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⚛️Physics8 min read

The Double Slit Experiment

Fire a single particle at two holes and it goes through both at once. Watch it to find out which hole, and it suddenly stops doing that. Reality changes based on whether you are looking. This is not a metaphor.

In plain English

Stand in front of a wall with a bucket of sand. Throw handfuls of sand at the wall. Some sand goes through the gaps; most hits the wall. On the other side, you see two piles of sand, one behind each gap.

Now do the same with light. Shine a laser beam at a barrier with two narrow slits. On a screen behind the barrier, you might expect two bright stripes, one behind each slit.

Instead, you see many stripes. Light and dark bands alternating across the screen. This is an interference pattern, the kind of pattern you see when two waves overlap and sometimes reinforce each other, sometimes cancel each other out. Light, it turns out, behaves like a wave.

Fine. That is surprising but manageable.

Now dim the light source until it emits only one photon at a time. One particle. Send it through. Let it land on the screen. Do it again. And again.

After many, many individual photons, each sent through separately with long pauses in between, build up a picture of where they landed.

They form the same interference pattern.

Each single photon, on its own, somehow went through both slits simultaneously, interfered with itself, and landed according to the same wave pattern as if millions of photons were travelling together. The photon acted like a wave spread across the entire apparatus, despite being a single particle.

Now add a detector to find out which slit each photon goes through.

The interference pattern disappears. The photons start behaving like sand: two piles, one behind each slit. The act of measuring which path the photon took changed what the photon did.

This is not a metaphor. This is not a limitation of measurement technology. This is how physics works.

Five things to file under "wait, what?"

  • The experiment works with electrons, atoms, and even large molecules. The double-slit experiment was first performed with light. It has since been performed with electrons, neutrons, atoms, and molecules containing hundreds of atoms. In 2019, it was performed with molecules large enough to see under a scanning electron microscope. All showed quantum interference. Quantum weirdness is not just a property of tiny particles — it scales.

  • The measurement problem is not about physical disturbance. You might think that placing a detector at the slit physically disturbs the particle and ruins the interference. This is not true. Physicists have devised experiments where the "which path" information is acquired and then erased before any particle reaches the screen. When the information is erased, the interference pattern comes back. The availability of information about the path, not any physical disturbance, is what matters.

  • Richard Feynman called it "the only mystery." The Nobel Prize-winning physicist said the double-slit experiment is the single experiment that captures everything strange about quantum mechanics. "Any other situation in quantum mechanics," he wrote, "it turns out, can always be explained by saying, 'You remember the case of the experiment with the two holes? It's the same thing.'"

  • No one agrees on what the experiment means. The mathematics of quantum mechanics predicts the experimental results with extraordinary precision. But the interpretation, what is actually happening, is violently disputed. The Copenhagen interpretation says there is no reality before measurement. Many Worlds says the particle goes through both slits in parallel universes that split at the moment of measurement. Pilot wave theory says a real wave guides a real particle. There is no scientific consensus.

  • Delayed choice experiments make it stranger still. In variants designed by John Wheeler in the 1970s and confirmed experimentally in 1984, the decision about whether to measure which slit the particle went through can be made after the particle has passed through the barrier. Changing the measurement decision retroactively changes what the particle did. This is called the delayed choice experiment. It appears to suggest that a particle's past behaviour depends on future measurement decisions.

The full story

Thomas Young and the wave nature of light

The experiment was first performed by Thomas Young in 1801, not with particles, but with light. Young's demonstration that light formed interference patterns was decisive evidence that light was a wave, not a stream of particles as Newton had proposed. His experiment became famous in physics circles, and the pattern of alternating bright and dark bands is still called a Young's interference pattern.

For over a century, light-as-wave was the consensus view. Then Einstein's explanation of the photoelectric effect in 1905 showed that light also behaved as particles, photons, under certain conditions. The wave-particle duality of light was disturbing but eventually accepted.

What was not anticipated was that the same duality would apply to matter.

Louis de Broglie and matter waves

In 1924, French physicist Louis de Broglie proposed, in his doctoral thesis, that matter had a wave-like character just as light did. His thesis committee did not know what to think of the idea and sent it to Einstein for comment. Einstein confirmed it was serious physics. De Broglie won the Nobel Prize in 1929.

The double-slit experiment with electrons, confirming de Broglie's prediction, was first performed in 1927 and definitively in 1961. The result was identical to the optical version: interference patterns appeared, and they disappeared when the path of each electron was measured.

The measurement problem

The central mystery is not that particles behave like waves. It is that measuring them changes that behaviour.

Quantum mechanics describes unmeasured particles in terms of a wave function, a mathematical object that encodes the probability of finding the particle at various locations. Before measurement, the particle has no definite position. It is spread across space in a probabilistic smear. When a measurement is made, the wave function "collapses" to a definite outcome.

What causes this collapse? The equations of quantum mechanics do not tell you. This is the measurement problem, and it has never been solved. Different interpretations of quantum mechanics address it in different ways:

Copenhagen interpretation: The wave function is not a real physical object. It is a mathematical tool for calculating probabilities. Asking what happens "before measurement" is a meaningless question. Reality exists only when it is observed.

Many Worlds interpretation: The wave function never collapses. Instead, at every quantum event, the universe splits into branches, one for each possible outcome. The interference pattern is caused by the particle going through both slits in branches that are briefly entangled before the split becomes permanent. You, the observer, only experience one branch.

Pilot wave theory: There is a real particle that follows a real path, and there is also a real wave (the pilot wave) that guides it. The interference pattern arises from the wave; the particle follows the pattern without going through both slits. Measurement disturbs the pilot wave. This is deterministic but requires the wave to be physically real and non-local.

QBism (Quantum Bayesianism): The wave function is a description of the observer's knowledge, not of the world. Measurement updates the observer's beliefs. "Collapse" is just belief revision. Reality is agent-relative.

None of these interpretations makes different experimental predictions. They are different stories about what quantum mechanics means, not different physical theories.

Why does it matter outside physics?

The double-slit experiment has influenced philosophy of science, metaphysics, and consciousness research, sometimes productively, sometimes speculatively. The genuine physics is strange enough to warrant the philosophical attention: the universe appears to be such that certain properties of objects simply do not have definite values before they are measured. This is not ignorance on our part. It is the structure of reality.

John Bell proved in 1964 that any attempt to explain quantum mechanical results by hidden variables (pre-existing definite properties we just hadn't measured) requires accepting that distant events instantaneously influence each other, what Einstein called "spooky action at a distance." Experiments since 1982 have confirmed that Bell's inequalities are violated. Definite pre-existing properties cannot explain what we observe.

The universe is, at a fundamental level, not the way common sense suggested. The double-slit experiment is where that became undeniable.

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