Straight from today’s Quotle — here’s who really said it, and the story behind it. Play again

Who really said it

If you think you understand quantum mechanics, you don't understand quantum mechanics

Feynman’s idea — but not Feynman’s words Richard Feynman 1918–1988 · theoretical physicist
Disputed In copyright

This is a right-person, wrong-words case. Feynman really did say that quantum mechanics defeats understanding — but not in these words, and nobody who repeats them has ever produced a source. The sentence appears in none of the sourced sections of his Wikiquote entry, and editors there have been asking for a citation since 2011 without one surfacing. What he actually wrote is ‘I think I can safely say that nobody understands quantum mechanics’, in The Character of Physical Law (1965), chapter 6, page 129 — the book of the Messenger Lectures he gave at Cornell in 1964.

Last verified 10 Aug 2026 · how we verify · spot an error?

For presentations

Put it on a slide

Disputed wording — copy with the correction

“If you think you understand quantum mechanics, you don't understand quantum mechanics” Feynman is not on record saying this. What he wrote was: "I think I can safely say that nobody understands quantum mechanics." — The Character of Physical Law (1965), ch. 6, p. 129. No source has been located for the "if you think you understand" version.

Can you use it?

Still under copyright (rights held by The estate of Richard Feynman and MIT Press). A short, credited quote is generally fine for talks, teaching, and internal decks; for commercial, published, or large-scale reuse, get permission.

Image direction — grounded in this quote’s real context; paste into your image tool

In context

Evoking the themes of knowledge, humility and doubt. Theme: "If you think you understand quantum mechanics, you don't understand quantum mechanics". Atmospheric and restrained, cinematic directional light, a muted period-appropriate palette, fine grain and texture. Compose with generous empty space for the quote and a small credit line. No lettering in the image itself.

Minimalist

Minimalist editorial slide for the quote "If you think you understand quantum mechanics, you don't understand quantum mechanics". Clean background, a single muted accent color, generous negative space, subtle paper or linen texture, no literal illustration — let the words carry it. Reserve clear space for a serif quote and a small credit line. No lettering in the image itself.

Provenance

The source

Genuine line
‘I think I can safely say that nobody understands quantum mechanics’
Document
The Character of Physical Law (1965), ch. 6, ‘Probability and Uncertainty — the Quantum Mechanical View of Nature’, p. 129
Delivered
Messenger Lectures, Cornell University, 1964; recorded by the BBC
Source for the popular wording
None located

There might have been a time when only one man did, because he was the only guy who caught on, before he wrote his paper. But after people read the paper a lot of people understood the theory of relativity in some way or other, certainly more than twelve. On the other hand, I think I can safely say that nobody understands quantum mechanics. The Character of Physical Law, 1965

The genuine passage, with the sentence in the position that gives it its force. He is contrasting quantum mechanics with relativity, which he says plenty of people understood once they had read the paper. The joke lands on the mock-cautious “I think I can safely say” — a man measuring exactly how large a claim he is about to make.

Wikiquote — the sourced Feynman line, with page citation

How we traced it

  • The genuine sentence has a clean citation: chapter 6 of The Character of Physical Law, ‘Probability and Uncertainty — the Quantum Mechanical View of Nature’, page 129, as recorded in the sourced section of Feynman’s Wikiquote entry the citation ↗.
  • The popular epigram has no such anchor. It is absent from every sourced section of that same entry, and the talk page carries a thread opened in February 2011 by an editor who found roughly 16,500 exact-phrase matches for it online, skimmed them, saw almost all crediting Feynman by name — and could not find one that actually gave a source the sourcing thread ↗.
  • The epigram is often assumed to descend from Niels Bohr, and that suggestion is made on the Feynman talk page itself, where an editor proposes it is a paraphrase of a line credited to Bohr the suggestion ↗. Bohr’s own saying — ‘anyone who is not shocked by quantum theory has not understood it’ — is filed by Wikiquote under Disputed as well, resting on Werner Heisenberg’s recollection of a 1952 conversation in Physics and Beyond (1971), p. 206. It is a separate saying with its own attribution problem Bohr, disputed ↗.
  • We could not open the book itself to re-check page 129: the Internet Archive’s copy of the 1965 M.I.T. Press edition is lending-restricted, so the page number here rests on Wikiquote’s citation rather than on our own reading the edition record ↗.
In copyright

The genuine sentence is in copyright. The Character of Physical Law was first published in 1965 and has been continuously in print, so it is protected in the United States, with rights administered through Feynman’s estate and his publishers. Short quotation with attribution is ordinary practice; reproducing the passage at length is not. The popular epigram has no identified author, so no term can be calculated for it — that is a gap in the record, not a licence. This is not legal advice.

Rights status is our research, not legal advice — and copyright varies by country and edition. For commercial or published use, confirm with the rightsholder. See terms.

Fact-check

The attribution problem

Nothing here was invented from nothing. A real remark by a real physicist was tightened into a paradox, and the paradox travelled better than the original.

Right man, wrong words no source located

It appears in none of the sourced sections of Feynman’s Wikiquote entry. A talk-page thread opened in February 2011 records an editor finding roughly 16,500 exact-phrase matches online, almost all naming Feynman — and reports being unable to find one that gave a source the thread ↗.

A documented sentence, one step away modern paraphrase

‘I think I can safely say that nobody understands quantum mechanics’ — The Character of Physical Law (1965), ch. 6, p. 129. The popular version rewrites this as a self-referential trap, which is wittier and says something he did not say the citation ↗.

A confession became a test meaning drift

Feynman’s sentence includes him: nobody understands it, himself included, said a year before he shared a Nobel Prize for work in the field. The epigram turns that into a standard other people fail — if you feel you understand, you are disqualified. Same subject, different posture.

A different saying, routinely conflated conflation

Bohr is credited with ‘anyone who is not shocked by quantum theory has not understood it’, which Wikiquote also files as Disputed, resting on Heisenberg’s account of a 1952 conversation in Physics and Beyond (1971), p. 206. It is not the source of the Feynman epigram Bohr, disputed ↗.

No one who quotes these words has produced a source for them. Feynman’s documented line is ‘I think I can safely say that nobody understands quantum mechanics’ — The Character of Physical Law (1965), chapter 6, page 129 the citation ↗.

Context

Why it spread

Quantum mechanics is the one part of physics whose predictions are impeccable and whose meaning is still argued over, so a licence to stop trying to picture it arrives as relief. That is most of the appeal.

The rewrite adds something the original refuses. Feynman was describing a limit on intuition, his own included. The epigram converts that into an entrance exam with no pass mark — anyone who feels they understand is thereby wrong. It is a better joke and a worse thought.

He said nobody understands it, and counted himself in. The rewrite makes it a test you fail by passing.

The documented remarks, in order

1964: Feynman gives the Messenger Lectures at Cornell; the BBC records them.

1965: they are published as The Character of Physical Law. Chapter 6, page 129 — ‘I think I can safely say that nobody understands quantum mechanics.’

1971: Heisenberg’s Physics and Beyond reports Bohr, in 1952, on not being shocked by quantum theory — the other saying this one gets confused with.

February 2011: a Wikiquote editor documents the epigram in wide circulation, crediting Feynman, with no source anyone can supply.

Explore

Filed under

More provenance-checked quotes on the same theme, each with its reuse status.

Knowledge Humility Doubt

Go further

Dig deeper

Trusted places to keep pulling the thread — primary archives, fact-checks, and scholarship. We show how we used each one before you leave.

Richard Feynman — sourced quotationsen.wikiquote.org

The genuine line with its full citation: The Character of Physical Law (1965), ch. 6, p. 129.
Talk:Richard Feynman — sourcing discussionen.wikiquote.org

The 2011 thread in which editors hunt for a source for this exact epigram, and fail to find one.
The Character of Physical Lawen.wikipedia.org

Background on the 1964 Messenger Lectures at Cornell, the BBC recording, and the 1965 book.
Niels Bohr — disputed quotationsen.wikiquote.org

The ‘not shocked by quantum theory’ saying often confused with this one, and its own sourcing problem.
The Character of Physical Law (1965 M.I.T. Press edition)archive.org

The edition record in the Internet Archive’s lending library — borrowing is required to read the text.

The credited author

About Richard Feynman

Richard Feynman

1918–1988 · theoretical physicist

Richard Feynman (1918–1988) shared the 1965 Nobel Prize in Physics for his work on quantum electrodynamics, and was probably the most quotable physicist of his century — a lecturer who talked about the hardest subject in science with cheerful irreverence. That reputation is exactly why this sentence sticks to him: it sounds like something he would say. He did say something like it. He just said it with more modesty and less paradox.

For writers & researchers

Cite this

Feynman, The Character of Physical Law (1965), ch. 6

Feynman, Richard P. The Character of Physical Law. Cambridge, MA: M.I.T. Press, 1965. Chapter 6, ‘Probability and Uncertainty — the Quantum Mechanical View of Nature’, p. 129 (page citation per Wikiquote). Delivered as the Messenger Lectures, Cornell University, 1964. No source has been located for the ‘if you think you understand’ wording.

This verification page

Quotle.info. “Who really said ‘If you think you understand quantum mechanics, you don't understand quantum mechanics’?” Last verified 10 Aug 2026. https://quotle.info/who-said/if-you-think-you-understand-quantum-mechanics-you-dont/

Something wrong on this page?

Tell us what’s off. A link helps but isn’t required — “this is the wrong person” is a useful report on its own. Nothing here changes the page directly; it goes to our review queue and a human checks it.

Only used to tell you if we act on your report. We never sell or share it.

Copied