What is the Correct Value of the Square of a Momentum Operator?

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Discussion Overview

The discussion revolves around the correct value of the square of the momentum operator in quantum mechanics, specifically examining the relationship between the momentum operator and kinetic energy in the Hamiltonian. The scope includes mathematical reasoning and conceptual clarification regarding operator algebra.

Discussion Character

  • Technical explanation, Mathematical reasoning, Debate/contested

Main Points Raised

  • One participant notes that squaring the momentum operator p = -iħ d/dx leads to a term that seems inconsistent with the expected form of the kinetic energy in the Hamiltonian.
  • Another participant attempts to clarify the algebra of squaring the imaginary unit, stating that (-i) squared results in +1, which is a point of contention.
  • A different participant asserts that the kinetic energy can be expressed as K = (p^2)/(2m) and provides a calculation that leads to K = (-ħ^2/2m)(d^2/dx^2), suggesting a connection between the squared momentum operator and kinetic energy.
  • One participant reiterates the algebraic properties of the imaginary unit, emphasizing that both i*i and -i*-i yield -1, which is relevant to the discussion of the momentum operator.

Areas of Agreement / Disagreement

Participants express differing views on the algebraic manipulation of the momentum operator and its implications for kinetic energy. There is no consensus on the correct interpretation of the squared momentum operator.

Contextual Notes

Participants have not fully resolved the assumptions underlying the algebraic steps, and there are unresolved questions regarding the definitions and interpretations of the momentum operator in the context of quantum mechanics.

Who May Find This Useful

Readers interested in quantum mechanics, operator algebra, and the mathematical foundations of kinetic energy in the Hamiltonian may find this discussion relevant.

hnicholls
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know I'm missing something obvious.

for a momentum operator p = -iħ d/dx

if I square the -iħ part I get (+1)ħ2

but I believe the correct value (as in the kinetic energy of the Hamiltonian) is

-ħ/2m d2/dx2.

how is the value of the term -ħ/2m where the square of -i = +1?

Thanks!
 
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i * i = -1 and so -i * -i = -1 * i * -1 * i = 1 * i * i = i * i = -1
 
Kinetic energy is given by
K = (1/2)*m*v^2 = (p^2)/(2m).
(-ih)^2 is actually equal to -h^2, because (-i)*(-i) = -1.
Hence, K = (p^2)/2m = (-h^2/2m)*(d^2/dx^2)
 
jedishrfu said:
i * i = -1 and so -i * -i = -1 * i * -1 * i = 1 * i * i = i * i = -1

this suggests, i x i = -1 and -i x -i = -1 x i x -1 x i = 1 x -1 = -1

right?
 

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