain when it and the atomic system meet.If I'm still being too vague, please say so. Yes. Penrose for example thinks it is gravity which causes it. Others think it has something to do with interactions with the environment and thermal properties like entropy. (That is, due to the subatomic system and the measuring equipment both being in a room with highly disordered thermal air or the disordered thermal nature of the equipment itself.) Currently, I wouldn’t subscribe to any of them as the truth. For instance Many-Worlds makes the least assumptions and modifications, literally taking QM as is and applying to both the subatomic and macroscopic world. On the other hand its implications sound crazy. They all suffer from this, being very convincing and difficult to argue against from one specific angle and yet sounding deficient from another. I should clearly say my main problem is that I find all of them convincing. Whenever I read something about one of them I always half-think “Yeah, there is something to this”. They all have very good points and not many bad ones.However when I’m thinking/visualising QM I definitely picture Modern Copenhagen. That is I actually do think of atoms as doing stuff by themselves somewhere away from equipment, like in outer space. (Something Bohr specifically said you should not do.) I also picture something suddenly jumping to a specific state when measured, but I never mentally picture a process behind this.To be honest I really don’t know what to think, it’s a very difficult issue. However I think possibly we need more theorems about the ontological structure of QM like the Kochen-Specker Theorem. I wouldn’t underestimate insights from those who are working with the practical consequences of quantum measurement, so there may also be insights from Quantum Computation and the recently introduced Measurement theory.

For anybody unfamiliar with delayed choice there is a description of the experiment: hereWell 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. 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.

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.

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"? 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. What are these uncomfortable aspects? Yes, proposing a paradox or confusing situation for the "collapse acutalises things" interpretation. (To give it some name) 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. 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.

Your descriptions are perfect. I'm glad, the difference between the two can sometimes be hard to get across. 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. 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. Exactly when Many-Worlds became the popular interpretation. I know it was taught to undergraduates in Imperial College around that time.

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. 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. I hope it is useful. Very good read regardless.

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.

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. From speaking to them the common view is, to use your words, "choices are just statistical branches along an ever branching timeline". The latter.Hopefully my answers aren't too brief.

Penrose's idea required the brain to be quantum mechanical. In the past few years we've become much better at telling when a given system is influenced by quantum mechanics or not. From this the brain is known to be classical and Penrose's idea is falsified. Even more importantly neurologists say it doesn't hold up to what we know about the brain. There is very little new in the second edition just some corrected typos and an afterword.One of highlights of the book for me was the interview with Schwinger. Although I would be at odds with some physicists I think Schwinger and Feynman were the greatest physicists of the second half of the twentieth century.. Unfortunatly even though one can learn from Feynman's conception of physics through all he wrote, Schwinger rarely said anything or published texts, so it was good to see what he was like.