Brian Green's Beam Splitter Experiments

[MojaveJoe]

Hi all,

I read about the beam splitter experiments in Brian Green's most excellent book The Fabric of the Cosmos. I am obviously missing something and thought I'd ask here. Forgive me if I sound ignorant; I am well educated but lacking in advanced physics (although fairly mathematically advanced).

There were a lot of things about these experiments that were quite amazing (past depends on the future?? huh??) but there was something that struck me as quite significant that I thought was left unexplained in the book.

The particles were reacting (their wave functions were collapsing) based on OUR KNOWLEDGE of the which-path information. Whether they erased the "tags" on the particles or indirectly detected the particles - the particles only seemed to react (collapse) when we had determined the which-path. By erasing or indirectly detecting the which-path, this meant, I assume, that the determination of the which-path information was not interfering with the particle in any manner, thus it could only mean that the particle was simply reacting to our knowledge of the which-path?

How is that possible?

I figure I am either misunderstanding the experiment or there is an explanati+on that wasn't in the book.

Can someone help me to answer this?

Thank you!

MojaveJoe

 

[beautiful1]

I've not read the book, or the passage, you discuss, but I noticed in your desciption that you emphasized "OUR KNOWLEDGE of the which-path information".

In general, explicit knowledge of the which-path information is not necessary for an interference effect to be compromised. The key distinction is that which path information (generally speaking) implies that you could, in principle, distinguish between the superimposed quantum mechanical amplitudes.

Whether you actually take the time to make such a measurment is ultimately irrelevant.

 

[MojaveJoe]

Yes. I emphasized OUR KNOWLEDGE because that's what it seemed to me be the reason for the behavoir.

He (Brian Greene) basically describes the beam splitter experiments done by Scully and Druhl. They use different ways of determining the which path including tagging (and erasing) the spin (he didn't describe it in detail if I recall) and indirect use of detectors and, provided I understood the experiments correctly, the only factor that seemed to matter was our knowledge of the which path. In other words, when we KNEW which path the beam traveled it behaved like a particle, when we DIDN"T KNOW it behaved like a wave.

I was wondering if I misunderstood it...Are we interfering with the observation when we determine the which path? If so, that would be a legitimate answer. However, it didn't appear to me that that was possible (was I wrong in this assumption - which is what I think you are implying in your answer?). What caused the collapse was simply our knowledge. It struck me as deeply weird.

Regards,

MojaveJoe

 

[beautiful1]

As I understand this, the "tags", or distinguishing information, you describe are themselves observables, which means that they can be measured. And, in fact, if you make a "tag" measurement, you expect the result to identify the path with which that outcome corresponded (assuming of course that the tags are distinct). In effect, your measurement has collapsed the superposition state, and any "wave-like" effects vanish.

But even if you do not explicitly make such a measurement, an interference experiment will make the measurement for you; upon interfering the two possible paths, the two tags will also interfere. If the tags are orthogonal (truly distinct), then the interference between them is identically zero and again the "wave-like" effects vanish.

 

[MeJennifer]

I figure I am either misunderstanding the experiment or there is an explanation that wasn't in the book.

Well there is a third option...

Basically, no-one understands what is going on here!

Oh, don't get me wrong it is a fascinating topic to think about and to discuss, I could not live without it, but getting anywhere with it is another matter altogether.

For about one hundred years the best brains on planet have thought about it and have not come one single step further with an explanation.

Oh sure there are theories, some of them far fetched, like multiple universes and such or (at least in my view) more down to Earth ones, like the one developed by David Bohm.

But understanding it, nobody does!

Can someone help me to answer this?

Well I certainly cannot, perhaps there are some bold folks here who pretend to understand it, I keep my eyes open...

But, we can all discuss the details and intellectually drool over this fascinating facet of nature!

 

[Rach3]

Nothing good can come from 'learning' physics by popular literature. Brian Greene is not trying to teach you about quantum mechanics - he is entertaining you with useless analogies and 'explanations' in order to sell his book. Your questions cannot be answered from him - you need actual physics for that, for example an introductory textbook on QM (e.g., Griffiths).

The particles were reacting (their wave functions were collapsing) based on OUR KNOWLEDGE of the which-path information.

Not at all - this is a common 'analogy' used in popular writing, and it's very misleading. "Measurement" does not need a conscious observer at all - roughly speaking, most interactions between a quantum system and a macroscopic system can be some kind of measurement, with or without observer. An electron hitting a phosphorescent screen is a common example - in the QM framework the electron's wavefunction collapses as it's position is "measured" by interaction with the screen. A truly complete quantum explanation of this would be statistical - else it would have to describe each of the 10^23 (!) atoms in the detecting screen - obviously this is far beyond calculation! At the same time, it is well within the law of large averages - which is what our very simple QM framework exploits, when it talks about "measurement". There's no magic here.

Oh sure there are theories, some of them far fetched, like multiple universes and such or (at least in my view) more down to Earth ones, like the one developed by David Bohm.

MWI interpretation and Bohm interpretation are not distinct theories from QM - they all describe exactly the same physics and predict exactly the same results (so far). Their theoretical framework is different; however insofar as they all make identical predictions, it is a mostly philosophical matter of no consequence to experiments (though very interesting, and provocative).

 

[Chaos' lil bro Order]

Yes. I emphasized OUR KNOWLEDGE because that's what it seemed to me be the reason for the behavoir.

He (Brian Greene) basically describes the beam splitter experiments done by Scully and Druhl. They use different ways of determining the which path including tagging (and erasing) the spin (he didn't describe it in detail if I recall) and indirect use of detectors and, provided I understood the experiments correctly, the only factor that seemed to matter was our knowledge of the which path. In other words, when we KNEW which path the beam traveled it behaved like a particle, when we DIDN"T KNOW it behaved like a wave.

I was wondering if I misunderstood it...Are we interfering with the observation when we determine the which path? If so, that would be a legitimate answer. However, it didn't appear to me that that was possible (was I wrong in this assumption - which is what I think you are implying in your answer?). What caused the collapse was simply our knowledge. It struck me as deeply weird.

Regards,

MojaveJoe

I suggest you dig up the papers he referenced on arivx to read them for yourself. Also, your question is an open one, even to top physicists, so don't feel disgruntled at not knowing the answer. Brian Greene, while a very good popularizer of physics, is not above quoting things he himself does not understand. I would suggest private messaging your question to an Physics Forum expert, they can answer it better than me.

 

[nrqed]

Nothing good can come from 'learning' physics by popular literature. Brian Greene is not trying to teach you about quantum mechanics - he is entertaining you with useless analogies and 'explanations' in order to sell his book.

Having known Greeene personally, I take offense at this. I don't think that he wrote his books with the intention of making money. He was genuinely interested in transmitting some sense of what string theory, QM, relativity and so on are all about to the general public.

I also take offense at the tone of this statement. I personally think that it is a great and noble goal to try to convey the ideas of modern physics to the general public. It is not easy but it's a worthwhile enterprise.
I know that many physicists have the attitude that the general public does not deserve to be told about the *ideas* of advanced physics and that one can only appreciate and grasp the concepts of physics only after years of mathematical training. I think this is arrogant and I think that Einstein himself would have disagreed heartily. Most people do not need or actually want to get deeper than the level of, say, Greene's books. They have other things to do (like building houses, taking care of sick people, raising families and other very important non-physics stuff!). Does that mean that we, as physicist, should not try to reach to them and to give them some idea of why physics is so exciting?

Finally, beyond the simple worthy goal of giving the general public an idea of the exciting and marvelous fundamental ideas of physics, there is a more pragmatic reason for conveying to the masses what physics is all about. Some of the young people reading those books today will become the policy makers of tomorrow. If they develop a sense of wonder and appreciation for pure research, then how can that this be bad? Even those will not go into policy making or politics are voters and play a role in how much money goes to science in general, and to fundamental research in particular. So it is good to give a sense of appreciation for the fundamental concepts of physics so that they will appreciate the desire we, physicists, have for doing fundamental research.


Patrick

 

[moving finger]

The particles were reacting (their wave functions were collapsing) based on OUR KNOWLEDGE of the which-path information. Whether they erased the "tags" on the particles or indirectly detected the particles - the particles only seemed to react (collapse) when we had determined the which-path. By erasing or indirectly detecting the which-path, this meant, I assume, that the determination of the which-path information was not interfering with the particle in any manner, thus it could only mean that the particle was simply reacting to our knowledge of the which-path?

it gets even more weird than your description -

In a quantum eraser experiment, one arranges to detect which one of the slits the photon passes through, but also construct the experiment in such a way that this information can be "erased" after the fact. It turns out that if one observes which slit the photon passes through, the "no interference" or particle behavior will result, which is what quantum mechanics predicts, but if the quantum information is "erased" regarding which slit the photon passed through, the photons revert to behaving like waves.

However, Kim, et al. have shown that it is possible to delay the choice to erase the quantum information until after the photon has actually hit the target. But, again, if the information is "erased," the photons revert to behaving like waves, even if the information is erased after the photons have hit the detector.

for details, see http://xxx.lanl.gov/PS_cache/quant-ph/pdf/9903/9903047.pdf

Best Regards

 

[MojaveJoe]

Not at all - this is a common 'analogy' used in popular writing, and it's very misleading. "Measurement" does not need a conscious observer at all - roughly speaking, most interactions between a quantum system and a macroscopic system can be some kind of measurement, with or without observer. An electron hitting a phosphorescent screen is a common example - in the QM framework the electron's wavefunction collapses as it's position is "measured" by interaction with the screen. A truly complete quantum explanation of this would be statistical - else it would have to describe each of the 10^23 (!) atoms in the detecting screen - obviously this is far beyond calculation! At the same time, it is well within the law of large averages - which is what our very simple QM framework exploits, when it talks about "measurement". There's no magic here.

I did not say there was magic (although I still think its weird!) - I am simply wondering if there was an explanation. Obviously these are statistical values, but that's not what I'm asking. What I am asking is why do the stastical averages change depending on whether we know the which-path or not?

Regards,

MojaveJoe

 

[MojaveJoe]

As I understand this, the "tags", or distinguishing information, you describe are themselves observables, which means that they can be measured. And, in fact, if you make a "tag" measurement, you expect the result to identify the path with which that outcome corresponded (assuming of course that the tags are distinct). In effect, your measurement has collapsed the superposition state, and any "wave-like" effects vanish.

But even if you do not explicitly make such a measurement, an interference experiment will make the measurement for you; upon interfering the two possible paths, the two tags will also interfere. If the tags are orthogonal (truly distinct), then the interference between them is identically zero and again the "wave-like" effects vanish.

Thanks!

MojaveJoe

 

[kvantti]

omg

moving finger: I got shocked by your post... I knew that QM is strange but not that it is that strange

And you finally made me decide which interpretation to believe in: I go with the MWI...

 

[MojaveJoe]

it gets even more weird than your description -
Best Regards

Wow!

MojaveJoe

 

[moving finger]

moving finger: I got shocked by your post... I knew that QM is strange but not that it is that strange

And you finally made me decide which interpretation to believe in: I go with the MWI...

I prefers (Cramer's) Transactional Interpretation myself

Best Regards

 

[selfAdjoint]

http://www.bottomlayer.com/bottom/kim-scully/kim-scully-web.htm" is a careful paragraph by paragraph analysis and discussion of the Kim et a. Delayed Quantum Eraser paper.

A thought about this: I think it was Bogoliubov who pointed out that you can't do QM math without distributions, and relativistic distributions have to be defined over the closure of a spacetime neighborhood with some test function. Consider that this picture could be taken "seriously" given that the choice of the test function is completely arbitrary given the obvious conditions on it, so a set of equivalence classes. But the neighborhood doesn't have to be arbitrary; it can be chosen by experimental design. In the usual Aspect-type entanglement experiment the neighborhood is short in the time direction and fat in at least one space direction; we want to demonstrate spacelike correlations.

But in this delayed quantum eraser experiment the neighborhood is long in the time direction (as long as the travel time of the photons, or longer) but skinny in the space directions, just those few paths.

So the distribution class describing the amplitudes is defined through time, and should be regarded as unitary. It doesn't have "parts" and responds as a whole. (As in the paper, it's an integral.) This doesn't mean the amplitudes are "real things"; the distribution equivalence class rather belongs to the kind of things like observation frames without which we cannot describe nature.

 

[MojaveJoe]

http://www.bottomlayer.com/bottom/kim-scully/kim-scully-web.htm" is a careful paragraph by paragraph analysis and discussion of the Kim et a. Delayed Quantum Eraser paper.

This is an excellent paper. They made a comment at the end of the paper:

Time 6. Upon accessing the information gathered by the Coincidence Circuit, we the observer are shocked to learn that the pattern shown by the positions registered at D0 at Time 2 depends entirely on the information gathered later at Time 4 and available to us at the conclusion of the experiment.

This describes my question. Why is the information/knowledge the critical factor? Anyway, I need to think more about your response as it sounds promising, but not sure I understand it entirely yet.

Thank you!

 

[MojaveJoe]

So the distribution class describing the amplitudes is defined through time, and should be regarded as unitary. It doesn't have "parts" and responds as a whole. (As in the paper, it's an integral.) This doesn't mean the amplitudes are "real things"; the distribution equivalence class rather belongs to the kind of things like observation frames without which we cannot describe nature.

Amplitudes are not "real" things? I'm not sure I understand. It seems to me that an amplitude is an abstract representation of real data. I want to know why the data changes (and subsequently the amplitude) based simply (as far as I can see) on our knowledge of the which-path.

I guess I'm asking, am I wrong to see the connection? It seems very clear to me. Am I misunderstanding something?

Thanks!

 

[Farsight]

Well said nrged.

 

[kvantti]

that is the question...

I want to know why the data changes (and subsequently the amplitude) based simply (as far as I can see) on our knowledge of the which-path.

The thing is, you CAN'T know it... nobody can... there are just different explenations (= http://en.wikipedia.org/wiki/Interpretation_of_quantum_mechanics" ) of the phenomenom, but you can't get absolute certainty.

"One explanation of this paradox would be that this is a kind of time travel. In other words, the delayed "choice" to "erase" or "observe" the which-path information of the original photon can change the outcome of an event in the past. Another explanation would be that in fact both outcomes occur. The universe itself exists in a superposition of states in which either the original photon goes through slit A or slit B and in which the which-path information either "observed" or "erased". This is described in detail in the Everett many-worlds interpretation of quantum mechanics."

Source: http://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser"

 

[moving finger]

"One explanation of this paradox would be that this is a kind of time travel. In other words, the delayed "choice" to "erase" or "observe" the which-path information of the original photon can change the outcome of an event in the past. Another explanation would be that in fact both outcomes occur. The universe itself exists in a superposition of states in which either the original photon goes through slit A or slit B and in which the which-path information either "observed" or "erased". This is described in detail in the Everett many-worlds interpretation of quantum mechanics."

Exactly. The former explanation accords with the Transactional Interpretation of Cramer; the latter with the MWI. For philosophical reasons, I prefer the Transactional Interpretation.

Best Regards

 

[Rake]

This is an excellent paper. They made a comment at the end of the paper:

"Time 6. Upon accessing the information gathered by the Coincidence Circuit, we the observer are shocked to learn that the pattern shown by the positions registered at D0 at Time 2 depends entirely on the information gathered later at Time 4 and available to us at the conclusion of the experiment. "

This describes my question. Why is the information/knowledge the critical factor? Anyway, I need to think more about your response as it sounds promising, but not sure I understand it entirely yet.

Thank you!

I think that the paper answers your question in the very last concluding paragraph. It states:

The position of a photon at detector D0 has been registered and scanned. Yet the actual position of the photon arriving at D0 will be at one place if we later learn more information; and the actual position will be at another place if we do not.

So the position at D0 is registered at T2 and this never changes. However, until you have the twin idler info at >T2, you have no information of how the twin signal got to D0.

So the information/knowledge is only critical in that without it, there is no way to tell how the signal photon got to D0.

I thought it was pretty funny how they included T6 as part of the experiment summary to begin with, since it's totally subjective, but then went on to state "Ho-hum. Another experimental proof of QM.". If it was so "ho-hum" shouldn't T6 have read that they were not shocked at all or is this evidence of experimenter belief/disbelief duality? lol

 

[Farsight]

Time Travel? Parallel Universes?

Hmmn. I think I'll read this paper properly.

 

[DrChinese]

This is an excellent paper. They made a comment at the end of the paper:

Time 6. Upon accessing the information gathered by the Coincidence Circuit, we the observer are shocked to learn that the pattern shown by the positions registered at D0 at Time 2 depends entirely on the information gathered later at Time 4 and available to us at the conclusion of the experiment.

Gosh, one might conclude from this that the future influences the past. That view would certainly make a mess of interpretations or theories that rely on non-local mechanisms, n'est pas?

 

[kvantti]

Yup

Gosh, one might conclude from this that the future influences the past. That view would certainly make a mess of interpretations or theories that rely on non-local mechanisms, n'est pas?

This is why I prefer the MWI/multiple histories + multiple futures interpretation

 

[Rake]

Gosh, one might conclude from this that the future influences the past. That view would certainly make a mess of interpretations or theories that rely on non-local mechanisms, n'est pas?

I think the paper is saying that without information from the future there are some features of past events that they are not privy to. Non-local interpretations would predict or account for this as well.

 

[selfAdjoint]

Gosh, one might conclude from this that the future influences the past. That view would certainly make a mess of interpretations or theories that rely on non-local mechanisms, n'est pas?

I think it is rather that the interaction isn't complete until the path interactions are well-defined. We want to see the "photon hitting the target" as the end, but this experiment shows it isn't, and the reason it isn't is that we're dealing with entangled particles here, and there's that essential two-step reduction of the amplitudes.

BTW, CarlB are you following this thread? How would you represent this experiment in density matrix form?

 

[kvantti]

I think it is rather that the interaction isn't complete until the path interactions are well-defined.

But why does OUR knowledge influence the outcome, even if the detectors have already measured the paths of the photons? It would seem that if we erase the info without checking it, it would be the same as even if there were no measurements made by the detectors.

So, do we "define" the path interactions?

 

[selfAdjoint]

But why does OUR knowledge influence the outcome, even if the detectors have already measured the paths of the photons? It would seem that if we erase the info without checking it, it would be the same as even if there were no measurements made by the detectors.

So, do we "define" the path interactions?

The experiment is set up to seem to demonstrate that ("Delayed Choice"), but consider; if automatic software were designed to (pseudo-)randomly scan or not scan with no human the wiser (double-blind) as to which "choices" had been made and only long after did anyone look at the results. Do you think the same patterns of behavior would result, or not? I believe they would. I think the "our knowledge" issue is a red herring, an artifact of the experimental design.

 

[beautiful1]

I think the "our knowledge" issue is a red herring, an artifact of the experimental design.

Well said.

 

[kvantti]

I think the "our knowledge" issue is a red herring, an artifact of the experimental design.

I can't agree with you after reading this:

"It would seem that the 'choice' to observe or erase the which-path information can change the position where the photon is recorded on the detector, even after it should have already been recorded."
- Wikipedia

Unless I'm missing a crucial point or the wikipedia article is misleading... if this is the point, please clarify.

Cheers.

Edit: what if we wouldn't erase the which-path information, but wouldn't observe it (ourselves) either? Would there be a interference pattern? If I understood you correctly, there wouldn't be a interference pattern? If this is what you meant, I agree with you.

I feel a bit stupid now