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Brian Green's Beam Splitter Experiments

  1. Jul 5, 2006 #1
    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!

  2. jcsd
  3. Jul 7, 2006 #2
    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.
  4. Jul 7, 2006 #3
    Yes. I emphasized OUR KNOWLEDGE because thats 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 wierd.


  5. Jul 7, 2006 #4
    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.
  6. Jul 8, 2006 #5
    Well there is a third option.....

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

    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. :smile:

    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!

    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! :smile:
    Last edited: Jul 8, 2006
  7. Jul 8, 2006 #6
    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).

    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.
    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).
    Last edited by a moderator: Jul 8, 2006
  8. Jul 8, 2006 #7

    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.
  9. Jul 8, 2006 #8


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    Having known Greeene personally, I take offense at this. I don't think that he wrote his books with the intention of making money. :grumpy: 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. :grumpy: 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.

  10. Jul 10, 2006 #9
    it gets even more weird than your description -

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

    Best Regards
    Last edited by a moderator: Apr 22, 2017
  11. Jul 10, 2006 #10
    I did not say there was magic (although I still think its wierd!) - 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?


  12. Jul 10, 2006 #11

    Last edited: Jul 10, 2006
  13. Jul 10, 2006 #12

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

    And you finally made me decide which interpretation to believe in: I go with the MWI...
  14. Jul 10, 2006 #13

  15. Jul 10, 2006 #14
    I prefers (Cramer's) Transactional Interpretation myself :smile:

    Best Regards
  16. Jul 11, 2006 #15


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    http://www.bottomlayer.com/bottom/kim-scully/kim-scully-web.htm" [Broken] 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.
    Last edited by a moderator: May 2, 2017
  17. Jul 11, 2006 #16
    This is an excellent paper. They made a comment at the end of the paper:

    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!
    Last edited by a moderator: May 2, 2017
  18. Jul 11, 2006 #17
    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?

  19. Jul 11, 2006 #18
    Well said nrged.
  20. Jul 11, 2006 #19
    that is the question...

    The thing is, you CAN'T know it... nobody can... there are just different explenations (= http://en.wikipedia.org/wiki/Interpretation_of_quantum_mechanics" [Broken]) 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" [Broken]
    Last edited by a moderator: May 2, 2017
  21. Jul 12, 2006 #20
    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
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