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Do particles possess properties before we observe them?

  1. Aug 3, 2010 #1
    My question is about which properties a particle can be said to possess. I understand that in accordance with Heisenberg's Uncertainty Principle, one can never know precisely a particle's location and its momentum at a single moment in time. I'm wondering whether it can be said that:

    A. The particle has a location, and it's just that we can't know it
    B. That it certainly does not have a specific location
    C. Or that we don't know if it does or if it doesn't
    D. Or that it is inherently impossible for such a thing to be known (which brings up a host of philisophical and symantic questions of its own)

    To try to answer my own query: Am I wrong in thinking that Bell's Inequality proves that particles truely do not have specific properties until we observe them? Because if they did, then Bell's inequality would not be violated and it would be obvious that the properties were there all along, and merely unbeknownst to us at the time?

    Am I on the right track here? Am I confusing properties like spin with ones like location and momentum? Am I lost completly? :redface: Any clarification is much appreciated.

    Thanks for your time
  2. jcsd
  3. Aug 3, 2010 #2


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    A particle can certainly have a specific property, provided the corresponding quantum state has been prepared.

    The Heisenberg uncertainty relation is about two mutually "incompatible sharp properties", Bell's theorem is about properties of entangled particles.
  4. Aug 3, 2010 #3
    If every observer in the universe suddenly ceased to exist, I would venture to guess that all of physical nature would continue to function just fine.
  5. Aug 3, 2010 #4
    A universe w/o observers is a universe w/o matter(atoms)...
    or should I say, correct me on this?
  6. Aug 3, 2010 #5
    "Observers" in this context of my post is with regards to humans or other real/potential observing "animals"; terrestrial or not.
  7. Aug 3, 2010 #6
    And prior to that?..

    Thanks. Yeah I see now how I'm conflating a couple things there. I really appreciate you sticking with me enough to reply, as I'm not used to talking about these things. So momentum and position are just a different kind of property from atomic spin? I guess I was trying to draw a (false?) analogy between different limits on our knowledge about the world. Somewhere in there was the thought that the act of observation (e.g. reflecting photon) was at the root of the problem in both cases.

    So taking the whole bit about momentum and location out of it, at least with orientation of spin, can't we say (once again because of Bell's Inequality) that it certainly does not exist before it is observed? The only way would be some hidden-variable theory revelation?

    This point is well taken. I always try to steer clear of theories that posit a special, exalted place for human beings.
  8. Aug 3, 2010 #7
    Unmeasured things are like waves and have wave properties...
    Measured things are like particles and have particle properties.

    Properties of waves include interference, reflection and refraction
    Properties of particles include interaction, position and momentum.

    Help, I've hit a brick wall.....
  9. Aug 3, 2010 #8

    George Jones

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    What about the Kochen-Specker theorem?

    Chris Isham, in his wonderful little book "Quantum Theory: Mathematical and Structural Foundations," writes:

    "The central position of standard quantum theory is not that a quantity like A has a value which we happen not to know, but rather that, in a typical quantum state, it is not meaningful to say that A possesses any value at all."

    Isham agrees with this specific type of scenario, and writes

    "a situation in which it arguable is meaningful to say that a system 'possesses' this value for A."

    I think that the eloheim is asking about the more general situation of "a typical quantum state."
  10. Aug 3, 2010 #9
    Thanks, I may have a look into the book you mention. Also I looked up the Kochen-Specker theorem, and I see it relates to arguments against hidden variable theories, but it takes me time to digest this stuff.

    Yeah that sounds like what I was trying to say. It's funny how vocabulary/jargon not only allow communication, but help one to actually think as well.

    I should have made clear earlier, my invocation of bell's inequality was in order to use one correlated particle as a control for the other.
  11. Aug 4, 2010 #10
    It is impossible to (dis)prove any of these statements in experiments. So, you can choose whichever answer you like most. It just doesn't matter for physics.

  12. Aug 4, 2010 #11


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    The correct answer is C. There is a theory (Bohmian mechanics) showing that A is also a possibility, but we don't know if this theory is actually realized in nature.

    You are wrong. It is possible that particles have such properties, but then these properties must change during a measurement in a nonlocal way. For more details see e.g.
  13. Aug 4, 2010 #12
    Fully agree...

    If particles didn't have properties then how can they perform certain function? And yes the property of particles changes not only during the measurement but also during non-measurements. Just because you are not measuring a particle, it doesn't mean it's doing absolutely nothing.
  14. Aug 4, 2010 #13


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    Great questions, Jeff, and there are no easy answers. I would say that your A. is ruled out if you are speaking in classical terms, as Einstein would have thought of things for example. That being that the moon is there when you are not looking at it, and there is no spooky action at a distance. Past that, you take the next leap yourself. Bell's Theorem (and others like K-S and GHZ) show that particles do not simulataneously possess properties as fully independent entitites.
  15. Aug 4, 2010 #14


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    There's another alternative here, which is actually the more 'mainstream' opinion. Which is the philosophical/ontological shift that's taken place on what the fundamental properties are. The classical view is that position and momentum are basic properties of a particle. Thus, the wave function is just some abstract object for calculating these properties. The more modern view is that the wave function itself is the fundamental property of the system, and classical properties such as position and momentum emerge as you go towards the classical limit.

    It's not just philosophical, but practical. In practice, the concept of an electron having a definite position within an atom or molecule is meaningless, since the uncertainty principle dictates that any attempt to determine that position to a meaningful level of accuracy (say 1 Å) would promptly impart so much momentum on the electron that it'd be kicked out of the atom. So the classical-mechanical problem of determining a electron's position x at time t is, from the practical standpoint, not very meaningful even if it could be determined. The state of electrons in an atom are instead viewed in terms of the possible states of their wave functions, aka 'orbitals'. Since this corresponds more-or-less directly to the charge density around the atom, this is a more meaningful observable quantity.

    Since the advent of QM, the theory of chemistry went from a set of fairly ad-hoc rules, to a rigorous physical theory with models based on the orbital picture, not in terms of electron position/momentum. Orbitals in chemistry are about as fundamental as the concept of species in Biology (and about equally debated). Orbitals have given us more insight than any picture involving definite 'orbits' ever did, and likely ever will. Because if you accept, for instance Bohmian mechanics, and calculate the corresponding trajectories or 'orbits' of the electrons, they end up with the same general shape as the orbitals. Which is interesting because it's not something you'd necessarily expect, but it also means you don't gain much additional insight, and mathematically, things get quite complicated. As opposed to the opinion of Bohmian enthusiasts, I actually find the consequences of that picture much weirder as well. (For instance, the electrons in some orbitals will be completely stationary, and in others, they move)

    This isn't to say the questions you raise aren't important. (In fact, if you look at this particular message board, you'd think it was the most important thing in the world, as it's constantly being discussed) They'll continue to be important for those who study theoretical physics and the foundations of physics. But as I hope I've conveyed, the practical benefits of answering these questions (if we can) is actually fairly limited. As soon as you get to applied QM, we're pretty comfortable just working with wave functions, and will most likely continue to do so, even if it turns out there's another layer to the onion, so to speak.
    Last edited: Aug 4, 2010
  16. Aug 4, 2010 #15


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    Yep, that's indeed the solution of all quibbles with quantum theory! Not a point in phase space defines the state of a point particle but the quantum-mechanical state, i.e., the Statistical Operator (or in case of preparation in a pure state, equivalently to the Stat. Op. the corresponding ray in Hilbert space). This has nothing to do with "philosophy" but just with physics! That's precisely what quantum theory teaches us to understand under the state of a system.

    The relation to observables is however through Born's probabilistic interpretation of the state, and that makes the trouble for our mind, which is used to classically behaving macroscopic systems. This classical behavior, however, is an emergent phenomenon and explained by quantum mechanics through the mechanism of environment-induced decoherence.
  17. Aug 4, 2010 #16
    Thanks everyone for such detailed replies. I now have plenty to think about!

    I actually have Bohm's "Wholeness and the Implicit Order", but I read it a while ago and if I remember correctly I was reduced to more skimming than I'd like to admit.

    This I'll have to think about. (And thanks for supplying information to support your point.)

    Yeah it's difficult to change your thinking on such a fundamental level. I'd love to see how a society of truly "native thinkers," a couple generations in, would see these things...

    The alternative sure does sound like that whole "I'm special" thing again. Although, I suppose, decoherence might put the question in a more reasonable light(?).

    This is getting toward the roots of my pondering. When writing the original post, I thought, "Where does the wave aspect fit into all of this?," and ended up dismissing it as 'just' a guide to the odds of observing a particle in this or that state. So you're right to point out my implicit, "the particle is fundamental", perspective (which I find interesting, considering I've generally found the opposite approach to be much more agreeable).

    You know what? I think the above is what I was trying to get after in the first place! This thread is helping me greatly to organize my thoughts on these matters...

  18. Aug 4, 2010 #17

    George Jones

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    But doesn't decoherence only take us so far, to a statistical mixture of orthogonal classical states. Doesn't going further, to the single classical state that we experience, require further interpretational baggage. See


  19. Aug 5, 2010 #18


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    I agree. Decoherence explains why we see what we see (namely classical states instead of weird quantum superpositions), but it does NOT explain why we see a specific classical state and why exactly THIS specific state has been "selected by the measurement" - or however we call the process of opening Schrödinger's box.
  20. Aug 5, 2010 #19
    I think there may be some confusion in this thread between the question of whether or not a particle has any properties when not observed, and the one of whether or not it has any certain value of a given property, although I may be making a false distinction here. The latter of the two cases would fit better with the idea of "superposition", like where the particle possesses both values at once. Then again it seems like one could ask those same epistemological/ontological questions about states of superposition as well. And now I feel like I'm talking myself in circles...
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