The Arrow of Time and the Reversibility of Decay Processes in Physics

[Abdullah Qasmi]

Hello!
I'm an Engineer by profession with less knowledge of physics, so please be basic with me.
After recently being intrigued by the double slit experiment, I started to read more and more about the same.
At most of the places, it says that as soon a sensor is kept near one of the slit to find out which slit the electron passes from, the wave nature of the electron collapses and the interference pattern disappears, i.e. a "conscious mind" collapses the wave function.
My question is, is it the act of measurement that is causing the change or is it just the sensor that is altering the particle nature?
I mean, if I shoot a particle and a sensor is mounted at one of the slits, but I am not measuring which slit it is passing through; will I see an interference pattern? Will the electron behave as a wave like it normally would, or will the wave function collapse and particle like properties will be seen?

It may be a basic question but please bear with me and explain.
Thank you.

 

[M Tayyab]

Dear Abdullah..
there is always a wave associated with a particle; according to de brogli's equation the associated wave is inversly propotional to mass... as electrons are really tiny fundamental particles so they posses wave nature. there is always a probability of existence of electrons we don't know exactly their positions; we can predict only as we solve by using schrodingers wave equation..
so in my point of view there is no such a sensor that can detect exactly the pessage of electron.
regarding detection we know that when light strikes the particle and reflect back to us then we can detect the existence od something but for the case of electron when we try to detect the photons (light packets) we fall will interect with the wave of electron and by interference it behaves like a particle otherwise like a wave.

 

[BvU]

If you don't measure you get the interference pattern.

 

[A. Neumaier]

The interference pattern solely depends on the experimental setting, not on an act of observation or on the presence of a conscious mind.

However, a sensor placed at one of the slits changes the experimental setting, and hence the pattern observed. Since the sensor interacts with the electron, the electron is influenced by it (action=reaction) and the response observed can be calculated by taking this interaction into account. It will reduce the quality of the interference pattern or even completely destroy it, depending on the amount of interaction.

 

[DrChinese]

I mean, if I shoot a particle and a sensor is mounted at one of the slits, but I am not measuring which slit it is passing through; will I see an interference pattern? Will the electron behave as a wave like it normally would, or will the wave function collapse and particle like properties will be seen?

For an electron, if you have the sensor, there will be no interference. The presence of a conscious observer is not required, as A. Neumaier mentions.

But there is another way to further understand the situation using photons and the double slit. Mind you, the same issues apply to electrons but it is bit harder to set it up appropriately.

Using photons as the particle source: You place 2 polarizers (which are somewhat analogous to the sensors), one each, behind the 2 slits, and there is no other sensors to otherwise record the path taken: If they are parallel, there WILL be interference. If they are perpendicular, there will be NO interference. The reason is that it would be possible to observe the polarization of photons as they hit the screen and determine the path when the polarizers are perpendicular. You don't actually need to take that step though, to eliminate the interference.

The moral is: you cannot pin the issue of interference on the fact that you are "disturbing" the particle by measuring it. Clearly, in this case, the only thing that changes is the relative orientation of the polarizers.

 

[mikeyork]

The short answer is that your sensor is an observer. A conscious being could come along later and check it and get the information. So the logic behind all science says that any later observations by any being or apparatus in the universe must be consistent with the information from your sensor (and anything else that happens).

 

[rkastner]

For some thoughts on how to define the act of measurement, and avoid dependence on an ill-defined 'external conscious observer,' see: https://transactionalinterpretation...t-measurement-is-not-necessarily-observation/

 

[secur]

It's interesting to note that no one knows whether the conscious mind has anything to do with the collapse of the wavefunction. Many physicists think it does. When people tell you otherwise, ask them to prove it. You'll find, they can't.

 

[Nugatory]

It's interesting to note that no one knows whether the conscious mind has anything to do with the collapse of the wavefunction. Many physicists think it does. When people tell you otherwise, ask them to prove it. You'll find, they can't.

It is impossible to prove a negative, so of course no one can prove that the conscious mind has nothing to do with the collapse of the wave function. However, that's not especially relevant to the original poster's question: will an interference pattern form if there's an unexamined detector at one of the slits?

Please try to keep the the thread on topic here; interpretation discussions are hard enough to moderate effectively already.

 

[rkastner]

This depends on how one defines 'unexamined', and that's why in the standard approach one always ends up having to define what it means to be 'examined'--an observer-dependent term. This is why I posted the link above concerning how to define the act of measurement. The ambiguity remains in Feynman's comments in his Lectures:
“Suppose you only want the amplitude that the electron arrives at x, regardless of whether the photon was counted at [detector 1 or detector 2]. Should you add the amplitudes [for those detections]? No! You must never add amplitudes for different and distinct final states. Once the photon is accepted by one of the photon counters, we can always determine which alternative occurred if we want, without any further disturbance to the system…do not add amplitudes for different final conditions, where by ‘final’ we mean at the moment the probability is desired—that is, when the experiment is ‘finished’. You do add the amplitudes for the different indistinguishable alternatives inside the experiment, before the complete process is finished. At the end of the process, you may say that ‘you don’t want to look at the photon’. That’s your business, but you still do not add the amplitudes. Nature does not know what you are looking at, and she behaves the way she is going to behave whether you bother to take down the data or not.” [Feynman 1965 Vol 3, 3-7; original italics and quotations]
The ambiguity about what constitutes a 'final condition' -- 'at the moment the probability is desired' is removed if absorption is taken into account, which is what is noted in the linked blog post. Then one can define when the experiment is 'finished'--at absorption (detection) of the relevant quantum. Absorption is what makes alternatives distinguishable. It doesn't have to depend on anyone 'desiring' a probability.
Regarding the original question, without appeal to whether anything is 'examined' or not, or whether anyone 'desires a probability', an interference pattern will form at the final screen if the original momentum superposition of the photon is retained at the point of detection of the photon. In principle one could even have individual detector screen elements that could couple to the momentum of the photon, but as long as there is no detection corresponding to that coupling, the superposition of momentum states is retained and the pattern is preserved. If in contrast you provide for physical detection of a momentum eigenstate at the screen elements, there will be no pattern. It is the mode of absorption/detection that defines the distinguishable alternatives, not anyone having to desire anything or examine anything,

 

[secur]

Please try to keep the the thread on topic here; interpretation discussions are hard enough to moderate effectively already.

Sorry, I certainly don't want to make extra work for moderators, whose efforts on our behalf are, or should be, appreciated by all.

It is impossible to prove a negative, ...

I've said this myself many times, but actually it's not entirely true. For instance Bell proved "there is no realist, local HV model for QM". Indeed all so-called "no-go" theorems prove a negative! I could give more examples from physics, and elsewhere.

... so of course no one can prove that the conscious mind has nothing to do with the collapse of the wave function.

True; nor can one prove the opposite. That's how one can tell it's an interpretation - not science, really, but philosophy. No interpretation can, at this time, be proven one way or the other. However some posts, for instance @rkastner's link above, assert definitely that consc has nothing to do with it. Since that's impossible to say, I thought it worth correcting.

However, that's not especially relevant to the original poster's question: will an interference pattern form if there's an unexamined detector at one of the slits?

Actually, OP did ask specifically about "conscious mind":

At most of the places, it says that as soon a sensor is kept near one of the slit to find out which slit the electron passes from, the wave nature of the electron collapses and the interference pattern disappears, i.e. a "conscious mind" collapses the wave function. My question is, is it the act of measurement that is causing the change or is it just the sensor that is altering the particle nature?

But let's ignore that. Still there is a point worth clarifying here. OP assumes "interference pattern disappears" and "collapse" are the same thing. But they're not. The detector introduces decoherence to destroy the interference pattern. It's irrelevant whether anyone looks at the result. But that's not the same as collapsing the wavefunction, reducing it to a single eigenvector. Instead both eigenvectors (|particle detected in slit> + |particle not detected>) are still present in the wavefunction, but not interfering. The collapse happens sometime between this event and the observer's involvement, no one knows where. Anyway, decoherence answers OP, and "collapse" just confuses the issue.

 

[rkastner]

I'd like to correct that I never asserted as a matter of 'impossibility' that 'consciousness has nothing to do with it'.
Rather, I presented an argument opposing the many extant categorical claims that consciousness is required in order to define measurement. That's a very different thing. For the sake of making progress in understanding what measurement is, I hope that my arguments will not be mischaracterized in the future. Thanks!

 

[rkastner]

Actually, OP did ask specifically about "conscious mind":
But let's ignore that. Still there is a point worth clarifying here. OP assumes "interference pattern disappears" and "collapse" are the same thing. But they're not. The detector introduces decoherence to destroy the interference pattern. It's irrelevant whether anyone looks at the result. But that's not the same as collapsing the wavefunction, reducing it to a single eigenvector. Instead both eigenvectors (|particle detected in slit> + |particle not detected>) are still present in the wavefunction, but not interfering. The collapse happens sometime between this event and the observer's involvement, no one knows where. Anyway, decoherence answers OP, and "collapse" just confuses the issue.

No, decoherence and unitary-only evolution does not resolve the issue of measurement (this is an ongoing debate in the literature). See, e.g., https://arxiv.org/abs/1406.4126 , which shows how the whole Quantum Darwinism program, relying only on 'decoherence' in a unitary-only dynamics, is circular.
To assert that 'collapse confuses the issue' is to adhere to a particular interpretation of measurement in QM (unitary-only, Everettian relying on decoherence) that is arguably inadequate.
There is in fact a consistent theory out there involving well-defined physical collapse, and there is nothing necessarily "confusing" about it, unless perhaps one has not read the relevant peer-reviewed literature with an open mind. See, e.g., https://arxiv.org/abs/1204.5227 , https://arxiv.org/abs/1608.00660

 

[secur]

No, decoherence and unitary-only evolution does not resolve the issue of measurement (this is an ongoing debate in the literature). See, e.g., https://arxiv.org/abs/1406.4126 , which shows how the whole Quantum Darwinism program, relying only on 'decoherence' in a unitary-only dynamics, is circular.

I agree.

To assert that 'collapse confuses the issue' is to adhere to a particular interpretation of measurement in QM (unitary-only, Everettian relying on decoherence) that is arguably inadequate.

No, you misunderstand. My point was it "confuses the issue" of OP's question, which is answered by decoherence, which removes interference pattern. In my judgment he's not really asking about collapse. Of course issue of collapse (apart from OP) is very important. I hope that my arguments will not be mischaracterized in the future. Thanks!

There is in fact a consistent theory out there involving well-defined physical collapse, and there is nothing necessarily "confusing" about it, unless perhaps one has not read the relevant peer-reviewed literature with an open mind. See, e.g., https://arxiv.org/abs/1204.5227 , https://arxiv.org/abs/1608.00660

AFAIK TI is a valid interpretation.

 

[rkastner]

We seem to be having a miscommunication. I'll try to clarify.
You seem to be saying (again) here that decoherence is necessary and sufficient to remove an interference pattern. I know that's a standard position, but that's what I"m taking issue with.
Interference, reflecting nonzero off-diagonal terms in the density matrix, can be understood without appeal to any decoherence argument--even though I know that is the 'mainstream' approach that everyone is taught now. Unfortunately it is quite inadequate, and many of those who originally championed it (e.g., Max Schlosshauer) have now realized that. Zurek still clings to the idea that decoherence-only can account for phenomena such as whether or not there is interference, but one can see from his response to my letter in Physics Today (https://arxiv.org/abs/1412.7950) that he continues to assume what he claims to demonstrate. (Basically, his response was just to say that of course cats aren't in superpositions. I.e.--to appeal to our macroscopic experience, which is what he claims to be deriving from unitary-only QM via decoherence.)
Recall that decoherence invokes entanglement of the system with many environmental degrees of freedom which are then 'traced over' to get an (almost) diagonal density matrix for the system. But that is neither necessary nor sufficient (since the off-diagonal terms are never really zero in that approach, and one also has to assume classical distinguishability of the environment as an ad hoc assumption).
What I tried to point out in my post with the Feynman excerpt is that all you need (rather than 'decoherence') is the response of absorbers, which lead to non-unitary collapse. That is what would destroy interference in the case of a two-slit experiment in which the relevant absorbers could respond only to a particular momentum eigenstate (as opposed to the superposition)--that would collapse the photon to only one of the momentum eigenstates. And that's why collapse is relevant to this issue.
I do appreciate your saying that TI is a valid interpretation. With TI you don't need decoherence arguments to explain the measurement process, including whether or not there is interference. That's not to say that we don't see decoherence (diminishing of off-diagonal terms due to entanglement of a system with other systems) occurring in certain situations--it does occur under certain conditions. But decoherence alone is neither sufficient nor necessary to explain the measurement process and what we see as a result of measurements.
Best wishes,
RK

 

[OCR]

True; nor can one prove the opposite. That's how one can tell it's an interpretation...

Yeah, I said something about like that, concerning interpretations, almost a month ago, right here...[COLOR=#black].[/COLOR] lol

No interpretation can, at this time, be proven one way or the other.

I informally, more or less, said the same thing... right here .

Obviously, it wasn't in this thread, so it might not count...[COLOR=#black].[/COLOR]

 

[secur]

We seem to be having a miscommunication.

Right.

You seem to be saying (again) here that decoherence is necessary and sufficient to remove an interference pattern.

Not necessary. There are many other possible explanations, such as predestination, absorber response, "God did it", and so on. To justify "necessary" one would have to deny every one of them. To justify that denial, one would have to study them all, and prove them all wrong - a never-ending, hopeless task. Decoherence is sufficient to remove interference pattern. AFAIK.

You point out that it only explains how off-diagonal elements become near 0, but not exactly 0. That's right. But if they're near enough to 0 you can't get an interference pattern, FAPP.

I do appreciate your saying that TI is a valid interpretation.

As far as I know, it is. But it makes the crazy (to use Bohr's word) assumption that influence can travel backward in time. Maybe so, who knows?

But decoherence alone is neither sufficient nor necessary to explain the measurement process and what we see as a result of measurements.

That's right. Decoherence doesn't explain, at all, what we see as a result of measurements, nor did I say it did. That's because it doesn't address, at all, the collapse. But collapse is why we see one particular result out of all the possibilities. Or, more precisely, it's the word we use to describe that fact.

BTW I reviewed your website a bit. The only thing that puzzles me is why you don't like the idea that consc may collapse wavefunctions (of course, it may not). I'd like to discuss it further but this isn't the place for it - especially, not this particular thread.

Yeah, I said something about like that, concerning interpretations, almost a month ago, right here... I informally, more or less, said the same thing... right here.

Great minds think alike!

 

[stevendaryl]

I'd like to correct that I never asserted as a matter of 'impossibility' that 'consciousness has nothing to do with it'.
Rather, I presented an argument opposing the many extant categorical claims that consciousness is required in order to define measurement.!

I would say even more strongly that there is absolutely no reason to think that there is something special about consciousness in QM. A measurement made by a machine seems to act exactly like a measurement made by a human being. I think that Von Neumann/London/Bauer/Wigner (whoever's idea it was to put the collapse at the conscious observer) did a disservice to physics by giving fuel to those who would connect quantum mechanics to mystical notions of the mind creating reality. QM is weird enough without that.

Having said that, it does seem to be the case that QM has a macroscopic/microscopic distinction, which is almost (but not quite) as weird as the consciousness distinction. Once a physical property such as a particle's spin has been amplified to make a macroscopic difference, then the value seems to be definite in a way that never happens for microscopic values.

 

[OCR]

...it does seem to be the case that QM has a macroscopic/microscopic distinction...

Yeah... that does seem to be the case .Oh snap!... I forgot.[COLOR=#black]..[/COLOR]

Stuff like this is the reason that wikipedia is not an acceptable source here.

In fact, just yesterday, I made a $20.00 donation to Wikipedia®...

I wonder if I can get my damn money back ?[COLOR=#black]...[/COLOR]

 

[rkastner]

Right.
Not necessary. There are many other possible explanations, such as predestination, absorber response, "God did it", and so on. To justify "necessary" one would have to deny every one of them. To justify that denial, one would have to study them all, and prove them all wrong - a never-ending, hopeless task. Decoherence is sufficient to remove interference pattern. AFAIK.

You point out that it only explains how off-diagonal elements become near 0, but not exactly 0. That's right. But if they're near enough to 0 you can't get an interference pattern, FAPP.
As far as I know, it is. But it makes the crazy (to use Bohr's word) assumption that influence can travel backward in time. Maybe so, who knows?
That's right. Decoherence doesn't explain, at all, what we see as a result of measurements, nor did I say it did. That's because it doesn't address, at all, the collapse. But collapse is why we see one particular result out of all the possibilities. Or, more precisely, it's the word we use to describe that fact.

BTW I reviewed your website a bit. The only thing that puzzles me is why you don't like the idea that consc may collapse wavefunctions (of course, it may not). I'd like to discuss it further but this isn't the place for it - especially, not this particular thread.
Great minds think alike!

There is still some distortion of my statements--and possibly yours--going on here. What I did was contest your replies to the original question of loss of interference, which repeatedly asserted that decoherence is what answers it" That is an unsatisfactory answer; it's really NOT sufficient--that's my point. There is much in the published literature pointing out why it's problematic, so I'm just asking people to stop categorically asserting it as if it's settled science. It's not. Take a look at the Stanford Encyclopedia entry on 'decoherence' to see all the problems with it as a putative explanation for all cases of loss of interference.
Re consciousness collapsing the wf: that fails to explain the form of the measurement transition, and it relies on an ill-defined appeal to a 'conscious observer' necessarily outside the scope of the theory. That's why I find it inadequate. The appeal to 'consciousness' as separate from the quantum system under study is a failure to solve the measurement problem, which can be solved if one takes absorption into account.

 

[rkastner]

I would say even more strongly that there is absolutely no reason to think that there is something special about consciousness in QM. A measurement made by a machine seems to act exactly like a measurement made by a human being. I think that Von Neumann/London/Bauer/Wigner (whoever's idea it was to put the collapse at the conscious observer) did a disservice to physics by giving fuel to those who would connect quantum mechanics to mystical notions of the mind creating reality. QM is weird enough without that.

Having said that, it does seem to be the case that QM has a macroscopic/microscopic distinction, which is almost (but not quite) as weird as the consciousness distinction. Once a physical property such as a particle's spin has been amplified to make a macroscopic difference, then the value seems to be definite in a way that never happens for microscopic values.

Yes. TI provides an explanation for that.

 

[secur]

I would say even more strongly that there is absolutely no reason to think that there is something special about consciousness in QM. A measurement made by a machine seems to act exactly like a measurement made by a human being. I think that Von Neumann/London/Bauer/Wigner (whoever's idea it was to put the collapse at the conscious observer) did a disservice to physics by giving fuel to those who would connect quantum mechanics to mystical notions of the mind creating reality.

I disagree. If it's allowed, I could explain why you're wrong, in a new thread. If not, that's fine also.

Take a look at the Stanford Encyclopedia entry on 'decoherence' to see all the problems with it as a putative explanation for all cases of loss of interference.

Stanford Encyclopedia is good. I agree with approximately everything in its decoherence article. In particular, decoherence does NOT answer the "measurement problem", although many people say it does. Quite likely, I didn't state my position correctly above, according to modern terminology. If so, the reason would be: I haven't been involved in Consciousness Studies, and related issues, since the last century. Back then I gave up, after more than two decades arguing with people like Daniel Dennett, etc. It's like beating one's head against a wall, but less productive.

... ill-defined appeal to a 'conscious observer' ... The appeal to 'consciousness' as separate from the quantum system under study is a failure to solve the measurement problem ...

I don't agree. Of course I don't "know" the answers: I could be wrong. Certainty about these issues is thoroughly unreasonable, unless you're under 30 or so.

 

[stevendaryl]

I disagree. If it's allowed, I could explain why you're wrong, in a new thread. If not, that's fine also.

Could you explain your reasons in a message?

 

[Mentz114]

I disagree. If it's allowed, I could explain why you're wrong, in a new thread. If not, that's fine also.

Start a new thread - I'm interested in your argument and I'm sure there are others.

 

[rkastner]

Thanks everyone. I agree that dogmatism has no place in science. I'm trying to counter the dogma that 'decoherence explains' whatever aspects of the results of measurements that are under discussion. A critique of the decoherence 'solution' is not dogma or categorical 'certainty' itself--so I can't apologize for suggesting that people consider a model that breaks the linearity of the evolution, and thereby allows an observer-independent account of measurement where the off-diagonal elements are truly zero and an epistemic intepretation of the mixed state is legit ;) Best wishes to all.

 

[bhobba]

No, decoherence and unitary-only evolution does not resolve the issue of measurement (this is an ongoing debate in the literature). See, e.g., https://arxiv.org/abs/1406.4126 , which shows how the whole Quantum Darwinism program, relying only on 'decoherence' in a unitary-only dynamics, is circular.

Correct.

Exactly what it does and does not do is examined carefully in the following text:
https://www.amazon.com/dp/3642071422/?tag=pfamazon01-20

It has however morphed the question a bit - now its, colloquially, why do we get any outcomes at all.

In some interpretations like that proposed by Rkastner (transactional interpretation) it's trivial, in mine (ignorance ensemble) its a big problem. Why do I believe in ignorance ensemble? Its a matter of debate if the problem is really a problem (nature may just be like that) - but that is a whole new thread.

Thanks
Bill

 

[bhobba]

I'm trying to counter the dogma that 'decoherence explains' whatever aspects of the results of measurements that are under discussion

Don't worry - me and many others on this forum make sure people know that one. Its false, and standard texts explain exactly why. But some still believe it - its an issue.

Thanks
Bill

 

[bhobba]

Unfortunately it is quite inadequate, and many of those who originally championed it (e.g., Max Schlosshauer) have now realized that.

Don't worry - his textbook explains clearly decohoence is not enough. But you may be referring to some of his claims about what it does do. He breaks the measurement problem into 3 parts. He agrees it explains the first two - but not the third. Are you saying even the first two are in doubt? Its on page 50 of his book.

If he has changed his position on the first two then me and others here would like to know about that.

Thanks
Bill

 

[bhobba]

Zurek still clings to the idea that decoherence-only can account for phenomena such as whether or not there is interference, but one can see from his response to my letter in Physics Today (https://arxiv.org/abs/1412.7950) that he continues to assume what he claims to demonstrate. (Basically, his response was just to say that of course cats aren't in superpositions. I.e.--to appeal to our macroscopic experience, which is what he claims to be deriving from unitary-only QM via decoherence.)

Oh no - not that damnable factorization problem again. Its an issue - yes - but really one needs to show that different factorizations leads to different results.

I mean do you really believe in analysing a ball rolling down an incline that if you chose a kooky factorization other than the incline and the ball you would get a different result. I don't think there are any theorems proving otherwise but its just so damn obvious.

But yes strictly speaking in many problems in physics unconscious factorizations are assumed and you really need to show they do not subtlety assume what you are trying to show from the start. Its just that most physicists are like me - their eyes sort of roll back and say - you need to start somewhere.

BTW it's not needed to show that the decoherence program is a failure in explaining the measurement problem - all that is being argued about is some of the things it does explain.

Thanks
Bill

 

[rkastner]

Oh no - not that damnable factorization problem again. Its an issue - yes - but really one needs to show that different factorizations leads to different results.

I mean do you really believe in analysing a ball rolling down an incline that if you chose a kooky factorization other than the incline and the ball you would get a different result. I don't think there are any theorems proving otherwise but its just so damn obvious.

But yes strictly speaking in many problems in physics unconscious factorizations are assumed and you really need to show they do not subtlety assume what you are trying to show from the start. Its just that most physicists are like me - their eyes sort of roll back and say - you need to start somewhere.

BTW it's not needed to show that the decoherence program is a failure in explaining the measurement problem - all that is being argued about is some of the things it does explain.

Thanks
Bill

Sorry I didn't see this earlier. It is crucial to recognize that this is a different situation from 'having to start somewhere' in classical physics. Classical physics isn't in conflict with the basic phenomena: determinate positions etc. The Quantum Darwinism/Decoherence program specifically claims to explain how we get classical determinacy out of quantum indeterminacy. So it cannot presume classical determinacy in order to 'explain' it. It would also be a mistake to conclude, as some seem to have, that 'the lesson of quantum theory' is that either (i) there is NO explanation for the quantum/classical transition or (ii) in order to 'explain' it one has to assume it.
There is in fact a perfectly viable explanation: real physical collapse in the energy/momentum basis, since at the relativistic level energy/momentum is unambiguously the preferred observable (there is NO position or time observable at the relativistic level). People seem to have overlooked this--they treat the nonrelativistic quantum theory as the complete picture, mistakenly concluding that ''position' is an observable when it is not. It has long been known that time is not an observable, so neither space nor time are really quantum observables! Thus there is really no symmetry between the dynamical conserved quantities (E,p) and the spatiotemporal 'quantities'--the latter are not real physical quantities at all. They are parameters characterizing the symmetries of a manifold, not properties of systems.
This approach, real collapse, is not being considered by orthodoxy since it is not a mainstream approach, being based on the long-denigrated direct-action theory (even though John Wheeler was tring to resurrect the direct-action theory in 2003, http://tinyurl.com/hjn228x ! )
See my https://arxiv.org/abs/1406.4126 for a diagnosis of the fallacy inherent in unitary-only decoherence arguments allegedly leading to 'einselection' of pointer states in order to allegedly explain macroscopic classical experience. That whole program presupposes crucial aspects of macroscopic experience (distinguishability in various forms) in order to 'derive' the distinguishability and determinacy of macroscopic experience. That is circular. It can't get off the ground in explaining what it purports to explain if it presupposes what needs explaining. Under decoherence-only, one can only get 'apparent collapse' and approximately diagonal density matrices if one presupposes a proto-classical initial condition. Yet that is now considered the standard explanation for why we find determinate measurement results. This situation (the dogmatic appeal to 'decoherence' as a catch-all explanation for the transition to macroscopic phenomena) needs remedying. One needs something else to pick out the relevant basis for distinguishability--and we can do that if we recognize that energy/momentum is fundamental. Another dividend: putting the 2nd Law of Thermo on a firm foundation. In connection with that, see https://rekastner.files.wordpress.com/2016/12/second-law-of-thermo-arxiv-version.pdf

 

[bhobba]

(there is NO position or time observable at the relativistic level)

Hmmmm.

Not so sure of that one:
http://arnold-neumaier.at/physfaq/topics/positionQFT

There are a couple of others as well but let's start with that one.

It was written by Dr Neumaier so questions are best directed at him.

Thanks
Bill

 

[rkastner]

Thanks, I am aware of this attempt to define a position operator by N-W. You only get a localized state in a particular frame, so it's not a covariant operator description as is required for a relativistic theory. Again, spatiotemporal parameters (x,t) are symmetry parameters, not conserved physical quantities. This is why quantum field theory is done by 'demoting' position to the same status as time, and replacing spacetime coordinate operators with field operators. Another intuitive way to understand the situation is in terms of particle creation: smaller spatiotemporal localizations involve higher momenta and energies, which create particles, so we can't be sure we have a single particle at a singular position. The most natural way to go is to admit that spatiotemporal quantities are not fundamental the way energy and momenta are. The form of the propagator (simpler in E,p representation) supports this as well. I'll add here a paper that valiantly tries to preserve position eigenstates but confesses that they would involve infinite and even imaginary masses: http://asg.sc.edu/sites/asg.sc.edu/files/attachments/Position%20Operators%201969.pdf (p. 181, below eq 40) They try to wave that fact off by saying 'one never achieves exact localization in nature'--which essentially confesses that there is no system possessing a property of well-defined position, which is my point.

 

[bhobba]

'one never achieves exact localization in nature'

Agreed - but I don't see the problem with that. I don't see the decoherence program claiming that. Sure our models have exact positions but everyone knows that's just a good approximation - everyone knows exact positions are not possible.

Thanks
Bill

 

[rkastner]

The significance of noting that energy/momentum is a real observable, while spacetime coordinates are not, is that it solves at least one aspect of the 'factorization problem'. I.e., many researchers who are trying to explain the appearance of the macroscopic realm from the microscopic realm assume that all observables are on the same footing. But at the relativistic level, which is the real level at which Nature operates, energy/momentum is more fundamental--more physically real--than the spacetime coordinate description. So an explanation for emergence of the macroscopic world of appearance needs to take that into account. That is what PTI (now called RTI for relativistic TI) does. In contrast, standard 'decoherence & unitary-only' approaches just help themselves to primal distinguishability in order 'derive' distinguishability. They think you have to do this because they have no way to solve the 'factorization' problem given the supposed equivalence of all the usual observables.

 

[bhobba]

I am having trouble following some of your concerns.

First - no question the factorization issue is a genuine problem - we need theorems that it doesn't matter how you factorize a system it makes no difference - you get the same result. I believe they will eventually be forthcoming but until they are its an issue.

However some points don't make sense to me.

1. It is well known from a number of texts that standard QM is the 'dilute' limit of QFT eg:
https://www.amazon.com/dp/9812381767/?tag=pfamazon01-20

2. Standard QM has, in its domain of applicability, proven correct without exception. Only phenomena such as spontaneous emission, which since it deals with electrons coupled to the quantum EM field is really part of QFT are an issue.

BTW I have no time for those dismissive of TI - its legit and those that deny it are simply letting their prejudices getting in the way of the truth. That's not science.

Thanks
Bill