“Buyin’ it”
Comment
The uncertainty principle can’t be a “law” in the absolute sense, it is a expression of pragmatic limitation.
Indeed measuring something perforce interacts with it, and thus changes the state in ways that invalidate the measuremnt.
But irregradless of our ability to measure both qualities simultaneously a particle at any given instance is moving on some vector, and it is somewhere.
Written by: Anonymous at 2007/12/08 - 02:23:00
Discussion
The Quantum Zeno Experiment (described in the post you responded to) isn’t some thought game to illustrate a paradox. It’s actually been carried out. It’s an illustration of δTδE<
, which is another way of stating the principle, but picturing one form of complementarity doesn’t necessarily help you to picture another. I’ve found lots of experiments which relate to this conjugate pair, and to wave/particle duality, but you seem to have trouble with δVδMV<
(sometimes written as δVδm<
) specifically. The fact that the two are mathematically equivalent doesn’t mean that picturing one helps to picture the other.
When I was in high school I was taught that electrons were tiny hard spheres which travel through space-time, which was a smooth and featureless substrate. The ‘pool ball’ analogy was often used, to try to get students to picture the particles travelling through space like pool balls rolling across a table. This is the situation you’re describing as how things ‘must be’. What happens if you try to picture space, time, and energy as being somehow ‘pixilated’ entitites? Does that get you closer to the concept that an object can ‘jump’ in space or energy level? Or can ‘jump’ between two positions without passing the intervening space? Or that two points which are a measurable distance from each other might be connected by quantum level wormholes?
I can picture it, actually, but I can’t relate it to the scale on which we live. That’s OK, though, because I can’t relate the results of various quantum experiments to the scale on which we live. We’re in good company, though. Einstein couldn’t grok it, either.
EPR Paradox
(paraphrased from a paper at University of California, Riverside)
One problem with demonstrating the complementarity principle with position and momentum is that the two properties are (macroscopically) on a continuum, rather than being discrete values. Spin is a much easier measurement to work with. A particle can have ’spin’ along three different axis, corresponding to X, Y, Z (three spatial directions). In any one of these axis it will be spin + or spin -. The three sets are complementary, in that the Uncertainty principle states that if you know one you do not, cannot, know the other two.
Einstein, together with Boris Podolsky and Nathan Rosen, set up a thought experiment to demonstrate that attributing this to the nature of reality rather than a limitation of measurement led to a paradox.
They imagined a physical system which created two ‘entangled’ photons - that means two particles which must have complementary spins. So, one would imagine, that measuring the X axis spin of one photon and the Z axis spin of the other photon would give you a two axis state of both photons. The uncertainty principle says it won’t, that measuring the state of one of the photons would change the state of the other, even though the photons are moving apart at the speed of light, and information travelling between them would violate the relativity principle. The only logical explanation is that both photons carry complete information about their spin state, and it’s ‘really’ there all the time.
Bell’s Theorm
(also using an article at Wikipedia)
So what happens when you take measurements along random axis of the two different photons? One would expect that they would be 100% correlated for X/X axis measurements, and 0% correlated for X/Y axis measurements.
What if the two axis of measurements were rotated by 45°? Classical physics (and logic,) which assumes that spin values along both axis exist at the time that the photons are emitted predict a different value of correlation than quantum physics does. Unfortunately, the only table I could find which represented the results is at wikipedia, so whether or not it will have changed by the time you read this is uncertain. I’m going to copy it here, attributed to the site linked above:
| Classical model: | highly correlated variables | less correlated variables | ||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Hidden variable for 0° (a): | + | + | + | + | - | - | - | - | + | + | + | + | - | - | - | - |
| Hidden variable for 45° (b): | + | + | + | - | - | - | - | + | + | - | - | - | + | + | + | - |
| Hidden variable for 90° (a’): | + | + | - | - | - | - | + | + | - | + | + | - | + | - | - | + |
| Hidden variable for 135° (b’): | + | - | - | - | - | + | + | + | + | + | - | + | - | + | - | - |
| Correlation score: | ||||||||||||||||
| If measured on a-b, score: | +1 | +1 | +1 | -1 | +1 | +1 | +1 | -1 | +1 | -1 | -1 | -1 | -1 | -1 | -1 | +1 |
| If measured on a’-b, score: | +1 | +1 | -1 | +1 | +1 | +1 | -1 | +1 | -1 | -1 | -1 | +1 | +1 | -1 | -1 | -1 |
| If measured on a’-b’, score: | +1 | -1 | +1 | +1 | +1 | -1 | +1 | +1 | -1 | +1 | -1 | -1 | -1 | -1 | +1 | -1 |
| If measured on a-b’, score: | -1 | +1 | +1 | +1 | -1 | +1 | +1 | +1 | -1 | -1 | +1 | -1 | -1 | +1 | -1 | -1 |
| Expected average score: | +0.5 | +0.5 | +0.5 | +0.5 | +0.5 | +0.5 | +0.5 | +0.5 | -0.5 | -0.5 | -0.5 | -0.5 | -0.5 | -0.5 | -0.5 | -0.5 |
Quantum Mechanical models predict a correlation of 0.71.
Guess which one the experimental data supports? I’m not talking about ‘cold fusion’ here. This isn’t a single experiment, never to be repeated, it’s been done again and again and again.
Future Topics
Strangely enough, I find uncertainty a lot easier to deal with if I look at it through the lense of string (Membraner) ‘theory’ - the concept that there is a real minimum distance or time period because the ‘point’ is actually a tiny, moving squiggle of reality which doesn’t have a precise position is a little easier to deal with, but, as I said before, there is no way to design an experiment to test this… yet.