certain directions, and they figure out what the particle’s “spin” is. Particles with ½ spin are mass
particles, while particles with spins of 0, 1, and 2 make the forces that occur between matter
particles. It’s also been found that matter obeys the exclusion principle, which means that 2
similar particles can’t have the same location and velocity. Hawking goes on to describe the
gravitational, electromagnetic, weak nuclear, and strong nuclear forces, which are known as the
four fundamental forces. Gravity is the weakest force (at the quantum level), while the
electromagnetic force is the strongest. The weak nuclear force is why we some things are
radioactive, and the strong nuclear force holds particles together. Hawking then talks about the
past attempts to create a single theory to unite the four forces, and how so far, only ¾ have
been unified, gravity being the force that’s being a nuisance to unify. Hawking ends this chapter
with “the three symmetries”: C, P, and T, which were believed to have been separate up until
1956. C: charge conjugation says that laws are the same for particles and antiparticles. P: parity
transformation, says the laws are the same for any situation and its mirror image, T: time
reversal, means the laws are the same in the forward and backward directions of time. They’re
important since it’s been found that any theory combining quantum mechanics and relativity
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