What Are We Really Measuring in Quantum Mechanics?

[friend]

In quantum mechanics what quantities are actually measured? Measurements of voltage and current are macroscopic measurements of accumulations, right? But when you measure scintillations or spots on a screen that's a point measurement of a particle. I don't suppose you actually measure energy, for example, but derive it from other quantities, right?

 

[bhobba]

There are devices that measure energy directly.

There was once this idea that everything boiled down to positions of a pointer and distance was the fundamental thing but digital technology put that to rest.

Simply accept measurements exist that measure fundamental things like position, energy, momentum etc and don't worry about what it boils down to as far as how such devices operate - that's a dead end. The deep answer, as far as what it means, if its really important to you has to do with symmetry and Noethers Theorem - but that's not required for a first brush with QM.

Thanks
Bill

 

[Fredrik]

I think that the only thing a measuring device can really do is tell you that an interaction has taken place. This information is then interpreted as a result of a measurement of some observable. Since you can always infer a position from the location of the relevant parts of the measuring device, I think that every measurement must at least approximately determine the position.

 

[Bill_K]

I don't suppose you actually measure energy, for example, but derive it from other quantities, right?

Experiments at the LHC, for example, have calorimeters which are used to measure particle energies.

"Electromagnetic calorimeters measure the energy of electrons and photons as they interact with the electrically charged particles in matter. Hadronic calorimeters sample the energy of hadrons (particles containing quarks, such as protons and neutrons) as they interact with atomic nuclei. Calorimeters can stop most known particles except muons and neutrinos."

Here's an overview.

 

[friend]

I think that the only thing a measuring device can really do is tell you that an interaction has taken place. This information is then interpreted as a result of a measurement of some observable. Since you can always infer a position from the location of the relevant parts of the measuring device, I think that every measurement must at least approximately determine the position.

I like the way you think. There at least has to be an interaction before you could measure anything. Then what? Is it that macrosopic indicators give signals proportional to how those single interactions propagate throughout macroscopic materials? So we are really measuring macroscopic effects and inferring what microscopic interaction must have caused them?