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Originally Posted by Len Moran
As I understand it, atomic particles, unlike macro objects, are not said to possess attributes independently of the experimental set up, so I was wondering in what context you say momentum and position are "present".
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I don't mean present in some transcendent way, I simply mean that they are available parameters to be teased out by the appropriate experiment. Your understanding is quite right.
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It does seem to me that there is no momentum or position of a particle outside of the experimental setup (it is weakly objective in nature), unlike, for example, a traveling ball where we can say the measured momentum is a strongly objective quantity - we do not have to measure it in order to ascribe a momentum to it.
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I would not quite say there's
no momentum or position, I'd say they don't have precise values. What you generally do is prepare the particle in a given state, which means it has a wave function. The wave function embodies the uncertainty-- there are now possible ranges to any momentum or position measurement. So before you do such an experiment, you have some fuzzy concept of
both position and momentum, but you can't make either one precise without messing up the other.
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In terms of quantum mechanics, I would say the complementarity aspect does not give two strongly objective views, each view is weakly objective, consisting holistically of the object and the experimental set up which always carries the notion of an observer.
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Yes, this piece also comes in when you "prepare" the particle, which is tantamount to starting off with one measurement and later doing another. That's what physics really does, it connects the results of two measurements, on we call the "initial state" and the other we call the "final state".
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And whatever attributes we assign in this measurement process, those same attributes do not exist independently of this setup.
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Right, there is no definition outside the setup-- the devices that prepare the state and those that measure it.
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So I think that in terms of quantum mechanics, complementarity quite importantly refers to a notion of underlying reality that can never be accessed objectively.
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I'd say it refers to a notion of the limits one invokes automatically when one seeks objectivity. There is no precise notion there of something that cannot be accessed objectively, because we have no means to describe such a notion. That's the real point, I'd say, that we cannot even talk about anything that is deeper than complementarity-- words fail us, because we base the words on objectivity.
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Hence the use of the term complementarity differs I think quite importantly between quantum mechanics and biology for the reasons suggested, but having said that, I have read that Bohr never gave any explicit definition of what he meant by complementarity, and the OP quote does refer specifically to Bohr as well as quantum mechanics, but I would say that the nature of quantum mechanics is such that the description I have outlined of weak objectivity follows naturally from experiment.
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One can only guess what Bohr meant, but it seems the Dyson thinks he might have been talking about a complementarity between what is objectively describable by prescribing the location (say, form) of a bunch of molecules, and how they are dynamically interacting (say, function). To study their interaction is to study their motion, but when considered to be in motion, they no longer have a clearly defined spatial configuration at the quantum scale. I think it is interesting that we do not use quantum thinking to describe living systems, because many of the processes occur on larger scales (the atomic scale is normally treated as an "interaction" only). But if the key to the process is indeed happening on that interaction scale, he may have a point. Certainly there are molecules with a certain "shape" because it is convenient for quantum effects like tunneling, yet if one is considering tunneling, one is accepting that no system configuration is being precisely specified. That may be along the lines of his thinking, I don't know-- we normally just think of the protons classically and the electrons quantum mechanically.