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There's a famous experiment displaying Heisenberg's Uncertainty Principle

https://www.youtube.com/watch?v=KT7xJ0tjB4A

Where a beam of light is subject to increasingly narrow slit (constraining its position), eventually causing it's momentum to be increasingly unclear (to maintain the inequality) spreading it out.

If I wanted to do the inverse of this experiment, where I have particles, I constrain the particles' momentum (somehow), and see that their positions are increasingly undefined, how would I do that?


One idea:

If have a light beam originating from a source A to a target strip B,

the strip B is made increasingly small, so that if you spot a photon that landed on B, you would know the momentum to an increasing degree, thus the location of the photon at the source A should be increasingly unclear.

Now the question is how to measure this?

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Momentum measurement is performed by observing the radius of curvature in a magnetic field.

As a practical matter many configuration require at least one collimator, but if you have spatially dense detection (say from a TPC, or a high resolution solid detector system) then you can even do without that.


Fun fact: the formula you learned in the intro class $$ r_c = \frac{p}{qB} \;,$$ remains correct for relativistic particles provided you use the momentum in the numerator rather than $mv$ (which—sadly—is the way many intro texts express it; tsk tsk).

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