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Name: Bruce
Status: other
Grade: other
Location: OH
Country: USA
Date: Spring 2013


Question:
In particle physics how are "observed" and "non-observed" behaviors defined experimentally? For example when electrons behave as waves at one time and particles at another.

Replies:
Bruce, To measure something, you have to interact with it. You have to change it. To measure the position of an electron, you must somehow change the electron. If an experiment does not interact with a particle such that a quantity can be measured, then the electron does not have to ?choose? a one specific value for that quantity. The quantity can be a probability distribution of several values. If an experiment does interact, then the particle must select only one quantity. An electron passing through two very close parallel slits is an example. If there is no way to tell which slit the electron passed through, if no interaction forced the electron to choose one slit or the other, then the electron can pass through both. The electron does not have to ?choose?. Now consider a setup that can tell the difference. When the electron passes through the left slit, a needle moves. When the electron passes through the right slit, the needle does not move. The needle cannot move and not move at the same time. The electron must ?choose? one of the two slits.

Dr. Ken Mellendorf Physics Instructor Illinois Central College


Hi Bruce,

Thanks for the question. From an experimental perspective, observed behaviors are some property (usually charge or kinetic energy) that is measured by a detector. From a theoretical perspective: Quantum mechanics only allows for certain quantities such as energy to be an observable. As an example, in quantum theory, the velocity of an electron is not an observable quantity while the kinetic energy is. Whether something is or is not an observable depends upon the nature of the quantum mechanical operator that corresponds to it. A quantity that is an observable is described by a linear, Hermitian operator. That's very technical language, but it is the real definition of an observable quantity in non-relativistic quantum theory.

I hope this helps. Please let me know if you have more questions. Thanks Jeff Grell


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