QED: The Strange Theory of Light and Matter (Princeton Science Library)

I only have high school physics (which completely left out quantum mechanics), but managed to learn something about quantum mechanics in this book (after reading it twice, while taking down notes). The introduction was very helpful in explaining the 3 main ways to understand quantum mechanics and putting Feynman's lecture on QED in some context. The lecture itself was really illuminating, without much math involved. I had not previously come across anyone else explain quantum mechanics in the way that Feynman does in this lecture.BTW, I personally don't have any great difficulty with a probabilistic view of quantum particles. I'm a little mystified why this idea of reality as probabilistic is objectionable to many physicists (including apparently Einstein). Perhaps this reflects my own ignorance, but for now, I don't find it objectionable. On the contrary, it seems wonderful to me that quantum particles are probabilistic. I would be troubled if the probabilities cannot be calculated or predicted, but (as far as I understand it) this is not the case. In fact, it's amazing that physicists can calculate the probabilities with such precision.

Feynman was a world famous, Nobel Prize winning physicist. He gave four lectures on QED, quantum electrodynamics, to explain the theory to a non scientific friend Alix Mautner, and wrote this book based on these lectures. QED has the best agreement between theory and experiment- accurate to 12 significant figures, of any scientific theory. I read the book first then watched his QED lectures on YouTube. It would have worked better to watch the lecture, read that chapter in the book, then watch the next lecture, etc. The questions and answers at the end of the videos are particularly helpful since the audience asks many of the same questions that I had while reading and watching the lectures.In his first lecture he starts with light reflecting off a glass surface and puts a detector next to the light source to measure reflected light. For every 100 photons that leave the light source, 96 enter the glass and 4 are reflected. If all the photons are the same, how do 4 of them know to be reflected? Next he considers a very thin piece of glass where again the reflected light is measured but there are two reflecting surfaces, the top and the bottom of the glass. The number of photons reflected now varies from 0 to 16. As the thickness of the glass is increased the number of photons reflected goes up till it reached 16 then decreases to 0 and oscillates between 0 and 16 as the thickness increases.To explain this variation he uses an arrow with a certain length to represent the reflected photon, where the length squared is the probability that the photon will hit the detector. A stopwatch with a large hand is added to measure how long it takes for the photon to be detected, and the arrow representing the photon is given the same angle as the large hand on the stopwatch. The arrow representing the photon from the front face is reversed in direction from the stopwatch hand, while the bottom surface is given the same direction as the stopwatch hand. The arrows are now vectors with size and direction. To get the probability of a photon reflecting from the top and bottom surfaces you add the arrows for each tail to head. The photons interfere with each other and when their directions are opposite they cancel and no photons are measured. I find it easier to picture the light as sine waves which can interfere constructively and add together, or destructively to go to zero. As a result you can get any percentage between zero and 16 percent for the intensity.Since 4% of the photons are reflected from a single surface the arrow length is 0.2 (0.2 squared is 0.04 or 4% probability for constructive interference), and if they destructively interfere then 0.2 - 0.2 = 0 no photons are seen. For two reflecting surfaces when both the arrows are in the same direction the squares add to 16% ( 0.2 + 0.2 = 0.4, then squared = 0.16 or 16% ). We calculate the observed 4% reflected photons from the experiment with a single reflecting surface. The interference effect is responsible for changing the percent of reflected light when we have two reflecting surfaces and is confirmed by other experiments. Note that we still do not know why 4% of the photons are reflected from the single glass surface, but using this straightforward method we can make very accurate predictions on the overall behavior of photons. We can only talk about probabilities for individual photons. The individual photons that are going to be reflected cannot be identified beforehand. Only the probability that a photon will be reflected can be measured. You can generalize this procedure for a number of reflecting glass surfaces. QED does not explain why 4% of the photons are reflected, but it does allow us predict what will happen very accurately.In his second lecture he describes the light that hits a mirror and is reflected at the same angle that the light hit the mirror, i.e., the angle of incidence equals the angle of reflection of the light. He takes into account light reflecting from every part of the mirror, and shows that the contribution of reflection from areas distant from the center of the mirror can be ignored. This is not because no light is being reflected, rather the interfering light cancels itself out. If a large number of parallel lines are etched on the mirror surface, they prevent light from hitting the lines and being reflected. This is a diffraction grating that reflects light separated as different colors, and looks like a rainbow, for example a CD or DVD reflects light this way. Numerous diagrams are used to make his arguments clear, and they help. He winds up the several interesting ideas. Light doesn't really travel only in a straight line, somehow it senses the neighboring paths around it and uses a small core of nearby space. The Heisenberg Uncertainty Principle, that we cannot know both the position and momentum of a photon, is no longer needed under QED. The usual graduate level explanation of this principle is that we need to use light of a certain energy to probe a photon or electron to find its position and in the process we added an unknown quantity of energy so we no longer know the energy of the particle. More advanced explanations consider this insufficient and that there is a basic unavoidable uncertainty in making the measurement.The way we look at things is usually too simple, such as thinking that light travels in a straight line. It actually travels many ways, most of which cancel each other out, which results in light looking like it is traveling in a straight line. When we take into account all possible paths some problems like the Heisenberg Uncertainty Principle disappear. Feynman is applying the principle of least action that he learned in high school and was first formulated by Pierre- Louis de Maupertuis in 1744. The principal says if you take the trajectory of a particle or artillery shell the most likely path is the one with the least action. To get the least action you measure the difference between the kinetic energy and the potential energy of the particle over the entire path, that is mathematically integrate the least action function over the possible paths of the particle. The path with the smallest difference between the two energies is the least action and is the one that is followed by the particle. Feynman uses this method of least action for many of his breakthrough ideas.The way a magnifying glass works is that all the light going through the lens will meet at the same point. In order to be in focus, every path must take the same amount of time. A ray of light passing through the edge of a lens to the focal point has a much longer path than one going through the center of the lens to the focal point. The light going through the center has to be slowed down so the times for the two beams are equal. This is accomplished by having a lot of glass at the center of the magnifying lens (it is double convex). The glass always appears to slow the speed of light, and more of the glass in the center slows the light more- the effect is that all the light from various parts of the lens reaches the focal point at the same time.The probability of an event happening is the absolute square of its arrow, the arrow represents a complex number. When the event can happen in separate, alternative ways you add the complex numbers or arrows. When an event can happen only as a succession of steps you multiply the complex numbers for each step. Most of his explanations are clearer with his diagrams, but they are rather detailed. You can see his reasoning, he breaks the reflection of light into simple steps, then considers in detail what happens at each step and finds a way to combine the different steps to be able to predict what will happen. One of his favorite tricks it Is to simplify the problem by figuring which of the very large number of possibilities cancel each other out or contribute very little so they can be ignored.In the third lecture he shows that photons don’t bounce off a glass surface, they are absorbed by an electron then reemitted anyplace within the glass. In making a simple graph or model he limits the motion to the x-axis and use time for the vertical axis. He uses solid lines to represent electrons, and wiggly lines for photons. At any point an electron can absorb or emit photons. Electrons and photons can travel forward and backward in time and in space. An electron traveling back in time is a positron (a positive electron). You can begin to see that he is making a model that allows all possibilities, whether a beam of light traveling in an unlikely zigzag pattern to get to the second point, or an electron traveling back in time, everything is possible and is included at first. Then consideration is made as to how likely a given event is and if it is not very likely it is excluded.At this point Feynman begins to talk about the amplitude of a photon that is moving a certain way or in a certain direction, he is relating this to the amplitude squared which is the probability of the event happening. Although you can predict accurately what will happen for an incoming ray from anywhere and what percent of it will reach a detector by just considering the front and back surface of the glass, what actually happens is that the photons are absorbed by the electrons in the glass and reemitted somewhat randomly. This is called scattering and causes some well known effects such as the sky appearing blue (blue light has a shorter wavelength than other visible colors and scatters the most). Although it is a bit tedious the detailed analysis shows how the repeated application of three simple actions explain the world around us.The actions are:A photon goes from place to place. P(A to B)An electron goes from place to place. E(A to B)An electron emits or absorbs a photon. j (-0.1) coupling numberIt's helps to visualize photons and electrons as waves, like a sine wave. These waves can interfere with each other. When they interfere constructively the waves add together and you get a strong signal since the arrows from the stopwatch are parallel and point in the same direction. When they are out of phase they essentially destroy each other. The arrows are parallel and point in the opposite directions and you get no signal. There can be any degree of interference between these two extremes. He cheats a little, thankfully, by not doing the actual calculation, which could require years of graduate school to understand throughly. There is still a lot of detail to wade through. The reward is that you get to travel along with one of the best minds in the area and learn how he perceives and solves problems.The agreement between the calculated and experimental values of the Dirac electron magnetic moment, an indication of the validity of QED, is now agreeing to about 12 significant figures. The calculation is difficult since it involves including terms such as j*j*j*j*j which involves hundreds of thousand of calculations.When arrows representing the amplitude of reflections from a glass surface are attached head to tail, they form a small part of a circle when the glass is thin, and up to a half circle when the glass is thicker. As the glass gets still thicker the arc starts getting smaller till it goes to zero then increases again. The resultant arrow from the first tail to the last head is the amplitude or probability for the whole set of reflections. If you connect the tail of the resultant with a radius from the center of the circle, then to the head of the resultant from the center of the circle you have two amplitudes of 0.2 apiece. Squaring and adding these gives probability 4% for the reflection mentioned in the beginning that agrees with experiment (see Fig 68 p 104). This reflected light is due to photons being absorbed by electrons and reemitted in random directions (this is scattering). This absorption and reemission process takes a small amount of time, causing the reflected photons to take longer traveling through the glass, and making them appear to be going slower. They are still traveling at the speed of light while moving between electrons, but it takes time and causes a pause when being absorbed and reemitted.In laser materials the electrons in the lasers atoms are held in a higher energy state by an external electric field (or heat). When they absorb a photon of the correct frequency (the energy equal to the difference in energy between the energy level the electron is in and the higher energy state), this photon and another identical photon are then emitted. Each collision of a photon results in an additional identical photon being emitted and traveling in exactly the same direction. After a numerous collisions there are a large number of identical photons traveling together in exactly the same direction. These beams of photons bounce back and forth between two parallel mirrors. One mirror is a perfect reflector, while the other only partially reflective. The beam, after bouncing back and forth between the mirrors, eventually exits through the partially reflective mirror as a laser beam.There are four types of polarization of light and it is sometimes ignored in general discussions. An important point is made that if two electrons have the same polarization they exhibit destructive interference if they try to occupy the same position. This is the basis of the Pauli Exclusion Principle. It is also the reason we have so many elements in the universe. As protons and neutrons combine to make heavier atoms, electrons are added to keep the atom neutral and stable. Since electrons can’t all have the same energy they have to populate other higher energy levels within atoms, This produces all of the known elements which have different properties. The chemical and physical properties of a given element are mostly due to the number of the outermost electrons. Feynman sees the principle of least action at work in the interaction of electrons and photons with external electric and magnetic fields. This interaction is responsible for all the effects we see around us in our everyday world, and are predicted and explained very accurately by QED.The renormalization technique that allows calculations with QED is tricky and is based on the odd fact that if you use m and e, the mass and charge on the electron, to determine n and j, the coupling constants in QED, then use these n and j to solve another problem your answer is accurate. If you do it again with another n and j that are close then the answer to the other problem is again close.In the fourth lecture he mentions chromodynamics which is used to explain the large number of subatomic particles. Unlike QED its ability to predict accurately is very limited - only 2.7 plus or minus 0.3 (2 significant figures) for the magnetic moment, instead of 12 significant figures using QED. It does provide a framework to discuss three of the four main interactions in Nature. The “strong interactions” of quarks and gluons (strong force that holds the paste okaynucleus together), “weak interactions” of W’s (weak force that causes most radiation), and the “electrical interactions” of photons (electromagnetic forces). The fourth interaction, gravity, is so weak at the subatomic particle level that it can be ignored unless you need about 40 significant figures. At the moment we are not capable of experiments to this type accuracy.The interaction between theoreticians and experimentalists is constantly changing. Sometimes the theoreticians suggest an experiment, such as Dirac predicting the positron. Other times experimentalists find values that force theories to be updated and improved. At the moment the very smallest particles, quarks, cannot be measured. At the very largest objects, the cosmology of universe, no one has been able to think of experiments to test some of the theories. We could use another Feynman.The lectures on QED, that the book is based on, are available on the internet and Wikipedia has an article on QED that includes good explanations and animations that improve understanding.

I read from other reviews about how Feynman didn't compromise on correctness on this book/lectures. I was afraid that meant that at least some parts of the contents would be out of reach for someone not familiar with advanced physics but bought it anyway, assuming that at least a good chunk of the book would be comprehensible.I was wrong. The whole book is comprehensible! Feynman sets a high standard for other authors by showing a way to make a very specific and technical topic approachable for the general public, at least in terms of main principles.

Feynman had a rare ability to explain things. I worked in nuclear physics research (but am an electrical engineer) and various people tried to give me a gloss of QED. Why they didn’t just give me this book is a mystery.Feynman explains how to do all the phase and related calculations using geometry (little arrows). I can’t say if that is enough if you don’t have some clue what is really happening. If you have a math/engineering background, it’s pretty easy to see this is just complex addition and multiplication on objects of the form exp(it) where i is the square root of -1. The way he explains this is itself as fascinating as the actual content.If you are impressed with Feyman’s approach, you might like Lakoff’s book “Where Mathematics Comes From”. He makes an amazing argument that essentially all math is just a clever series of imagination tricks using our ability to count, measure, and imagine rotating things in our heads.

As an electrical engineer, I've always been an amateur physicist. This book introduced me finally to this amazing theory. Electrons exchanging photons with protons? Really made me look at my understanding of things differently.

This is a easy read for the average college graduate. It concentrates on a description of the physics and skips the mathematics that were necessary to develop the theory. A great review of the basics of the quantum theory of electrodynamics. Feynman had a great way with words and an amazing ability to make very technical subjects simple to grasp the basic concepts. Highly recommended as either an introduction or a review of the quantum theory of light. This book is not a textbook on quantum theory…no math.

I really love all physics. I just wasn’t smart enough to understand this at all

The subject matter is presented in an understandable manner!

I received this book recently. I have only read a few pages but already I am finding it hard to put down. This is one of the better books I have bought.

It's really good, I couldn't stop once I started reading it

Ótimo,realmente Feynman é um dos melhores mestres de todos os tempos.Realmente muito bom e o recomendo aos alunos e mestres!

Feynman at his best. A classical presentation of QED for lay people, explaining it very clearly but not giving up on presenting the real stuff.

I read the book without taking breaks. Yes, I had my meals and slept. It took 21 hours. The first two lectures were easy and entertaining. The third lecture really took off. And very difficult to understand. I did not expect to understand the fourth lecture. Thanks Feynman.