Allen Mills’s career trajectory took a sudden turn when, one day in graduate school at Brandeis University in the 1960s, he tightened the screws too much on one of the few lasers in existence at the time, breaking the glass. Thus compelled to switch labs, Mills joined the only other experimental physics group on campus and began studying positrons.
Positrons are the antimatter twins to electrons, identical in every respect except for their positive charge. If you get an electron and a positron together, they may unite for a brief moment as a hydrogen-like atom called positronium—before ending it all in a burst of energy. Theoretical physicists first predicted a two-atom molecular form of positronium, called di-positronium, in 1946. But it took more than six decades before Mills and his team at the University of California, Riverside, captured the first evidence of di-positronium formation after blasting a thin film of silica with a stream of positrons.
The research has laid the groundwork for further study of positronium atoms en masse. The weird creations reveal new aspects of fundamental physics, while at the same time inspiring science-fiction-like applications. Mills himself envisions lasers that channel the energy of electron-positron annihilations.
Before speaking at the February 2011 meeting of the American Association for the Advancement of Science in Washington, D.C., Mills took a moment to discuss his recent adventures in antimatter with SciCom’s Catherine Meyers.
Let’s start with the basics. What, exactly, is antimatter?
In today’s theory, every particle of matter has an antiparticle. And an antiparticle is the opposite of a particle. Now, you can’t say, “What is the opposite of an apple?” But particles of matter are simple enough that they only have a few properties, and you can change one of them and have an opposite. For example, take a proton, which has a charge of plus one, a mass of one proton, and a spin of one-half. If you change the charge to minus, you have an anti-proton. The positron is the antiparticle version of the electron.
How did you get into the antimatter field?
I guess it’s because I’m left-handed and I like electronics. Antimatter, with its opposites, is sort of the epitome of being left-handed.
Why is there more matter than antimatter in the universe?
I don’t think we know why, but that seems to be the fact. There seems to be some asymmetric force [that created a surplus of matter] that we haven’t really discovered yet.
And where is antimatter found?
Well, there aren’t any large collections of antimatter that we know of, and there aren’t any places to go get it. So you have to make it one particle at a time in an accelerator. Because you can only make antimatter in high-energy events, the amount that you get is very, very small.
"Maybe someday, someone will find a mine on an antiasteroid where we can get some antimatter dilithium crystals for a starship. Until then, antimatter is not going to be used as a primary fuel."
But in your experiments, you don’t use an accelerator, correct? How did you create positrons in the lab?
Positrons can also be created by beta decay of a radioactive source [the spontaneous disintegration of an atomic nucleus during which an electron or positron is emitted]. So the short answer is I buy a radioactive source from a company and collect the positrons.
When matter and antimatter particles meet, they annihilate each other. Could you describe this process?
Annihilation is the transforming of a particle-antiparticle pair into something else. In the case of a positron and an electron, the two particles get very close together and all their opposite qualities cancel, leaving neither of the original particles. However, the sudden disappearance of the positive and negative charges that were very close together causes the emission of a pair of high-energy electromagnetic waves—gamma rays—just like the radio antenna on your cell phone sends out radio waves.
The things we call particles, like electrons, positrons, protons or antiprotons, are not different from the things we call waves, like light or X-rays, except the particles are usually endowed with a mass. Really, everything is a wave, and they all appear to be particles when they interact with something here or there. Therefore it is no surprise when one pair of waves turns into another bunch of waves. Thus when an electron and a positron annihilate, their existence terminates abruptly, in about 0.01 micro-micro-microseconds, and whatever qualities they possessed in total before the annihilation event are passed on to a pair of gamma ray wavelike things.
So how it is possible to form an ‘atom’ out of an electron and a positron?
It turns out you can make a sort of stable atom out of a positron and electron because, although annihilation is very powerful, it is not so quick. The particles have to get within a very tiny distance before they can annihilate. It can take them a nanosecond to annihilate, and during that time the electron and positron orbit each other. This is a positronium atom.
And the di-positronium molecule?
If you have a high density of positronium, and the atoms collide with something and give up a little of their energy, two atoms can bond to form di-positronium. But you have to have a very high density of positronium and a surface.
You used silica as the surface.
That’s what we used. It provided a lot of surface area, with lots of nooks and crannies.
Di-positronium’s existence was predicted decades ago. Why did it take so long to create it in the lab?
It took a long time for a guy named Cliff Surko to invent a positron trap to collect large numbers of positrons. For our experiments, we needed high-density bursts of positrons hitting a sample. After that it took years of trying to finally make it work. Actually, it’s embarrassing how long it took us to get it to work. I always think it’s going to take just a couple of weeks, but it always takes much, much longer.
What were the main challenges you had to overcome?
The main challenge is getting enough positrons together at one instant and in a small volume. [If it were not for this challenge], positrons would be a common probe that would be as widely used as neutrons and X-rays for medical, commercial, and scientific applications.
Speaking of applications, what are some possible uses of antimatter?
People who read science fiction comic strips would probably think of spaceship fuel. The weight is 100 million times less than ordinary chemical fuels, so it makes it very attractive for compact space travel. However, the cost for a unit of energy from antimatter is roughly one million times the cost from gasoline or coal.
So, no hope for an antimatter-fueled trip to Mars?
Maybe someday, someone will find a mine on an antiasteroid where we can get some antimatter dilithium crystals for a starship. Until then, antimatter is not going to be used as a primary fuel except for special applications where weight reduction is essential and cost is unimportant, and that will only be when we have attained a much higher efficiency for antimatter production.
Have there been efficiency gains in making antimatter from ordinary matter?
From 1958 to 1988, the efficiency of making slow positrons improved by a million times. Thirty years to get a factor of a million improvement, with low-funded, single-investigator work, is not so bad.
Tell me about some other applications for antimatter.
At the moment, more realistic applications might be using positrons or anti-hydrogen [an antimatter version of hydrogen, composed of an antiproton and a positron] for scientific experiments. Positrons, especially, could be used as probes to study material structures. There are also medical uses, for example using anti-protons to kill cancer cells.
What about the annihilation gamma-ray laser that’s mentioned in your AAAS abstract?
Antimatter and matter can be turned into radiation directly. The electron and positron in positronium annihilate into two photons. If you could have positronium atoms that are cooled down so much that they are not moving, then everything will have the same energy and there is the possibility of stimulated annihilations and emissions of gamma rays. Once annihilation starts at one end of the collection of positronium and aims for the other, it encourages other atoms to follow, like a military band picking up recruits as it marches through town. You could get a lot of photons all going in the same direction with the same energy.
How would an annihilation gamma-ray laser compare with an optical laser?
Optical and X-ray lasers are described by the same Einstein law of stimulated emission, but the annihilation gamma-ray photons are 100,000 times more energetic than typical optical photons. They are much harder to manipulate by mirrors and lenses, which are so common to the optics field.
Are there potential military applications?
Anything that can project something energetic at a distant object can be used to make changes in that object. The first gamma-ray laser would project less than 1 Joule, the energy of a hand clap, so it might not be useful for anything but metrology [the science of measurement]. One thousand Joules properly aimed could be lethal. A million Joules, which is the energy contained in a few hundred grams of dynamite, could disable mechanical devices or possibly ignite a fusion reaction.
How dangerous, or not, is it to work with antimatter?
The amount of antimatter we have at any one time is very small, equivalent to a few micro Joules, and so it is not very dangerous. As one works with larger amounts of antimatter, it will have to be more remote from the people working on it.
What are the next steps in antimatter research?
I would like to do precision measurements of a positronium atom’s energy levels. The anti-hydrogen people would like to do the same sorts of things. They’d like to test whether matter and antimatter are identical in their electrical properties, and to test anti-gravity.
Anti-gravity?
The opposite of gravity. Positronium or anti-hydrogen might go up instead of down. We don’t think this will happen, but it’s not an entirely foolish idea. All it means it that there is another field that you haven’t thought of before. But usually when you look where nobody has looked before, you find something new.
What do you enjoy most about your work?
I am not sure whether I enjoy it or not, but I cannot help trying to find out how the world works. I prefer thinking about things like how many postulates are needed for quantum theory, when does an event become a measurement, and how can I detect dark matter, but I am too stupid to get far in these directions. So I content myself with looking in places that are a little different from where others have looked, hoping to stumble upon something that will give new insight to the constant question, why?
Catherine Meyers, a 2011 graduate of the Science Communication Program at UC Santa Cruz, earned her bachelor's degree in engineering from Harvey Mudd College. She served in the U.S. Peace Corps in Ukraine. She has worked as a reporting intern at the SLAC National Accelerator Laboratory news office, the Monterey County Herald, ScienceNOW (the daily online news service of Science magazine). She is now a news writer for the American Institute of Physics in College Park, Maryland.
© 2011 Catherine Meyers
