Problem
In a double-slit experiment, electrons are sent one at a time. The final screen still shows the interference pattern. What happens if a detector is placed near the slits to “see” which slit the electron passes through?
Solution
Without a detector. Even sending one electron at a time, after many electrons the interference pattern emerges on the screen. Each electron passes through “both” slits as a probability wave and interferes with itself: this is the heart of wave-particle duality.
With the detector (path measurement). If a detector establishes which slit the electron passes through, the interference pattern disappears: two simple blobs appear on the screen (the sum of the two slits, as with classical particles).
Why. Determining the path means measuring the system: this destroys the coherence/superposition between the two paths (collapse of the state). You cannot have both the “which way” information and the interference pattern at the same time — this is Bohr’s complementarity, closely tied to the uncertainty principle: measuring the position at the slit introduces an uncertainty in the momentum that washes out the fringes.
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Topics: Quantum physics Concepts: Wave-particle duality · Interference · Uncertainty principle