What are the implications of this experiment?

[unusualname]

Fig 1 (on page 9) of http://arxiv.org/abs/quant-ph/0102071

corresponds to the trajectories constructed in this experiment http://www.aip.org.au/Congress2010/Abstracts/Monday%206%20Dec%20-%20Orals/Session_3E/Kocsis_Observing_the_Trajectories.pdf

Now, is there a calculation from standard QM or EM to reproduce the plot?

 

[unusualname]

And I don't mean, can the intensities in each plane be reconstructed. That's not the same thing as constructing trajectories.

Obviously Maxwell's equations predict the correct intensity as standard QM does (otherwise one of them would be wrong), so SpectraCat above is probably thinking of intensity calculations.

Unless there is some simple limit process where the trajectories as plotted above come out in the limit of zero areas for the intensities?

 

[IllyaKuryakin]

Fig 1 (on page 9) of http://arxiv.org/abs/quant-ph/0102071

corresponds to the trajectories constructed in this experiment http://www.aip.org.au/Congress2010/Abstracts/Monday%206%20Dec%20-%20Orals/Session_3E/Kocsis_Observing_the_Trajectories.pdf

Now, is there a calculation from standard QM or EM to reproduce the plot?

Yes, an interesting extension of Bhomian Mechanics from fermions to bosons.

But your question is valid, "Now, is there a calculation from standard QM or EM to reproduce the plot?"

I've looked for such a calculation in published papers, but I can find none. Until I see such a calulation, with all due respect, I have to consider claims that the same average trajectories can be calculated from standard QM suspect, and claims that they can be calculated using classical e-m theory even more suspect. My appoligies if there is such a paper and I've simply missed it.

 

[IllyaKuryakin]

And I don't mean, can the intensities in each plane be reconstructed. That's not the same thing as constructing trajectories.

Obviously Maxwell's equations predict the correct intensity as standard QM does (otherwise one of them would be wrong), so SpectraCat above is probably thinking of intensity calculations.

Unless there is some simple limit process where the trajectories as plotted above come out in the limit of zero areas for the intensities?

Yes, you phrased this better than I did. There is a difference between intensity-EM, probable density-QM and avgTrajectory-deBB.

 

[unusualname]

We must be sure the experiment distinguishes between intensity calculations and trajectories as in the Bohmian Analysis.

If the experimental data is at a resolution such that a simple intensity calculation from classical EM in each plane would explain it then the experiment isn't that great.

 

[SpectraCat]

And I don't mean, can the intensities in each plane be reconstructed. That's not the same thing as constructing trajectories.

Obviously Maxwell's equations predict the correct intensity as standard QM does (otherwise one of them would be wrong), so SpectraCat above is probably thinking of intensity calculations.

Unless there is some simple limit process where the trajectories as plotted above come out in the limit of zero areas for the intensities?

Have you read the paper? Do you understand how the AVERAGE trajectories are reconstructed? They are reconstructed, as I already said, by comparing the interference patterns measured in the two polarization channels, and extracting the AVERAGE transverse component of the momentum. The equation used is (cf. eqn 2 on p.1172)

$$\frac{<k_x>}{|\vec{k}|}=\frac{1}{\zeta}\frac{I_R~-~I_L}{I_R~+~I_L}$$

where I_R and I_L are the intensities in the right and left-handed detection channels, at a particular point in the interference pattern, and zeta is an experimentally determined constant.

Note that the authors omitted the <> around k_x in the text of the paper, but it is there in their derivation provided in the supporting information.

Anyway, the point is that the only experimental observables used to create the AVERAGE trajectories are the interference patterns. So, if you can reproduce those, you are done .. using the same reconstruction produce will produce the same AVERAGE trajectories.

 

[unusualname]

Have you read the paper? Do you understand how the AVERAGE trajectories are reconstructed? They are reconstructed, as I already said, by comparing the interference patterns measured in the two polarization channels, and extracting the AVERAGE transverse component of the momentum. The equation used is (cf. eqn 2 on p.1172)

$$\frac{<k_x>}{|\vec{k}|}=\frac{1}{\zeta}\frac{I_R~-~I_L}{I_R~+~I_L}$$

where I_R and I_L are the intensities in the right and left-handed detection channels, at a particular point in the interference pattern, and zeta is an experimentally determined constant.

Note that the authors omitted the <> around k_x in the text of the paper, but it is there in their derivation provided in the supporting information.

Anyway, the point is that the only experimental observables used to create the AVERAGE trajectories are the interference patterns. So, if you can reproduce those, you are done .. using the same reconstruction produce will produce the same AVERAGE trajectories.

So go on then, reproduce the average trajectories they found, by instead using standard QM or EM calculations.

I'm feeling that nobody has bothered because nobody thinks it's been worthwhile. But now that we have an actual experiment with actual experimental data and a plot of these trajectories, perhaps someone should show how the trajectories can be explained more simply. And in particular, refute the outrageous claim that this paper is in any way vindicating Bohmian calculations of the trajectories.

 

[SpectraCat]

Fig 1 (on page 9) of http://arxiv.org/abs/quant-ph/0102071

corresponds to the trajectories constructed in this experiment http://www.aip.org.au/Congress2010/Abstracts/Monday%206%20Dec%20-%20Orals/Session_3E/Kocsis_Observing_the_Trajectories.pdf

Now, is there a calculation from standard QM or EM to reproduce the plot?

As far as I can tell, the resemblance is coincidental, but I am far from an expert on BM. Perhaps Demystifier could comment further on this? The plot from the BM paper shows actual single-particle trajectories, where as the experimental data is for average trajectories. My guess is that the dependence of the average momenta measured experimentally on the phase of the interference pattern is similar to the dependence of the Bohmian velocities of the individual photons on the phase of the pilot-wave interference pattern in the BM simulation.

One additional question that seems like the elephant in the room to me is the fact that Bohmian trajectories seem to predict that photons NEVER cross the center line between the slits. Thus, photons going through the left slit contribute ONLY to the left half of the interference pattern, and vice versa. So what I don't understand is how that is compatible with the usual double-slit interpretation, which says that knowing which path the photons went through should destroy the interference pattern. Maybe I should ask that question on another thread ...

 

[unusualname]

Yes probably, but if anyone wants internet fame for a few days and a publication, just calculate the plots using standard QM or classical EM.

 

[Varon]

Yes probably, but if anyone wants internet fame for a few days and a publication, just calculate the plots using standard QM or classical EM.

If there's someone who can do it. It's Ken G. I asked him to participate here as he is not aware this thread exists.

Ken. How hard or long would it take to calculate the plots using standard QM or classical EM? Come on, do it to demystify the latest experiment as it confused the hell out of many quantum enthusiasts.

 

[StevieTNZ]

I like the graph on page 111 of 'The Quantum Challenge' (2nd edition).

http://books.google.co.nz/books?id=...uantum Challenge&pg=PA110#v=onepage&q&f=false - starts on page 110.

with additional information from the article in question in this thread:

However, it is possible to “weakly” measure a system, gaining some information about
one property without appreciably disturbing the future evolution (7); although the information obtained from any individual measurement is limited, averaging over many trials determines an accurate mean value for the observable of interest, even for subensembles defined by some subsequent selection (perhaps even on a complementary)."

Some information = still a bit of interference, as the book link in the first quote says.

 

[Demystifier]

Good question. I haven't seen anyone else derive this result with any deterministic model other than Bohmian Mechanics. Someone claimed they could derive the same results with orthodox QM, but I haven't seen the math. I'm not sure if that's possible either really, since orthodox QM doesn't contain the equation for the particle positions interpreted as the pilot wave in Bohmian Mechanics. Perhaps someone else here knows the answer?

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

 

[Demystifier]

Trajectories of ensemble is detected.

No, it is not. The experiment does not measure any trajectories. Instead, it measures the vector velocity field v(x). In other words, it measures the average velocity for each possible position. From this velocity field, the trajectories are then CALCULATED as integral curves of the vector field.

 

[Demystifier]

Will someone please correct me if I'm wrong, but I don't believe orthodox QM could have predicted Steinbergs results? I believe that orthodox QM would have predicted a random probability distribution of photons according to schrodinger's wave equation, yielding NO ensemble trajectories. Have I got that wrong somehow?

You are wrong. See my two posts above.

 

[Fyzix]

So this experiment has basically yielded no new information that will affect the debate in either direction?

 

[Demystifier]

As far as I can tell, the resemblance is coincidental, but I am far from an expert on BM. Perhaps Demystifier could comment further on this?

No, it is not coincidence. It is a theorem in standard QM that weakly measured trajectories will always coincide with Bohmian trajectories. See
https://www.physicsforums.com/showthread.php?t=252491

 

[Demystifier]

So this experiment has basically yielded no new information that will affect the debate in either direction?

As we see, it affects debates a lot. But only for those who do not understand what they are talking about. Unfortunately, most of those who debate on it belong to that group.

 

[Demystifier]

So go on then, reproduce the average trajectories they found, by instead using standard QM or EM calculations.

I'm feeling that nobody has bothered because nobody thinks it's been worthwhile.

You are talking nonsense. The average trajectories have indeed been calculated with standard QM. See
https://www.physicsforums.com/showthread.php?t=252491

 

[Fyzix]

As we see, it affects debates a lot. But only for those who do not understand what they are talking about. Unfortunately, most of those who debate on it belong to that group.

So what was the real purpose on this experiment?
If nothing new is learned from it?


How exactly does MWI predict these trajectories?

 

[DevilsAvocado]

Demystifier, I haven’t had the time to read all previous post, and maybe you’ve already provided this link, but in case not, the explanation on your blog is excellent:

Bohmian trajectories are no longer "hidden variables"
https://www.physicsforums.com/blog.php?b=3077

 

[Demystifier]

So what was the real purpose on this experiment?

To publish a paper in a respectable journal, and to get attention of people who think that QM is cool, but do not really understand it.

How exactly does MWI predict these trajectories?

In exactly the same way as Copenhagen or ensemble interpretation.

 

[Fyzix]

To publish a paper in a respectable journal, and to get attention of people who think that QM is cool, but do not really understand it.


In exactly the same way as Copenhagen or ensemble interpretation.

I really really hate science journalism, hyping everything up when this was infact not important AT ALL.
Like Deutsch said about this experiment: a cool trick, but a waste of time, the math already told us this.

 

[Demystifier]

Demystifier, I haven’t had the time to read all previous post, and maybe you’ve already provided this link, but in case not, the explanation on your blog is excellent:

Bohmian trajectories are no longer "hidden variables"
https://www.physicsforums.com/blog.php?b=3077

Thanks DevilsAvocado! I hope the others will read it too.

 

[Varon]

Demystifer. Is it true that classical picture of waves going through two slits could generate the same figure? See message #135, #137 here from Ken that argued how classical waves could generate the same figure. Do you agree with it 100% or 80% (if so, which 20% don't you agree?). Thanks.

 

[Demystifier]

I really really hate science journalism, hyping everything up when this was infact not important AT ALL.
Like Deutsch said about this experiment: a cool trick, but a waste of time, the math already told us this.

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.

 

[DevilsAvocado]

Thanks DevilsAvocado! I hope the others will read it too.

You’re welcome!

Please excuse an ignorant layman, but I have to ask you about Bohmian trajectories. This is the result from Kocsis measurement:

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

And this is a calculation of Bohmian trajectories:

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

As we all see, there are some similarities, but they are not identical.

Does this mean anything at all for dBB? Or was this 'expected'...?

 

[Demystifier]

Demystifer. Is it true that classical picture of waves going through two slits could generate the same figure?

No, classical waves could never do that. You need quantum waves, which differ from quantum ones by having a probabilistic interpretation.

 

[Demystifier]

You’re welcome!

Please excuse an ignorant layman, but I have to ask you about Bohmian trajectories. This is the result from Kocsis measurement:

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

And this is a calculation of Bohmian trajectories:

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

As we all see, there are some similarities, but they are not identical.

Does this mean anything at all for dBB? Or was this 'expected'...?

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.

If the measured trajectory did not coincide with the Bohmian trajectory, it would imply that STANDARD QM is wrong.

 

[DevilsAvocado]

... 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.

This is obvious for an avocado!

P.S: Please... don’t eat me...

 

[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.

Is there any graph available online for calculated photon Bohmian trajectories?

(just curious)

 

[Varon]

No, classical waves could never do that. You need quantum waves, which differ from quantum ones by having a probabilistic interpretation.

Gee. I'm confused now of what Ken was talking about. The following is the complete context of what he was describing. Do you know what he was talking about when he talked about "detection densities", "divergence-free field", etc. which he claimed was related to the experiment?

(the rest is Ken comment, what part is wrong?)

Ken G: "It doesn't mean that... Nothing in that experiment is the trajectory of an individual photon, instead, what they have seems to me is equivalent to what you'd get if you put the detecting screen at various different places and create a field of detection densities, attribute the detection densities to trajectory densities such as could be done with any divergence-free field, and draw the "field lines" and call them average trajectories. I'll wager doing that would generate precisely the same figure. Much ado about nothing.

What they seem to be missing is that the classical picture of waves going through two slits could generate the same figure. What makes the quantum realm so weird is the quantization-- not the averaged behavior. I really don't see what "weak measurement" is adding to the question, it still is not true that you can say which slit any of those electrons went through."


"What I'm saying is, I'm not convinced that "weak measurement" is any different from "compiling average trajectories from treating the wave energy flux like a divergenceless scalar field and drawing 2D lines of force for that field." I maintain you could get that exact same picture by measuring the energy flux of a classical wave passing between two slits, and drawing trajectories such that the line density is proportional to the energy flux density. This would be completely consistent with a macroscopic treatment of an energy flux as a photon number flux. Those trajectories don't really mean anything beyond a statistical treatment of where photons go in large aggregations, that they could get the same picture with "weak measurement" of "one photon at a time" doesn't strike me as being at all profound.

Let me put it another way. The key statement that we don't know the trajectory of an individual photon is that we cannot know which slit it went through, and still have that photon participate in an interference pattern. Does this experiment tell us which slit any of those photons went through? No. So what? There are still no trajectories in the physical reality of what happened to those photons, and it's not at all clear that an "average trajectory" is anything different from the usual macro aggregate measurement in the classical limit. To me, all this experiment is is a kind of consistency check that "weak measurement" can recover statistical aggregates, but I see no threat to the CI interpretation that the reality is still only what you measure and not what happens between the measurements. So they can create weak measurements that don't completely collapse the wave function, then recover the aggregate behavior in the same way that complete measurements that do collapse the wavefunction could easily do also. What does that tell us? That weak measurements don't mess up aggregate results? Why should we be surprised-- the weak measurements don't tell us the trajectories of any of those particles."

 

[f95toli]

So what was the real purpose on this experiment?
If nothing new is learned from it?How exactly does MWI predict these trajectories?

As I wrote in my earlier post: one nice thing about this experiment is that it tells us something what is possible using weak measurements. Weak measurements are a "hot" topic at the moment because they are potentially very useful in certain applications, such as reading out quantum computers, quantum metrology etc.
Hence, the fact that experiment does not tell us anything "fundamental" does not mean that it was not worth doing.

 

[Demystifier]

(the rest is Ken comment, what part is wrong?)

I don't want to comment statements of someone who is not here to clarify what he meant.

 

[Demystifier]

Weak measurements are a "hot" topic at the moment because they are potentially very useful in certain applications, such as reading out quantum computers, quantum metrology etc.

Can you give some references or links on these applications?

 

[Varon]

I don't want to comment statements of someone who is not here to clarify what he meant.

Hope he'll get here. He is an extreme Copenhagenist. I was discussing with him QM in the Philosophy thread as he is already bored with this forum. I also mentioned how calcite with angles and photons and momentum is different from classical waves. The following is his reply yesterday (maybe it clarifies what he means such that if he didn't get here.. you know how to comment on it as it is very relevant to this thread).

Ken G wrote:

"I'm saying the details of how they generate that figure doesn't matter, what matters is its information content, which I can get in much easier ways. 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", then the question to ask is: what additional information are they extracting with their clever measurements if they end up with the exact same figure I get?

Note that it makes no difference how clever their measurements are-- if they can tell which slit the photon went through, they won't get that photon to participate in an interference pattern anywhere. That is all the CI needs to hold."