The double-slit experiment is the most famous result in quantum mechanics. Richard Feynman called it "the only mystery" — the single phenomenon that, if you understand it properly, contains everything strange about quantum physics. That was not modesty. He meant it.
And yet there is a question at the centre of it that almost nobody answers. Not because the answer is unknown. Because the answer is uncomfortable.
Here is the experiment. Here is what it shows. And here is the question.
The Setup
The experiment is simple. You take a source of particles — originally electrons, though it works with photons, atoms, and molecules — and you fire them at a barrier with two narrow slits cut into it. Behind the barrier is a detector screen that records where each particle lands.
If particles behaved like bullets, you would expect two bands on the screen: one behind each slit. The particles go through one slit or the other, hit the screen, and pile up in two lines.
That is not what happens.
What you get instead is an interference pattern — a series of alternating bright and dark bands, like the pattern you see when two sets of ripples cross each other on water. This is the signature of waves, not particles. Waves spread out, pass through both slits simultaneously, and interfere with themselves: where the peaks coincide, you get brightness; where a peak meets a trough, they cancel.
So electrons are waves. Fine. Except they are not. Because if you fire the electrons one at a time — so slowly that only a single electron is in the apparatus at any moment — the interference pattern still builds up, dot by dot, over thousands of individual detections. Each electron lands in a single spot, like a particle. But the pattern of where they land, accumulated over time, is the wave interference pattern.
A single electron, fired alone, interferes with itself.
What This Means
This is not a metaphor. It is not an approximation. It is what the experiment shows.
The electron does not go through the left slit. It does not go through the right slit. It goes through both slits simultaneously, as a wave of probability, and then — when it hits the detector screen — it collapses into a single point, like a particle.
Before detection, the electron exists in a superposition: a genuine physical state in which it is, in some sense, in multiple places at once. The wave function — the mathematical object that describes this state — passes through both slits, interferes with itself, and produces the probability distribution that the detector screen gradually reveals.
This is not a gap in our knowledge. It is not that the electron "really" went through one slit and we just don't know which one. We know this because if you try to find out — if you place a detector at the slits to see which one the electron goes through — the interference pattern disappears. The act of measuring which slit the electron passes through destroys the superposition. The electron now goes through one slit or the other, and you get two bands, not an interference pattern.
The measurement changes the result. Not because the detector disturbs the electron mechanically. Because the act of gaining information about which path the electron took forces the electron to have taken a definite path.
This is the observer effect. And it is where the comfortable explanations run out.
The Question Nobody Answers
Here is what the textbooks say at this point: the wave function collapses when a measurement is made. The superposition resolves into a definite state. The interference pattern disappears.
Here is what the textbooks do not say: what counts as a measurement?
This is not a minor technical question. It is the central unresolved problem of quantum mechanics, and it has been unresolved for a century. The question is this: at what point in the chain from quantum particle to macroscopic detector does the superposition collapse? Is it when the particle interacts with the detector? When the signal reaches the amplifier? When it is recorded on a computer? When a human looks at the screen?
The standard formulation of quantum mechanics — the one taught in every physics course — does not answer this question. It simply says that a measurement occurs when a classical apparatus interacts with a quantum system, and that the wave function collapses at that point. But it does not define what makes something a classical apparatus. It draws a line between the quantum world and the classical world without explaining where the line is or why it exists.
This is called the measurement problem, and it is not solved.
The Interpretations
Physicists have proposed various ways of thinking about what is actually happening. None of them is universally accepted. All of them have serious problems.
The Copenhagen interpretation — the oldest and still the most widely taught — says that the wave function is not a description of physical reality but a tool for calculating probabilities. When you ask what the electron is doing before measurement, Copenhagen says the question has no answer. There is no fact of the matter. Reality is created by measurement, not revealed by it.
The many-worlds interpretation says that the wave function never collapses. Instead, every possible outcome occurs, in branching parallel universes. When you measure which slit the electron went through, the universe splits: in one branch, the electron went left; in another, it went right. You only experience one branch, which is why it looks like a collapse.
Pilot wave theory (de Broglie-Bohm) says that particles are real and always have definite positions, but they are guided by a real wave — the pilot wave — that passes through both slits and creates the interference pattern. The particle goes through one slit; the wave goes through both. This restores determinism and avoids the measurement problem, but it requires the pilot wave to be non-local.
Objective collapse theories propose that the wave function collapses spontaneously, through a physical process that happens at a certain scale. This is testable — there should be a scale at which superposition breaks down — but no such breakdown has been observed.
Each interpretation preserves the predictions of quantum mechanics exactly. They are empirically indistinguishable with current technology. The choice between them is not a scientific question in the usual sense. It is a question about what kind of reality you are willing to accept.
Why It Matters
The double-slit experiment is taught as a curiosity — a strange result that shows quantum mechanics is weird, before the course moves on to practical applications. The weirdness is noted and then set aside.
But the question it raises does not go away. If measurement collapses the wave function, and if we cannot define what counts as a measurement, then quantum mechanics does not have a complete account of when and how definite reality emerges from indefinite superposition. The theory that underlies all of modern physics — semiconductors, lasers, MRI machines, every piece of technology built in the last century — does not have a complete account of what is actually happening.
Feynman was right. The double-slit experiment contains everything strange about quantum physics. What it contains, specifically, is a question about the relationship between observation and physical reality that the theory raises and does not answer.
That question is still open.