Richard Feynman Quotes (181 quotes, page 2)

“In general, we look for a new law by the following process: First we guess it. Then we – now don't laugh, that's really true. Then we compute the consequences of the guess to see what, if this is right, if this law that we guessed is right, to see what it would imply. And then we compare the computation results to nature, or we say compare to experiment or experience, compare it directly with observations to see if it works. If it disagrees with experiment, it's wrong. In that simple statement is the key to science. It doesn't make any difference how beautiful your guess is, it doesn't make any difference how smart you are, who made the guess, or what his name is. If it disagrees with experiment, it's wrong. That's all there is to it.”

Richard Feynman book The Character of Physical Law

same passage in transcript: video http://www.youtube.com/watch?v=-2NnquxdWFk&t=16m46s
The Character of Physical Law (1965)
Variant: In general we look for a new law by the following process. First we guess it. Then we compute the consequences of the guess to see what would be implied if this law that we guessed is right. Then we compare the result of the computation to nature, with experiment or experience, compare it directly with observation, to see if it works. If it disagrees with experiment it is wrong. In that simple statement is the key to science. It does not make any difference how beautiful your guess is. It does not make any difference how smart you are, who made the guess, or what his name is – if it disagrees with experiment it is wrong. That is all there is to it.

Law , Beauty , Nature , Science

“I think I can safely say that nobody understands quantum mechanics.”

Richard Feynman book The Character of Physical Law

Variant: I think I can safely say that nobody understands quantum mechanics.
Source: The Character of Physical Law (1965), chapter 6, “Probability and Uncertainty — the Quantum Mechanical View of Nature,” p. 129

Understanding , Thinking

“Whenever you see a sweeping statement that a tremendous amount can come from a very small number of assumptions, you always find that it is false. There are usually a large number of implied assumptions that are far from obvious if you think about them sufficiently carefully.”

Richard Feynman

volume II; lecture 26, "Lorentz Transformations of the Fields"; section 26-1, "The four-potential of a moving charge"; p. 26-2
The Feynman Lectures on Physics (1964)

Thinking , Falseness

“I hope … that you will find someday that, after all, it isn’t as horrible as it looks.”

Richard Feynman

volume III, "Feynman's Epilogue", p. 21-19
The Feynman Lectures on Physics (1964)

Hope

“You will have to brace yourselves for this — not because it is difficult to understand, but because it is absolutely ridiculous: All we do is draw little arrows on a piece of paper — that’s all!”

Richard Feynman book QED: The Strange Theory of Light and Matter

Source: QED: The Strange Theory of Light and Matter (1985), p. 24

Understanding , Drawing

“It has been a mystery ever since it was discovered more than fifty years ago, and all good theoretical physicists put this number up on their wall and worry about it. Immediately you would like to know where this number for a coupling comes from: is it related to π, or perhaps to the base of natural logarithms? Nobody knows. It's one of the greatest damn mysteries of physics: a magic number that comes to us with no understanding by man. You might say the "hand of God" wrote that number, and "we don't know how He pushed his pencil." We know what kind of a dance to do experimentally to measure this number very accurately, but we don't know what kind of dance to do on the computer to make this number come out — without putting it in secretly!”

Richard Feynman book QED: The Strange Theory of Light and Matter

On the numerical value of α, the fine-structure constant, p. 129
QED: The Strange Theory of Light and Matter (1985)

God , Understanding , Nature , Dance

“I can live with doubt, and uncertainty, and not knowing. I think it's much more interesting to live not knowing than to have answers which might be wrong. I have approximate answers, and possible beliefs, and different degrees of certainty about different things, but I'm not absolutely sure of anything. There are many things I don't know anything about, such as whether it means anything to ask "Why are we here?" I might think about it a little bit, and if I can't figure it out then I go on to something else. But I don't have to know an answer. I don't feel frightened by not knowing things, by being lost in the mysterious universe without having any purpose — which is the way it really is, as far as I can tell. Possibly. It doesn't frighten me.”

Richard Feynman book The Pleasure of Finding Things Out

Source: No Ordinary Genius (1994), p. 239, from interview in "The Pleasure of Finding Things Out" (1981): video http://www.youtube.com/watch?v=NEwUwWh5Xs4&t=48m10s

Way , Feeling , Thinking , Universe

“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.”

Richard Feynman

" Simulating Physics with Computers http://www.cs.berkeley.edu/~christos/classics/Feynman.pdf", International Journal of Theoretical Physics, volume 21, 1982, p. 467-488, at p. 486 (final words)

Nature , Problem

“There are 1011 stars in the galaxy. That used to be a huge number. But it's only a hundred billion. It's less than the national deficit! We used to call them astronomical numbers. Now we should call them economical numbers.”

Richard Feynman

from a 1987 class, as quoted in David L. Goodstein, "Richard P. Feynman, Teacher," Physics Today, volume 42, number 2 (February 1989) p. 70-75, at p. 73
Republished in the "Special Preface" to Six Easy Pieces (1995), p. xx.

Star

“The next question was — what makes planets go around the sun? At the time of Kepler some people answered this problem by saying that there were angels behind them beating their wings and pushing the planets around an orbit. As you will see, the answer is not very far from the truth. The only difference is that the angels sit in a different direction and their wings push inward.”

Richard Feynman book The Character of Physical Law

Source: The Character of Physical Law (1965), chapter 1, “The Law of Gravitation,” p. 18: video http://www.youtube.com/watch?v=j3mhkYbznBk&t=17m10s

People , Truth , Angels , Question

“There were certain things I didn't like, such as tipping. I thought we should be paid more, and not have to have any tips. But when I proposed that to the boss, I got nothing but laughter. She told everybody, "Richard doesn't want his tips, hee, hee, hee; he doesn't want his tips, ha, ha, ha." The world is full of this kind of dumb smart-alec who doesn't understand anything.”

Richard Feynman

Part 1: "From Rockaway to MIT", "String Beans", p. 25
Surely You're Joking, Mr. Feynman! (1985)

World , Understanding , Laughter

“The chance is high that the truth lies in the fashionable direction. But, on the off-chance that it is in another direction — a direction obvious from an unfashionable view of field theory — who will find it? Only someone who has sacrificed himself by teaching himself quantum electrodynamics from a peculiar and unfashionable point of view; one that he may have to invent for himself.”

Richard Feynman

"The Development of the Space-Time View of Quantum Electrodynamics," Nobel Lecture http://nobelprize.org/physics/laureates/1965/feynman-lecture.html (11 December 1965)

Truth , Fashion , Chance

“It is to be emphasized that no matter how many [amplitude] arrows we draw, add, or multiply, our objective is to calculate a single final arrow for the event. Mistakes are often made by physics students at first because they do not keep this important point in mind. They work for so long analyzing events involving a single photon that they begin to think that the arrow is somehow associated with the photon”

Richard Feynman book QED: The Strange Theory of Light and Matter

rather than with the event
Source: QED: The Strange Theory of Light and Matter (1985), p. 75-76

Thinking , Drawing

“Well, we’re getting a little philosophical and serious, ok? Let’s go back to what we’re doing. One day we look at a map and this capital is K-Y-Z-Y-L and we decided it would be fun to go there because it’s so obscure and peculiar. It’s a game. It’s not serious. It doesn't involve some deep philosophical point of view about authority or anything. It’s just the fun of having an adventure to try to go to a land that we’d never heard of, that we knew was an independent country once, no longer an independent country, find out what it’s like. And discover as we went along that nobody went there for a long time and it’s isolated made it more interesting. But, you know, many explorers liked to go to places that are unusual. And, it’s only for the fun of it. I don’t go for this philosophical interpretation of "our deeper understanding of what we’re doing."”

Richard Feynman

We haven’t any deep understanding of what we’re doing. If we tried to understand what we’re doing, we’d go nutty.
Source: No Ordinary Genius (1994), p. 236, from interview two weeks before his death in "The Quest for Tannu Tuva" (1989): video http://www.youtube.com/watch?v=Mn4_40hAAr0&t=51m49s

Game , Understanding , Time , Country

“If we take everything into account — not only what the ancients knew, but all of what we know today that they didn't know — then I think that we must frankly admit that we do not know.”

Richard Feynman

The Value of Science (1955)

Thinking

“Therefore psychologically we must keep all the theories in our heads, and every theoretical physicist who is any good knows six or seven different theoretical representations for exactly the same physics.”

Richard Feynman book The Character of Physical Law

Source: The Character of Physical Law (1965), chapter 7, “Seeking New Laws,” p. 168

“Shut up and calculate!”

Richard Feynman

Probably a misattribution which instead originated with David Mermin; in "Could Feynman Have Said This?" http://scitation.aip.org/journals/doc/PHTOAD-ft/vol_57/iss_5/10_1.shtml?bypassSSO=1, by N. David Mermin, in Physics Today (May 2004), p. 10, he notes that in an earlier Physics Today (April 1989), p. 9, he had written what appears to be the earliest occurrence of the phrase:
If I were forced to sum up in one sentence what the Copenhagen interpretation says to me, it would be "Shut up and calculate!"
Disputed and/or attributed

“Principles
You can't say A is made of B
or vice versa.
All mass is interaction.”

Richard Feynman

note (c. 1948), quoted in Genius: The Life and Science of Richard Feynman (1992) by James Gleick, p. 5 (repeated p. 283)

“Light is something like raindrops — each little lump of light is called a photon — and if the light is all one color, all the "raindrops" are the same.”

Richard Feynman book QED: The Strange Theory of Light and Matter

Source: QED: The Strange Theory of Light and Matter (1985), p. 14

Color , Light

“In fact, the science of thermodynamics began with an analysis, by the great engineer Sadi Carnot, of the problem of how to build the best and most efficient engine, and this constitutes one of the few famous cases in which engineering has contributed to fundamental physical theory. Another example that comes to mind is the more recent analysis of information theory by Claude Shannon. These two analyses, incidentally, turn out to be closely related.”

Richard Feynman

volume I; lecture 44, "The Laws of Thermodynamics"; section 44-1, "Heat engines; the first law"; p. 44-2
The Feynman Lectures on Physics (1964)

Problem , Science

“I don't know anything, but I do know that everything is interesting if you go into it deeply enough.”

Richard Feynman book The Pleasure of Finding Things Out

From Omni interview, "The Smartest Man in the World" (1979) p. 203
The Pleasure of Finding Things Out (1999)

“Nature's imagination far surpasses our own.”

Richard Feynman book The Character of Physical Law

Source: The Character of Physical Law (1965), chapter 7, “Seeking New Laws,” p. 162: video http://www.youtube.com/watch?v=-2NnquxdWFk&t=29m20s

Imagination , Nature

“One time I was in the men's room of the bar and there was a guy at the urinal. He was kind of drunk, and said to me in a mean-sounding voice, "I don't like your face. I think I'll push it in."I was scared green. I replied in an equally mean voice, "Get out of my way, or I'll pee right through ya!"”

Richard Feynman

Part 4: "From Cornell to Caltech, With a Touch of Brazil", "Any Questions?", p. 177
Surely You're Joking, Mr. Feynman! (1985)

Men , Way , Guy , Thinking

“There is one feature I notice that is generally missing in cargo cult science. … It's a kind of scientific integrity, a principle of scientific thought that corresponds to a kind of utter honesty — a kind of leaning over backwards. For example, if you're doing an experiment, you should report everything that you think might make it invalid — not only what you think is right about it; other causes that could possibly explain your results; and things you thought of that you've eliminated by some other experiment, and how they worked — to make sure the other fellow can tell they have been eliminated. Details that could throw doubt on your interpretation must be given, if you know them. You must do the best you can — if you know anything at all wrong, or possibly wrong — to explain it. If you make a theory, for example, and advertise it, or put it out, then you must also put down all the facts that disagree with it, as well as those that agree with it. There is also a more subtle problem. When you have put a lot of ideas together to make an elaborate theory, you want to make sure, when explaining what it fits, that those things it fits are not just the things that gave you the idea for the theory; but that the finished theory makes something else come out right, in addition. In summary, the idea is to try to give all of the information to help others to judge the value of your contribution; not just the information that leads to judgement in one particular direction or another.”

Richard Feynman

" Cargo Cult Science http://calteches.library.caltech.edu/51/2/CargoCult.htm", adapted from a 1974 Caltech commencement address; also published in Surely You're Joking, Mr. Feynman!, p. 341

Idea , Problem , Thinking , Science

“Our imagination is stretched to the utmost, not, as in fiction, to imagine things which are not really there, but just to comprehend those things which are there.”

Richard Feynman book The Character of Physical Law

Source: The Character of Physical Law (1965), chapter 6, “Probability and Uncertainty — the Quantum Mechanical View of Nature,” p. 127-128

Imagination

“I do feel strongly that this is nonsense! … So perhaps I could entertain future historians by saying I think all this superstring stuff is crazy and is in the wrong direction. I think all this superstring stuff is crazy and is in the wrong direction. … I don’t like it that they’re not calculating anything. … why are the masses of the various particles such as quarks what they are? All these numbers … have no explanations in these string theories – absolutely none! … I don’t like that they don’t check their ideas. I don’t like that for anything that disagrees with an experiment, they cook up an explanation—a fix-up to say, “Well, it might be true.” For example, the theory requires ten dimensions. Well, maybe there’s a way of wrapping up six of the dimensions. Yes, that’s all possible mathematically, but why not seven? When they write their equation, the equation should decide how many of these things get wrapped up, not the desire to agree with experiment. In other words, there’s no reason whatsoever in superstring theory that it isn’t eight out of the ten dimensions that get wrapped up and that the result is only two dimensions, which would be completely in disagreement with experience. So the fact that it might disagree with experience is very tenuous, it doesn’t produce anything.”

Richard Feynman

interview published in Superstrings: A Theory of Everything? (1988) edited by Paul C. W. Davies and Julian R. Brown, p. 193-194

Way , Feeling , Mathematics , Idea

“You know, the most amazing thing happened to me tonight. I was coming here, on the way to the lecture, and I came in through the parking lot. And you won't believe what happened. I saw a car with the license plate ARW 357. Can you imagine? Of all the millions of license plates in the state, what was the chance that I would see that particular one tonight? Amazing!”

Richard Feynman

from a public lecture, as quoted in David L. Goodstein, "Richard P. Feynman, Teacher," Physics Today, volume 42, number 2 (February 1989) p. 70-75, at p. 73
Republished in the "Special Preface" to Six Easy Pieces (1995), p. xxi.
Republished also in the "Special Preface" to the "definitive edition" of The Feynman Lectures on Physics, volume I, p. xiv.

Cars , Way , Imagination , Chance

“A person talks in such generalities that everyone can understand him and it's considered to be some deep philosophy. However, I would like to be very rather more special and I would like to be understood in an honest way, rather than in a vague way.”

Richard Feynman book The Character of Physical Law

Source: The Character of Physical Law (1965), chapter 1, “The Law of Gravitation,” p. 13: video http://www.youtube.com/watch?v=j3mhkYbznBk&t=7m53s

Way , Understanding , Personality

“My mother … had a wonderful sense of humor, and I learned from her that the highest forms of understanding we can achieve are laughter and human compassion.”

Richard Feynman book What Do You Care What Other People Think?

"The Making of a Scientist," p. 19
What Do You Care What Other People Think? (1988)

Understanding , Sense , Laughter , Learning

“So far as we know, all the fundamental laws of physics, like Newton’s equations, are reversible.”

Richard Feynman

volume I; lecture 46, "Ratchet and Pawl"; section 46-5, "Order and entropy"; p. 46-8
The Feynman Lectures on Physics (1964)

Law

“The imagination of nature is far, far greater than the imagination of man.”

Richard Feynman

The Value of Science (1955)

Imagination , Nature

“Perhaps you will not only have some appreciation of this culture; it is even possible that you may want to join in the greatest adventure that the human mind has ever begun.”

Richard Feynman

volume III, "Feynman's Epilogue", p. 21-19 (closing sentence)
The Feynman Lectures on Physics (1964)

“We absolutely must leave room for doubt or there is no progress and no learning. There is no learning without having to pose a question. And a question requires doubt. People search for certainty. But there is no certainty. People are terrified — how can you live and not know? It is not odd at all. You only think you know, as a matter of fact. And most of your actions are based on incomplete knowledge and you really don't know what it is all about, or what the purpose of the world is, or know a great deal of other things. It is possible to live and not know.”

Richard Feynman book The Pleasure of Finding Things Out

from lecture "What is and What Should be the Role of Scientific Culture in Modern Society", given at the Galileo Symposium in Italy (1964)
The Pleasure of Finding Things Out (1999)

People , World , Search , Question

“Incidentally, psychoanalysis is not a science: it is at best a medical process, and perhaps even more like witch-doctoring. It has a theory as to what causes disease—lots of different “spirits,” etc. The witch doctor has a theory that a disease like malaria is caused by a spirit which comes into the air; it is not cured by shaking a snake over it, but quinine does help malaria. So, if you are sick, I would advise that you go to the witch doctor because he is the man in the tribe who knows the most about the disease; on the other hand, his knowledge is not science. Psychoanalysis has not been checked carefully by experiment.”

Richard Feynman

volume I; lecture 3, "The Relation of Physics to Other Sciences"; section 3-6, "Psychology"; p. 3-8
The Feynman Lectures on Physics (1964)

Disease , Knowledge , Science , Help

“Energy is a very subtle concept. It is very, very difficult to get right.”

Richard Feynman

address " What is Science? http://www.fotuva.org/feynman/what_is_science.html", presented at the fifteenth annual meeting of the National Science Teachers Association, in New York City (1966), published in The Physics Teacher, volume 7, issue 6 (1969), p. 313-320

“I don't like honors. … I've already got the prize: the prize is the pleasure of finding the thing out, the kick in the discovery, the observation that other people use it. Those are the real things.”

Richard Feynman book The Pleasure of Finding Things Out

Source: No Ordinary Genius (1994), p. 82, from interview in "The Pleasure of Finding Things Out" (1981): video http://www.youtube.com/watch?v=NEwUwWh5Xs4&t=24m55s

People , Pleasure

“Since then I never pay attention to anything by "experts". I calculate everything myself.”

Richard Feynman

After having been led astray on neutron-proton coupling by reports of "beta-decay experts".
Part 5: "The World of One Physicist", "The 7 Percent Solution", p. 255
Surely You're Joking, Mr. Feynman! (1985)

“Do not read so much, look about you and think of what you see there.”

Richard Feynman

letter to Ashok Arora, 4 January 1967, published in Perfectly Reasonable Deviations from the Beaten Track (2005) p. 230

Thinking , Reading

“The Quantum Universe has a quotation from me in every chapter — but it's a damn good book anyway.”

Richard Feynman

Review blurb for the first edition of The Quantum Universe http://www.cambridge.org/catalogue/catalogue.asp?isbn=0521564573 (1987)

Books , Universe

“Every instrument that has been designed to be sensitive enough to detect weak light has always ended up discovering that the same thing: light is made of particles.”

Richard Feynman book QED: The Strange Theory of Light and Matter

Source: QED: The Strange Theory of Light and Matter (1985), p. 15

Light

“I took this stuff I got out of your [O-ring] seal and I put it in ice water, and I discovered that when you put some pressure on it for a while and then undo it it doesn't stretch back. It stays the same dimension. In other words, for a few seconds at least, and more seconds than that, there is no resilience in this particular material when it is at a temperature of 32 degrees. I believe that has some significance for our problem.”

Richard Feynman

statement at hearing by Rogers Commission, 11 February 1986, Report of the PRESIDENTIAL COMMISSION on the Space Shuttle Challenger Accident, volume 4, p. 680 http://history.nasa.gov/rogersrep/v4part4.htm#4; also quoted in Genius: The Life and Science of Richard Feynman (1992) by James Gleick, p. 423

Problem , Water

“If we suppress all discussion, all criticism, proclaiming "This is the answer, my friends; man is saved!" we will doom humanity for a long time to the chains of authority, confined to the limits of our present imagination. It has been done so many times before.”

Richard Feynman

The Value of Science (1955)
Context: We are at the very beginning of time for the human race. It is not unreasonable that we grapple with problems. But there are tens of thousands of years in the future. Our responsibility is to do what we can, learn what we can, improve the solutions, and pass them on.
... It is our responsibility to leave the people of the future a free hand. In the impetuous youth of humanity, we can make grave errors that can stunt our growth for a long time. This we will do if we say we have the answers now, so young and ignorant as we are. If we suppress all discussion, all criticism, proclaiming "This is the answer, my friends; man is saved!" we will doom humanity for a long time to the chains of authority, confined to the limits of our present imagination. It has been done so many times before.
... It is our responsibility as scientists, knowing the great progress which comes from a satisfactory philosophy of ignorance, the great progress which is the fruit of freedom of thought, to proclaim the value of this freedom; to teach how doubt is not to be feared but welcomed and discussed; and to demand this freedom as our duty to all coming generations.

Imagination , Time

“If, in some cataclysm, all of scientific knowledge were to be destroyed, and only one sentence passed on to the next generation of creatures, what statement would contain the most information in the fewest words? I believe it is the atomic hypothesis (or the atomic fact, or whatever you wish to call it) that all things are made of atoms — little particles that move around in perpetual motion, attracting each other when they are a little distance apart, but repelling upon being squeezed into one another. In that one sentence, you will see, there is an enormous amount of information about the world, if just a little imagination and thinking are applied.”

Richard Feynman

volume I; lecture 1, "Atoms in Motion"; section 1-2, "Matter is made of atoms"; p. 1-2
The Feynman Lectures on Physics (1964)

World , Imagination , Knowledge , Thinking

“When you're thinking about something that you don't understand, you have a terrible, uncomfortable feeling called confusion. It's a very difficult and unhappy business. And so most of the time you're rather unhappy, actually, with this confusion. You can't penetrate this thing. Now, is the confusion's because we're all some kind of apes that are kind of stupid working against this, trying to figure out [how] to put the two sticks together to reach the banana and we can't quite make it, the idea? And I get this feeling all the time that I'm an ape trying to put two sticks together, so I always feel stupid. Once in a while, though, the sticks go together on me and I reach the banana.”

Richard Feynman

Video interview http://www.youtube.com/watch?v=lytxafTXg6c dated 1963

Stupidity , Feeling , Idea , Understanding

“While in Kyoto I tried to learn Japanese with a vengeance. I worked much harder at it, and got to a point where I could go around in taxis and do things. I took lessons from a Japanese man every day for an hour.
One day he was teaching me the word for "see." "All right," he said. "You want to say, 'May I see your garden?' What do you say?"
I made up a sentence with the word that I had just learned.
"No, no!" he said. "When you say to someone, 'Would you like to see my garden?' you use the first 'see.' But when you want to see someone else's garden, you must use another 'see,' which is more polite."
"Would you like to glance at my lousy garden?" is essentially what you're saying in the first case, but when you want to look at the other fella's garden, you have to say something like, "May I observe your gorgeous garden?" So there's two different words you have to use.
Then he gave me another one: "You go to a temple, and you want to look at the gardens…"
I made up a sentence, this time with the polite "see."
"No, no!" he said. "In the temple, the gardens are much more elegant. So you have to say something that would be equivalent to 'May I hang my eyes on your most exquisite gardens?"
Three or four different words for one idea, because when I'm doing it, it's miserable; when you're doing it, it's elegant.
I was learning Japanese mainly for technical things, so I decided to check if this same problem existed among the scientists.
At the institute the next day, I said to the guys in the office, "How would I say in Japanese, 'I solve the Dirac Equation'?"
They said such-and-so.
"OK. Now I want to say, 'Would you solve the Dirac Equation?'”

Richard Feynman

how do I say that?"
"Well, you have to use a different word for 'solve,' " they say.
"Why?" I protested. "When I solve it, I do the same damn thing as when you solve it!"
"Well, yes, but it's a different word — it's more polite."
I gave up. I decided that wasn't the language for me, and stopped learning Japanese.
Part 5: "The World of One Physicist", "Would <U>You</U> Solve the Dirac Equation?", p. 245-246
Surely You're Joking, Mr. Feynman! (1985)

Guy , Idea , Problem , Learning

“Anyway, I have to argue about flying saucers on the beach with people, you know. And I was interested in this: they keep arguing that it is possible. And that's true. It is possible. They do not appreciate that the problem is not to demonstrate whether it's possible or not but whether it's going on or not.”

Richard Feynman book The Meaning of It All

lecture III: "This Unscientific Age"
The Meaning of It All (1999)

People , Problem , Flying

“We scientists are clever — too clever — are you not satisfied? Is four square miles in one bomb not enough? Men are still thinking. Just tell us how big you want it!”

Richard Feynman

note (c. 1945), quoted in Genius: The Life and Science of Richard Feynman (1992) by James Gleick, p. 204

Men , Thinking

“The "paradox" is only a conflict between reality and your feeling of what reality "ought to be."”

Richard Feynman

volume III; lecture 18, "Angular Momentum"; section 18-3, "The annihilation of positronium"; p. 18-9
The Feynman Lectures on Physics (1964)