Double slit experiment and Interaction

[kith]

My thinking was different - dBB and string theory both introduce new physics

dBB is widely considered to be an interpretation of QM, so its predictions are considered to be identical to Copenhagen's. If this is the case then dBB doesn't introduce new physics. If not, dBB is a different theory. This is possible but my impression so far is that whenever dBB deviates from QM, the effects are ultimately not observable.

dBB deals with the Heisenberg classical/quantum cut, and string theory deals with the Wilsonian UV cut. So having string theory as the next theory is not against dBB thinking.

This is an interesting analogy. Unfortunately, I don't know much about renormalization and String theory. To me, the motivation and accomplishments of String theory seem to be mostly physical by being able to make predictions for physical situations where previous theories break down. But since actually making these predictions -let alone verifying them- seems to be very difficult, you may be right that the main advancement is conceptual.

I think it is completely consistent with the spirit of dBB to have MWI, if it works.

That doesn't make sense to me. In dBB, there's only one world. Wrt to Copenhagen, the hidden variables of dBB represent a more fundamental reality. Wrt to MWI, the hidden variables specify which world is the real one. So the hidden variables give a more complete picture for both interpretations.

As I understand it, considering Copenhagen complete would be like saying subtracting infinities is mathematically sound, ie. the Wilsonian idea is fundamentally wrong.

This doesn't seem to fit into what you wrote above: your analogy was Copenhagen<->Wilson, dBB<->String theory. So wouldn't considering Copenhagen complete correspond to the Wilsonian idea being fundamental?

 

[kith]

Are they not just possible outcomes though?, and only one of them actually IS realized?

bhobba already answered this but just to be clear: we were talking specifically about the Many Worlds interpretation. In the Copenhagen interpretation, only one outcome is realized.

 

[atyy]

dBB is widely considered to be an interpretation of QM, so its predictions are considered to be identical to Copenhagen's. If this is the case then dBB doesn't introduce new physics. If not, dBB is a different theory. This is possible but my impression so far is that whenever dBB deviates from QM, the effects are ultimately not observable.

Yes, here I am assuming that dBB points to the possibility of "quantum non-equilibrium" and so it is a different theory. I consider pure dBB to be not real, just as the ensembles of statistical mechanics are not real.

This is an interesting analogy. Unfortunately, I don't know much about renormalization and String theory. To me, the motivation and accomplishments of String theory seem to be mostly physical by being able to make predictions for physical situations where previous theories break down. But since actually making these predictions -let alone verifying them- seems to be very difficult, you may be right that the main advancement is conceptual.

I think they are mainly conceptual. And both still have ways to go in describing known reality. String theory is shaky for cosmology, especially with positive cosmological constant, and dBB is shaky for chiral fermions interacting with non-Abelian gauge fields. An interesting point here is that one idea for getting chiral fermions in lattice gauge theory is by introducing extra dimensions. If that actually works, then dBB and string theory will both introduce extra dimensions.

That doesn't make sense to me. In dBB, there's only one world. Wrt to Copenhagen, the hidden variables of dBB represent a more fundamental reality. Wrt to MWI, the hidden variables specify which world is the real one. So the hidden variables give a more complete picture for both interpretations.

Yes, I should elaborate. It is not so much that dBB is directly consistent with MWI, rather dBB and MWI are both motivated by naive realism and the assumption that Copenhagen is complete. Each introduces an additional assumption (hidden variables or multiple outcomes) to complete Copenhagen.

(Perhaps BM is in a very technical sense consistent with MWI, since we can imagine Bohmian Many-Worlds, which seems to make all the difficulties of MWI go away.)

This doesn't seem to fit into what you wrote above: your analogy was Copenhagen<->Wilson, dBB<->String theory. So wouldn't considering Copenhagen complete correspond to the Wilsonian idea being fundamental?

Copenhagen <-> Subtracting infinitities (It works FAPP!)
Naive realism, incompleteness of Copenhagen <-> Wilsonian framework, incompleteness of QED, quantum general relativity
dBB <-> string theory (new physics)
MWI <-> Asymptotic Safety (no new physics)

 

[kith]

I like your point of view but I don't share it yet. ;-) I still tend to think that Copenhagen tells us something important about physics and that although dBB is valuable it won't lead to new physics. But my current thinking is strongly rooted in non-relativistic QM and I will definitely have your analogies in the back of my head when I explore QFT and physics beyond the Standard Model more.

 

[atyy]

I like your point of view but I don't share it yet. ;-) I still tend to think that Copenhagen tells us something important about physics and that although dBB is valuable it won't lead to new physics. But my current thinking is strongly rooted in non-relativistic QM and I will definitely have your analogies in the back of my head when I explore QFT and physics beyond the Standard Model more.

I do think Copenhagen is telling us something. But what? There are two ways to go. The first is that naive realism always holds, but at some point we need a cut (or MWI), because the aim of physics is to predict the future, whereas as the Wiener saying goes - the best model for a cat is another cat, preferably the same cat - at some point we cannot model the whole universe and our theories must necessarily be incomplete. So perhaps all useful post-quantum theories will have a cut, maybe something like http://arxiv.org/abs/1105.4464. I think dBB is consistent with this view, since naive realism is privileged by assumption. However, dBB also suggests that we are not necessarily at this stage yet by providing a toy counterexample in the case of a universe in which non-relativistic quantum mechanics is a good approximation.

The second view is that there is something fundamentally wrong with naive realism. Maybe consistent histories in the Griffiths's style or Wheeler's universe observing itself or Penrose's consciousness is a fundamental element (Penrose is usually considered a naive realist, so he wouldn't put himself here, but I do). Let's call this the Wheeler-Penrose-Chopra approach :)

Is your view about what Copenhagen is teaching us one of the above, or something else entirely? Myself I do Copehagen on weekdays, dBB on Sundays (Bell: "I am a Quantum Engineer, but on Sundays I have principles") and Wheeler-Penrose-Chopra on Friday and Saturday evenings.

A bit more seriously, here is an analogy for naive realism in mathematics. In mathematics we have the intuitive natural numbers and Peano's axioms. Goedel's incompleteness theorem says that there will always be statements that are true about the intuitive natural numbers that cannot be captured by any axiomatic system. Here the intuitive natural numbers live in naive reality. Well, can we get rid of naive reality? We can at least try to get rid of the intuitive natural numbers. Instead of using the intuitive natural numbers, we can formalize the natural numbers in ZFC. Then we assert that the intuitive natural numbers do not exist, and we only ever mean the natural numbers in ZFC. However, naive realism still survives, because to define ZFC itself, we need a metalanguage which lives in naive realism.

What is interesting about the above argument is that it both argues that we need naive realism, and that mathematics has an unavoidable cut. But I don't know how that cut relates to the Heisenberg cut of Copenhagen, since I think the cut of mathematics should also be in a classical theory like GR. Maybe, as you say, even the old theories have a observer/system cut?

 

[zonde]

No, all outcomes are realized in some universe. So after a spin experiment, there is one universe where the spin has been measured as up and one where it has been measured as down.

Newer understood this about MWI. Take Mach–Zehnder interferometer, photons are detected only at one output in case of interference. If you say that all outcomes are realized then you contradict experiment.

 

[StevieTNZ]

Newer understood this about MWI. Take Mach–Zehnder interferometer, photons are detected only at one output in case of interference. If you say that all outcomes are realized then you contradict experiment.

But there is only one possible outcome? How does that contradict Many Worlds?

 

[zonde]

But there is only one possible outcome? How does that contradict Many Worlds?

Obviously there are two possible outcomes (beamsplitter has two outputs) but we observe only one in idealized case.
There are two possible outcomes after the first beamsplitter. And then there are two possible outcomes after second beamsplitter if we consider only single path.
Basically two worlds have to interact to determine outcome or the "wrong" worlds have to cease to exist (after they somehow discover that they are wrong).

 

[GasolineRainbow]

I am a bit ignorant about all this so can people clear something up for me...
You can see the interference pattern in the double slit experiment until you try to measure it with, for example, polarised light.
This is because the 2 part wavefunction becomes one part once you measure which slit the light photon has gone through, i.e. it's no longer in a superposition state
Surely that doesn't actually mean there is no interference pattern?? Of course you can't observe it when you're measuring which photons went through which slit since where they end up, aside from some places being more likely than others, is essentially random. So the interference pattern hasn't gone, you're just not observing it? If you marked on all the photons that, from your measurements, passed through the right, and then all the photons that passed through the left, would you not be able to see the interference pattern again? Or can you not?
I think I'm just confused by superposition states. I can't help but think of wavefunctions as hypothetical descriptions of where the photon might pass, rather than the photon passing through both slits at once. So the superposition state, by my definition, does not collapse. It's an imaginary state that describes a possibility rather than a reality and when you measure the realities you can't see the interference pattern, but when you look at the photons without knowing which slit they passed through, you see the possibilities, and that equates to the interference pattern. I don't think I'm explaining myself very clearly but if anyone gets my point, can they help me out?

 

[StevieTNZ]

Obviously there are two possible outcomes (beamsplitter has two outputs) but we observe only one in idealized case.
There are two possible outcomes after the first beamsplitter. And then there are two possible outcomes after second beamsplitter if we consider only single path.
Basically two worlds have to interact to determine outcome or the "wrong" worlds have to cease to exist (after they somehow discover that they are wrong).

I'm pretty sure after the 2nd beam splitter one of the paths has zero probability for the system to go along.

 

[zonde]

I'm pretty sure after the 2nd beam splitter one of the paths has zero probability for the system to go along.

Yes, this is QM prediction in idealized specific case. But this prediction is non trivial and you have to explain it if you claim that you have meaningful QM interpretation.

 

[StevieTNZ]

Things with 0 probability of happening don't make the universe branch out so it does happen, only then to realize it shouldn't and cease to exist.
No branching occurs to make a universe where the particle goes along that 0 probability path. (I would have thought?)

 

[Feeble Wonk]

I know that I shouldn't pursue this, but I can't help myself. I am sometimes confused as to whether (and why) decoherence is consistently as limiting to potential quantum states as I've often heard it argued. I think I understand the general decoherence concept... the interaction between the environment, the prepared system and the "apparatus" by which potential quantum states are differentiated. But, it seems to me that IF we consider that the quantum state in question as being that describing the entire universe, and no effort is made to manipulate (prepare) the system, then the definition of "environment" becomes somewhat arbitrary and/or ambiguous.

Even a few stray photons from the CMBR is enough to decohere a dust particle...

Yet, does anything change if the quantum state of the "few stray photons" are also unresolved? Again, if what we are considering is the quantum state of the cosmos as a whole, and no experimental controls are in place, does that change anything. Is every subatomic particle necessarily an apparatus by which a measurement is made?

 

[atyy]

I know that I shouldn't pursue this, but I can't help myself. I am sometimes confused as to whether (and why) decoherence is consistently as limiting to potential quantum states as I've often heard it argued. I think I understand the general decoherence concept... the interaction between the environment, the prepared system and the "apparatus" by which potential quantum states are differentiated. But, it seems to me that IF we consider that the quantum state in question as being that describing the entire universe, and no effort is made to manipulate (prepare) the system, then the definition of "environment" becomes somewhat arbitrary and/or ambiguous.

I think this is a problem for pure decoherence. It is often said that decoherence has rephrased the measurement problem, and removed the need for a subjective observer-dependent cut, and that the measurement problem is mainly one of why there are definite outcomes. In simple systems, it may be that pure decoherence plus an additional criterion can give an "objective" cut, in the sense the all observers who use the criterion will place the cut in the same location. However, a hallmark of the Copenhagen interpretation is that different observers can place the cut in different places, and the cut is not objective. Because decoherence plus an additional criterion is too objective, I don't think decohence can place an objective cut - or rather if it can, the cut will be wrong for some observer.

Schlosshauer's very good review on decoherence http://arxiv.org/abs/quant-ph/0312059 does phrase the measurement problem mainly as one of definite outcomes, but he does mention (p15): "Finally, a fundamental conceptual difficulty of the decoherence-based approach to the preferred-basis problem is the lack of a general criterion for what defines the systems and the “unobserved” degrees of freedom of the environment (see the discussion in Sec. III.A). While in many laboratory-type situations, the division into system and environment might seem straightforward, it is not clear a priori how quasiclassical observables can be defined through environment-induced superselection on a larger and more general scale, when larger parts of the universe are considered where the split into subsystems is not suggested by some specific system-apparatus surroundings setup."

 

[harrylin]

[..] Those that go on about this conciousness stuff often are influenced by gutter trash like What The Bleep Do We Know Anyway:


Its junk of the first order trying to justify new age stiff like The Secret. [..]

That one I have seen completely - not because I wanted to, but because my sister saw it on TV and asked my opinion about it. It's obviously manipulated by some kind of sect, making it appear as if serious physicists support wacky ideas.
See: https://en.wikipedia.org/wiki/What_the_Bleep_Do_We_Know!?#Featured_individuals

 

[Feeble Wonk]

Schlosshauer's very good review on decoherence http://arxiv.org/abs/quant-ph/0312059 does phrase the measurement problem mainly as one of definite outcomes, but he does mention (p15): "Finally, a fundamental conceptual difficulty of the decoherence-based approach to the preferred-basis problem is the lack of a general criterion for what defines the systems and the “unobserved” degrees of freedom of the environment (see the discussion in Sec. III.A). While in many laboratory-type situations, the division into system and environment might seem straightforward, it is not clear a priori how quasiclassical observables can be defined through environment-induced superselection on a larger and more general scale, when larger parts of the universe are considered where the split into subsystems is not suggested by some specific system-apparatus surroundings setup."

This appears to be precisely the point of my question. So, in the absence of a definitive "line in the quantum sand" determined by an obvious system-apparatus-environment situation, how open can the cosmological quantum state remain? At the risk of extending this question to the seemingly absurd, can we return to the unobserved moon scenario for a moment? If the presence of the moon on a macroscopic level is manifested by various parameters such as gravitational effects and visual identification among others, without any additional manipulation of apparatus-system-environment relationships in place, would the EXACT location of the moon become infinitesimally less determined if no one was looking?

 

[atyy]

This appears to be precisely the point of my question. So, in the absence of a definitive "line in the quantum sand" determined by an obvious system-apparatus-environment situation, how open can the cosmological quantum state remain? At the risk of extending this question to the seemingly absurd, can we return to the unobserved moon scenario for a moment? If the presence of the moon on a macroscopic level is manifested by various parameters such as gravitational effects and visual identification among others, without any additional manipulation of apparatus-system-environment relationships in place, would the EXACT location of the moon become infinitesimally less determined if no one was looking?

Are you asking in the context of a particular interpretation?

 

[Feeble Wonk]

Well... I'm open to in-put from any interpretation I guess. But I suppose the question was specifically with regard to standard Copenhagen interpretation, and the limitations imposed by decoherence.

 

[atyy]

Well... I'm open to in-put from any interpretation I guess. But I suppose the question was specifically with regard to standard Copenhagen interpretation, and the limitations imposed by decoherence.

That's easy. Copenhagen says nothing about the moon when you don't look. Copenhagen has 3 things it needs the observer to do

(1) Choose a system/apparatus divide (ie. the Heisenberg cut or the classical/quantum cut)
(2) Choose an observable (ie. a preferred basis)
(3) Choose when the observable is measured

In simple cases, it does seem that decoherence plus additional objective criteria (eg. the probability sieve) can do all 3, so there is some advance in the sense that although we still have bizarre things like a Heisenberg cut and wave function collapse, we no longer need an observer to subjectively do these jobs. This is why it is often said that decoherence rephrases the measurement problem. However, as we discussed above in posts #83 and 84, decoherence cannot do (1) one in general, because a hallmark of Copenhagen is that the cut is not objective, and can be shifted. (Schlosshauer uses different terminology in his review. What I am calling Copenhagen with a movable cut is called the "standard interpretation" by him, while he uses "Copenhagen" for an objective unmovable cut.)

 

[Rajkovic]

In Qbism, the interpretation of the state is subjective, right.. then what changes in the macro world?

 

[bhobba]

Yet, does anything change if the quantum state of the "few stray photons" are also unresolved? Again, if what we are considering is the quantum state of the cosmos as a whole, and no experimental controls are in place, does that change anything. Is every subatomic particle necessarily an apparatus by which a measurement is made?

Scratching head here. Why exactly do you think the state of the photons will not be changed by the interaction?

And not every object in the universe is not an observational apparatus - only those capable of causing decoherence eg a single photon is not enough to decohere a dust particle - you evidently need a few.

.Thanks
Bill

 

[bhobba]

In Qbism, the interpretation of the state is subjective, right.. then what changes in the macro world?

The outcome of the observation.

Thanks
Bill

 

[zonde]

Things with 0 probability of happening don't make the universe branch out so it does happen, only then to realize it shouldn't and cease to exist.
No branching occurs to make a universe where the particle goes along that 0 probability path. (I would have thought?)

Let me use analogy.

Say you are standing on a street corner and you want to get to the diagonally opposite corner of the block. You can go around the block by going left (L path) or right (R path). And as you go by either path you have to count front doors of the buildings that you pass. So if you choose L path you will count X number of doors but if you choose R path you will count Y number of doors.
But the trick is that I want you to tell me at the end if the difference between X and Y is odd or even number (that will decide which one of the further paths will be 0 probability path and which one - 1 probability path).

Can you do that if you are allowed to make two copies of the world?
Say two worlds - L and R. In L world you take L path but at the end you know X but don't know Y. And vice versa for R world. And if you do not allow worlds to communicate some time after splitting you can't tell me the quantity I am asking, right?

 

[kith]

Newer understood this about MWI. Take Mach–Zehnder interferometer, photons are detected only at one output in case of interference. If you say that all outcomes are realized then you contradict experiment.

No. If the probability to detect the photons at one detector is 100%, then there is only a single possible outcome and therefore only a single world.

The MWI doesn't simply assign worlds to all terms in a superposition. This wouldn't make sense because whether a state is a superposition or not depends on the basis and every state is a superposition of eigenstates of some observable.

In the most basic version, the splitting into different worlds occurs whenever you have different possible outcomes in a measurement. More sophisticated, the splitting occurs whenever a superposition is turned into the corresponding mixed state by decoherence.

 

[Feeble Wonk]

Scratching head here. Why exactly do you think the state of the photons will not be changed by the interaction.

And, again, how does the environment know if the photon(s) interacted with the dust particle? My question was making the assumption that the dust particle is not being observed in anyway other than through its "potential" random environmental interaction, with no system preparation and no experimental controls. Furthermore, the dust particle and the photon(s) are only being considered as part of the entire cosmological quantum state. So, would the quantum state of both the photons and the dust particle be less "determined" in such a way as to delay the environmentally induced collapse secondary to decoherence?
I

 

[Feeble Wonk]

Apologies... I botched that lady posting. I'll try again.

And not every object in the universe is not an observational apparatus - only those capable of causing decoherence eg a single photon is not enough to decohere a dust particle - you evidently need a few.

.Thanks
Bill

I'm sorry to be dense about this, but that's what I'm trying to understand. Which subatomic particles perform as an observational apparatus, and why sometimes rather than others. Is it totally dependent on the preparation state of the system being observed? And if the system is not "prepared", and there are no experimental controls in place, such that both system and "environment" are maximally "open", does this change things? It seems that under certain circumstances quantum superpositions can be maintained for a functional period of time, even in warm and "noisy" environments. At least, the demonstration of quantum effects in photosynthesis would appear to suggest that.
http://m.phys.org/news/2012-01-role-quantum-effects-photosynthesis.html

 

[Feeble Wonk]

Scratching head here. Why exactly do you think the state of the photons will not be changed by the interaction?

But, how does the environment know if the photon(s) interacted with the dust particle? My question was making the assumption that the dust particle is not being observed in anyway other than through its "potential" random environmental interaction, with no system preparation and no experimental controls. Furthermore, the dust particle and the photon(s) are only being considered as part of the entire cosmological quantum state. So, would the quantum state of both the photons and the dust particle be less "determined" in such a way as to delay the environmentally induced collapse secondary to decoherence?

 

[bhobba]

Which subatomic particles perform as an observational apparatus, and why sometimes rather than others.

It depends on the situation - specifically if the object or objects interact with what is being observed and the degree of interaction. You must analyse each situation in detail to determine if the decoherence (ie interaction) is enough to give a definite outcome in some observable. Of course its a bit arbitrary what is counted as a definite outcome eg what precision we count the position to be known before we say it has that position. It turns out to nearly always be the position observable and when you chug through the math the reason is virtually all interactions are of the radial type.

Thanks
Bill

 

[bhobba]

But, how does the environment know if the photon(s) interacted with the dust particle?

The environment has nothing to do with it in that situation ie the few stray photons is the environment.

Thanks
Bill

 

[zonde]

No. If the probability to detect the photons at one detector is 100%, then there is only a single possible outcome and therefore only a single world.

The MWI doesn't simply assign worlds to all terms in a superposition. This wouldn't make sense because whether a state is a superposition or not depends on the basis and every state is a superposition of eigenstates of some observable.

In the most basic version, the splitting into different worlds occurs whenever you have different possible outcomes in a measurement. More sophisticated, the splitting occurs whenever a superposition is turned into the corresponding mixed state by decoherence.

Yes, with a bit of help from Wikipedia I understand what I missed about MWI - world splits only at measurement and not before.

And it makes MWI even more bizarre. Quantum system in MWI actually is a wave. So it is non-local.
And then the split. It is non-local - how else if the quantum system is non local?
And it creates only appearance of particle. So there is no such a thing as particle in MWI, right?

 

[bhobba]

So it is non-local.

That doesn't follow nor is the quantum system in MW actually a wave. Its exactly the same as in the formalism - the only difference is the interpretation of decoherence.

Thanks
Bill

 

[kith]

And it makes MWI even more bizarre. Quantum system in MWI actually is a wave.

The quantum system in the MWI is the whole universe and the universal wavefunction is the state of the universe.

 

[Edward Wij]

I think this conciousness stuff is a very very big crock of the proverbial, but it must be said a fully coherent interpretation can be built from it - just a very very weird one. We even have people that believe in solipsism despite the fact it leads to just as weird a view. Most people, correctly IMHO, reject such - but it can't be proven incorrect.

Thanks
Bill

Bill. Do you have any references or papers detailing or summarizing this consciousness interpretation where you said "a fully coherent interpretation can be built from it - just a very very weird one"? Can you give an example how weird? In physics weirdness must not be taken as arguments not to explore clues.. in 16th century, people can be burnt at stake or feed to the dogs for believing in special relativity or superstrings for example.

 

[bhobba]

Bill. Do you have any references or papers detailing or summarizing this consciousness interpretation where you said "a fully coherent interpretation can be built from it - just a very very weird one"?

Don't know of any. But the kind of things it would need to do is pretty obvious eg do a double slit experiment, record the output to computer memory, make a million copies and send them all except one to different distant parts of the universe. Then a century later view the one you kept. At that time the observation becomes real and all those copies contain real data by some strange unspecified means.

Thanks
Bill

 

[zonde]

That doesn't follow nor is the quantum system in MW actually a wave. Its exactly the same as in the formalism - the only difference is the interpretation of decoherence.

As I see it wave function collapse is a bridge between wave function and particles. If you take away this bridge and try to explain everything using only wave functions then it changes a lot of things. Say quantum interference certainly relays on the idea that single wave function can be spatially discontinuous (can be at two places simultaneously) while particles are localized.