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Is superposition widely accepted?

  1. Sep 13, 2013 #1
    I've been wondering and asking questions about entanglement lately.

    I am very dissatisfied with the answers I've been getting - not necessarily because any of the answers were incorrect, but more likely, because the answers were of the sort which reminded me that i was asking about interpretations, while quantum physics really, or strictly, or maybe only, supplies answers to questions about the probable outcome of measurements.

    So here i go again, with a very basic, conceptual question that may help me to understand my dissatisfaction and confusion.

    My understanding is that prior to measurement, two entangled particles are in a state of superposition. Neither is, for example, spin-up or spin-down. Instead, their state is that they are BOTH spin-up AND spin-down. ONLY after a measurement are they in a coherent state.

    Is that an accepted truth? Or is it a "mere" interpretation of probabilities?

    If that is an accepted truth, that "really" the particles are not (yet) in a coherent state, then it seems that measurement of one "really" affects the state of the other.

    But the answers I get seem to indicate that prior to measurement of the nearby particle, the distant particle is in a definite state, which is revealed by the local measurement. Is that true? Or were both in a state of superposition prior to the first measurement?

    Or is "superposition" an interpretation? Are there any theories which deny that unmeasured particles are in a state of superposition?
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  3. Sep 13, 2013 #2


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    If it were just up and down, you could interpret it as probability. Entanglement goes further, you are free to choose the direction in which you measure the polarization. And that cannot be interpreted as probability any more.

    I guess you mean incoherent here. They are in a coherent state before a measurement is done.
    That depends on the interpretation now.

    That is certainly wrong, unless the distant particle has been measured.

    I think that was your starting point? That you have entangled particles?
    It is possible to have un-entangled photons, but then all those questions are not interesting.

    I don't think so.
  4. Sep 13, 2013 #3
    I think you should forget about "truth" and "really" when it comes to science. That is the realm of religion, opinion and wider philosophy. Science is more about observations, models, predictions and descriptions.

    Note that "superposition" is a basic concept that applies even where entanglement does not. The simple double slit experiment appeals the the superposition of the particle to explain the interference pattern. Superposition is a key component of quantum theory and it absolutely necessary to make the powerful kinds of predictions and descriptions that quantum theory is able to do. In that sense its just as "real" and any other piece of powerful scientific theory. But again, I would caution against thinking about science along the lines of "truth" and "really".
  5. Sep 13, 2013 #4


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    It's pretty much an integral part of the quantum mechanics of spin (it's true of other properties, as well, but it's a little more complicated to show in those cases).

    Spin is relative to a direction. The mathematics of spin works this way:

    If a particle has spin-up in the z-direction, then it is in a superposition of spin-up and spin-down in the x or y direction.
  6. Sep 13, 2013 #5
    OK. Thanks for all of the replies.

    So do i understand correctly that the state of a distant entangled particle is in superposition prior to it's local cousin being measured? And that it is no longer in superposition, but rather, in some definite state after the local measurement?

    And do i need to study the concept of superposition more deeply, to disabuse me of the notion that it refers to some real, physical state of matter? Might my problem stem from a basic misunderstanding of what is meant by superpositon?
  7. Sep 13, 2013 #6


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    If it were representative of specific values, then the nature of an observation on a particle here would not affect the correlation statistics for an observation on an entangled particle there. But short of FTL action, Bell's Theorem tells us that there is not a statistical distribution of values that will match experiment in these cases.
  8. Sep 13, 2013 #7
    I think the biggest misconception about entanglement is that the two particles are in these different states. The idea is that they are both part of the same state, and this is why measuring one forces us to understand the observables of the other. Basically, since both particles are considered part of the same state, measuring one will collapse the state and tell you about the other particle as well. QM doesn't answer any old question that seems reasonable from our very classical understanding of the universe.

    As for superposition: this is much more fundamental than even physics. It comes about from the fact that two solutions to a differential equation may be added to yield yet another solution. This is the principle behind a lot of the math that is used to develop QM and many other branches of physics.
  9. Sep 13, 2013 #8


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    Superposition is built right into the foundations of QM - its about as accepted as you can get.

    However semantics like what you expressed above most certainly are not. In QM saying a quantum system has any property outside a measurement context is not really what the theory is about - its about the results of measurements, observations, etc - not what it is otherwise. Saying it is both spin up and spin down is not correct - it doesn't have properties like that until measured.

    Don't worry too much though - it takes a while to get used to this type of thinking - I have read a LOT of books on QM and thought I knew this stuff pretty well until I posted on this forum - I fell into similar semantic problems saying things like particles are literally in two positions at once. It was wrong, and I needed to be corrected. It takes a little while and practice to think correctly regarding QM.

    Keep at it - slowly but surely whats going on will dawn - as much as its possible to understand what going on in QM is possible anyway.

  10. Sep 13, 2013 #9
    At the risk of being called out for technical inaccuracies, I often hear people question how something can be in two states at once. How can it be both spin up and spin down at the same time? Or a zero and a one, as in quantum computers. I like to point out the wave-particle duality of matter. People are generally accepting of the fact that light can act like either a particle or a wave. They don't have a problem with that. Well electrons and other things can act like waves too. A particle has a definite state, but a wave doesn't. Have you ever seen a wave that was all peaks and no troughs? If it ain't got both, it ain't a wave. A wave by definition is both up and down at the same time.

    So if you can imagine that a particle can at times act like a wave, then perhaps it would be easier to understand, how it can be in two opposite states at once. That's what makes it a wave. With entangled particles, if you measure one particle to be up, the other will always be down. So the idea of superposition isn't so difficult to understand, if you just think of the particle as acting like a wave.

    I realize that I've probably only made things more confusing, and that I'm not really giving the correct explanation of a particle in superposition, but the idea might be helpful to some of us less educated people.
  11. Sep 14, 2013 #10
    Given the superposition state |state>=|O> + |E> (|O> photon on ordinary path, |E> photon on extraordinary path) [the photon has just come out of a birefringent crystal] I state what GianCarlo Ghirardi says on what superposition is:
    You can tell why 2 is true. The photon exists as a potentiality, not an actuality, in both paths. Its not acting as a particle located in some place in the universe. It doesn't exist in the universe as a particle.

    That might clarify the issue.
  12. Sep 14, 2013 #11


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    When a quantum object is LIKE a wave (meaning it is wavelike when expanded in the basis relevant to the context being examined) it is in just as definite a state as when it isn't.

    A state is definite - its expansion in a certain basis is measurement context dependent.

    I know you mention you may not be technically accurate, but it must be said a lot of semantic confusion surrounding this stuff makes matters worse in trying to understand whats going on. Even though its difficult I believe it necessary to be accurate.

  13. Sep 21, 2013 #12

    It is often stated that photons (for example) are both waves and particles. It seems to me that while they sometimes mimic the objects of these concepts, and while they sometimes behave in a manner similar or identical to these objects, they nevertheless cannot possibly be either waves or particles. I say this because in important respects, such objects do NOT behave like a wave or a particle would behave.

    Is it at all useful to conceptualize things this way? That they are NOT waves and that they are NOT particles, but something else altogether? Will this mode of analysis lead to insight or confusion?
  14. Sep 21, 2013 #13
    That's the mainstream interpretation, they they are not waves nor particles but something else entirely that acts a little like each. When you say you often hear that photons are both waves and particles that is probably somebody being ignorant or lazy. You generally wont hear that in scientific circles.
  15. Sep 21, 2013 #14


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    It is not a classical wave or classical particle. I will repeat it again - it is not a wave or particle in the classical sense.

    It is a quantum object - that's it - that's all. If it has the observable property of a definite position then it is considered a particle - but a quantum particle.

    It's a strange but true fact that all of QM is derivable from just two axioms - you can find the detail in Ballentine:

    The so called wave particle duality follows from Schrodinger's equation which follows from the Galilean POR - exactly as classical mechanics is derived from Galilean Relativity and the Principle Of Least Action (PLA). The two axioms of QM imply the PLA, so really non relativistic QM is based on exactly the same fundamental principle as Classical Mechanics. The essence of QM is not the wave particle duality - its the two axioms detailed in Ballentine.

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  16. Sep 21, 2013 #15


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    Check out the FAQ - it comes up often enough to have its own entry:
    'So there is no duality – at least not within quantum mechanics. We still use the “duality” description of light when we try to describe light to laymen because wave and particle are behavior most people are familiar with. However, it doesn't mean that in physics, or in the working of physicists, such a duality has any significance.'

    What we think of as the wave-particle duality (ie that in certain physical situations wavelike solutions naturally occur) in fact follows from more fundamental principles that lie at the foundation of QM.

    As mentioned previously you will find the detail in Ballentine.

    I also want to point out its easy to be confused by this stuff - even people who have taken undergraduate courses in QM from books like Griffiths (and it is a good book - just a bit pricey) get confused. What's really going on is found in more advanced books like Ballentine - and even then you have to think a bit - he doesn't spell it all out - but that's the way with more advanced textbooks in any subject - they assume - whats the word used - maturity :tongue::tongue::tongue::tongue:. Translation - you need to persevere and think.

  17. Sep 22, 2013 #16


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    One cannot stress enough that particularly photons are a very bad example to start to learn quantum mechanics, because as a massless quantum with spin 1, it's more complicated than massive quanta. There is not even a position operator for photons in the strict sense!

    "Wave-particle duality" is an old-fashioned idea which is obsolete for nearly 90 years now and should not be taught anymore. Unfortunately, many textbooks start with photons in an old-fashioned inadequate way and with "wave-particle duality" for electrons. This then sticks with the students's minds and one has a hard time to forget this wrong ideas again, when learning about modern quantum theory later.
  18. Sep 22, 2013 #17


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    But guess what - that's the way its taught in the first brush of typical HS courses on it. That was my first exposure. Then I read this book called - In Search Of Schrodinger's Cat. It raised more questions than it answered but said Dirac and Von Neumann was the books to answer them. Right - good books for sure - and I learned a lot - but for me, having a math rather than physics background, it raised all sorts of issues like what the hell is this Dirac Delta function and why the hell weren't physicists using Von Neumann's approach which sent me down the road of Rigged Hilbert spaces and what not. I emerged the other side after sorting it out and then came across Ballentine and was impressed - very impressed. Everything was clear. But its a graduate text.

    IMHO what we need is a undergrad and even high school version of Ballentine - anyone want to step up to the plate?

  19. Sep 22, 2013 #18
    some really good insights already posted!

    Is that an accepted truth? Or is it a "mere" interpretation of probabilities?[/QUOTE]

    it's a 'mere' interpretation of the math; what is real is what we measure....the rest is math, our best model of what we think underlies the measurement. We measure particles; the underlying fields and virtual particles, for example, are mathematical theory.

    The famous quote still reigns: "SHUT UP AND CALCULATE' .....We may disagree on what the all the math means, but we can arrive at the proper probabilistic quantum answer....

    Quantum entanglement refers to the mathematics of QM. What it 'means' remains a mystery so far without complete explanation. The math explains what we observe, not precisely why.
  20. Sep 28, 2013 #19

    Is anyone besides me less enamored with uantum physics the more one learns about it?

    I read secondary sources and short papers by the originators, and I get fascinated. I think of the implications and mysteries revealed by the experiments. I put two and two together and search for understanding.

    And then, when I get to the crux of the matter, people who study this stuff and who have mastered it tell me that nobody has a clue as to what is going on, and that the entire discipline is centered solely on calculating numbers with no real understanding of any underlying mechanisms and no real insight into what any of it means.

    My interest in physics stems from wondering about the nature of reality - both our perceived consensual reality and any underlying reality that escapes our senses.

    But with uantum physics, it seems, those who are in the trenches care little about, and are uick to point out that they know nothing about, any underlying reality that escapes our measurements.

    I dunno. I'm increasingly disappointed. Thanks for listening.
  21. Sep 28, 2013 #20


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    Dude, get that "q" key fixed :smile:

    My perspective, which may be somewhat influenced by a side interest in the history of science:
    Being quick to point out that we don't understand the underlying reality is, I think, well-placed humility. It signifies not that people don't care about it but rather that the people who know the most know how much we still don't know. Indeed, I view the enormous volume of work on interpretations of quantum mechanics, produced by some of the top thinkers of the past century, as very compelling evidence that people care, a lot.

    It's true that when we're using QM as a predictive tool in day-to-day work, we tend to be a bit impatient with discussions of interpretations. But again I don't see this as evidence of disinterest in the problem so much as disinterest in the conversation when no one has anything new to say. After all, when someone does come up with a major insight so there is something new to say, attention and excitement is quickly rekindled. We saw this happen when the Copenhagen interpretation was first solidified, when Bell discovered his theorem (and stimulated a wave of experimentation), when decoherence was discovered. There's no reason to expect that we're at the end of that road.
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