Fire one electron at two slits. It goes through both. Watch which slit it uses. It goes through one. The universe is paying attention to you.
Fire photons — or electrons — through two slits one at a time, with no other particles present. Over thousands of shots, an interference pattern builds up. Each individual particle somehow "knew" about both slits. A single particle passes through both slits simultaneously.
Add a detector to find out which slit it used — the interference pattern vanishes. The universe responds to the acquisition of information, not physical disturbance.
Many alternating bright and dark bands. Wave behaviour. The particle went through both slits.
Just two bands — one per slit. Particle behaviour. The interference vanishes the moment we look.
Take a light source (or an electron gun). Place a solid barrier with two narrow slits in front of it. Behind the barrier, set up a detector screen that records where particles land. Simple enough.
If light is made of particles — bullets — they should go through one slit or the other and accumulate into two bright bands, one behind each slit. This is what common sense predicts. This is what Newton would have predicted.
Instead: alternating bright and dark stripes across the whole screen. This is an interference pattern — the signature of two waves overlapping. Where the waves reinforce each other: bright. Where they cancel: dark. This can only happen if the light is behaving as a wave and passing through both slits simultaneously.
Here's where it gets truly strange. Turn the source down until only one particle is fired at a time — with no other particles present. Each particle lands as a single dot. But over thousands of shots, the dots accumulate into… an interference pattern. Each particle interferes with itself. It passed through both slits simultaneously.
Place a detector at the slits to record which one each particle goes through. The moment you do this, the interference pattern disappears. You now get two bands — one per slit. Particle behaviour, not wave behaviour. The pattern is gone — not because the detector physically disturbed anything, but because information about the path was recorded.
Remove the detector. Don't look at the path. The interference pattern returns. This has been verified with photons, electrons, atoms, and even large molecules like buckminsterfullerene (C₆₀). The effect is universal.
Toggle the detector on and off. Watch the pattern build up one photon at a time in single-particle mode.
[ DOUBLE-SLIT SIMULATOR ]Shows the steady-state interference pattern. The particle exists as a spread-out probability wave until it hits the screen.
Fires one particle at a time. Watch the interference pattern emerge dot by dot — with no other particles present.
It isn't the physical act of placing a detector that destroys the interference. It is the fact that information about the path exists. The universe responds to knowledge.
Physicists tried increasingly gentle detectors — even ones that interact barely at all with the particle. As long as path information is recorded, the interference pattern vanishes. You can use a detector that absorbs zero energy from the particle. It doesn't matter. Information, not disturbance, is the key.
Turn the source down to one particle per hour. No other particles are present. The interference pattern still builds up, shot by shot, over days or weeks. A single particle has no other particle to interfere with. It interferes with itself. One particle, two slits, simultaneously.
In a quantum eraser experiment: let particles go through the slits with detectors on. Pattern disappears. Then erase the detector's record before you read it. The interference pattern returns — even though the particle has already hit the screen. The future act of erasing information affects the outcome of an experiment that already happened. Time, apparently, is also negotiable.
Richard Feynman called the double-slit experiment "the only mystery" of quantum mechanics. He argued that it contains all of quantum mechanics in it — that if you really understood this one thing, you'd understand all of quantum theory.
The experimental results are settled. What they mean — how to interpret them — is one of the deepest open questions in physics. Here are the three leading interpretations. Click each to expand.
The Copenhagen interpretation says the particle does not have a definite position — or any definite property — until it is measured. Before measurement, it exists as a wave function: a mathematical description of all possible outcomes and their probabilities. Measurement "collapses" the wave function to a single definite state.
On the double-slit: The particle goes through both slits — not as two copies, but as a single wave function distributed across both. When the screen detects it, the wave function collapses to a point.
On the detector: The detector forces a "measurement" which collapses the wave function to one slit — so there's no longer a wave passing through both slits to interfere.
Criticism: What counts as a "measurement"? When does a wave function collapse? Is the moon there when nobody looks? Copenhagen says "don't ask" — which many physicists find deeply unsatisfying.
Everett's radical proposal: the wave function is always real and never collapses. Instead, every quantum event causes the universe to branch into multiple parallel worlds — one for each possible outcome. All outcomes happen. You just happen to be in one branch.
On the double-slit: The particle goes through both slits in both branches. The interference pattern exists in each branch. When the detector fires, the universe splits — in one branch the particle went through slit 1, in another slit 2. You are in one branch and see no interference.
Appeal: Mathematically the cleanest. No arbitrary collapse. No vague "measurement." The Schrödinger equation applies everywhere, always.
Criticism: Where does the energy come from for infinite parallel universes? How do we derive the probability rule (Born rule) from branching? And: is this science, if the other branches are in principle undetectable?
Pilot wave theory says there is always a real particle with a definite position, and a real wave that guides it. The particle always goes through one slit. But the wave goes through both slits and creates an interference pattern that then guides where particles are likely to land.
On the double-slit: The particle goes through one slit. The wave goes through both. The wave's interference pattern steers the particle away from destructive interference zones and toward constructive ones. The overall distribution looks like an interference pattern — because it is one.
On the detector: The detector disturbs the wave, collapsing the guiding field to one slit. Without both-slit wave information, the particle is no longer steered into an interference pattern.
Criticism: Requires non-local "hidden variables" — the wave must somehow know about the whole experimental setup instantly. Bell's theorem shows this non-locality is real and unavoidable (confirmed 2022 Nobel Prize). Some find this more troubling than Copenhagen.
Young performed the original double-slit experiment in 1801, passing light through two adjacent pinholes and observing interference fringes on a screen. His results were attacked by supporters of Newton's corpuscular theory, but the mathematics was irrefutable. Young was also instrumental in deciphering the Rosetta Stone, helping unlock Egyptian hieroglyphics — a man of extraordinary range.
Bohr was the central architect of quantum mechanics as it is taught today. His Copenhagen interpretation, developed with Heisenberg in the late 1920s, became the standard framework: quantum objects don't have definite properties until measured, and asking about their state before measurement is meaningless. He famously clashed with Einstein, who found Copenhagen deeply unsatisfying. Bohr won every argument — not because he convinced Einstein, but because the experiments kept proving him right.
Feynman developed the path integral formulation of quantum mechanics — a way of computing quantum probabilities by summing over all possible paths a particle could take simultaneously. In this framework, the double-slit experiment is natural: the particle takes all paths, including both paths through both slits, and the amplitudes interfere. Feynman was also famous for his teaching ability and his insistence on intellectual honesty, including his famous line that nobody understands quantum mechanics — an admission he meant as a challenge, not a defeat.
Everett proposed the Many-Worlds interpretation in his PhD thesis at Princeton in 1957. His advisor John Wheeler presented it to Bohr, who dismissed it. Everett left academia, deeply discouraged, and spent his career in defence research. His interpretation was largely ignored until the 1970s, when physicist Bryce DeWitt rediscovered and popularised it. Today it is arguably the most popular interpretation among working physicists — 25 years after Everett's death.
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