The Double-Slit Experiment: From Thomas Young to Single Electrons, and What 'Observation' Really Means

The popular version of the double-slit story goes like this: in 1801 Thomas Young shone light through two slits and proved it was a wave; a century and a half later physicists fired electrons at two slits, and the moment anyone looked, the pattern vanished. About half of that is accurate. The history is more interesting than the legend, and the physics of “looking” is more concrete than the mysticism that usually surrounds it.

What Young actually did

Young was a physician, a linguist who worked on the Rosetta Stone, and a physicist on the side. His case for the wave theory of light rests on two Bakerian Lectures at the Royal Society.

• 12 November 1801, “On the Theory of Light and Colours”, published in the Philosophical Transactions vol. 92 (1802). Young treated light as a wave in the ether and set out the principle of interference: two overlapping waves can reinforce or weaken each other. He used it to explain the colours of thin films such as soap bubbles and of finely grooved surfaces.
• 24 November 1803, “Experiments and Calculations relative to Physical Optics”, published in vol. 94 (1804). The experiment he describes here is not the tidy two-slit diagram in textbooks. He let a beam of sunlight in through a small hole in a window shutter, held a thin slip of card edge-on in the beam to split it in two, and saw fringes inside the card’s shadow. Block the light passing one edge of the card and the fringes disappear. The fringes need both beams at once, which is exactly what interference predicts.

Two narrow parallel slits became the standard form later because they show the effect most cleanly. Young’s argument was not accepted quickly either. Newton’s particle theory had strong support in Britain, and the wave theory only became widely accepted after Fresnel’s work on diffraction in the late 1810s.

What the fringe spacing tells you

With slit separation d, slit-to-screen distance L and wavelength λ, bright fringes are spaced roughly λL/d apart. Turn that around and you can measure a wavelength far too small to see using a ruler. Young estimated wavelengths for different colours from interference measurements and found red light has a longer wavelength than violet. The double-slit simulator walks through where the formula comes from and where the small-angle approximation breaks down, using the numbers on its own screen. To see the same effect with water waves, try the double-slit preset in the ripple tank.

Whether the pattern survives at extremely low intensity was asked early on. In 1909 Geoffrey Ingram Taylor photographed the diffraction fringes around the shadow of a needle using light so feeble that the longest exposure took months, and found the fringes no less sharp than with bright light. There were no single-photon counters then, so it is not really a one-photon-at-a-time experiment, but it is often cited as the first evidence that interference does not depend on how bright the light is.

One electron at a time: Bologna 1974, Hitachi 1989

In the 1920s de Broglie proposed that electrons also behave as waves, and electron diffraction was confirmed in 1927. But an electron always arrives at a detector as a single point. Would a pattern still form if electrons were sent one at a time? Feynman presented this in his 1960s lectures as an experiment you should not try to do, because the apparatus would have to be impossibly small.

The first experiment usually credited with doing it came from Bologna, Italy. Pier Giorgio Merli, Gian Franco Missiroli and Giulio Pozzi submitted their paper to the American Journal of Physics in May 1974; it appeared in 1976. They fitted an electron microscope with an electron biprism: a very fine wire between two grounded plates. A voltage on the wire bends the electron beam into two paths that then overlap again. Electrons appeared one by one as flashes on a television monitor, and their film shows them gradually forming fringes.

In 1989 Akira Tonomura and colleagues at Hitachi’s Advanced Research Laboratory published a similar experiment using a position-sensitive electron-counting system, which produced much cleaner images (Am. J. Phys. 57, 117). The first few dozen dots look random. After thousands, the stripes are unmistakable. Those are the images you will most often see reproduced today.

Strictly speaking, neither experiment used two slits; both used a biprism. Physically it is the same arrangement: an electron has two available paths, and the paths meet again at the screen. An electron interference experiment with real slits was done earlier, by Claus Jönsson in 1961, but it did not record the pattern building up one electron at a time.

The idea has since been pushed to much larger objects. In 1999 Markus Arndt’s group in Vienna obtained interference with C60 fullerene molecules, each made of 60 carbon atoms.

What “observation” means

This is the part pop science distorts most. “The electron behaves like a particle when someone watches” is not how physicists describe it.

In this context an observation is any physical interaction that leaves a record of which path the particle took (which-path information). It does not matter whether anyone ever reads that record. Put a device next to one slit that responds when an electron passes, or bounce a photon off the electron so the direction the photon scatters reveals the path, and that information now exists in the detector or in the scattered photon. Once it does, the two paths no longer interfere.

The trade-off can be written down exactly. In 1996 Berthold-Georg Englert showed that fringe visibility V and path distinguishability D obey V² + D² ≤ 1. If you can only partly tell which path was taken, the fringes are only partly washed out. Observation does not flip a switch; it is a continuous exchange between knowing the path and seeing the fringes.

Why the fringes fade: a kick, or entanglement?

The older explanation goes like this: to find out where the electron is you must hit it with a photon, the hit disturbs the electron’s momentum, and the fringes smear out. This is the textbook appeal to Heisenberg’s uncertainty principle. It is not wrong, but it is not the whole story.

In 1991 Marlan Scully, Berthold-Georg Englert and Herbert Walther proposed a way to mark the path without giving the particle any significant kick, and in 1998 Stefan Dürr, Tilman Nonn and Gerhard Rempe at the University of Konstanz did it with an atom interferometer. They tagged the path in the atoms’ internal state; the momentum disturbance was far too small to wash out the fringes, and yet the fringes vanished. What erases the pattern is not the jolt but the fact that the path has become entangled with something else that can tell the two paths apart.

Decoherence: fringes fade even when nobody is watching

Which-path information does not only leak into detectors that experimenters install on purpose. A collision with a stray gas molecule, or a photon emitted by the particle itself, carries it into the environment. The loss of interference that results is called decoherence.

The Vienna group demonstrated this directly in 2004. They heated C70 molecules with lasers before sending them through an interferometer. Hotter molecules emitted more thermal photons, and the fringe visibility dropped. As the emitted photons’ wavelength becomes short compared with the separation between the two paths, each photon acts as a witness to where the molecule was. No person looked at any of those photons. This is the main reason everyday objects never show interference: anything constantly struck by air molecules and light spreads its path information into its surroundings almost instantly.

Five common misconceptions

1. “A conscious observer changes the outcome.” What removes the fringes is an interaction that records the path. If a detector writes a record that nobody ever reads, the fringes still fade.

2. “The electron splits in half at the slits.” The screen always registers one whole electron. Nobody has ever detected half an electron. What spreads out like a wave is the quantum state that sets the probability of where the electron will be found.

3. “Electrons interfere with each other.” In Tonomura’s experiment the electron current was so low that there was almost never more than one electron in the apparatus at a time, and the fringes still formed. The interference is between the two paths of a single particle.

4. “The delayed-choice quantum eraser proves the future changes the past.” In the 2000 experiment by Yoon-Ho Kim and colleagues, fringes appear only after the data are sorted according to measurements made on partner photons. The unsorted data show no fringes, so no signal can be sent backward in time.

5. “Young proved light is a wave and Einstein proved it is made of photons, so one of them was wrong.” Both describe the same thing. Dim a light source until photons arrive one at a time and the individual dots still add up to Young’s fringes. You can watch this in the simulator’s Real experiment setup panel by lowering the photon detection rate. Bear in mind those dots are sampled at random from the computed wave intensity; the tool has no which-path detector.

Try it

In the double-slit simulator, switch from two slits to one and back. The two-slit pattern is clearly not the sum of two single-slit patterns. Complete which-path information amounts to exactly that sum: the result of opening each slit separately and adding them. The smooth, stripeless hump you get that way is what remains once observation has destroyed the interference.