Trapping Antihydrogen

23 January 2011

Interview with

Jeffrey Hangst, CERN

Chris -   In November 2010, researchers at CERN announced that they had managed to make and trap the antimatter equivalent of hydrogen, so-called antihydrogen.  Jeffrey Hangst is one of the scientists behind that breakthrough.  He joins us now from Geneva.  Hello, Jeffrey.

Jeffrey -   Hello.

Chris -   So first of all, tell us why were you wanting to trap antihydrogen?  What was the reason for doing the experiment?

Jeffrey -   Well, our long term goal is to study if there really is a difference between the physics for matter and antimatter.  We'd like to do that by comparing hydrogen and antihydrogen.  Hydrogen is something we understand very, very well.  It's kind of the model system for quantum mechanics.  It's how we learned how to do the physics with atoms.  So, we now know how to make antihydrogen atoms and we've just recently learned how to trap them, so we'd like to shine some light on them if you will, to see if hydrogen and antihydrogen behave in the same way.  The laws of physics, as we understand them now, say that those two things should behave identically.

Chris -   So it's one of those things, you start with something very simple and work out how the simplest thing works before trying to get clever and looking at more complicated things.

Jeffrey -   In this case, it's also the only thing in terms of atoms that you can imagine making out of antimatter.

Chris -   Indeed.  Well perhaps you could tell us then how you actually went about making antihydrogen.

Jeffrey -   Okay.  First, you need the components which are antiprotons, the nucleus, and positrons which are antielectrons as has just been explained by Professor Parker.  We are at CERN because CERN has a machine that makes antiprotons.  It's called the antiproton decelerator.  You run protons into a target and you collect antiprotons which were some of the particles that come out of that interaction.  So CERN has this wonderful facility.  It's unique in the world where they can collect some antiprotons for us and in addition, they can slow them down.  These antiprotons are made at high energy and we'd like to have our antihydrogen atoms essentially at rest.  We'd like to have antimatter in a bottle, if you will.  So this machine does the first step.  It takes the antiprotons and slows them down to an energy where we can manage them.  What we then do is slow them even further and trap them in a kind of electromagnetic bottle, in almost sort of the best laboratory vacuum you can make.  Matter and antimatter don't like each other so you need to deal with your antimatter in a vacuum.  So we have a device that slows and stops antiprotons.  The same device can also slow down and stop positrons from a radioactive source and then we combine the two.  So, negative antiproton meets positive positron, they can combine and form an atom, and that atom is antihydrogen.  Now, we've been doing that particular thing since 2002.  What's new is that we've managed now to hold on to the antihydrogen atom.

Chris -   I was just going to ask you about that because the first steps that you explained are relatively easy because those particles have got charges on them, which means you can guide them with magnetic fields and things like that because they will experience forces in magnetic fields, won't they?  But once you actually combine them and you make your antihydrogen, they're now neutral.  So, how do you then confine and constrain, and control them then?


Jeffrey -   Yes.  Relatively easy is exactly the right word because the first step took about 15 years, the making of antihydrogen.  But what you do now, although the antihydrogen is neutral, it has a small magnetic character.  You can think of an atom as a minute compass needle, so it can be deflected by very strong magnetic fields.  So we can use that interaction to hold on to the antihydrogen atom, even though it's neutral.  The rub is that, that interaction is so weak that when we take the best magnets we can fabricate today, the deepest bowl, if you will - imagine you're holding your antihydrogen in a magnetic bowl.  The depth of that bowl will only hold an antihydrogen atom that is 0.5 degrees Kelvin above absolute zero, right?  So it has to be extremely cold.  Otherwise, it just runs away.

Chris -   Because otherwise, it's moving too quickly and trying to hold on to it when it's going that fast is very hard.

Jeffrey -   Again, imagine a marble rolling in a bowl.  If the marble was going too fast, it goes over the lip.  Our bowl is very shallow, so these antihydrogen atoms need to be moving very slowly and in temperature equivalent, slowly means half a degree above absolute zero.

Chris -   Pretty chilly.  Once you get it down to that sort of level, how long can you constrain and confine the antihydrogen so that you can study it?

Jeffrey -   In the article we published, it's a bit misleading because we were just trying to show that this was possible.  So, we produced our antihydrogen in the bottle and then released it as quickly as we could to see if it was still there.  In that experiment, it was about 2/10th of a second.  We can now hold on to it much, much longer, a thousand seconds.

Chris -   That's a very long time.

Jeffrey -   Yes, that's a very long time.

Chris -   So long enough in other words, to begin to ask some important scientific questions about the characteristics of the stuff you're making and constraining.

Jeffrey -   Exactly.  That's the next step which we hope to start on in May when the accelerator starts up again.

Chris -   In the last couple of minutes, let's talk then about what you can learn from this because one of the interesting things that we see when we look out into space, we can use physics that Bunsen of 'Bunsen burner' fame taught us, which is that we can work out what things are made of by looking at the way they interact with light.  They absorb or emit light at characteristic wavelengths and that's how we know what distant stars are made of, for example.  Does an antimatter material like antihydrogen - does that have the same absorption spectrum that normal hydrogen does?

Jeffrey -   That's exactly the question that we'd like to answer. The standard model of physics says that it must, but there are no measurements yet on antihydrogen.  So, we would like to start that as quickly as we possibly can.  We're going to shine both light and/or microwaves on antihydrogen atoms to see if they absorb and emit exactly the same colours or frequencies if you will, of electromagnetic radiation.  So that's exactly the question we would like to answer and to see if there's some asymmetry between those two systems.  The difference between high energy physics and this type of physics is we can make precision measurements - the hydrogen atom is so well known, to one part in 10 to the 15 - that's a lot of zeros. So we can make precision comparison rather than going to a very high energy and looking for something new, we look for very small differences in a system that's extremely well understood in the matter sector.

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