The double slit: the experiment that broke intuition

Take a light source so faint it releases one photon at a time. In front of it, a barrier with two narrow slits; behind it, a screen. Each photon leaves a single dot on the screen — a point hit, like a bullet. And yet after thousands of hits the dots arrange themselves into fringes: bands crowded with hits separated by bands with none at all. Start the source below and watch the pattern emerge dot by dot.

Bullets do not behave this way

If photons were ordinary bullets, this would be simple: some fly through the left slit, some through the right, and two blurred bands build up on the screen — one per slit. The sum of two piles. There would be no fringes at all, because how would a bullet passing through the left slit "know" that the right one is open?

And yet the fringes are there. What is more, they appear even when only one photon is in the apparatus at a time — the next departs long after the previous one has hit the screen. So the particles cannot collide or "conspire". Each photon interferes with itself.

A wave of probability

Quantum mechanics describes the photon not as a point with a trajectory but as a wave of probability. That wave passes through both slits at once — like a water wave through two gaps — and overlaps with itself beyond the barrier. Where crests reinforce, hits are frequent; where a crest meets a trough, the wave cancels and photons never land. Each dot on the screen is a single draw from that distribution.

The same formula describes water waves and sound waves. The quantum novelty is that here it is the probability of a single, indivisible particle that waves — and yet the on-screen pattern is as sharp and predictable as in a bathtub.

Look — and the fringes vanish

As long as you do not ask which way it goes, the photon takes both paths. Ask — and it picks one.

The strangest part starts now. Place a detector at the slits that checks which way each photon went. Switch the "observer" on in the figure: the fringes disappear, leaving two blurred bands on the screen — exactly as if photons were bullets. The mere possibility of telling the paths apart destroys the interference. It is not about "disturbing" a delicate particle with a nudge: it is enough that the which-way information exists in the world at all.

This is not a philosophical anecdote but a repeatable laboratory result — with photons, electrons, atoms, even molecules of thousands of atoms. The boundary between the "quantum" and the "ordinary" world is not drawn at some size; it is drawn where the which-way information is lost.

A simplificationThe figure samples hits from a ready-made interference distribution and omits single-slit diffraction, which modulates the fringe brightness. The "observer" is in practice a subtle coupling of the photon to its environment (decoherence), not a human eye — consciousness plays no role here. The principle stays exactly this: which-way information exists — fringes do not.