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# What is Heisenberg's uncertainty principle?

Question #112153. Asked by star_gazer. (Jan 17 10 12:30 AM)

sameer638

The Heisenberg uncertainty principle states both position and momentum of an electron cannot be determined accurately.

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

 Jan 17 10, 2:13 AM
looney_tunes

Actually, as stated in the link above, the principle applies to everything, not just electrons. Because the amount of uncertainty in the two quantities is so small, it is only a noticeable issue for small particles (or wave/particle entities) such as the electron, positron, photon, proton, neutron, etc.

 Jan 17 10, 2:50 AM
star_gazer

The more precisely the position is determined, the less precisely the momentum is known in this instant, and vice versa.
--Heisenberg, uncertainty paper, 1927

http://www.aip.org/history/heisenberg/p08.htm

 Jan 17 10, 2:56 AM
Baloo55th

Baloo isn't sure. But seriously, you can't use the Principle as an excuse for speeding. You can't say, "I was absolutely sure where I was, so I couldn't know my speed.".

 Jan 17 10, 6:49 AM
looney_tunes

I'm not sure in what sense you could say that you were absolutely sure where you were - even before we talk measurement errors, we need to decide exactly what point of the car is 'where you were'. If you can decide that, and measure the position to an accuracy of a micrometre (1/1000 of a millimeter), then the uncertainty of the speed measurement at the relevant instant would be 5 x 10-29 m/s. I am pretty sure that a speed of 80 km/hr isn't going to be under the 60 speed limit because of the uncertainty principle!

 Jan 17 10, 1:13 PM
looney_tunes

For the above: Heisenberg's Uncertainty principle states that (sorry about the words, but the neat symbols are too hard to get up!) the uncertainty in the measurement of an objects momentum multiplied by the uncertainty in the measurement of its position is always at least as great as Plank's constant divided by 4*pi.

And I just realised that I forgot to change the calculated uncertainty from momentum to velocity correctly - it is not valid to assume a car mass of 1 kg! Let's make it a big car, a 1-tonner, and we get a speed uncertainty of roughly 2.5 x 10^-34 m/s.

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

 Jan 17 10, 1:18 PM

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