Quotes from 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... 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 Feynman, quote from 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 Feynman, quote from QED: The Strange Theory of Light and Matter

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

“Figure 18. As the thickness of a layer increases, the two surfaces produce a partial reflection of monochromatic light whose probability fluctuates in a cycle from 0% to 16%. Since the speed of the imaginary stopwatch hand is different for different colors of light, the cycle repeats itself at different rates. Thus when two colors such as pure red and pure blue are aimed at the layer, a given thickness will reflect only red, only blue, both red and blue in different proportions (which produce various hues of violet), or neither color (black). If the layer is of varying thicknesses, such as a drop of oil spreading out on a mud puddle, all of the combinations will occur. In sunlight, which consists of all colors, all sorts of combinations occur, which produce lots of colors.”
― Richard Feynman, quote from QED: The Strange Theory of Light and Matter

“Figure 36. A "trick" can be played on Nature by slowing down the light that takes shorter paths: glass of just the right thickness is inserted so that all the paths will take exactly the same time. This causes all of the arrows to point in the same direction, and to produce a whopping final arrow-lots of light! Such a piece of glass made to greatly increase the probability of light getting from a source to a single point is called a focusing lens.”
― Richard Feynman, quote from QED: The Strange Theory of Light and Matter

“To understand this better, we need to know that the cycle of zero to 16% partial reflection by two surfaces repeats more quickly for blue light than for red light. Thus at certain thicknesses, one or the other or both colors are strongly reflected, while at other thicknesses, reflections of both colors is cancelled out (see Fig. 18). The cycles of reflection repeat at different rates because the stopwatch hand turns around faster when it times a blue photon than it does when timing a red photon. In fact, that's the only difference between a red photon and a blue photon (or a photon of any other color, including radio waves, X-rays, and so on)-the speed of the stopwatch hand.”
― Richard Feynman, quote from QED: The Strange Theory of Light and Matter

“With the thinnest possible layer of glass, we find that the number of photons arriving at A is nearly always zero-sometimes it's like 1. When we replace the thinnest layer with a slightly thicker one, we find that the amount of light reflected is higher-closer to the expected 8%. After a few more replacements the count of photons arriving at A increases past the 8% mark. As we continue to substitute still "thicker " layers of glass-we're up to about 5 millionths of an inch now-the amount of light reflected by the two surfaces reaches a maximum of 16%, and then goes down, through 8%, back to zero-if the layer of glass is just the right thickness, there is no reflection at all. (Do that with spots!)

With gradually thicker and thicker layers of glass, partial reflection again increases to 16% and returns to zero-a cycle that repeats itself again and again(see Fig. 5). Newton discovered these oscillations and did one experiment that could be correctly interpreted only if the oscillations continued for 34,000 cycles! Today, with lasers (which produce a very pure, monochromatic light), we can see this cycle still going strong after more than 100,000,000 repetitions-which corresponds to glass that is more than 50 meters thick. (We don't see this phenomenon every day because the light source is normally not monochromatic.)

So it turns out that our prediction of 8% is right as an overall average (since the actual amount varies in a regular pattern from zero to 16%), but it's exactly right only twice each cycle-like a stopped clock (which is right twice a day).”
― Richard Feynman, quote from 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, quote from 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, quote from 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, quote from 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, quote from 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, quote from QED: The Strange Theory of Light and Matter

“the price of gaining such an accurate theory has been the erosion of our common sense.”
― Richard Feynman, quote from 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, quote from 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, quote from 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, quote from QED: The Strange Theory of Light and Matter

“Thus, some things that satisfy the rules of algebra can be interesting to mathematicians even though they don't always represent a real situation. Arrows on a plane can be "added" by putting the head of one arrow on the tail of another, or "multiplied" by successive turns and shrinks. Since these arrows obey the same rules of algebra as regular numbers, mathematicians call them numbers. But to distinguish them from ordinary numbers, they're called "complex numbers." For those of you who have studied mathematics enough to have come to complex numbers, I could have said, "the probability of an event is the absolute square of a complex number. When an event can happen in alternative ways, you add the complex numbers; when it can happen only as a succession of steps, you multiply the complex numbers." Although it may sound more impressive that way, I have not said any more than I did before-I just used a different language.”
― Richard Feynman, quote from 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, quote from QED: The Strange Theory of Light and Matter

“To summarize, where the time is least is also where the time for the nearby paths is nearly the same; that's where the little arrows point in nearly the same direction and add up to a substantial length; that's where the probability of a photon reflecting off a mirror is determined. And that's why, in approximation, we can get away with the crude picture of the world that says that light only goes where the time is least (and it's easy to rpve that were the time is least, the angle of incidence is equal to the angle of reflection, but I don't have the time to show you).

So the theory of quantum electrodynamics gave the right answer-the middle of the mirror is the important part for reflection-but this correct result came out at the expense of believing that light reflects all over the mirror, and having to add a bunch of little arrows together whose sole purpose was to cancel out. All that might seem to you to be a waste of time-some silly game for mathematicians only. After all, it doesn't seem like "real physics" to have something there that only cancels out!”
― Richard Feynman, quote from QED: The Strange Theory of Light and Matter

“Throughout these lectures I have delighted in showing you that the price of gaining such an accurate theory has been the erosion of our common sense. We must accept some very bizarre behavior: the amplification and suppression of probabilities, light reflecting from all parts of a mirror, light travelling in paths other than a straight line, photons going faster or slower than the conventional speed of light, electrons going backwards in time, photons suddenly disintegrating into a positron-electron pair, and so on. That we must do, in order to appreciate what Nature is really doing underneath nearly all the phenomena we see in the world.”
― Richard Feynman, quote from 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, quote from 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, quote from QED: The Strange Theory of Light and Matter

“Thus 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 size.”
― Richard Feynman, quote from 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, quote from QED: The Strange Theory of Light and Matter

“In this way, great globs of physics theory were synthesized into a simplified theory. The theory of gravitation, on the other hand, was not understandable from the laws of motion, and even today it stands isolated from the other theories. Gravitation is, so far, not understandable in terms of other phenomena.”
― Richard Feynman, quote from QED: The Strange Theory of Light and Matter

“It is to be emphasized that no matter how many 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. But these arrows are probability amplitudes, that give, when squared, the probability of a complete event.”
― Richard Feynman, quote from QED: The Strange Theory of Light and Matter

“Most of the phenomena you are familiar with involve the interaction of light and electrons-all of chemistry and biology, for example. The only phenomena that are not covered by this theory are phenomena of gravitation and nuclear phenomena; everything else is contained in this theory.”
― Richard Feynman, quote from QED: The Strange Theory of Light and Matter

“When we deal with probabilities under ordinary circumstances, there are the following "rules of composition": 1) if something can happen in alternative ways, we add the probabilities for each of the different ways; 2) i the event occurs as a succession of steps-or depends on a number of things happening "concomitantly" (independently)-then we multiply the probabilities of each of the steps (or things).”
― Richard Feynman, quote from 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, quote from QED: The Strange Theory of Light and Matter

About the author

Popular quotes

“Shit. He was perfect. And he was a killer.”
― Tijan, quote from Carter Reed

“She was afraid to be angry and that made her angry.”
― Marilynne Robinson, quote from Home

“Their other hands flipped up, palm to palm, and Merik’s only consolation as he and the domna slid into the next movement of the dance was that her chest heaved as much as his did. Merik’s right hand gripped the girl’s, and with no small amount of ferocity, he twisted her around to face the same direction as he before wrenching her to his chest. His hand slipped over her stomach, fingers splayed. Her left hand snapped up—and he caught it. Then the real difficulty of the dance began. The skipping of feet in a tide of alternating hops and directions. The writhing of hips countered the movement of their feet like a ship upon stormy seas. The trickling tap of Merik’s fingers down the girl’s arms, her ribs, her waist—like the rain against a ship’s sail. On and on, they moved to the music until they were both sweating. Until they hit the third movement. Merik flipped the girl around to face him once more. Her chest slammed against his—and by the Wells, she was tall. He hadn’t realized just how tall until this precise moment when her eyes stared evenly into his and her panting breaths fought against his own. Then the music swelled once more, her legs twined into his, and he forgot all about who she was or what she was or why he had begun the dance in the first place. Because those eyes of hers were the color of the sky after a storm. Without realizing what he did, his Windwitchery flickered to life. Something in this moment awoke the wilder parts of his power. Each heave of his lungs sent a breeze swirling in. It lifted the girl’s hair. Kicked at her wild skirts. She showed no reaction at all. In fact, she didn’t break her gaze from Merik, and there was a fierceness there—a challenge that sent Merik further beneath the waves of the dance. Of the music. Of those eyes. Each leap backward of her body—a movement like the tidal tug of the sea against the river—led to a violent slam as Merik snatched her back against him. For each leap and slam, the girl added in an extra flourishing beat with her heels. Another challenge that Merik had never seen, yet rose to, rose above. Wind crashed around them like a growing hurricane, and he and this girl were at its eye. And the girl never looked away. Never backed down. Not even when the final measures of the song began—that abrupt shift from the sliding cyclone of strings to the simple plucking bass that follows every storm—did Merik soften how hard he pushed himself against this girl. Figuratively. Literally. Their bodies were flush, their hearts hammering against each other’s rib cages. He walked his fingers down her back, over her shoulders, and out to her hands. The last drops of a harsh rain. The music slowed. She pulled away first, slinking back the required four steps. Merik didn’t look away from her face, and he only distantly noticed that, as she pulled away, his Windwitchery seemed to settle. Her skirts stopped swishing, her hair fluttered back to her shoulders. Then he slid backward four steps and folded his arms over his chest. The music came to a close. And Merik returned to his brain with a sickening certainty that Noden and His Hagfishes laughed at him from the bottom of the sea.”
― Susan Dennard, quote from Truthwitch

“لم يكن الشعور بالذنب هو دافعه الوحيد للمخاطرة، بل كره المذلة التي تجعل من الحياة نصف حياة، ومن الإنسان نصف إنسان. أراد أن يضع حياته في كفة الميزان، والموت في الكفة الأخرى. كان يود أن يقيس كل فعل من أفعاله، بل كل يوم من أيامه، وكل ساعة، وكل ثانية، بالمعيار الأسمى الذي هو الموت.”
― Milan Kundera, quote from Life is Elsewhere

“These people were content with their environment, and felt no particular objection to an impersonal steel and concrete landscape, no qualms about the invasion of their privacy by government agencies and organizations, and if anything welcoming these intrusions, using them for their own purposes. These people were the first to master a new kind of 20th century life. They thrived on the rapid turnover of acquaintances, the lack of involvement with others, and the total self-sufficiency of lives which, needing nothing, were never disappointed. Alternatively, their real needs might emerge later.”
― J.G. Ballard, quote from High-Rise

Interesting books

About BookQuoters

BookQuoters is a community of passionate readers who enjoy sharing the most meaningful, memorable and interesting quotes from great books. As the world communicates more and more via texts, memes and sound bytes, short but profound quotes from books have become more relevant and important. For some of us a quote becomes a mantra, a goal or a philosophy by which we live. For all of us, quotes are a great way to remember a book and to carry with us the author’s best ideas.

We thoughtfully gather quotes from our favorite books, both classic and current, and choose the ones that are most thought-provoking. Each quote represents a book that is interesting, well written and has potential to enhance the reader’s life. We also accept submissions from our visitors and will select the quotes we feel are most appealing to the BookQuoters community.

Founded in 2023, BookQuoters has quickly become a large and vibrant community of people who share an affinity for books. Books are seen by some as a throwback to a previous world; conversely, gleaning the main ideas of a book via a quote or a quick summary is typical of the Information Age but is a habit disdained by some diehard readers. We feel that we have the best of both worlds at BookQuoters; we read books cover-to-cover but offer you some of the highlights. We hope you’ll join us.