Going Interstellar (collection)

Here is the rub. As these primeval nucleons were created out of the energetic stew of pre-nuclear matter, standard, well-established nuclear physics predicts an exactly equal number of nucleons and anti-nucleons. Many or all of these particles and anti-particles should have been converted into gamma rays as they annihilated each other. In fact, the universe should be absolutely empty as a result of these matter/antimatter annihilation events!

Clearly, this is not the case. Matter exists, but what became of the antimatter? Did the early universe divide during its inflationary phase into a matter-half and an antimatter-half? If so, then why don’t we detect annihilation gamma rays from regions where these two sub-universes come into contact?

The giant black holes that became luminous, quasi-stellar objects and now reside quietly at the centers of spiral galaxies (such as our Milky Way) also evolved in the early universe. Some suggest that in some unknown fashion, a bit more of the universe’s early antimatter fell into these cosmic maws than did normal matter. But no one can suggest a mechanism. If this hypothesis turns out to be correct, though, there are some interesting science-fiction concepts. How might we travel to the huge black holes? And how might we get the antimatter out of them?

Another possibility is that there is a slight asymmetry in the production scheme for matter and antimatter. This scheme might slightly favor the production of normal matter. Experimental evidence for such an asymmetry is sparse. One reason for the development and construction of the Large Hadron Collider at the CERN is to search for such asymmetries. But even this enormous and energetic proton accelerator may not have sufficient energy to duplicate conditions in the very early universe.

The Antimatter-Matter Interaction

It was originally believed that the interaction of a particle and its antiparticle twin would instantaneously result in gamma ray photons. This would not be great for space travel since gamma rays are not easy to deflect. But nature is actually a bit kinder to us in this respect. Yes, gamma rays are the end product. But along the way, many of the intermediate, short-lived particles are electrically charged.

Early antimatter rocket pioneers had no idea regarding the charged-particle decay scheme for matter-antimatter annihilation products. In the early 1950s, the German rocket scientist Eugen Sanger proposed that a spacecraft propelled by the matter-antimatter reaction would be a photon rocket emitting gamma rays. But focusing these gamma rays so that they emerged as an exhaust seemed to be a nearly insurmountable problem. Sanger’s thought experiments centered upon an electron gas that might reflect the gamma rays. But he was never able to solve the problem.

It was a flamboyant and dynamic American physicist and science fiction author, Robert Forward, who brought the charged-particle decay scheme of the proton-antiproton annihilation reaction to the attention of the space propulsion community. An imposing figure, Forward was famous for his colorful vests. Legend has it that he never wore any of his vests more than once!

In 1983, Forward conducted a research effort on alternative propulsion techniques. This was published in a December 1983 report for the United States Air Force Rocket Propulsion Laboratory. According to this report, the immediate products of proton-antiproton annihilation are between three and seven electrically neutral and charged pions. (A pion is one of the many subatomic particles found to comprise the matter around us.)

A magnetic nozzle can be used to focus these electrically charged particles and expel them out the rear of a matter/antimatter rocket as exhaust. A large fraction of the energy produced in the proton/antiproton annihilation is transferred to the kinetic energy of this charged particle exhaust. Although an operational matter/antimatter annihilation rocket will not have the one hundred percent efficiency of Sanger’s photon rocket (probably thirty to fifty percent according to Forward), it will be much more effective than a fission or fusion rocket. And charged particles, even short-lived charged particles, are much easier to handle than gamma rays.

Antimatter Factories

To date, no repositories of antiprotons or anti-hydrogen have been found. But antimatter is routinely produced in nature and also by humans. In this section, we deal with various types of antimatter factories.

First, let’s consider nature’s factories. Then, we will look at antimatter production in our largest existing nuclear accelerators. Finally, we treat antimatter production facilities that might be constructed by a future solar-system wide civilization.

Natural Antimatter Factories