The inner life
Double-Slit Experiment
Direct answer
Double-Slit Experiment: The double-slit experiment sends light or matter through two narrow openings; a pattern of bands builds up as from waves, and disappears when the path of each particle is recorded.
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At a glance
- Also called
- double slit experiment, two slit experiment, young's experiment, double slit experiment explained
- Found in
- Consciousness & Spiritual Psychology
- Related
- Observer Effect, Wave Function, Complementarity, Quantum Entanglement
The double-slit experiment sends light or matter through two narrow openings; a pattern of bands builds up as from waves, and disappears when the path of each particle is recorded.
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What does the double-slit experiment show?
A source fires particles, photons or electrons, at a barrier with two slits, and a screen beyond records where each one lands. Each arrives as a single dot. Yet over thousands of arrivals the dots gather into bright and dark bands, the pattern two overlapping waves would make, even when the particles are sent one at a time.
If a device is added that registers which slit each particle passes, the bands vanish and two plain heaps remain. This is the fact most often retold as proof that looking changes reality. What changes the result is the physical interaction that records the path. It works the same with no one in the room.
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What sets Double-Slit Experiment apart
Its force lies in what it rules out. Send particles one at a time and the bands still form, so they are not made by particles jostling one another. Block one slit and the bands go, so each particle's landing depends on both openings being available. Add a path detector and the bands go again, even if the detector's record is never read. Each variation removes an easy explanation, which is why physicists return to this one set-up to teach the whole theory.
- Light: Thomas Young, about 1801 to 1803
- Electrons: Claus Jönsson, Tübingen, 1961
- One electron at a time: Merli, Missiroli and Pozzi, Bologna, 1974; Tonomura, 1989
- Large molecules: carbon-60 molecules through a grating, Vienna, 1999
- Feynman's verdict: it holds 'the only mystery' of quantum mechanics
Often mistaken for something else: Ripples through two gaps in a harbour wall also make bands, but water arrives everywhere at once; in the quantum version single particles land at single points and the bands appear only in the count.
The story behind Double-Slit Experiment
Thomas Young showed the interference of light in the first years of the nineteenth century, and the two-slit version bears his name. Claus Jönsson performed it with electrons in 1961. In 1974 a group in Bologna, and in 1989 Akira Tonomura's team in Japan, recorded the pattern building up from single electrons. Richard Feynman said the experiment contains the only mystery of quantum mechanics.
Why Double-Slit Experiment matters
Double-Slit Experiment is the experiment behind nearly every claim that the observer creates reality, so its plain result is the best answer to that claim.
How to read Double-Slit Experiment
Readers return to it to check what a teacher means by observation: a recording device, which the experiment needs, or a mind, which it does not.
Double-Slit Experiment in practice: an example
Sent one at a time through two slits, electrons still land in stripes, as if each had gone through both openings.
Where readings of Double-Slit Experiment go wrong
Double-Slit Experiment shows nothing about wishes, prayers or attention; the pattern depends on the apparatus alone.
Sources and editorial method
This entry summarizes the tradition in the Almanac's own words; where no source is listed, it is an editorial summary rather than a primary text, and symbolic readings are named as the tradition's, not as fact.
- Source
- The Great Almanac Dictionary
- Category
- Consciousness & Spiritual Psychology
- Aliases
- double slit experiment, two slit experiment, young's experiment, double slit experiment explained
- Last updated
- 2026-09-26
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Common questions
What does the double-slit experiment show?
That light and matter behave in a way no ordinary wave or particle does: single particles land one by one, yet together they form the banded pattern of interfering waves.
Does observing change the result?
Recording which slit each particle takes removes the banded pattern. The recording is a physical interaction with the particle. A human observer is not needed, and looking at the data later changes nothing.
Who first did the double-slit experiment?
Thomas Young demonstrated the interference of light shortly after 1800. The version with electrons was done in 1961, and its build-up one electron at a time was recorded in 1974 and again in 1989.
