The antimatter episode was first published July 27, 2018.

Physicists love symmetry. Symmetry means that you can change a system in some way and it will still be the same. So reflectional symmetry is the one most people understand; something that’s the same when you reflect it in a mirror. But there are other symmetries, too. It’s not just some kind of weird fetish, although, to hear some physicists talk about it, you might think that it was… when they start drooling about the glorious and beautiful symmetries that underlie everything, you can begin to question whether the interest is purely mathematical… But symmetry is mathematically useful. It can hugely simplify your equations, and the problem you’re trying to solve. Imagine mapping a world that’s symmetrical; you only need to see half of it, and understand the symmetry, to have a map of the whole. The symmetry cuts your work in half; it can simplify so many problems. By mapping the laws of physics in our little corner of the universe, and then expecting a “translational symmetry” — systems that work in a similar way in different locations — to allow us to make calculations that apply across the world, across the Universe. If the laws of physics weren’t the same pretty much everywhere, we would struggle to calculate anything. In fact, most of the laws of physics as we know them can be derived by just looking at a system and thinking of its symmetries. We’ll come onto this in a future episode.

And, more than this — even though physicists might say that they’re motivated only by the empirical evidence — that’s not strictly true. All of them have a view that an ideal world should be symmetric. Maybe this is because the pool of people who become physicists is a little bit biased; you might want to study physics because you find mathematics beautiful, or the world to be beautiful, or symmetry to be beautiful, or all three. Then you’d want to discover a universe with symmetry, because symmetry is beautiful. From a philosophical standpoint, if the universe was created, we hope that the creator would make it logical, and make sense; create something that is beautiful. And if it wasn’t created — if you’re the most cynical person ever, who doesn’t believe in gods or reasons or any of this namby-pamby stuff about things being beautiful or ugly — then there’s still a reason to expect symmetry, in a lot of cases. It’s kind of like the anthropic principle. We exist, humans exist. And although it might not always seem like it, we’re fairly orderly and fairly structured creatures. If the Universe, and the laws of physics, were a confusing asymmetrical mess, it probably wouldn’t be good news for stable order to exist. Let me give you a concrete example. Imagine that particle physics wasn’t symmetric — so that, say, the charge on the proton was much higher than the charge on the electron. Then, you’d struggle to have electrically neutral atoms, and you wouldn’t have the complex chemistry that gives rise to life. Similarly, you can imagine that the force law for charges might work differently. What if like charges repelled stronger than opposite charges attracted? Disturbances to symmetry do more than make the Universe ugly — they could make it completely unworkable.

But we shouldn’t be so fast to discount asymmetry. Imperfection. Little imbalances that don’t make sense. Happiness, joy, ecstasy, love; these are all chemical imbalances in our brains that will, in the fullness of time, be corrected and return to a baseline, a zero. Your personal zero may vary, but that’s the way of things. All of our exuberant joys, and all of our heartrending sorrows: in other words, the very things that make us human; all of these are asymmetries. As are we, as is everything. Because one of the unsolved problems of physics is why the hell anything actually exists at all.



See, everything we know is made up of matter — all the physical objects, all the clouds and sheep and people and chemicals and rocks and water and trees and buildings, the stars, the galaxies, everything; all composed of these subatomic particles that make up matter. And there’s a conversion, between matter and radiation. A highly energetic photon of radiation can produce matter; so, if the universe began with energy and radiation, matter can follow from that. This equivalence between energy and matter is what E = mc² means; the energy intrinsically in something that’s massive is the same as the mass of that object multiplied by the speed of light, squared. That was what Einstein was on about, all these years.

But there’s a problem. When photons turn into matter, they produce two particles. The photon needs to have twice as much energy as the particle — whether it’s a proton, electron, whatever — that you want to produce. Every particle is produced with its antiparticle; for each electron, there’s a positron, and for each proton produced, there’s an anti-proton. The antiparticles have the same mass; they’d look the same, if we could see them; they behave in similar ways under most of the laws of physics, but they have the opposite electrical charge.

And this makes perfect sense, too. Because we believe that things like electrical charge should be conserved in the Universe — they can’t be created or destroyed. That’s a form of symmetry, too. We can’t have photons, which have no charge, randomly turning into protons, which have a positive charge. Then we’d end up with an imbalance where there’s more positive charge than negative, which would ruin everything. One of the ways to see this is by realizing that, on large scales, the dominant force seems to be gravity. For example, in our very first episodes, we talked about how stars are born due to gravitational collapse. But gravity is a weak force compared to electromagnetism. If, for some reason, the universe was making more positive charges than negative — even if it was just one in a million — stars like the Sun couldn’t possibly exist. The electromagnetic repulsion would tear them apart. The Universe as a whole would probably ‘explode’ — with everything driving everything else apart far faster than gravity. Only a tiny, tiny imbalance could possibly be permitted without changing all of our observations — so it makes a lot of sense for the overall charge of the Universe to be zero. And if that’s true, then we can’t have any processes that make charge or destroy it. Every process has to keep the charge the same. And every process we see in the Universe does keep charge the same; we never see electrical charge being made or destroyed.

One way of looking at antiparticles, that was popular originally, is like “holes” in particle space. Imagine that there’s a huge sea of electrons, say, that all have negative energy. When a photon interacts with this sea, it can push an electron from this negative energy sea into existing, with positive energy. But the hole is left behind, and if the electron ever hits the hole, it will fall back down again, releasing its energy once more as photons. That’s what we call annihilation.

So the fact that photons turn into a particle and antiparticle with opposite charges is pretty crucial; it means that radiation can turn into matter without messing up charge conservation. The photon has zero charge; it turns into a particle with a positive charge, like a proton, and one with negative charge, the antiproton, that exactly cancels it out, so you have zero again. But there is a serious problem. When a particle collides with its anti-particle, they annihilate each other, and turn back into photons of energy again. This process is symmetrical, too; it’s symmetrical in time, if you like. You run it forwards, and the photons turn into an electron (say) and a positron: pair production. Run it backwards, and the electron and positron turn into a photons again. Nice and symmetrical.


But you can see the problem. Why is there any matter at all? Why didn’t all the antimatter that was produced just annihilate it? After all, we’re not running around in fear of annihilation from antimatter. (Just nuclear war.) In fact, when we look at the Universe around us, there are no large-scale structures made of anti-matter. There are no anti-stars, no anti-people, no anti-planets. Antimatter is so incredibly rare on Earth, and usually only exists for a few fleeting seconds. We only discovered antimatter by looking at rare cosmic rays from outer space. And when it’s produced on earth, for example in our particle colliders, it doesn’t take long before the antimatter collides with some matter and annihilates, disappearing again into energy. This is very destructive; if you and anti-you ran into each other, the result would be an explosion with the power of ten thousand nuclear bombs. When nuclear weapons are used, only a small fraction of the rest mass energy is converted into the explosive power behind the bomb; with annihilation, you get all of the rest mass energy converted into radiation, which is far more powerful. It would be a massive explosion, and a serious problem. So it’s a good job antimatter is rare in our part of the Universe.

But this is weird. Because the processes we know, and see in the world around us, produce equal amounts of matter and antimatter. So why does matter dominate? Why is there more matter than antimatter in the Universe? Why aren’t we just a soup of particles and antiparticles, annihilating each other, and being pair-produced, in a symmetric — but ultimately boring — mess?

The whole fact that anything at all exists is down to some unexplained asymmetry in the early universe. At some point, things weren’t neat and tidy and symmetrical. Something happened to make sure that slightly more matter than antimatter was produced. Most of the matter and antimatter produced around the time of the big bang annihilated each other, but a tiny leftover due to an asymmetry in the laws of physics meant that we ended up with matter. Maybe it was only one particle in a billion, or one particle in a trillion, but the universe — for a while — had a slight favourite. It favoured matter. And hence, due to this little asymmetry, everything around us exists. I’m reminded whenever I think about this of that brilliant song lyric by Neutral Milk Hotel. “How strange it is to be anything at all.”

By the way — one of the things you might have thought, and that I remember thinking originally, is — why do we need to assume there isn’t antimatter out there? Why do we assume that there isn’t loads of antimatter in the Universe, and we just somehow got separated from it, rather than having asymmetric physics? Could there be anti-stars and anti-atoms out there? Well, we actually know that anti-atoms can exist. In fact, we’ve made some of them in our particle accelerators. The only problem is that everything in our universe wants to annihilate them, since it’s made of matter… you can see how this might be a pretty big problem. But we have created atoms of antihydrogen. Hydrogen is the simplest element — it’s just one electron whizzing around one proton — so its anti-atoms were the easiest to create. But if you’re hoping to annihilate your enemies, you’re probably better off buying a gun; because anti-hydrogen costs $90 trillion dollars per gram to produce. It is the most expensive material on the planet.

[how can we store antimatter/antihydrogen. Explanation of antiantimatter?]

So while the physics of anti-matter seems to check out, and in principle you can imagine an anti-universe with anti-people and anti-politicians (who maybe occasionally tell the truth), it probably doesn’t exist in our universe. Almost everything would be the same for these anti-people in their anti-worlds, except what they call positive charge would be our negative charge, and a few similar things along those lines. It’s a tantalizing prospect. Because of the symmetry, antimatter looks exactly the same as matter — it’s not like it would be the opposite colour, or emit anti-light, or anything like that. If you had a telescope, observing an anti-galaxy would probably look the same as observing a galaxy. And this makes sense, right? We’ve talked about how electromagnetic photons turn equally into matter, and antimatter. Electromagnetism treats matter and antimatter like the same objects. And since light is just electromagnetic radiation, we know that they should look the same. So it might not be obvious if we were looking at antigalaxies. Paul Dirac, the genius quantum physicist who first theorized that antimatter should exist, speculated that there might be regions of the Universe that were dominated by antimatter. [MORE ON THIS?]

But the problem is that if there were big chunks of the universe made out of antimatter, there would be some frontier where the matter part of the Universe, that we live in, and the antimatter bit, collided with each other. And that frontier would produce huge amounts of high-energy gamma rays from **** hitting the fan, when all these particles collided with each other and annihilated. Most of space is… um… empty space. Between the galaxies, in intergalactic space, there’s around one atom per cubic metre. Compare this to air, which contains We’d be able to see this with our space telescopes. So it seems very likely that there aren’t big regions of the Universe that are dominated by antimatter. Although it’s not completely disproved yet — after all, we can only observe a fraction of what might exist in the Universe. Physicists launched the Alpha Magnetic Spectrometer into space a few years back; and what this can do is detect different types of particles; it’s like a particle physics observatory in outer space. If the AMS had detected a big hunk of antimatter — for example, an anti-helium — that would change everything. Anti-helium is too complicated to reasonably be produced by natural collisions and processes that make antimatter in the Universe around us. If we saw anti-helium, it’s far more likely that it’s drifted from some part of the universe dominated by antimatter, or that large amounts of antimatter did survive the processes in the early universe — around the time of the Big Bang. So far, nothing.

There is always the possibility that there is an anti-Universe out there — somewhere beyond the part of the Universe we can see. Since it’s impossible for us to observe anything outside of the observable Universe, there could well be antimatter out there. But then you have to explain how it managed to end up separated from all of the matter — what process drove them apart before they could mix?

Luckily for physicists, there’s finally starting to be some evidence that there are processes which — ever so slightly — favour matter over antimatter. And, if these processes — or similar ones — occurred in the early Universe, we might not need to imagine universes of antimatter somewhere over the rainbow to explain the world that we live in. For this, we turn to particle physics experiments like the Large Hadron Collider, in search of something called CP violation.

CP violation is a process where charge + something called parity aren’t symmetrically conserved in the process. In other words, there might be some processes where the matter process occurs at a slightly faster rate than its corresponding antimatter process; the laws of physics aren’t quite the same for matter and antimatter, and so we can see how there might end up being a universe full of matter rather than a universe full of annihilation. And they have seen evidence for some of this in particle physics. At the large hadron collider, they smash protons together at incredible speeds and energies. The result is a shower of secondary particles, produced as the energy from the collision gets turned into the rest mass of the particles. Physicists fine-tune the energy of the collision by changing how much they accelerate the protons; and, depending on the energy, they’ll see different secondary showers of particles. Based on how these behave, they can work out things like the energy and the lifetime of the particles that are being produced. The LHC has discovered dozens of very unstable, very short-lived particles — or resonances — that decay extremely quickly. By studying how a particular pair of these particles — the Lambda B 0 and its antiparticle — decay, physicists are pretty confident they’ve seen CP violation — in other words, the particle and its antiparticle don’t behave in the same way. Could this kind of behaviour, in the crazy hot and dense conditions of the early universe, explain why there’s more matter than antimatter? Maybe. This is just one example — there are several different processes that seem wildly obscure, but do exhibit this strange asymmetry which may be responsible for the fact that… well, anything exists at all. Philosophically, I quite like the idea that some obscure corner of physics might have such weighty consequences; after all, otherwise, would anyone care that some particle lived ever so slightly longer than its antiparticle, or whatever the case may be? But no one has a convincing model for it yet — and these little bits of CP violation might not be enough to explain the excess of matter, which, after all, is big enough to create everything in the universe that we can see. The jury is still out. If you can solve the problem, there’s a Nobel Prize in it.






[CP Violation]


[needs finishing — what else to include?]


So aside from being mysterious — or, maybe, mysteriously absent — what is antimatter good for? And if it annihilates everything it touches, how can we make it and store it?

Let’s talk about the recipe for antihydrogen. When you smash particles together in a particle accelerator like CERN, you can get a shower of anti-protons produced as a result — along with loads of protons. And electrons, and their anti-particles positrons, as well. But these are all produced at incredibly high energies, travelling close to the speed of light. To capture them before they smash into matter and annihilate, you have to be smart. You need to rely on how charged particles act in magnetic fields.

When you apply a magnetic field to a charged particle that’s moving, the particle feels a force, that’s called the Lorentz force. The force is always at right angles to the direction the particle is travelling, and the direction of the field, and the force depends on the charge of the particle, too. So you can imagine that, by applying a carefully-chosen magnetic field, you can separate out these particles — which are being forced at different rates, and have different masses which means they accelerate at different rates — into different zones. The trick, then, is to slow down the antiprotons until they’re slow enough to capture. And they do this by ramming them through a gas of electrons, which slows them down through collisions and the electromagnetic force. At this point, they’re cool enough, and slow enough to form stable atoms of anti-hydrogen — if you put them in a zone with lots of positrons. The naked anti-protons can capture positrons, and then you have this strange anti-hydrogen atom; a positron, with positive charge, whizzing around an anti-proton, with negative charge. The antiproton atom has particular magnetic properties — and a carefully shaped magnetic field, or a “magnetic bottle”, as it’s sometimes called, can keep them confined in a particular region for a while. (This is much easier when they’re moving slowly, which is why decelerating them is so important!) So you make sure the region has no pesky matter to annihilate with — you make sure it’s a good vacuum. With no matter to touch, they won’t annihilate — for a little while at least. In 2011, physicists got very excited because they’d managed to store antihydrogen for sixteen minutes — so you’d better take your data readings fast! It doesn’t sound like much, but that was 10,000 times longer than the previous attempt — long enough that they could actually hope to study the atoms. Back in 2011, the only way to know for sure that you’d captured antihydrogen was… well, the moment you’d no longer captured it. Doesn’t it always seem to go, you don’t know what you’ve got till it’s gone? Physicists could detect the flash of energy when the antihydrogen annihilated on the walls of its magnetic prison, and they figured… okay, we musta had one there for a bit.

Since then, things have advanced, and they’ve now been able to measure how it reacts to light by zapping it with photons from a laser. And they’ve confirmed that, in this respect at least, matter and antimatter are exactly symmetric — they respond to light in the same way; the antihydrogen looks exactly the same as hydrogen would; it has the same ‘spectrum’. It took 20 years since the first time they created antihydrogen to measure its spectrum — and only to confirm what we suspected already — which just goes to show the amazing persistence of physicists. Plus, experimentalists like to look for Black Swan events — the really exciting experiment is one that smashes up your theory, because it means you’ve discovered new physics! Or you’ve made a terrible mistake. But, you know, maybe new physics.

If it had been different, perhaps the antimatter-matter asymmetry problem would be solved… but not yet.

So — I know what some of you are probably thinking. There’s this incredibly destructive substance that can convert all of its matter into energy. It explodes with more force than an atomic bomb; we say that it literally annihilates matter. So… to quote Professor Death from those Mitchell and Webb sketches… could there be some kind of… military application?

Of course people have thought about it. Dan Brown, author of the world’s most charity-shopped novel Da Vinci Code (until it was cruelly knocked off by 50 Shades of Grey) — he even wrote a book where someone tries to blow up the Vatican with an antimatter bomb, powered by a simple gram of antimatter. In principle, the idea works, and it’s pretty scary. But with the way we produce antimatter currently? There’s simply no way. I’ll quote a CERN physicist explaining why not:

“Take Dan Brown’s hypothetical 1 gram of antimatter,” said Rolf Landua. “With present CERN technology, we would be able to produce about 10 nanograms of antimatter per year, at a cost of about $10–20 million. Then we would have to deal with the problem of how to store so many particles (about 10,000,000,000,000,000 antiprotons). Obviously, it would take 100 million years and $1,000 trillion to make 1 gram. This appears ambitious even for the US military.”

Obviously nuclear weapons exist that could do the same job waaay cheaper. It would take CERN 2 million years to produce antimatter enough for a Hiroshima type explosion… and they’d have to be really careful not to drop it. Even if you imagine that you can suddenly mass-produce antimatter, in a really cheap way that allows you to cram it all into a bomb… it’s still not much better than nukes. One thing you can say for nuclear weapons, at least the newer ones — they’re mostly fail-safe. You might remember from our TEOTWAWKI episodes on nukes that a nuclear weapon is set off when a core of radioactive material is explosively compressed, setting off a chain reaction that leads to an explosion. It used to be the case that this was done with some conventional explosives, but even then, the explosives had to be very carefully shaped in order to ensure that the chain reaction would go supercritical and you’d get a nuclear explosion. Nowadays, they’re much safer; there’s very few ways this could happen by mistake: it requires an intentional detonation. In other words, if you leave a nuclear bomb alone for a hundred years, or kick it, it won’t explode — although I can’t advise it. But antimatter is a completely different story. If, even for a fraction of a second, you stop powering the magnets that hold it in place — it will hit the cell walls and annihilate. That’s just fine if you have a tiny amount. But if you have a Hiroshima bomb yield of antimatter explosives, it’s fail-dangerous. Fail very dangerous. And given that the sum total of antimatter that’s been produced so far is about as destructive as lighting a match, I can’t see people investing in these inefficient death-traps.

But there is some hope for you, if you’re in love with the asymmetry of antimatter. For a start, in limited doses, it has already had some incredible uses. In PET scanners, for example, positrons — the antiparticles for electrons — are already used to great effect. People dream that in the future, someday, antimatter could even be used to fuel spaceships that will go on interstellar voyages. The main problem with this will probably already be obvious to you — after all, if whatever contains your fuel needs to be very carefully managed to prevent a catastrophic explosion, and your spaceship is fuelled by tiny explosions, there are obvious engineering problems that may preclude this from being ideal.

So, there we have it. The ghostly alternative to ordinary matter. An unsolved mystery at the beginning of the Universe. A strange anomaly which allows, well… anything to exist at all. Annihilation as a force for destruction, but also — possibly — as a tool we could harness in the future. Antimatter; the antisymmetric counterpart to the world around us; continues to fascinate and delight. Just don’t shake hands with anti-you if you ever happen to spot them.

Thanks for listening to this episode of Physical Attraction…