What's Happening With Antimatter at CERN? Scientists Are Stumped Again
Summary
TLDRThe video explores the enigmatic world of antimatter, delving into its discovery, theoretical predictions, and experimental observations. It highlights the historical detection of antimatter, the predictions made by Dirac's quantum field theory, and the subsequent experimental confirmations. The video also discusses the baryonic asymmetry of the universe and the ongoing quest to understand the differences between matter and antimatter, focusing on the weak force's distinct interaction with particles and antiparticles. The discussion culminates with the ALPHA experiment at CERN, which investigates the gravitational pull on antimatter, revealing that it behaves similarly to matter, thus dismissing speculative theories of antimatter falling upwards.
Takeaways
- 🌌 The fundamental building blocks of matter, protons, neutrons, and electrons, are consistent across the observable universe, including distant galaxies like Andromeda.
- 🔍 The mystery of the universe lies not in unknown phenomena but in the unexpected scarcity of antimatter, which is a significant part of the Standard Model of particle physics.
- 🏅 Antimatter's existence was first detected in 1932 by Carl D. Anderson, who observed positively charged particles with the mass of an electron but opposite charge, known as positrons.
- 📈 Paul Dirac's quantum field theory predicted the existence of antimatter before its experimental detection, leading to the understanding of particles and their corresponding antiparticles.
- ⚛️ Antimatter can theoretically form entire celestial bodies, appearing similar to matter构成的 planets, but the asymmetry between matter and antimatter in the universe remains unexplained.
- 🔬 Experiments at CERN aim to uncover differences between matter and antimatter, focusing on the properties of antiparticles and their interactions with the four fundamental forces of nature.
- 🔄 The weak nuclear force affects particles and antiparticles differently, with 'handedness' playing a key role in their interaction, but this doesn't sufficiently explain the universe's matter-antimatter imbalance.
- ⚖️ The strong nuclear force, contrary to early predictions, appears to treat particles and antiparticles identically, further deepening the mystery of the baryonic asymmetry.
- 🌐 Gravitational experiments at CERN, such as those conducted by the ALPHA group, suggest that gravity may not differentiate between matter and antimatter, as previously assumed in the Equivalence Principle.
- 🔵 The ALPHA experiment found that antimatter experiences gravitational acceleration at 75% the rate of matter, with a margin of error that still allows for the possibility of a full 1g of gravitational force.
- 🚀 Continued research into the gravitational effects on antimatter is necessary for a more precise understanding, and the search for new forces and particles that could explain the baryonic asymmetry persists.
Q & A
What are the three building blocks of matter mentioned in the script?
-The three building blocks of matter are protons, neutrons, and electrons.
What is the significance of the discovery of anti-electrons or positrons?
-The discovery of anti-electrons or positrons is significant because it confirmed the existence of antimatter, which is a critical part of the Standard Model of particle physics and has been observed in experiments for nearly a century.
How did Paul Dirac predict the existence of antimatter?
-Paul Dirac predicted the existence of antimatter by realizing that in order to describe electrons as quantum fields in a way that was physically consistent with special relativity, they had to be part of a larger mathematical structure known as a Dirac spinor, which inevitably gave rise to both positively and negatively charged versions of the same particle.
What is the 'baryonic asymmetry of the universe' and why is it a mystery?
-The 'baryonic asymmetry of the universe' refers to the observed dominance of matter over antimatter in the universe. It is a mystery because if antimatter were too similar to matter, it would be impossible to explain why our universe contains so much of one and so little of the other.
What are the four fundamental forces of nature?
-The four fundamental forces of nature are electromagnetism, gravity, the weak nuclear force, and the strong nuclear force.
How do particles and antiparticles interact differently with the weak force?
-Particles and antiparticles interact differently with the weak force in that ordinary particles can only feel the weak force if they are 'left-handed' and antiparticles can only feel it if they are 'right-handed.' Additionally, right-handed antiparticles experience a different strength of the weak force compared to left-handed ordinary particles.
What was the result of the 1963 experiment by James Cronin and Val Fitch?
-The 1963 experiment by James Cronin and Val Fitch observed a fundamental asymmetry between particles and antiparticles in their interactions with the weak force. This discovery earned them a Nobel Prize.
What is the current understanding of how the strong nuclear force affects particles and antiparticles?
-Current understanding and experiments suggest that the strong nuclear force treats particles and antiparticles exactly the same, which is surprising given the differences observed in other fundamental forces.
What is the ALPHA experiment at CERN investigating?
-The ALPHA experiment at CERN is investigating the gravitational properties of antimatter, specifically the gravitational acceleration of antimatter on Earth's surface.
What was the outcome of the ALPHA experiment regarding the gravitational acceleration of antimatter?
-The ALPHA experiment found that antimatter experiences a gravitational acceleration that is approximately 0.75g of the strength of gravity acting on ordinary matter, indicating that gravity does not treat matter and antimatter differently, at least to the extent observed in the experiment.
What are the two major sources of uncertainty in the ALPHA experiment's results?
-The two major sources of uncertainty in the ALPHA experiment's results are the uncertainty in the applied magnetic field bias and possible errors in alignment, as well as other systematic and statistical uncertainties.
What implications do the results of the ALPHA experiment have for theories of gravity and antimatter?
-The results of the ALPHA experiment rule out speculative theories that suggest antimatter might have a negative gravitational charge or that it might fall upwards instead of downwards. The findings also confirm that, aside from the weak force, the other fundamental forces, including gravity, appear to treat particles and antiparticles in a similar manner.
Outlines
🌌 The Building Blocks of Matter and the Mystery of Antimatter
This paragraph introduces the fundamental particles that make up all matter, protons, neutrons, and electrons, and extends the discussion to the cosmic scale, highlighting that matter is composed of these same particles throughout the observable universe. The central mystery presented is the unexpected absence of an equally abundant presence of antimatter. The video's host, Alex McColgan, sets the stage for an exploration of antimatter, its interactions with particles and gravity, and the frustration among physicists that antimatter isn't as 'weird' as it should be to explain the observed scarcity of it in the universe. The historical context is established with the mention of Carl D. Anderson's 1932 experiment that first detected antimatter, specifically positrons, and his subsequent Nobel Prize for this discovery. The paragraph also introduces Paul Dirac's theoretical prediction of antimatter as part of his quantum field theory, which posited that every particle has a corresponding antiparticle with the same mass but opposite charge. This theoretical framework was confirmed just four years later by Anderson's experiment.
🔬 Experiments and Theories: Unraveling the Properties of Antimatter
This paragraph delves into the scientific understanding of antimatter, summarizing what is currently known about its properties and how they compare to those of regular matter. It explains that while the intrinsic properties of antiparticles, such as mass, mirror those of their matter counterparts, their behavior under electromagnetic forces is characterized by their opposite charge, as demonstrated by Anderson's cloud chamber experiment. The discussion then moves to the weak nuclear force, which affects particles and antiparticles differently based on their 'handedness' or chirality, and the surprising discovery that these two types of particles experience the weak force with different strengths. This asymmetry was first observed by James Cronin and Val Fitch, earning them a Nobel Prize. The paragraph also touches on the strong nuclear force, which, contrary to early theoretical predictions, appears to treat particles and antiparticles identically. The focus then shifts to the ongoing experiments at CERN aimed at understanding the gravitational interaction with antimatter, questioning whether it could differ from that with matter and potentially offering insights into the baryonic asymmetry of the universe.
🧪 CERN's Experiments on Antimatter and Gravity
This paragraph discusses the experimental efforts at CERN to study the gravitational properties of antimatter, questioning the long-standing assumption that antimatter falls downward under the influence of gravity, just like matter. It describes the intricate experimental setup involving the creation of antihydrogen atoms, which are composed of positrons and antiprotons, and their behavior within a magnetic field trap. The experiment aimed to measure whether the gravitational acceleration experienced by antimatter is the same as that experienced by matter. The ALPHA experiment's results indicated that antimatter does indeed fall downward, with approximately 75% of the antihydrogen atoms escaping through the bottom of the chamber. However, the gravitational acceleration measured for antimatter was found to be 0.75g, suggesting it could be three-quarters the strength of gravity acting on matter. The paragraph also addresses the limitations and uncertainties of the experiment, such as the applied magnetic bias and systematic errors, which prevent a definitive conclusion but rule out the speculative theories that antimatter might have a negative gravitational charge.
🔄 The Search for the Baryonic Asymmetry Solution Continues
The final paragraph wraps up the video's exploration of antimatter by reflecting on the experimental findings and their implications for understanding the baryonic asymmetry of the universe. It acknowledges that while the weak force has been shown to differentiate between particles and antiparticles, the observed differences are not sufficient to explain the predominance of matter over antimatter in the universe. The paragraph leaves open the possibility of undiscovered forces or particles that could provide a more significant distinction between matter and antimatter interactions. It invites the viewer to consider whether the preponderance of matter is merely a coincidence and encourages further exploration into the mysteries of antimatter. The video concludes with a call to action for viewers to share their thoughts and interests in the topic, fostering engagement and a sense of community around the scientific pursuit of knowledge.
Mindmap
Keywords
💡Antimatter
💡Protons, Neutrons, and Electrons
💡Standard Model
💡Quantum Field Theory
💡Dirac Spinor
💡Baryonic Asymmetry of the Universe
💡Weak Force
💡Gravitational Acceleration
💡AEgIS, GBAR, ALPHA
💡Equivalence Principle
💡Antihydrogen
Highlights
Everything from the skin of your hand to the screen you're watching this video on is made of the same three building blocks of matter: protons, neutrons, and electrons.
The mystery of the universe is not that we've seen something we can't explain, but rather that we haven't seen something we were expecting: a universe full of antimatter.
Antimatter is very real and forms a critical part of the Standard Model of particle physics, with particles of antimatter having been observed in experiments for nearly a century.
The first detection of antimatter dates back to 1932 when Carl D. Anderson discovered anti-electrons, or positrons, for which he won the Nobel Prize in Physics in 1936.
Paul Dirac predicted the existence of positrons in 1928 as part of a larger mathematical structure, the Dirac spinor, which gives rise to both positively and negatively charged versions of the same particle.
Other particles of matter, like quarks, each have their own anti-quark counterparts, allowing for the formation of anti-protons, anti-neutrons, anti-atoms, and anti-molecules.
The cosmic mystery known as the 'baryonic asymmetry of the universe' has sent physicists on a quest to find differences between matter and antimatter.
In the 1960s, it was discovered that particles and antiparticles are affected differently by the weak force, with 'handedness' playing a role in how they experience this force.
The famous experiment by James Cronin and Val Fitch in 1963 observed a fundamental asymmetry between particles and antiparticles, for which they were awarded a Nobel Prize.
Theoretical models predicted differences in how left-handed particles and right-handed antiparticles feel the strong force, but experiments to date suggest that the strong force treats them the same.
The ALPHA experiment at CERN has provided real-world data on the gravitational acceleration of antimatter on Earth's surface, challenging assumptions after decades.
Antihydrogen atoms, composed of positrons and antiprotons, were used in the ALPHA experiment to measure the effects of gravity on antimatter.
The ALPHA experiment found that roughly 75% of antihydrogen atoms escaped through the bottom of the chamber, indicating a preference for downward-pulling gravity.
The best-fit gravitational acceleration for antimatter was reported as 0.75g, meaning three-quarters of the strength of gravity acting on ordinary matter.
Despite the experimental results, a full 1g of gravitational acceleration is still fairly consistent with the collected data due to uncertainties in the applied bias and other factors.
The ALPHA experiment rules out speculative theories that antimatter falls up instead of down, and the weak force is the only fundamental force that applies differently to particles and antiparticles.
The search continues for new forces and particles that interact more weirdly with antimatter, as explaining the baryonic asymmetry of the universe would require more drastic differences between matter and antimatter.
Transcripts
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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