The big physics-y news story of the moment is the trapping of antihydrogen by the ALPHA collaboration at CERN. The article itself is paywalled, because this is Nature, but one of the press offices at one of the institutions involved was kind enough to send me an advance version of the article. This seems like something that deserves the ResearchBlogging Q&A treatment, so here we go:
OK, what’s the deal with this paper? Well, the ALPHA collaboration is announcing that they have created antihydrogen atoms– that is, a single antiproton orbited by a single positron– at low temperatures, and confined them in a magnetic trap for something like 172 ms.
Awesome! When can we blow up the Vatican? Settle down. We’re not talking huge quantities of antimatter, here. In 335 runs with their apparatus, they detected all of 38 atoms of antihydrogen. You’re not going to be blowing anything up soon.
What’s the point of making antimatter if you can’t use it to blow stuff up? The point is to understand the laws of physics better. If you can do spectroscopy of anti-atoms, it will tell us a lot about whether antimatter obeys the same laws as ordinary matter, which might provide a clue as to why everything we see seems to be made of ordinary matter. You could also use it to test how antimatter interacts with gravity, which is something we don’t currently have any way to test.
OK, fine, it’s all about basic physics. So, this paper is the first time people have made antihydrogen? No, the first observation of antihydrogen was back in 1995, followed by another observation in 1997. The first cold antihydrogen was at CERN in 2002, and again the same year.
So what’s today’s article about? this is the first time that antihydrogen has been magnetically trapped. Which is a big step, because if you want to do spectroscopy of antihydrogen, you need the atoms to stick around for a while so you can interrogate them with lasers.
OK, so how do they make this stuff? They make it basically by sticking a large number of antiprotons and positrons in the same region of space, and waiting a while. If you’ve got enough antiprotons and positrons, eventually three of them will collide and form an atom.
Three? Don’t you only need one positron and one antiproton? That’s what you need to make an atom, but if you want the atom to stick around in a bound state, you need a third body to carry off some excess energy and momentum.
OK, so the creation just sort of happens. How do they trap it, if it annihilates when it comes into contact with normal matter? Well, you can’t let it come into contact with any normal matter, that’s for sure. What they do is they use strong magnetic fields to confine the atoms to a small region of space. Then they use electric fields to push any remaining loose positrons and antiprotons out of the trap, so they just have antihydrogen in the trap.
Wait, how does that work? Well, the positrons and atiprotons, being charged, respond very strongly to electric fields, so they rush toward or away from a big electric field, depending on their charge. Bound atoms of antihydrogen, though, are neutral, and electric fields just make a small shift in their energy levels. A neutral atom will behave like a magnet, though, so you can push them around with magnetic fields much more effectively than you can with electric fields. Thus, you can push the charged particles away, and keep the neutral atoms around.
OK, so how do you know when you’ve trapped one? Well, you turn off the trap, and let it fly out and collide with normal matter. When it does, it spits out a bunch of high-energy particles, which then get detected by instruments surrounding the trap. They can tell when they got antihydrogen by tracing the particle tracks backward to show that they originated in the trap region.
Yeah, but aren’t there a lot of things that can create tracks in the detectors? Yes, but you can rule most of them out by looking at the shape of the tracks. When the particles are coming from inside, and have the right number and character, you can say that it was due to an antihydrogen atom annihilating on the edge of the trap chamber. A cosmic ray particle, or a radioactive decay somewhere else would produce tracks that start outside the chamber.
OK, but what about the leftover antiparticles? Couldn’t an annihilation on the edge of the chamber be due to a leftover antiproton that didn’t get pushed out? It could, but they can test that by applying different fields and looking at the distribution of counts. The key figure from the paper is this one:
These are two plots of the distribution of the detected antihydrogen atoms in time (vertical axis) and position along the detector (horizontal axis). The large colored symbols represent the positions of the 38 probable atoms they detected, and are the same in both plots.
The haze of grey dots in the upper picture show the locations of detected atoms in 2000-ish simulated experiments, and show that all the points they’re calling atoms fall in the right region. The smaller colored blurs in the bottom pictures represent the locations of detection events if they were looking at leftover antiprotons, for three different configurations of applied fields. You’ll notice that with two exceptions, none of the detections they’re calling antihydrogen atoms fall in the places you would expect antiprotons, which strongly suggests that they’re not leftover antiprotons, but real, true, neutral atoms.
So I’m supposed to believe this is antihydrogen just because they simulated what they think ought to happen and the real data sort of look right? Isn’t that kind of weak? Yeah, well, welcome to experimental particle physics. Enjoy your stay.
This is a fairly standard method of evaluating data in the particle and nuclear physics communities, and these plots are really nice as this sort of thing goes. They also have some other evidence, namely that when they prepare the antiprotons and positrons at a much higher temperature, where they don’t expect much antihydrogen to form, they don’t see any. That, again, is strongly suggestive that they have real antihydrogen.
OK, I guess I’ll buy that. So, is this good enough to work with for the basic physics tests you mentioned? It’s tough to say without knowing more about the details of their experiment, and how they plan to proceed. It’s not clear, for example, where you would put the lasers to do spectroscopy of the antihydrogen atoms.
I can tell you this much, though: the ATTA group at Argonne National Lab did spectroscopic measurements of the size of heavy helium nuclei using an apparatus that trapped a single atom at a time, with atoms having a lifetime of about 0.2 s. So if they can make antihydrogen atoms stick around for 0.17s, they can probably do the sort of spectroscopy you would really like to do with these atoms.
So, how long would it take to get enough of this stuff to blow up the Vatican, anyway? A really long time. For an antihydrogen annihilation equal to a tone of TNT, you’d need a few time 1019 atoms. Each cycle of their experiment takes a couple of seconds, so their 38 atoms in 335 attempts is something like 0.05 atoms/s. At that rate, you would need to run the experiment for about 1500 times the current age of the universe to get enough to blow up the Vatican. It’s not going to happen any time soon.
A long, long time before you could make a weapon out of this stuff, though, they’ll have enough to tell us more about how the universe is put together. And that’s the really exciting part of this result.
Andresen, G., Ashkezari, M., Baquero-Ruiz, M., Bertsche, W., Bowe, P., Butler, E., Cesar, C., Chapman, S., Charlton, M., Deller, A., Eriksson, S., Fajans, J., Friesen, T., Fujiwara, M., Gill, D., Gutierrez, A., Hangst, J., Hardy, W., Hayden, M., Humphries, A., Hydomako, R., Jenkins, M., Jonsell, S., JÃ¸rgensen, L., Kurchaninov, L., Madsen, N., Menary, S., Nolan, P., Olchanski, K., Olin, A., Povilus, A., Pusa, P., Robicheaux, F., Sarid, E., Nasr, S., Silveira, D., So, C., Storey, J., Thompson, R., van der Werf, D., Wurtele, J., & Yamazaki, Y. (2010). Trapped antihydrogen Nature DOI: 10.1038/nature09610