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!
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