About a hundred years ago, the brilliant British physicist Paul Dirac predicted the existence of a mysterious substance in the universe, which he called "antimatter." These particles include things like positrons (antielectrons), antiprotons, and antineutrons.
Dirac stated that antimatter was an elusive counterpart to ordinary matter. While ordinary matter is made up of atoms in which negatively charged electrons orbit around a positively charged nucleus, antimatter consists of positively charged electrons that orbit around a negatively charged nucleus.
Since then, scientists have been trying to understand antimatter in relation to physics, particularly its response to gravity.
Some scientists believe that, while gravity causes ordinary matter to fall, it causes antimatter to do the exact opposite—levitate.
However, recent experimental research published in the leading multidisciplinary science journal Nature suggests that antimatter cannot, in fact, escape gravity, which has raised question marks about several unconventional theories (and sci-fiction depictions).
Recent experimental research published in the leading multidisciplinary science journal Nature suggests that antimatter cannot, in fact, escape gravity.
What is antimatter?
To understand the relationship between antimatter and matter, it's important to know this: symmetry governs both the universe and the laws of physics.
As such, antimatter can be described as the "opposite" of matter. For every microscopic piece of matter, like an electron, proton, or neutron, there's a matching piece of antimatter with the same mass but an opposite charge, like a mirror image.
When the universe was born with a massive explosion (the Big Bang), it produced roughly the same amount of matter and antimatter. Perhaps the obvious assumption would be that the two would have cancelled each other out. But something strange happened.
Matter took over, and antimatter disappeared in its shadows. As a result, antimatter is no longer visible in large quantities in our world today. Even stranger, no one can say why.
Recently, scientists artificially produced antimatter in labs using particle accelerators and other high-energy methods.
In a series of experiments conducted at CERN (European Organisation for Nuclear Research), they created antihydrogen atoms (consisting of an antiproton and a positron) and observed how they behaved in the presence of gravity.
The big question they wanted to answer was: Does antimatter react to gravity the same way that regular matter does? According to Albert Einstein's general theory of relativity, all objects, regardless of their composition, fall into a vacuum at the same speed under gravity's influence.
What happens if you drop an anti-apple?In a paper published today in @Nature, the ALPHA collaboration at CERN's Antimatter Factory shows that, within the precision of their experiment, atoms of antihydrogen – a positron orbiting an antiproton – fall to Earth in the same way as... pic.twitter.com/oo83GbZT2Z
Speaking to Al Majalla, Jeffrey Hangst, a physicist at Aarhus University in Denmark and the lead author of the study, told Al Majalla: "This principle, known as the equivalence principle, has been tested and confirmed countless times in ordinary matter, but the question remains: Does it apply to antimatter?"
The results were groundbreaking. Just like ordinary hydrogen atoms (consisting of a proton and an electron), antihydrogen atoms appeared to respond to gravity in the same way as their ordinary matter counterparts.
The results of recent research were groundbreaking. Just like ordinary hydrogen atoms (consisting of a proton and an electron), antihydrogen atoms appeared to respond to gravity in the same way as their ordinary matter counterparts.
This was a resounding confirmation of the equivalence principle as well as a testament to the accuracy of experimental physics. It also provided further support for Einstein's general theory of relativity, which has been tested time and again.
Einstein proven right, again
Formulated in 1915, Einstein's general theory of relativity remains one of the most significant achievements in the history of physics. This revolutionary theory changed our understanding of gravity in particular, and the fabric of the universe in general.
Over a century later, the theory and its predictions have endured many experiments that have only served to prove their accuracy.
One of the most famous experiments took place during the 1919 solar eclipse. British astronomer Sir Arthur Eddington led an expedition to measure the deflection of starlight due to the gravity of the Sun. The results confirmed Einstein's predictions and catapulted the physicist to global acclaim.
Einstein's theory has been subjected to rigorous testing, including measurements of gravitational time dilation, gravitational redshift, and the precise orbits of planets and satellites.
However, the most recent test came in the form of gravitational waves, first observed in 2015 by the Laser Interferometer Gravitational-Wave Observatory (LIGO). These ripples in space-time, created by cataclysmic events such as black hole mergers, have provided direct evidence for the existence of gravitational waves, as predicted by Einstein's theory.
Nonetheless, modern cosmology has revealed perplexing mysteries such as antimatter, dark matter, and dark energy, which have profound implications for our understanding of gravity.
One head-scratcher is how antimatter behaves. Einstein had no knowledge of antimatter in 1915, but in 1928, Paul Dirac came up with a theory to describe it. Meanwhile, the first observation of an antimatter particle, the positron (the antimatter equivalent of the electron), was in 1932.
Einstein had no knowledge of antimatter in 1915, but in 1928, Paul Dirac came up with a theory to describe it. Meanwhile, the first observation of an antimatter particle, the positron (the antimatter equivalent of the electron), was in 1932.
Although the consensus is that antimatter is attracted to matter, some hypotheses in the past suggested the possibility of "antigravity" effects.
On the other hand, the weak equivalence principle, a cornerstone of Einstein's theory, assumes that all masses interact with gravity in the same way, regardless of their internal structure.
The recent experiment with antihydrogen atoms demonstrates that these antimatter particles behave consistently with Earth's gravity, ruling out the concept of "antigravity" for antimatter and reinforcing Einstein's predictions.
Re-visiting Galileo's famed experiment
While these results are not exactly a shock to physicists, they have called into question several unconventional theories. To solve some of the biggest mysteries of cosmology, some posited that gravity pushes rather than pulls antimatter. This no longer holds merit.
Centuries ago, Italian scientist Galileo Galilei rolled balls (made of different materials) down a small incline (contrary to the legend of dropping objects from the Leaning Tower of Pisa). He showed that all objects, regardless of their composition, fall at the same speed. Based on this principle, Einstein concluded that gravity arises when massive objects distort space-time.
At the time, however, nobody had tested whether the principle of weak equivalence applies to both matter and antimatter.
As such, physicists working at the Antihydrogen Laser Physics Apparatus (ALPHA) at the European Laboratory for Particle Physics decided to conduct an updated version of Galileo's drop test.
If you drop a piece of antimatter it will fall down, not up. We toured the antimatter "factory" at the CERN particle physics laboratory and saw the ALPHA-g experiment that was set up to test gravity's effect on antihydrogen.https://t.co/2qcFB011yPpic.twitter.com/0mAEbLebMp
To start, the team used an electric field to trap antimatter protons and positrons generated by particles colliding. They manipulated these particles to create antihydrogen atoms and captured them with a magnetic trap, which was built around the electric field.
The team then released about a hundred atoms of antimatter per experiment to determine whether they would fall up or down.
However, the requirements of the experiment were much more complex.
The antihydrogen atoms produced were relatively hot and fast-moving and tended to fly up and down in the elongated cylindrical magnetic trap, making it difficult to discern the effects of gravity.
Additionally, the magnetic field could cause the atoms to scatter upward in the trap, creating a false sense of antigravity.
To control these effects, the team used further magnetic fields to slightly push the antihydrogen atoms, either up or down, as they were released. In other words, the team applied an additional force that compelled the antihydrogen atoms to move in the same direction as gravity.
Then, they altered the force's direction to push the atoms upward against gravity's pull. Interestingly, they noticed that when applying this force, the atoms didn't move upward but seemed to hang in place.
This suggests that the antihydrogen atoms must be influenced by a force pulling them downward, namely Earth's gravity.
An assumption, debunked
French physicist Gabriel Chardin was among those who believed that antimatter was subject to antigravity.
In 2012, he proposed that the visible universe might contain equal amounts of matter and antimatter. Despite having some basis in science, this idea was largely unfounded because astronomers have never observed galaxies made up of antimatter.
In 2012, French physicist Gabriel Chardin proposed that the visible universe might contain equal amounts of matter and antimatter. This idea was largely unfounded because astronomers have never observed galaxies made up of antimatter.
If, as Chardin had posited, antimatter was truly subject to antigravity, it would have solved two of the biggest mysteries in cosmology: the mysterious dark matter that maintains the gravity holding galaxies together, and the even more mysterious dark energy that fills space and accelerates the expansion of the universe.
In this scenario, opposing gravitational forces would cause matter and antimatter to separate. The matter would then clump together to form galaxies, while antimatter would spread as far apart as possible between galaxies, behaving like dark energy, swirling around galaxies, and opening cavities of empty space. Both matter and antimatter would continue to exist.
However, this theory no longer holds weight in light of recent discoveries. "We now have to find another explanation to solve these mysteries," Hangst told Al Majallah.
Looking into dark matters
This recent discovery has profound implications for our understanding of the universe. For one, it bolsters our confidence in one of the most fundamental principles of general relativity.
It also opens the possibility of conducting more precise experiments to study other aspects of antimatter, such as the interaction of gravity with dark matter, which remains one of the greatest question marks in astrophysics.
Dark matter, which is thought to make up much of the universe's total mass, neither emits nor absorbs electromagnetic radiation (like light), making it invisible and difficult to detect directly.
It's thought to play a crucial role in the formation and structure of galaxies and galaxy clusters, but its exact role remains largely unknown.
In addition to scientific implications, this will also challenge common science fiction narratives that describe antimatter as a strange substance that defies the laws of physics.
Ultimately, although the annihilation of matter and antimatter remains a powerful concept for generating energy, the behaviour of antimatter in gravity appears to be consistent with our physics as we know it.