The Observer Problem

Here's where quantum mechanics gets genuinely unsettling. Place a detector at the slits to see which one each electron passes through. The interference pattern vanishes.

The act of measurement changes the outcome. The loss of interference is tied to the availability of which-path information, not just to using a clumsy detector. Whether and how momentum disturbance enters the story is subtle, which is why wavefunction collapse remains one of the deepest puzzles in science.