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The following quip is often attributed to the physicist Richard Feynman:

If all of mathematics disappeared, physics would be set back by exactly one week.

See for example Quote Master, quotefancy, AZ Quotes, or Internet Pillar.

But did Feynman actually say this? None of the above websites gives a source. It sounds like something Feynman might have said. He did say other provocative things about mathematics, such as, "I bet there isn't a single theorem that you can tell me ­­what the assumptions are and what the theorem is in terms I can understand ­­where I can't tell you right away whether it's true or false." But I haven't been able to locate a source for the above quotation.

As an aside, my interpretation of the quip is that physicists are highly capable of reinventing whatever mathematics they need for the physics they are working on. So if they encounter some new situation in which unfamiliar mathematics is needed, then they have two options. They can either check the mathematics literature to see if the relevant mathematics has already been developed by mathematicians, or they can roll up their sleeves and reinvent the requisite mathematics themselves. The quip suggests that the latter approach will take at most one week longer than the former approach. I mention this interpretation because it might help search for the source of this quotation, or at least something similar to it.

Timothy Chow
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    One book (Music of Matter) said it came from [QED](https://archive.org/details/qedstrangetheory0000feyn_x8n8/mode/1up?q=Disappeared), but I couldn't find anything there. (The book's other quote about shell games was, however.) – Laurel Aug 12 '23 at 13:31

1 Answers1

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The quote is reported in:

Cohen, Joel E. A Life of the Immeasurable Mind. Annals of Probability 14(4): 1139-1148 (October, 1986). DOI: https://dx.doi.org/10.1214/aop/1176992358

which is a review of mathematician Mark Kac's autobiography Enigmas of Chance. Cohen writes on page 1147:

One cannot find here one of the most famous Kac witticisms, which I verified with him the summer before he died.

Kac went to Pasadena to lecture at the California Institute of Technology. Richard Feynman was in the audience. After the lecture, Feynman got up and announced: "If all mathematics disappeared, it would set physics back precisely one week." Without a pause, Kac responded: "Precisely the week in which God created the world."

I guess this is still secondhand (Kac was there, and personally told Cohen, who published the account) but it seems to me about as good as we can expect for a purely verbal quote.

(It's too bad that all your sources reported Feynman's barb without Kac's retort, which I think makes it much funnier.)

Cohen repeats the anecdote in his book Absolute Zero Gravity with Betsy Devine, which is where I remembered having seen it, and cites his AoP review. This book is a collection of science jokes and humorous anecdotes, and is pretty well referenced, so it may be a helpful source for future questions like this. A full PDF of the book is currently available on Cohen's website.

Nate Eldredge
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    I guess this might also explain why Feynman doesn't seem to have mentioned this in his own memoirs, seeing as he ended up on the receiving end of the sick burn. – Nate Eldredge Aug 13 '23 at 04:11
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    I guess the other bit of context is that Feynman and Kac had been colleagues at Cornell and famously collaborated on the Feynman-Kac formula. So they knew each other. And so while Feynman's comment sounds on its face like a fairly harsh insult to a mathematician, in context it could have been more like friendly teasing. – Nate Eldredge Aug 13 '23 at 12:58
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    Here is a [chat](https://chat.stackexchange.com/rooms/147887/feynman-kac-repartee) for all further discussion of what Feynman and/or Kac meant by their comments, whether they are clever or funny, the relationship between mathematics and physics, etc. Comments can still be used for corrections or notes relevant to the question at hand, which is simply "did Feynman say it?". – Nate Eldredge Aug 14 '23 at 15:00
  • Decimated comments not related to improving or clarifying the answer. – Oddthinking Aug 16 '23 at 14:42
  • @Oddthinking I think you missed something. – JimmyJames Aug 16 '23 at 14:44
  • @JimmyJames: Spell it out in a flag, please. – Oddthinking Aug 16 '23 at 14:50
  • @Oddthinking We still don't know what about Kac's presentation led to Feynman's outburst. I suppose that doesn't directly address whether he made the statement, but I would think is a reasonable thing to include here and in theory, there might be some record of what Kac's lecture was about and what led to Feynman's statement. – JimmyJames Aug 16 '23 at 15:02
  • I don't think it can be understated just how unreliable this is as evidence. Eyewitness testimony is already notoriously unreliable, and now we have a second-hand account of that, and we're expecting an exact word-for-word quote, when people are especially bad at recalling exact words (small changes in phrasing or context could change meaning a lot). Cohen claims that it's "one of the most famous Kac witticisms", which strongly suggests that there would be supporting sources dated earlier than Cohen's writing. – NotThatGuy Aug 16 '23 at 15:04
  • @JimmyJames: I am reviewing my actions, and I can't see where you think I went wrong. [I am not defending myself here; just explaining I am confused.] – Oddthinking Aug 16 '23 at 15:09
  • @Oddthinking Let me ask this: do you think knowing why Feynman reacted that way to Kac's lecture is relevant? – JimmyJames Aug 16 '23 at 15:51
  • I just noticed that in the "Absolute Gravity" book, it has a slightly different version of what Feynman said i.e.: "it would set the *world* back a week" [emphasis mine] which makes a little more sense to me given the retort. It also claims that Feynman "loved to take potshots at what he considered the petty attachments to rigor of mathematicians" – JimmyJames Aug 16 '23 at 16:07
  • @JimmyJames: I am not here to play games. If you think I did something wrong, tell me exactly what. – Oddthinking Aug 16 '23 at 16:20
  • @Oddthinking I thought I already did. You deleted comments related to why Feynman would say such a thing. I'm not sure how much more I can spell it out. – JimmyJames Aug 16 '23 at 16:22
  • @Oddthinking Here's my guess as to what JimmyJames is saying. When he says you "missed something," he doesn't mean that you failed to delete something that was relevant. He means that you deleted something that *was* relevant, namely comments asking about what Kac said that prompted Feynman's remark. – Timothy Chow Aug 16 '23 at 22:05
  • Oh! @JimmyJames, do you mean the unreferenced speculation about the motives of the physicists which was controversial enough to spark discussion in the comments? No, I deliberately deleted those. I left a few from the answer author that had been voted up and stood on their own. There's a chat room for that discussion if you want to continue it. – Oddthinking Aug 17 '23 at 01:59
  • @Oddthinking I didn't think you needed references to discuss a potential improvement to an answer which is what I thought comments were for. – JimmyJames Aug 17 '23 at 14:08
  • @NotThatGuy: I would assume the quote was "famous" in the sense that it had circulated by word of mouth. There may well be earlier sources, but I suspect they would have been people who heard it from a friend of a friend, etc. Eyewitness accounts may be unreliable, but they're still better than accounts from people who weren't there at all. I'm really not sure what else we can hope for - it's not like there was a stenographer at the Caltech seminar. But by all means, if you find something better, post it. – Nate Eldredge Aug 17 '23 at 21:34