Postby **Psnarf** » Wed Jan 09, 2013 2:50 pm

Thank you, Dr. Nemiroff, for lifting the veil of uncertainty regarding Heisenberg's Uncertanty Principal! It is all too easy to stumble upon someone invoking the HUP for all manner of twaddle, even to the point of posting random brain detritus as fact. On the youtube page for your lecture someone waxes eloquently upon the subject without having read Dr. Feynberg's easy-to-remember point d'appui. The uncertainty is in our ability to predict future events, not measure position and momentum at the present. Dr. Feynman explains in his QED lecture that one of Maxwell's Equations is true only for the case of one photon and the result of the calculation is the probability of finding a photon at a particular place; you can't apply that equation to the case of more than one photon and any interpretation of the result beyond the probability of finding one photon is incorrect. Thank you for explaining that you can indeed measure both position and momentum for an event that already happened.

Comments like if one person measures the position and another measures the momentum, then they tell each other their results and thereby destroy the universe are preposterous. The uncertainty is in predicting future events, not measuring events in the present or past. That fact helped me to better understand probability vectors and why you have to add up all of the probability vectors for each possible way a photon can get from point A to point B. All of the vectors at the extremes cancel each other out and we are left with the results that are the pretty close to the measurements of laser reflection in undergrad physics labs (angle of incidence vs reflection and all).

--

Obquote:

The shades of night were falling fast,

As through an Alpine village passed

A youth, who bore, 'mid snow and ice,

A banner with the strange device,

"The uncertainty relation refers to the predictability of a situation, not remarks about the past!" -Dr. Richard Feynman, apologies to Henry Wadsworth Longfellow.