Book

The Feynman Lectures on Physics, Vol. III: Quantum Mechanics

Name: The Feynman Lectures on Physics, Vol. III: Quantum Mechanics
Author: Richard P. Feynman, Robert B. Leighton, Matthew Sands

by Richard P. Feynman, Robert B. Leighton, Matthew Sands · 1965 · 2 reading cards

AuthorRichard P. Feynman, Robert B. Leighton, Matthew Sands →ShelvesQuantum physics made simple — slits, waves and tunnelling

2 cards

01
If you place a detector that finds out which slit each electron goes through, the bands vanish and two stripes remain, as with balls. What matters is not a person looking but the fact that the information about the path exists somewhere in the world.
The Feynman Lectures on Physics, Vol. III: Quantum Mechanics · 1965
Either you know which way it went or you see the bands, never both.
The natural question after the last lesson: if the electron seems to go through both slits, let us look. We put a small detector next to each slit that signals when an electron passes there. The result is clear: the detector always fires at one slit only, never at both at once. And, at the same moment, the bands on the screen vanish. Two stripes remain, exactly as with balls. In other words, you can have one of two things. Either you know which way the electron went, and then there are no bands. Or you have bands, and then you cannot know which way it went. Niels Bohr called this complementarity: particle behaviour and wave behaviour are like the two faces of a coin, you never see both at the same time. Feynman explained the loss of the bands by saying that any detector gives the electron a little push. Later experiments showed something even more interesting: the bands vanish even when the detector barely touches the particle. What matters is whether, somewhere in the detector, a trace remains that says which path it took. No person needs to read the trace. A device left alone in an empty room spoils the bands just as well.
“It is impossible to design an apparatus to determine which hole the electron passes through, that will not at the same time disturb the electrons enough to destroy the interference pattern.”
Open the card →
02
When you send electrons through two slits one at a time, each hits the screen at a single point, like a ball. But after thousands of electrons, the points line up in bands, like a wave. Each electron, alone, behaves as if it had gone through both slits.
The Feynman Lectures on Physics, Vol. III: Quantum Mechanics · 1965
Each electron arrives as a particle, but all of them together draw a wave.
The electron is a particle: it has mass, it has electric charge and it leaves a single point on a screen, in a single place. So we would expect it to behave like a tennis ball and make two stripes. Physicists actually ran the experiment, and in 1989 Akira Tonomura's team at Hitachi sent electrons so sparsely, about ten per second, that there was never more than one inside the apparatus at a time. At first the points look scattered at random. After a few hundred, something begins to show. After tens of thousands, the picture is clear: bright and dark bands, exactly the interference pattern of a wave. Nobody can say where the next electron will land. But all of them together follow the band pattern precisely. Why is it so strange? Because almost no electron reaches the dark bands, even though electrons would arrive there if only one slit were open. Open the second slit and some places receive fewer electrons, not more. A lone electron has nothing to interfere with, so it seems to interfere with itself. Richard Feynman called this experiment the place where the only mystery of quantum physics lives. Quantum mechanics does not say which way the electron went; it only says, very precisely, with what chances it lands in each place.
“We choose to examine a phenomenon which is impossible, absolutely impossible, to explain in any classical way, and which has in it the heart of quantum mechanics. In reality, it contains the only mystery.”
Open the card →

If you place a detector that finds out which slit each electron goes through, the bands vanish and two stripes remain, as with balls. What matters is not a person looking but the fact that the information about the path exists somewhere in the world.

When you send electrons through two slits one at a time, each hits the screen at a single point, like a ball. But after thousands of electrons, the points line up in bands, like a wave. Each electron, alone, behaves as if it had gone through both slits.