In a major breakthrough, scientists have proved that antimatter, an ultra-rare counterpart to regular matter, obeys the same laws of gravity as other masses in the universe. A new study reports that the discovery confirms Albert Einstein’s general relativity theory and solves a mystery regarding antimatter’s motion in gravitational field.
Researchers with the Antihydrogen Laser Physics Apparatus (ALPHA) collaboration, an experiment based at the European Center for Nuclear Research (CERN), observed far more antimatter atoms falling out of the bottom of a vertically oriented machine than the top, indicating that these bizarro particles are subject to the gravitational pull of Earth. These results disprove fringe theories about a force that was called “antigravity”, which could be used to enable levitation or other strange processes.
Antimatter atoms, or anti-atoms, are composed of subatomic particles that have the opposite charges of the regular atoms we experience in our daily lives. Whereas normal atoms are composed of positively-charged protons and negatively-charged electrons, antimatter contains positively-charged versions of electrons called positrons, and negatively-charged versions of protons called antiprotons. When antimatter and matter collide, they annihilate each other in catastrophic microexplosions that can be detected by extremely precise equipment, such as the ALPHA experiment.
Einstein’s theory of general relativity, which he formulated more than a century ago, includes a concept called the weak equivalence principle (WEP), which predicts that all objects with mass are subject to the laws of gravity. Most scientists have presumed that the WEP extends to antimatter, but it has never been demonstrated experimentally before because antimatter is extremely finicky stuff to work with, even in the world’s most sophisticated laboratories.
Now, ALPHA collaboration members have snagged the first direct measurement of the force of gravity on neutral antimatter with their special ALPHA-g experiment, beating out other competitors at CERN who were racing to achieve the same milestone. According to a Wednesday study in Nature, the team reported that the “motion of antimatter within the Earth’s gravitational field finally has a promising experimental foothold.” This breakthrough “paves way for precision measurements of the gravity acceleration between antiatoms and Earth in order to test the WEP.”
“Everything about gravity and antimatter until now has been speculation,” said Jeffrey Hangst, an experimental particle physicist at Aarhus University and founder and spokesperson of the ALPHA collaboration, in a call with Motherboard. “It’s based on some model, and some calculation that you do based on a theory. That’s an important part of physics, but you don’t know anything until you actually observe it.”
“That’s what’s important here,” he continued. “We can actually now say that, as far as we can tell within the uncertainty of the experiment, [antimatter] behaves as it should under general relativity. That’s a very profound statement to be able to make because if you go online, you would find literally hundreds of thought experiments trying to say what should happen with gravity and antimatter because there has never been a direct measurement.”
Antimatter is extremely rare in the universe, a situation that is considered to be one of the biggest mysteries in particle physics and cosmology. The Big Bang theory suggests that both matter and antimatter were produced at the same rate. It is unclear, however, why the matter was so abundant. The ALPHA collaboration is part of a worldwide effort to shed light on this question, and others, by forging antimatter in laboratories, and studying its properties, which is no easy task.
“Antimatter is this really horrible stuff that you have to make, and as soon as you make it, the entire environment is conspiring against you to kill it,” Hangst said. Antimatter can never be near regular matter. So, you first have to produce it, then you have to hold on to it and keep it away from normal matter, and you have to learn how to manipulate it. All of that is really more than 30 years of work.”
While the researchers had previously focused on mapping out the internal structures of antimatter, their ALPHA-g experiment was designed to isolate the effects of gravity on these strange particles. Instead of their normal horizontally-oriented device, Hangst and his colleagues built a tall vertical machine that trapped antihydrogen particles, released them at the top of the apparatus, and measured how many fell through the bottom instead of the top.
After delays due to COVID and other projects, the team was finally able to run this experiment in 2022. The results revealed that most of the antihydrogen accelerated downward through the bottom of the device at close to 1 g, which is the rate that normal matter freefalls on Earth. According to the WEP, the antimatter follows the gravity laws. This is in keeping with what was expected, but it’s a vital step to understanding antimatter and its relationship with the cosmos.
“As experimenters, we must approach this with an open-mindedness,” Hangst said. “We built a machine that would measure any possible outcome, and we went to great lengths to do that. Even in the one-in-a-million chance that it would go up [out of the apparatus], we would have been able to tell that, but nobody seriously thought that that was a likely outcome of this experiment. You write about it, just to rule it out.”
“But how cool would it have been?” he added.
Now that researchers have demonstrated that antimatter can be subjected to gravity, the hope is to take even more accurate measurements of its behavior in freefall. However, those experiments won’t begin until 2024; in the meantime, Hangst said the collaboration is content to enjoy the fruits of their labor with ALPHA-g.
“We’re ecstatic obviously; we don’t like coming in second,” he said. “We’re really happy to get there first. For me, it’s been 30 years of doing this kind of thing, with every step necessary to get to this stage. It’s really gratifying to have such a great team that can pull this off the way we’ve done. We love this, it’s what we live for–to be the first to see something like that.”
“We’ve opened up a whole new avenue of inquiry here, where now we know what to do, how to do it, and we hope to improve it,” Hangst concluded.
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