Author
Richard P. Feynman
4 reading cards from 4 books · 1965–1985.
4 cards
The Character of Physical Law · 1965
Quantum physics is strange, not magic: if you swap ‘quantum’ for ‘magic’ and the meaning does not change, it is not physics.
Feynman's line, ‘nobody understands quantum mechanics’, is often quoted to justify anything at all. He meant something else: the rules of the theory work with astonishing precision, but they are hard to square with our everyday intuition. ‘Strange’ does not mean ‘anything goes’. Quantum theory makes very precise predictions, and many ‘quantum’ claims in advertising and self-help books flatly contradict them. Three common abuses. First: ‘the observer creates reality’ or ‘your thoughts change matter’. You saw in the double-slit lessons that ‘observing’ means the which-path information ends up somewhere — in a detector, in a wisp of air, in an instrument with no consciousness. Second: entanglement as telepathy or an ‘energetic link’ between people. You saw in lesson 16 that no message passes through entanglement. Third: ‘quantum healing’, ‘quantum’ water, bracelets or supplements. Delicate effects such as superposition vanish extremely fast in a large, warm, wet object like the human body. There is serious quantum biology, studying for example photosynthesis or how birds navigate, but it works at the scale of molecules and publishes measurements; it does not sell products. A short checklist. Does the claim name a measurable quantity, with units? Does it cite a published experiment, with numbers? Is it trying to sell you something? Does it use ‘frequency’, ‘vibration’ or ‘energy’ without saying how much? If you replace ‘quantum’ with ‘magic’, does the meaning change at all? If nothing changes, you are looking at a buzzword, not physics. What is genuinely open is something else: what is real behind the rules. The Copenhagen interpretation says the theory only gives the odds of results. The many-worlds interpretation says every result happens, in different branches. Pilot-wave theory says particles always have a position, guided by a wave. The relational interpretation says properties exist only relative to other systems. All of them predict the same results for every experiment done so far; physicists do not argue about what works, but about what it means.
“There was a time when the newspapers said that only twelve men understood the theory of relativity. I do not believe there ever was such a time. [...] On the other hand, I think I can safely say that nobody understands quantum mechanics.”
Cargo Cult Science · 1974
What cannot be shown to a stranger did not happen, as far as the audit is concerned.
Feynman's 1974 line describes, without meaning to, why management systems ask for documented information. Not because paper has value in itself, but because one's own memory is the most indulgent auditor anyone has. A process everyone agrees simply works that way cannot be contradicted by anything; a written process can be compared with what actually happened. From this come the two halves of the discipline: write what you do and do what you write. The first alone produces a shelf of procedures nobody follows, and every deviation becomes a nonconformity. The second alone produces good practice that cannot be demonstrated or handed on. Objective evidence is an artefact with a date, an author and content someone else can verify: a log, an approval, a test result, an attendance list. What cannot be shown to a stranger did not happen, as far as the audit is concerned.
“The first principle is that you must not fool yourself—and you are the easiest person to fool.”
Simulating Physics with Computers · 1982
A quantum computer does not try every answer at once; it makes the wrong answers cancel each other out.
An ordinary bit is like a coin lying on a table: heads or tails, 0 or 1. A qubit is more like the electron in the double-slit experiment. Until it is measured, it is described not by a single answer but by two ‘waves’, one for each possible result. These waves can add up, like waves meeting in step, or cancel, like waves meeting crest to trough. When you measure it, the qubit gives a single answer, 0 or 1, at random, with odds set by the waves. This is where the most common mistake comes from. People say a quantum computer ‘tries every option in parallel’. If that were all, at the end you would read one result at random — that is, nothing useful. The real trick is different: a quantum algorithm is a choreography of interference. Its steps are chosen so that the waves of the wrong paths cancel, like the dark bands on the screen, and those of the right answer reinforce, like the bright bands. That choreography can be written only for problems with a special structure. Two clear examples: simulating molecules and materials, which is exactly what Feynman proposed in 1981, and breaking large numbers into factors (Peter Shor's algorithm, 1994), which would break part of today's cryptography. That is why, in August 2024, the US standards institute NIST published the first ‘post-quantum’ cryptography standards. For most everyday things — email, games, spreadsheets, films — a quantum computer would bring nothing. The hard part is fragility. You saw in the lesson on observing that any leak of information to the environment destroys interference. For a qubit, that means errors. The fix is error correction: one ‘logical’ qubit is spread over many physical qubits that check one another. It works only if the physical qubits are good enough, below a certain threshold. In December 2024 Google's Willow chip showed convincingly, for the first time, that below the threshold a larger grid makes errors smaller, not larger. Breaking today's keys would need, by a 2025 estimate, close to a million qubits; today's chips have around a hundred.
“Nature isn't classical, dammit, and if you want to make a simulation of nature, you'd better make it quantum mechanical, and by golly it's a wonderful problem, because it doesn't look so easy.”
QED: The Strange Theory of Light and Matter · 1985
Ordinary chances only add up, but quantum arrows can cancel — which is why two open slits can make a place impossible.
In the double-slit experiment we saw something odd: an electron sent on its own lands at a single spot, yet thousands of electrons draw stripes. The theory cannot say where the next electron will land. It can only say how likely each place on the screen is. This list of chances, for every possible place, is what physicists call the wave function. Max Born proposed in 1926 how to read the list. Richard Feynman later explained it with arrows, like the hands of a clock. For each possible path of the electron, through the left slit or through the right one, you draw an arrow. You put the arrows head to tail and get a final arrow. The chance that the electron arrives there is the length of that arrow multiplied by itself. If the arrows point the same way, they reinforce and the spot is bright. If they point in opposite directions, they cancel and the spot stays dark. Here is the difference from dice. Ordinary chances only add up: two open doors cannot make a room less visited than one. Quantum arrows can cancel, so two open paths can make a place impossible even though each path alone would reach it. The clock-hand analogy has a limit: the arrows are not objects you can see, but a calculating tool that gives exactly the numbers that are measured.
“The theory of quantum electrodynamics describes Nature as absurd from the point of view of common sense. And it agrees fully with experiment. So I hope you can accept Nature as She is — absurd.”