Daniel and Jorge Explain the UniverseDaniel and Jorge talk about the weirdness of muons and how they can let us see inside things.
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Hi.
I'm David Eagleman.
From the podcast Inner Cosmos, which recently hit the number one science podcast in America.
I mean neuroscientists at.
Stanford, and I've spent my career exploring the three pound universe in our heads.
Join me weekly to explore the relationship.
Between your brain and your life, because the more we know about what's running under the hood, better we can steer our lives. Listen to Inner Cosmos with David Eagleman on the iHeartRadio app, Apple Podcasts or wherever you get your podcasts.
Guess what Will?
What's that Mango?
I've been trying to write a promo for our podcast, Part Time Genius, but.
Even though we've done over two hundred and fifty episodes, we don't really talk about murders or cults.
I mean, we did just cover the Illuminati of cheese, so I feel like that makes us pretty edgy. We also solve mysteries like how Chinese is your Chinese food? And how do dollar stores make money? And then of course can you game a dog show?
So what you're saying is everyone be listening.
Listen to Part Time Genius on the iHeartRadio app or wherever you get your podcasts.
Hey, Daniel is particle physics actually useful for anything.
I mean it's good for like understanding the universe for sure.
Yeah, but what can I use particles for?
Can I use a charm quark charm my way into a better life?
I think you're plenty charming already without any charm quarks. But we might be able to, like use muons to help us get to the moon.
What just because they start with an M? Is there one letter off from moon and muon?
I'm just reaching here, man.
Can you use a muans to feed cows? You know? Or grow more corn?
More?
Muse less corn? I mean, nothing is certain in science, but that's probably a no.
Why not don't cows eat muans? Don't they eat muons?
I think muons actually cause cows to mutate and make new kinds of cows.
Oh well, maybe we'll get a taste of cow out of it, In which case particles would.
Be useful better steaks through physics, that's.
Right, better particle burgers.
Him MC cartoon, I and the author of Oliver's Great Big Universe.
Hi, I'm Daniel. I'm a particle physicist and a professor at UC Irvine, and I like believing that physics raises the steaks.
The steaks like the cow steaks.
Yeah, we've got to raise some steaks or the burgers maybe. I mean you're always talking about setting the steaks and.
Stories, right, Yeah, that's always important, but usually makes the emotional steaks, not the raw or well done.
Kime.
Well, I like to get my steaks at the restaurant called mcguffin's.
So do you like them rare or well done?
I rarely eat steaks. Actually, it's the truth.
He eats steaks rarely or rarely eat steaks.
Yeah, almost never eat steaks. My son is a big fan of protein, but he prefers chicken and turkey. He's a poultry man.
Oh I see, he likes it lean.
He likes it with wings.
Nice.
Nice to stay lean and flighty as well.
But anyways, welcome to our podcast Daniel and Jorge Explain the Universe, a production of our Heart Radio in.
Which we help your brain to take flight and trim all the fat from your understanding of the universe. We think it's possible to zoom out there with our minds and understand everything that happens in the universe, from the tiniest little particles to the biggest, most massive black holes, and our goal is to break it all down and explain it to you.
That's right.
We try to prevent your brain from having a cow thinking about the amazing and vast universe we live in, with all the complete physics and mechanics that are happening. We try to boil it all down to make it digestible and lean. We trim all the fat out of science communication while trying to keep it still plenty juice. And it's all one hundred percent organic, right, These no chemicals in this podcast.
I mean, I guess everything's a chemical, So yeah, I mean I do use growth horribones to inflate my intelligence a little bit. But one of the reasons we're talking about such practical matters is because one of the criticisms of particle physics is that it can be kind of abstract, Like, are the questions of particle physics really useful to you on an everyday basis or is it more of a philosophical search for understanding of the nature of the universe.
Yeah, you got to kind of wonder what is smashing all those particles together, spending billions of dollars? What it is that all useful for how is that helping humanity move forward and maybe eat better as well?
And of course there are lots of indirect benefits. Just understanding the nature of the universe is its own prize and is priceless. But every dollar we invest in basic research comes back to us in terms of technological advancements and economic output and education and employment. So it's definitely a worthy way to spend money, I say, with absolutely no conflict of interest whatsoever.
I was gonna say it definitely means employment for certain people like physicists.
Perhaps it certainly does, but it benefits everybody because investment in basic research always leads to revolutions and our understanding and in technology and all sorts of stuff.
Yeah.
I guess without physics there wouldn't be this podcast, which sort of employs us right.
And improves the lives of everybody on Earth.
I guess if physics wasn't around, we'd have to talk about something else, or toxplaining the universe using other things.
Absolutely, but sometimes particle physics can be more directly useful. Things we learn about weird particles exotic can actually be put to use to help us solve everyday earthly.
Mysteries, and they might actually also help us have X ray vision in a way. So today on the podcast, we'll be taxing the question can we use muons to see inside of things? What kinds of things are we talking about? Daniel m All kinds?
Boxes?
Yes, absolutely, escape rooms, people's pockets, safes and banks. Yeah, oh boy?
What's inside the burgers at McDonald's.
Perhaps nobody wants to know that for real, that's not why you go to McDonald's.
I don't think we'll get grand funding for that question.
Now that's a situation where knowledge can ruin something.
Yeah, yeah, but it is an interesting question whether we can use meons to see inside of things?
You mean, is this sort of like using muons as X rays?
Kind of Yeah, it's a similar idea. Can we use penetrating radiation to reveal something that is hidden from us? Can we look inside something without opening it up?
Can we just use X rays that was already invented?
We can use X rays, but X rays also have their limits, and some muons might open up the possibility to see inside things that are otherwise still close to us even with X rays.
Mmm.
Interesting, All right, won't dig into it, but first, as usual, we were wondering how many people out there had thought about using muons to see inside of things and how we might be able to do that.
Thanks very much to everybody who plays the game for this section of the podcast. We love hearing your voice, and if you would like to participate, it's very easy. It all happens over email. Just write to me too questions at Danielandjorgey dot com.
So think about it for a second. Do you think we can use muons to see inside of things? Here's what people had to say.
No, idea, absolutely, But I know that there's some talk of making a muon's collider or something like that. I read about that in some news reports. So I'm going to say, yeah, uh, why not. You know, if you can accelerate them enough and they don't dissipate energy like electrons, there should be a way to create collisions.
I'm going to say, yes.
I listened to your whole podcast about muons, but I've completely forgotten what they are. So I am going to take a kiss and say, yes, you can use muons to say inside something.
I would imagine using muons to look inside things would be what the same principle is using an electron microscope. I suspect muons are smaller than electrons, so for them to bounce off something and give an image, they have to be bouncing off very small subatomic particles.
I don't know what a mion is, so I don't know, all right.
I imagine a lot of people are like that person who said they don't know what a muon is. They don't know, but it sounds like a reasonable question. A lot of people seem pretty optimistic about this.
Yeah, if a mune is some new kind of particle, maybe it's got some kind of powers or abilities or properties that lets you do new kinds of stuff. I guess that's the optimism.
All right.
Well, let's put a stake through this question, and they start at the basics, Daniel, what is exactly a muon? A lot of people seem to have heard us talk about it, but maybe forgotten.
What it is.
A muon can best be understood is like a heavier version, a more massive version of the electron. It's very very similar to the electron, has a lot of the same properties, same kinds of relationships as the electron, but it's more massive.
See.
So it's a particle and I guess maybe we should mention that the universe has particles, or at least the potential to create particles or further to exist particles, and a muon is one of these particles.
Yeah, there are lots of particles that make up me and you and all the normal matter that's out there. If you drill inside of us, you find molecules and atoms, and those atoms are made of protons and neutrons and electrons. The protons and neutrons are made out of quarks. So at the most fundamental level, everything that you and I are made out of, and everything that you and I eat, including steaks and cows, are made up of upquarks and down quarks and electrons. So those are the three basic building blocks of normal matter. But there are other kinds of particles out there that the universe can make. They're sort of on the menu, but they're not stable and they're not involved in building normal, everyday atomic matter. So there's sort of various categories of particles out there. Ones that can be made and exist all over the universe, and ones it can be made but only.
Exist briefly, or at least in the current universe that we have, right. I think we talked about maybe before, like maybe in the early universe, the particles like nuons were common and they would hang out.
Yeah, the frequency of which you find these particles definitely depends on the temperature of the universe because the unstable particles muons, charm quarks top quarks are a lot more massive than the other particles that take more energy. These days, it's rareer to create that kind of energy because the universe is more spread out and colder. Back in the early days of the universe, it wasn't as hard to get enough energy together to make a muon or a top quark. They always have a short lifetime, though they still don't last very long, but they're made much more often in the early universe. These days, it takes more specialized conditions like humans smashing particles together or cosmic rays hitting the atmosphere to create the conditions to make these weird particles. They still don't last for very long.
So like the meon you said only lives for a few microseconds, right.
Yeah, the muon lives for two point two microseconds before it decays into an electron and a couple of neutrinos and we call the muon like a cousin of the electron, because it has a lot of the similar properties. It's negatively charged like the electron is. It's paired with the neutrino the way an electron is. So in our sort of table of particles, we put the quarks in one category and these other particles we call leptons in another category because the muon, and like the electron, also doesn't feel the strong nuclear force that the quarks do.
I see.
So it's basically an electron, but somehow it just has a more mass to it, like the label that says this is how much an electron. Way, it just happens to be more for the muon, but other than that, it's almost exactly the same, Like it has the same electrical charge and all the other quantum values.
Right, Yeah, it's about two hundred times more massive than the electron, and nobody knows why that is, Like why does the electron have this mass and the muon have that mass? These are just numbers that we've discovered in the universe without any explanation. You might think that the Higgs gives an explanation for why some particles have more mass than some have less. And it's true that the muon has more mass than the electron because the Higgs interacts with it more, giving it more mass, But that doesn't explain why there's a difference. It just kicks the can down the road. Instead of asking why does the muon have more mass than the electron, we now ask why does the muon interact with the Higgs more than the electron does. The Higgs explains what mass is, but not why some particles have more or less of it. It's still just two numbers without an explanation. Now those two numbers are interaction strength instead of mass. And there's a third version of the electron called the tao, which is even more massive. And this is the general pattern of the particles. Each of the particles we talked about, the electron, the upcork, the down cork has two copies of it which are more massive. So this is some deep symmetry, some structure to the universe that we've observed. We've organized, we've seen the pattern, we've laid it out on the table, but we've not understood it. And the mew one was like one of the first clues we had that there was more out there to the universe than just the particles that made up our matter.
But I guess you know, what does it mean that it only lists or two point two microseconds? Like does that even count as existing?
You know?
Like why I didn't call it a thing if it's only around for two point two micro seconds? You know, like can it move around that much? Or or is this one of these like gorillativistic things where to us it lists or two point two microseconds, but maybe it's going really fast it lives for a really really long time.
I think yes to all of that, although you know, the timescale is always relative, like we only live for one hundred years on the timescale of the universe, that's basically nothing. Do we even count as existing? I would say yes, right, because time scales are relative relative to some other particles, like the top quark lives for ten to the month twenty three seconds, but we still think that it's a thing. Like the neutron lasts for I think eleven minutes before it decays, So these timescales are all relative. What we actually mean by two point two microseconds is in the muon's rest frame, Like if you had a muon in front of you at rest, and you started a clock when it was created, and you waited until a decayed, that would be two point two microseconds. But you're right, relativity plays a big role. Muons are often moving really really fast, especially when they're created in the atmosphere, So if they're moving near the speed of light, then a clock that's moving with them is slowed down. And so the reason muons can actually survive from the top of the atmosphere where they're made to hit us on the ground is because their time is slowed. So from our perspective, they can last for much much longer than two point two microseconds, long enough to make it to the surface of the Earth.
And what does it mean that it decays or does it disintegrade? Does it like the energy just diffuses or transforms it to something else?
What does that actually mean?
Yeah, sometimes we think about decay as like something breaks up and you get the component bits. It's like cracking something open, breaking it into its basic legos. Not an atom broken up into its protons and neutrons. That's not what's happening here, because when a muon decays, it turns into an electron and two neutrinos. But It's not like the electron and those neutrinos were inside the muon. It's not like the muon is made of the electron and the two neutrinos. Instead, think of that energy is passing from the muon field to the electron field and the neutrino fields. Remember that all these particles are really just ripples in universe spanning fields that feel all of space. Every part of space has a muon field, an electron field, and the three different neutrino fields. So it's happening here is that those fields are coming into contact. They're interacting in the muon field. Oscillations in that field are not stable. They like to decay down into the electron field and the neutrino fields. So that's what's happening here.
But maybe a question is like why is it so unstable, Like what makes the muon field, which makes muons prone to be to basically dissipating or disappearing, and not, for example, the electron field, which seems super duper stable.
The electron would like to decay, but there's nothing for it to decay into because it's the lowest mass particle in this chain. It's the lightest charged particle, and so the muon can decay to an electron, which is a lower mass particle, and so it does because in doing so, it spreads out the energy. The universe doesn't like to have a lot of energy concentrated in one place, likes to spread it out. It's like entropy at a most basic level. And so a high mass particle will tend to decay into lower mass particles if it can, because that provides more arrangements of that energy. Instead of having all of it just in mass, now you have it in a lower mass particle, plus lots of different possible momentum states. So the quantum mechanical probabilities are just much more for lower mass particles, and so they're more likely to happen.
But I guess maybe why doesn't Why can't the electron break into something smaller? Is it just like we just haven't seen it do that, or maybe it's impossible.
Well, we haven't seen an electron decay. We think electrons are stable. Though it's possible that electrons live for like a trillion years, we just never seen one decay because they just last for a long long time. Right, It's the same with the proton We think the proton is stable, but we don't know. We've never seen one decay, so we think it might be stable or very very very long lived. But for the electron to decay, there would have to be something for it to decay into that also has electric charge. Because electric charge is conserved, it can't just go away. We don't know if any lower mass charged particle than the electron, So it's sort of like the bottom rung of the ladder, which is why energy sort of gets stuck there.
I see, okay, so well, then the mewon decays because it can decay in to other particles. Does it get triggered by something, or if you just leave a meon there, it'll be like okay, I'm done, and then it breaks apart.
If you just leave a muon in the vacuum, it will decay. Some muons flying through space will decay on their own. They can also interact with stuff because they have charge. They can interact with electrons, and they can interact with protons and all sorts of stuff. So if you slam them into a block of lead, for example, they will interact and that can also trigger the decay. But muons on their own will also just decay.
Can they appear out of nowhere? Like, what does it take to make a muon? Or how are they made if they're a thing in the universe. Is it just whenever you have enough energy concentrated into one spot or what's the origin story of a muon?
So the origin stories that you get enough energy into sort of a higher mass field to feel that can decay into muons. Energy likes to flow down the lower mass fields like rungs down the ladder, So you got to get enough energy into a higher mass field and then it can decay into muons. So the typical way that muons are made naturally in our environment is that you have a cosmic ray, which is like a proton slamming into some particle in the atmosphere, which creates a lot of energy density in one space, and then you create some very massive, unstable particle, and a lot of particles decay into muons, So you might create like a pion or a chaon. These are more massive particles that like to decay into muons, and then those decay in the atmosphere, giving you a muon which flies down to the surface of the Earth.
But then you need Where does the charge come from?
Protons are charged, right, so the charge comes from the cosmic ray, and also there's loss of charge in the upper atmosphere. Even a neutron slamming into a particle in the upper atmosphere, and like disintegrating an oxygen molecule can create showers of charged particles.
M Where does the negative charge come from? Isn't a photon neutral?
Well, first of all, we have two flavors of muons. We have negative muons or to the normal ones, and then anti muons, which are positively charged. In particle physics we don't really care so much about it, and both of them are created in the upper atmosphere. So we have anti muons and muons created in the upper atmosphere. But your question is a good one. If you start from a positively charged proton, how you end up making like a negatively charged muon. The answer is that there's just a lot more stuff involved in this interaction than we're describing, because a proton is a big complicated bag of quarks and it slams into something else in the atmosphere, which is a big complicated bag of other protons and neutrons. So there's plenty of charges around to create something which decays into a negatively charged particle and balance it out with all the rest of the stuff. So a proton can turn into a huge shower of negative and positive particles with a total charge of plus one. So you can have lots of muons and anti muons created in these showers.
All right, Well, whether you're pro or anti muon maybe is the question of the episodes. Can we use a muon to see inside of things and maybe put these giant particles to use. That's the question. Let's dig into that. But first let's take a quick break.
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Hi.
I'm David Eagleman from the podcast Inner Cosmos. Which recently hit the number one science podcast in America. I'm a neuroscientists at Stanford, and I've spent my career exploring the three pound universe in our heads. We're looking at a whole new series of episodes this season to understand why and how our lives look the way they do. Why does your memory drift so much? Why is it so hard to keep a secret, When should you not trust your intuition? Why do brains so easily fall for magic tricks? And why do they love conspiracy to I'm hitting these questions and hundreds more because the more we know about what's running under the hood, the better we can steer our lives. Join me weekly to explore the relationship between your brain and your life by digging into unexpected questions. Listen to Inner Cosmos with David Eagleman on the iHeartRadio app, Apple Podcasts or wherever you get your podcasts.
All right, We're talking about the electrons, cousin, more massive cousin, the muon, and whether it can be used to see inside of things like steaks and cows perhaps and.
Also maybe solving mysteries of archaeology.
Ooh, you mean like ancient buried cows.
Yes, maybe ancient buried cows.
Literally did early man eat steak or not?
Or were they van Can you ages take for thousands of years and still have it be tasty?
Paleoman actually follow the paleodiet We might use meons for that, all right, So we talked about what the meon is. It's the more massive cousin of the electron, and that it rarely lasts more than two point two microseconds in nature in the universe. So if it's so elusive and unstable, how did we discover this heavy particle.
Well, it turns out that muons are everywhere because cosmic grays are constantly slamming into the upper atmosphere, creating showers of particles, a lot of which turned into muons. So there are ten thousand muons per square meter per minute at the surface of the Earth.
By cosmic rays, you mean, like just other particles going really really fast somehow hitting the Earth exactly.
Sometimes people think that space is a vacuum. It's emptiness, there's nothing out there, but the Sun is pumping out protons and electrons and all sorts of stuff, and the galaxy has lots of sources of high energy particles, So we're really flying through a wind of particles, meaning that you can think of the is like tiny little meteors hitting the upper atmosphere, one proton at a time, or maybe an iron and nucleus at a time, in creating a little shower of energy. Just the same way that a meteor hitting the atmosphere will interact with the atmosphere and get friction and break up and slow down. A tiny particle like a proton, with enough energy will create a shower of particles which eventually reaches the surface of the Earth, and a lot of those are muons. There are also photons and electrons and other stuff in there, but muons are the most penetrating. They tend to pass through matter without interacting, so a lot of them make it to the surface of the Earth.
And sort of a good thing, right, Like we didn't have the atmosphere and we were getting hit directly by cosmic grays, we might not be around today, right. These costomic grays are very harmful, so it's a good thing they're being kind of broken up into muons.
Yeah, the atmosphere is like a big blanket that protects you from the radiation of outer space. When astronauts go up into space, they have to take special precautions to avoid being slammed into by all of this radiation. When there's like a solar storm, the astronauts have like a panic room they can go into with extra shielding to protect themselves from all that radiation. But the higher up you go in the atmosphere, the more radiation you're exposed to, because more these particles survive. So every time you take a flight, for example, you're exposing yourself to more radiation. This is one reason why like flight attendants and pilots are limited to how many days a month they can work.
All right, So then the atmosphere breaks up the cosmic grays and you said, turns them mostly into mulons or rarely into meons. How often are muons created by these cosmic rays.
It's sort of like a chain. The proton creates a bunch of particles which then decaynes is something, which then decayned is something. And the muon is like an end product and it tends to last the longest. So saw like the muon dominates the production of particles. You also make electrons, and you make neutrinos, and you make photons, the neutrinos, and the muons are the ones that make it through the rest of the atmosphere. They tend to interact a little bit less than electrons and photons, so you see them on the surface of the Earth more often.
Oh, I see, you're also making a lot of electrons and other particles. Put maybe like the electrons get stopped by all the remaining air in the atmosphere exactly.
Electrons like to interact with stuff. The electrons passing through air will interact with those molecules much more often than muons do. Muons are more penetrating.
And why is that? Are they just more antisocial?
It actually has a really fascinating explanation that has to do with special relativity, and this is the power that muons have to let us see through things. Muons are more penetrating because they have more mass, so they're two hundred times more massive than the electron. Otherwise, from a particle physics perspective, they're very similar. They feel the weak force, they feel electromagnetism, they don't feel the strong force. But if you shoot a beam of muons into like a block of lead, you'll get a lot more out on the other side than if you did with electrons. And the reason is their mass.
Is it like they have more inertia?
Is that kind of what you're getting at, Just like you know, if I shoot a small pebble into a pool or something, or if I throw shoot a bowling ball through it, like the bowling bull will get through the pool further or is it other kind of mechanism.
It's another mechanism. It's actually because they are interacting less because they see less of the material. It's a special relativity effect. If you have an electron and a muon at the same energy, the muon is actually going slower because it's more massive, like more of the energy is taken up creating the mass of the muon. So if you give them the same energy, the muon is moving slower as a lower velocity than the electron at the same energy because it has more mass.
So then you're sort of constraining things to be all the same energy.
Yeah, exactly, because it's the typical energy that these particles are produced at in these showers. So if you have an electron and a muon at the same energy, the muon is going slower and that affects how it interacts because it sees less of the material. To an electron moving at nearly the speed of light, everything in front of it is squeezed by special relativity. Remember we talked about how things moving near the speed of light look shorter. That's also true from their perspective. An electron whizzing through the atmosphere sees the distance to the surface of the Earth as close then we see it because it's moving fast relative to the surface sphere, so things are squeezed. As a result, it can interact with more the atmosphere. Or another way to think about it is like the atmosphere is denser because all that gas is like the Lorentz contracted in front of it into something a little more dense. So it can interact with more of the atmosphere because it's moving at a higher speed and it has more of this special relativity enhancement. Wait, that doesn't make a whole lot of sense to me. Like you're saying, like the rest of the atmosphere to an electron, because it's moving fast, the atmosphere looks thinner and more dense, and so it's harder to get through it. But it's still the same length to us. Isn't it like it's squeezed, but it's still the same It's going through the same amount of stuff as the slower nuon.
No.
Yeah, but it sees more of the material at once. It's like it has more atoms to interact with. So this is a quantum mechanical process and it has like a probability to interact with an atom. An electron flies by an atom, there's a chance it's going to interact in a chain. Is that it's not. The more atoms that flies by, the more likely it's going to interact and lose some of its energy. So if you squeeze more atoms into the same space, then it's got a higher chance of interacting. And what special relativity does is because the electron is moving faster, it Lorentz contracts the stuff in front of it basically squeezes in more atoms at once.
I see, you sort of have to change the way you're thinking about how these particles interact. Like you're saying, like you know, an electron when it hits a wall, it's not actually touching the wall, It just gets close enough to it that there's some sort of quantum mechanical transmission between the two that makes them technically interact, right.
Yeah, exactly, And that's why, for example, neutrinos can pass through a light year of lead. They're passing through the same material, and they're not like dodging around those particles. It's not a mechanical physical interaction of things touching. It's a quantum mechanical interaction of forces. The neutrino just doesn't interact with those particles at all, like phases right through that stuff, because it doesn't feel electromagnetism. It only has a smaller chance to interact with every single particle. So that's why neutrinos pass through almost everything, and that's why there's a difference between the penetrating power of muons and electrons. Muons, being more massive at the same energy, are effectively moving slower, so they have less of this special relativity boost where they can interact with otherwise further away atoms. Then now look closer to them, and so they can feel their fields.
So as the electron is a showering down coming down the atmosphere, you're saying, it sees the bottom of the atmosphere as closer, which might make it more likely do it right, But I guess the weird thing is that, you know, if it does interact with the bottom of the atmosphere, wouldn't it mean it made it through the atmosphere? And so it's really isn't it sort of the same thing? Probability.
It's a cool way to look at it. But it can interact with the bottom of the atmosphere while still not being that far through the atmosphere because to it, the bottom of the atmosphere is not that far away, so it can still feel those fields.
Right right, it feels it closer.
But if it interacts with the bottom of the atmosphere, isn't it the same as making it through the atmosphere? Like it skipped everything above and it made it to the bottom of the atmosphere, that means it made it through the atmosphere.
It doesn't have to make it to the bottom of the atmosphere in order to interact with things at the bottom of the atmosphere. Remember, all of these things are action at a distance. You're feeling the fields of things. Two electrons don't have to touch each other in order to interact. They just have to feel their.
Field or I guess maybe, But I mean It's like, what's the difference between an electron that makes it through the atmosphere and interacts with the bottom of the atmosphere and an electron that sees the bottom of the atmosphere is closer and interacts with it. Aren't they both the same result? And that don't both mean that they made it through the atmosphere.
So higher speed electron is more likely to interact because it sees more of the atmosphere, and it's going to interact at a higher altitude than a lower velocity electron, which doesn't see as much of the atmosphere because a special relativity boost. And so even if you're interacting with things that are lower down, your actual location is still high.
Oh, I see you were talking about it might decay before it reaches the bottom of the atmosphere. It's kind of not necessarily interacting with the bottom of the atmosphere. Like, if it touches the bottom of the atmosphere, it means it made it through the atmosphere, doesn't it.
Well, electrons don't decay, right, All they can do is interact. But again, you can interact with something at the bottom of the atmosphere without being there, right. The same way, like the Earth is interacting with the Sun without touching the Sun, because if you can feel its gravity at a distance.
All right, well, let's assume that then that that's the case. And so you're saying neuons can make it through more of the atmosphere or anything in particular, just because they're moving, they tend to be moving slower, although if you had a fast moving meon that wouldn't be the case.
Exactly, And we actually see those at the Large Hadron Collider. We can make muons with enough energy that they're moving at very relativistic speeds and they interact with matter like electrons do, so we can see like muon created showers when we happen to make a really really high velocity muon. It's just a feature of muons and electrons at the energies that they tend to be produced at in our co mcrays here on Earth because of the ratio of their masses.
I guess, couldn't you just use a slower moving electron. Wouldn't that be the equivalent of a slow moving muon? Then then the electron could penetrate things more.
Yeah, it's a good question. You can slow down electrons, but then there are other effects that are going to come into play that are going to make it interact more so, there isn't a window there for electrons to do the same trick that muons can do.
I think what you're really saying is like you're trying to use muons, not as a general concept, but meons that are particularly created in the cosmic rays when they interact, when they slam into the atmosphere. You're trying to put forward the idea of using these mions that are showering us as maybe like an X ray machine.
Yeah, exactly. Muons have this window of energy in which they can penetrate really really deeply. If they move more slowly, then they run into the same atomic physics that electrons have. They can get captured. They move faster, then they get the relativistic effects and they interact just like electrons. But muons have this special window, this energy range in which they can pass through a lot of matter, much more than X rays can. X rays can pass through some kinds of matter, which is why you can use them to see your bones and inside your body, but muons can pass through a lot more matter than X rays can. X rays, for example, cannot pass through huge blocks of granite, But do you.
Sort of skipped through something?
Which is you said, electrons, even if you slow them down, are not as good as muons for X rays applications, And why is that?
Well, they'll get captured by atoms. Like electrons moving slowly, we'll just get.
Captured, but not a muon.
A muon moving really slowly also will get captured. Yeah, so a muon has a window, it's got a minimum energies do this and a maximum energy in order to do this penetrating trick.
So then you were saying, how were these muons discovered?
So these muons were discovered in cosmic rays. People were studying electrons and somebody had even discovered the anti electron, and they're studying these particles by watching them move in magnetic fields and seeing how they curve. And they saw some thing which looked kind of like an electron and had a charge like an electron. They incurred in a magnetic field the same direction as an electron, but it didn't curve as much, and it penetrated much more deeply. Like you could put slabs of lead in front of your detector and you would still see it. So nineteen thirty six, Physicistic Caltech first discovered these.
Things and they bend less in a magnetic field because of their mass, right, basically their innership. Or is it also some weird quantum interaction. No, no, it's a very classical thing. It's just because of their mass. Yeah, mmmmm, I see. All right, Well, let's get into how you might use muons to penetrate things, see inside of things, maybe discover ancient artifacts inside of pyramids.
So let's dig into that. But first let's take another quick break. Hi.
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Or We're talking about neons and how you can use them to see inside of things. And we talked about how neons sort of have an extra penetrating effects more than its cousin, the electron, because it's heavier, and so the ideas than to use this like an X ray, basically shoot it at something, and if it gets through, then that tells you what's inside of the thing.
Yeah, you can sort of use it as a way to measure the density of something. If you have an object and you don't know if inside of it is nothing like a vacuum or a huge block of super dense uranium, you can try to shoot it with a bunch of muons and count how many come out. By figuring out how many make it through, you can tell what the density of something is. This only works if you have something which has a chance to make it through. Right. If you just shoot photons at a block, then none of them are going to make it through. They're all going to get absorbed. Doesn't tell you anything about what's inside. But if you have a particle which has a chance to make it through for some densities, then you can measure the rate at which does make it through and figure out what the density of that stuff.
Was, Right, Right, I guess it's sort of like X rays.
Like X rays, if I just shine of flashlight onto my body, it's going to bounce off the skin or at least most of the photons because the light is at a certain wavelength. But if I change the wavelength to that of an X ray, it'll go through my body sort of.
Yeah, exactly. Some of the X rays will make it through, and if you have an X ray detector on the other side, you can pick that up and by looking at the pattern of where the X rays made it through and didn't make it through, you can tell what the density of stuff is. And that's how you can tell the difference between like bone or metal and soft tissues, which have different densities and therefore different absorption for the X rays. So it's exactly the same principle for muons, except that muons will make it through things that X rays will not survive, which allows you to effectively X ray or muon ray other kinds of things that you couldn't otherwise see inside.
So in the case of an X ray, an X ray can go through my body because it's a different wavelength, which what makes it go through my body more than say the life from a flashlight.
So X rays have more energy their higher frequency, right, and the interaction with a photon with the materials in your body depends on the energy. But a whole episode about transparency why photons can go through some things and can't go through other things, and it's all about whether they will interact. Photons can interact with matter depend on their energy. They can get absorbed if there are atoms out there that can.
Eat them, because atoms only like to eat photons that are a particular frequency, right, Yeah, exactly, like they don't just like any photon, They have to be a special frequency because of quantum mechanics.
Yeah, they have various energy levels. They have these ladders of energies, so they can absorb photons of like just the right energy, and that affects what photons can pass through your body or through glass, or through metal or any kind of stuff.
So that's why X rays can go through things more than regular light. And we talked about how muons can do that too. Why is that because they don't they have a specific energy range that makes them go through but not interact with the atoms, say inside my body.
Yeah, exactly. At certain energy range, they won't be captured by atoms, and they're not quite going fast enough to have a special relativistic boost where they interact with lots more atoms than otherwise, and so they can make it through a lot of this material. And so you can see muons. If you're like deep underground, you put a muon detector like meters and meters underground, those muons will pass right through that solid rock and hit your muon detector.
Now, is the idea that you're shooting these muons like you're creating them and shooting them with like an X ray gun or a mewray gun and then catching them on the other side, Or is the idea that you're using the ones that are showering down on us from the atmosphere.
In principle, you could do both, right, If you have a muon beam, then you could put stuff in the muon beam in order to do these kind of tests. There is a muon beam. It'scern and we've put cell phones in it and stuff like that. It's a lot of fun. But it's hard to build a muon beam. It's hard to point a muon beam. It's hard to bring stuff to a muon beam.
Why why is that?
Why is it hard to bring stuff to the muon beam?
Now?
Like, why is it hard to make a muon shoot a gun?
Yeah? Great question. Muons are created from the decays of other particles. So the way you make a muon beam is actually you smash protons into like a block of material like graphite, which creates a shower of other stuff. It's basically stimulating what's happened in the upper atmosphere. Then a lot of those things decay into muons. So you need a proton accelerator of sufficient energy, and there just aren't that many of those. They're not that portable. You need like a linear accelerator. You need magnets to filter some of this stuff out.
How big would it have to be, Like can you make it a handheld version, or do you need like a building size anything to shoot muons?
Yeah, that's a great question. What's the smallest muon gun in the world. Definitely the size of a large physics laboratory, not something you could pick up and carry, though you might be able to put it in the back of a flatbed truck. But mostly it's unnecessary because the world is filled with muons from cosmic rays. Like there's a constant stream of these things just naturally produced in the atmosphere and you can just use those.
M What do you mean, like there's we're surrounded or being penetrated by muons from all directions all the time.
Not from all directions from above, right, these things are made in the upper atmosphere and are streaming down to us. Again, there's ten thousand muons per square meter per minute, so there's not a small number of muons passing through us. And so if you want to measure the density of something, you just put like a muon detector underneath it and count how many muons are making it, and then you can tell how many were absorbed by the material, and that tells you what the density.
Of it was.
Mmm.
But are muons coming at us from the sides as well? Like, aren't their cosmic rays hitting us from all directions?
There's definitely an angler dependence, but most of them come straight down. That's the most likely.
Direction, all right.
So then the idea is that if I want to see through something, I just put a meon detector under it. And so what are these meon detectors made out of? How do you make a muon detector If muons go through things so easily.
The original sort of old school ones are these films. Now, muons are hard to stop, but they're not that hard to see, Like, they will leave a little trail of evidence as they go. For example, you can build a cloud chamber in your garage, which is just like a transparent box filled with water vapor super saturated in the air, and as muons fly through it, they won't be stopped, they won't lose a lot of energy, but they will interact with those things and create little a stream of droplets. So you can actually build a muon detector like at home with simple materials. There's all sorts of fun instructions on YouTube that you can follow, so they'll leave like breadcrumbs for where they were. The original ones were like film and moulsion blocks. These days we use like charged gases or scintillating plastics in order to see these muons.
I see, you don't stop the muons, You just kind of see the evidence of them going through.
Yeah, exactly. It's hard to stop the muon for them to interact in a significant enough way to get slowed down to deposit all of their energy, but they will leave a little trace of energy as they go by if you have the right setup, So it's not that hard to detect muons.
Interesting, all right, So then what kinds of things have we seen with a meon ray? Have we seen a instead of a cow are McDonald's hamburger.
I don't know that anybody's tried that, you know, put a cloud chamber under a cow to see what it's eaten. I do not know if that experiment has been done, so I don't know if we have muon rayed cow.
Hey, there's an instruction on YouTube to do that.
One of the first applications of this was to measure like how much rock and the density of rock over a tunnel. Like you're building a tunnel through a mountain. You can put a muon detector in the tunnel and you can use it to measure the total mass of the rock or effectively the density of the rock that's above you, to measure your overburden because you're basically shooting through the rock with the muons and you can tell by counting how many muons make it to your tunnel the density of the rock.
So for like construction projects.
That was the first application. But then in the sixties a physicist thought, ooh, let's use this to basically X ray the pyramids, because you know, a lot of people wonder like if there's something in the pyramids, or are there hidden chambers in the pyramids. Nobody wants to take the pyramids apart because they're obviously treasures of humanity. But we would like to see inside the pyramids in a non invasive way. So in the sixties, Louis Alvarez thought, oh, let's use muons to see inside the pyramids, to see if there's like an opening or a gap, or like a big void somewhere that nobody's discovered.
Ooh, wouldn't that require you to put the meon detector under the pyramid?
Yes, exactly. So you do need some access to the pyramids, and there are some openings, but this is limiting factor. You can't just like drill under the pyramid and put up muon detectors everywhere. There are some shafts and some chambers we know about. What you can do is put the muon detector there in the bottom of the as far below the pyramid as you can, and then measure the rate of the muons and compare it to calculations you do, like how many muons should I see if there are no additional chambers, or how many muons going in this direction versus that direction, if there's a chamber here or a chamber there.
Wait, if I put a detector under a pyramid. Let's say it's like a tile the size of like a one by one foot square. It can only detect the muons that are coming from right above that one square foot area, or can it detect muons from all directions?
If you have like a one foot tile, it'll detect any muon that passes through that tile, you know, coming from any direction. And so if you have a few of those, then you can start to get directional information. If there's like a difference in how many muons you see in one place versus another, what do you mean, Well, the way you can tell, like the difference between parts of your body is that you have an X ray detector. That's not just a point, it's like a whole array, or it takes an image. You can tell how many X rays came through this part of your body versus that other part of your body. So imagine if you could put X rays all over the bottom of the pyramid, then you could like muon X ray the whole pyramid. You can't do that, but you can put a few here and a few there based on what access points you do have, and you can get like a very rough image of what's going on inside the pyramid from your various detectors.
But wouldn't I just give you like a couple of pixels basically of an image.
Yeah, it's very rough, but it's better than nothing. Right right now, we have basically no image, and so this is like a way to crack it open a little bit and give you some very rough idea of what might be there.
M I guess alternately you could create a Mion ray gun and shoot it from the side, right, wouldn't that be more convenient?
Yeah? Absolutely, you had a big Meon detector on one side and a big Meuon gun on the other side, then you could really muanograph the pyramids. That would be awesome.
Were you about to take Ameo on the heck out of it? Are these muons dangerous? Like if I create a muon gun and I aim it at somebody, is it going to harm them? Just like X rays are sort of harmful if you take an X ray gun and shoot it as a person for too long.
Absolutely, these are radiation and muons are responsible for mutations in our DNA. They're part of the natural radiation of our environment and they do cause mutations. So yes, in principle they can cause cancer, right, So an intense dose of muons from a beam could definitely give you cancer. It's not something to play around.
With and does sound like a great idea to make ameon gun or a good idea is for certain applications perhaps.
Yeah, exactly, and the difficult to shield. Right, once you start that muon beam, it's going to pass right through your pyramid and then through your detector and then it's just going to keep going for kilometers and kilometers. So it's not like you can have a beam dump or something to protect people from the other side.
Well, I guess they would just shoot off into space, right, because the Earth is curved or with gravity pull them back down.
No, you're right there, showed off into space. So maybe you just don't need to angle it up a little bit.
Oh interesting, But then you might be like shooting it maybe an alien silization out there.
They might take offense.
Yeah, and you could accidentally be sending them a mewantograph of our pyramids. I don't know how they would.
Interpret that, that's right, or a picture of cows. They'd be like, oh, that looks tasty, let's go invade them.
But people have actually done this for the pyramids without building you on gun. They've just used cosmic rays and measured the rate at which the constant rays make it through the pyramids to see are there new cavities inside.
The pyramids and what have they found.
So the first time they looked, they looked in one pyramid, they didn't find anything unusual. But then later on, actually in twenty fifteen, they did this for the Great Pyramid, and they found what they called the Big Void, and then another opening they labeled maybe a corridor. What they're seeing is a region of the pyramid that seems to have lower density than the rest of the pyramid. So this could be like a big opening, maybe a treasure chamber filled with all sorts of jewels and fascinating information about ancient Egypt. Or maybe it's just like a gap they left in the pyramid to reduce the pressure on the rest of it. You know, it could just be like a construction trick. We don't exactly know, but there's some sort of large cavity within the Great Pyramid.
Interesting.
I guess what you're saying is making me feel a little skeptical, just because you needed like a lot of space underneath the pyramid to create these to be certain that there's something there, right, you need to basically put a lot of these neon detectors under a pyramid. Like just putting like a couple doesn't seem like you'd be able to find or resolve any kind of real details, can you.
Yeah, your resolving power definitely improves as you have more detectors.
Or just more space to put these detectors.
But you'd be surprised what you can accomplish with a few detectors, the same way that like a radio array is just a few detectors scattered over kilometers and kilometers. By measuring the difference between signals received by one antenna and another, you can get a lot of directional information and resolving power almost as if you had the detector of the same size as a full array. Not quite, but almost as if. So you do some complex data analysis and you can recover a lot of information with just a few measurements.
Right, right, But there's a raise. There're antennas right which you can focus and point in the certain directions to kind of get the equivalent of a giant lens. This feels like, you know, laying out a bunch of photographic negatives, a film out on the ground and trying to get an image from that.
Yeah, it's difficult. And if you look at the reconstruction of the void, you see it's very fuzzy. They're very uncertain but exactly where it is, how big it is. They have no idea what shape it is. This is not like a crystal clear image the way an X ray is at all. This is just like a hint that there's an under density somewhere inside this pyramid.
All right, well, it seems like a pretty cool application that maybe let's us see through mountains, internments and potential of bovine animals.
Especially if they're the size of pyramids or mountains.
What else can you use these mion rays for to detect.
People have used it actually to see inside mountains like Vesuvius, for example, the famous volcano. They've used muons to try to understand what's going on inside Vesuvius to maybe do a better job predicting of when it's going to blow.
But don't you need to get under Vesuvius to do this?
The best case scenario is to have a bunch of mean detectors under Vesuvius. But if you put a bunch around it. Then you can get muons which shoot through sort of at an angle. Especially if you can measure the angle of the muons, so you can tell whether they came through the mountain or whether they came from the other side, then you can get some good information.
Wait, you can angle these detectors.
Yeah.
Absolutely. The detectors are not just like a flat sheet. They can be fick and so you can see a whole track of a muon. You can tell which direction it was going, not just that a muon was there, but the direction of its motion.
Hmm.
Interesting. So you can angle these then kind of like an antenna.
Yeah, kind of like an antenna exactly.
Okay, it seemed like maybe you're saying you can't.
Can the thicker they are, the better angle measurement you can make like a cloud chamber that you can build in your garage. You can see the whole track of the muon flying through. It's really pretty.
Cool, all right, So geology and archeology those are pretty cool uses for particle physics.
Yeah, exactly. So maybe particles will not just teach us about the nature of the universe. They might teach us about what's going on inside mountains and what humans have hidden away inside pyramids.
All right, well, another great justification for Daniel's job at the university.
I'm not mu anything, but I'm definitely a favorite of it.
I feel like half of these episodes are just a commercial for your job in particle physics.
They're a commercial for particle physics and for physics in general and trying to understand the nature of the universe, and yeah, why it matters?
Should we have a disclaimer here at the bottom?
Every episode is indirectly Daniel's self promotion. Yes, there you go. Yeah, absolutely, I'm totally transparent about that.
All right, Well, engineers, please clip that and put it at the bottom of every episode.
It'll be like the fine print and.
Every conversation I have basically with.
Everybody unless you're talking about something else.
Perhaps it's all particles, man, everything is made of particles.
Oh interesting, even non particles. All right, Well, we hope you enjoyed that. Thanks for joining us. See you next time.
For more science and curiosity. Come find us on social media, where we answer questions and post videos. We're on Twitter, Discord, Instant, and now TikTok. Thanks for listening, and remember that Daniel and Jorge explain the Universe is a production of iHeartRadio. For more podcasts from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever you listen to your favorite shows. When you pop a piece of cheese into your mouth, you're probably not thinking about the environmental impact. But the people in the dairy industry are. That's why they're working hard every day to find new ways to reduce waste, conserve natural resources, and drive down greenhouse gas emissions. How is us dairy tackling greenhouse gases? Many farms use anaerobic digestors to turn the methane from manure into renewable energy that can power farms, towns, and electric cars. Visit you as dairy dot COM's Last sustainability to learn more.
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Hi.
I'm David Eagleman from the pod cast Inner Cosmos, which recently hit the number one science podcast in America. I mean neuroscientists at Stanford, and I've spent my career exploring the three pound universe in our heads.
Join me weekly to explore the relationship.
Between your brain and your life. Because the more we know about what's running under the hood, better we can steer our lives. Listen to Inner Cosmos with David Eagleman on the iHeartRadio app, Apple Podcasts, or wherever you get your podcasts.
