Last week I finished blogging through Thirty Years That Shook Physics, a great book by George Gamow on the first thirty some years of the 1900s, when the groundwork of modern physics was laid.
If I were to pick, I would pick Einstein as the dominant figure of the first twenty years of the twentieth century. Max Planck may have suggested the quantum, but Einstein ran with it and, in the meantime, established both special (1905) and general (1915) relativity.
For the next twenty some years, 1920-40, I pick Paul Dirac as the greatest mind in physics. There are lots of candidates. I pick Niels Bohr as the most influential figure, but I don't think the greatest mind. Heisenburg, Schrodinger came up with the most central concepts of quantum mechanics, but at times it seems like they just were the ones who did what someone else would have done anyway. It was Dirac who combined Schrodinger's famous wave equation with relativity, the last time that longed for synthesis was successfully accomplished.
But I'm not sure that any of those I've mentioned were as genius as Richard Feynman. I don't feel like I know enough to have a firm opinion yet, but Feynman may have been the most brilliant physicist of the modern era. The book I'm reading for the next few weeks for my Science Fridays is Lawrence Krauss' biography of Feynman, Quantum Man.
Krauss begins with some fun memories of his own personal engagement with Feynman. A book given to him in high school about Feynman's notion that antiparticles were particles moving back in time (it was originally John Archibald Wheeler's idea). In college he had Feynman's lectures on hand. And of course he mentioned Feynman's moment in the spotlight, when he showed everyone why the Challenger blew up. (He dropped an O ring into ice water on television)
1. Feynman in High School
Feynman was born in 1918. He was obviously a genius. Unlike Dirac, from whom you could hardly extract a word, Feynman was flamboyant and was loud. But he could also tunnel into a problem for long periods of time. He kept notebooks with multiple solutions to the same problems. he was methodical. He taught himself subjects years before he got to them in school.
He was incredibly gifted at math--not all physicists are, Einstein for instance. Of course you have to remember that when I say, "not gifted at math," I mean that Einstein didn't invent a new branch of math. Heisenburg reinvented matrix mechanics on his own. Dirac made major contributions to linear algebra for his relativistic quantum mechanics. So Feynman would invent path integrals. Meanwhile, Einstein couldn't have worked out general relativity without the help of David Hilbert.
One principle Feynman learned in high school that he didn't like but that would be key to his later fame was Fermat's principle of least time: light takes the path through various media that gets it most quickly to its ultimate destination. It is a weird concept because it seems to imply planning on the part of light, as if it knows where it wants to go and plans accordingly.
Apparently, another way to state this principle is this: an object will take that path where the total sum of the difference between its kinetic energy at each point and its potential energy at each point is lowest. This sum is called the "Lagrangian" and the "action" of an object.
2. Feynman at MIT and Princeton
Feynman declared as a physics major at the end of his first year at MIT. It was just right for him between the non-concrete pure math and the all concrete engineering. He and a student named Ted Welton started taking advanced graduate physics courses their sophomore year. One of those professors met with the two of them their junior year to teach them quantum mechanics. This was about 1938.
Then he turned down Harvard to go study at Princeton, which at that time had Einstein and John Archibald Wheeler. Feynman was Jewish, and his parents worried both about whether he could make a go at physics and whether he would be accepted as a Jew there. He and Wheeler (the guy that coined the phrase, "black hole") worked well together.
It was during this period that Wheeler suggested some particles might move back in time. The specific problem they tackled was the self-energy of an electron. They unsuccessfully tried to eliminate that factor from the energy equation because it went to infinity according to calculations at that time. Their suggestion was to try to eliminate the idea of a electromagnetic field. The idea was that interactions would only take place by the direct interaction of charged particles, by the exchange of photons. The prevailing wisdom was that forces interact by "action at a distance," which is what fields allegedly do.
Wheeler and Feynman were apparently wrong, but it was exactly the kind of interaction that fed Feynman's growing sense of possibilities.
Krauss takes a couple pages in this chapter to give a quick two points on the nature of quantum mechanics. The first point is that, in QM, objects can be in different places doing different things simultaneously. The second is the Heisenberg uncertainty principle. There are certain pairs of qualities (position and momentum, energy and time) where the more we know one, the less we know the other. The product of our uncertainty in knowing one and the uncertainty of knowing the other will never be smaller than Planck's constant.
That's probably enough for one day...