What a trapped antimatter particle could tell us about reality

Researchers have trapped a positron and held it at extremely cold temperatures for months, allowing them to measure its quantum properties.

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:00Speaker 00

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• 0:14Flora Lichtman?

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• 1:00Ira Flatow?

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• 1:19Flora Lichtman?

  Hey, it's Flora, and you're listening to Science Friday. Anti-matter. It sounds like something from science fiction, but it's very real. Almost all matter has corresponding particles of antimatter. Protons have an antiproton, neutrons have an antineutron, and so on. And when matter and antimatter bump into each other, they destroy each other and release energy. Because of that, it's been hard to pin down antimatter because it generally disappears within a tiny fraction of a second, which is why it's a big deal that scientists were able to trap, hold, and study a positron, that's an antimatter version of an electron, for months, creating what they call

• 2:03Flora Lichtman?

  a one-positron quantum cyclotron. Here to tell us exactly what that means is the lead author on the paper, Dr. Gerald Gabriels. He studies antimatter at Northwestern University. Thanks for joining me today.

• 2:17Speaker 03

  I'm glad to be here.

• 2:19Flora Lichtman?

  Is it difficult to trap antimatter and keep it alive?

• 2:24Speaker 03

  Well, it's difficult enough that not so many people on Earth do it or have done it. Though we did it the first time, people told me it was impossible. Now people say, well, of course you can do it because you've done it before. So it's a nuanced answer I'd have to give.

• 2:43Flora Lichtman?

  Okay, so you were on the show on Science Friday 27 years ago, and you told us that you were able then to hold antiprotons. So is there something that is more tricky or finicky about antimatter versions of electrons, about positrons?

• 3:01Speaker 03

  No, I would say not. And in fact, I would stress that what we did recently that's creating some general interest around the world, it's not interesting so much for the fact that we trapped antimatter, but that we trapped antimatter at this positron and made it so cold that we could probe its quantum structure. That's really what we're psyched about.

• 3:27Flora Lichtman?

  Okay, I want to get into that in a second. But I think first, what is antimatter? Like, we know it has a mass, right? But do you, so do you have a family gathering way? You know, you're all at the Thanksgiving dinner table, like a way of talking about it?

• 3:42Speaker 03

  Well, yeah, at my family, it doesn't always stimulate conversation, but this is how I do it. You know, the remarkable thing is you and I, and President Trump, and others are all made out of the same particles, right? For that matter, garbage is made out of the same three particles, the proton, the neutron, and the electron. That's an unusual thing, right? If I was making reality, I would have done it differently. I would have had more variety. But to make all this variety out of three particles is astounding.

• 4:17Speaker 03

  There are three antimatter particles that correspond to those three. So to the proton, there's the antiproton. To the neutron, there's the antineutron. For the electron, there's the antielectron, which we typically call a positron for historical reasons. These particles have the same mass, so far as we've been able to tell, and they have the opposite sign of charge. And they're predicted to act like magnets, which are oppositely directed to the magnets that are associated with the matter particles. Now, as far as we know, the whole universe could have been made out of antimatter particles instead of matter particles. It's a mystery why it's matter rather than antimatters,

• 5:01Speaker 03

  and there's more mysteries involved here where the universe is involved.

• 5:05Flora Lichtman?

  Well, that's what I want to hear a little bit more about. So you said the cool part wasn't that you held this positron. It's that you could learn something about its quantum state? So what sort of data are you getting from it?

• 5:17Speaker 03

  Okay, well, let me just, first of all, I think it's really cool whenever we trap a particle. So I didn't mean to say that it wasn't. But what's even niftier is to see a positron in your apparatus for months at a time, making quantum jumps between its lowest energy levels. Now, most everyone in high school chemistry has studied energy levels of atoms. We made our own atom, which is our trap, which is on the outside with a positron inside. And that bound system is so cold that we can see jumps between one energy level and the next. That's even cooler than knowing there's a positron inside and observing it, to my way of thinking.

• 5:59Flora Lichtman?

  Why do you care about the jumps? What do they tell you?

• 6:02Speaker 03

  Okay, the jumps, well, first, it's a great kick to sit there and watch a particle making quantum jumps. If you haven't done it, you should try it sometime. It's just fun. But the really important thing is what we can do with that. We can measure the number of jumps that happen between energy levels as we put radio or microwave signals into the system. And that's the method that we actually use to measure the size of the magnet that's in the positron. Okay, and so that's what's so exciting, that it's potential for really comparing the positron and the electron to see if the prediction that they're completely opposite is right or not.

• 6:47Flora Lichtman?

  Okay, so you're learning about the positron's properties. Like, is it the exact mirror image of an electron or not?

• 6:55Speaker 03

  That's right, yes.

• 6:57Flora Lichtman?

  And are they? What'd you learn?

• 6:59Speaker 03

  Well, so far, the exciting thing about our report last week was that we now have a positron that we can study using these quantum methods that we've developed with an electron. That was never possible before. With an electron, we demonstrated a couple of years ago that we could make quantum measurements of its magnet and do it at a precision of 1 part in 10 to the 13, which is more precise than just about anything else is measured, right? And so now that we have the positron in the same environment making quantum jumps, we should be able to use the same quantum methods

• 7:41Speaker 03

  to measure its magnetic moment to a part in 10 to the 13. Then we can compare and see if indeed they are exactly opposite as predicted.

• 7:51Flora Lichtman?

  And if they're not, will that be a big deal?

• 7:54Speaker 03

  Oh, it'll be a huge deal, okay. It'll be a huge deal for several reasons. We haven't come up with really convincing arguments as to how we could make a theory in which they weren't exactly opposite so far. But I always say it's important not to be limited by theoretical imagination. Another saying I like to do is God decides and we measure, right? That's our place in the universe. And so in the end, it's an experimental question. So we try and look, and if theorists can't now imagine a way that it could happen, if we observe it different, there'll be 100 papers in the first year explaining possible ways that it could happen,

• 8:39Speaker 03

  and we'll have to investigate those. That's how science works.

• 8:42Flora Lichtman?

  It's going to be a shakeup, if that's the case.

• 8:44Speaker 03

  It'd be a shakeup if it happens, that's right.

• 8:47Flora Lichtman?

  Where does the particle come from? Do you have to trap it? Do you make it?

• 8:50Speaker 03

  Oh, that's the challenge of this experiment. And that's why we celebrated it with a report. Because the positron comes from the nuclear decay of a radioactive sodium atom. Okay, and sodium gives off these positrons, but they're going at a really high energy and a high rate of speed. And we want to get it to absolute zero all within the space of, you know, a couple of feet. You know, there's this cold volume that we can keep near absolute zero. So that's the challenge of this experiment, indeed, to take the positron from the radioactive source and coax it into our trap at nearly absolute zero.

• 9:34Flora Lichtman?

  Wow. You were on Science Friday talking to Ira back in 1999, as we said. You joined us on the phone from CERN, and I just want to play this little clip.

• 9:43Speaker 04

  As I speak, I look over a new experiment that we're building in which we hope to make anti-hydrogen atoms. So the core of this atom would be an antiproton, the antimatter counterpart of a proton. And in orbit around it would be a positron, the antimatter counterpart of the electron. So we'd have real antimatter as opposed to just antimatter particles for the first time.

• 10:07Ira Flatow?

  How long will it take before that experiment shows some success, do you think?

• 10:11Speaker 04

  Oh, boy. My optimistic or my pessimistic response?

• 10:15Ira Flatow?

  It's a ballpark.

• 10:16Speaker 04

  In my fondest dreams, we will make cold anti-hydrogen for the first time next year.

• 10:22Flora Lichtman?

  Did your fondest dreams pan out?

• 10:24Speaker 03

  No, but we did do it.

• 10:28Flora Lichtman?

  How long did it take?

• 10:30Speaker 03

  Oh, it took us, I think, I'd have to look back, another half a dozen years. In part because in the measurement that we were trying to do it the first time, we just weren't sensitive enough to detect the antihydrogen. I'm quite sure we made it, but we didn't, weren't sensitive enough to detect it. And then the facility that was making the antiprotons for us went offline because they made a significant change in the storage rings at CERN. And so when that came back on, and by the time we got our act together and got it working, an entirely new apparatus was a number of years later. So you have to be optimistic in this business, but it's not, it's not so uncommon that one,

• 11:15Speaker 03

  that it takes longer, shall we say.

• 11:17Flora Lichtman?

  So if there are antimatter atoms, is there like an antimatter me out there?

• 11:25Speaker 03

  If there was, it would be a really interesting and dangerous experience to hug yourself. Okay. Because the annihilations that would happen would be enough to blow up a whole city. Okay. So people, science fiction writers do great, have great, play great games with that notion. Fortunately, in the lab, we have such a little amount of antimatter that if you took all of the antimatter that we've ever studied in the history of the universe, I don't think we would have enough energy if we annihilated it to boil a cup of tea. So it's relatively safe.

• 12:01Flora Lichtman?

  I mean, speaking of science fiction, in the movies antimatter, you know, is either like the source of infinite energy or most, the most powerful weapon ever made. Is there a practical use to this research?

• 12:13Speaker 03

  Is there a practical use? Well, I like to say it makes God happy, so he sends us all rain. That's pretty practical. No, it's, it's, we're not, we're not application driven for this. I mean, we invent things that are then used by our culture, you know, a technological society. That always happens in these fundamental measurements. But no, we're really just trying to study how reality is put together. And usually what happens is if you find a new wrinkle, a new paradigm, there's a shift, then there are new applications that come from that. But it's hard to predict that, you know, hard to predict that with a time scale. Okay.

• 12:57Speaker 03

  Our, our contribution to society is to contribute these cutting edge sorts of ideas that then turn into technologies on a longer time scale.

• 13:07Flora Lichtman?

  Well, I would also say that understanding the nature of reality is practical, too.

• 13:13Speaker 03

  Well, I like your definition of practical. It's not the usual one I encounter, but I'll take it.

• 13:20Flora Lichtman?

  Dr. Gerald Gabriels is a Board of Trustees Professor in Physics at Northwestern University. Thank you so much for joining us again.

• 13:27Speaker 03

  You're welcome. It was great fun.

• 13:29Flora Lichtman?

  I agree.

• 13:38Flora Lichtman?

  This podcast was produced by Charles Burquist. And if you have a question about, I don't know, the underlying nature of reality or something else, call us 8774-SCIFRIGHT is the number. Thank you for listening. I'm Flora Lichtman.