Many propulsion systems designed for interstellar travel are last-ditch, desperation schemes with very small chances of a payoff. The decidedly iffy status of some of the propulsion concepts so far discussed – the Alcubierre Drive, Sonny White’s warp drive – have led some star travel proponents to conceive of other exotic, “alternative,” or overly imaginative propulsion methodologies: flying through wormholes, for example, or crackpot faster-than-light schemes such as tachyon drives. But those concepts are so far-out and unlikely as to be well beyond even Hail Mary desperation status. There are some further theoretically possible systems, however, that just might work. The least implausible of them all is the controlled nuclear fusion drive. It was this type of engine that would supposedly propel the otherwise unworkable Bussard Interstellar Ramjet as well as the second stage of the Project Daedalus starship. In its favor is the fact that nuclear fusion is the single Hail Mary propulsion technology that is currently under active development.

Back in Berkeley, Weinberg reconsiders how we understand some of the theories of physics, that is, why they actually are true. He begins teaching a general relativity course starting from physical principles, rather than the usual geometric approach. These course notes later became the basis for his book Gravitation and Cosmology. Around this time, Louise was pregnant, so Weinberg avoided opportunities to travel to spend more time at home. He begins working on functional analysis, but discovers the Russian Faddeev has already done foundational work in this area. Weinberg then reexamines what he knows about quantum field theory, and jettisons the Heisenberg–Pauli canonical formalism, taking particles as his starting point. This led him to a clearer understaning of antimatter. He embarks on a series of papers about massless particles. in 1964, he is promoted to full professor. Louise applied to Harvard Law School, prompting a move to Cambridge, Mass.

This chapter introduces the formal second quantization method for fermions in quantum field theory, and the connection to second quantization of bosons is shown. The picture of fermions as rotations between two states is presented, which helps the reader to see where the Pauli exclusion rule comes from. Finally, Dirac’s original derivation of his equation for relativistic motion of fermions is given.

This chapter gives a brief but quantitative introduction to the method of Feynman diagrams in quantum field theory, sufficient for the reader to understand what these diagrams mean. The concept of “vacuum energy” is discussed in this context.

There are many representations of time reversal symmetry, including PT, CT, and CPT, but only the standard time reversal operator T is associated with an arrow of time itself.