Saturday, March 22, 2008

Anti-Hydrogen Atoms

There is antimatter, and then there is antimatter. The former (individual antiparticles like positrons) has been known since early in the last century and used in huge quantities in accelerators for decades. The latter (actual atoms made of anti-protons and anti-electrons) is a bigger challenge because matter is neutral and cannot be manipulated the way individual anti-particles can be.

The subject of anti-matter must be one of the most common side-questions that come up in my physics class, given its prevalence in Star Trek. (That would be one of the curriculum issues in a previous comment and a planned followup, since modern physics is not on the list of key topics for pre-engineering physics.) A new publication reports a major advance related to anti-atom production.

A new article in Physical Review Letters by the research group at Harvard led by Gerald Gabrielse reports on improvements in their ability to trap anti-hydrogen atoms. The best overview might be on the web page of the ATRAP collaboration, although their most recent "breaking news" is 6 years old.

Any progress by this group interests me because it is a field of research where the project could very well take an entire career. I first heard a talk by Gabrielse at least 20 years ago. The ultimate objective, to make anti-hydrogren atoms and store them at rest in the lab so you can study the atomic levels in detail, is a heroic one. His work is interesting because it uses AMO techniques (the sort of thing Chad Orzel does to study normal atoms) in pursuit of a particle physics goal: to find out just how good the symmetry of "Time reversal invariance" is. Does it extend as far as the levels (and the relativistic effect known as the Lamb shift) of an atom made entirely of anti-particles.

AMO = atomic, molecular, optics

Along the way, Gabrielse worked with Hans Dehmelt, who shared in the Nobel Prize for key developments in trapping ions (charged particles) that might not have been possible if he had been captured in the Battle of Stalingrad in WW II. I think there is little doubt that Gabrielse deserves the same prize if his research program succeeds ... although he has competition.

As you might guess, the challenge in working with antimatter is that everything you use in the lab is made up of matter. If any matter, include any gas that leaks into your equipment, touches what you are studying, it is gone in a flash of energy. That makes Chad's professional interest in the esoterica of quiet and noisy vacuum pumps even more important than when working with ordinary matter. Everything, including slowing down and probing the antimatter, has to be done with photons (lasers) and electromagnetic fields. This is a lot harder when the thing you want to work with is not electrically charged.

One related material is positronium, which is an "atom" made up of an electron and an anti-electron. It lives a short life because the antimatter is never far away from the matter, but it has provided lots of insight into antimatter and time reversal invariance. We know the properties of these particles to an astounding precision.

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