Why One Electron Builds an Interference Pattern by Itself

Why One Electron Builds an Interference Pattern by Itself

Fire electrons at a wall with two slits — one at a time, so there's nothing else around to interfere with — and they still build a wave interference pattern, dot by dot. This is Newton, in for Bear, walking through Thomas Young's double-slit experiment and the strangest part of it: a single electron somehow interferes with itself.

We start with water waves through two slits to see where stripes come from, then swap in single electrons fired one at a time. Classically, you'd expect two piles of dots — one behind each slit. Instead, thousands of individual, particle-like clicks slowly assemble into the same interference stripes the water waves made. The explanation: each electron's wave passes through both slits at once, and where that wave reinforces itself, dots pile up; where it cancels, almost none land.

The video closes with the de Broglie wavelength (λ = h/p) and the real number behind it — 54 eV electrons, the exact energy from the 1927 Davisson–Germer experiment that first confirmed electron wave behavior by diffracting off a nickel crystal.

0:00 Cold open — the setup 0:15 Water waves through two slits 0:30 Single electrons, fired one at a time 0:45 The classical prediction: two piles 1:00 Dot by dot, the stripes assemble 1:15 Self-interference, explained 1:30 De Broglie's equation + the 54 eV number 1:45 Your turn

Your turn: tonight, try to ask which slit the electron went through — and notice how the question itself breaks down.

This is Newton, in for Bear.

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What this video covers

• 0:00 — Cold open — the setup
• 0:15 — Water waves through two slits
• 0:30 — Single electrons, fired one at a time
• 0:45 — The classical prediction: two piles
• 1:00 — Dot by dot, the stripes assemble
• 1:15 — Self-interference, explained
• 1:30 — De Broglie's equation + the 54 eV number
• 1:45 — Your turn

Transcript

A lightly formatted transcript of the video, with timestamps so you can jump to any section on YouTube.

Cold open — the setup

[0:01] This is Newton In Fra Red. Today, Thomas Young's double slit one electron at a time. Fire single electrons at two narrow slits and something impossible happens.

Water waves through two slits

[0:15] First, water. Waves through two slits overlap. They add where crests meet, cancel where they clash. The result is stripes. Remember the stripes.

Single electrons, fired one at a time

[0:32] Now, electrons fired one at a time. Nothing to interfere with. Each lands as a single dot. One click, one place. Perfectly particle like.

The classical prediction: two piles

[0:46] So, two slits should mean two piles of dots. One behind each slit. That is the classical prediction. Hold that thought and watch the real screen.

Dot by dot, the stripes assemble

[1:00] Dot by dot. 10 100 10,000 Stripes appear. The interference pattern assembles itself from electrons that were always alone. Each electron's wave passed

Self-interference, explained

[1:17] Through both slits and interfered with itself. Where the waves reinforce, dots are likely. Where they cancel almost none.

De Broglie's equation + the 54 eV number

[1:30] The spacing follows De Broglie. Wavelength equals H over momentum. At 54 electron volts it matches the spacing of atoms. Measured on this very machine.

Your turn

[1:46] Your turn. Tonight, ask which slit it went through and notice the question itself breaks. That was why one electron builds an interference pattern by itself. Newton in for bear.