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Super-position / Duality Physically Real?

  1. Jun 11, 2009 #1
    Hi, please excuse what is probably a very basic question. I'm abit confused about some of the things I've read about quantum mechanics (can you blame me?). The main thing I'm trying to understand is the reality of duality and superposition. I've heard it said that these concepts are merely mathematical models that provide a (please excuse my non-technical terms) probability map of where a single point particle is. The notion being that, until observed the particle could be anywhere, and its actual position is unknown. This 'probability map' gives us an idea of where it is likely to be. At this point the mathematical model would treat the particle as being in many places at once (super-position) but in actuality it is only really in one place.

    This seems to contradict what I have been told that if the particle is left unobserved, its location remaining unknown, and is allowed to carry on propagating through an enclosed ('two slit') system it displays the physical property of superposition (and duality) in the form of an interference pattern. The fact that the single point particle has behaved like a wave and interfered with itself indicates that it did in-fact exist in superposition and pass through both slits simultaneously in the from of a wave; how else could it create the wave-like interference pattern?

    I have read that the areas of high probability within a wave-function (i believe to be called the absolute square of the wave) it not the probability of the particle being in that place but is actually the probability of you finding it there (a very subtle yet crucial difference) and it dose in fact exist in superposition... at this point it would make sense to say that this is merely a mathematical model describing something imaginary, and in reality the particle of course only exists in one place... Except that if the particle is allowed to pass through the two slit experiment it is shown to be physically in a super-position state (or have the properties of a wave).

    So my question is this... Are super-position and duality real physical propertys of the microscopic world? Or are they just the conceptual representation in a mathematical model?

    Please note I'm not a professional physicist (in case that isn't obvious) and struggle to understand some of the mathematics involved so an explanation avoiding maths would be appreciated. Also, I think I may have blurred the distinction between super-position and duality in my question but hopefully the Q is still valid.

  2. jcsd
  3. Jun 11, 2009 #2
    I'm not at all a specialist on this subject, but I do think that it depends on which interpretation of QM you 'believe in'.

    For example, I think the Copenhagen interpretation says that particles do not have a position until measured. If you say that particles do have a position, but we simply don't know it, then you are describing a hidden variable interpretation. The position of a particle is then a hidden variable. This suggests that our current QM theory is incomplete, because we don't know the particle's position, yet it is well defined.

    Which of the many interpretations is correct? As far as I know there is no way to tell, since they are, by construction, impossible to tell apart after you do a measurement on something.

    For example, take the cat in a box 'experiment': is the cat alive, or dead, or in a superposition between the two when the box is closed and nobody is looking? Well, there is no way to tell, because the only way to know would be to look in the box, which is not allowed in the first place.
  4. Jun 11, 2009 #3
    Hi, thanks for the reply. I understand that there have been different interpretations of the same observed results but it seems to me that this particular aspect (superposition & duality) aren't open to interpretation.

    I keep seeing it being stated that superposition and duality aren't real physical properties, they are only an abstract mathematical representation. Am I not right in thinking that the two slit experiment dispels any notion that super-position / duality is false? The fact that the wave function interferes with itself in this situation and has a direct affect on physical reality (i.e where the particle ends up) is indirect proof that super-position & duality really exist. No? True we can't see the particle go through both slits at once, but we can see the resulting effects.
    This seems clear-cut and isn't open to interpretation. Superposition & Duality are real, physical properties. Is this right?
  5. Jun 11, 2009 #4
    There are interpretations that can describe the dual slit experiment without using superposition. I think the Bohm interpretation is one, although I know nearly nothing about it. There was a thread about it here recently, try looking through the threads you might find it. I haven't followed it but I think it said that the particles travel non-straight paths. There was a nice plot of the paths which resulted in something looking alot like a double slit experiment result, but I have no idea how they got to that plot or whether or not it is valid...

    EDIT: This is the topic I was referring to:

    Other than that, I'm afraid I know just as much as you do so I cannot help you.
  6. Jun 11, 2009 #5
    Hey thanks, Will read it now. I'm vaguely aware of the Bohm Interpretation but don't really know what it is. Guess I have allot of catching up to do. I may come back and re-iterate my question if I'm still confused. Thanks again Nick89
  7. Jun 11, 2009 #6
    Ok, I just read the topic you linked and I think it basically answered my question. It was pretty technical but the one point that stood out, made by Mr. Zenith8, was, to quote directly...

    'While each particle track passes through just one of the slits, the wave passes through both; the interference profile that consequently develops in the wave generates a similar pattern in the particle trajectories guided by the wave.'

    This is based on Broglie/Bohm interpretation and has shown me how an interference pattern can be explained without using superposition. Marvelous. Now I must go away and broaden my horizons by learning about the Broglie/Bohm (and other) interpretations.

    Thanks again, Tom
  8. Jun 11, 2009 #7
    "Real" is a weasel word. Whenever you talk about whether some is "real" or not in physics, you're probably getting caught up in nonsensical philosophy. So tread carefully =-)

    The real truth is that your every day understanding of the universe is built on a lie. A useful lie that let you and your ancestors survive until the present, but a lie nonetheless. You see an apple in a tree, you see it has a place in the world just above your head. You hear a story about how the world is made up of little balls called atoms, and you assume they work just like very small apples. But like any analogy, there is a limit to how similar two different situations are.

    No one has ever seen a bare electron. No one ever will. The way you interact with an electron is very different from how you interact with an apple. You can't "touch" it. You can taste it. But you can push it around with light and matter. You can see it's ghost when it smashes against an atom of phosphorous. But an electron and its ghost are still not the same thing.

    We can watch the ghosts move around. In doing so, we hope to figure out how the actual electrons move around. But it's funny. An individual ghost seems unpredictable. By unpredictable, we simply mean the rules aren't as obvious as for the apple. If we watch the behavior of many ghosts, we see a pattern emerge about their motion. But the individual results of their motion don't seem to follow a straightforward pattern. But you know all that. My point is that we can only see their ghosts. We still can't see the electron at all.

    What about randomness? Random looking results of a quantum experiment are not interesting. The "interpretations" of QM are nonsense philosophy. If we can believe in a universe that computes insoluble differential equations, imagining an immeasurably large source of entropy isn't hard to imagine.

    The curious thing about QM is it's personality. It's like a cheater that never gets caught. The universe doesn't mind doing things that are impossible -- under the condition that you can't prove it did it. An electron as a particle can't go through two holes at once. Except that as long as you aren't "watching", it will do just that! It "cheats" at every possible instance. If you entangle two particles, separate them, and measure the state of the first, guess what? The universe breaks Einstein's commandment and sends information far, far faster than the speed of light. But since the result is random, you can't prove it broke any rules.

    In a sense, it all boils down to artifacts of our mathematical formalism for the subject. It's probably easier to talk about operators and hamiltonians rather than particles and waves because we have fewer preconceived notions over the former. There's a lot of weird stuff that happens in physics, and allowing a particle to exist without putting your finger on where it is exactly is just the tip of the iceberg. Don't get too caught up in how it does it, because we don't yet understand the "big picture". And that's probably something we won't know until we have all the rules figured out.
  9. Jun 11, 2009 #8
    I'm not sure how the poster got from

    yes they are....but quantum reality and classical reality are not identical "realities" anymore than two observers in relative motion experience the same "reality".

    because the later reflects superposition.....it does not negate superpsoition....

    Wikipedia, for example:
    at: http://en.wikipedia.org/wiki/Quantum_superposition#Formal_interpretation
  10. Jun 11, 2009 #9
    Right, then I think I misunderstood. I thought the thread about bohm had stated that superposition could be done away with if the particle in question was considered to have two parts to it.
    Based mainly on these two statements...

    "...While each particle track passes through just one of the slits, the wave passes through both; the interference profile that consequently develops in the wave generates a similar pattern in the particle trajectories guided by the wave..."

    "...All such paradoxes emerge from the assumption that photon is ONE object. In the Bohmian interpretation there are actually two objects (wave function AND particle) and all such paradoxes trivially disappear..."

    I thought this was a counter-concept to duality and superposition. The particle being comprised of two separate components which affect each other (the wave part influencing the path of the particle), rather than one component which exists in multiple states.
    Maybe you can see my reasoning. And maybe they are both saying the same thing and I'm confused.

    I think I need to go back to square one with you if possible. When we speak of reality do we not mean an objective reality that all observers can agree on? I understand some aspects can be tricky and seem subjective. For example the order of events can seem different for two observers moving in relative motion. It may be impossible to say at what point in time and space the event occurred, but it can be agreed upon that the event did take place. The reality of the situation would be that the event happened. This we can all agree on. Pinning it down to a location in 4d space, however, is subjective.

    So in the context of my original question, when I ask is duality and super-position real I'm asking does it exist in objective reality? In way we can all 'see' and agree on and is a definate physical property. Or is it something that is in-itself immeasurable? Simply a model that explains something.
    I understand the scale at which we are working and the natural limitations of working on the smallest scale. If you imagine quantums to be the smallest possible scale, then affecting them by your measurement has to be expected as you are using other things (quantums) of the same scale to do the measuring. I understand that there is an inherent uncertainty in QM. I know your dealing with 'ghosts', or only 'seeing' the particle indirectly, so some aspects cannot be said to be objectively real, as all you see are the by-products. I guess I'm asking are duality and superposition ghosts?

    I understand the dangers of entering the philosophical realm. This is what attracts me to science (and physics in particular) as it treats the world with pure objectivity. I'm hoping that my original question is viewed not as a philosophical one but an objective one.
    I long to know how the world I live in works, not just speculate about it. And am fully prepared for the revelation that it doesn't work in the way that I may perceive. The Big Picture is a very exiting prospect. I have massive respect for anyone who dedicates themselves to understanding these wonders. I'm sure there must be hundreds of people like me who come on PF and ask 'how dose the world work?'. Thanks for taking the time to explain.
  11. Jun 11, 2009 #10


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    Do a search on here for the Delft/Stony Brook SQUID experiments, which I've referred to several times already.

  12. Jun 13, 2009 #11
    Hi, thanks for that. I've had a look at the Delft/Stony Brook SQUID experiments, and whilst I don't really understand the more technical details of it, it seems that they have manged to setup superposition in a measurable fashion, via the path a single electron. Which they have been able to show moving in multiple directions simultaneously and therefor 'prove' the existence of superposition. Or at least they have found compelling evidence that support the existence of superposition. That was really what I was asking, thank you.
  13. Jun 13, 2009 #12
    I'm not expert on reality, but your comments which follow the above staement clearly show "reality" is not something all can agree upon.

    here's an example from Wikipedia:

  14. Jun 13, 2009 #13

    QUOTE]"...All such paradoxes emerge from the assumption that photon is ONE object. In the Bohmian interpretation there are actually two objects (wave function AND particle) and all such paradoxes trivially disappear..."[/QUOTE]

    I don't know where that came from, but it seems to me an over simplification....if that's a trival paradox explanation, I'm Brad Pitt!!

    For example, Wikipedia says:

    at http://en.wikipedia.org/wiki/Wave_function_collapse

    which is consistent with my own (limited ) understanding....and caused my prior post....

    In other words, you can think of a photon, for example, as a particle or a wave...but implicit in the wave explanation are multiple states...superpositions.....conditions not reflected in classical physics.

    That's my story and I'm sticking to it until somebody posts something better!!!!!
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