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How to interpret QM

  1. Apr 9, 2005 #1
    This may be inappropriate for this forum. If so, please suggest where I should post a question like this. Thanks.


    Brian Green writing in the NY Times:

    <<
    Instead, the proponents of quantum theory claimed, reality consists of a haze of all possibilities - all trajectories - mutually commingling and simultaneously unfolding. And why don't we see this? According to the quantum doctrine, when we make a measurement or perform an observation, we force the myriad possibilities to ante up, snap out of the haze and settle on a single outcome. But between observations - when we are not looking - reality consists entirely of jostling possibilities.

    Quantum reality, in other words, remains ambiguous until measured. The reality of common perception is thus merely a definitive-looking veneer obscuring the internal workings of a highly uncertain cosmos. Which is where Einstein drew a line in the sand. A universe of this sort offended him; he could not accept, as he put it, that "the Old One" would so profoundly incorporate a hidden element of happenstance in the nature of reality. Einstein quipped to his quantum colleagues, "Do you really think the Moon is not there when you're not looking?" and set himself the Herculean task of reworking the laws of physics to resurrect conventional reality.
    >>

    When I was a young student of elementary physics, the above description of quantum theory is what I called "uncertainty of fact". Events on a small scale have many outcomes, and only when someone looks at such an event, do the many outcomes resolve into a single observed outcome. Schrödinger's cat is the famous outcome of this thinking, or Einstein's questioning of the moon's existence when no one is looking at it.

    I have always believed that this was BS, and that the issue was uncertainty of knowledge rather than uncertainty of fact. Small events have one outcome, even when no one is looking. This one outcome is not predictable in advance, since the theory allows for many possible outcomes, each one with a probability, and the sum of these probabilities being 1. I find this much easier to swallow than Schrödinger's cat.

    I never went on to master physics, but I have continued to wonder about this question. Recently I asked a friend who is a professor of physics at a university, and he agreed with the second interpretation. Now I wonder why Brian Green and other certified geniuses apparently buy the first interpretation.
     
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  3. Apr 9, 2005 #2

    ZapperZ

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    While what you ask is certainly a good question, it, unfortunately, has been discussed and debated to death on here (and no, this isn't your fault and I'm not putting any blame on anyone). The schrodinger cat-type effect (i.e. the illustration of superposition where orthorgonal states exist simultaneously) HAS experimental verifications (read H2 molecule, Stony Brook/Delft SQUIDs experiments, etc.) And heisenberg uncertainty principle (HUP) is also an observational fact (example: diffraction from a single slit - I have covered this at length in one of my journal entry if you wish to read it. Furthermore, the de Boer effect especially in noble gasses would be unexplainable without this principle).

    There are many indications that QM is describing something intrinsic, rather than simply our state of knowledge of a system. This is very much unlike classical statistical mechanics whereby the statistics merely reflect our ignorance of the system. Classical statistics does not contain observables that is a result of superposition of states. QM does (bonding-antibonding states producing an energy gap between the two).

    So no, I would disagree with your (and your professor's) interpretation, simply because of all the experimental observations that we already have.

    Zz.
     
  4. Apr 9, 2005 #3
    Actually, I believe it was Enrico Fermi who used to jokingly ask his collegues if the moon was there if no-one was looking at it. He knew it was a silly question but what the quantum mechanical answer was to it is what he wanted to know.

    Many brilliant physicists such as Gell-Mann and Hartle have been working on answering this question and removing the related ideas that "measurements" and "observers" are in any way important or that wavefunctions "collapse".

    Schrodinger's cat is now just dead or alive and it stays that way, measurements or not. Decoherence enforces this.

    I'm struggling away to learn what it all really means, though, so don't ask me for answers, as I can provide only directions. :smile:
     
    Last edited: Apr 9, 2005
  5. Apr 9, 2005 #4
    HUP is a technological limitation. True or False?

    In quantum physics, the HUP expresses a limitation on accuracy of simultaneous measurement of observables such as the position and the momentum of a particle and is sometimes explained by claiming that the measurement of position necessarily disturbs a particle's momentum. In other words what this is saying is that our method of gathering information (eg. bouncing a photon(s) off of it) will disturb the wavefunction of the particle we are detecting, we alter its position and momentum - think classical mechanics of colliding objects if you want, in a QM sense it could be charge or whatever based but the effect is comparable. We cannot yet achieve a "look but dont touch" method of gathering information. If we could, then that solves alot of questions/problems. So to answer my own question, I would say Yes the HUP is a technological limitation, that eventually I believe we will get around. It should be as simple as substracting the jolt given to the particle upon detection to get the state of the particle had we not influenced it (anyone know what I'm saying?) but that in itself is not so simple.

    On the idea regarding a myrid of possibilities converging to a single outcome, a single result. Well what are you saying? That another answer at another location does not exsist? You have to consider that this comes about as a function of time(s) and the exact relationship between time(s) and your angle or method of detection - this is not something unique to the QM world. A particles position and momentum are ever changing and perhaps even repeating so if we have our focus on an area that we know it has or will pass through, any information we gather regards it as what was the system (part of the system) like at t time at l location and if possible can we then go on to predict the future or past - but again our measurement affects the future hence quite an uncertainty in QM arises.

    QM collisions are different than classical mechanics macro objects collisions where we have a good idea of all the meaningful varibles involved (including gravity which QM doesn't) and not disturb the system because any disturbance of observation (force of photons, or whatever particles coming into contact) is neglegible.

    I wanted to comment on diffraction patterns from a slit also. It is my belief that this pattern (multiple bands/patterns actually) comes about due to the device that creates and emits electrons rather than anything else. Think about it this way, if all electrons that were emited were 'exactly similar' and exactly similar in position and momentum along its wavelength, then wouldnt it be reasonable to assume that they would all take the same path given no extra dynamic varibles to source? In a vacuum for example, the environmental influences- if any- are fairly constant so the paths taken should be identical if identical (in all aspects) electrons are emited. But thats not the case is it? The device that emits electrons will emit unlike electrons therefore the paths taken by them are different and effectively random, therefore an definitive aspect of the HUP becomes realized and observed.
    (The above assuming a non random operation of an electron in motion - eg. a predictable period and frequency according to given energy.)
    (Also what pattern would you get if they all took the same path? A dot, thats it, no verticle or horizontal displacement, just a dot.)
     
    Last edited: Apr 9, 2005
  6. Apr 9, 2005 #5
    your friend the professor he isnt a professor in Quantum Mechnics right ? One thing that you can be absolutley sure about is that there are such things as entanglement which make it possible to teleport qm states. Now I dont think entanglement or teleportation is possible in the classical world ?
     
  7. Apr 9, 2005 #6

    ZapperZ

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    This is false, and you have a misunderstanding of what HUP is based on the rest of your posting.

    The HUP is NOT the limitation of measuring a quantity. This is the biggest misunderstanding of the HUP. The accuracy of measuring a position and momentum of a particle depends entirely on the resolution of my instruments. The uncertainty in such a measurement is NOT the HUP. This is something people need to clearly understand. I can measure the position of an object, and then measure as accurately as my instrument allows, the momentum of that object. In none of these two measurements are they limited by the HUP.

    What the HUP actually says is that if you prepare an identical system, and measure the identical position of the object, your ability to predict what momentum it will acquire will get worse as you knowledge of the position becomes more accurate. Again, read my essay on the single slit experiment (which, by the way, is NOT due directly to the light source but rather the geometry of the slit). If your certainty of the position get better, your prediction on what value the momentum of that object will be will become worse. So this has nothing to do with exact measurement of position and momentum, but rather the ability to predict what these values will be given the IDENTICAL situation.

    What this means is that if you do this repeatedly, there will be a statistical spread of both the position and the momentum. THIS is what is reflected in the HUP, and NOT the measurement of a SINGLE position and momentum.

    This is very different than the classical description, in which if you have an identical system, knowing where it is will always give you the same momentum. The statistical spread is only a reflection of uncertainty in measurement and other random errors that one gets in a typical experiment. They certainly do not become worse as one parameter becomes better.

    Zz.
     
  8. Apr 9, 2005 #7
    "The HUP is NOT the limitation of measuring a quantity"

    I absolutely did not mean to imply that...

    What your describing is the UP Method and (for the most part) it is how I also understand it, there is no confusion. What I am describing is the UP of Interpretation within the context of the Copenhagen Intepretation and it's underling relation to the uncontrol of randomness- a technological limitation. The HUP was never defined or stated with regards to any specific experiment or measurement technique so I can, if I want, use the underling principle however I wish and according to a specific experiment, such as the 2-slit, Feyman himself did this.

    "Within the widely but not universally accepted Copenhagen interpretation quantum mechanics, the uncertainty principle is taken to mean that on an elementary level, the physical universe does not exist in a deterministic form—but rather as a collection of probabilities, or potentials. For example, the pattern (probability distribution) produced by millions of photons passing through a diffraction slit can be calculated using quantum mechanics, but the exact path of each photon cannot be predicted by any known method. The Copenhagen interpretation holds that it cannot be predicted by any method."

    < I am saying that it can eventually given technological advances, be predicted, and that the reason that the path is not currently predictable is because none thought of perhaps that it is actually the device that we need to focus on because as it stands, generates effectively random path electrons.

    Just my theory of course. Sorry I should have been more clear.

    "which, by the way, is NOT due directly to the light source but rather the geometry of the slit"

    Which by the way what? What are you talking about? What it sounds like you just said is that the geometry of the 2-slits or the actual 2-slits themselves are somehow the "measurement" the thing that sets off the collapse, because you go on to say not due to the light source - the light source is typically related to the measurement.

    I may read it. However I already think I know what it is going to say. Let me guess, the particle(s) wave bounces off the edges and thus deflect the particle giving it a unique path or something to that effect. That is covered briefly as one possible viewpoint in Feyman lectures if I remember. I happen to believe the geometry of the obstruction plays a significant role myself, not just a passive non varible as if the particle just "slips by" unaffected by the geometrical dimentions as I'm sure you agree - in other words perhaps a hole isnt just a hole. In any case, the second path though is what is really important.
     
    Last edited: Apr 9, 2005
  9. Apr 9, 2005 #8
    Well, I read the rest of the Brian Greene's article. It's very textbook. Which is a problem if the textbooks are out of date. And they are. Making it sound like measurements are important is a classic sign of someone who's not been reading anything recent about interpretation.

    Also, Greene gives Einstein too much credit for EPR, which is actually believed to have been Podolsky's idea. Greene also seems to think EPR was about locality when it was about realism and the consequences of believing wave function collapse is a physical effect.

    Anyhow, for the person who started this thread, decoherence is the subject to read about. It really is about uncertainty of knowledge now for Schrodinger's cat and the Moon. Facts stay facts in the sense most people mean and whether someone observes them or not.

    The ambiguity of what happens between one event and another is still around of course, as is the mystery as to why one event actually happens and not another but the former can be tamed and the latter is only to be expected from a probabilistic theory, I guess. :smile:
     
  10. Apr 9, 2005 #9
    no, no, no, I don't know if the uncertainty principle have been taken to any experimental test but things such as hidden variables in QM have been disproved mathematically, and in fact, the statistical nature of QM (Copenhagen) has predicted experimental results much better than any more deterministic interpretation (notably Bohmian mechanics).

    the only thing that can predict any microscopic object deterministically is a theory which essentially overthrows standard QM right now and NOT technology. Does an electron gun generates random electrons, then what about a tennis ball shooter? I assume you look at them at the same light. The tennis ball shooter is not random because the tennis ball has well-defined position and we can calculate forces and all using classical mechanics and predict where the ball would end up.

    People have tried using the same way to calculate trajectories of micorscopic objects and have failed, esp. before the birth of Quantum theory. Look at it this way, there is nothing really stopping you from confining any wavefunction in a particular space (rigid box), the important thing you have to note is that standard QM says that "everything possible is compulsory", so the electron or whatever small particle takes every possible route and not just 1 like the tennis ball.

    i think that what Zapperz was saying is that the geometry of the 2 slits influences the interference pattern. And now how is the light source related to measurement? And there's no bouncing off the edges thing going on. No, look at every microscopic object not as a tennis ball but as a probability cloud.

    Feynman's path integral formulation of QM is the sum of all possible trajectories to give a (somewhat) classical trajectory, but such classical trajectories shown in Feynman diagrams are simply simplifications and what really happens in a quantum system is best described using the schrodinger equation to describe a particle's wavefunction and this method is indeterminate. (am i right on this one ?, hope someone can correct or back me up).
     
  11. Apr 10, 2005 #10

    ZapperZ

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    But you should also read Feyman's own path integral method and realize that the double slit interfence pattern has nothing to do with photons, but with the superposition of PATHS! This means that if you have ANY object, and you could design an experiment in which the path the object going through cannot be detected, you WILL get the identical interference pattern. And we do (example: supercurrent in SQUIDs). So it is puzzling that you are saying that we CAN have a "technological advances" to know the exact path and still maintain the same effect. That's like removing "indistinguishibility" in "indistinguishable statistics" for fermions and bosons. You will get a completely different set of statistics and the original phenomena that it has already described will be gone! If you know the path, there will no longer be a superposition of paths, and what are you left with to be able to get back the interference phenomena?

    In the earlier posting, you said:

    Thus, you are attributing the diffraction pattern to the SOURCE. This is false. I can change the shape of the slit and change the diffraction pattern. In fact, the diffraction pattern is actually the fourier transform of the geometry of the slit! All the source affects (if it's monochromatic) is the location of the interference pattern, i.e. the spacing between the patterns, but NOT the nature of the pattern.

    And I'm sure that if you have already read my entry, you would have discovered that your guess was wrong. The single slit in a diffraction experiment is equivalent to a position measurement, while the diffraction pattern on a screen or detector is the momentum measuring device. I make no mention of a wave "bouncing off the edges" etc. because such a scenario cannot explain why the diffraction pattern gets larger as the slit is made smaller.

    Zz.
     
  12. Apr 10, 2005 #11
    Can we actually construct a measurement tool which can measure a state which is in a superposition of some orthogonal states?
     
  13. Apr 10, 2005 #12

    ZapperZ

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    Yes, you can. The Schrodinger Cat-type experiments conducted by Delft/Stony Brook measured the energy gap, or the energy observable. This is a non-commuting observable to the position observable. Thus, the act of measurement does not destroy the position superposition and you can get the effects of such superposition.

    Both experiments (and Tony Leggett, who first proposed such experiment) claim to have the supercurrents flowing in opposite directions SIMULTANEOUSLY. So you have a superposition of current directions. This is the only scenario (at least so far) to explain the phenomenon. I have listed all the relevant references in one of my latest Journal entries if you wish to take a look at them.

    Zz.
     
  14. Apr 10, 2005 #13
    Just to further clarify.. are you saying that the measurement actually projects the state onto its exact state (which is a superposition of orthogonal states)?

    PS: Just checked out your journal. Are you refering to this work:
    A.J. Leggett "The Quantum Measurement Problem", Science v.307, p.871 (2005).
     
  15. Apr 10, 2005 #14

    ZapperZ

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    Er... hum. Let me put it this way. If you measure the angular momentum, you have determined Lz, for example, but Lx and Ly are still maintaining their superposition with each other. This is what I mean.

    Yes, but read his paper on J. Phys. Condens. Matt. That one has an extremely clear exposition on this whole thing, and includes his analysis of the Delft/Stony Brook experiments (both of which I listed also in the same journal entry).

    Zz.
     
  16. Apr 10, 2005 #15
    I see. Thanks for the interesting paper. Will check it out first :smile:
     
  17. Apr 10, 2005 #16
    misogynisticfeminist

    "the only thing that can predict any microscopic object deterministically is a theory which essentially overthrows standard QM right now and NOT technology."

    Not a theory, just insite into why things are deterministic - specifically with regards to the random path (1 electron fired at a time) observed in the 2-slit experiment, and I stand by that it Is the device that creates the electrons.

    "Does an electron gun generates random electrons, then what about a tennis ball shooter? I assume you look at them at the same light."

    Ha no. You people tend to assume alot of things, most noteable that I have no idea what I'm talking about.

    "The tennis ball shooter is not random because the tennis ball has well-defined position and we can calculate forces and all using classical mechanics and predict where the ball would end up."

    Gee really? Isnt that what I kinda said?

    "so the electron or whatever small particle takes every possible route and not just 1 like the tennis ball."

    every possible route according to what? I know what your saying I've heard it before. Superposition of paths. Thank you for your insite. Superposition of paths is in actuality the wave function displacement, whether this involves 1 particle or more than one- entanglement. It does not travel a path that its wave function does not displace.


    "i think that what Zapperz was saying is that the geometry of the 2 slits influences the interference pattern."

    I agreed with this.

    "And now how is the light source related to measurement?"

    This was maybe misunderstood. I am refering to the detection area light source (behind the 2-slits), not the device that creates the electrons.


    ZapperZ

    "So it is puzzling that you are saying that we CAN have a "technological advances" to know the exact path and still maintain the same effect."

    Not maintain the same effect. I asked, what the pattern would be given identical electrons fired, and I said a dot.

    "If you know the path, there will no longer be a superposition of paths, and what are you left with to be able to get back the interference phenomena?"

    Not true, Like I said, if the device created exactly similar electrons they should all take the same 'superposition of paths'
    In that sense, the path is predicable and known. What is left to create the interference? Read above, the pattern is now a dot.

    "Thus, you are attributing the diffraction pattern to the SOURCE"

    Both you and misogynisticfeminist misinterpreted. Actually I am in a sense but realize the whole picture.

    "I can change the shape of the slit and change the diffraction pattern."

    Only because a pattern, no matter what it may be, comes about as the result of uncommon electrons being fired - and subsequently interacting with the obstruction so that over time a pattern results DUE to the different paths taken. I cant make this anymore clear.
     
    Last edited: Apr 10, 2005
  18. Apr 10, 2005 #17

    ZapperZ

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    Then you have a lot of explaining to do to account for the interference pattern from the supercurrent in a superconducting interference devices. These are electrons in a SINGLE, DEFINITE, and COHERENT state. Yet, we see no "dot".

    Zz.
     
  19. Apr 10, 2005 #18
    link? Are these electrons by chance spatially independent of one another? Fired at the same time? - as in a beam of light. I can tell you right now where you are again failing to see my point.

    Your use of the word Coherent (as in coherent light) implies mutiple particles and spatically independent - in phase, an entanglement of sorts.
     
  20. Apr 10, 2005 #19

    ZapperZ

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    I take that back. It appears that you have to start, instead, by reading the whole chapter on superfluids and superconductivity. I am suprised that you think this has anything to do with electrons being "fired".

    Zz.
     
  21. Apr 10, 2005 #20
    ;) you got me, wasnt paying attention. Im familiar with the subject already. Electrons being fired or electrons traveling through a material, Whatever, point is they are in motion and have a relative path. Completely different subject if you ask me anyway. There are numerous types of interference so I think we should really stick to a common ideal.

    Still though I have no idea what the setup your talking about involves, appreciate a link.
     
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