The Double-Slit Experiment

Light — or electrons, or whole molecules — leaves the source on the left, passes through two narrow openings in the barrier, and piles up on the screen at the right.

Classically you would expect two bands, one behind each slit. Instead you get stripes: bright, dark, bright, dark, spreading out across the whole screen. Stripes are what waves do. Where the two paths arrive in step they add and you get a bright fringe; where they arrive out of step they cancel and you get nothing at all. Widen the slit separation and the stripes crowd together; stretch the wavelength and they fan apart.

Turn on Fire one particle at a time and watch. Now the source releases a single quantum, waits, and releases another. Each one arrives as a single dot in a spot you cannot predict — the individual impacts look like pure noise.

Keep going. After a few hundred dots the stripes appear anyway, exactly where the wave picture said they would. The pattern was never a property of the crowd. Each particle interferes with itself: it goes through both slits as a spread-out amplitude, and there is nothing else in the apparatus for it to interfere with.

Now switch on Watch which slit. Put a detector at each opening so you always know which one the particle used. The stripes vanish. What is left is two plain overlapping blobs — the classical answer, the one that was wrong a moment ago.

The usual story is that the detector nudges the particle off course. That is not it. The real reason is that once which-path information exists anywhere in the world, the two routes are no longer alternatives that add as amplitudes; they are distinct outcomes whose probabilities simply add. Interference needs the paths to be indistinguishable in principle. Erase the information and the fringes come back.

This is the whole mystery of quantum mechanics in one picture: what exists depends on what can be known.