# Measurement in Quantum Mechanics

*Most recent answer: 07/10/2019*

Q:

In copenhagen interpretaion of quantum mechanics , i know that position of a particle doesn't exist until measurement,but what about momentum and energy,does momentum(or energy) of a particle exist before me make measurement? or particle doesn't have a definite energy before measurement

- sai kiran alvala (age 23)

hyderebad,telangana,india

- sai kiran alvala (age 23)

hyderebad,telangana,india

A:

Every real quantum state has a spread of both position and momentum. The math describing each of these variables is essentially the same. A measurement can result in a state with much less spread in either poition or momentum (not both!), depending on what is measured.

Energy is somewhat different because there are (at least at the usual level of description) some states with definite energy. If the initial state is already one of those "energy eigenstates", then measuring energy won't change it.

Mike W.

*(published on 07/10/2019)*