Going Interstellar (collection)

Antimatter belongs to a mirror world. The anti-electron or positron, for example, has the same mass as the electron but an opposite (positive) electrical charge. Because their electric charges are opposite and opposite charges attract, electrons and positrons attract each other. When they touch, they mutually annihilate one another and their energy appears in the form of a gamma-ray photon.

It was the British physicist Paul A. M. Dirac who predicted in the 1930s that such a mirror world would exist. In his development of a relativistic theory of the electron, Dirac may have been the first to realize that the vacuum is far from empty.

The concept of a dynamic vacuum is hard to swallow by most people schooled in classical physics. After all, we are all taught in secondary school that a perfect vacuum is totally empty—devoid of all matter. And everyone who has followed extra-vehicular activity in space or seen the science fiction movie 2001: A Space Odyssey, knows how quickly a human astronaut would die if exposed without a spacesuit to the hard vacuum of interplanetary space.

But Dirac chose to view the universal vacuum on the tiny scales of quantum mechanics. In very small portions of space and on infinitesimal time intervals, a better model for the vacuum is the dynamic sea. Think of an ocean wave—the peak of the wave corresponding to a positive vacuum energy state and the trough analogous to a negative vacuum energy state. In Dirac’s theory, every sub-atomic particle in the “positive-energy” universe that we inhabit has a “negative-energy” analog. The negative-energy analog of the electron (also considered a “hole” in Dirac’s “sea”) is the positron. When the two meet, the result is a neutral state corresponding to calm water in the ocean.

Science, unlike deductive philosophy, requires experimental or observational confirmation of brilliant theoretical ideas. It was the American physicist Carl Anderson, working at Caltech, who discovered the track of a positively charged electron in cloud chamber photographs of cosmic ray tracks in 1932. For this discovery, which was confirmed by others, Anderson shared the 1936 Nobel Prize in Physics.

Positrons actually can be found in other places. For example, they are produced when carbon-11 naturally decays into boron-11. But there are no known radioactive decay schemes that release the positron’s big brother, the antiproton.

Because protons and antiprotons are almost two thousand times as massive as electrons and positrons, a more energetic strategy was required to search for the antiprotons. The instrument used to do the trick was the 6.5 billion electron volt proton accelerator called the Bevatron at the Lawrence Radiation Lab, which was at University of California at Berkeley.

Antiprotons were initially produced by bombarding a stationary target with a high-energy proton beam accelerated by the Bevatron. The discovery was announced in the November 1, 1955 issue of Physical Review Letters by Owen Chamberlain, Emilio Segre, Clyde Wiegand and Thomas Ypsilantis. Chamberlain and Segre shared the 1959 Nobel Prize for this discovery.

It is known today that most or all particles have corresponding antiparticles. This is even true for electrically neutral particles such as the neutron. The antineutron is also electrically neutral, but it has other properties opposite that of the neutron.

Because of the inefficiencies involved in antimatter production, matter-antimatter reactors will almost certainly never be a solution to the energy requirements of our global civilization.

Antimatter in the Early Cosmos

Since antimatter is essentially non-existent on Earth, one might hope that we will someday locate a cosmic repository for it. Unfortunately, since cosmic-ray studies put an upper limit on the universal antimatter/matter ratio under 0.0001, the odds do not look very good for locating such a source.

But this presents us with a cosmological mystery. According to the Big Bang Theory, which is well supported by observational evidence, all of the matter, energy and space/time in our universe originated from a fluctuation in the universal vacuum that somehow became stabilized approximately 13.7 billion years ago.

In this early universe, things were very compact and very hot. Three of the four universal forces—electromagnetic, nuclear strong and nuclear weak—were united in one “super force.” Instead of nucleons, atoms, stars and planets, the early universe’s matter was a soup of tremendously energetic subatomic particles called quarks and gluons.

As things cooled and inflated, the universe went through a number of phase changes. At some point, nucleons such as protons, deuterons and alpha particles were created.