Briefing Chat: How physicists successfully trapped the electron's antimatter equivalent

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1:06Hi, listeners. Benjamin here. It's the Friday Show, the podcast where we talk about a couple of stories we've read about in the Nature Briefing. And joining me this week is none other than Lizzie Gibney. Lizzie, how are you doing today? Hello, Ben. I'm very well, thank you. Very happy to be here. Excellent. Well, listen, why don't you go first this week? You've been writing a story and its headline is rather intriguing, I have to say. It's about a trapped single antimatter particle that's been kept in a cage for months. Now, you and I have covered antimatter many times on the podcast, but this is a particular particle that maybe we haven't discussed quite so much.

1:40That's right. This is a positron. So that's the antimatter equivalent of an electron. In the past, we've often talked about antiprotons and how they compare to protons because they are the particles that are produced at CERN. It's kind of a byproduct of the LHC. This instead is looking at a completely kind of different area within particle physics. We're looking at what are called leptons, so that's electrons and positrons. And the big goal here is to do what physicists have been doing now for decades, which is to try and compare antimatter, so in this case the positron, with matter and electron, in some property and study them so precisely until you try and try and hopefully find some difference between the two.

2:24In this case, when they finally are able to do it, it will be at about one part in a trillion that they will be making this comparison. So, you know, if there is some tiny difference between matter and antimatter, they're hoping to try and find it. Right, because that's one of the big questions in physics. Why does matter prevail? We're obviously surrounded by it. We can reach out and touch stuff. Everything is built in matter. Antimatter is there, but very, very scarce. This is a big mystery because according to the standard model, which by now you'll have heard me talk about many, many times, but you know it's like the best framework, the best kind of model that we have in particle physics that describes all the particles and the forces.

2:58It's very elegant, it works extremely well, it makes incredibly precise predictions that then fit with observations but one thing that's part of the standard model is that it seems like it should make just as much antimatter as matter and then you have this question well going back to the Big Bang if we had together matter and antimatter produced in equal amounts, boom, the whole thing would have just basically spectacularly annihilated with each other. And you'd have nothing left. Podcast is dumb. No podcast, no, no anything, unfortunately. And that obviously didn't happen. But we still don't know why that is.

3:34The laws of physics don't tell us why that should be the case. So there's this hunt that's going on to say, well, look, there must be some difference between their behaviour, their properties that explain why this didn't happen. And we are trying really, really hard to find it. Which leads us to this experiment then, right? So they're looking at this positron, then the anti-electron, if I put it that way, to see if they can find whatever this discrepancy might be. And in your article, there is a picture of their setup, right, which understandably is incredibly complicated. Presumably, this was quite a hard thing to do.

4:08Very, very hard to do. It's one of these situations where it's literally this one team in the world who at the moment has the skills and the equipment to be able to do this. So the big challenge really is it's not that difficult to make positrons but they're traveling very very fast and so to do these studies you need to have them under your exquisite control. You're kind of reducing their energy by I think it's about 10 billion times like they fly out of actually radioactive decay and the way that this team did it was they passed the positrons through a crystal of tungsten and there it kind of scatters and a lot of them annihilate but when you come out the other side the ones that you have left are much much much slower than they were and then they very weakly bind them to an electron to make a positronium and that enables them to feed into this trap because it's very temporarily neutrally charged so they feed it into the trap despite the fact that this trap has huge electric fields and then they crank up the electric field, rip the electron from the positron and you just have left your perfectly caged little positron and they chill it down as well so it's just 0.3 degrees above absolute zero so extraordinarily cold and they have as close to a vacuum as they can possibly get and it's just pinned there in midair by electric and magnetic fields and it's in this very low energy state because like you know how electrons in an atom have different energy levels and you can apply energy to pump them up or down.

5:39Well, this positron, even though it's not attached to an atom, kind of behaves like that. It's in its lowest quantum state. And then the researchers are able to feed in, kind of zap it with precisely controlled microwaves in order to put it in these different energy states that they then can use to measure this property that's called its magnetic moment. Right. So the magnetic moment is where they're looking for any possible discrepancies. Now tell me a little bit about what that actually means. So the magnetic moment is almost like kind of the little tiny bar magnet inside each of these particles and it affects how they behave in a magnetic field.

6:14In a magnetic field the particle will spiral but the magnetic moment refers to how it all kind of like wobbles, it processes as it goes around in that spiral and it's a fundamental property of a particle. So an electron has a magnetic moment a positron has a magnetic moment and the point is here to do an experiment that can as precisely as possible measure what the magnetic moment is because then you can compare the two and see if you have any difference between matter and antimatter the team have already done it with electrons and they had a highly precise result and they have now got their setup ready to go this poor positron is trapped in its magnetic cage for months on end and they actually have started to doing their experiments.

6:57How long will it take then until they work out what the magnetic moment is for this positron? I asked, so Jerry Gabriolese, who is leading this team at Northwestern University, he said they're doing it. It takes a lot of effort to figure out all of your sources of error when you're doing an experiment that is this precise. But he kind of said, maybe next year, hopefully next year, we'll have the actual result that is the comparison between the two. Goodness, how exciting. And you and I have obviously talked about stuff like this many times. And the standard model seems fairly robust, right? Any question that's thrown at it, it seems to be resolute against it.

7:30Like, what would it mean if a difference is found? It would be the biggest result in a long, long time. It would be revolutionary. So the thing that we know about the standard model, we know two things. We know that it's incredibly good and very, very good at predicting and it's very precise. We also know it's not the full picture. We know it's incomplete because it doesn't explain this difference between matter and antimatter. it also doesn't explain many, many other things that are observed in physics and the world around us. For instance, dark matter is one that we, again, we've spoken about before.

7:58We think dark matter exists because we see the way that galaxies behave in space and we can extrapolate from the amount of gravitational field they seem to have that there must be a lot of matter there that we just cannot see. It doesn't seem to interact in any other way. Dark matter is kind of almost a given that it exists, but it's not in the standard model and it's never been seen. So there's kind of a hope that if, you know, an experiment like this could find a crack in the standard model, could tell us there's some way in particular that it's wrong, that would then open up the standard model and lead to potentially solving a whole host of other issues that we know is not the full story on.

8:35I guess the flip side of that, though, is if it doesn't show anything, right, that they're exactly the same, where does that leave everything? We keep trying. I mean, this is the thing is you can always get more precise, pretty much. So at the moment, physicists are looking in every different area they can. So this is the first time this has been done in positrons. We've talked about already. It's been done with antiprotons. You know, you can probe the standard model in many, many ways. Neutrinos, again, is something we've talked about a lot. And there's a lot of experiments that are just cranking up there that could also help to show where the standard model is going wrong.

9:08But I think usually as well, we end up just getting some offshoots. You know, the fact that we can control a positron so well is probably going to lead to some other kind of experiment that leads to some other kind of experiment. So even if we don't find here that there is a difference between matter and antimatter, it's kind of told us where not to look or at least not to look at this level of precision. And it's going to open the door to many other fields of physics. We will be covering the result, I am sure, when it comes out. But let's move on to my story this week. And I read the technical details about it in Aerospace America.

9:40America. And we love a space launch, Lizzie, right? I'm still buzzing from Artemis 2, right? One of my highlights of the year. But at the start of this week, as you're listening to the podcast listener, the Japanese space agency JAXA successfully launched a mission to a moon, not just any moon, Phobos, one of Mars's moons. Yeah, I'm excited about this because we've had a lot of bad news about Mars missions lately, right? You know, the US is kind of leaning away from or even cancelled their sample return mission. So any mission that actually goes to a Martian moon, I think is really exciting. Right, and this one has some unique challenges, which I will lay out for you.

10:18Now, Phobos is one of Mars's two moons, right? There's Phobos and Deimos. Now, these were only discovered in the late 1800s. And Phobos is much smaller than our moon. It's only about 30 kilometres long. Apparently, it's the shape of a potato, Lizzie, so I'm led to believe. It's so often R. Well, absolutely right. But how it came to be, it's been a bit of a mystery. And that's something that this mission is trying to figure out. Now, this is called the Martian Moon's Exploration Mission, MMX. And its spacecraft is on its way there as we speak. And it'll get there in about a year. And what it does, it's going to do a bunch of stuff.

10:54So firstly, it's going to orbit Mars itself, and then it's going to hop over to Phobos. And this is when things get kind of exciting, because in 2029, MMX will release a tiny rover onto the surface of Phobos. And this rover, well, it's got some issues that it needs to overcome, as I say. What are the issues? The issues are there's basically no gravity on Phobos whatsoever. It's a fraction of a fraction. So this little rover is called E-Day Fix, and it's courtesy of the French and German space agencies. Now, E-Day Fix is the little dog in the Asterix comics. We know him as Dogmatics, but in other countries, they're known as E-Day Fix.

11:32But what's going to happen is the spacecraft part of MMX is going to get really close to Phobos and let the rover go. Now, it has to get quite close so it doesn't gain too much energy as it falls down, right? because if it's going too fast, it will bounce and potentially bounce out of Phobos's gravitational pull and disappear into deep space. God, has it got some kind of, can it break? Like, can it slow itself down as it comes in to try and land? It's going to bounce a few times, as I understand, and then it will sort of stop. It's got a variety of mechanisms to help it right itself. And then the plan is this little rover will spend 100 days trundling very slowly around Phobos, slowly so it doesn't sort of pitch over forwards and end up on its back.

12:13Because as we say, Mars missions are really, really hard. And once your solar panels aren't pointing at the sun, it's kind of game over. So very slowly around Phobos. And then we head back to the MMX spacecraft. So what the rover is going to do is look for landing sites. And once it's found a suitable one, the spacecraft is going to come down, very briefly kind of kiss the surface of Phobos and scoop up 10 grams of the soil of this moon and then send it back to Earth where it will come back a few years later. And this should help answer a fair few questions about what's going on with the red planet and its moons.

12:51That's amazing. So that's the advantage, of course, of going to a moon that has very low gravity is you can do something like that. So this seems like a really smart way of getting a bit of Mars. Of course, we're getting a Martian moon rather than the Mars surface itself. But you're going to learn, I would think, a lot about the whole system by doing that. Do we think that Phobos kind of chipped off Mars at some point? You know, was it hit by something and then this little chunk ended up coalescing in its orbit? Because if that's the case, which is what we think happened with our moon around Earth, we would be able to learn a lot about Mars just from studying its moon.

13:25Well, this is one of the fundamental questions that this mission is trying to answer. How did these two moons come to be? And yeah, it could be that something a long time ago smashed into Mars and Phobos and Deimos were sort of split off and there they are. But it could also be that these were ancient asteroids that were captured and there they stick. And from what I understand, once the scientists get a look at this soil, they should be able to answer that question pretty much straight away, right, relatively quickly. But even if this never was part of Mars proper, if you see what I mean, there's a chance that if something else hit Mars, it would kick up some dust and that dust would make its way to Phobos.

14:02So they might find some actual Martian soil on there as well. It might get two for the price of one. Absolutely right. And as you are obviously aware, Lizzie, we do have Martian rock here on Earth, right, that has been fired off as a result of an impact. But as soon as that stuff gets to Earth, it gets contaminated by our atmosphere and by the ground and all the rest of it. So this is a potential chance to really get around all of that and look at stuff from the source itself. Obviously, fantastically complicated and many space missions, particularly missions to Mars, don't necessarily go the distance.

14:32but this would be potentially the first time that samples come back. Although there is a mission from the Chinese space agency that's due to launch in 2028, I think, that may, if the timing works out, come back a little bit ahead of this Japanese mission. And of course, as you said at the start there, the US space agency, NASA, has been trundling around Mars for a long time. They've collected a bunch of samples, but because of cost and technical issues and political will, those samples are sitting there in tubes, potentially in perpetuity. So it could be that I think around the early 2030s, which seems like the far future, but actually isn't that far away, we may really get a handle on what Mars looks like.

15:10I'm so happy that space agencies are still doing these kind of missions, which, as you say, is really quite risky, risky in the sense it's still for a rover, but it's very, very hard to land, to operate around Mars. But there's such a wealth of science and we've got some exciting missions to come. Absolutely and hopefully information about the soil will help inform future missions if we are to send people to Mars and all the rest of it and of course there is that one big question that David Bowie sang about that people would quite like to know the answer to is their life on Mars. I'm not saying this mission will help answer that but hey one can dream right.

15:43Well listen let's leave it there for this week's edition of the briefing chat. We'll put links to both of those stories and where you can sign up to The Nature Briefing to get more of them delivered to your inbox in the show notes. So head over there to read them. But for the time being, all that's left to say is Lizzie Gibney, thank you as always for being with me. Thank you so much, Ben.

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