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 patrner - when matter
encounters anti=matter- the two can pair-annihilate, canceling each other out completely, and 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 are
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 very 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 compared to anti-particles?
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
are 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, besides the charge and spin thing —
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 enough - 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. 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.
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
atoms. 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, ALPHA's 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. The CPT theorem states 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.
We like to give shoutouts to our Patreon supporters, but today we have a special request to forgo
the direct shoutout - let’s assume our extreme gratitude is implied - and instead
to wish a very VERY happy birthday to Kristie Meiklejohn. Kristie, we tried to bake you
a cake made of antimatter - guaranteed to age you minus one year if per slice! Unfortunately
it also anhillates you unless you’re also made of antimatter. But Kristie, you can’t
be antimatter because obviously you matter very, very much to someone. We’ll keep working
on the cake, but in the meantime have a wonderful next orbital revolution - from me and all
the space time crew.