Warped Passages - Lisa Randall [79]
Here’s a rough argument for why antiparticles follow from special relativity. Charged particles can travel forwards and backwards in space. Naively, special relativity would therefore tell us that those particles should be able to travel forwards and backwards in time as well. But so far as we know, neither particles nor anything else we are aware of can actually travel backwards in time. What happens instead is that oppositely charged antiparticles replace the reverse-time-traveling particles. Antiparticles reproduce the effects the reverse-time-traveling particles would have so that even without them, quantum field theory’s predictions are compatible with special relativity.
Imagine a movie of a current of negatively charged electrons traveling from one point to another. Now imagine running the movie in reverse. Negative charge would then travel backwards, or, equivalently (so far as the charge is concerned), positive charge would travel forwards. A current of positrons, the positively charged antiparticles of electrons, produces this positively charged forward-traveling current and therefore acts like a time-reversed electron current.
Quantum field theory tells us that if any type of charged particle exists, such as an electron, so must a corresponding antiparticle with opposite charge. For example, since an electron carries charge -1, the positron has a charge of +1. The antiparticle is like the electron in all repects aside from its charge. A proton also has a charge of +1, but it is 2,000 times heavier than an electron and therefore could not be its antiparticle.
As Stoppard said, antiparticles do indeed annihilate particles when the two come into contact. Because the charges of a particle and its antiparticle always add up to zero, when a particle meets an antiparticle, they can annihilate each other and be destroyed. The particle and antiparticle together carry no charge, so Einstein’s relation E = mc2 tells us all the mass can convert into energy.
On the other hand, energy can convert into a particle-antiparticle pair when the energy is sufficient to produce them. Both particle annihilation and particle creation occur in high-energy particle accelerators, where physicists conduct the experiments that study heavy particles, particles too massive to be found in ordinary matter. In these colliders, a particle and an antiparticle meet and annihilate each other, thereby creating a burst of energy from which new particle-antiparticle pairs emerge.
Because matter—and atoms in particular—are composed of particles and not antiparticles, antiparticles such as positrons are generally not found in nature. But they can be produced temporarily at particle colliders, in hot regions of the universe, and even in hospitals, where positron emission tomography (PET) is used to scan for signs of cancer.
Gerry Gabrielse, a colleague of mine in the Harvard physics department, makes antiparticles all the time in the basement of Jefferson Laboratories, where I work. Thanks to the work of Gerry and others, we know at a very high level of precision that antiparticles really are like their particle counterparts in mass and gravitational pull, despite their opposite charge. But there aren’t enough of them to do any harm. I can assure science fiction fans that these antiparticles do far less damage to the building than the perpetual construction of new labs and offices, which is always preceded by a large amount of visible and audible destruction.
Electrons, positrons, and photons are the simplest and most accessible particles. It is no coincidence that electric forces and electrons were the first Standard Model ingredients that physicists understood. The electron, positron, and the photon are not the only particles, however, and electromagnetism is not the only force.
I listed the known particles and nongravitational forces* in Figures 32 and 33. I left gravity out of the picture