What are the implications of this experiment?

[StevieTNZ]

Quantum physics first: Researchers observe single photons in two-slit interferometer

http://www.physorg.com/news/2011-06-quantum-physics-photons-two-slit-interferometer.html

http://www.sciencedaily.com/releases/2011/06/110602143159.htm

http://www.sciencemag.org/content/332/6034/1170.abstract

 

[StevieTNZ]

An article about the work: http://www.aip.org.au/Congress2010/Abstracts/Monday%206%20Dec%20-%20Orals/Session_3E/Kocsis_Observing_the_Trajectories.pdf

 

[DAlden]

Please see the following article on BBC News: http://www.bbc.co.uk/news/science-environment-13626587

Researchers have apparently side-stepped one of the conclusions of the double-slit experiment, namely that we cannot know which slit a photon went through if we want to produce the diffraction pattern. By performing weak measurements they are able to observe both the particle nature and wave nature of light at the same time.

My question is does this effectively disprove the "many-worlds" interpretation of quantum mechanics, in which the photon was said to go through both slits in alternate quantum realities? It seems they are averaging over large numbers of photons, so I'm not sure they can say what any individual particle does, but it does seem to point to the notion that each particle does take a definite path. What do you think?

 

[f95toli]

It doesn't really change anything. The results agree with ''standard'' QM and is independent of interpretation.
Note that they are performing a weak measurements, meaning their results do not tell you anything about the trajectory of an individual photon

From the paper (latest issue of Science)

For the experimentally reconstructed trajectories for our double slit (Fig. 3), it is worth stressing that photons are not constrained to follow these precise trajectories; the exact trajectory of an individual quantum particle is not a well-defined concept.

This experiment is actually more about the nature of weak measurements (which is still somewhat controversial), than about the nature of photons or even the double-slit experiment.
It is interesting work, but not in any way revolutionary.

 

[Demystifier]

I agree with f95toli. See also my arguments against weak measurements on my blog
https://www.physicsforums.com/blog.php?b=1225
https://www.physicsforums.com/blog.php?b=1226

 

[ibysaiyan]

Wave-Particle duality observed

Hi all
I just came across this article in which from what I have read is that the wave-particle nature has been observed though the whole technicality confused me a little.
http://www.bbc.co.uk/news/science-environment-13626587"
On the whole it seems they took the average photons into consideration ,can someone with more knowledge enlighten me ?
Regards,
ibysaiyan

 

[phinds]

Hi all
I just came across this article in which from what I have read is that the wave-particle nature has been observed though the whole technicality confused me a little.
http://www.bbc.co.uk/news/science-environment-13626587"
On the whole it seems they took the average photons into consideration ,can someone with more knowledge enlighten me ?
Regards,
ibysaiyan

THere's already a thread on this experiment. Don't remember subject title but it's in QM so should be easy to find.

 

[OmCheeto]

Note that they are performing a weak measurements, meaning their results do not tell you anything about the trajectory of an individual photon

hmm... My sister had me all atwitter today when she sent me the article.

Though it looks like there's nothing to see.

Odd that their abstract seems to imply something contrary to what you are saying. I guess I'll have to read the full article.

We sent single photons emitted by a quantum dot through a double-slit interferometer and reconstructed these trajectories by performing a weak measurement of the photon momentum, postselected according to the result of a strong measurement of photon position in a series of planes.

And go back to school to find out what a "weak measurement" is.

 

[Truecrimson]

An explanation: http://scienceblogs.com/principles/2011/06/watching_photons_interfere_obs.php

 

[IllyaKuryakin]

Really good stuff. If this holds up, and I expect it will, it answers many more questions than some might realize at first glance. Really foundational work. Congrats to Aephraim Steinberg and his team at the University of Toronto!

 

[yoda jedi]

good news for Two-State Vector model and Aharonov.

 

[StevieTNZ]

It doesn't really change anything. The results agree with ''standard'' QM and is independent of interpretation.
Note that they are performing a weak measurements, meaning their results do not tell you anything about the trajectory of an individual photon

Yeah, I was thinking that when I read the article (wording "on average"). Plus something to do with momentum having an approximate value.

 

[Demystifier]

An explanation: http://scienceblogs.com/principles/2011/06/watching_photons_interfere_obs.php

As said there, these MEASURED trajectories look exactly like Bohm trajectories. Which, of course, is not surprising:
https://www.physicsforums.com/showthread.php?t=252491

 

[Phrak]

This line of reasoning will eventually bust quantum mechanics wide open---finally! A more primitive theory will emerge of continuous field. It won't be MWT nor Bohmian mechanics nor any of the host of suppositions commonly bantered about these days, including Aharanov's latest musings--though he is close. Temporal symmetry is at the heart of it. All of this is just my less-than-humble opinion, of course.

 

[AntonL]

trajectories mapped of single photons in double slit experiment

a fascinating article with lively reader contribution

http://physicsworld.com/cws/article/news/46193

An international team of researchers has, for the first time, mapped complete trajectories of single photons in Young’s famous double slit experiment. The finding takes an important first step towards measuring complimentary variables of a quantum system – which until now has been considered impossible as a consequence of the Heisenberg uncertainty principle

PS: I just noticed this is a double posting of the thread "What are the implications of this experiment?"

 

[SpectraCat]

a fascinating article with lively reader contribution

http://physicsworld.com/cws/article/news/46193




PS: I just noticed this is a double posting of the thread "What are the implications of this experiment?"

That quote is somewhat misleading .. in fact, I am pretty sure that in that article, the authors themselves say that mapping trajectories of individual photons is impossible. The paper describes the results of *weak* measurements, which still provide information about the average trajectories over large numbers of measurements. Another way of saying this is that the authors don't get "which path" information for the photons, so the interference pattern is still preserved.

So even though the particles do travel through the interferometer one at a time, saying that "...mapped complete trajectories of single photons", makes it sound like they can say, "Ok, photon 1 went this way, photon 2 went that way, ..." etc., and that is not correct.

 

[Demystifier]

Spectracat is right.

For me, the main implication is the following:
Bohmian particle trajectories are much more than a part of a controversial interpretation of QM. Bohmian particle trajectories are a part of QM itself, irrespective of the interpretation. However, what different interpretations disagree on, is what these trajectories really "are".

In this sense, trajectories play a role in QM similar to the role of the wave function. All interpretations involve the wave function, but different interpretations disagree on what this wave function really "is".

 

[Demystifier]

Or let me further refine the comment above. I believe that the main implication of this experiment is that Bohmian trajectories are no longer "hidden variables". Or more precisely, not more hidden than the wave function. For a more elaborated argument see my blog
https://www.physicsforums.com/blog.php?b=3077

 

[Lost in Space]

So does it mean Heisenberg got it wrong, or what?

 

[Demystifier]

So does it mean Heisenberg got it wrong, or what?

Heisenberg is still right in the sense that you cannot STRONGLY measure both position and momentum. But you can do it WEAKLY.

 

[IllyaKuryakin]

Heisenberg is still right in the sense that you cannot STRONGLY measure both position and momentum. But you can do it WEAKLY.

Yes, the HUP principle still applies and always will. As I understand it, Heisenberg was one of the most vocal of opponets to deBB Theory, due to his understanding that particle position can never be completely determined. Essentially, I believe Bohmian Mechanics uses the exact position of the particle as a NON-LOCAL Hidden variable, and of course, the HUP prevents that from ever being completely known, so one can only come close, and the Non-Local Hidden Variable of exact position will always remain hidden to the extent HUP demands.

The nice thing is, de Broglie-Bohm theory resolves Schrodinger's problem, where he complained his wave formula predicts a wave spread out over the detecting screen and he sees particles impacting the screen in little specks. John Bell was very clear that Bohmian Mechanics, or deBB if you want to give proper recognition to the genius of Luis de Broglie, turns the indeterminate statistical QM into a determinate process, eliminating the mysticism of the observer's special position in the Statistical QM and thereby doing away with the Copenhagen interpretation. This does not mean that statistical QM is invalid. Both theories use Schrodinger's Wave Equation and if deBB theory is valid then statistical QM must also be valid. In a sense, it just means that the moon is there, even if we aren't looking at it, and it took a specific trajectory to get where it is.

Of course, we can't measure the exact positions and momentum of individual photons to experimentally verify their trajectories are just as deBB theory predicts due to the HUP restrictions, so there is a limit to how much we can know about the trajectory of anyone particle, which is very little actually. But, by making measurements on 31,000 photons, and gaining just a little information off each, we can see the average trajectory is precisely what deBB theory predicts.

It's all explained here: http://plato.stanford.edu/entries/qm-bohm/

If you look at the deBB predicted photon trajectories in figure 1. of the above referenced article, and look at the average trajectories measured by this experiment, you get an exact match. In hindsight, it seems all so obvious. The only mystery is why it took so long for this approach to be accepted. Especially when someone with the stature of John Bell was practically shouting from the rooftops that we should look at deBB theory more closely:

(Bell 1987, p. 160):

"But in 1952 I saw the impossible done. It was in papers by David Bohm. Bohm showed explicitly how parameters could indeed be introduced, into nonrelativistic wave mechanics, with the help of which the indeterministic description could be transformed into a deterministic one. More importantly, in my opinion, the subjectivity of the orthodox version, the necessary reference to the ‘observer,’ could be eliminated. ... "

Perhaps the first step in good science is to dispel mysticism. deBB theory dispels the mysticism of the observer's special position in the Copenhagen interpretation. The Copernican Principle holds. Ding dong, the witch is dead.

As usual, the above is my own humble opinion. Feel free to correct me if I have made a mistake.

 

[StevieTNZ]

I've just read what Matthew Francis has written on Scientific American - http://www.scientificamerican.com/blog/post.cfm?id=what-does-the-new-double-slit-exper-2011-06-07 - which says no valid interpretation of QM has been falsified.

Does this mean that the particles in the first posts experiment could not have gone through just one slit, in accord with the Copenhagen interpretation? Is this related to the fact that we only obtained statistical figures in position for the esemble of particles, which do not necessarily imply one particle path was followed?

 

[Varon]

Why do you guys keep mentioning about particles. Isn't it already clear in quantum field theory that field is primary and particles are just momentum and energy of the field? According to the QFT interpretation. It is wrong to think of particles going either the left or right slit. There is no particle. There is only the field. And the field passes both slits and in the detector, the wave are smeared all over, there is no collapse. One of the existing electron out of the millions that is used in the detector is simply triggered making us wrongly conclude it represents the original particle which never even exist (because there is no particle). Doesn't this QFT (Neumaier) Interpretation make sense? What's the flaw?

 

[Fyzix]

Why do you guys keep mentioning about particles. Isn't it already clear in quantum field theory that field is primary and particles are just momentum and energy of the field? According to the QFT interpretation. It is wrong to think of particles going either the left or right slit. There is no particle. There is only the field. And the field passes both slits and in the detector, the wave are smeared all over, there is no collapse. One of the existing electron out of the millions that is used in the detector is simply triggered making us wrongly conclude it represents the original particle which never even exist (because there is no particle). Doesn't this QFT (Neumaier) Interpretation make sense? What's the flaw?

Varon, you need to stop this.
Every other day you ask 50 questions, then you pretend to understand the things and then you start critizing people for not understanding what you pretend to understand, but don't understand at all!
It's like 5th grade...

Just a few weeks ago you were talking about talking about killing people in other univereses and wanting MERGE completely with this model.
What happened to this?

 

[Varon]

Varon, you need to stop this.
Every other day you ask 50 questions, then you pretend to understand the things and then you start critizing people for not understanding what you pretend to understand, but don't understand at all!
It's like 5th grade...

Just a few weeks ago you were talking about talking about killing people in other univereses and wanting MERGE completely with this model.
What happened to this?

killing people in other universe? You may have mistaken me for someone... although there is this movie by Jet Li called the One that just do it.

I'm just asking above whether with the new experiment one can see a flaw in the QFT Interpretation emphasized by Neumaier. Here, there are no particles but only fields. Unless you or others are saying that these particles being momentum or energy of the field is just another interpretation that may not be right? If it is right, then there is no particles.. so why do we keep speaking about particles?? Copenhagen Interpretation occurred at a time when particles being momentum of the field is not yet popular.

 

[StevieTNZ]

killing people in other universe? You may have mistaken me for someone... although there is this movie by Jet Li called the One that just do it.

No, Varon. I clearly remember you posting such comments. You also commented that you practice voodoo, which seems to confirm my original suspicion you're trying to marry physics with spirituality. Your talk of faster-than-light communication doesn't help, either.

 

[Varon]

No, Varon. I clearly remember you posting such comments. You also commented that you practice voodoo, which seems to confirm my original suspicion you're trying to marry physics with spirituality. Your talk of faster-than-light communication doesn't help, either.

Let's not discuss about it but directly the issues at hand. I can't dismiss Neumaier easily. He knows mathematics as good as von Neumann. And QFT says particles are just momentum of the field. Therefore if Neumaier was right. Then it refutes Bohmian Mechanics, Many worlds in one blow. It's almost a century already of endless debate. I think it is possible to derive at the right interpretation. All may conform to QM in predicting experimental results but there may be subtle differences that can make us distinguish what is the correct one, and this latest experiment is a good one.

Can it be done with electrions? If so, and it shows the same results, then it can refute the QFT interpretation.

 

[vanhees71]

Let me put some doubts on all this "interpretation" business. What is the point of interpretation? Isn't it a waste of time to discuss on these purely philosophical "ontology" questions?

Personally, I stick to the only non-nonsense interpretation of quantum theory I know of, and that's the minimal statistical interpretation, which goes back to Born's probabilistic interpretation of the state vectors (more precisely the pure states as rays in Hilbert space). As far as I can see all other interpretations make additional assumptions, which are either not empirically verifiable (like Bohm's trajectories or the infinitely many parallel universes within the MWI interpretation that are created only because I watch an electron hitting a detector ;-)). Claims about empirically unverifiable features of models are just empty words without any scientific significance since science, by definition, deals with objective, reproducible (and in the ideal case quantifiable) observations of Nature.

Particles within QFT have a clear meaning as asymptotically free (or if long-range forces like the Coulomb force are present distorted-wave) one-particle Fock states. Observable are (statistical!) quantities like the lifetime of an unstable particle or cross sections for scattering processes, etc.

 

[Demystifier]

Why do you guys keep mentioning about particles. Isn't it already clear in quantum field theory that field is primary and particles are just momentum and energy of the field? According to the QFT interpretation. It is wrong to think of particles going either the left or right slit. There is no particle. There is only the field. And the field passes both slits and in the detector, the wave are smeared all over, there is no collapse. One of the existing electron out of the millions that is used in the detector is simply triggered making us wrongly conclude it represents the original particle which never even exist (because there is no particle). Doesn't this QFT (Neumaier) Interpretation make sense? What's the flaw?

I'm not sure about the Neumaier interpretation, but your explanation of the Neumaier interpretation certainly does not make sense at all. For example, if there is only field that goes through both slits and nothing else, and if there is no collapse, then why on the screen we see localized objects?

 

[Demystifier]

Can it be done with electrions? If so, and it shows the same results, then it can refute the QFT interpretation.

Weak measurements of particle trajectories can be done with electrons too. It is a matter of time when somebody will do it.

 

[Demystifier]

Let me put some doubts on all this "interpretation" business. What is the point of interpretation? Isn't it a waste of time to discuss on these purely philosophical "ontology" questions?

Thinking about interpretations may inspire research resulting in more concrete results. For example, thinking about the Bohmian interpretation inspired Bell to find Bell inequalities. Similarly, thinking about many worlds inspired Deutsch to make concrete results on quantum computers. Or thinking about the meaning of wave-particle duality inspired delayed-choice experiments. Even Feynman path integrals was inspired by Feynman's philosophical thoughts on quantum ontology. Etc, etc ...

The minimal statistical interpretation you prefer is too sterile to inspire such innovative ways of thinking about QM. This sterile interpretation is sufficient for those who only want to make routine calculations in conceptually already understood domains of quantum physics, but not for those who attempt to be more creative.

 

[Varon]

Thinking about interpretations may inspire research resulting in more concrete results. For example, thinking about the Bohmian interpretation inspired Bell to find Bell inequalities. Similarly, thinking about many worlds inspired Deutsch to make concrete results on quantum computers. Or thinking about the meaning of wave-particle duality inspired delayed-choice experiments. Even Feynman path integrals was inspired by Feynman's philosophical thoughts on quantum ontology. Etc, etc ...

The minimal statistical interpretation you prefer is too sterile to inspire such innovative ways of thinking about QM. This sterile interpretation is sufficient for those who only want to make routine calculations in conceptually already understood domains of quantum physics, but not for those who attempt to be more creative.

Also note that the correct interpretation would produce an emergence that won't be found in the incorrect interpretations. It's like the correct understanding of atomic theory has led to superconductivity. Without understanding electron wave functions. We won't know how cooper pairs behave. Superconductivity is an emergence. This is also true for the correct interpretation which would produce a breakthrough as stunning as the birth of the quantum in year 1900. In fact I think that it is within a decade that we would see its arrival. That is why we have to accelerate quantum understanding and also cooperation with other groups that can give clues to what form it may take.

 

[Phrak]

Where is the original research article, rather than just internet sites jaw-boning about it third hand--like this one?

 

[Demystifier]

Where is the original research article, rather than just internet sites jaw-boning about it third hand--like this one?

http://www.sciencemag.org/content/332/6034/1170.full

 

[Demystifier]

The minimal statistical interpretation you prefer is too sterile to inspire such innovative ways of thinking about QM. This sterile interpretation is sufficient for those who only want to make routine calculations in conceptually already understood domains of quantum physics, but not for those who attempt to be more creative.

Vanhees, in the meanwile I have seen that you are doing particle physics phenomenology, and that you are doing it very well. I was not surprised at all, because phenomenology is exactly the kind of physics where minimal statistical interpretation is sufficient. But contrary to what most phenomenologists think, physics is not only phenomenology. For example, Feynman would never discover path integrals (now very useful in particle phenomenology) if he didn't think about ontological questions as well.