The Experiment That Broke Reality
Fireparticlesatawallwithtwoslits.Theycreateawavepattern.Butwatchthem,andthepatterndisappears.Nobodyfullyknowswhy.
A Simple Setup, An Impossible Result
Thomas Young first performed a version of this experiment in 1801, using sunlight. He showed that light creates interference patterns — proving light is a wave, not a particle as Newton had believed.
A century later, quantum mechanics revealed that light is also made of particles (photons). And then things got strange.
In the 1920s, physicists repeated the experiment with electrons — tiny particles of matter. Electrons, fired at two slits, produced the same interference pattern as light. Matter, it seemed, was also a wave.
One At A Time
The truly mind-breaking version came when physicists fired electrons one at a time. Each electron hits the screen at a single point, like a bullet. But after thousands of individual electrons, the points accumulate into the wave interference pattern.
Each electron, travelling alone, somehow "knows" that both slits are open. It interferes with itself — with the version of itself that took the other path.
This isn't a limitation of our instruments. It's not that we're measuring poorly. The mathematics of quantum mechanics — confirmed by every experiment ever conducted — says that before measurement, the electron genuinely doesn't have a definite position. It exists in a "superposition" of all possible paths simultaneously.
The Observer Effect
Now add a detector at the slits. Any device that can tell you which slit the electron passed through — even one that barely disturbs the electron — destroys the interference pattern.
It's not the physical disturbance that matters. Experiments using incredibly gentle "which-path" detectors still destroy the pattern. What matters is whether information about the path exists.
The electron "decides" to be a particle the moment its path can be known. Not when someone looks at the detector. Not when a conscious mind processes the data. The instant information about which slit becomes available — even in principle — the wave behaviour collapses.
What Does It Mean?
Physicists have been arguing about this for a century. The main interpretations:
Copenhagen Interpretation: The electron has no definite state until measured. Measurement "collapses" the wave function into a single outcome. This is the standard textbook answer, and it explains nothing about why.
Many-Worlds Interpretation: The electron goes through both slits. The universe splits. In one branch, it went left; in another, right. You only experience one branch.
Pilot Wave Theory: The electron is always a particle, but it's guided by an invisible "pilot wave" that passes through both slits. The wave creates the pattern; the particle rides it.
None of these interpretations has been proven. None has been disproven. The experiment works. The math works. The meaning remains one of the deepest unsolved problems in physics.
The original experiment
Thomas Young's 1801 experiment that proved light is a wave — and accidentally started a 200-year argument about the nature of reality.
But it got much weirder when they tried it with single particles.
Feynman explains it
Richard Feynman called the double-slit experiment "the only mystery" of quantum mechanics. His explanation is the clearest anyone has ever given.
Now here's the part where physicists start disagreeing with each other.
The interpretations
Copenhagen, Many-Worlds, Pilot Waves — three completely different explanations for the same experiment, and no way to tell which is right.
What you now know
- Thomas Young proved light is a wave in 1801 — then quantum mechanics showed it's also particles
- Single electrons, fired one at a time, still create wave interference patterns — each particle interferes with itself
- Measurement doesn't need to physically disturb the particle — the mere availability of path information destroys the wave pattern