Are we ever going to use antimatter to drive a starship? The question is tantalizing because while chemical reactions liberate about one part in a billion of the energy trapped inside matter — and even nuclear reactions spring only about one percent of that energy free — antimatter promises to release what Frank Close calls ‘the full mc2 latent within matter.’ But assuming you can make antimatter in large enough amounts (no mean task), the question of storage looms large. We know how to store antimatter in so-called Penning traps, using electric and magnetic fields to hold it, but thus far we’re talking about vanishingly small amounts of the stuff.
Moreover, such storage doesn’t scale well. An antimatter trap demands that you put charged particles into a small volume. The more antimatter you put in, the closer the particles are to each other, and we know that electrically charged particles with the same sign of charge repel each other. Keep pushing more and more antimatter particles into a container and it gets harder and harder to get them to co-exist. We know how to store about a million antiprotons at once, but Close points out in his book Antimatter (Oxford University Press, 2010) that a million antiprotons is a billion billion times smaller than what you would need to work with a single gram of antimatter.
Antihydrogen seems to offer a way out, because if you can make such an anti-atom (and it was accomplished eight years ago at CERN), the electric charges of positrons and antiprotons cancel each other out. But now the electric fields restraining our antimatter are useless, for atoms of antihydrogen are neutral. Antimatter that comes into contact with normal matter annihilates, so whatever state our antimatter is in, we have to find ways to keep it isolated.
A Novel Antihydrogen Trap
One solution for antihydrogen is being explored at CERN through the international effort known as the ALPHA collaboration, which reported its findings in a recent issue of Nature. Here positrons and antiprotons are cooled and held in the separate sections of what researchers are calling a Minimum Magnetic Field Trap by electric and magnetic fields before being nudged together by an oscillating electric field, forming low-energy antihydrogen. Keep the anti-atoms at low energy levels and although they are neutral in charge, they still have a magnetic moment that can be used to capture and hold them. Says ALPHA team member Joel Fajans (UC-Berkeley):
“Trapping antihydrogen proved to be much more difficult than creating antihydrogen. ALPHA routinely makes thousands of antihydrogen atoms in a single second, but most are too ‘hot’”—too energetic—“to be held in the trap. We have to be lucky to catch one.”
Image: Antiprotons and positrons are brought into the ALPHA trap from opposite ends and held there by electric and magnetic fields. Brought together, they form anti-atoms neutral in charge but with a magnetic moment. If their energy is low enough they can be held by the octupole and mirror fields of the Minimum Magnetic Field Trap. Credit: Lawrence Berkeley National Laboratory.
Clearly we’re in the earliest stages of this work. In the team’s 335 experimental trials, 38 antihydrogen atoms were recorded that had been held in the trap for about two-tenths of a second. Thousands of antihydrogen atoms are created in each of the trials, but most turn out to be too energetic and wind up annihilating themselves against the walls of the trap. In this Lawrence Berkeley National Laboratory news release, Fajans adds a progress update:
“Our report in Nature describes ALPHA’s first successes at trapping antihydrogen atoms, but we’re constantly improving the number and length of time we can hold onto them. We’re getting close to the point where we can do some classes of experiments on antimatter atoms. The first attempts will be crude, but no one has ever done anything like them before.”
Taming the Positron
So we’re making progress, but it’s slow and infinitely painstaking. Further interesting news comes from the University of California at San Diego, where physicist Clifford Surko is constructing what may turn out to be the world’s largest antimatter container. Surko is working not with antihydrogen but positrons, the anti-electrons first predicted by Paul Dirac some eighty years ago. Again the trick is to slow the positrons to low energy levels and let them accumulate for storage in a ‘bottle’ that holds them with magnetic and electric fields, cooled to temperatures as low as liquid helium, to the point where they can be compressed to high densities.
One result is the possibility of creating beams of positrons that can be used to study how antiparticles react with normal matter. Surko is interested in using such beams to understand the properties of material surfaces, and his team is actively investigating what happens when positrons bind with normal matter. As you would guess, such ‘binding’ lasts no more than a billionth of a second, but as Surko says, “the ‘stickiness’ of the positron is an important facet of the chemistry of matter and antimatter.” The new trap in his San Diego laboratory should be capable of storing more than a trillion antimatter particles at a time. Let me quote him again (from a UC-SD news release):
“These developments are enabling many new studies of nature. Examples include the formation and study of antihydrogen, the antimatter counterpart of hydrogen; the investigation of electron-positron plasmas, similar to those believed to be present at the magnetic poles of neutron stars, using a device now being developed at Columbia University; and the creation of much larger bursts of positrons which could eventually enable the creation of an annihilation gamma ray laser.”
An interesting long-term goal is the creation of portable antimatter traps, which should allow us to find uses for antimatter in settings far removed from the huge scientific facilities in which it is now made. Robert Forward was fascinated with ‘mirror matter’ and its implications for propulsion, writing often on the topic and editing a newsletter on antimatter that he circulated among interested colleagues. But he was keenly aware of the problems of production and storage, issues we’ll have to solve before we can think about using antimatter stored in portable traps for actual space missions. Much painstaking work on the basics lies ahead.
The antihydrogen paper is Andresen et al., “Trapped antihydrogen,” Nature 468 (2 December 2010), pp. 673–676 (abstract). Clifford Surko described his work on positrons at the recent meeting of the American Association for the Advancement of Science in a talk called “Taming Dirac’s Particle.” The session he spoke in was aptly named “Through the Looking Glass: Recent Adventures in Antimatter.”