Light, two slits, and a pattern that shouldn't be there
In 1801, Thomas Young shone light through two closely spaced slits onto a screen. If light were simply a stream of particles, you'd expect two bright bands on the screen, one behind each slit. Instead, Young saw a series of alternating bright and dark bands — an interference pattern, the signature of waves overlapping and reinforcing or cancelling each other. The experiment became the classic demonstration that light behaves as a wave, decades before anyone understood what, physically, was waving.
One electron at a time, and the pattern still shows up
The genuinely strange part of the story came over 170 years later, when the experiment was run with individual electrons sent through one at a time — never more than one particle in the apparatus at once. The first version of this was run by Pier Giorgio Merli, Gian Franco Missiroli, and Giulio Pozzi, with data collected in 1974 and published in 1976; a later, widely cited version by Akira Tonomura and colleagues at Hitachi in 1989 produced the now-famous sequence of images: individual electrons land on the detector screen one at a time, each landing point looking essentially random, with no visible pattern — until enough of them accumulate, at which point the interference bands emerge, built up dot by dot.
- Single-Electron Build-Up
- Tonomura, Endo, Matsuda, Kawasaki, Ezawa — "Demonstration of single-electron buildup of an interference pattern," American Journal of Physics 57(2), 1989
- Earlier Realization
- Merli, Missiroli, Pozzi — data collected 1974, published 1976; voted the "most beautiful experiment in physics" in a 2002 Physics World poll of over 200 physicists
No individual electron can be interfering with another electron — there isn't another one present. Whatever is producing the interference pattern is happening on the level of each single particle, which is the detail that makes this result so often (and so understandably) read as deeply strange.
It isn't consciousness. It's decoherence.
Here is the part most popular retellings get wrong. If you set up a way to detect which slit each particle actually went through, the interference pattern disappears — the particles start behaving like simple particles again, landing in two clumps instead of an interference pattern. This is real, and it's the basis of the common claim that "observation" or "consciousness" causes the collapse.
The actual physics has nothing to do with a mind looking at anything. What destroys the interference is decoherence: the moment which-path information becomes physically available anywhere — recorded in a detector, imprinted on a stray photon, correlated with literally any other physical system in the universe — the particle becomes entangled with that system, and the interference is destroyed. This happens whether or not anyone, or anything with a mind, ever reads the result. An unplugged, unread detector left sitting in the path still kills the interference pattern the instant it physically interacts with the particle, because the information became available in principle, not because someone checked it.
There's also a continuous version of this, not just an on/off switch: the more which-path information becomes available, the more the interference pattern degrades, in a precise, measurable tradeoff between path-distinguishability and interference visibility.
- Decoherence & the Visibility Tradeoff
- "Measurement-induced decoherence and information in double-slit interference" — American Journal of Physics 84(7), also on arXiv:1606.09442
The honest short version: the double-slit experiment shows that reality at this scale genuinely depends on what physical information exists about a particle's path — a fact strange enough on its own without adding a claim about consciousness that the physics doesn't require or support.
A real experiment, and a real limit on what it can mean
Physicist John Wheeler proposed a thought experiment in 1978 asking what would happen if the choice to measure (or not measure) which-path information were made after a particle had already passed through the slits. The actual experimental version was run two decades later by Yoon-Ho Kim, Rong Yu, Sergei Kulik, Yanhua Shih, and Marlan Scully, published in Physical Review Letters in January 2000, using pairs of entangled photons produced by a nonlinear crystal.
The result is real: depending on choices made about a second, correlated photon measured after the first, researchers can select subsets of the data that show or don't show interference. This gets popularly described as "retroactively changing the past" — it does not do that. No information ever travels backward in time, and the experiment never violates the no-signaling theorem that forbids faster-than-light or backward-in-time communication in quantum mechanics. The apparent paradox only appears because the "which pattern do you see" question can only be answered by sorting the first photon's data after the fact, using the second photon's later measurement as a key — a form of statistical post-selection, not a causal loop.
- Original Experiment
- Kim, Yu, Kulik, Shih, Scully — "A Delayed Choice Quantum Eraser," Physical Review Letters, January 2000
- Correcting the Retrocausality Myth
- arXiv:1905.03137, "The 'Delayed Choice Quantum Eraser' Neither Erases Nor Delays," and arXiv:1908.03920, "Demystifying the Delayed-Choice Quantum Eraser"
What it actually proved — and the headline that overstated it
The 2022 Nobel Prize in Physics went to Alain Aspect, John Clauser, and Anton Zeilinger "for experiments with entangled photons, establishing the violation of Bell's inequalities and pioneering quantum information science." Their work, building on decades of refinement, closed off remaining loopholes in tests showing that quantum mechanics' predictions cannot be reproduced by any theory combining local realism — the specific, technical combination of locality (no influence travels faster than light) and realism (particles have definite properties before being measured) that Einstein had hoped would eventually replace quantum mechanics.
That's a profound result about the structure of physical law. It is not a demonstration that the universe "isn't real" in any colloquial sense — and a real, widely circulated headline claiming exactly that ("Nobel Prize Just Given for Proving the Universe Isn't Real") is a clean example of exactly the kind of overreach this page is trying to correct. The Nobel committee's own citation is specific and technical; it rules out a particular class of alternative theories, not reality itself.
- Official Citation
- The Nobel Prize in Physics 2022 — Press Release
What this is, and isn't
The double-slit experiment, in its single-particle form, is one of the most repeated, most solid results in physics — not a fringe claim, not disputed, not awaiting confirmation. What's genuinely open to interpretation is why quantum mechanics works the way it does at the deepest level (that's a real, unresolved debate among physicists, known as the interpretation problem). What is not open to interpretation, because the physics is well understood and precisely measured, is the specific claim that a conscious mind is what causes the pattern to change. That part is a myth, retired by the decoherence explanation above, regardless of which deeper interpretation of quantum mechanics you find most persuasive.
Other edge-of-knowledge topics, told the same way
This page is part of Resonant Inquiry, an ongoing series on this site covering genuinely strange, real research — sourced, with honest caveats, and no claims of knowing more than what's actually been shown. The double-slit experiment is frequently cited as evidence for the simulation hypothesis — this page is the factual foundation for evaluating that claim on its own merits.