Now here's where the hotel gets especially strange. The rooms don't stop at the ground floor—there's a basement, and that basement is full of rooms. And beneath that is another subsurface floor, equally stuffed full of rooms. And down and down and down—there is technically an infinite number of rooms beneath the surface, but that's not especially important for now either.
When you arrive at Hotel Dirac, the hotel may look empty, but really all the subsurface floors are already occupied with guests, one per room. So what you see as an unoccupied ground floor actually sits on top of countless millions of underground guests.
When I say that any guest in the hotel occasionally moves up to a random higher level, I really mean it—that includes the underground rooms. Someone down there can get motivated, find the nearest elevator, and briefly get to enjoy the views out the window—until the surly hotel staff finds them and bounces them back down.
When such a guest gets bumped up to a higher room, they may find that all the room doors are unlocked—they can wander from room to room on their floor, flicking on the lights as they go, checking out the layout and seeing all the possible views. They can't change floors without permission, but the rooms on each floor are fair game.
What about the empty room they left behind, on one of the underground floors? Down there all the rooms are unlocked too, and curious looky-loos will float in and out of that unoccupied room, one at a time.
What does Hotel Dirac look like from a distance when an underground guest gets bumped to a higher floor? That guest, floating from room to room, turns on the lights when they enter and off when they leave. From far enough away, you'd see a single light on an upper floor, shifting from room to room as the guest explores.
If you could see through the ground, what would the subterranean floors look like? As the guest left, they dutifully turned off the light—that room looks like a hole in an otherwise unbroken sea of lights. But as other nosy guests switched to the unoccupied room, the “hole” would appear to shift around.
For as long as the guest got to remain upstairs, a light would move on the aboveground floors, and a hole would move on the underground floors. Once the staff noticed the discrepancy, however, the guest would reluctantly slink back down beneath the ground level, find the nearest unoccupied room, and turn on the light, closing the hole and returning life to normal.
This is the picture of the world as painted in the 1920s by Paul Adrien Maurice Dirac, who accidentally discovered antimatter as a result of trying to solve another problem—reconciling the burgeoning description of quantum physics with the already-established theory of special relativity.1
This marriage was attempted and then abandoned by Erwin Schrödinger, who instead settled for a less general formalism—his eponymous wave equation, which students across the world grapple with in frustration on a daily basis.2 But Dirac managed to nail it; all it took was a completely new mathematical description of the world and a serious rethink about the nature of reality, so you can see why Schrödinger shied away.
And buried within the mathematics of the theory was a surprising little symmetry: a new kind of particle, positioned like a mirror to our everyday world. Every fundamental particle, like an electron, was matched by a new particle with identical properties (mass, spin, etc.) but with perfectly opposite charge. For the electron, its antimatter twin is called the positron. Exactly like the electron, but with a positive sign in front of its charge.
The positron was experimentally discovered a couple of years later, followed shortly by the twins (antitwins?) of all the other known particles.
The scenario of Hotel Dirac is a useful way to explain a surprising situation: if you have a bit of light—a photon—at high enough energies, it can spontaneously split into an electron and a position. After traveling for a bit, the two particles will find each other again, collide, and disappear in a flash of light, releasing back the original photon.
In this picture, what we view as the “ground state” of the electron in any situation—the lowest possible energy state—actually sits on top of an infinite pancake stack of negative energy states, already occupied by a subterranean hotel full of electrons. A photon of sufficient energy can knock one of these negative-energy electrons into a positive-energy state, where it runs around doing all the things that electrons do. But it leaves behind a “hole” in the sea of negative-energy electrons, and that hole looks, acts, and smells like a typical electron—except it has the opposite charge.
Eventually the electron gets tired of wandering in the positive-energy world and falls back down into its hole, releasing the energy that originally promoted it. The photon returns, and everything is back to status quo.
This picture isn't exactly correct—our more modern view of the process is different in some subtle and important ways, and I'll get to that in a another chapter, but it does serve a very useful point here: matter and antimatter are symmetric. Or at least, ought to be symmetric. For every piece of matter—an electron over here, an atom over there—there ought to be a matching twin with the opposite charge out there, somewhere.
This symmetry of antimatter is baked into the same mathematics that predicted its existence in the first place. It appears completely unavoidable.
But look out there, somewhere, anywhere. See any antimatter? No, you don't. From one end of the Milky Way to the other, from the earliest moments of the universe that we can observe to the present day, matter rules the cosmos. Almost all the stuff is normal, not anti. If there were, say, a galaxy composed entirely of antimatter, then as it swam through the thin soup of particles between the galaxies, it would be releasing enormous amounts of energy—the most energetic events ever known.
One ounce of antimatter annihilating with one ounce of normal matter would release the energy equivalent of about a good-sized H-bomb. One galaxy of antimatter annihilating with one galaxy of matter would release—let me see here—ah, right, a lot of energy.
We don't see it. We don't have the suspicion of seeing it. We don't even have a hint of a suspicion of seeing it.
Matter, not antimatter, dominates the universe, and has for an incredibly long time—essentially its entire history. We're obviously misunderstanding something.
Where did all the antimatter go?
When we last left the story of the universe, it had reinvigorated itself after exhausting its energy in the most rapid expansion yet known—and ever to be known. Inflation had ballooned the observable cosmos to the preposterous dimensions of an apple, in the slightest sliver of a second. That inflation was triggered by the splitting of the strong nuclear force away from the others, and the process spread out all the matter into a cold, thin soup.
This soup was somehow reinvigorated as whatever drove inflation shook itself off (cosmologists are still working out that detail of the story). But that reinvigoration itself creates a new problem (sensing a common theme here?). Let's say the universe at this stage is filled with high-energy radiation at a temperature of 1015 K. Some of those photons can transform into pairs of electrons and positrons (and a host of other particles), but they will always do so symmetrically. For every bit of matter that pops into existence, a matching bit of antimatter will be along for the ride. Eventually they'll find each other, tragically end their lives in a fury of mutual self-destruction, and return back to radiation.
But our universe, today, is not filled with only radiation. There's matter all over the place. To create the amount of matter that we observe, the imbalance in the newborn universe didn't have to be much: just one part in a billion extra in the ratio of regular matter to antimatter would do it.3 That's a small number, but even small numbers are huge compared to the totally zero predicted by particle physics.
Long side note: Unfortunately, when dealing with high-energy physics, which is the realm of the early universe, the jargon comes fast and furious. We have to keep track of the names for all the particles, the antiparticles, the hypothetical particles, the forces, the hypothetical
forces, groups of particles, families of particles, and on and on. Plus it's all twisted up because some processes got their names assigned before we fully settled on a definition. Hence I've been trying to avoid most of that messiness, but just in case you want to look this stuff up on the internet later (you masochist), the name given to the process of the domination of matter over antimatter is baryogenesis, and no, my spell-checker doesn't recognize that as a real word either. Baryon in most contexts means particles made of three quarks, like the familiar proton and neutron. And quarks are—well, I'll just save that for later.
Anyway, to solve this riddle, we have a few options. Take your pick:
Option 0: Asymmetry is a lie! There is actually tons of antimatter out there; we just live in a little patch of regular matter. But as I talked about earlier, the implications of that kind of universe seem kind of (a) violent and (b) obvious, so essentially nobody finds this palatable.
Option 1: The universe had an asymmetry between regular matter and antimatter baked in since the beginning. I know, I know, there may not be a “beginning,” but in this argument, there is some overarching rule that says the two kinds of matter are not really in balance, and it's been in place for the entire history of the universe, in a similar vein as a rule that says, “By the way, there's a force of gravity.” This is generally unappealing because it's totally just sweeping the problem under the rug and pretending it doesn't exist, and because we see no evidence of this grand law operating in the present-day universe. You would think something that was that big of a deal ought to hang around for longer than a second.
Option 2: Hey, I know, there were lots of crazy physics happening in the preinflation madhouse era, so maybe that's the key! We don't really understand the physics, so maybe tucked into an equation here or slipped into a term over there is an imbalance, and that will do the trick. In this story, before inflation even got rolling, the stage was set for baryons to dominate. Our understanding of this epoch is fuzzy enough to accommodate lots of wacky ideas (something the theorists among us love) but not clear enough to actually separate one idea from another (something the theorists hate, because none of them can get the validation to win a Nobel Prize). While a valid choice, this option is basically the community saying, “Let's have the next generation of scientists solve this one.”4
Option 3: Maybe the imbalance came later, after inflation, as the universe was steadily expanding and cooling. It's still a crazy mess of a place, and there's plenty of particle wiggle room to get up to some funky stuff. For example, the weak nuclear force still hasn't split off from the electromagnetic force, and while we largely understand that process, there might be something interesting there.
Just for fun, and because it's our most solid lead, let's follow Option 3.
I really need to introduce the weak force properly to show how it might play a rather unexpected role in disrupting the delicate balance between matter and antimatter. Let's face it: nobody treats the weak force with any respect. I mean, just look at the name! The other forces have ancient, complex, or assuredly self-descriptive names. The weak nuclear force is indeed weak, but it's far, far stronger than gravity. And it does play a role in nuclear reactions, but not in the same way as the strong force.
In essence, the strong nuclear force is a binder: it glues things together (except when it repels—it's complicated5). The weak nuclear force is a transformer: it can change one kind of particle into another. That may not seem impressive, but it lies at the heart of radioactive decay and the synthesis of heavy elements. So yeah, kind of important.
And when it comes to matter versus antimatter, the weak nuclear force has a favorite. It's not immediately obvious, and for the effect to show up it requires piles of particle collision data. It also came as a big surprise to particle physicists when it was discovered in the 1950s and ’60s, but that's just life.
I won't go into the details here, since I'm trying my best not to make this a textbook on particle interactions,6 but in a collider you can make some exotic combinations of particles. These exotic combinations don't hang around for long—they're unstable and quickly decay into a shower of smaller, longer-lasting particles. Two of these bizarre characters, called the pion and kaon (pro tip for any wannabe particle physicists: if you need to name something new, just take letters from another alphabet and add “on”), decay into various children particles with various rates.
But they don't decay into exactly the same particles at the same rates every time. They show an ever-so-slight preference for decaying into one combination of charges versus the opposite. In the jargon that we are now enmeshed in, their decays violate C-symmetry, or symmetry of charge. These decays also violate another apparent symmetry of our universe called parity, which means that all fundamental interactions at the deep particle level look the same in a mirror. Well, almost all: these pion and kaon decays are an exception.
So both C (charge) and P (parity) are not essential symmetries in the cosmos, even though for a long time we thought they were. If you're curious, the ultimate combo of charge-parity-time (CPT) is thought to be persistent: if you take a particle interaction of your choice, flip all the charges, run it in a mirror, and run it backward in time, you shouldn't see any difference. But let's not get carried away here.
This symmetry violation in the weak nuclear force is important because it provides a known channel for favoring one kind of electric charge. And since the weak nuclear force doesn't get to become a player in cosmic history until after inflation, that's why we think it's a prime candidate for making the universe—wait for it—matter.
Violating this central symmetry, however, isn't the only part to the story. I know, just as you thought we were getting out of the woods. Two other conditions must be satisfied if you want more matter than antimatter, and they are harsh conditions indeed.
One is that there must be a process that produces a raw excess of matter over antimatter. Wait, what? Why didn't we just, you know, start with that? You might think this is the only condition you need, and you're almost certainly questioning my decision to regale you with tales of kaons and symmetry violations. However, you need both conditions (a favor for matter and a favor for charge) to get the desired result.
Now would be a good time to take a break. Go on, I'll wait.
You can have a process that makes an abundance of matter all you want, but if charge symmetry is enforced, it must be matched by a process that generates more antimatter than the regular kind. In other words, you might think you're cleverly generating an imbalance, but nature will sneakily slip in some back-channel reaction when you're not looking to make sure everything evens out. Then all the particles will end up back in balance, despite your best efforts, and you'll be stuck with a radiation-only universe again. So you need a channel for generating extra matter, and you need to make sure that channel isn't negated by its evil charge twin. Only then can you flood the universe with regular matter.
Unfortunately, creating excess matter doesn't happen in our normal everyday universe. Fortunately, the initial moments into the history of the cosmos aren't our normal everyday universe.
The culprit is once again the crafty weak nuclear force. We know that every once in a while, a weak interaction can produce an excess of matter, but the channels available to do it are highly suppressed—they are so rare that they essentially never happen at low energies. But at high energies, especially energies high enough to merge the weak and electromagnetic forces, these processes can operate at full blast. So it's certainly possible to transmute radiation into a matter-filled early universe, using hidden tricks and trapdoors built into the nature of the weak force itself.7
Unless your universe is in equilibrium. If you have a hot ball of gas or plasma, and it's left totally to its own devices, then all allowable processes and interactions within that hot ball will happen, canceling out each other. That's the very definition of equilibrium. So if the young universe were in such a state, any method that produced extra nor
mal matter would be competing against other methods that produced extra antimatter, and nobody would win, ending in a draw (i.e., no matter at all, anywhere).
Back to square one.
Ever ready for a three-peat, the humble weak force comes to the rescue to satisfy this last of the necessary conditions to tip the cosmic scales in favor of normal matter. But not the force itself; here, the splitting of the unified electroweak interaction, while not quite as violent as the inflation-inducing cataclysms of earlier epochs, was just as spontaneous and scattered. The cooling from electroweak to electromagnetism-plus-weak didn't happen all across the universe simultaneously, but rather as bubbles sparked at random places, each spreading outward.
It's like bubbles in boiling water. Except this happens at a temperature of a thousand trillion Kelvin in the first picosecond into the history of the universe as we know it. Outside the bubbles, the universe is in equilibrium. Inside the bubbles, evenness prevails as well, but in a new state. But the boundaries of the bubbles are different beasts altogether, and here a fully nonequilibrium state (basically by definition) occurs.
And it's there, in these exotic bubble boundaries, that all the conditions can be met within the realm of known physics: excess matter is produced, it's preserved by asymmetries in charges, and there are no competing processes there to fight against it.
Problem solved! Except that our best guess at the details of this process predict about one-billionth the expected amount of matter. Whoops.
So, yeah. After all the buildup and explanation and excruciating jargon, we don't have much. Or do we?
At first blush, this chapter so far, plus the earlier chapter discussing the earlier epochs, seems like it could be three words: “We don't know.” But I hope, if I've spun this tale the way I intended, you're starting to see something interesting emerge. The further we get into the history of the universe—the older, larger, and cooler it becomes—more recognizable shapes and patterns begin to emerge from the mist.
Your Place in the Universe Page 7