What are the implications of this experiment?

[Demystifier]

Ken G wrote:

Let me ask if you agree that the "average trajectories" that they plot are indeed exactly the same as we would get via my method #2 above-- running one photon at a time through exactly their configuration, and just putting the wall at different distances, and collect the aggregate detections. Then build up a concept of the aggregate photon flux by taking those measurements, normalizing the total detection numbers to be a constant total for every wall distance used (zero divergence), and then drawing the "field line density" for that divergenceless detection field? That's exactly how we would generate a concept of "aggregate photon flux" in this very two-slit experiment, in a completely classical limit of many iterations of slightly different experimental setups (the distance to the wall being the sole variable).

If we can agree that I can get the exact same figure my way, with no subtle "weak measurements",

OK, I'll comment, but only briefly. In that way, one cannot obtain the same figure.

 

[Fyzix]

I both agree and disagree with Deutch. I agree that it is a waste of time for those who already understand it. But I think that this paper is playing a very important role by motivating people to pay more attention to Bohmian mechanics. This is a first step towards a PROPER understanding of its TRUE value.

This is like saying to children: "If you eat vegetables, you will be strong as Supermen". Of course it's not true, but it will make them eat vegetables which is good for other reasons.

Why would this motivate anyone to pay anymore attention to BM when this result was predicted by every interpetation?
I don't see how this experiment changes anything in regards to dBB being more likely to be a correct interpretation now...

 

[IllyaKuryakin]

Average quantities in orthodox QM satisfy deterministic equations. So yes, it's possible to explain it with orthodox QM.

I haven't seen this done and I'm skeptical it can be done.

 

[IllyaKuryakin]

You are wrong. See my two posts above.

You've offered no proof of your statements. I do not believe you are correct.

 

[Demystifier]

Why would this motivate anyone to pay anymore attention to BM when this result was predicted by every interpetation?

First, because he/she does not understand that, as can be seen on this thread.

Second, because the role of interpretation is not only to predict, but also to give an intuitive explanation. Even if all interpretations have equal prediction powers, some interpretations are more appealing than others for intuitive explanations.

 

[Demystifier]

You've offered no proof of your statements. I do not believe you are correct.

It's not a matter of belief. It's a matter of reading papers such as those in
https://www.physicsforums.com/showthread.php?t=252491
You can find proofs there, if you want to read ...

 

[Lost in Space]

As well as the distance of the surface behind the slits, what about the distance between the slits? Is there a maximum and minimum distance where these results can be seen and is it determined by the wavelength of the particle?

 

[Demystifier]

Is there a maximum and minimum distance where these results can be seen and is it determined by the wavelength of the particle?

No (at least in principle).

 

[Lost in Space]

So would I be correct then, Demystifier, in saying that there would still be a recognisable diffraction pattern even if the slits were very far apart or that the wall was very close and that it's probability alone that determines it?

 

[Demystifier]

Well, if the wall was too close, then there would be no diffraction pattern. The answers to other questions are - yes. But all this has not much to do with the subject of this thread.

 

[Lost in Space]

Well, if the wall was too close, then there would be no diffraction pattern. The answers to other questions are - yes. But all this has not much to do with the subject of this thread.

Sorry if I was off thread. Thanks for answering my questions. I find the whole subject of probability fascinating - if baffling!

 

[Ken G]

I'm sorry that I have not reviewed this whole long thread, so perhaps this was covered, but it seems like a central objection to the idea that this experimental result has anything important to say about the two-slit experiment or about Bohmian trajectories.

So let me ask this. Can anyone argue (and now that I am around to clarify, I would expect an actual argument, not just making the claim) that I would not get the exact same trajectory figure the following way: I set up their exact apparatus, but I put my detecting wall at various different distances, repeating over and over until I map out not just a 1D detection-rate function, but a full 2D detection-rate field. The field is of course normalized to represent a divergence-free photon flux. Then I simply draw the divergence-free lines of flux of that 2D field. Why am I not getting their exact same figure, using no "weak measurement" at all? Why is their figure nothing but the 2D divergence-free lines of flux of a completely classical wave?

 

[Demystifier]

So let me ask this. Can anyone argue (and now that I am around to clarify, I would expect an actual argument, not just making the claim) that I would not get the exact same trajectory figure the following way: I set up their exact apparatus, but I put my detecting wall at various different distances, repeating over and over until I map out not just a 1D detection-rate function, but a full 2D detection-rate field. The field is of course normalized to represent a divergence-free photon flux. Then I simply draw the divergence-free lines of flux of that 2D field. Why am I not getting their exact same figure, using no "weak measurement" at all? Why is their figure nothing but the 2D divergence-free lines of flux of a completely classical wave?

I've already asked a question on it in the other thread, but if you want you can answer it here as well.

 

[SpectraCat]

I'm sorry that I have not reviewed this whole long thread, so perhaps this was covered, but it seems like a central objection to the idea that this experimental result has anything important to say about the two-slit experiment or about Bohmian trajectories.

So let me ask this. Can anyone argue (and now that I am around to clarify, I would expect an actual argument, not just making the claim) that I would not get the exact same trajectory figure the following way: I set up their exact apparatus, but I put my detecting wall at various different distances, repeating over and over until I map out not just a 1D detection-rate function, but a full 2D detection-rate field. The field is of course normalized to represent a divergence-free photon flux. Then I simply draw the divergence-free lines of flux of that 2D field. Why am I not getting their exact same figure, using no "weak measurement" at all? Why is their figure nothing but the 2D divergence-free lines of flux of a completely classical wave?

Hmmm ... I wonder what you mean here. You say the "exact apparatus", but then you say there is no weak measurement in your treatment. The weak measurement comes from the inclusion of the thin calcite crystal in their apparatus ... the birefringence of that crystal is what provides the weak measurement in the experiment. They also physically separate the left and right circularly polarized components of the signal at the detection screen, so that two separate patterns are observed. Vide infra, you seem to not be considering those aspects in your treatment, so either your hypothetical apparatus is not exactly the same, or you are just glossing over those details somehow. Can you please clarify whether you are incorporating the two detection patterns in your analysis, or just considering the sum of the two patterns that would be measured in the absence of the separation of the two polarization components?

Also, I guess you are aware that the authors of the paper did use 41 separate "detection wall distances" (to use your term) in order to reconstruct their average trajectories in the paper? So it seems like they are already using the same "2D detection rate field" (to use your term) that you are proposing to measure.

 

[Ken G]

Hmmm ... I wonder what you mean here. You say the "exact apparatus", but then you say there is no weak measurement in your treatment. The weak measurement comes from the inclusion of the thin calcite crystal in their apparatus ... the birefringence of that crystal is what provides the weak measurement in the experiment.

True, but I maintain it makes little difference. One could try it either way, either with the calcite, or without it, you'll get virtually the same figure either way. This is my central thesis: the weak measurement is not really doing much of anything at all, other than causing us to confuse an "average trajectory" as somehow different from the standard classical notion of the lines of photon flux. So far, I have not seen any demonstration that these are two different things, so that is what needs to be established before any claim can be made that "weak measurement" is doing anything interesting or profound, beyond beyind a clever but unnecessarily complicated way to achieve a mundane result.

They also physically separate the left and right circularly polarized components of the signal at the detection screen, so that two separate patterns are observed. Vide infra, you seem to not be considering those aspects in your treatment, so either your hypothetical apparatus is not exactly the same, or you are just glossing over those details somehow.

They provide one figure at the end, right? So do I. So yes, it's exactly the same apparatus, just no attention to polarization-- the orientation of the "trajectories" is fixed by the divergence-free requirement.

Can you please clarify whether you are incorporating the two detection patterns in your analysis, or just considering the sum of the two patterns that would be measured in the absence of the separation of the two polarization components?

They give the trajectories of all the photons. So do I. There's no need to distinguish the polarizations, they do not alter the photon trajectories-- all that is just part of the "weak measurement" that I claim is not doing anything of importance if I can get the same figure with strong detections.

Also, I guess you are aware that the authors of the paper did use 41 separate "detection wall distances" (to use your term) in order to reconstruct their average trajectories in the paper? So it seems like they are already using the same "2D detection rate field" (to use your term) that you are proposing to measure.

All right, that's interesting. But the question remains-- what "trajectories" do they get if they ignore polarizations and do it the way I suggest? If we simply don't care what the polarizations are, we have a photon flux going through this apparatus, and that photon flux can be resolved into "lines of flux". Why is that not the "average" trajectories of the photons?

ETA:
What I'm saying is really pretty simple. We can insert our hand into their apparatus anywhere we like and just count the photons hitting it. This is going to have to agree with their "trajectories" picture, or else their trajectories obviously don't mean anything. But if they have to agree, then they also have to be the lines of flux for those photons, which is determined by the divergence-free requirement. It just seems like a very roundabout way to calculate lines in a figure that have a much simpler meaning, and so far I have not seen any arguments that is not what they are getting, nor would it make sense if they got something different.

 

[DevilsAvocado]

These pictures differ because they refer to different wave functions. In particular, one of them is a photon wave function, while the other is an electron one.

Okey, but now I’ve found one dBB graph for photons, and it looks even 'worse'...

[PLAIN]http://scienceblogs.com/principles/upload/2011/06/watching_photons_interfere_obs/photon_trajectories.png

[URL]http://tewlip.com/pics/photons-bohmian-trajectories-double-slit.jpg[/URL]

http://arxiv.org/abs/quant-ph/0102071"
Authors: P. Ghose (S.N.Bose Natl. Centr.), A. S. Majumdar (S.N.Bose Natl. Centr.), S. Guha (IIT Kanpur), J. Sau (IIT Kanpur)
(Submitted on 14 Feb 2001 (v1), last revised 11 Oct 2001 (this version, v2))

"Figure 2. Bohmian trajectories for a pair of photons passing through two identical slits. Note that there is no crossing of trajectories between the upper and lower half planes."​

Any explanation...?

 

[Ken G]

Those figures do not look to me like they have the same ratio of slit width to slit separation. Also, one would want to make sure there is the same ratio of wavelength to slit width. Whether they are electrons or photons should not make any difference, once you have the same de Broglie wavelengths.

 

[DevilsAvocado]

Those figures do not look to me like they have the same ratio of slit width to slit separation. Also, one would want to make sure there is the same ratio of wavelength to slit width. Whether they are electrons or photons should not make any difference, once you have the same de Broglie wavelengths.

I’m not sure about dBB... but I think mass and massless do make a difference...

Calculated dBB electrons:

[PLAIN]http://m1.ikiwq.com/img/xl/1CZnrM0l5IuTyWElY7ND6c.jpg

What bothers me though... is that Demystifier tries to slip out of this nice little trap that I’ve set up for him... by referring to new 'factors'...

I’m more or less ignorant on dBB, but my "naive intuition" tells me that if dBB is correct we should see 'more' similarities between theoretically calculated trajectories and physically measured...

Either the physical measurement is 'deceptive'... or 'something else' might be...

 

[DevilsAvocado]

Avocado! I've been missing your posts...

Hey DrMac&Cheese! I’ve been missing you and the entangled guys over here! Sooooo good to be back!

Kindest Regards
SatansGuacamole

(sorry for the late reply)

Me too.

thanks guys you are much too kind

 

[DevilsAvocado]

I agree DA but you should post this (or at least cc it) in his discussion thread https://www.physicsforums.com/showthread.php?t=490492.

His interpretation has not much hope to explain GHZ or any other multi-entangled states, or many of the other experiments by Zeilenger and co.

Thanks for reminding me. I’ll see what I can do about the "divine classical mess"...

 

[Ken G]

I’m not sure about dBB... but I think mass and massless do make a difference...

I fugured the diffraction would only care about the de Broglie wavelength. There are some subtleties (Wigner did something on this) about a relativistic particle not having a "localizable" wavefunction, but it seems to me we just have momentum eigenstates here-- which would mean the wavelength is all that counts. I haven't heard any claims that this result depends sensitively on the actual form of the particle wavefunction, beyond its central wavelength.

Either the physical measurement is 'deceptive'... or 'something else' might be...

When I see figures that look like they have different ratios of slit width to slit separation, I don't expect anything else in the figure to look the same either. Are you saying the slit widths and separations (and particle wavelengths) are the same in all these, or are they not?

 

[rgmcc]

If the trajectories were formed by connecting 'average' velocity vectors, does that mean that some 'actual' trajectories do in fact cross the center-line? (Since, an 'average' velocity of 'straight along the middle' implies that we have some 'actual' velocities going slightly up and some going slightly down, crossing the middle?)

Or am I thinking about 'average' in the wrong sense?

 

[unusualname]

It's funny how demystifier is criticising people for talking nonsense and misinterpreting the paper, the authors themselves say

"These results are the first observation of trajectories in a two-
slit interferometer that display the qualitative features
predicted in the de Broglie-Bohm interpretation [3,4]."

( see http://www.aip.org.au/Congress2010/Abstracts/Monday%206%20Dec%20-%20Orals/Session_3E/Kocsis_Observing_the_Trajectories.pdf )

Where they refer to calculations from this paper: (see fig 1 on page 9)
http://arxiv.org/abs/quant-ph/0102071 (Bohmian trajectories for photons)

So the experiment shows something predicted by dBB interpretation, right?

Er, well according to DeMystifier (a dBB believer), NO it doesn't. The authors are clearly misleading everyone, and should reword that passage.

But then DeMystifier goes on to post a https://www.physicsforums.com/blog.php?b=3077 making the astonishing claim that dBB trajectories are no longer hidden variables - they now have the same status as the wavefunction!

lol. Yeah, so an experiment which shows nothing that interpretations without trajectories can't explain has some effect in raising the status of the trajectories!

dBB trajectories are deterministic and have absolutely no useful predictive worth in science, stop pretending otherwise.

And I was actually trying to assist the dBB side by asking someone to point out how the trajectory lines could be otherwise calculated using standard QM or EM (ie NOT USING dBB mechanics), demystifier pointed me to a thread which didn't answer this, since I still haven't seen a calculation which gives the plots. You know, a calculation, no measurements, gives mathematical expression for trajectories.

More like Mystifier tbh.

 

[DevilsAvocado]

... I haven't heard any claims that this result depends sensitively on the actual form of the particle wavefunction, beyond its central wavelength.

I could be mistaken, but I interpreted Demystifier’s answer as there is a difference between photons and electrons in Bohmian trajectories, and since spin etc doesn’t 'exist' in dBB – I took it for granted (guessed) it had to be mass... maybe wrong...

These pictures differ because they refer to different wave functions. In particular, one of them is a photon wave function, while the other is an electron one.

When I see figures that look like they have different ratios of slit width to slit separation, I don't expect anything else in the figure to look the same either. Are you saying the slit widths and separations (and particle wavelengths) are the same in all these, or are they not?

You’re right, and I was too hasty in my conclusion in post #193 (sorry Demystifier). I’ll get back to that, but first let me elaborate my point.

Let’s pretend you have a theory saying that phenomena X is due to a triangle wave that we can’t see or measure. Then 60 years later someone comes up with a bright idea how to measure this wave, and the result turns out to be a sawtooth wave, and that’s fine because there could be some minor mistakes in the measurement.

But if the measurement shows a square wave, I say you’ve got some bigger 'troubles'...

Maybe this is completely nuts and naive... but it would be interesting to hear what Demystifier has to say about it.

As I said, I’ve found Bohmian photon trajectories that match the experiment much better:

Calculated Bohmian photon trajectories

[PLAIN]http://scienceblogs.com/principles/upload/2011/06/watching_photons_interfere_obs/photon_trajectories.png
Experimental (average) photon trajectories

Here you could at least imagine a 'connection' between theory and measurement.

The BIG question is: What happens if one would use the exact experimental setup (slit width/separation) for calculated Bohmian photon trajectories? Would we get an almost perfect match?

And why didn’t Steinberg et al do this in the paper??


(... please, don’t tell me "they have to match" ...)

 

[IllyaKuryakin]

It's not a matter of belief. It's a matter of reading papers such as those in
https://www.physicsforums.com/showthread.php?t=252491
You can find proofs there, if you want to read ...

I read the papers. It took me two days to work through them. No where in those papers do the authors claim that the same average trajectories can be determined by orthodox QM. There is a very simple reason for that. Orthodox QM is non-deterministic. There are no average trajectories in orthodox QM. In fact, there are not even specific trajectories to take an average of. You can't take an average of the trajectory of a density probability wave since it doesn't have a specific trajectory and doesn't even exist as a physical object until it's waveform collapses upon detection. That's specifically how orthodox QM describes the photon in it's path from emmision to detection.

In fact, I have searched for any paper that predicts the average trajectories measured by Steinberg and predicted by Bohmian Mechanics using simply orthodox QM and no such paper exists.

I'm really beginning to wonder of you truly understand the difference between a statistical non-deterministic theory and a deterministic theory?

 

[Ken G]

My claim is we have three identical ways to get that figure:
1) the weak measurement approach, which may be a hard way to do something simple.
2) the Bohmian approach, which invokes a pilot wave but in a more or less ignorable way.
3) the divergence-free flow-field approach, which is purely classical and the easiest by far to interpret.
I don't know for sure these are all the same, but I'd be very surprised if they aren't, and so far we have no good evidence they aren't.

 

[IllyaKuryakin]

I read the papers. It took me two days to work through them. No where in those papers do the authors claim that the same average trajectories can be determined by orthodox QM. There is a very simple reason for that. Orthodox QM is non-deterministic. There are no average trajectories in orthodox QM. In fact, there are not even specific trajectories to take an average of. You can't take an average of the trajectory of a density probability wave since it doesn't have a specific trajectory and doesn't even exist as a physical object until it's waveform collapses upon detection. That's specifically how orthodox QM describes the photon in it's path from emmision to detection.

In fact, I have searched for any paper that predicts the average trajectories measured by Steinberg and predicted by Bohmian Mechanics using simply orthodox QM and no such paper exists.

I'm really beginning to wonder of you truly understand the difference between a statistical non-deterministic theory and a deterministic theory?

Yea, well, that was cranky. But spending two days grinding through the math of two papers that I was told would prove orthodox QM produces the same average trajectories, only to find that the papers have nothing to do with that subject, tends to make me a bit cranky.

And to answer one of your questions, the reason the calcite crystal is necessary is it takes a tiny bit of information as to the path of a single photon via the polorization change. Yes, you could simply place a detector at that point and collapse the entire waveform to create a speck and definitely identify the position at that one point, But that tells you nothing of the trajectory, since QM says there is a very real probability that the next photon will hit the screen a little to the left, and the next will be a little to the right, until they scatter into the exact probability distribution predicted by schrodinger's equation. Remember that Bohmian mechanics depends on the same shrodingers equation, so if Bohmian mechanics is valid, QM must also be valid.

That does not however, mean QM addresses the very same things as deBB. Bohmian mechanics predicts very specific particle trajectories, even though they can never be specifically measured. QM simply doesn't predict specific particle trajectories, no way, no how.

 

[IllyaKuryakin]

The other question, is there a purely classical way to generate the same results? No. The results are created by non-local effects. No classical theory can explain the non-local effects of one photon on one side of the apparatus affecting the path of another photon on the opposite side of the apparatus. Some non-local effects can be explained by the math of QM. But when we are discussing non-local effects on the trajectory of a photon, we have to go to Bohmian Mechanics, or some variant of it.

 

[Ken G]

The other question, is there a purely classical way to generate the same results? No. The results are created by non-local effects. No classical theory can explain the non-local effects of one photon on one side of the apparatus affecting the path of another photon on the opposite side of the apparatus.

So you claim, but with no support. Until my previous post is resolved, I claim we have zero evidence that the information they present in their paper is the least bit non-classical. Even polarization averages can be done classically. The whole concept of an "average trajectory" involves taking a classical limit, ergo, it is a classical concept, masquerading as a quantum mechanical one. That is my claim, and cannot be refuted unless someone can answer my previous post.

 

[IllyaKuryakin]

So you claim, but with no support. Until my previous post is resolved, I claim we have zero evidence that the information they present in their paper is the least bit non-classical. Even polarization averages can be done classically. The whole concept of an "average trajectory" involves taking a classical limit, ergo, it is a classical concept, masquerading as a quantum mechanical one. That is my claim, and cannot be refuted unless someone can answer my previous post.

I see your point. So, the question is, are there non-local effects involved. If yes, then a version of Bell's inequality says there can be no classical explanation.

Here's a bit of discussion, but this probably needs some more in depth study. It seems the whole topic of weak measurement is somewhat controversial:

http://en.wikipedia.org/wiki/Weak_measurement

Do you think Steinberg's claim is false in some way?

 

[Demystifier]

Okey, but now I’ve found one dBB graph for photons, and it looks even 'worse'...

[PLAIN]http://scienceblogs.com/principles/upload/2011/06/watching_photons_interfere_obs/photon_trajectories.png

[URL]http://tewlip.com/pics/photons-bohmian-trajectories-double-slit.jpg[/URL]

http://arxiv.org/abs/quant-ph/0102071"
Authors: P. Ghose (S.N.Bose Natl. Centr.), A. S. Majumdar (S.N.Bose Natl. Centr.), S. Guha (IIT Kanpur), J. Sau (IIT Kanpur)
(Submitted on 14 Feb 2001 (v1), last revised 11 Oct 2001 (this version, v2))

"Figure 2. Bohmian trajectories for a pair of photons passing through two identical slits. Note that there is no crossing of trajectories between the upper and lower half planes."​

Any explanation...?

As the abstract of the theoretical paper says, they calculate the trajectories by using Kemmer-Duffin-Harishchandra formalism. Such trajectories can be thought of as a kind of alternative "Bohmian" trajectories.

Besides, you cannot compare trajectories obtained for different configurations (wave length, distance between the slits, etc.)

Finally, different pictures use different scales. In this way even two exactly identical results may look very different on different pictures. In fact, it could be the main reason for an apparent "difference" between the pictures.

 

[Demystifier]

No where in those papers do the authors claim that the same average trajectories can be determined by orthodox QM.

They do not say it explicitly. Yet, in their theoretical derivation of trajectories they use only orthodox QM. But to see that, it's not enough to read what they say. You need to UNDERSTAND it. Or if your understanding of QM is not good enough, the best you can do is to trust someone else (not necessarily me, of course). In any case, it's up to you to choose what you will believe. For me it is better that more people believe that this experiment proves that Bohmian interpretation is right, because it makes my own professional research on Bohmian mechanics more relevant in the scientific community.

 

[Demystifier]

It's funny how much nonsense was seen in the past arguments AGAINST Bohmian mechanics, and how much nonsense (often by the same people) is seen now in the arguments FOR Bohmian mechanics. And it's frustrating how useless are attempts to explain them anything subtle or nontrivial, to explain them that the truth is somewhere in between.

 

[SpectraCat]

The other question, is there a purely classical way to generate the same results? No. The results are created by non-local effects. No classical theory can explain the non-local effects of one photon on one side of the apparatus affecting the path of another photon on the opposite side of the apparatus. Some non-local effects can be explained by the math of QM. But when we are discussing non-local effects on the trajectory of a photon, we have to go to Bohmian Mechanics, or some variant of it.

No, that is not correct. There is nothing involving interactions of "one photon with another photon" in this experiment. The photons go through one at a time.

 

[DevilsAvocado]

... Besides, you cannot compare trajectories obtained for different configurations (wave length, distance between the slits, etc.)

Finally, different pictures use different scales. In this way even two exactly identical results may look very different on different pictures. In fact, it could be the main reason for an apparent "difference" between the pictures.

True, I hope you saw my (painful ) 'withdrawal' in post #199...

When it comes to "believing" in this or that, it’s completely irrelevant to me, I’m too 'young' to get 'married' to any interpretation soon , and personally I think that "believers" would be far better off inside a church – on the outside we all hope to someday know beyond any doubts.

For me, this is all about getting questions answered. Any 'fight' between different interpretations seems a little bit unwarranted. We all want to know the truth – don’t we?

Therefore, again: What’s 'wrong' with a calculation on the exact experimental setup (slit width/separation etc) for Bohmian photon trajectories?

How hard could be?
Why persist on that this information must be 100% compatible with experiment, before it has even been carried out?
AFAIK, if Steinberg et al would have added calculated Bohmian photon trajectories for the experiment – that paper would have been a real blockbuster (and cover on various science magazines etc)...

Or is it my ignorance? There is no way to make any use, and compare, average experimental trajectories with deterministic theoretical Bohmian trajectories??

So what is it?

• Completely Impossible task
• Completely useless information – because die-hard believers already know the result
• Stupid question by ignorant laymen – there’s no relation at all between experiment and theory in this case
• Not worth the time because ignorant laymen wouldn’t understand it anyway