Scientists get closer to solving mystery of antimatter
Pallab Ghosh - BBC Science correspondent
Wed, September 27, 2023
Scientists have made a key discovery about antimatter - a mysterious substance which was plentiful when the Universe began.
Antimatter is the opposite of matter, from which stars and planets are made.
Both were created in equal amounts in the Big Bang which formed our Universe. While matter is everywhere, though, its opposite is now fiendishly hard to find.
The latest study has discovered the two respond to gravity in the same way.
For years, physicists have been scrambling to discover their differences and similarities, to explain how the Universe arose.
Discovering that antimatter rose in response to gravity, instead of falling would have blown apart what we know about physics.
They've now confirmed for the first time that atoms of antimatter fall downwards. But far from being a scientific dead end this opens the doors to new experiments and theories. Does it fall at the same speed, for example?
During the Big Bang, matter and antimatter should have combined and cancelled each other, leaving nothing but light. Why they didn't is one of physics' great mysteries and uncovering differences between the two is the key to solving it.
Somehow matter overcame antimatter in those first moments of creation. How it responds to gravity, may hold the key, according to Dr Danielle Hodgkinson, a member of the research team at Cern in Switzerland, the world's largest particle physics laboratory.
"We don't understand how our Universe came to be matter-dominated and so this is what motivates our experiments," she told me.
Engineers adding liquid helium to the system to keep antimatter at minus 270 celcius, near to the lowest possible temperature, absolute zero
Most antimatter exists only fleetingly in the Universe, for fractions of seconds. So to carry out experiments, the Cern team needed to make it in a stable and long-lasting form.
Prof Jeffrey Hangst has spent thirty years building a facility to painstakingly construct thousands of atoms of antimatter from sub-atomic particles, trap them and then drop them.
"Antimatter is just the coolest, most mysterious stuff you can imagine," he told me.
"As far as we understand, you could build a universe just like ours with you and me made of just antimatter," Prof Hangst told me.
"That's just inspiring to address; it's one of the most fundamental open questions about what this stuff is and how it behaves."
What is antimatter?
Let's start with what matter is: Everything in our world is made from it, from tiny particles called atoms.
The simplest atom is hydrogen. It's what the Sun is mostly made from. A hydrogen atom is made up of a positively charged proton in the middle and negatively charged electron orbiting it.
With antimatter, the electric charges are the other way round.
Take antihydrogen, which is the antimatter version of hydrogen, used in the Cern experiments. It has a negatively charged proton (antiproton) in the middle and a positive version of the electron (positron) orbiting it.
These antiprotons are produced by colliding particles together in Cern's accelerators. They arrive at the antimatter lab along pipes at speeds that are close to the speed of light. This is much too fast for them to be controlled by the researchers.
The first step is to slow them down, which the researchers do by sending them around a ring. This draws out their energy, until they are moving at a more manageable pace.
The antiprotons and positrons are then sent into a giant magnet, where they mix to form thousands of atoms of antihydrogen.
The magnet creates a field, which traps the antihydrogen. If it were to touch the side of the container it would instantly be destroyed, because antimatter can't survive contact with our world.
When the field is turned off the antihydrogen atoms are released. Sensors then detect whether they have fallen up or down.
Some theorists have predicted that antimatter might fall up, though most, notably Albert Einstein in his General theory of Relativity more than a hundred years ago, say it should behave just like matter, and fall downwards.
The researchers at Cern have now confirmed, with the greatest degree of certainty yet, that Einstein was right.
But just because antimatter doesn't fall up, it doesn't mean that it falls down at exactly the same rate as matter.
For the next steps in the research, the team are upgrading their experiment to make it more sensitive, to see if there is a slight difference in the rate at which antimatter falls.
If so, it could answer one of the biggest questions of all, how the Universe came into existence.
The results have been published in the journal Nature.
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Mysterious antimatter observed falling down for first time
Daniel Lawler
Daniel Lawler
AFP
Wed, September 27, 2023
Physicists an CERN used a 25-centimetre-long cylinder, called ALPHA-g, to observe antimatter falling downwards due to gravity (Handout)
For the first time, scientists have observed antimatter particles -- the mysterious twins of the visible matter all around us -- falling downwards due to the effect of gravity, Europe's physics lab CERN announced on Wednesday.
The experiment was hailed as "huge milestone", though most physicists anticipated the result, and it had been predicted by Einstein's 1915 theory of relativity.
It definitively rules out that gravity repels antimatter upwards -- a finding that would have upended our fundamental understanding of the universe.
Around 13.8 billion years ago, the Big Bang is believed to have produced an equal amount of matter -- what everything you can see is made out of -- and antimatter, its equal yet opposite counterpart.
However there is virtually no antimatter in the universe, which prompted one of the greatest mysteries of physics: what happened to all the antimatter?
"Half the universe is missing," said Jeffrey Hangst, a member of CERN's ALPHA collaboration in Geneva which conducted the new experiment.
"In principle, we could build a universe -- everything that we know about -- with only antimatter, and it would work in exactly the same way," he told AFP.
Physicists believe that matter and antimatter did meet and almost entirely destroyed each other after the Big Bang.
Yet matter now makes up nearly five percent of the universe -- the rest is even less understood dark matter and dark energy -- while antimatter vanished.
- Newton's apple flying up? -
One of the key outstanding questions about antimatter was whether gravity caused it to fall in the same way as normal matter.
While most physicists believed that it did, a few had speculated otherwise.
A falling apple famously inspired Isaac Newton's work on gravity -- but if that apple was made of antimatter, would it have shot up into the sky?
And if gravity did in fact repel antimatter, it could have meant that impossibilities such as a perpetual motion machine were possible.
"So why not drop some and see what happens?" Hangst said.
He compared the experiment to Galileo's famous -- though likely apocryphal -- 16th-century demonstration that two balls of different mass dropped from the Leaning Tower of Pisa would fall at the same rate.
But this experiment -- the result of 30 years of work on antimatter at CERN -- was "a little bit more involved" than Galileo's, Hangst said.
One problem was that antimatter barely exists outside of rare, short-lived particles in outer space.
However in 1996, CERN scientists produced the first atoms of antimatter -- antihydrogen.
Another challenge was that, because matter and antimatter have an opposite electrical charge, the moment they meet they destroy each other in a violent flash of energy scientists call annihilation.
- A magnetic trap -
To study gravity's effect on antimatter, the ALPHA team constructed a 25-centimetre-long (10-inch) bottle placed on its end, with magnets at the top and bottom.
Late last year, the scientists placed around 100 very cold antihydrogen atoms into this "magnetic trap" called ALPHA-g.
As they turned down the strength of both magnets, the antihydrogen particles -- which were bouncing around at 100 metres a second -- were able to escape out either end of the bottle.
The scientists then simply counted how much antimatter was annihilated at each end of the bottle.
Around 80 percent of the antihydrogen went out of the bottom, which is a similar rate to how regular bouncing hydrogen atoms would behave if they were in the bottle.
This result, published in the journal Nature, shows that gravity causes antimatter to fall downwards, as predicted by Einstein's 1915 theory of relativity.
In more than a dozen experiments, the CERN scientists varied the strength of the magnets, observing gravity's effect on antimatter at different rates.
While the experiment rules out that gravity makes antihydrogen go upwards, Hangst emphasised it did not prove that antimatter behaves in exactly the same way as normal matter.
"That's our next task," he said.
Marco Gersabeck, a physicist who works at CERN but was not involved in the ALPHA research, said it was "a huge milestone".
But it marks "only the start of an era" of more precise measurements of gravity's effect on antimatter, he told AFP.
Other attempts to better understand antimatter include using CERN's Large Hadron Collider to investigate strange particles called beauty quarks.
And there is an experiment onboard the International Space Station trying to catch antimatter in cosmic rays.
But for now, exactly why the universe is awash with matter but devoid of antimatter "remains a mystery," said physicist Harry Cliff.
Since both should have annihilated each other completely in the early universe, "the fact that we exist suggests there is something we don't understand" going on, he added.
Antimatter isn't immune to gravity, landmark experiment confirms
Peter Weber
Thu, September 28, 2023
Antihydrogen Laser Physics Apparatus (ALPHA) lab at CERN.
Antimatter — the mysterious substance that's the mirror opposite of matter in most ways — falls downward in gravity like everything else in the universe, a team of physicists reported Wednesday in the journal Nature. In a delicate, groundbreaking experiment conducted at the European Center for Nuclear Research (CERN), the scientists pretty conclusively proved that antiparticles are not governed by antigravity.
The results are a bit of a wet blanket for science fiction. "The bottom line is that there's no free lunch, and we're not going to be able to levitate using antimatter," study coauthor Joel Fajans of the University of California, Berkeley, told The New York Times. But they are kind of a relief for science. The tug of gravity on antimatter conforms with Albert Einstein's general theory of relativity. If the antiparticles had floated upward in the experiment, as some scientists had hypothesized, it would have turned the world of physics on its head.
"Antimatter is just the coolest, most mysterious stuff you can imagine," Jeffrey Hangst, the particle physicist whose 30 years of work trapping antiparticles led to the discovery, told BBC. "As far as we understand, you could build a universe just like ours with you and me made of just antimatter."
For this experiment, Fajans and his colleagues collected and suspended antihydrogen — the antimatter version of hydrogen, with one positively charged electron (positron) orbiting a negatively charged proton (antiproton) — in a magnetic field inside a specially designed tube. When the magnetic force was turned down on the top and bottom of the tube, some of the antiparticles rose but about 80% fell, roughly in line with hydrogen atoms.
The experiment left a bunch of huge questions about antimatter unanswered, however. The big one: Where is it?
When matter and antimatter meet, they annihilate each other in flashes of pure energy. Scientists believe that when the universe was born, the Big Bang created equal quantities of matter and antimatter. And they don't understand why matter won out while antimatter all but disappeared, only fleetingly observed in cosmic ray showers or created by colliding particles in labs like CERN.
One theory to explain what happened posited that all the antimatter was drawn away from the matter by antigravity and formed its own mirror antigravity universe. That hypothesis looks more implausible now.
One of world’s greatest physics mysteries finally decoded
Vishwam Sankaran
Thu, September 28, 2023
One of world’s greatest physics mysteries finally decoded
Physicists have answered the long-standing question of whether antimatter falls up or down under gravity, an advance that could help crack one of the biggest mysteries of why almost everything in the universe is only made of matter.
The new study, published in the journal Nature on Wednesday, found that antimatter falls downwards under gravity – as expected by much of the scientific community,
Antimatter is made of particles possessing the opposite electric charge as ordinary particles.
For instance, the antimatter equivalent of the negatively charged electron is the positron, and the two annihilate each other to produce gamma radiation if they collide.
One of the strangest mysteries of the universe, scientists have observed, is that almost all the visible matter in the universe is made of ordinary matter and not antimatter.
“Right now, we don’t have an explanation about where all the antimatter in the universe is. To find a solution for this conundrum, what we do is test the elements of the physics of antimatter to see if we can find an inconsistency,” study co-author Robert Thompson from the University of Calgary in Canada said.
In the new study, scientists assessed the gravitational characteristics of antihydrogen – the simplest atom in antimatter that mirrors hydrogen.
Matter dropped from any height on Earth accelerates towards the planet’s surface at the constant rate of about 9.8m/s each second – a mathematical constant used in physics calculations, known as acceleration due to gravity g.
Now, with the latest groundbreaking study, physicists say, this value for antihydrogen – accounting for errors in the experiment – “is consistent with a downward gravitational acceleration of 1g” or about“32 feet per second per second”.
Graphic shows antihydrogen atoms falling and annihilating inside a magnetic trap, part of the ALPHA-g experiment at CERN to measure the effect of gravity on antimatter.
Isaac Schultz
Wed, September 27, 2023
In the 95 years we’ve known about antimatter, physicists have not tested how the elusive inverse of ordinary matter is affected by gravity, the force that pulls masses to Earth and seems to affect all things in the classical realm.
Now, a group of physicists have. Members of the Antihydrogen Laser Physics Apparatus (ALPHA) collaboration at CERN directly observed antihydrogen—hydrogen’s antimatter foil—free-falling in a container. The observations confirm the weak equivalence principle set forth by Einstein in his general theory of relativity, which holds that all masses react the same way to gravity, regardless of their composition. The team’s research was published today in Nature.
“In modern physics, inertial mass is encoded in the standard model of particle physics, whereas gravitational mass is dealt with in Einstein’s general theory of relativity,” wrote Anna Soter, a physicist at ETH Zürich, in an accompanying News & Views article. “The assumed equivalence of inertial mass and gravitational mass is incorporated in the weak equivalence principle, which is the cornerstone of general relativity—but no proposal has yet succeeded in unifying the theories.”
Plans to test gravity’s force on antimatter were first made by 2018, as Gizmodo reported at the time, when the ALPHA collaboration built ALPHA-g, a magnetic trap for antihydrogen atoms. The antimatter particles are suspended and then dropped inside the trap, in a 21st-century equivalent to the story of Galileo Galilei dropping objects off the Leaning Tower of Pisa.
The antimatter particles are cooled in order to slow their movement, giving the physicists precious time to observe and measure them. In 2021, the ALPHA collaboration announced that they managed to cool antihydrogen to near-absolute zero. “Slowing down the motion of antiatoms allows us to perform more precise measurements on its properties. In daily life, you can imagine things moving fast are harder to see than things moving slowly,” Makoto Fujiwara, a particle physicist with Canada’s TRIUMF particle accelerator team, told Gizmodo at the time. “The same thing happens in quantum physics…. The more time you have to observe a certain property, the more precise your measurement.”
As the cooled antihydrogen escaped the magnetic trap, the particles entered a vertical vacuum chamber; if the particles came in contact with the sides of the chamber or its ends (i.e., regular matter) they were annihilated. The researchers fiddled with the strength of the magnetic field, but found that when the fields were balanced on either side of the chamber, about 80% of the antimatter particles annihilated towards the bottom of the trap. Which is to say, gravity got them there.
It’s a good thing that gravity influences matter the same way it does the ordinary stuff—if it didn’t, it would mean that physicists were missing something pretty fundamental to the standard model. It’s not yet clear if antimatter was affected by gravity in the exact same way as ordinary matter, but the very fact that it’s affected affirms both Einstein and the standard model.
But mysteries remain. The magnitude of the gravitational acceleration on the antihydrogen was not precisely measured, but set the foundations for such future experiments. That can be done with even colder atoms—and better understand the weak equivalence principle. And beyond—which is to say below—the masses of the smallest particles, there is the quantum realm, which seems to be beyond the influence of gravity, at least as we know it.
Put in perspective, it seems that this confirmation of something long suspected is just a baby step in physicists’ understanding of the forces of nature and particle physics. But the baby steps are the most important ones to take.
More: Antimatter Could Travel Through Our Galaxy With Ease, Physicists Say
Wed, September 27, 2023
Physicists an CERN used a 25-centimetre-long cylinder, called ALPHA-g, to observe antimatter falling downwards due to gravity (Handout)
For the first time, scientists have observed antimatter particles -- the mysterious twins of the visible matter all around us -- falling downwards due to the effect of gravity, Europe's physics lab CERN announced on Wednesday.
The experiment was hailed as "huge milestone", though most physicists anticipated the result, and it had been predicted by Einstein's 1915 theory of relativity.
It definitively rules out that gravity repels antimatter upwards -- a finding that would have upended our fundamental understanding of the universe.
Around 13.8 billion years ago, the Big Bang is believed to have produced an equal amount of matter -- what everything you can see is made out of -- and antimatter, its equal yet opposite counterpart.
However there is virtually no antimatter in the universe, which prompted one of the greatest mysteries of physics: what happened to all the antimatter?
"Half the universe is missing," said Jeffrey Hangst, a member of CERN's ALPHA collaboration in Geneva which conducted the new experiment.
"In principle, we could build a universe -- everything that we know about -- with only antimatter, and it would work in exactly the same way," he told AFP.
Physicists believe that matter and antimatter did meet and almost entirely destroyed each other after the Big Bang.
Yet matter now makes up nearly five percent of the universe -- the rest is even less understood dark matter and dark energy -- while antimatter vanished.
- Newton's apple flying up? -
One of the key outstanding questions about antimatter was whether gravity caused it to fall in the same way as normal matter.
While most physicists believed that it did, a few had speculated otherwise.
A falling apple famously inspired Isaac Newton's work on gravity -- but if that apple was made of antimatter, would it have shot up into the sky?
And if gravity did in fact repel antimatter, it could have meant that impossibilities such as a perpetual motion machine were possible.
"So why not drop some and see what happens?" Hangst said.
He compared the experiment to Galileo's famous -- though likely apocryphal -- 16th-century demonstration that two balls of different mass dropped from the Leaning Tower of Pisa would fall at the same rate.
But this experiment -- the result of 30 years of work on antimatter at CERN -- was "a little bit more involved" than Galileo's, Hangst said.
One problem was that antimatter barely exists outside of rare, short-lived particles in outer space.
However in 1996, CERN scientists produced the first atoms of antimatter -- antihydrogen.
Another challenge was that, because matter and antimatter have an opposite electrical charge, the moment they meet they destroy each other in a violent flash of energy scientists call annihilation.
- A magnetic trap -
To study gravity's effect on antimatter, the ALPHA team constructed a 25-centimetre-long (10-inch) bottle placed on its end, with magnets at the top and bottom.
Late last year, the scientists placed around 100 very cold antihydrogen atoms into this "magnetic trap" called ALPHA-g.
As they turned down the strength of both magnets, the antihydrogen particles -- which were bouncing around at 100 metres a second -- were able to escape out either end of the bottle.
The scientists then simply counted how much antimatter was annihilated at each end of the bottle.
Around 80 percent of the antihydrogen went out of the bottom, which is a similar rate to how regular bouncing hydrogen atoms would behave if they were in the bottle.
This result, published in the journal Nature, shows that gravity causes antimatter to fall downwards, as predicted by Einstein's 1915 theory of relativity.
In more than a dozen experiments, the CERN scientists varied the strength of the magnets, observing gravity's effect on antimatter at different rates.
While the experiment rules out that gravity makes antihydrogen go upwards, Hangst emphasised it did not prove that antimatter behaves in exactly the same way as normal matter.
"That's our next task," he said.
Marco Gersabeck, a physicist who works at CERN but was not involved in the ALPHA research, said it was "a huge milestone".
But it marks "only the start of an era" of more precise measurements of gravity's effect on antimatter, he told AFP.
Other attempts to better understand antimatter include using CERN's Large Hadron Collider to investigate strange particles called beauty quarks.
And there is an experiment onboard the International Space Station trying to catch antimatter in cosmic rays.
But for now, exactly why the universe is awash with matter but devoid of antimatter "remains a mystery," said physicist Harry Cliff.
Since both should have annihilated each other completely in the early universe, "the fact that we exist suggests there is something we don't understand" going on, he added.
Antimatter isn't immune to gravity, landmark experiment confirms
Peter Weber
Thu, September 28, 2023
Antihydrogen Laser Physics Apparatus (ALPHA) lab at CERN.
Antimatter — the mysterious substance that's the mirror opposite of matter in most ways — falls downward in gravity like everything else in the universe, a team of physicists reported Wednesday in the journal Nature. In a delicate, groundbreaking experiment conducted at the European Center for Nuclear Research (CERN), the scientists pretty conclusively proved that antiparticles are not governed by antigravity.
The results are a bit of a wet blanket for science fiction. "The bottom line is that there's no free lunch, and we're not going to be able to levitate using antimatter," study coauthor Joel Fajans of the University of California, Berkeley, told The New York Times. But they are kind of a relief for science. The tug of gravity on antimatter conforms with Albert Einstein's general theory of relativity. If the antiparticles had floated upward in the experiment, as some scientists had hypothesized, it would have turned the world of physics on its head.
"Antimatter is just the coolest, most mysterious stuff you can imagine," Jeffrey Hangst, the particle physicist whose 30 years of work trapping antiparticles led to the discovery, told BBC. "As far as we understand, you could build a universe just like ours with you and me made of just antimatter."
For this experiment, Fajans and his colleagues collected and suspended antihydrogen — the antimatter version of hydrogen, with one positively charged electron (positron) orbiting a negatively charged proton (antiproton) — in a magnetic field inside a specially designed tube. When the magnetic force was turned down on the top and bottom of the tube, some of the antiparticles rose but about 80% fell, roughly in line with hydrogen atoms.
The experiment left a bunch of huge questions about antimatter unanswered, however. The big one: Where is it?
When matter and antimatter meet, they annihilate each other in flashes of pure energy. Scientists believe that when the universe was born, the Big Bang created equal quantities of matter and antimatter. And they don't understand why matter won out while antimatter all but disappeared, only fleetingly observed in cosmic ray showers or created by colliding particles in labs like CERN.
One theory to explain what happened posited that all the antimatter was drawn away from the matter by antigravity and formed its own mirror antigravity universe. That hypothesis looks more implausible now.
One of world’s greatest physics mysteries finally decoded
Vishwam Sankaran
Thu, September 28, 2023
One of world’s greatest physics mysteries finally decoded
Physicists have answered the long-standing question of whether antimatter falls up or down under gravity, an advance that could help crack one of the biggest mysteries of why almost everything in the universe is only made of matter.
The new study, published in the journal Nature on Wednesday, found that antimatter falls downwards under gravity – as expected by much of the scientific community,
Antimatter is made of particles possessing the opposite electric charge as ordinary particles.
For instance, the antimatter equivalent of the negatively charged electron is the positron, and the two annihilate each other to produce gamma radiation if they collide.
One of the strangest mysteries of the universe, scientists have observed, is that almost all the visible matter in the universe is made of ordinary matter and not antimatter.
“Right now, we don’t have an explanation about where all the antimatter in the universe is. To find a solution for this conundrum, what we do is test the elements of the physics of antimatter to see if we can find an inconsistency,” study co-author Robert Thompson from the University of Calgary in Canada said.
In the new study, scientists assessed the gravitational characteristics of antihydrogen – the simplest atom in antimatter that mirrors hydrogen.
Matter dropped from any height on Earth accelerates towards the planet’s surface at the constant rate of about 9.8m/s each second – a mathematical constant used in physics calculations, known as acceleration due to gravity g.
Now, with the latest groundbreaking study, physicists say, this value for antihydrogen – accounting for errors in the experiment – “is consistent with a downward gravitational acceleration of 1g” or about“32 feet per second per second”.
Graphic shows antihydrogen atoms falling and annihilating inside a magnetic trap, part of the ALPHA-g experiment at CERN to measure the effect of gravity on antimatter.
(U.S. National Science Foundation)
The experiment, which has never been done before, is a milestone in physics marking “a leap forward” in the world of antimatter research, they say.
Researchers used the new ALPHA-g apparatus in operation at CERN – Europe’s largest physics laboratory.
In the experiment, physicists created antimatter and trapped the neutral antihydrogen atoms in a magnetic bottle, making the environment as cold as possible.
They then released the antihydrogen within the vertical apparatus to witness and measure its gravitational behaviour under free-fall.
Scientists observed the physical properties of the antihydrogen by making precise measurements, including of its charge and colour spectrum.
The milestone study is the first step in taking precise measurements of the gravitational properties of antimatter to determine whether antimatter falls in the exact same way as matter.
“Here we show that antihydrogen atoms, released from magnetic confinement in the ALPHA-g apparatus, behave in a way consistent with gravitational attraction to the Earth,” scientists wrote in the study.
Researchers have also ruled out “repulsive ‘antigravity’” in this case – findings that can help better understand the lack of antimatter observed in the universe.
“We know there’s a problem somewhere in quantum mechanics and gravity. We just don’t know what it is. There has been a lot of speculation on what happens if you drop antimatter, though it’s never been tested before now because it’s so hard to produce and gravity is very weak,” Timothy Friesen, another author of the study, said.
The new finding, according to scientists, allows for more precise studies of the magnitude of the acceleration of anti-atoms under the influence of Earth’s gravitational force.
Antimatter Reacts to Gravity in the Same Way as Ordinary Matter, Physicists Find
The experiment, which has never been done before, is a milestone in physics marking “a leap forward” in the world of antimatter research, they say.
Researchers used the new ALPHA-g apparatus in operation at CERN – Europe’s largest physics laboratory.
In the experiment, physicists created antimatter and trapped the neutral antihydrogen atoms in a magnetic bottle, making the environment as cold as possible.
They then released the antihydrogen within the vertical apparatus to witness and measure its gravitational behaviour under free-fall.
Scientists observed the physical properties of the antihydrogen by making precise measurements, including of its charge and colour spectrum.
The milestone study is the first step in taking precise measurements of the gravitational properties of antimatter to determine whether antimatter falls in the exact same way as matter.
“Here we show that antihydrogen atoms, released from magnetic confinement in the ALPHA-g apparatus, behave in a way consistent with gravitational attraction to the Earth,” scientists wrote in the study.
Researchers have also ruled out “repulsive ‘antigravity’” in this case – findings that can help better understand the lack of antimatter observed in the universe.
“We know there’s a problem somewhere in quantum mechanics and gravity. We just don’t know what it is. There has been a lot of speculation on what happens if you drop antimatter, though it’s never been tested before now because it’s so hard to produce and gravity is very weak,” Timothy Friesen, another author of the study, said.
The new finding, according to scientists, allows for more precise studies of the magnitude of the acceleration of anti-atoms under the influence of Earth’s gravitational force.
Antimatter Reacts to Gravity in the Same Way as Ordinary Matter, Physicists Find
Isaac Schultz
Wed, September 27, 2023
In the 95 years we’ve known about antimatter, physicists have not tested how the elusive inverse of ordinary matter is affected by gravity, the force that pulls masses to Earth and seems to affect all things in the classical realm.
Now, a group of physicists have. Members of the Antihydrogen Laser Physics Apparatus (ALPHA) collaboration at CERN directly observed antihydrogen—hydrogen’s antimatter foil—free-falling in a container. The observations confirm the weak equivalence principle set forth by Einstein in his general theory of relativity, which holds that all masses react the same way to gravity, regardless of their composition. The team’s research was published today in Nature.
“In modern physics, inertial mass is encoded in the standard model of particle physics, whereas gravitational mass is dealt with in Einstein’s general theory of relativity,” wrote Anna Soter, a physicist at ETH Zürich, in an accompanying News & Views article. “The assumed equivalence of inertial mass and gravitational mass is incorporated in the weak equivalence principle, which is the cornerstone of general relativity—but no proposal has yet succeeded in unifying the theories.”
Plans to test gravity’s force on antimatter were first made by 2018, as Gizmodo reported at the time, when the ALPHA collaboration built ALPHA-g, a magnetic trap for antihydrogen atoms. The antimatter particles are suspended and then dropped inside the trap, in a 21st-century equivalent to the story of Galileo Galilei dropping objects off the Leaning Tower of Pisa.
The antimatter particles are cooled in order to slow their movement, giving the physicists precious time to observe and measure them. In 2021, the ALPHA collaboration announced that they managed to cool antihydrogen to near-absolute zero. “Slowing down the motion of antiatoms allows us to perform more precise measurements on its properties. In daily life, you can imagine things moving fast are harder to see than things moving slowly,” Makoto Fujiwara, a particle physicist with Canada’s TRIUMF particle accelerator team, told Gizmodo at the time. “The same thing happens in quantum physics…. The more time you have to observe a certain property, the more precise your measurement.”
As the cooled antihydrogen escaped the magnetic trap, the particles entered a vertical vacuum chamber; if the particles came in contact with the sides of the chamber or its ends (i.e., regular matter) they were annihilated. The researchers fiddled with the strength of the magnetic field, but found that when the fields were balanced on either side of the chamber, about 80% of the antimatter particles annihilated towards the bottom of the trap. Which is to say, gravity got them there.
It’s a good thing that gravity influences matter the same way it does the ordinary stuff—if it didn’t, it would mean that physicists were missing something pretty fundamental to the standard model. It’s not yet clear if antimatter was affected by gravity in the exact same way as ordinary matter, but the very fact that it’s affected affirms both Einstein and the standard model.
But mysteries remain. The magnitude of the gravitational acceleration on the antihydrogen was not precisely measured, but set the foundations for such future experiments. That can be done with even colder atoms—and better understand the weak equivalence principle. And beyond—which is to say below—the masses of the smallest particles, there is the quantum realm, which seems to be beyond the influence of gravity, at least as we know it.
Put in perspective, it seems that this confirmation of something long suspected is just a baby step in physicists’ understanding of the forces of nature and particle physics. But the baby steps are the most important ones to take.
More: Antimatter Could Travel Through Our Galaxy With Ease, Physicists Say
Gizmodo
