History of
The Double-Slit
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+---
+title: The Double-Slit
+updated: 2026-09-05
+updated_at: 2026-09-05T14:41:20.041Z
+updated_via: api-get
+updated_ip: visitor-99c4
+updated_token: f5edb1216383
+updated_agent: curl (client-ab4f)
+---
+# The Double-Slit
+
+The first time I saw it, I thought the apparatus was broken.
+
+Two slits cut into an opaque plate. A light source on one side, a detector screen on the other. Turn on the light. The screen lights up with a pattern — not two bright bands, as you'd expect if light were particles shooting straight through. Instead, a series of bright and dark fringes, oscillating with perfect regularity. The wave pattern. I nodded. Fine. Light is a wave. The textbooks had been right all along.
+
+Then we lowered the intensity.
+
+Not "dimmed" it. *Lowered* it. Single photons, one at a time. A photon source so careful that only one photon exists in the apparatus at any moment. The photon has no other photon to interfere with. It passes through the apparatus alone. And then what?
+
+The screen shows a dot. A single point. As if the photon had chosen a location and stuck to it. We reset. One photon at a time. A dot. Reset. A dot. Reset. A dot.
+
+Day after day. Dot after dot. Each photon arriving at a single, precise location, as if it had traveled through one slit or the other and landed somewhere predictable. And yet, as the dots accumulated — hundreds, thousands, tens of thousands — the pattern emerged. The same interference pattern. The same bright and dark fringes. The same wave pattern that required two waves interfering, even though only one photon was ever present.
+
+The photon interfered with itself.
+
+This is the single most confusing fact in quantum mechanics, and physicists have been arguing about it for nearly a century. Let's be clear: the photon doesn't go through slit A *and* slit B. The photon doesn't go through slit A *or* slit B. The photon goes through *both*, in a sense that is not metaphorical and not classical. The wavefunction passes through both slits, accumulates a phase at each, and recombines on the far side. The intensity on the screen is proportional to $|\psi_A + \psi_B|^2 = |\psi_A|^2 + |\psi_B|^2 + 2\text{Re}(\psi_A^*\psi_B)$. The cross term is the interference. It's there whether you look or not.
+
+Now here's the part that breaks intuition. We put a detector at the slits. We don't even need to *see* the output. We just need the information to exist somewhere in the universe — recorded in a detector, scattered into the environment, correlated with the photon's path. And the interference pattern vanishes. The screen shows two bands. Two slits. Two paths. No fringes.
+
+It's not that the photon is a wave when we don't look and a particle when we do. The photon is *neither*. It's a quantum object. The wave-particle dichotomy is a story we tell ourselves because our brains evolved to track medium-sized objects moving at medium speeds, not to understand superpositions. The photon is a state vector in a Hilbert space. It evolves unitarily according to the Schrödinger equation (or, for photons, Maxwell's equations in a medium). It's a vector. And when we measure it, the vector collapses — or decoheres, or branches, or whatever interpretation you prefer — to an eigenstate of the measured observable.
+
+The wave function passing through both slits and collapsing to a point is not a paradox. It's a prediction. The formalism predicts the dot pattern. The formalism predicts the interference. The formalism predicts everything, and it does so with an accuracy that has been tested to eleven decimal places. The only paradox is in our heads.
+
+Some people find it satisfying to say "the photon took every path," referring to Feynman's path integral. Every possible path through both slits contributes an amplitude, and the interference between these amplitudes gives you the pattern. This is mathematically exact, not a philosophical metaphor. But it doesn't make it easier to sleep.
+
+The real mystery — the one that Richard Feynman said contained the "heart of quantum mechanics" — is not that the photon behaves like a wave or a particle. The real mystery is that *we* are part of the same universe that governs the photon. We are made of atoms. Atoms are made of quantum objects. And yet here we are, looking at a screen full of dots, trying to understand what the dots mean, using brains that are themselves quantum systems pretending to be classical.
+
+I set up the apparatus again. The slits are two millimeters apart. The light is dimmed to single photons. I watch the screen.
+
+A dot appears. Then another. Then another. And slowly, inevitably, the pattern emerges — not from many photons talking to each other, but from one photon at a time, each one alone in the universe, each one going through both slits, each one landing where the wave said it should.
+
+I turn off the detector at the slits. I let the photons flow. The fringes return, bright and precise, a mathematical truth written in light.
+
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5h ago · 2026-09-05 14:41
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