Does the environment cause wave function collapse

[audioloop]

But apparently, if my understanding of the results of experiments done using an MZI, are correct, then the photon does reach the final BS via both paths.

Physicists ask photons 'Where have you been?
http://physicsworld.com/cws/article/news/2013/nov/26/physicists-ask-photons-where-have-you-been
http://arxiv.org/abs/1304.7469
http://prl.aps.org/accepted/27074Y6bS8a10b4901dc7d435d32e59308c3919be

....

 

[bhobba]

If interaction causes decoherence, which seems to be the position that you're advocating, then in the MZI, the photon enters the apparatus in a state of superposition, and should immediately decohere upon interacting with the beam splitter.

That does not follow.

You do not seem to understand that decoherence that leads to a particle and a photon having a position caused by the particular interaction they have in air (as I mentioned its related to the inverse square like interaction they have) and what goes on in transparent solids like glass are entirely different things - the fact its transparent and a dust particle isn't should be a hint something else is going on.

I gave the link previously from the FAQ and will give it again:
https://www.physicsforums.com/showthread.php?t=511177
'The process of describing light transport via the quantum mechanical description isn't trivial. The use of photons to explain such process involves the understanding of not just the properties of photons, but also the quantum mechanical properties of the material itself (something one learns in Solid State Physics).'

Even the way we have been discussing it here is not correct. A gas like air, while not a solid, really needs a much more sophisticated treatment, but to get a bit of an understanding we are ignoring that issue.

I have explained that a couple of times now and can't quite follow why its still an issue.

Thanks
Bill

 

[bhobba]

expectatives are not established facts. standard quantum mechanics until then expect a full validation (or invalidation)

Since no one is denying that exactly why you think its relevant I have zero idea.

Thanks
Bill

 

[bhobba]

Er, air is transparent and see-through. It what sense can you possibly have a issue with the explanation?

There is an issue here.

Both air and glass are transparent, but glass is solid.

I think he is having difficulty with the idea why solids like dust particles decohere with photons and glass doesn't.

The answer is what happens with glass is actually quite complex, beyond the simple models used to analyse photons and dust particles, or even air molecules. Or to put it another way the kind of interaction they have is different. Dust particles and air molecules have an interaction like an inverse square law and when you work through the math that leads to position decoherence - if you are interested in the details it can be found in Schlosshauer - the argument from what I recall wasn't that hard and I could probably dig it up and post it - but nothing would really be served in doing it. But glass has an entirely different interaction as indicated by the fact its transparent, the simple model used with dust particles is WRONG.

One can find simple explanations of what happens in glass etc, in say Feynman's QED, but they are in fact quite wrong, as explained in the FAQ link I gave.

I am afraid here the jig is up - unless you want to go deeply into solid state physics you simply have to accept that's the way it is.

I have been through that one before. I read about holes and electrons in conductors and that holes really requires a deep solid state QM treatment to understand. I decided to get to the bottom of it - and I did - but with a lot of work and heavy math in advanced tomes.

I am afraid their are some things we simply need to accept, unless you really are into self flagellation, a certified genius, or even one of those rare 'magicians' like Feynman or Von Neumann, that this stuff comes so easy to its second nature.

Thanks
Bill

 

[Fiziqs]

You do not seem to understand that decoherence that leads to a particle and a photon having a position caused by the particular interaction they have in air (as I mentioned its related to the inverse square like interaction they have) and what goes on in transparent solids like glass are entirely different things - the fact its transparent and a dust particle isn't should be a hint something else is going on.

I gave the link previously from the FAQ and will give it again:
https://www.physicsforums.com/showthread.php?t=511177

I have explained that a couple of times now and can't quite follow why its still an issue.

Bill, I hope that you don't think that I failed to read the FAQ article that you linked to. I did indeed read it, and I found it to be a novel idea that I hadn't encountered before. I also found it to be, not without merit, but I couldn't say that I'm sold on the idea either. I think that I can see how this difference in the way that photons behave in gas as opposed to solids, would explain why we see decoherence in the interactions that take place in air, but not in interactions that take place in solids. I guess that I was hoping that you would explain the difference in more detail on your own, so that I wouldn't run the risk of attributing an explanation to you, that you didn't actually intend.

Let me see if I can follow your line of reasoning. When a photon interacts with a particle in the air between the slits and the screen. A particle of dust for example. That particle behaves as a discrete individual object, and thus the interaction imparts to the photon a fixed position. And this fixed position causes the photon to decohere. All other paths become impossible, because the photon now has a definite fixed position.

However, when a photon interacts with the glass in the beam splitter and the mirrors, it doesn't interact with a single discrete object. It interacts with a "lattice", as you called it, of molecules, which have a collective behavior. Due to this collective behavior the photon doesn't take on a distinct, fixed position, and because it doesn't, it doesn't decohere. It's position never becomes fixed.

Before I go on, I would like to know if I have at least a simplistic understanding of your position. Have I at least got the gist of it?

 

[bohm2]

I gave the link previously from the FAQ and will give it again:
https://www.physicsforums.com/showthread.php?t=511177
'The process of describing light transport via the quantum mechanical description isn't trivial. The use of photons to explain such process involves the understanding of not just the properties of photons, but also the quantum mechanical properties of the material itself (something one learns in Solid State Physics).'

Some authors have used this difference to explain that the interference with large molecules is different than free particles:

It has been claimed, in a number of high-impact publications since 1999, that large molecules can be made to interfere on gold gratings, and that these experiments show both, the coherence of the molecules over macroscopic trajectories (range of cm), and that the "wavelength" of these molecules is equal to the de Broglie wavelength of their inertial mass. This is highly naïve and manifestly incorrect, as we show in the following...

So how does it really work? Most likely in the way sketched in the previous section. A polarizable molecule is excited by laser light so that most of its low lying vibrational excitations are activated. This molecule enters the interferometer with a time-dependent dipole moment in lateral direction. As the molecule interacts with the atomic environment of the interferometer, it induces electric dipoles into the slit system. These time-dependent dipole moments interact with the molecular dipole moments until the molecule has passed the interferometer. Due to the interaction the molecules acquire a distinct lateral momentum. The momentum leads to a deflection on the detector screen. The deflection is interpreted as the result of a de Broglie wave, because the distance from the point of no deflection to the point of impact is inverse proportional to the velocity of the molecule. Why is it inverse proportional to the velocity of the molecule?Because the time constant of the interaction duration depends on the time the molecule spent in the slit environment of constant depth. Then a faster molecule will spend less time, therefore acquire less lateral momentum, therefore end up closer to the point of no deflection. This, again, has nothing to do with a de Broglie wave, and all to do with the constant distance from the entry to exit of the interferometer (100nm). This whole scenario should be relatively easy to simulate with modern electronic structure methods. One could also try to pin down the actual effect by using non-polarizable molecules. The prediction here is that no periodic variation on the screen will be observed in this case.

Elements of physics for the 21st century
http://arxiv-web3.library.cornell.edu/pdf/1311.5470v1.pdf

 

[Fiziqs]

Physicists ask photons 'Where have you been?
http://physicsworld.com/cws/article/...-have-you-been
http://arxiv.org/abs/1304.7469
http://prl.aps.org/accepted/27074Y6b...2e59308c3919be

Thank you very much for the links. They were indeed very intriguing, and I had not seen them before. They went immediately into my bookmarks. The experiment is almost as beautiful as the original double slit experiment, and I am sure that I will spend a great deal of time pondering its implications. I found the authors conclusion to be both plausible, and logical, but I am withholding judgment for the time being. I have no problem with waves traveling forward and backward through time, in fact I find the idea to be quite reasonable.

Once again, thanks for the links.

 

[bhobba]

Before I go on, I would like to know if I have at least a simplistic understanding of your position. Have I at least got the gist of it?

Mostly - yes.

But a few points.

Its the interaction between the photon and the object that's important - it causes dechoerence, the splitting of the beam, or polarizing of the photon or whatever. That's the important thing.

Because of the inverse square like interaction between the photon and the dust particle the result of churning through the math is that its decohered in such a way its position is known.

However with objects like glass, polarizes etc the interaction is not that simple, it not a simple easily understood thing and position decoherence is not the result. You will find a simple explanation for objects like that in for example Feynmans QED, and you can read about it if you like, but they are WRONG, what is really going on is much more difficult and complex.

The bottom line is simply don't worry about it - simply accept that photons interact weakly when traveling through the air and the few that do interact are given a definite position and so do not participate in the interference effect. When photons encounter things like mirrors, prisms, polarizers etc simply accept they do things like split it, give it a definite polarizeation (actually 50% are absorbed and 50% come out with a definite polarization), reflect it, or whatever, and don't worry about exactly how they accomplish this feat - its not really important to the issue anyway.

Thanks
Bill

 

[Fiziqs]

Some authors have used this difference to explain that the interference with large molecules is different than free particles:

A polarizable molecule is excited by laser light so that most of its low lying vibrational excitations are activated. This molecule enters the interferometer with a time-dependent dipole moment in lateral direction. As the molecule interacts with the atomic environment of the interferometer, it induces electric dipoles into the slit system. These time-dependent dipole moments interact with the molecular dipole moments until the molecule has passed the interferometer. Due to the interaction the molecules acquire a distinct lateral momentum. The momentum leads to a deflection on the detector screen. The deflection is interpreted as the result of a de Broglie wave, because the distance from the point of no deflection to the point of impact is inverse proportional to the velocity of the molecule. Why is it inverse proportional to the velocity of the molecule?Because the time constant of the interaction duration depends on the time the molecule spent in the slit environment of constant depth. Then a faster molecule will spend less time, therefore acquire less lateral momentum, therefore end up closer to the point of no deflection. This, again, has nothing to do with a de Broglie wave, and all to do with the constant distance from the entry to exit of the interferometer (100nm). This whole scenario should be relatively easy to simulate with modern electronic structure methods. One could also try to pin down the actual effect by using non-polarizable molecules. The prediction here is that no periodic variation on the screen will be observed in this case.

This is an example of one of those times when I have no idea what this passage just said. But I assume that it's quite intelligent, and it makes me think that I should just shut up, and go pound rocks somewhere.

 

[bhobba]

This is an example of one of those times when I have no idea what this passage just said. But I assume that it's quite intelligent, and it makes me think that I should just shut up, and go pound rocks somewhere.

That's the problem with quotes.

Unless you are quite selective in it, it can look like out of context gibberish.

I will occasionally quote, but much prefer to link or explain in my own words - I find things easier to follow that way.

BTW I can't follow it either. If I applied myself to it and asked a lot of questions I may - but from experience know such is unlikely to really be productive.

Thanks
Bill

 

[DennisN]

I just remembered a paper and an experiment which I believe fits the discussion in this thread like a glove:

The Simplest Double Slit: Interference and Entanglement in Double Photoionization of H2
D. Akoury et al. (2007)

Abstract:

The wave nature of particles is rarely observed, in part because of their very short de Broglie wavelengths in most situations. However, even with wavelengths close to the size of their surroundings, the particles couple to their environment (for example, by gravity, Coulomb interaction, or thermal radiation). These couplings shift the wave phases, often in an uncontrolled way, and the resulting decoherence, or loss of phase integrity, is thought to be a main cause of the transition from quantum to classical behavior. How much interaction is needed to induce this transition? Here we show that a photoelectron and two protons form a minimum particle/slit system and that a single additional electron constitutes a minimum environment. Interference fringes observed in the angular distribution of a single electron are lost through its Coulomb interaction with a second electron, though the correlated momenta of the entangled electron pair continue to exhibit quantum interference.

Paper:
http://escholarship.org/uc/item/0mm6845j#page-1
http://www.sciencemag.org/content/318/5852/949

Article: The world's smallest double slit experiment (PhysOrg)
http://phys.org/news113822439.html

 

[Fiziqs]

Excuse me for dredging up this old thread, but I tend to go back and reread my old threads, in case I may have missed something. In doing so this statement from bhobba got me to thinking.

Photons that have been scattered by for example a dust particle have been decohered

It actually got me to thinking about a number of things, but two things in particular.

1. What exactly is decoherence?
2. In the double slit experiment, would interaction with a dust particle actually cause a photon to decohere?

After considerable thought, I have come to the conclusion that I must not have an adequate understanding of decoherence, because I don't see how a dust particle could cause decoherence. If I understand decoherence correctly, then in the double slit experiment, a photon interacting with the screen, doesn't experience decoherence. The interaction with the screen doesn't suddenly cause the photon to go through only one of the two slits. But if it still goes through both slits, in what sense has it decohered? I had always understood that decoherence occurred when the photon was forced to take one path or the other, but interaction with the screen doesn't seem to force the photon to do that.

Likewise, a dust particle is simply a small screen. The photon goes through both slits and then interacts with the dust particle, but this interaction shouldn't cause the photon to decohere any more than the larger screen would have. To me it seems logical that if you could map out the positions of a large number of dust/photon interactions, you would see an interference pattern. After all, weren't the early versions of this experiment done just this way, by moving a small screen around and measuring the number of detections at each location. The dust particles are simply acting as small screens, but collectively they should exhibit an interference pattern, and if they do, in what sense have they caused decoherence?

My initial thought was, that the photon still goes through both slits before interacting with the dust particle, so the photon obviously hasn't decohered. My second thought was, that I have no real understanding of what decoherence is. At the moment I'm betting that the second thought is the correct one, I just don't know why. How am I misunderstanding decoherence?

(FYI, when I say that the photon goes through both slits I am not speaking literally. The correct terminology would probably be to describe it as a probability wave, or a wave function, but hopefully everyone will understand what I mean.)

 

[bhobba]

What exactly is decoherence?

To understand decoherence you really need the math - no out here. It results from what's called tracing over the environment when particles are entangled (see section 1.2.3):
http://philsci-archive.pitt.edu/5439/1/Decoherence_Essay_arXiv_version.pdf

Intuitively when entangled with other particles they have random phase so when you average it out the superposition is in a sense destroyed and you have apparent collapse. That is intuitive - the detail is the math.

So, intuitively, say we have a whole heap of dust particles. The photon, which is in a superposition of position, becomes entangled with the dust particles and looses phase (via this tracing over the environment) so it now act as if, with a certain probability, its in the position of one of the dust particles, like the simple case of two systems with photons in section 1.2.3.

The same with the screen - except the particles of the screen are really close together so its a continuum.

Glass does not act in the sense of being absorbed then flashing etc, but rather refracts, partially reflects, and other stuff. The photon, because for some reason it is transparent to photons, doesn't behave the same way. If you want an intuitive view see Feynmans QED.

Thanks
Bill

 

[PhysicsStuff]

Machines can "measure" things, but what distinguishes a measurement from just another random decoherence? Why do our eyes cause collapse and simply not another factor of decoherence? Are we technically limiting the angle of trajectory to only one possibility? But still, why would that happen vs just a smaller limitation of the possible trajectories?

 

[kith]

I would say that the interpretation-independent difference between a measurement and simple decoherence is that measurement outcomes have to be readable by humans while the final states in simple decoherence may or may be readable by humans. In the case of the moon they are because we can "read" its position by looking at it. In other cases, it is necessary to correlate the states of the QM system of interest with the states of a macroscopic pointer in order to achieve human readability.

 

[PhysicsStuff]

I feel like "human readability" is a term I've never seen in particle physics, but rather computer science.

 

[audioloop]

1. What exactly is decoherence ?

After considerable thought, I have come to the conclusion that I must not have an adequate understanding of decoherence

your inquietude is justified.

a lot of people talk as if all that were definitely settled, but nobody know exactly anything, they lack modesty.
i can say you that decoherence is "enviroment induced superselection" or "loss of unitarity" and so on and explain and explain and lost you in the translation but us, just glimpse a quality of nature, we describe things, but answer "why", ohh god ! the thing maybe go beyond our capabilities (human).

maybe decoherence is caused by gravity or stochastic background fluctuations or have a context dependent cause i.e is polycausal, who knows, we need more experiments or more advanced theories, but some people is conformist or dogmatic

 

[bhobba]

Machines can "measure" things, but what distinguishes a measurement from just another random decoherence? Why do our eyes cause collapse and simply not another factor of decoherence? Are we technically limiting the angle of trajectory to only one possibility? But still, why would that happen vs just a smaller limitation of the possible trajectories?

What do you mean by random decoherence?

Generally these days its thought a measurement has occurred once decoherence has.

Most of the time decoherence has occurred well before the eyes eg when a dust particle is dcecohered to give position. Its only contrived situations you have to consider decoherence at the eyes. I can't think of any off the top of my head but they undoubtedly exist.

Have no idea what you mean by the trajectory stuff.

Thanks
Bill

 

[PhysicsStuff]

What do you mean by random decoherence?

Generally these days its thought a measurement has occurred once decoherence has.

Most of the time decoherence has occurred well before the eyes eg when a dust particle is dcecohered to give position. Its only contrived situations you have to consider decoherence at the eyes. I can't think of any off the top of my head but they undoubtedly exist.

Have no idea what you mean by the trajectory stuff.

Thanks
Bill

Well there is a distinction between decoherence and a particle going into an Eigenstate right? So what physical act makes the distinction between simply limiting the probability of the possible paths a particle travels versus collapsing it into a single outcome?

 

[bhobba]

I would say that the interpretation-independent difference between a measurement and simple decoherence is that measurement outcomes have to be readable by humans while the final states in simple decoherence may or may be readable by humans. In the case of the moon they are because we can "read" its position by looking at it. In other cases, it is necessary to correlate the states of the QM system of interest with the states of a macroscopic pointer in order to achieve human readability.

I think I get the gist of your thought and agree, but would like to expand on it a bit.

Decoherence creates in improper mixed state in a singled out pointer basis. Once that has happened then a measurement has been considered to have occurred. Of course if you set up an apparatus to observe the outcome in that pointer basis it will give a result consistent with it having collapsed - even though it really hasn't - as has been discussed many times in explaining the difference between in improper and proper mixed states and apparent collapse.

Thanks
Bill

 

[bhobba]

Well there is a distinction between decoherence and a particle going into an Eigenstate right? So what physical act makes the distinction between simply limiting the probability of the possible paths a particle travels versus collapsing it into a single outcome?

Do you mean the distinction between apparent collapse and real collapse? If so that has been discussed innumerable times on this forum - do a search for the gory detail. But basically there is no way to tell the difference between apparent collapse and real collapse.

Say, in the double slit experiment, you put a device to detect a particle going through a slit. This means you get, at the slits, an interaction between the detector and photon that changes it to an improper mixed state so it now has an actual probability of going through one slit or the other - coherence has been destroyed and you do not get an interference effect.

Can I ask you to be a bit more precise in your questions - I am finding it a bit difficult figuring out what exactly you are asking.

Thanks
Bill

 

[kith]

Traditional textbooks suggest that we have two types of interactions: unitary interactions and measurement interactions. Because in modern approaches, the latter are explained by the first via decoherence (at least to a certain degree) many people use "decoherence" and "measurement" interchangeably. I don't think this is good practice. I prefer the everyday meaning of "measurement" which involves an observer somehow. Using this terminology, decoherence occurs in all measurements, but not every decoherence process corresponds to a measurement.

Of course if you set up an apparatus to observe the outcome in that pointer basis it will give a result consistent with it having collapsed [...]

You don't have a pointer basis without the apparatus because the pointer is part of the apparatus.

 

[bhobba]

You don't have a pointer basis without the apparatus because the pointer is part of the apparatus.

Not necessarily. Usually the inverse square (or some power of distance) like nature of interactions singles out position as the pointer basis. You will find a discussion of this on page 83 of Schlosshauer - Decoherence And The Quantum To Classical Transition. The ubiquitous nature of these types of interactions is why objects are usually decohered to have positions.

Thanks
Bill

 

[kith]

I don't disagree that a basis of the system is singled out.

I thought that the expression "pointer basis" would refer to -well- the pointer of the measurement apparatus and not the system itself. But you are right, Schlosshauer doesn't make this distinction and his usage seems to be common. My background regarding decoherence is the theory of open quantum systems where the concept of measurements is not important. So the terminology used by foundations people like Schlosshauer is not my mother tongue. Thanks for clarifying!

off topic: I remember that you said that Schlosshauer doesn't mention the factorization problem. I just skimmed a few chapters and he does comment on it in section 2.14, although very briefly. I don't know if this is news to you, I just thought I'd mention it.

 

[bhobba]

I remember that you said that Schlosshauer doesn't mention the factorization problem. I just skimmed a few chapters and he does comment on it in section 2.14, although very briefly. I don't know if this is news to you, I just thought I'd mention it.

Just read it.

He does indeed - well picked up.

Thanks
Bill