Experiment #5 · Scientific experiment

The Double-Slit Experiment

Interference, measurement, and the role of the observer

Thomas Young (light, 1801); Davisson–Germer (electrons, 1927) · 1801 / 1927 · Quantum mechanics

First published: Thomas Young, "On the Theory of Light and Colours", *Philosophical Transactions* (1802); Davisson & Germer, "Diffraction of Electrons by a Crystal of Nickel", *Physical Review* 30 (1927): 705.

A single electron interferes with itself — until you watch which slit it goes through.

Particles (photons, electrons, even buckminsterfullerene molecules) fired one at a time through two slits build up an interference pattern on a detector — as if each particle went through both slits and interfered with itself. Place a detector that registers which slit each particle traversed, and the interference vanishes; the pattern becomes the sum of two classical distributions. Repeated across nine decades and many particle species, the result is the canonical demonstration that quantum systems do not have determinate trajectories between measurements — and the principal arena for disputes over what "measurement" *is*.

Formulation

Source → barrier with two slits → detection screen. With both slits open and no which-path information, single-particle detections accumulate into an interference pattern. With which-path information available (even in principle, even if not recorded by an observer), the pattern is destroyed.

Compare this experiment

Dimensions Engaged

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Matter

Forces Matter · Ontological Status into a choice: between measurements, is the particle a localised object that took one path (hidden-variable readings), a wave (Copenhagen / pilot-wave readings), or no determinate thing at all (strict instrumentalism)?

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Observer

The which-path / no-which-path asymmetry is the headline puzzle for Observer · Metaphysical Agency: does observation collapse the wave function (von Neumann–Wigner), select a branch (Everett), or merely correlate the observer with an already-decohered outcome?

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Space

Pushes on Space · Locality: a definite trajectory through a definite slit cannot recover the pattern; the wave function is non-local in configuration space even if observable predictions remain local.

Responses — How Schools Engage

Affirms / takes the bait 2

Quantum Realism

Take the wave function as physically real. The particle has no definite position between measurements; the interference pattern is what reality without definite trajectories looks like. The experiment is not a puzzle, it is the basic phenomenon.

Idealism

Some idealists (and the von Neumann–Wigner reading) take the experiment to suggest consciousness as the collapse trigger — the physical record is incomplete without an observing mind to actualise it. A minority reading, but the experiment is its locus classicus.

Denies / rejects the premise 1

Logical Positivism

Asking what the particle "really does" between measurements is empirically vacuous: only the distribution of detection events is meaningful. The Born rule is the theory; the rest is metaphysics.

Reframes the question 3

Multiverse Theory

Everettian/many-worlds: there is no collapse. Each detection outcome is realised on a separate branch; interference is between amplitudes of branches in which the particle "took" different slits. The observer-effect dissolves into decoherence.

Naturalism

Standard naturalism (in its post-Bohmian guise) accepts hidden variables — pilot-wave theory: particles do have trajectories, guided by a non-local quantum potential. The experiment shows the pilot wave is real, not that determinacy fails.

Structuralism

Ontic structural realism: what is real is the pattern of relations the experiment exhibits, not the "particle" supposed to bear them. The double-slit is the cleanest argument that structure outruns object.