Crucial Quantum Experiments

Four experiments — each one performed, replicated, undeniable — that forced physicists to give up something they had taken for granted about reality.

Quantum mechanics is not strange because the equations are difficult. The equations are, by the standards of theoretical physics, fairly tame. It is strange because of what its experiments do. Run them, and ordinary words like “particle,” “path,” “cause,” and “real” start to fray. Each of the four experiments below removed a piece of common sense that physics had inherited from the seventeenth century. None of them has ever come unstuck.

1. The double slit, with one particle at a time

You shine a beam at a barrier with two slits, and a banded interference pattern lights up on the screen behind. Thomas Young saw this with sunlight in 1801. It looked like proof that light is a wave. Then in the twentieth century, experimenters did the same thing one quantum at a time — one photon, one electron, eventually one whole molecule of buckminsterfullerene.

Each particle leaves a single point on the screen. But over thousands of runs, the points pile up into the same banded pattern. The conclusion is forced: each particle, on its own, somehow goes through both slits and interferes with itself. Cover one slit and the bands vanish. Place a detector at the slits to see which one a particle “really” took, and the bands also vanish — the interference is destroyed by the act of looking.

A single quantum, fired alone, still produces a wave-like interference pattern over many runs.

Richard Feynman called this “the only mystery” of quantum mechanics. Every other puzzle is, in some sense, a variation on it.

2. Stern–Gerlach — quantities that come only in lumps

In 1922, Otto Stern and Walther Gerlach fired a beam of silver atoms through a non-uniform magnetic field. Classically, each atom is a tiny bar magnet pointing in some random direction; the field should bend it slightly, smearing the beam into a continuous vertical line on the detector plate.

Instead, they got two spots. Just two. The atoms had only ever been “up” or “down” with respect to the field. The intermediate values that classical physics demanded simply did not occur. They had stumbled onto spin: an angular-momentum-like property that is fundamentally quantized.

The unsettling sequel came when they chained the apparatus. Send the “up” beam through a second magnet rotated sideways, and it splits again into two — left and right — in equal numbers. Take the “left” beam from that and feed it back into a vertical magnet, and it splits into up and down once more. The atoms had “forgotten” the up–down information they were sorted on a moment earlier. Measuring one direction does not just reveal a hidden value; it actively erases another. Properties that you would expect to coexist — like a person's height and weight — turn out to be incompatible at the quantum level. You cannot, in principle, know both at once.

3. Bell's test — locality is not optional

Einstein, with Podolsky and Rosen, argued in 1935 that quantum mechanics had to be incomplete. Surely, he thought, when two particles fly apart and we measure one, the answer is fixed by some property they both carried from the start — like a pair of gloves split in two boxes. The randomness in the equations would just be our ignorance of those hidden values.

In 1964, John Bell did something extraordinary: he turned this philosophy into arithmetic. Any theory in which (a) measurement outcomes are predetermined by local properties carried with the particle, and (b) influences cannot travel faster than light, must satisfy a specific inequality between the correlations of measurements taken at different angles. Quantum mechanics predicts a violation of that inequality.

Bell's setup. Two particles, two distant choices of angle. Their correlations exceed what any local hidden-variable theory can produce.

Starting with Alain Aspect's experiments in 1982 and tightened to a near-airtight standard by “loophole-free” tests in 2015, the inequality has been violated again and again. The conclusion is uncomfortable but inescapable: nature is not, at the deepest level, both local and definite. Either the values were not there before measurement, or something unmediated by light bridges the gap, or both.

No reasonable definition of reality could be expected to permit this. — Einstein, Podolsky, Rosen, 1935. The experiments since have permitted exactly this.

4. The delayed-choice quantum eraser

The double slit suggested that watching destroys interference. The delayed-choice quantum eraser asks: when does the watching matter? You set up a double slit so that each photon, after passing through, is entangled with a partner photon that you can route to different detectors. Some routes record which slit the partner passed through. Other routes deliberately scramble that information.

The trick is that you choose which route to take after the original photon has already hit the screen. When you later sort the original hits according to which path you chose for the partner, the bands appear or disappear accordingly. The interference pattern is not erased back in time; rather, what was always a single distribution of dots reorganizes itself into sub-patterns based on information collected later. There was never a fact of the matter about which slit it went through — only a fact about the joint statistics, settled when all the data is in.

Different from the others, this experiment doesn't add a new principle so much as drive home a single lesson the others were already teaching: in quantum mechanics, “what really happened” is not always a question with an answer prior to the measurement that asks it.

What survives

Take the four together. Particles travel multiple paths at once. Properties exist in incompatible pairs that cannot be jointly fixed. Distant outcomes are correlated more strongly than any local mechanism allows. The classical narrative — objects with definite properties moving along definite trajectories under local forces — cannot be salvaged. What replaces it is the quantum formalism: a wavefunction whose squared amplitude gives probabilities, evolving smoothly until measurement, after which only one outcome is recorded.

The interpretations — Copenhagen, many-worlds, pilot waves, QBism — differ on what is “really” going on between measurements. They all agree on what the experiments say. That, more than any equation, is what makes quantum mechanics the most thoroughly tested and most genuinely surprising theory we have.