Who is puzzled by the delayed choice?

[StandardsGuy]

Your notes must have left something out, because you cannot affect the outcome of an experiment by doing something to Pockels cells after the photon has passed. There are important details like what is the bandwidth of the laser, because by the HUP the inverse of the laser bandwidth tells you the precision in the time that you can say when the photon passed various places. If we shine photons one at a time, each photon will have a time-dependent wave function, and it will not be affected by changes in an apparatus that does not overlap with that wave function. If it did, it wouldn't be quantum mechanics, and we would all have heard about it.

Oh right, blame it on me. You can't accept the facts. Here is another book that eludes to it: http://books.google.com/books?id=VF...lit&source=bl&ots=rbrAHsmWqr&sig=ufBV3aOEv5yR


· "Those who are not shocked when they first come across quantum theory cannot possibly have understood it." Niels Bohr.
· "If you are not completely confused by quantum mechanics, you do not understand it." John Wheeler.
· "It is safe to say that nobody understands quantum mechanics." Richard Feynman.
· "If [quantum theory] is correct, it signifies the end of physics as a science." Albert Einstein.
· "I do not like [quantum mechanics], and I am sorry I ever had anything to do with it." Erwin Schrödinger.

 

[zonde]

You can't measure the photon itself, but the outcome of the experiment is changed. No FTL communication is possible IMO. From my Notes: This phenomenon is known as non-locality. Einstein called it ‘spooky action at a distance.’ The Austrian Erwin Schrodinger thought that all this was ridiculous, and came up with his story of the cat in the box, which appeared in print in 1935. Albert Einstein, Boris Podolsky, and Nathan Rosen also came up with a ‘thought experiment’ about the same time. It is known as the ‘EPR paradox.’ In the mid 60’s, John Bell, an Irish physicist, found a way to express the paradox in terms of an experiment that could be carried out on pairs of photons emitted from an atom simultaneously in two different directions. Several researchers took up the challenge. The most conclusive of these experiments was carried out by Alain Aspect & colleagues in the early 1980’s. They demonstrated beyond any reasonable doubt that non-locality really does rule in the quantum world, and that Einstein and the others were wrong.

First, I do not agree with you. Second, this is not relevant to question.

OK, this isn't the experiment I thought. From my notes: In the mid 1980s, a group from the University of Maryland, and a separate group from the University of Munich carried out a version of the two holes experiment. They used a beam of laser light that went through a beam-splitting mirror. One of the split beams was phase-shifted and then the two split beams were recombined to form the interference pattern. Detectors known as Pockels cells were placed in each of the split beams to monitor the passage of photons, and detectors at the far end looked to see if it was producing interference or not. The Pockels cells could be switched on or off within 9 billionths of a second. The length of each path of light took about 15 billionths of a second for light to travel that far, so the detectors could be switched on (or off) after the light had passed the detectors. The decision of whether the cells were on or off was made by a computer at random (with no human intervention). Both groups found that the light behaved as particles or waves depending on which choice was going to be made, even though the decision had not been made when the light passed the detectors! See John Gribbin’s, book Schrodinger’s Kittens and the Search for Reality.

Would not the Pockels cells be an apparatus in the path of the photocells? If I am off base here, please explain how.

Give reference to that experiment only then I can compare your interpretation with description given by experimenters.

 

[zonde]

OK, this isn't the experiment I thought. From my notes: In the mid 1980s, a group from the University of Maryland, and a separate group from the University of Munich carried out a version of the two holes experiment. They used a beam of laser light that went through a beam-splitting mirror. One of the split beams was phase-shifted and then the two split beams were recombined to form the interference pattern. Detectors known as Pockels cells were placed in each of the split beams to monitor the passage of photons, and detectors at the far end looked to see if it was producing interference or not. The Pockels cells could be switched on or off within 9 billionths of a second. The length of each path of light took about 15 billionths of a second for light to travel that far, so the detectors could be switched on (or off) after the light had passed the detectors. The decision of whether the cells were on or off was made by a computer at random (with no human intervention). Both groups found that the light behaved as particles or waves depending on which choice was going to be made, even though the decision had not been made when the light passed the detectors! See John Gribbin’s, book Schrodinger’s Kittens and the Search for Reality.

Would not the Pockels cells be an apparatus in the path of the photocells? If I am off base here, please explain how.

Ok, there are some things that does not seem right even without any reference.

From wikipedia Pockels effect
"Pockel cells are voltage-controlled wave plates."

So this phrase of yours looks wrong "Detectors known as Pockels cells were placed in each of the split beams to monitor the passage of photons".

This is unclear - "so the detectors could be switched on (or off) after the light had passed the detectors." With detectors you mean Pockels cells, right? But then they are not detectors.

This is wrong (given description from book you liked in post #61) - "even though the decision had not been made when the light passed the detectors!" The right phrase is "even though the decision had not been made when the light passed the slits!"

 

[harrylin]

[..]
· "Those who are not shocked when they first come across quantum theory cannot possibly have understood it." Niels Bohr.
· "If you are not completely confused by quantum mechanics, you do not understand it." John Wheeler.
· "It is safe to say that nobody understands quantum mechanics." Richard Feynman.
· "If [quantum theory] is correct, it signifies the end of physics as a science." Albert Einstein.
· "I do not like [quantum mechanics], and I am sorry I ever had anything to do with it." Erwin Schrödinger.

Sure, serious physicists should be puzzled by QM (I never saw those citations of Einstein and Schrodinger, thanks!). I even joined physicsforums because I would like to understand what may be behind the observations. However, I was not puzzled for a long time by "delayed choice" and I bet that also Einstein did not refer to delayed choice but to EPR/Bell.

Note that that book you refer to makes a common interpretation mistake: not its behaviour changed but the observation of its behaviour changed by changing the observation conditions. Similar: if you put on sunglasses you could conclude that the sky becomes dark, instantly. How did you do that to the sky?

 

[Ken G]

Oh right, blame it on me. You can't accept the facts. Here is another book that eludes to it: http://books.google.com/books?id=VF...lit&source=bl&ots=rbrAHsmWqr&sig=ufBV3aOEv5yR

Well, the blame is indeed with you (or more likely, with poorly worded popularized articles), for you have not understood that experiment. The pockels cells in that experiment are not inserted after the photon has passed them, they are inserted after the photon has passed the slits. Big difference!

Indeed, if you look at my post #34 above, you will see that I described a completely analogous situation with polarizers. Here are the key points:
1) whether or not interference occurred (colloquially described as whether or not it was wavelike or particlelike) is a matter of interpretation of the outcome, not the outcome itself.
2) what kind of interference you end up seeing depends on the full apparatus you choose to employ, not any individual piece of the apparatus, and you can change the apparatus in midstream as long as the photon hasn't arrived at the part you are changing.
3) nothing you do to a part of the apparatus that you know the photon has already passed matters a whit to the outcome.

Those are the facts, and that is what quantum mechanics says, which is also what harrylin and zonde are telling you. I think a large share of the blame for the misconceptions goes to popularized articles that are so vested in the "quantum mechanics is weird" position that they aren't happy with the actual weirdness in quantum mechanics and want to insert a bunch of incorrect weirdness as well.

 

[Dadface]

I am not sure whether that really helps, but to really understand DCQE it is necessary to understand the physics of spontaneous parametric down conversion. Some of the authors of one of the DCQE experiments have written a detailed review article on the physics and correlations in SPDC which was published in Physics Reports. The ArXiv version can be found here:http://arxiv.org/abs/1010.1236.

This is a detailed paper, but goes more into the details of SPDC and spatial correlation patterns and therefore only mentions DCQE partially. If you have some spare time this is a good introduction to get a feeling for what is important for understanding SPDC and DCQE experiments. If you, however, are more interested in a short review especially about DCQE just disregard this link.

Hello Cthugha.The paper is far too heavy duty for me at the moment and I think that a short review would be an easier introduction for me.Thank you very much for your post.

 

[harrylin]

The following may be helpful:
http://departments.colgate.edu/physics/research/Photon/Photon research/Quantumlan07/lab3eraser09.pdf
(a little disappointment: I have an earlier version of 2005 which contains more explanations but is not available anymore from their website. As a result, it has become less helpful).

Note that in view of the above discussion, in the last paragraph it would perhaps be clearer to put "we regain visible interference".

In addition, here attached is the older version.
Regards,
Harald

 

[StandardsGuy]

Ok, there are some things that does not seem right even without any reference.

From wikipedia Pockels effect
"Pockel cells are voltage-controlled wave plates."

So this phrase of yours looks wrong "Detectors known as Pockels cells were placed in each of the split beams to monitor the passage of photons".

This is unclear - "so the detectors could be switched on (or off) after the light had passed the detectors." With detectors you mean Pockels cells, right? But then they are not detectors.

This is wrong (given description from book you liked in post #61) - "even though the decision had not been made when the light passed the detectors!" The right phrase is "even though the decision had not been made when the light passed the slits!"

You make some good points. These are not my words, but Gribben's. I assume he is referring to the Pockels cells when he says detectors (confusion). I wish I had a link to the experiment to get the original comments.

 

[Demystifier]

One answer to the question given in the title is "me". ... Any recommendations please?

Yes. Try first to understand at least one of the interpretations in post #1.

 

[Dadface]

Yes. Try first to understand at least one of the interpretations in post #1.

Thank you,but for other things I understand,such as Schrodingers cat,I have my own interpretation.It doesn't fit in any of the categories listed in post one but is closest to "shut up and calculate".
I want to look at this for the fun of it but my main problem has been in finding an article which is at my level of understanding and which describes the experiment in enough detail.

 

[Dadface]

In addition, here attached is the older version.
Regards,
Harald

That's very kind of you to post the older version.It does go further but still leaves me with a load of questions.I do not wish to bother people here with my questions until and if I get a better understanding of the experiment.Thank you.

 

[audioloop]

seem to change the past.

What about this one, Demystifier ?

Can a Future Choice Affect a Past Measurement's Outcome?
http://arxiv.org/ftp/arxiv/papers/1206/1206.6224.pdf


not delayed choice of course

 

[Demystifier]

What about this one, Demystifier ?

Can a Future Choice Affect a Past Measurement's Outcome?
http://arxiv.org/ftp/arxiv/papers/1206/1206.6224.pdf


not delayed choice of course

Let me quote from the paper:
"Consequently, nonlocal effects between the two particles have been commonly accepted as the only remaining explanation. It is possible, however, to explain the results without appeal to nonlocality, by allowing hidden variables to operate within the Two-State Vector Formalism (TSVF). The hidden variable would then be the future state-vector affecting weak measurements at present."

In other words, they propose an alternative interpretation of certain EPR-like correlations, as being caused not by nonlocality but by backward causation. They do not claim that their alternative interpretation is the only possible one, but only that it is a possible one. Indeed, this type of alternatives to quantum nonlocality already exists on my list:
https://www.physicsforums.com/blog.php?b=3622

 

[Ken G]

In other words, they propose an alternative interpretation of certain EPR-like correlations, as being caused not by nonlocality but by backward causation. They do not claim that their alternative interpretation is the only possible one, but only that it is a possible one. Indeed, this type of alternatives to quantum nonlocality already exists on my list:
https://www.physicsforums.com/blog.php?b=3622

I was interested to see that you classify your solipsistic hidden variables approach as local but only describing the observers, because that's very much the intepretation I take-- that physics is the study of how physicists interact with, and learn about, nature. That's probably why we agree so much on the way to strip delayed choice of its mystical qualities. But I thought you were a Bohmian, who would therefore take a very realist perspective on nature outside of the physicist, so I'm surprised to hear you as applying local thinking to the observer only. Would you then say you are not really a Bohmian at all?

 

[audioloop]

ok.
listed under

.-backward causation - objective reality exists and is local, but there are signals backwards in time (transactional interpretation).ok. then but not transactional, if not two state vector formalism.
by aharonov.

 

[Demystifier]

But I thought you were a Bohmian, who would therefore take a very realist perspective on nature outside of the physicist, so I'm surprised to hear you as applying local thinking to the observer only. Would you then say you are not really a Bohmian at all?

First, I am open to various possibilities, not only to Bohmian and/or solipsistic.

Second, my solipsistic model is technically very similar to Bohmian mechanics; both are based on deterministic particle trajectories guided by the state of the system.

Third, in the paper I discuss also some unappealing features of the solipsistic model. Bohmian mechanics does not share these unappealing features. In fact, Bohmian mechanics is still my favored interpretation. But it has some unappealing features as well, and as I said, I am open to other possibilities as well.

 

[Demystifier]

I was interested to see that you classify your solipsistic hidden variables approach as local but only describing the observers, because that's very much the intepretation I take-- that physics is the study of how physicists interact with, and learn about, nature. That's probably why we agree so much on the way to strip delayed choice of its mystical qualities.

As you can see from the first post on this thread, my argument is based on the fact that NEITHER of the self-consistent interpretations I am aware of implies that delayed choice affects past. As you can see, many of those interpretations are very far from being solipsistic.

 

[jcsd]

I have to say Demystifier, I do like your approach, which seems to be let's make ontology work for us, rather than us working for ontology.

 

[Ken G]

Second, my solipsistic model is technically very similar to Bohmian mechanics; both are based on deterministic particle trajectories guided by the state of the system.

Well, I would have to say that on the surface, having an interpretation that is guided by deterministic particle trajectories certainly sounds like an external ontology-- which does not sound like an interpretation whose local elements are limited to the observers. Can you clarify that specific issue?

 

[Demystifier]

Well, I would have to say that on the surface, having an interpretation that is guided by deterministic particle trajectories certainly sounds like an external ontology-- which does not sound like an interpretation whose local elements are limited to the observers. Can you clarify that specific issue?

Have you actually read my paper? It's all explained there. In short, this particle ontology is not external but internal, describing only degrees of freedom which are ultimately responsible for emergence of consciousness. In a sense, it is assumed that consciousness is MADE of hidden point-particles.

 

[Demystifier]

I have to say Demystifier, I do like your approach, which seems to be let's make ontology work for us, rather than us working for ontology.

Thanks! Yes, that's one way to see it.

 

[Ken G]

In short, this particle ontology is not external but internal, describing only degrees of freedom which are ultimately responsible for emergence of consciousness. In a sense, it is assumed that consciousness is MADE of hidden point-particles.

OK, I can see that connection. Many might balk at attempting that extrapolation, but it makes sense to me as one potentially valid angle from which to attack the essential issue: that of connecting the physics to the physicist.

 

[bob900]

Not correct. In Bohmian interpretation wave function never really collapses. It only splits into separate branches that do not know about each other.

Not correct. There is only one particle which takes only one path, but the particle does not "materialize". Instead, it exists all the time, irrespective of the wave function. The purpose of the wave is only to guide the MOTION of the particle, not to create the particle.


People are irrelevant. What is relevant is the measuring apparatus. The wave function that guides the particle interacts with the wave function that guides the apparatus-particles. This interaction changes the particle-guiding wave function, which affects the particle trajectory AFTER the interaction.

But this interaction happens much later than when the signal particle is measured, so how can the guiding wave function resulting from this interaction possibly affect/guide the signal particle?

For reference, let's take the experimental setup depicted here. The first ("signal") photon is detected at D0, then, possibly at some much later time, its entangled partner ("idler") photon is detected one of the four detectors D1-D4.

Now going back to what you said :

The wave function that guides the particle interacts with the wave function that guides the apparatus-particles. This interaction changes the particle-guiding wave function, which affects the particle trajectory AFTER the interaction.

But if there is some guiding function that guides the signal photon to its location within D0, this guiding function must take into account (i.e. be changed as the result of) the interaction between the apparatus (D1-D4). Let's say we set up the experiment so that the we release the photon at time t0, D0 measurement happens at some time t1, and the D1-D4 interaction happens a year from t1. So if the guiding function is to guide the photon from the source (the laser) to D0, it must, at time t0, have information from an interaction that will happen a year later. Is that not the same as saying that this information somehow travels back in time, to influence the guiding function at t0, so as to guide the signal photon to the proper place at D0?

 

[Demystifier]

For reference, let's take the experimental setup depicted here. The first ("signal") photon is detected at D0, then, possibly at some much later time, its entangled partner ("idler") photon is detected one of the four detectors D1-D4.

That objection is not difficult to resolve. The signal photon is destroyed at D0 and ceases to exist. However, it doesn't mean that there is no longer any particle the idler photon could be entangled with. The destruction of signal photon does not destroy information carried by the signal photon (otherwise, QM would not be unitary). Instead, the information is transmitted from the signal photon to some of the particles of which the detector D0 is made of, so after a long time the idler photon is still entangled and correlated with those detector particles. Of course, the detector D0 is macroscopic, but it doesn't mean that it doesn't obey quantum laws. According to Bohmian mechanics, everything obeys quantum laws, even if sometimes these laws can be approximated by the classical ones.

 

[San K]

Of course, the detector D0 is macroscopic, but it doesn't mean that it doesn't obey quantum laws.

or perhaps it's entangled to only a microscopic part of the detector D0

 

[my_wan]

I'm a realist. Hence I find the various mechanisms invoked by these interpretations very interesting, including RBW. However, merely massaging my philosophical sensibilities is not sufficient grounds for acceptance, anymore than some LET interpretation of Relativity is sufficient grounds for accepting it.

I suspect experimentalist want something they can sink experimental teeth into. Much like I expect empirical extensions to the model. That doesn't mean the initial proposal must provide it, but at least provide foundation amenable to calculation. So my issue with interpretations is that they are too generally geared toward avoiding any new predictions. Still better than nothing though.

 

[Demystifier]

or perhaps it's entangled to only a microscopic part of the detector D0

Not so according to the environment-induced decoherence theory.