QED

• Very good! Learned a lot. Even thought I've read it before. Great description of complex numbers, index of refraction and the paradox of partial reflection.
• The phenomena of sounds was completely understood as the motion of atoms in the air, so sound was no longer something in addition to motion.
• The theory of quantum mechanics could explain all of chemistry and the various properties of substances, but there was still the problem of the interaction of light and matter.
• So leave out gravity and radioactivity (more generally nuclear physics) what have we go left? Gasoline burning, foam and bubbles, the hardness of salt or copper, the stiffness of steel. all of biology and chemistry
• The book explains what physicists are doing when they are predicting how Nature behaves, now the tricks to do in efficiently. This is how we skip 7 years of education.
• The color of light can be split in to polarization.
• It is critical to know that light behaves like particles, you were probably told something about light behaving like waves. I'm telling you the way it does behave – like particles.
• I'm going to pretend that the light is reflected by only the surface of the glass. In reality a piece of glass is a terrible monster of complexity – huge numbers of electrons moving around. What a photon comes down it interacts with all the electrons throughout the glass, not just on the surface. The photon and electron interactions have the net result which is the same as if the photon hit only the surface.

• Mystery of Partial Reflection: How should a photon "decide" where it should go ? This is already a deep mystery, was a very difficult problem for Newton.

  Quantum Mechanics

• Philosophers have said that science/rationality is only possible if the same circumstances give the same outcomes. Here is a case where identical circumstances (photons) produce different results (some reflect, some dont) We can only predict the probability of an event, not what exactly happens. Yet, science has not collapsed.

PhilosophyOfPhysics

• Partial reflection by a single surface is a deep mystery, partial reflection by two or more surfaces is absolutely mind-boggling! Even today we dont have a good model to explain partial reflection by two surfaces. We cannot explain how the photons decide, that is not known.

Quantum Mechanics

• The thin film of oil on a puddle of mud is something like a thin sheet of glass. It reflects light of one color depending on the thickness.
• Partial reflection can be explained by a theory of light as waves. But this wave theory cannot explain why only single photons are ever detected. QED resolves this.
• Where the time is least, is also where the time of near by paths is nearly the same; and the little arrows constructively add; this is why we can say to good approximation the light only goes where the time is the least.
• Diffraction Grating proves that the regions far from least time matter: Can scrape the mirror so there is a bias in the direction of the arrows. Such a mirror is called a diffraction grating: works like a charm. This proves we cant ignore the parts of the mirror that dont seem to be reflecting, this in turn implies that there is an amplitude for every way an event can happen, we have to add all the amplitudes, not just the ones we think are important.

Quantum Mechanics PhilosophyOfPhysics

• We dont have to assume light goes in straight lines when its in a uniform medium like air or water: even that is explainable by the general principle of quantum theory.
• The idea that light goes is a straight line is a convenient approximation to describe the world that is familiar to us. similar to the approximation that when light reflects off a mirror its only the path of least time that matters.

PhilosophyOfPhysics

• Where we expect to add probabilities under ordinary circumstances, we add arrows, where would have normally multiply probabilities, we multiply arrows.
• When we squeeze light too much the ordinary rules – eg: light goes in straight lines etc – fall apart.
• (Double Slit Experiment) Nature has it cooked up so we can never tell how she manages the interference. If put in detectors to see which way the photon went, we destroy the interference. The more you see how strangely nature behaves, the harder it is to make a model that explains how even the simplest phenomena actually work. So theoretical physicists have given up on that. (Many worlds)

Many Worlds Quantum Mechanics

• There are only three basic actions to produce all the phenomena associated with light and electrons.

  • A photon goes from place to place
  • An electron goes from place to place
  • An electron emits or absorbs a photon

  Each of these has an amplitude (an arrow) that can be calculated according to certain rules.

• Electrons were discovered in 1895 as particles: you can count them, you can put one on a drop of oil and measure its charge.
• The first basic action is for a photon to go from one place to another, it explains all about optics. There is an amplitude for light to go faster or slower than the conventional speed of light. (Not only does it not always go in straight lines, it also doesn't not always go at the "right" speed). The amplitudes for these "wrong speed" propagation cancels over long distances.

PhilosophyOfPhysics

• Great discussion of relationship of electron propagator to the photon propagator. Electron propagation is sum over many different number of intermediate steps. each with a factor of the mass included. Relation to the Higgs field.
• The backward moving electrons when viewed with time moving forward appears the same as the ordinary electron, except its attracted to normal electrons we say it has "positive" charge.
• In partial reflection, we can get the right answer for the probability by imagining (falsely) that all the reflection is from only the front and back surfaces. These surfaces are mathematical constructions that give the right answer. The "real" picture is that partial reflection is the scattering of light by electrons inside the glass.
• The amplitudes for the light scattering of electrons in a transparent substance is orthogonal to the incoming photon amplitude. After adding these scattering amplitudes, the final result has the same magnitude as the incoming amplitude, but is turned in a different direction. The thicker the glass the more turning. (This is really how a lens works) The same effect would be seen if photons went slower in the transparent material: there would be extra turning of the input amplitude (arrow). Light appears to go slower through glass or water, this slowing is the extra turning caused by the atoms in the glass scattering light. The same light-electron scattering amplitudes are responsible for partial reflection. Thus there is an intimate connection between partial reflection and the index of refraction.
• Great discussion of Bose statistics. When the photons end up at the same point, the arrows are identical and are thus always lined up. So interference then doesn't fluctuate according the relative separation of the starting points; it is always positive. (P(1->3)*P(2->3) == P(2->3)*P(1->3)) We might have expected that on average we should get twice the probability, on average. Instead we get four time the probability all the time. With more photons this more-than-expected probability increases even further. (ie: always constructive interference) We can say that photons tend to get in the same place or state. So eg: The chance that an atom emits a photon is enhanced if some photons are already present. This is "stimulated emission" and is how lasers work.

PhilosophyOfPhysics

• There are circumstances when the amplitude to emit a photon by a source is independent of whether another photon has been emitted. (eg: when the source is very heavy or when there are a lot of electrons moving together like in an antenna) Here, a large number of photons of exactly the same kind are emitted. The amplitude of an electron to absorb a photon in this case is independent of whether it or any other electron has already absorbed photons ("condensate") The entire behavior of the electron can therefore be given by just this amplitude which depends only on the electrons position in space and time. We say "the electron is moving in an external field"
• The energy required to break up a nucleus is much greater than that required to ionize an atom in the same proportion of the energy of dynamite is to the atomic bomb. Dynamite is the rearrangement of electron patters, an atomic bomb is the rearrangement of nuclear patterns
• Had Benjamin Franklin known about quarks he might have made the charge of the electron -3.

PhilosophyOfPhysics

• Quarks have a different kind of polarization that is not related to geometry. "color"
• With QCD we have a definite theory and hundreds of experiments, but we cant compare them. Its a situation that has never before existed in the history of physics.
• We have solved 99% of the phenomena of the world with electrons and photons. however explaining the remaining 1% turns out to need 10 or 20 times the complication. (eg: additional particles)

PhilosophyOfPhysics

• This mess of explaining the SM is much smaller than it would have been in the 50s with more than 400 particles. And think about the mess at the beginning of the 20C, when there was:

  • heat
  • magnetism
  • electricity
  • light
  • X-rays
  • UV rays
  • indices of refraction
  • coefficients of reflection

  and other properties that are now explained by quantum mechanics.