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Why does expectation value of momentum of a particle in one dimensional box box for state one comes to be zero?
Wiki User
∙ 12y ago
Recall that, in basic quantum mechanics, the "expectation value" of a quantity is the arithmetical mean you would get if you measured that quantity innumerable times. A particle in a one-dimensional box is basically bouncing back and forth within the box, with no change in momentum between bounces. Thus, it is just likely to have momentum in one direction (let's call it "to the left") as the other direction ("to the right"). If you take several measurements of the momentum, half will have a leftward momentum, half will have a rightward momentum -- and the size of all measurements will be equal (no loss of velocity in the bounce). If you sum up all such measurements, the half going left will thus exactly cancel the other half going right. Since the sum is zero, the arithmetic mean is zero, and thus the expectation value is zero.
Wiki User
∙ 12y ago
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