QED Quotes

“What I am going to tell you about is what we teach our physics students in the third or fourth year of graduate school... It is my task to convince you not to turn away because you don't understand it. You see my physics students don't understand it... That is because I don't understand it. Nobody does.”
― Richard P. Feynman, QED: The Strange Theory of Light and Matter

“There is a most profound and beautiful question associated with the observed coupling constant, e - the amplitude for a real electron to emit or absorb a real photon. It is a simple number that has been experimentally determined to be close to 0.08542455. (My physicist friends won't recognize this number, because they like to remember it as the inverse of its square: about 137.03597 with about an uncertainty of about 2 in the last decimal place. 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 pi 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 P. Feynman, QED: The Strange Theory of Light and Matter

“We cannot predict whether a given photon will arrive at A or B. All we can predict is that out of 100 photons that come down, an average of 4 will be reflected by the front surface. Does this mean that physics, a science of great exactitude, has been reduced to calculating only the probability of an event, and not predicting exactly what will happen? Yes. That's a retreat, but that's the way it is: Nature permits us to calculate only probabilities. Yet science has not collapsed.”
― Richard Feynman, QED: The Strange Theory of Light and Matter

“The lay reader only wanted to have the illusion of understanding and to catch a few buzzwords to throw around at cocktail parties.”
― Richard Feynman, QED: The Strange Theory of Light and Matter

“With quantum physics, who needs drugs?”
― Richard P. Feynman, QED: The Strange Theory of Light and Matter

“atoms in the air scatter light from the sun and make the sky blue”
― Richard Feynman, QED: The Strange Theory of Light and Matter

“Why are all the theories of physics so similar in their structure?

There are a number of possibilities. The first is the limited imagination of physicists: when we see a new phenomenon we try to fit it into the framework we already have-until we have made enough experiments, we don't know that it doesn't work.

Another possibility is that it is the same damn thing over and over again-that Nature has only one way of doing things, and She repeats her story from time to time.

A third possibility is that things look similar because they are aspects of the same thing- some larger picture underneath, from which things can be broken into parts that look different, like fingers on the same hand. Many physicists are working very hard trying to put together a grand picture that unifies everything into one super-duper model. It's a delightful game, but at the present time none of the speculators agree with any of the other speculators as to what the grand picture is.”
― Richard Feynman, QED: The Strange Theory of Light and Matter

“Because “extra-difficult popular physics books” scare publishers half to death. Hawking famously said that every equation halves the sale of a popular book.”
― Richard P. Feynman, QED: The Strange Theory of Light and Matter

“Mysteries like these repeating cycles make it very interesting to be a theoretical physicist: Nature gives us such wonderful puzzles! Why does She repeat the electron at 206 times and 3,640 times its mass?”
― Richard Feynman, QED: The Strange Theory of Light and Matter

“There are only two states of polarization available to electrons, so in an atom with three protons in the nucleus exchanging photons with three electrons-a condition called a lithium atom-the third electron is farther away from the nucleus than the other two (which have used up the nearest available space), and exchanges fewer photons. This causes the electron to easily break away from its own nucleus under the influence of photons from other atoms. A large number of such atoms close together easily lose their individual third electrons to form a sea of electrons swimming around from atom to atom. This sea of electrons reacts to any small electrical force (photons), generating a current of electrons-I am describing lithium metal conducting electricity. Hydrogen and helium atoms do not lose their electrons to other atoms. They are "insulators."

All the atoms-more than one hundred different kinds-are made up of a certain number of protons exchanging photons with the same number of electrons. The patterns in which they gather are complicated and offer an enormous variety of properties: some are metals, some are insulators, some are gases, others are crystals; there are soft things, hard things, colored things, and transparent things-a terrific cornucopia of variety and excitement that comes from the exclusion principle and the repetition again and again and again of the three very simple actions P(A to B), E(A to B), and j. (If the electrons in the world were unpolarized, all the atoms would have very similar properties: the electrons would all cluster together, close to the nucleus of their own atom, and would not be easily attracted to other atoms to make chemical reactions.)”
― Richard Feynman, QED: The Strange Theory of Light and Matter

“It is hard to believe that nearly all the vast apparent variety in Nature results from the monotony of repeatedly combining just these three basic actions. But it does. I'll outline a bit of how some of this variety arises.”
― Richard Feynman, QED: The Strange Theory of Light and Matter

“The probability of an event is always represented by a single final arrow-no matter how many arrows were drawn, multiplied, and added to achieve it.”
― Richard Feynman, QED: The Strange Theory of Light and Matter

“Le cose di cui parlerò le insegniamo agli studenti di fisica degli ultimi anni di università: ora, voi pensate che io riuscirò a spiegarle in modo da farvele capire? Ebbene no, non le capirete. Perché, allora, farvi perdere del tempo? Per convincervi a non andar via solo perché questa conferenza vi risulterà incomprensibile, vi dirò che anche i miei studenti di fisica non capiscono queste cose. E non le capiscono perché non le capisco nemmeno io. Il fatto è che non le capisce nessuno.”
― Richard P. Feynman, QED: The Strange Theory of Light and Matter

“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.”
― Richard P. Feynman, QED: The Strange Theory of Light and Matter

“What I am going to tell you about is what we teach our physics students in the third or fourth year of graduate school—and you think I’m going to explain it to you so you can understand it? No, you’re not going to be able to understand it. Why, then, am I going to bother you with all this? Why are you going to sit here all this time, when you won’t be able to understand what I am going to say? It is my task to convince you not to turn away because you don’t understand it. You see, my physics students don’t understand it either. That is because I don’t understand it. Nobody does.”
― Richard P. Feynman, QED: The Strange Theory of Light and Matter

“According to Feynman, to learn QED you have two choices: you can either go through seven years of physics education or read this book.”
― Richard P. Feynman, QED: The Strange Theory of Light and Matter

“In the acknowledgment, Feynman decried popular physics books as achieving “apparent simplicity only by describing something different, something considerably distorted from what they claim to be describing.” Instead, he posed himself the challenge of describing QED to the lay reader without “distortion of the truth.” Thus, you should not think of this book as a typical popular physics book. Neither is it a textbook. A rare hybrid it is instead.”
― Richard P. Feynman, QED: The Strange Theory of Light and Matter

“Richard Feynman (1918–1988) was not only an extraordinary physicist, but also an extraordinary figure, a swash-buckling personality the likes of which theoretical physics has not seen before or hence. Occasionally theoretical physicists will while away an idle moment comparing the contributions of Feynman and Schwinger, both nice Jewish boys from New York and almost exact contemporaries. This senseless discussion serves no purpose, but it is a fact that while Julian Schwinger was a shy and retiring person (but rather warm and good-hearted behind his apparent remoteness), Dick Feynman was an extreme extrovert, the stuff of legends. With his bongo drums, showgirls, and other trappings of a carefully cultivated image enthusiastically nurtured by a legion of idolaters, he is surely the best-loved theoretical physicist next to Einstein.”
― Richard P. Feynman, QED: The Strange Theory of Light and Matter

“Nature permits us to calculate only probabilities. Yet science has not collapsed.”
― Richard P. Feynman, QED: The Strange Theory of Light and Matter

“Throughout this entire story there remains one especially unsatisfactory feature: the observed masses of the particles, m. There is no theory that adequately explains these numbers. We use the numbers in all our theories, but we don't understand them-what they are, or where they come from. I believe that from a fundamental point of view, this is a very interesting and serious problem.”
― Richard Feynman, QED: The Strange Theory of Light and Matter

“Whether the proton decays or not is not known. To prove that it does not decay is very difficult.”
― Richard Feynman, QED: The Strange Theory of Light and Matter

“Underneath so many of the phenomena we see every day are only three basic actions: one is described by the simple coupling number, j; the other two by functions-P(A to B) and E(A to B)- both of which are closely related. That's all there is to it, and from it all the rest of the laws of physics come.”
― Richard Feynman, QED: The Strange Theory of Light and Matter

“You might wonder how such simple actions could produce such a complex world. It's because phenomena we see in the world are the result of an enormous intertwining of tremendous numbers of photon exchanges and interferences. Knowing the three fundamental actions is only a very small beginning toward analyzing any real situation.”
― Richard Feynman, QED: The Strange Theory of Light and Matter

“The aversion of two electrons to occupy the same place in space-time is called the "Exclusion Principle," and accounts for the great variety of atoms in the universe.”
― Richard Feynman, QED: The Strange Theory of Light and Matter

“The chance that an atom emits a photon is enhanced if some photons (in a state that the atom can emit into) are already present. This phenomenon of "stimulated emission" was discovered by Einstein when he launched the quantum theory proposing the photon model of light. Lasers work on the basis of this phenomenon.”
― Richard Feynman, QED: The Strange Theory of Light and Matter

“In this intuitively easy analysis, the "front surface" and "back surface" arrows are mathematical constructions that give us the right answer, whereas the analysis we just did-with the space-time drawing and the arrows forming part of a circle-is a more accurate representation of what is really going on: partial reflection is the scattering of light by electrons inside the glass.”
― Richard Feynman, QED: The Strange Theory of Light and Matter

“A source of white light-many colors mixed together-emits photons in a chaotic manner: the angle of the amplitude changes abruptly and irregularly in fits and starts. But when we construct a monochromatic source, we are making a device that has been carefully arranged so that the amplitude for a photon to be emitted at a certain time is easily calculated: it changes its angle at a constant speed, like a stopwatch hand. (Actually, this arrow turns at the same speed as the imaginary stopwatch we used before, but in the opposite direction-see Fig. 67.)”
― Richard Feynman, QED: The Strange Theory of Light and Matter

“Now, let's look again at the partial reflection of light by a layer of glass. How does it work? I talked about light reflected from the front surface and the back surface. This idea of surfaces was a simplification I made in order to keep things easy at the beginning. Light is really not affected by surfaces. An incoming photon is scattered by the electrons in the atoms inside the glass, and a new photon comes back up to the detector. It's interesting that instead of adding up all the billions of tiny arrows that represent the amplitude for all the electrons inside the glass to scatter an incoming photon, we can add just two arrows-for the "front surface" and "back surface" reflections-and come out with the same answer. Let's see why.”
― Richard Feynman, QED: The Strange Theory of Light and Matter

“The backwards-moving electron when viewed with time moving forwards appears the same as an ordinary electron, except it's attracted to normal electrons-we say it has a "positive charge." (Had I included the effects of polarization, it would be apparent why the sign of j for the backwards-moving electron appears reversed, making the charge appear positive.) For this reason it's called a "positron." The positron is a sister particle to the electron, and is an example of an "anti-particle."

This phenomenon is general. Every particle in Nature has an amplitude to move backwards in time, and therefore has an anti-particle. When a particle and its anti-particle collide, they annihilate each other and form other particles. (For positrons and electrons annihilating, it is usually a photon or two.) And what about photons? Photons look exactly the same in all respects when they travel backwards in time-as we saw earlier-so they are their own anti-particles. You see how clever we are at making an exception part of the rule!”
― Richard Feynman, QED: The Strange Theory of Light and Matter

“So now, I present to you the three basic actions, from which all the phenomena of light and electrons arise.

-ACTION #1: A photon goes from place to place.

-ACTION #2: An electron goes from place to place.

-ACTION #3: An electron emits or absorbs a photon.

Each of these actions has an amplitude-an arrow-that can be calculated according to certain rules.”
― Richard Feynman, QED: The Strange Theory of Light and Matter