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Wave superposition nothing more than abstraction?

  1. Aug 9, 2011 #1
    I was recently watching a video about the double slit experiment that said if we fire electrons one by one, the QM superposition principle says that the single electron goes through both holes, goes through one, vice versa, and none all at the same time. i.e a Superposition of states.

    However, it really, really bothers me how much this seems to not be a true depiction of reality, but instead nothing more than a mathematical abstraction.

    Think of when we calculate population averages, and we come up with decimal values. Obviously, we can't have fractional humans (we could, but it's not truly allowed) so the answer isn't physical. It's only a mathematical way to visualize the solution. Much like the superposition principle, it doesn't occur in reality but only serves to help physicists wrap their head around the intangible.

    Thus, I get the feeling that physics is all about "making the equations conform to observation", kind of like filling in a puzzle piece as one sees fit. That's a very distasteful, boring way to go about science, in my opinion.

    ..or Maybe quantum mechanics is not supposed to be an entirely physical theory, at least not yet?
     
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  3. Aug 10, 2011 #2

    xts

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    I see you are entering the field of metaphysics, touching such subjects as 'what is physical', 'what is real', etc.
    I fully agree that wavefunctions are just mathematical tools to predict observable results. I am afraid, you want to think about electrons as about something real, physically passing one slit or another like football ball is passing the goal. Such view is hard to maintain, but some people like it. The most appealing interpretation of QM in terms or 'real' particles is Bohmian interpretation, you may start from Wiki and follow references http://en.wikipedia.org/wiki/Bohm_interpretation
    Personally, I don't like this interpretation - I definitely prefer non-realistic model, in which it makes sense to speak about 'particles' only at the very moment of their detection/emission/interaction.
     
  4. Aug 10, 2011 #3
    The timeline of your argument is wrong, the mathematical framework of quantum mechanics was developed before many of the predictions it made were actually observed (eg. quantum entanglement I believe was predicted 80 years before we ever saw it in a lab). It's a model, it agrees extremely well with physical observation, why throw it out on the basis that it "bothers" you? That would be a far bigger crime.
     
  5. Aug 10, 2011 #4
    What's the difference? Consider this: it would be possible to take just the mathematical equations of QM, plug them into a computer, and simulate a mini-universe whose physics is identical to our own in all respects. Given that the math is all you need to describe reality, why do you think there is anything to understand besides mathematics?
     
  6. Aug 10, 2011 #5
    Don't you think this should be the goal of physics, and not just meta-physics/philosophy? Physics should be about deducing the rulebook of the Universe. Why should we leave out thoughts about "what's physical and what's real?" How else are we supposed to "figure out the mind of God?" if we leave physical interpretations out? After all, the Universe is a REAL thing, at least I hope.



    I didn't mean for abstractions to be the same thing as equations. I'm fine with that. In fact, I believe I'm one of the few that agrees that the Universe is nothing more than a complex computing machine.

    What I have a problem with is physicists taking mathematical superposition to be literal and saying "it's both in this state and that state at the same time." No, it's really not, that's just a mathematical abstraction so that when we square quantities and do other things with them , those abstractions cancel out and disappear to produce a physical model that fits with observation. I think people are reading into abstraction to much. Abstraction is just abstraction, don't take it literally. Instead we get in this whole mess of violating causality just because people take things literally.
     
  7. Aug 10, 2011 #6

    DrChinese

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    Physical theory is a useful description of reality. (In fact, read my tag line.) I do not know of many scientists who confuse the description with the thing being described. So when someone says it is in both states simultaneously, they mean it is "as if".

    There are many interpretations and hypotheses regarding significance of the mathematical nature of physical laws such as Relativity and Quantum Theory. As has been pointed out, this is really a philosophical point. No one really has a deeper answer. By the way, your viewpoint ("no it's really not...") has even less basis than many others - since there is no actual evidence to support it.
     
    Last edited: Aug 10, 2011
  8. Aug 10, 2011 #7
    I know there's not much evidential basis for my argument. However, I did recently stumble upon an article in which Penrose seems to argue the same point.


    ---

    http://discovermagazine.com/2009/se...quantum-mechanics/article_view?b_start:int=1"


    In quantum mechanics an object can exist in many states at once, which sounds crazy. The quantum description of the world seems completely contrary to the world as we experience it.

    It doesn’t make any sense, and there is a simple reason. You see, the mathematics of quantum mechanics has two parts to it. One is the evolution of a quantum system, which is described extremely precisely and accurately by the Schrödinger equation. That equation tells you this: If you know what the state of the system is now, you can calculate what it will be doing 10 minutes from now. However, there is the second part of quantum mechanics—the thing that happens when you want to make a measurement. Instead of getting a single answer, you use the equation to work out the probabilities of certain outcomes. The results don’t say, “This is what the world is doing.” Instead, they just describe the probability of its doing any one thing. The equation should describe the world in a completely deterministic way, but it doesn’t.


    Erwin Schrödinger, who created that equation, was considered a genius. Surely he appreciated that conflict.


    Schrödinger was as aware of this as anybody. He talks about his hypothetical cat and says, more or less, “Okay, if you believe what my equation says, you must believe that this cat is dead and alive at the same time.” He says, “That’s obviously nonsense, because it’s not like that. Therefore, my equation can’t be right for a cat. So there must be some other factor involved.”

    When you accept the weirdness of quantum mechanics, you have to give up the idea of space-time as we know it from Einstein. You come up with something that just isn’t right.


    So Schrödinger himself never believed that the cat analogy reflected the nature of reality?


    Oh yes, I think he was pointing this out. I mean, look at three of the biggest figures in quantum mechanics, Schrödinger, Einstein, and Paul Dirac. They were all quantum skeptics in a sense. Dirac is the one whom people find most surprising, because he set up the whole foundation, the general framework of quantum mechanics. People think of him as this hard-liner, but he was very cautious in what he said. When he was asked, “What’s the answer to the measurement problem?” his response was, “Quantum mechanics is a provisional theory. Why should I look for an answer in quantum mechanics?” He didn’t believe that it was true. But he didn’t say this out loud much.
     
    Last edited by a moderator: Apr 26, 2017
  9. Aug 10, 2011 #8

    DrChinese

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    No theory is really "true". Simply that some theories may be more useful some of the time. Ever heard the saying, "it's the exception that proves the rule"? Most theories work except when they don't.

    At any rate, one major flaw in your hypothesis is that a double slit setup has slit 1 and slit 2 interfering, even when there is one particle. That shouldn't happen if superpositions are not "real".
     
  10. Aug 10, 2011 #9

    JK423

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    CyberShot, i understand your objections on the *being in 2 places at once* thing. As Dr. Chinese say, in these expressions what is actually meant is *AS IF it were in 2 places at once*. Whats happening in 'real'? We do not know. We dont have a deeper understanding of what a superposition of states really is. The sure thing is that we dont meet these in the classical world, and there are good reasons for this (Check: Decoherence)
    I dont know what your views are in general, but if you think that particles are just tiny tiny things, like tiny balls, think again.
    I'll just tell you this:
    Have you ever seen an electron? Have you ever seen ANY particle? No matter how big it is?
    You'll say: *No, i havent seen an electron because its too small but i see my cellphone, which is a huge system of particles*.
    But realize that you actually dont *see* the particles (or your cellphone), you just detect the photons scattered and emmited by them. So what you see... is just something created by photons in your eye! You dont actually observe the *actual* particle, if such thing can be defined!
    And here is the thing:
    Quantum mechanics can explain everything. You may know that QM describes particles as, spread in space, wavefunctions. We do not know anything more about what particles are, most that we know is their wavefunction. You may be confused with the fact that the classical world has (at first sight) nothing to do with the quantum mechanical world, since particles are wavefunctions in QM and not something that is *here* and you can *touch*! However.... QM can (in principle) model every macroscopic object, like your cellphone, and predict exactly what you observe!
    For example, you observe that your phone is located in a specific place on your desk. QM predicts that as well even thought it treats the cellphone as a huge wavefunction! Again, the reason is called "Decoherence".

    My point is that everything you see around you is as QM predicts using the wavefunctions. So, take it a little more seriously, and stop thinking that QM is wrong because particles are *actually* located somewhere specifically and they are not a wavefunction as QM says. You have never seen a particle, so you're false in saying such things.
     
  11. Aug 10, 2011 #10

    BruceW

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    I agree that Quantum Mechanics isn't a very aesthetically pleasing theory, but it is better than classical mechanics.
     
  12. Aug 12, 2011 #11
    That's generally true from the perspective that we seem to have more mathematical theories than we do observations....so only some, so far, "fit" observations.

    When you "solve" a problem in physics, you often have to pick out the correct equation to use.....most will not solve your particular, specific, problem. Newtonian mechanics won't work well at speeds close to that of light.

    And only some of our mathematical theories fit THIS world. But if there are really an infinite number of universes [parallel worlds] we don't have nearly enough math!! And as noted above in previous posts, there are a number of situations where the math was discovered first and made predictions which were, remarkably, later observed.

    The bending of light was forecast in GR and observed later by Arthur Eddington as another example where math came before observation....And it turned out, oddly, that the Euler beta function had unusual characteristics related to the strong nuclear force....but more importantly, string theory!!!. You just never know for sure where that "abstract" math will turn out to be useful in this universe.
     
  13. Aug 12, 2011 #12

    ZapperZ

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    When someone says things like "superposition is an abstraction, why do they ALWAYS ignore evidence from chemistry (bonding/antibonding), and from the Delft/Stony Brook SQUID experiments? Why?

    Is it because these people are ignorant of the physics involved in those observations, but yet, they are so ready to draw up a conclusion regarding what they don't know well? I find this practice to be orders of magnitude more dubious than what QM says about superposition.

    Zz.
     
  14. Aug 12, 2011 #13
    Well, Albert Einstein and many other phycisist thought the same... the last century.

    Nowadays there is strong experimental evidence that support superposition as "physically real"
     
  15. Aug 12, 2011 #14
    ..
    Dear CyberShot,

    seeking to encourage you in your studies, you might like to have a look at:

    http://www.nature.com/ncomms/journal/v2/n4/full/ncomms1263.html?


    QUOTE: "One can find various definitions in the literature for what a true Schrödinger cat [27] should be and a number of intriguing experiments have reported the generation of photonic [28] or atomic cat-states [29, 30]. In as far as the term designates the quantum superposition of two macroscopically distinct states of a highly complex object, the molecules in our new experimental series are among the fattest Schrödinger cats realized to date. Schrödinger reasoned whether it is possible to bring a cat into a superposition state of being 'dead' and 'alive'. In our experiment, the superposition consists of having all 430 atoms simultaneously 'in the left arm' and 'in the right arm' of our interferometer, that is, two possibilities that are macroscopically distinct. The path separation is about two orders of magnitude larger than the size of the molecules."
     
  16. Aug 12, 2011 #15
    As a neutral comment: I find it interesting to note that the "pro-QM side" (truly just by lack of a better term) seems to be (unknowingly?) split up into the "as if"-camp and the "really"-camp
     
  17. Aug 13, 2011 #16

    JK423

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    ZapperZ and Gordon Watson,

    The OP does not deny the superposition principle. His worries are about the interpretation of it. The experimental data have little to do with interpretational issue of QM.
     
  18. Aug 13, 2011 #17
    ..
    I was responding to the OP's – "Much like the superposition principle, it doesn't occur in reality but only serves to help physicists wrap their head around the intangible" – via reference to some recent interesting and seemingly relevant experimental data.

    That is: I was offering the OP additional "food-for-thought" -- that was readily accessible -- adding to the examples offered by ZapperZ.

    For now, I'll happily leave it to Zapper (and others) to expand on the subject.
     
    Last edited: Aug 13, 2011
  19. Aug 13, 2011 #18

    ZapperZ

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    But in some sense, it does!

    The SQUID experiments[1,2] were a clear attempt to check if the supercurrent were flowing in both directions simultaneously based on the formalism. This is no different than saying that a particle is in both locations at the same time. A formal description of it can be found in Leggett's paper[3].

    But what is rather puzzling here is that, if you look at the OP's philosophy, then one could argue that ALL of physics is an "abstraction"! So why pick on QM, or superposition for that matter? After all, what are Maxwell equations, for example?

    The problem I always have in discussion such as this is that it isn't based on physics, but rather based on a matter of TASTES. It comes down to "Oh, I find it difficult to accept that something can be in two places at once, so it must not be real". Nowhere in there is physics. As I've often said, we might as well argue for favorite colors.

    Zz.

    [1] C.H. van der Wal et al., Science v.290, p.773 (2000).
    [2] J.R. Friedman et al., Nature v.406, p.43 (2000).
    [3] A.J. Leggett "Testing the limits of quantum mechanics: motivation, state of play, prospects", J. Phys. Condens. Matt., v.14, p.415 (2002).
     
  20. Aug 13, 2011 #19

    xts

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    Dear ZapperZ, and other Mentors:
    Half of threads like this are moved to lounge->general->philosophy, half stays in Physics threads, no one posts such problems in 'philosophy' branch, all posters decide that such issues are more related to Physics than to General-Philosophy.
    Wouldn't it to be wise to make a subforum: Physics -> Philosophy of Physics (QM interpretations, nature of physical reality, etc.) ?
     
  21. Aug 13, 2011 #20

    ZapperZ

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    The ones that stay in physics tend to have more physics content. The ones that don't tend to have less physics content.

    It all depends on the intent of the topic, and how the participants respond to it. If a lot of physics is used, then the thread stays here. If there's a lot of handwaving, metaphysical stuff, it goes to you-know-where.

    There is no need to create another forum. The Philosophy forum isn't a hot-bed of posts and new topics.

    Zz.
     
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