Does the environment cause wave function collapse

[bhobba]

What is more important is that the physical model describing how a given system's density matrix evolves appropriately includes any influence of its environment." He also has a very interesting discussion in section 3.6.5 on distinguishing between decoherence and classical dephasing.

Actually that's a VERY important point - thanks for sharing.

Thanks
Bill

 

[bhobba]

because, we don't know, what causes it, just we describe what we see.
no explanation at all.

Just because there is no generally agreed definition on exactly when decoherence has occurred, it does not follow that in many many cases we can't tell it has occurred.

Thanks
Bill

 

[zonde]

And exactly how that invalidates my claim that in your usual double slit experiment, the reason you get an interference pattern is because photons interact weakly with the air, dust particles etc that is usually what lies between it and the screen, and in that situation have long decoherence times, as well as there are a huge number of them so those that do is negligible, is beyond me. Of course they interact STRONGLY with polarizes, that the randomly polarized photons that go through such are in effect observed, and only those of a certain polarization in effect get through.

I take your claim that "the reason you get an interference pattern is because photons interact weakly with the air, dust particles etc" and make a prediction that follows from that statement - when photons interact strongly with the medium on their way interference pattern should disappear.
The first stage of experiment with markers at the slits demonstrates that photons indeed interact strongly with markers and the second stage with additional polarizer at 45° demonstrates that despite strong interaction with makers interference is still observable i.e. prediction falsified.

 

[audioloop]

everything is now quantum.
Bill

just a claim, has to be proved over very wide range of experimental facts.
example; on 10²⁰ atoms.
so, not proved yet.


.

 

[audioloop]

Just because there is no generally agreed definition on exactly when decoherence has occurred, it does not follow that in many many cases we can't tell it has occurred.

Thanks
Bill

when or what are different questions, sir.


.

 

[bhobba]

when or what are different questions, sir.

Which is of relevance exactly how?

My point is simple in the extreme - while there is no generally agreed way to determine if decoherence has occurred, there are many cases where for sure we know it has.

Many of these such as decohereing dust particles by photons are discussed in the reference I gave earlier by Schlosshauer.

Thanks
Bill

 

[bhobba]

just a claim, has to be proved over very wide range of experimental facts. example; on 10²⁰ atoms. so, not proved yet

It's not an experimental issue - its if a pointer basis is determined by decoherence. It is generally thought it is, but some key theorems are lacking, and the definitive answer needs to wait until then.

Thanks
Bill

 

[DennisN]

Regarding "proofs" in science:

I don't intend to nitpick, but I think it's nevertheless important for others who may read this thread that there are no proofs in science, proofs are for mathematics.

In science there are evidence, e.g. strong evidence, weak evidence, no evidence. A hypothesis can be confirmed or ruled out (or not confirmed, if the evidence is irrelevant w.r.t. to the hypothesis).

From Introduction to the Scientific Method (University of Rochester):

As just stated, experimental tests may lead either to the confirmation of the hypothesis, or to the ruling out of the hypothesis. The scientific method requires that an hypothesis be ruled out or modified if its predictions are clearly and repeatedly incompatible with experimental tests.

 

[bhobba]

prediction falsified.

I think you need to explain, very carefully, what you are getting at, and I do mean carefully, because I have zero idea what prediction has been falsified and if it is of any relevance at all.

Thanks
Bill

 

[bhobba]

I don't intend to nitpick, but I think it's nevertheless important for others who may read this thread that there are no proofs in science, proofs are for mathematics.

The context of proofs in science is in the area of the logical consequences of theories eg if QM is true such and such follows.

Here the idea is does QM, as a theory, single out a pointer basis. If so then a very important part of the measurement problem is solved. The theorems at present are not general enough to decide, but it is generally thought it does.

Thanks
Bill

 

[DennisN]

The context of proofs in science is in the area of the logical consequences of theories eg if QM is true such and such follows.

I don't disagree with you in general - I just instinctively frown when I see the use of the word "proof" in science, particularly when it is used when talking about experiments (which was done before in the thread). QM, the Standard Model, relativity etc. as theories/models are not proved. They are however confirmed with very, very strong evidence supporting them. It is an important distinction, IMO - we should remember that we are talking about empirical evidence in science - not proofs.

 

[bhobba]

I don't disagree with you in general - I just instinctively frown when I see the use of the word "proof" in science, particularly when it is used when talking about experiments (which was done before in the thread). QM, the Standard Model, relativity etc. as theories/models are not proved. They are however confirmed with very, very strong evidence supporting them. It is an important distinction, IMO - we should remember that we are talking about empirical evidence in science - not proofs.

And I agree its important to raise it so people understand exactly what's going on.

I have been discussing physics online for I would say nearly 15 years now and know there can be confusion about things that are obvious to those that learn it from standard textbooks. They in fact are so obvious until questioned you aren't even aware they are made.

A prime example is what a mathematical model is, and that physics is really just mathematical models.

Thanks
Bill

 

[Fiziqs]

Photons that have been scattered by for example a dust particle have been decohered - actually both the dust particle and photon are decohered, and in effect both given a position. The reason position is usually what's 'observed' by decoherence is tied up with the inverse square like nature of most interactions and you will need to consult the technical literature, such as the textbook on decoherence mentioned previously, for the detail. Since they now have a definite localized position they have lost 'which path' information as you put it.

The reason you still can get an interference pattern is the massive number of photons that make it to the screen without being decohered.

Also this is a very rough and ready description, photons traveling through a medium like air is a very much more complicated process than this.

Thanks
Bill

After considerable thought, and the consulting of various referenced sources and materials, something is puzzling me about your explanation of why photons traveling through an environment made up of air molecules, do not undergo decoherence. Your explanation, if I understand it correctly, is that the photons rarely interact with the air molecules, and this lack of interaction accounts for the lack of decoherence. If on the other hand a photon should interact with anything on its way through the environment, then it will inevitably undergo decohence. But the rarity of such events is why we see an interference pattern when passing photons through an environment consisting mainly of air. I actually have a number of concerns with this explanation, but I will focus on one in particular.

If instead of using a classical double slit setup, we use a Mach–Zehnder interferometer, then there is little doubt that the photons will interact with some part of the environment before reaching a detector. Yet these interactions do not seem to cause decoherence. If interaction is the cause of decoherence, then how do photons pass through an MZI without apparently undergoing it?

 

[bhobba]

If instead of using a classical double slit setup, we use a Mach–Zehnder interferometer, then there is little doubt that the photons will interact with some part of the environment before reaching a detector. Yet these interactions do not seem to cause decoherence. If interaction is the cause of decoherence, then how do photons pass through an MZI without apparently undergoing it?

Can't follow you there. In the Mach–Zehnder interferometer between the parts of the setup things are exactly as I describe. That's all that's required - the rest acts exactly the same regardless.

In other words it makes no difference if it was done in air or a vacuum.

Is your issue how do beam spliters, polarizes etc work?

That is a very difficult issue as a link from the FAQ's I gave before on light in solids attested to - its even difficult in air - although probably not as bad as solids - but I suspect difficult enough that our discussion here is not really correct - but the best that can probably be done at this level. For this purpose we need to simply accept they do work.

Thanks
Bill

 

[Jilang]

After considerable thought, and the consulting of various referenced sources and materials, something is puzzling me about your explanation of why photons traveling through an environment made up of air molecules, do not undergo decoherence. Your explanation, if I understand it correctly, is that the photons rarely interact with the air molecules, and this lack of interaction accounts for the lack of decoherence. If on the other hand a photon should interact with anything on its way through the environment, then it will inevitably undergo decohence. But the rarity of such events is why we see an interference pattern when passing photons through an environment consisting mainly of air. I actually have a number of concerns with this explanation ...

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

 

[Fiziqs]

Can't follow you there. In the Mach–Zehnder interferometer between the parts of the setup things are exactly as I describe. That's all that's required - the rest acts exactly the same regardless.

In other words it makes no difference if it was done in air or a vacuum.

Is your issue how do beam splitters, polarizes etc work?

Let me see if I can explain. 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. Even if the photon somehow exits the BS along both possible paths, it should again decohere upon interacting with the mirrors. If interaction causes decoherence then it would seem logical that it is impossible for the photon to reach the second BS via both paths at the same time. Thus we should end up with a 50/50 likelihood of the photon being observed at each detector.

I will grant you that I may have overlooked something, even something completely obvious, but it would seem to me that by using an MZI to perform the double split experiment, it can be shown that interaction alone does not cause decoherence. Thus why should we assume that interaction with the air molecules in a traditional double slit experiment does cause decoherence, when the beam splitter and mirrors in an MZI apparently don't?

 

[audioloop]

It's not an experimental issue . It is generally thought it is and the definitive answer needs to wait until then.
Thanks
Bill

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



.

 

[Maui]

Let me see if I can explain. 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. Even if the photon somehow exits the BS along both possible paths, it should again decohere upon interacting with the mirrors. If interaction causes decoherence then it would seem logical that it is impossible for the photon to reach the second BS via both paths at the same time. Thus we should end up with a 50/50 likelihood of the photon being observed at each detector.

You are right, it can't reach the second beam splitter via both paths. People here are continuously on a daily basis hitting the same roadblock - the particle myth. There are no particles, if there were, people would learn to manipulate them and force them to precise positions without the need for probabilistic predictions and limiting postulates. But sure they can't because there are no particles. This is the root of all conceptual problems in qm and you've been misled as many others in believing in a framework that leads to nowhere.

As soon as you get rid of the particle concept as something existing in a realistic space and time and interacting, you'd be on the right track for certain. Yes, nobody really understands how the world works, this isn't news. All attempts at realistic ontologies are crippled and quite unsatisfactory which likely shows that they are wrong and/or incomplete.

PS. I am not trying to sell you anything but only point out what does not work.

 

[Fiziqs]

You are right, it can't reach the second beam splitter via both paths.

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. Something bhobba's concept, of interaction causing decoherence, would seem to forbid. As of now, I am patiently waiting for bhobba's response.

As soon as you get rid of the particle concept as something existing in a realistic space and time and interacting, you'd be on the right track for certain.

Forgive me if my use of terms like photon, and paths, give the impression that I view them as particles, I do not. Just to be clear, I have absolutely, positively, no preconceptions of photons or electrons being "particles". As I mentioned in an earlier post, I am heavily influenced by Richard Feynman's sum over histories method. I break everything down into waves, not particles. I sometimes think that it is actually due to my lack of formal education, (9th grade), that I am forced to conceptualize things in this manner, but it also makes me quite good at it. There are technical issues and concepts of which I lack an understanding, but that's why I ask questions.

This lack of education does at times make it difficult to understand the ideas that other people are trying to convey, because they are using terminology and concepts that I do not understand. It also makes it difficult for me to convey what I see, to them. It's not that I'm right and their wrong, it's just that we're seeing things from a different point of view, and having difficulty communicating those views to each other. But hopefully they will have the patience to try, and I will have the patience to listen. They may gain nothing from the exchange, but whether I gain anything, is up to me.

People may think at times, that I disregard what they say, but I try not to. I may think that what you believe is wrong, and even openly say so, but I will still attempt to ascertain why you believe it.

I'm blathering again. Sorry folks, for another off topic post.

If anyone would like to contribute their thoughts please feel free to do so. In single photon MZI experiments, how can the photon take both paths, if interaction with any of the objects along those paths, will cause decoherence? The photon should be forced to take only one of the available paths. Unless of course it takes more than mere interaction to cause decoherence.

 

[Maui]

It's not that I'm right and their wrong, it's just that we're seeing things from a different point of view, and having difficulty communicating those views to each other. But hopefully they will have the patience to try, and I will have the patience to listen. They may gain nothing from the exchange, but whether I gain anything, is up to me.

This is a valuable bit imo - many of the knowledgeable here believe reality is mathematical and does not need an interpretation - even though it's not stated explicitly, it's implicit in their statements on this board. Hence a lot of the arguments of people pushing for a classical picture over people thinking in terms of relationships. You can't reach any agreement unless you discuss in common terms and most of the times we don't. I am not a physicist and can only hope to partially get to know their religion(now I need to go put on my flamesuit :) )

 

[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