Ask HN: Could someone explain the Heisenberg uncertainty principle to me? | Hacker News Reader

Ask HN: Could someone explain the Heisenberg uncertainty principle to me?

2 pointsXcelerate4 comments

I've taken quantum chemistry. Everyone in the class can rote memorize the formulas and their derivations and write down the expected answers on the test. I know how to derive [r, p] = (i)(h_bar) from the postulates of QM. But I still have no clue what this means experimentally.

If someone tells me you can't measure the position and momentum of a bowling ball accurately at the same time, I'll say "yeah right" and drop it on the floor. Its position is the center of the indentation and the momentum can be determined by its depth.

But if someone says "you can't do it at the particle level", well, why not? I've got a "position probe". It reads out the value 0.2535230598235... When it shows one more decimal place, the "momentum probe" explodes to prevent me from knowing its value precisely. Sorry for the hyperbole, but I've been grappling with this for a while (and even asked some physicists on Stack Exchange) and have still been unable to understand this.

It seems you're laboring under a misapprehension -- that quantum effects can be understood in the same way that a pendulum, or a planetary orbit, can be understood. Quantum effects can be modeled and predicted very precisely, but they're not open to everyday comprehension like macroscopic physics. Niels Bohr once said, "Anyone who is not shocked by quantum theory has not understood it."

Heisenberg's Uncertainty Principle can be summarized by saying that there is an irreducible level of uncertainty in measuring a particle's position and momentum -- if you increase the momentum measurement accuracy, you sacrifice the position measurement accuracy, and vice versa. To put this in the simplest terms, if you multiply momentum and position accuracy together, the product of the two accuracies cannot be smaller (more precise) than a certain well-defined lower threshold.

You can force a greater accuracy of either of the two possible measurements, but only at the expense of the other. In the final analysis, if you measure, say, position with an arbitrarily high degree of accuracy, you sacrifice the ability to measure momentum at all. And vice versa.

It's important to say that this fact about measurement at the microscale affects macroscopic measurements as well, but with a progressively lower probability of affecting events, as size and mass increase. To say this in the simplest way, quantum effects are present at all scales, but with a decreased probability of influence as sizes and momenta increase.

http://en.wikipedia.org/wiki/Uncertainty_principle

This isn't a 100% explanation but it's how one of my professors explained it as a way to get comfortable with it: let's measure an electron and assume said electron has a well defined position and momentum that we just don't know. Let's say I want to know the position. How am I going to do that? Well, by throwing a photon at it. If the wavelength is small we can find the position fairly accurately but it's going to transfer momentum to the electron and we really don't know how much and it's a relatively major effect. Hmph. Alright, let's measure the momentum instead. We need a longer wavelength so we don't disturb the electron too much. Great, we get a very good value for the momentum! But, unfortunately, the long wavelength is not very accurate for determining the position.

Other intrinsic problems arise even sticking with the photon idea but hopefully this is enough of something to latch onto to help you start to understand.

A lot of people seem to think that according to the uncertainty principle it is impossible to measure the momentum and position of a particle together accurately since the more you try to measure one, the more difficult it becomes to try to measure the other.

This is wrong.I used to think this too until I came upon this quora answer.

http://www.quora.com/How-would-the-uncertainty-principle-be-...

Basically accurate information about the momentum and position of a particle does not exist fundamentally.Observational interference has nothing to do with it.

(http://www.scientificamerican.com/article.cfm?id=heisenbergs...)

(http://theconversation.edu.au/explainer-heisenbergs-uncertai...)

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