Does Anti-Matter Explain Why There Is Something Rather than Nothing? (script) (Patreon)

The most precious substance in our universe is not gold, nor oil. It’s not even printer ink. It’s antimatter. But it’s worth every penny of it’s very high cost, because it may hold the answer to the question of why anything exists in our universe at all.

Each particle in our universe has its exact counterpart: an anti-particle identical in every way, but with the opposite charge and spin. An electron has a positron; a proton, an anti-proton; and so on. And when a particle encounters its anti-particle - when matter encounters anti=matter- the two can pair-annihilate, canceling each other out completely, leaving only two photons to carry away the energy. And it works in reverse too. Particle and anti-particle pairs can be created from pure radiation. In fact that’s how we think the first particles were created in the very early universe. But if matter and anti-matter is always created in pairs, then in the beginning of time there should have been exactly the same amount of both. So where is all the anti-matter? The better question is: why is there any matter at all? Shouldn’t everything just have annihilated again, leaving only a vacuum bathed in light? 

The most likely answer seems to be that the universe started out with a little more matter compared to anti-matter. If there were slightly more particles than anti-particles, then almost everything would have annihilated, leaving a universe full of photons and only the few particles that couldn’t find an annihilation partner. These days there are around a billion times more photons than there are particles of matter - so we estimate that for every billion particles of matter that annihilated, only one survived. 

And there’s the mystery: why were particles created with that 1-in-a-billion overabundance? It seems there must be something inherently different in the way the universe interacts with particles versus anti-particles. The universe must not treat the two symmetrically. Indeed, many physicists think the answer lies in the fundamental symmetries of the universe, or, rather, in the breaking of these symmetries. We’ve discussed this before - there are three symmetries of the universe that physicists once believed were fundamental. You should be able to perform any of these transformations, or all of them, and the laws of physics should be unchanged. We have charge conjugation, where positive and negative charges are swapped; we have parity inversion, where the universe is reflected through a mirror; and time reversal, where all particles have their direction of motion and spins exactly reversed.

If you apply all three of these transformations to a particle - if you apply a CPT transformation - then it becomes its own antiparticle. Because we expect the universe to be CPT symmetric, we expect it to treat antimatter in exactly the same way as regular matter. 

But one by one, these presumed symmetries failed. The first to fall was parity, with Chien-Shiung Wu’s famous cobalt-60 experiment proving that a mirror image of our universe would be distinguishable from our own. Then, charge and parity combined, or CP, also fell, with the observation of the peculiarity in the decay of K-mesons. 

That CP violation may have contributed to the asymmetry of matter and anti-matter in the early universe, in a process called “electroweak baryogenesis.” But, at least at the level of CP violation that we’ve observed, this isn’t enough to explain the level of baryon asymmetry that does exist. 

But remember that antimatter is what you get when you do a full CPT transformation of matter. So maybe the violation of full CPT symmetry is needed to explain this imbalance. We saw in our previous episode that so far CPT symmetry looks safe - and that’s because we know that both CP AND T symmetries are separately broken, and the breaking of T symmetry could actually counteract CP violation, preserving CPT symmetry. In principle, at least. If CPT symmetry really IS violated however it may explain why we live in a universe of matter, and would undo a lot of what we think we know about quantum mechanics. So, good news bad news I guess. 

The CPT theorem says that, assuming our knowledge of special relativity and the Standard Model is correct, then CPT symmetry must hold. But, we already know that our current understanding of the universe is incomplete. It doesn’t explain dark energy, dark matter, or this baryon asymmetry problem. So, if there exists some underlying more fundamental theory, maybe string theory for example, then CPT symmetry may no longer be a foregone conclusion. 

But how do we test for CPT violation? The CPT theorem demands that an anti-particle must have the exact same properties as its matter counterpart—it must have the same mass, the same quantum energy levels, and the same interactions with its environment. So we need to make some antimatter and test it. That’s not exactly a simple process. Well, actually making anti-particles is straightforward - for example, positrons - or anti-electrons - are created all the time in nature - for example in the Sun, or in radioactive decay, or when cosmic rays hit the atmosphere, which is how antimatter was discovered in the first place. And more exotic antimatter like anti-protons can be created in particle accelerators - just by smashing regular matter together. The problem is, antimatter immediately annihilates with any matter it encounters, so it’s hard to keep the stuff around for long. And that’s particularly true of anti-matter atoms, which are electrically neutral and so are hard to even store using electric and magnetic fields.

To give you an idea of the effort involved, take the simplest type of anti-matter atom: anti-hydrogen. It consists of just a single anti-proton plus a positron, instead of the proton + electron of regular hydrogen. In 1999, NASA estimated that when taking all expenses into account, just one gram of anti-hydrogen cost 62.5 trillion dollars. And you thought gas was expensive. So we’re going to wait a while to be powering starships with antimatter engines anytime soon. Fortunately, you don’t need anything like a whole gram of the stuff to do CPT experiments. A handful of atoms is enough, and so the cost of doing these experiments is many orders of magnitude lower.

One of the few facilities in the world capable of making anti-hydrogen is at CERN in Switzerland.

There, the ALPHA experiment is testing the behavior of anti-hydrogen in multiple ways to see if it deviates from regular hydrogen - deviations that could point to the violation of CPT symmetry. ALPHA uses CERN’s proton synchrotron to get their anti-protons. The synchrotron accelerates protons to 10’s to 100’s of G-electron Volts of kinetic energy, corresponding to over 99% of the speed of light. These high-energy protons then hit a metal target and produce a zoo of particles and anti-particles. Some of these by-products are anti-protons. 

Voltages applied to electrodes around the outside of the chamber direct the anti-protons to a storage ring called the Antiproton Decelerator, where they are slowed down by pulses of radiofrequency electric fields as they travel around the ring. They can then be redirected to a number of different experiments, including ALPHA. There, the negatively-charged anti-protons are trapped by a combination of electric and magnetic fields in a so-called Penning trap. Positrons also fly into this trap from a radioactive sodium-22 source, and pair up with the anti-protons, creating anti-hydrogen. 

In previous experiments, the now-neutral anti-hydrogen was no longer confined by the Penning trap, and so drifted to the walls of the trap where it annihilated with the matter it encountered. But although anti-hydrogen is electrically neutral, it does have a small magnetic moment - like a tiny bar magnet. ALPHA introduces a new magnetic field that forces the anti-matter to the center of the chamber. In this way they’ve managed to keep anti-hydrogen in the trap for several days. The longer they can keep an anti-atom around, the more tests they can conduct on it, and so the more precise their measurements will be.

With the anti-hydrogen securely in the trap, scientists measure the difference in energies between the various positron orbitals in the anti-atoms using laser spectroscopy. The exact energy of one of these states is determined by many different factors: the precise mass and charge of the particles, their orbital angular momentum, their magnetic and electric dipole moments, and even the strength of the coupling between the particles and the quantum fluctuations of the vacuum. Any difference in these properties in anti-matter versus matter will cause a shift in the laser frequency necessary to stimulate a transition in the atom. 

Scientists measure this frequency, then compare to the corresponding frequency in regular hydrogen. The best-measured transition in hydrogen is known with 15 digits of precision, so even a tiny variation between hydrogen and antihydrogen might be detected. 

In a recent measurement, they’ve been able to test CPT invariance down to 16 parts per billion. So far no evidence of CPT violation. 

But they’ve already made plans to push boundaries even further, testing CPT with higher sensitivity. Another anti-proton decelerator, ELENA, will come online next year and be able to deliver much slower anti-protons, which will mean way more anti-hydrogen in their trap. They expect this to dramatically improve their measurement precision.

ALPHA scientists are also building a new experiment: similar to the original ALPHA apparatus, but rotated by 90 degrees, ALPHA-g will allow the anti-atoms to be dropped from the trap and free-fall for some time. CPT theorem says that the acceleration of an anti-atom in Earth’s gravitational field should be exactly the same as for an atom, but scientists want to test this. They’ve even designed the contraption to allow room for the anti-atoms to move up in the extremely unlikely, but maybe not completely ruled-out, off-chance that anti-atoms experience “anti-gravity.” Such a discovery would be a huge surprise and would cause all sorts of terrible consequences to our current model of the universe, so scientists aren’t holding their breath. They’re pretty sure anti-matter interacts with gravity in the exact same way as matter, but why not test it?

OK, so. Apparently we have another one of these Space Time episodes where scientists busted ass to break physics and … didn’t. But as I’ve said before, that’s cool and amazing in itself. We now understand the symmeties of nature to much greater precision - which means we have a better idea of where to look for this strange BROKEN symmetry that leads to matter-antimatter imbalance and to the fact that we have matter in the universe at all. Maybe it’ll come from CPT violations measurable only in future experiments. Or maybe it won’t—instead proving beyond a doubt that CPT truly is an underlying symmetry of Space Time.