Why is matter and antimatter symmetric? What happened in the early universe to make matter win? Do we have any clue as to why? What do puppies have to do with it? I discuss these questions and more in today’s Ask a Spaceman!
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Music by Jason Grady and Nick Bain. Thanks to WCBE Radio for hosting the recording session, Greg Mobius for producing, and Cathy Rinella for editing.
Hosted by Paul M. Sutter, astrophysicist at The Ohio State University and the one and only Agent to the Stars (http://www.pmsutter.com).
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EPISODE TRANSCRIPTION (AUTO-GENERATED)
I want you to imagine that you're standing in front of a staircase. Nothing fancy about the staircase. It's just a staircase. But sitting on the steps here and there are a bunch of puppies. Why puppies?
Because once you get the visual of the staircase full of puppies in your head, it's impossible to forget about it. The staircase is tall. It stretches all the way up into the sky, past the clouds, possibly into space so far that you can't see the top. And everywhere you look, puppies. Sometimes a step will have a few puppies on it, some just one, some are totally empty.
And and you're at the base of the staircase, the ground level. And puppies can move around on their step doing the usual puppy thing that is totally adorable, but they can only jump to higher steps if they're given a treat, if they've given a little bit of energy. If you toss a puppy a little treat, it can just jump up a single step. If you give it a bunch of treats, it can jump up a bunch of steps. But the puppies don't like being very high up, so they don't spend a long time at the higher steps.
They all want to be at the ground level so that they can play with you, but there's only so much room on a step. Once puppies fill up the bottom step and there's no more room and no more puppies will fill on it fit on it, so they start filling up the second step. And then once that's filled, they fill up the third and fourth and so on until all of the puppies on the staircase are as close to the ground level, you know, they're as close to you as possible, as much as the steps will let them fit. And you can give a treat to any of the puppies, and that puppy will jump to a higher step leaving behind a gap in its step, and the other puppies will jostle around until that higher one drops back down to fill the gap. Now I want you to imagine that same staircase full of adorable puppies extending down below ground level, down into the basement.
It goes down as far as you can see. You have an unreasonably deep basement here. This staircase goes up as much as it goes down. As far as you can tell, there are an infinite number of steps going up and an infinite number of steps going down. Now, the puppies want to be as low as possible, So why don't they start going down into the steps below ground level into the basement?
They can't because the basement steps are already full of puppies. Seriously, you look down and it's puppy infinity. Each step is packed full of puppies. There aren't any gaps left in any of the steps. So our above ground puppies can't get any lower.
But what would happen if you were to give a treat to one of the below ground puppies? Well, it would jump up to one of the above ground steps. Right? And a gap would be left behind in the steps below ground and a small gap in an otherwise infinite sea of puppies. An anti puppy, if you will.
But if you waited long enough, eventually a puppy from above ground level would see the empty gap and want to jump down to it because the puppies want to be as low as possible. And as soon as the puppy jumps, the gap disappears and everything is back to normal. What's going on? Well first, no puppies were harmed in the making of this metaphor, and second, the puppies are electrons. And third, the steps are quantum energy states, and fourth, this is a show about antimatter.
So how does it all work? Like, quantum energy states are like steps. You start at the ground level or the ground state and you put as many electrons in it as you can. But because of the rules of quantum mechanics, you can only fit so many electrons in that ground state, in that bottom step. So you start adding electrons to the next level, in the next, in the next until you're you're done placing electrons.
And the electrons can jump from state to state if they're given energy or they release energy just like the puppies can jump up and down the steps. And so far this is pretty normal, not weird at all. Well, not weird when it comes to quantum mechanics, which is generally overall weird, but, you know, that's alright. So that helps us understand what's going on with the above ground staircase. What about the underground puppies?
What about the underground steps? The visual, not the actual puppies, but the source of the visual comes courtesy of the one and only only Paul Adrienne Maurice Dirac, one of the most awesome physicists ever. He was the one who was able to unite special relativity and quantum mechanics together into a single picture. He also had a fantastic name that I love saying every time he comes up, Paul Adrienne Maurice Dirac. That single picture where he united special relativity and quantum mechanics was able to explain this weird quantum thing called spin, and if you haven't listened to the episode on spin, you should wait for it give it a spin.
But in the process of solving that little conundrum of merging relativity, special relativity with quantum mechanics, understanding spin, in that process, Dirac discovered something remarkable. He discovered antimatter. And this is interesting. We didn't discover antimatter in a lab or an experiment. We discovered antimatter by thought alone.
We discovered it in the mathematics of our universe. And Paul Adrian Maurice Dirac was the one who able to do it. In this view, in his view of antimatter is that we have all these energy states, all the possible energy states, all the possible steps where electrons can exist, but we're sitting on top of a sea of electrons sitting in negative energy states. That just like energy states can go from here to infinity, energy states can go from here to minus infinity. And all these states are already filled with electrons.
And if you pluck one of them out, if you if you find an electron down there in the sea of negative energy and give it some energy, it pops up to our positive energy world, and it leaves behind a hole. So now you have an electron floating around doing electron things, and you have a hole in the sea of negative energy electrons, which this hole floats around as all the electrons down there, you know, jostle about this hole moves around it wiggles, and it does all the same things that an electron would do, has all the same properties, but it has opposite charge. In this picture, we can understand what's happening when we say that, say, a beam of light, a high energy beam of light, is capable of producing a pair, an electron, and its antimatter twin called the positron. This gives us a visual for understanding it. The high energy light just strikes one of these electrons in the negative energy c, promotes it, gives it a boost of energy.
The light is gone, and now you're left with a pair, an electron floating around and a hole floating around, the electron and the positron. And eventually they meet. The electron jumps back down, fills the hole, and they disappear. And that process by transitioning to a lower energy releases energy in the form of light. I have to say that in modern physics, we don't really view antimatter like this.
Dirac's picture isn't 100% accurate. We have other ways of understanding the relationship between matter and antimatter. But I still like sharing this visual metaphor because it's so striking and I don't often get to talk about puppies in this show, and this gave me an excuse. And Dirac's picture tells us something important. It looks like matter and antimatter, you know, the particles and the holes they left behind are symmetric.
They're paired up. For every bit of matter, there should be a corresponding bit of antimatter. For every particle promoted into the above ground world, there's a corresponding hole in the below ground world. And even though Dirac's picture isn't 100% accurate, as far as the math can tell us, matter and antimatter are twins. Antimatter is simply normal matter with opposite charge.
So you take an electron with its mass, its spin, and charge. You keep the mass the same. You keep the spin the same. You keep the size the same. You flip that charge.
Boom. You got a positron. Same for a top quark. Same for a neutrino. Same for every particle, there is a twin, an antimatter twin.
And the processes they're able to generate matter and antimatter always make them in pairs. They're always symmetric. And then when they touch, when matter and antimatter touch, they explode. They release energy and they disappear. And in the picture of that sea, you can see why.
Because it takes energy to promote one of the electrons from the below ground sea. Now you have the electron and its hole, a k a the positron, and then when they finally meet, they go away as distinct entities, and energy is released. So if matter and antimatter are always twins, if matter and antimatter always come in pairs, where's all the antimatter? We look around the universe, and it's filled with stuff. Where's the anti stuff?
We look around the universe and we see the normal everyday same things that we're made of, electrons and top quarks and new all that stuff. Where's the anties? Where's the antie? Our universe today is filled with a lot of puppies, but where are the antie puppies? Well, maybe antimatter is just, you know, out there, but not around here.
But that's that's hard to like, maybe there are entire, antimatter galaxies or or, you know, little isolated regions full of antimatter. But that's hard to reconcile with observations because when matter and antimatter meet, they release a lot of energy, and stuff is constantly mixing around in the universe like cosmic rays from distant supernova, gas falling onto galaxies, gas getting blown out of galaxies, etcetera, etcetera. If our universe were half antimatter, we probably would've we we wouldn't have noticed by now. So that's not gonna work. Well, maybe our universe was just born this way?
It just came out of the womb with more matter than antimatter and that's just the way it is? I mean, that's technically an answer, but in science circles, that doesn't sit very well. We we like to not just have answers, but we like to have explanations. We we want something that fits into coherent knowledge and is has predictive power and just saying, well, yep. Nope.
Nope. That's just the way it is. There's more matter than any matter and that's all I'm gonna say about it. That doesn't that doesn't really satisfy us scientifically. So we want an explanation.
Maybe. The explanation is in Patreon. Go to patreon.com/pmsutter to learn how you can keep this show going. And I swear I swear if I reach my next goal, puppy metaphors in every single episode. Don't please don't hold me to that, but and it's just a joke I swear.
But maybe at least one more puppy metaphor in a future episode. Patreon.com/pm sorry. It's how you keep this show going. Maybe something caused the universe to have more matter than antimatter. Maybe there used to be a balance between matter and antimatter, but something upset the equilibrium.
Something broke the symmetry. Some obscure process made more matter than antimatter. And this process had to have happened in the very, very early universe because, as far as we can tell, very early on, matter's more matter than antimatter. And we know that the early days of the universe were wild and wonderful with all sorts of exotic physics at work before it settled down and became, you know, our universe, so maybe something funky went on in the early Big Bang. Because lots of funky things went on in the early Big Bang.
Like, you just run the clock backwards, and when our universe was very very small and very very hot and very very dense, physics gets weird. Maybe it gets weirder. Maybe it did start off perfectly symmetric between matter and antimatter. You know, collisions all over the place and interactions, but but everything's balanced, everything's balanced. Something funky happens and matter ends up winning.
More matter gets generated than antimatter in the early universe that leads to our present day cosmos. Well, what's it gonna be, folks? Well, that's fine. That's fine. Like, this is a plausible route.
This is a way we can hook our ideas and math into that lead to some sort of understanding and predictive power, blah blah blah. Okay. Let's say you come up with a way to solve this. You have an idea, a process, an interaction that can lead to an imbalance between matter and antimatter. But if you want to generate more matter than antimatter in the universe, you have to overcome three challenges, three tests, three riddles.
You must solve all three, otherwise you will end up with equal matter and antimatter. And before we get to the challenges, I need to warn you. Like everything else in particle physics, it's all jargon all the time. Sorry I couldn't find puppy based metaphors for every one of these challenges. Actually, none of the challenges.
There will be a lot of unfamiliar terms and concepts. I'll do my best to guide you through it. If something is confusing, just ask and I'll redo it. Oh, wait. That's right.
This isn't a conversation. It's a podcast. And this train won't stop, so hold on tight. Challenge the first. Your exotic process that leads to more matter than antimatter in the universe must generate more matter than antimatter in the universe.
This seems, a, totally obvious and, b, the only challenge, but here's why it's a big deal. As far as we can tell, and we've tested this a lot, is that if you have some sort of process, some sort of interaction with fundamental particles, then the total amount of matter going into the interaction must equal the total amount of matter coming out. And the exact same for antimatter. The matter can change forms and transform into different kinds of particles and the antimatter can change forms and transform into different kinds of antiparticles, but it has to stay the same. The total amount.
If if you put 10 matter particles and seven antimatter particles into a box and shake it up and or collide them at near the speed of light, then when you open the box, you'll still have 10 matter particles and seven antimatter particles. They might be different arrangements within their groups, but they won't mix together. It'll all add up. In the lingo, we call this baryon number conservation, and this appears to be true in basically every single interaction we've ever seen. So if you want more matter than antimatter, your process has to violate this, which isn't exactly easy.
Challenge the second. The reason the previous challenge is just a starting point and not the only thing you need to do when you come up with a crazy process that generates more matter than antimatter is that there's another rule of the universe. Just like there's baryon number conservation that applies to all interactions, the total amount of charge going into a reaction must equal the total amount of charge coming out. Just like before, you put your particles in a box. You count up all the charges, and you remember antimatter has an opposite charge, so you keep track.
You count, okay. One, two, three, oh, minus one, minus one. Okay. We're back at one, two, three, minus one. You you add it all up.
You shake your box. You open it. You count again. Your particles may have changed character. They may have transformed.
There's all sorts of crazy stuff that can happen in particle physics, but the total amount of charge stays the same. So if you have a process that generates, so you come up with some exotic interaction that generates more matter than antimatter, this restriction of charge in the lingo we call charge symmetry or c symmetry means that nature will conspire to cook up a mirror of your process which will generate more antimatter than matter, maintaining the charge balance and totally spoiling your efforts. So if you want more matter than antimatter, you now have to break two rules of the universe. You need to make extra matter and to make it stick, you have to snuff out the mirror of that process that will naturally develop. There's another symmetry called Peritree symmetry that also needs to be broken, but I'm just going to lump that in with this challenge so we don't get too far down the rabbit hole and I can keep it at three challenges, which sounds way nicer.
But feel free to ask about pear tree symmetry someday if you like. Challenge the third. The final challenge. The hardest one. The challenge of equilibrium.
If you have a box of particles doing their thing with no other influence on them, they will do their thing. Interactions will be balanced. The movement here will be balanced by movement there. A raising of energy in one corner will be balanced by a lowering of energy in another. And a process that completes the first two challenges will, if the system is in equilibrium, be defeated.
Why? Because if you have a process that starts playing with the amount of antimatter, then a system in equilibrium will, by definition, simply float around the different amounts of matter and antimatter. Sometimes you'll get an interaction that makes an excess of matter and other times there'll be a deficit. And at the end of the day, you'll get nothing. All matter will have found an antimatter partner and annihilate leaving the universe totally empty and we don't live in an empty universe so the system must be outside of equilibrium to make it stick.
It can't be perfectly balanced. And do we have any contenders that satisfy all three challenges? Well, yes. We know that when the universe was around zero point zero three seconds old, some pretty funky stuff happened. At this time, the weak nuclear force, which still deserves its own podcast episode, became a force.
It was born here at this moment and its birth was sudden, violent, and messy, like most births tend to be. And we know that the weak nuclear force in extreme rare conditions is able to produce more matter than antimatter and it is able to violate charge symmetry. And when the force emerged when our universe was just three hundredths of a second old, it didn't do so uniformly and evenly throughout the universe. It emerged in patches that spread. And for a brief, brief, brief window in time, the universe was not in equilibrium.
Is this it? Is it is this the birth of matter? When the weak nuclear force arrived on the scene, is this what caused the imbalance? Well, we can do the math, follow the energies and temperatures and interactions and predict how much matter this process would have generated. And it's less than a billionth of what we observe in the universe.
This interaction with the weak nuclear force that we know of that satisfies all the conditions produces about one billionth the amount of matter that we actually see in the universe. Well, okay. It was a nice try, and I know I didn't get into details about the weak nuclear force and how it emerged, so feel free to ask. So that's not gonna work. I mean, do we have any answers?
Well, maybe it's dark matter. Right? Dark matter does weird stuff. Maybe in dark matter was around in the early universe, maybe it caused some exotic interaction. Maybe it's something even weirder.
Maybe it happened in an earlier epochs before our universe was three hundredths of a second old, but we don't have really good physics there, do we? Here's the thing. We don't have an answer. This is an outstanding problem, something that we're very interested in because the answer is outside of known physics, which means if you can find an answer, then you can understand something new about the universe. But when it comes to the difference, the asymmetry between matter and antimatter in our universe, that's all we've got for now.
In the meantime, we'll, we'll just have to go play with some puppies. Thank you to one eight five transformer on YouTube, William g on email, Matthew a on email, Bogdan v on email, Dave b on email, Eric d on email, your lord and savior on YouTube, Ken b on email, and Kahandran on YouTube for asking the questions that led to today's episode. You can ask your own questions. Go to askaspaceman@gmail.com, ask a spaceman dot com. Hit me up on social.
Use the hashtag ask a spaceman. I will find those questions. And especially so thanks to all of you for the amazing questions and for listening. Thanks to the top Patreon contributors this month, Robert r, Dan m, Matthew k, Evan t, Justin g, Kevin o, Christy, Helgeby, Barbara k, Matt w, Kirkby, and Duncan m for keeping this show and all of my education outreach efforts going. I'll just end the show now because, man, we got some puppies to play with.
I will see you next time for more complete knowledge of time and space.