A new force of nature?
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This is Unexplainable. I'm Noah Hassenfeld. Last month, a group of scientists found what seems like a crack in the foundations of physics. Experiments at Fermilab in Batavia, Illinois, showed that a certain subatomic particle disobeys the laws of physics as scientists have written them. Physicists are excited and they say this could be a major breakthrough in our understanding of the universe. I think it's quite mind-boggling and they have the potential to turn physics on its head.

I think the whole physics community is in it to see. Jessica Esquivel is a physicist who works at Fermilab in Illinois. We're very, very close to potentially new particles that exist beyond the standard model.

The Standard Model. It's one of the most important ideas in physics. So the Standard Model is an attempt to catalog all of the building blocks of the universe. And the way I like to kind of explain it is it's sort of like the periodic table. The periodic table of elements lists all types of atoms in the universe. But even those are made up of other, smaller things.

You have atoms made up of electrons, protons, and neutrons. And for a long time we thought that that was it. Physicists thought they'd hit the bottom. And then we dug some more and realized that protons and neutrons are actually made up of more stuff called quarks. They're a building block of the universe. Physicists found that electrons couldn't be broken into smaller parts either, so they count as building blocks too.

And then over time, they started finding some weirder particles outside atoms that also couldn't be broken down. You have electrons heavier cousin, the muon, and weird things like neutrinos. So these are the kinds of particles that make up the standard model. Basic building blocks of the universe that we know of.

But physicists are still looking for more. And to do that, they need to find holes in the Standard Model where new particles could fit. And we're at a point right now where we might have found a hole. And it all has to do with the strange wobble of one of these Standard Model particles called a muon. Have you heard of a muon?

No. Muons are sort of like less stable versions of electrons. How do you spell it? M-U-O-N. And these weirdly wobbling muons could change the future of physics. That's why it's so exciting. It hints towards something that we haven't seen before. Open that standard model. Help us open that standard model. Open it up. Let me hear it for the new.

Okay, so Jessica, this muon experiment you worked on at Fermilab might be pointing toward this hole in the standard model. So does that mean the standard model is incomplete?

I mean, we've known for a good minute that it's incomplete. And the reason why we keep poking at it is to try and figure out where the hole is. One of the big reasons why we know it's incomplete is because we know of this idea of gravity and there is no...

particle or force carrier in the standard model that describes this notion of gravity. But we know it exists, right? Because apples fall from trees and I'm not floating off my seat. Okay, so one gap could be a particle to help explain gravity. Yes. But then also, when we look at actually everything that's out there,

The standard model only consists of 5% of everything. And the rest of that is dark matter and dark energy. And we still haven't figured out how that falls into our theories and how that falls into our standard model. So there's a whole bunch of questions that we know are there. And there's a whole bunch of things that we know exist, but we haven't been able to kind of fit it into our

this standard model. So how exactly does this muon experiment point to a hole in the standard model or a new particle to fill that hole? So the muon g-minus 2 experiment is actually taking a very precise measurement of this thing that we call the precession frequency. And what that actually means is that we shoot a whole bunch of muons into a very, very precise magnetic field

And we watch them dance. They dance? Yeah.

When muons go into a magnetic field, they precess or they spin like a spinning top. Why do muons dance? So one of the really weird, quantum-y, sci-fi things that happen is that when you are in a vacuum or an empty space, it actually isn't empty. It's filled with this roiling, bubbling sea or...

of virtual particles that just pop in and out of existence whenever they want, spontaneously. So when we shoot muons into this vacuum, they're not just muons that are going around in our magnet. These virtual particles are popping in and out and kind of changing how the muon wobbles. Wait, sorry, what exactly are these virtual particles popping in and out?

So virtual particles, I like to see them as kind of like ghosts of actual particles. So, you know, we have photons that kind of pop in and out and they're just kind of like there, but not really there. And I think a really good kind of depiction of this like weirdness of quantum mechanics is Ant-Man.

Oh no. The Marvel movie? So there's this scene where he shrinks down to the quantum realm. As you shrink, as you shrink, as you shrink. And everything is kind of like wibbly wobbling and something's there, but it's really not there. Gone forever.

That's kind of like what virtual particles are. It's just kind of hints of particles that we're used to seeing, but they're not actually there. They just kind of pop in and out and just mess with things.

So quantum mechanics says there are these virtual particles, sort of like ghost particles we already know about in the standard model, popping in and out of existence. And they're bumping into muons, making them wobble? Yes. But again, theoretical physicists know this. And they've come up with a really good theory of how the muon will change with regards to which particles are popping in and out.

So we know specifically how every single one of these particles interacts.

with each other and within a magnetic field, and they build their theories based on what we already know. So what is in the standard model? Got it. So even though there are these virtual particles popping in and out, as long as those particles are things we know, like versions of particles in the standard model, then physicists can predict exactly how they're going to make muons wobble. So did something different happen? Were the predictions off? So what...

we just unveiled is that precise measurement doesn't align with the theoretical predictions of how the muons are supposed to wobble in a magnetic field. It wobbled differently. And the idea is that you have no idea what's making it do that extra wobble, so it might be something that hasn't been discovered yet? Something outside the standard model? Yeah, exactly. So does this break the standard model?

I've seen that in a bunch of headlines. No, I don't think I would say the standard model is broken. I mean, we've known for a long time that it's missing stuff. So it's not that what's there doesn't work as it's supposed to work. It's just that we're adding more stuff to the standard model.

So just like back in the day when scientists were adding more elements to the periodic table, even back then they had spots, right? Of where they knew an element should go, but they haven't been able to see it yet or they haven't been able to like create it yet. That's essentially where we're at now is that we know we have the standard model, but we're missing things. So we have holes that we're trying to fill. Okay.

Okay, I want to try and figure out what might fill those holes after the break. But before we go, one last important science question. If you had to guess what song the muons were dancing to, what would you say? That's a very good question. And I have the perfect answer for it. Hit me. It's the Wobble song. The Wobble song. Wobble baby, wobble baby, wobble baby, wobble baby. Wobble baby, wobble baby, wobble baby, wobble baby, wobble baby, wobble.

Up next, what all of this wobbling might be pointing to. Something new that physicists have never seen before. Support for Unexplainable comes from Greenlight. People with kids tell me time moves a lot faster. Before you know it, your kid is all grown up. They've got their own credit card and they have no idea how to use it.

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Unexplainable. Unexplainable, we're back. In the first half, we were talking about these wobbling muons, which wobbled so much that scientists think they point to something completely new. A particle outside the standard model, this chart of all the stuff that makes up the universe.

I wanted to try and figure out what that new particle might be, so I called up Nashin Shah. I am a professor of physics and astronomy at Wayne State University. Just to be safe, I wanted to make sure this tiny extra wobble on these tiny little muons couldn't just be some sort of mistake. Like, maybe they just measured it wrong, or maybe someone spilled coffee on the particle accelerator. Ha ha ha!

No, we are sure. Nashin is pretty sure because the most recent muon experiment matches one that scientists have done before at another particle collider called Brookhaven. 20 years ago, we did this experiment at Brookhaven and we, you know, set up these muons and made them go around in a magnetic field and we looked at how much they wobble.

And it turned out that they seemed to wobble a tiny bit more than they should. The results were exciting, but they weren't certain. You do need to be very careful about, you know, what if there is some thing that you have not thought about that impacted the experimental measurement that you did, which is why you always, always need to validate.

So that was the whole reason for setting up the Fermilab muon g-2 experiment. New experiment, new detectors, new location, all to see if they can still get that extra muon wobble. To make sure the original experiment replicates and that first one wasn't just a coffee spill. So you hopefully are not going to spill the same coffee in the same place.

And the results are very consistent with the ones from Brookhaven. So what we are pretty sure about is that there's no screw up

in the experiment. There are still conversations happening right now about some of the absurdly complicated math here. But Nashin says that no matter what, something weird is probably happening. Exactly, exactly. Which is why I think that I find this, what's happening right now, like super exciting, right? Because something's going on somewhere, right? So it's like, all right, we got to hunt to see where it is.

Now, Sheen highlights three explanations for what's causing this extra muon wobble that are worth discussing. First, there's something called a leptoquark, which would be a new particle we haven't seen before. Then there's supersymmetry, which would give us a whole set of new particles. And finally, there's a possibility of an entire new force we haven't discovered yet. So, option one. Leptoquarks. Leptoquarks are particles that would be able to interact with muons and quarks, or even turn a muon into a quark.

Physicists have talked about these in theory, but this extra wobble could be a sign that they're real. Or the wobble could be a sign of option two, supersymmetry. I really like supersymmetry. It says that for every standard model particle, there needs to be what we call a superpartner associated with it.

It's called supersymmetry because it gives every particle in the Standard Model a mirror particle that's almost, but not quite the same. So it's actually an idea that's been there for a long time. And we have been looking for the signs of

this type of theory for a while and we haven't seen anything yet. But I personally still find it one of the most compelling stories. If these particles were discovered, it would be enormous. It would essentially double the number of particles in our standard model. And these new superpartners already have some pretty great names. We decided that we're going to call all the superpartners by putting an S in front of the name.

So for example, the electron must have a selectron and a muon would have a smuon associated with it.

My favorite is definitely the squark, which would be the supersymmetric partner of the quark. And we have very serious, very technical seminars and colloquia and discussions with all of these names. But supersymmetry is more than just funny names. The nice thing about these supersymmetric models is that they come with a particle which can actually be a dark matter candidate.

So if this muon wobble leads us to supersymmetry, then supersymmetry might lead us to discovering the dark matter particle. Right.

So that's option two: an entire set of new particles. Option three gets way weirder. There is a third prong, which is, for example, an additional force. Not just a new particle or a set of new particles, but an entirely new force. Something like electromagnetism that we haven't discovered yet. And that could be making the muons wobble so much. So apart from just our electromagnetic

and weak and strong force, maybe there's an additional force that we don't know about.

And this new force would also come with its own new particle. Well, it's coupled together in the sense that usually new forces also come with new force carriers. Right? So just like, you know, the electromagnetism, right, that's the electric force, that's mediated by a photon, by light. Right? That's an exchange of photon is what mediates the force. So option three, this new force, along with its new particle, could be wobbling the muon.

So if you had a new force, then that could be causing some sort of little wiggle there. Okay, deep breath. We've got these three possibilities to explain the extra muon wobble. It could be caused by a lepto quark, this new particle, supersymmetry, a whole set of new particles, or maybe even a new force we haven't discovered yet.

This wobbling muon in the Fermilab experiment, it's like a little breadcrumb of a clue. And we've got these three ideas of what could be making these crumbs. So the next step is for scientists to try to figure out what the whole loaf could be. What we have to do is figure out the ingredients of this breadcrumb. Right? Did it have a little cinnamon on it? Or maybe some vanilla? So you say, okay, this hints to me of a particle which has this type of characteristics.

If there existed a particle with these type of characteristics, I should be able to do this experiment and be able to produce it directly. Essentially, scientists need to keep doing more experiments to try and head down each one of these paths and see if these breadcrumbs lead to a loaf. We actually do have a whole bunch of different experiments running right now.

Right? Which are in fact looking at, you know, all of these different types of theories in different ways. It's a constant dynamic process. That constant dynamic process is right on the edge, just peering off into the unknown of new particles, new forces, new physics.

Look, do you really need to know what a lepton or a lepto quark is? Probably not. But all of this amounts to the fact that physicists are still trying to figure out what our universe is made of. And the Fermilab experiment might just be pointing in the right direction. I hope so. I really hope so. Nashin isn't the only physicist hoping.

I've been doing particle physics for maybe 15 years and there's been a bunch of things that have come and gone. And this is really the first thing that's come and stayed. And so to be honest, I don't even really know how to feel in these situations. We're sort of trained to always be very skeptical. It's the first time that it's like, oh, how are we going to respond to this thing that is kind of unexpected? The fact that we're chasing new physics and we're so close we can taste it, it's...

It's the unknown. It's like the first bite of a really good cookie and you know that the next couple of years we get another bite and there are so many things we don't understand about the universe. You know, what's going on with muons? What's going on with super symmetry? Where does dark matter fit into all of this? Why is the universe the way it is? Why are we the way we are and not some other way?

I think there's something that's really innate in people to want to know about who we are and where we come from and what our place in the world is. And I think that there are a lot of different ways that we can answer that, whether it's through stories or music or film. But I think also through physics that we can actually peer into what's at the heart of our universe. And it's exciting that this might give us a clue as to what is really going on. And the fact that the work that I'm doing

could potentially be in textbooks in the future. People can be learning about the dark matter particle that G-2 had a role in finding. That's it. It gives me chills just thinking about it.

Thanks to all the physicists we just heard from. So that's Jessica and Nashin, but also Brendan Keeberg, Priska Cushman, Brian Chauvet, Jessica Muir, Sarah Demers, and Rodolfo Capdevilla. This episode was produced and co-reported by me, Bird Pinkerton, and it was edited by Brian Resnick and Meredith Hodnot. We had music from Noam, and sound design and mixing from Christian Ayala.

Mandy Nguyen backtracked this episode. And Liz Kelly Nelson is the VP of Vox Audio.

Jessica and Nashin talked a little bit about the hunt for the dark matter particle in this episode. But if you want to hear lots more about that, you can check out our very first episode because it is all about that search. We have physicists looking for the particle at the bottom of a mine, also trying to find it in deep space. That episode is called Most of the Universe is Missing.

And if you're looking for a standalone version of the Mewon song that you heard in the first half of the show, please sign up for our newsletter. We'll have a link to it there. You can do that at vox.com slash unexplainable. And please, please, please, please feel free to send any thoughts you have to unexplainable at vox.com. Read and try to respond to all your emails.

Also, if you like the show, please tell a friend or leave us a rating or a review wherever you listen, because whatever you do, when you do stuff like that, it helps us find new listeners and we really appreciate it. In the meantime, Unexplainable is part of the Vox Media Podcast Network and we'll be back in your feed next Wednesday.