Wave-particle duality is not a poetic licence: it is an experimental fact verified down to the smallest detail. The cleanest experiment — the one Feynman called “the heart of all of quantum mechanics”, the phenomenon that contains the only mystery of the theory — is the double slit carried out with electrons arriving one at a time.
The apparatus. A source emits electrons at controlled energy (typically a few keV, with de Broglie wavelength around m). The beam strikes a barrier with two closely spaced slits; behind it, a detector screen — for example a pixel sensor — records the position of every single electron that arrives, as a point, a click, a lit-up pixel.
The double-slit apparatus with electrons. The dashed lines show the two possible paths; on the screen the points — each a single detected electron — cluster into alternating bands, the interference pattern.
The most startling observation. Even if the electrons are fired one at a time, with intervals long enough to guarantee that there is never more than one electron in the apparatus, an interference pattern progressively appears on the screen: the alternation of bright and dark bands identical to that of a wave crossing two slits. The first electrons arrive at apparently random positions; but already after a few thousand the pattern can be glimpsed; after a million it is very sharp. The information about the wave was in every single electron.
On the left: a few dozen electrons, apparently random impacts. On the right: after tens of thousands, the fringe pattern emerges with precision. Each single electron “knew” where to go to build the bands.
The slit paradox. Since at the moment of impact every electron is a “point” — leaving a single click, a single pixel — it is natural to ask: which slit did it go through? Quantum mechanics’ answer is among the least classical things one can hear: we don’t know and, worse, it doesn’t even make sense to ask. As long as we don’t place a detector at the slits, the electron “goes through both”, in the sense that its wave function — its probability wave — passes through both openings, and the two waves interfere with each other.
The plot twist of measurement. Let us try to cheat: we place a small detector next to each slit, which signals the passage of the electron. Now we know, for every electron, which one it went through. What then happens is the most surprising fact of the experiment: the interference pattern disappears. The screen becomes covered with a flat distribution — two overlapping broad bands — indistinguishable from the result that classical bullets would give.
Distribution of impacts on the screen. On the left, without knowing which slit the electron passes through: interference fringes. On the right, with a detector at the slits: the fringes vanish and only the simple sum of two humps remains.
Why? The simple fact of measuring which slit the electron passes through — even with an ideally minimally invasive detector — makes the wave function “collapse” onto one of the two alternatives: the electron has gone through a single slit, and so there is no longer a second wave to interfere with. The destruction of the pattern does not depend on how invasive the measurement is: the fringes disappear as soon as the which-slit information becomes accessible, even only in principle.
Principle — Complementarity (Bohr, 1928)
Wave and particle are two complementary descriptions of the same quantum object: they cannot be observed at the same time. An experiment that reveals the wave-like character (interference fringes) precludes knowledge of the corpuscular trajectory (which slit); an experiment that determines the trajectory destroys the wave-like pattern.
"Not existing until measured"
The single-electron double slit shows very cleanly that, before measurement, the electron does not have a definite position. It is not that we simply don’t know it: it is that it genuinely does not exist as an attribute of the object. In the state that produces the fringes, the electron has not gone through one slit or the other — it has gone through both. Only upon arrival at the screen (or as soon as there is a slit detector) is it “decided” where, with probability governed by the wave function. It is a way of thinking about reality that has no analogy in everyday experience, and lies at the heart of measurement theory: the microscopic world is not “classical in disguise”, but genuinely different.
Increasingly refined versions of this experiment have been carried out over time: single electrons (Tonomura, 1989), then neutrons, atoms, and molecules as large as the fullerene C (Zeilinger, 1999), up to molecules of more than 2000 atoms in 2019. In no case has a deviation from quantum predictions ever been observed. The world really is like this.
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Topics: Quantum physics Concepts: Interference · Wave-particle duality · De Broglie wavelength · Uncertainty principle
Related exercises: Detector at the slits and interference · Problem — Wave-particle complementarity · Double slit, which intensity graph