Have a look around you. Everything you see, from the skin of your hand to the screen you’re
watching this video on, is a different combination of the same three building blocks of matter:
protons, neutrons, and electrons. Now let’s look a little farther, say at Mars, or the Andromeda
galaxy, or even halfway across the observable universe… and still, there is matter made of
protons, neutrons, and electrons, as far as the eye can see. At first, this might not sound all
that surprising—but for once, the mystery here isn’t that we’ve seen something we can’t explain,
but rather that we haven’t seen something we were expecting: a universe just as full of antimatter.
I’m Alex McColgan, and you’re watching Astrum. Join me today as we explore the
world of antimatter and learn about its interactions with other particles and even
with gravity. By the end of this video, you’ll probably agree that antimatter is a bit weird,
but you’ll also see why some physicists are frustrated that it isn’t weird enough.
Let’s get one thing out of the way first: although it might sound like something straight out of
science fiction, antimatter is very real. It forms a critical part of the Standard Model of
particle physics, and particles of antimatter have been observed in experiments going back nearly a
century. The very first detection of antimatter dates back to a 1932 experiment conducted by Carl
D. Anderson at Caltech using a cloud chamber immersed in a magnetic field. When charged
particles from outer space—broadly called cosmic rays—intercept the Earth’s orbit and fly through
this chamber, the magnetic field curves their paths according to the charge and mass of each
particle, and the clouds show a visible imprint of their resulting trajectories. Anderson was
hoping this experiment would help determine just what kinds of particles were streaming
into the Earth from the cosmos, and he may have found just a little bit more than he bargained
for! What Anderson saw was that these cosmic rays included both positively and negatively charged
particles. The masses of the negatively charged particles lined up exactly with the known mass of
an electron, but some of the positively charged particles were far too light to be protons.
Instead, they appeared to also have the mass of an electron, despite having the opposite charge,
and so these never-before-seen particles came to be known as anti-electrons, or later positrons
for short. In 1936, Anderson would win the Nobel Prize in Physics for this discovery.
Meanwhile, a British physicist, who was also destined to win a Nobel,
had been developing a description of electrons that would fit nicely within the framework of
quantum field theory. His name was Paul Dirac. By 1928, Dirac had realized that in order to describe
electrons as quantum fields in a way that was physically consistent with special relativity,
they had to be a part of a larger mathematical structure— later known as a Dirac spinor—that
inevitably gave rise to both positively and negatively charged versions of the same
particle. In this way, Dirac had predicted the existence of positrons before Anderson
had even built the cloud chamber that would detect them four years later.
What’s even more incredible is that electrons aren’t the only fundamental particle to come
in a two-for-one "Dirac spinor" package. Other particles of matter, like the quarks that make
up protons and neutrons, each have their own anti-quark counterparts. These anti-quarks can
come together to form anti-protons and anti-neutrons, which can then bond with
positrons to form anti-atoms and anti-molecules. You could make a whole planet out of antimatter,
and from the outside, it would look quite similar to an ordinary planet made of ordinary matter!
But if anti-matter were too similar to matter—if the only difference were the
sign of its charge—then it would be impossible to explain why our
universe contains so much of one and so little of the other. This cosmic mystery,
known as the “baryonic asymmetry of the universe,” sent physicists on a decades-long quest to try and
find as many differences as they could between matter and antimatter. That quest lives on today,
spearheaded by particle colliders at CERN that are capable of producing, trapping,
and studying both positrons and anti-protons. But before we talk about these experiments,
let’s try to summarize what we already know about the properties of antimatter.
When studying antiparticles in isolation, experiments have confirmed with ever-greater
precision that their intrinsic properties - namely their masses - are exactly the same as
for ordinary particles. And when studying how antiparticles are affected by electromagnetic
forces, experiments have again found that they behave the same exact way as ordinary particles,
except with the opposite electric charge - just as Anderson had observed in his cloud
chamber. But electromagnetism is just one of the four fundamental forces of nature,
alongside gravity and the weak and strong nuclear forces. And as
physicists began to better understand the weak force in the 1950s and ‘60s,
they realized that particles and antiparticles are actually affected by it quite differently.
The first surprise was that ordinary particles could only feel the weak force if they were
"left-handed" and antiparticles could only feel it if they were "right-handed." The concept of
handedness (or "chirality") is subtle and difficult to conceptualize for particles
with mass. But a loose analogy can be drawn with a particle's helicity, which describes whether
a particle is spin-up or spin-down along its direction of motion. In this analogy,
a spin-up particle is called right-handed, while a spin-down particle is called left-handed.
The second and even crazier surprise was that right-handed antiparticles experienced a different
strength of the weak force, as compared to left-handed ordinary particles. In practice,
this means that the quantum probabilities for radioactive decay in ordinary nuclei are
somewhat different from the probabilities of the analogous decay processes in anti-nuclei. This
fundamental asymmetry between particles and antiparticles was first observed in
a 1963 experiment run by James Cronin and Val Fitch of Princeton University,
who would be awarded yet another Nobel Prize for their discovery.
When this asymmetry was discovered, there was some hope that it would explain the baryonic asymmetry
of the universe: perhaps these differences in the weak force were responsible for the
abundance of matter and utter lack of antimatter around us. But the maths didn't quite work out.
There simply wasn’t enough of a difference between the strength of the weak force acting
on particles versus antiparticles. That was when physicists began to turn their attention
to the strong nuclear force. Theoretical models predicted that, just like in the weak interaction,
there should be some differences in how left-handed particles and right-handed
antiparticles feel the strong force. But antimatter just keeps surprising us! Every
experiment to date suggests that the strong force treats particles and antiparticles just the same.
This brings us to the last of the four fundamental forces and
the subject of today's ongoing experiments at CERN: gravity.
To be honest, suggesting that gravity might treat matter and antimatter differently is
kind of a long shot. Think back to the popular legend of Galileo tossing stones of different
sizes and materials from the Tower of Pisa: they all fell at the same rate,
because the gravitational acceleration on Earth is 9.8 meters per second squared, regardless
of which object is falling. Of course, the experiment works even better in a vacuum chamber,
where air resistance is taken out of the equation. Newton expanded on this idea and showed in the
17th century that your gravitational acceleration anywhere in space depends only on the mass of the
object pulling you and your distance from it, but not on any of your personal properties—not
even your own mass. This famous result—known as the Equivalence Principle—is the foundation of
Einstein’s theory of General Relativity, our most accurate and successful model of gravity to date.
With that in mind, physics is still an experimental science at its core,
and we can’t know for sure whether matter and antimatter obey the same
laws of gravity unless we check for ourselves. The physicists at CERN set out to do just that,
motivated not only by the baryonic asymmetry of the universe, but also by a few speculative
papers suggesting that the cosmological mysteries of dark matter and dark energy
could be more easily explained if antimatter were to have a negative gravitational charge—or
put more simply, if antimatter were to fall up, rather than down. There are
several ongoing experiments at CERN testing the gravitational properties of antimatter,
including AEgIS (“Aegis”), GBAR (“g-bar”), and ALPHA (“Alpha”). Today we will focus
specifically on a key experiment coming out of the ALPHA group that was published in the
journal Nature this past September. After decades of assumptions, this experiment has
brought us real-world data on the gravitational acceleration of antimatter on Earth’s surface.
But before we show you the results, let’s take a moment to appreciate just how intricately
this experiment was designed in order to isolate and measure the effects of gravity.
The first step in the experiment is to secure a beam of several million positrons per second
emitted from a radioactive isotope of sodium (Na-22). Most of these positrons
end up colliding with ordinary matter in the experiment, causing miniature explosions in
which positrons and electrons annihilate each other and release place a small burst of energy
in the form of light. But a small fraction of the positrons survive as they are guided through the
experimental apparatus [figure below], where they are cooled by low-pressure gases and trapped by
electric and magnetic fields. But observing the effects of gravity on these positrons would be
nearly impossible: their masses are so small, that the tiny force of gravity felt by each particle
is overshadowed by even the smallest fluctuations in the surrounding electromagnetic fields. That’s
why this collection of positrons is merged with a separate container of antiprotons, where they bond
and form neutral anti-hydrogen atoms that are much less responsive to stray electromagnetic fields.
And where did the antiprotons come from? Suffice it to say that they were produced by firing
ordinary protons into a block of metal really, really fast. Yeah—Physics is awesome like that.
Once the antihydrogen atoms are created, they behave like tiny, weak magnets that can remain
trapped by a complicated arrangement of external magnetic fields. But now, this
magnetic interaction is weak enough that it no longer overwhelms the gravitational effects that
we’re trying to measure. The chamber containing these antihydrogen atoms is nearly a vacuum:
there are just about 200,000 atoms of ordinary gas per cubic centimeter, compared to a typical
atmospheric density of 20 quintillion atoms per cubic centimeter. Under these conditions,
the trapped antihydrogen atoms almost never collide or annihilate with atoms of ordinary
matter. Instead, they can more or less just float around the chamber for minutes or longer. But
as the magnetic fields used to vertically trap the antihydrogen atoms are weakened,
this random floating eventually allows the antihydrogen atoms to escape through either the
top or bottom of the chamber [figure below], where they can collide with a wall of the apparatus,
annihilate with some ordinary atoms, and release a small burst of light. In the ALPHA experiment,
this happens over the course of about 20 seconds.
The theory behind the experiment is that if gravity really pulls antimatter downwards,
more of the antihydrogen atoms escape through the bottom than the top. The stronger the
gravitational force, the more atoms escape through the bottom. The simulations the ALPHA
team ran showed that under normal gravitational attraction, about 85% of the antihydrogen atoms
should escape through the bottom, whereas only 20% of them would escape through the
bottom if gravity pulled antimatter upwards. If there were no gravitational force at all,
the simulations showed a more even distribution of 55% escape through the bottom (probably only
differing from 50% due to asymmetries in the experimental apparatus itself).
What did the actual experiment find? Well, roughly 75% of antihydrogen atoms escaped
through the bottom of the chamber, showing a clear preference for downward-pulling gravity.
As any thorough scientist would, the ALPHA team repeated this experiment to collect a variety
of data points that tell a more complete story. They redid the procedure under various levels of
magnetic field bias, which applied external upward or downward magnetic forces on the
antihydrogen atoms. On this graph, a bias of -1g means that just enough magnetic force is applied
to counteract normal gravity, while a bias of +1g means an extra “g” of magnetic force is applied to
push the antihydrogen atoms downward, and so on. The team made predictions through simulations for
each bias and for various possible gravitational interactions, which produced the orange, green,
and purple curves shown here [figure below]. As you can see, the experimental data points,
shown in blue, best match the orange curve, which represents the “normal”
simulation where gravity pulls antimatter downwards. But because the data falls
just a bit below this curve, the best-fit gravitational acceleration was only 0.75g:
three-quarters of the strength of gravity acting on ordinary matter.
Does this mean that gravity affects matter and antimatter particles differently after all?
Not necessarily. Let’s have a quick look at the error bars. They indicate that there
are two major sources of uncertainty in the results, including an uncertainty in
the applied bias (highlight horizontal error bars on screen), possible errors in alignment,
and other systematic and statistical uncertainties (highlight vertical error bars for these last 3).
When accounting for these uncertainties, the best-fit gravitational acceleration is actually
reported as 0.75g±0.13g±0.16g. This means that a full 1g of gravitational acceleration is still
fairly consistent with the collected data. Future experiments will be able to determine
more precisely how strongly gravity acts on antimatter. But … we can already rule
out speculative theories that rely on antimatter falling up instead of down.
In the end, despite how weird and backwards the world of antimatter is, it seems that only the
weak force actually applies differently to particles and antiparticles. But explaining
the baryonic asymmetry of the universe would require much more drastic differences between
the two— so scientists aren’t done looking for them. Could there be new forces and particles
that interact even more weirdly with antimatter? Or would you be willing to accept that having so
much more matter than antimatter around us is a mere coincidence? In any case,
let us know if you’ve learned something new about antimatter from watching this video, and whether
this is a topic you’d like to hear more on! We generally applaud it when hundreds of eyes turn
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