Math 6612- The Feynman Lectures on Physics

  • Why the Laws of Physics Are Inevitable
    - By considering simple symmetries, physicists working on the “bootstrap” have rederived the four known forces. 

    - The existence of a spin-2 particle leads inevitably to general relativity — Albert Einstein’s theory of gravity.-The photon can participate in interactions involving other types of particles, however, such as spin-1/2 electrons. These constraints on the photon’s interactions lead to Maxwell’s equations.

    - Gluons are also massless spin-1 particles. Constraints on these gluon self-interactions match the description given by quantum chromodynamics, the theory of the strong force.- The breaking of the Higgs symmetry created massive spin-1 particles called W and Z bosons, the carriers of the weak force that’s responsible for radioactive decay.

    - Then “for spin-2, a miracle happens,”  That solution is the graviton: a spin-2 particle that couples to itself and all other particles with equal strength.

    - Thus, by thinking through the constraints placed on fundamental particle interactions by basic symmetries, physicists can understand the existence of the strong and weak forces that shape atoms, and the forces of electromagnetism and gravity that sculpt the universe at large.

    - In addition, bootstrappers find that many different spin-0 particles are possible. The only known example is the Higgs boson, the particle associated with the symmetry-breaking Higgs field that imbues other particles with mass.

    - The spin spectrum stops at 2 because the infinities in the four-particle interaction equation kill off all massless particles that have higher spin values. Higher-spin states can exist if they’re extremely massive, and such particles do play a role in quantum theories of gravity such as string theory. But higher-spin particles can’t be detected, and they can’t affect the macroscopic world.

    - Spin-3/2 particles could complete the 0, 1/2, 1, 3/2, 2 pattern, but only if “supersymmetry” is true in the universe — that is, if every force particle with integer spin has a corresponding matter particle with half-integer spin.