Uncertainty About Uncertainty Principle

Most recent answer: 9/3/2015

Q:
Hi. My question seems to me the most obvious question about the uncertainty principle, yet I have yet to find a direct answer. Perhaps I am missing something obvious. What I wish to know is, does the uncertainty principle of quantum mechanics exist because - a) Every known (and perhaps possible) way of measuring particles interfers with the particle and therefore changes its position/momentum (such as being hit by a photon used to measure it). b) Fundametally a particle does not possess both absolute position and momentum at a point in "time", regardless of being measured or not. c) A particle DOES possess both position and momentum at a point in time, but somehow "knows" if it is being measured even if that measurement does not physically change the particle’s position or momentum. d) None of the above. e) No one knows. Thanks.
- Brian (age 28)
Australia
A:
Brian- That’s a really nice question. All of the answers you suggest have been advocated at some time or other by some serious physicists. The interpretation of quantum mechanics has not yet fully settled down, but we can at least now eliminate some of the possibilities.

In most reasonable interpretations, the particle does not have completely sharply defined values of position and momentum, ever. After a measurement, one or the other can be pretty sharply defined, but not both.

It’s tempting to think that all the standard physical variables actually have definite values, but that somehow we just don’t know them or that they are somehow perturbed by the measurement. However, experiments following an idea by John Bell have shown that if there are any ’hidden variables’ determining the seemingly random outcomes of quantum experiments, those variables are not local- i.e. they don’t exist in any particular position but rather all over the place at any time! These days almost all of us who worry about the interpretation of quantum mechanics, no matter how much we argue with each other, believe that your answer "b" is closest to the truth.

If you were to broaden your question to something like "What actually goes on in a ’measurement’?" I think the closest answer would be "e".

Mike W.

(published on 10/22/2007)

Follow-Up #1: Bell Inequalities

Q:
My original question, which I believe is very similar to a previously-answered question on this site is as follows: Is the claim that certain particles can "exist" in two or more places at once to be taken at face value, or is this idea merely a concept that expresses our limited ability to unobtrusively measure quantum particles? Put another way, is there evidence to suggest that these particles behave in such a strange way as to occupy two points in space simultaneously, or do we just say this because our best methods for measuring these particles without altering location or momentum are imperfect.Taken at face value, quantum behavior seems magical and exciting. However, as I read more about our limited measuring ability, the "magic" of existing two places at once began to feel like mere uncertainty.
- Zachary (age 29)
Lansing, Michigan, USA
A:

You can search our site for descriptions of how nature violates the Bell Inequalities. These results are inconsistent with anything like having a unique true but hard to measure value of those uncertain variables. About the closest one can come to that is a recent idea about a swarm of different nearly identical universes, with all the particles in each interacting with all those in the others. Then you could sort of say that in any one universe there was a definite value. But I don't think that's the sort of familiar answer you were seeking.

Mike W.


(published on 09/03/2015)