We all know an apple will fall down because of the Earth’s gravity. But what about an “anti-apple”?
For the first time, scientists have “dropped” antimatter in the form of anti-hydrogen atoms, and found that antimatter feels the same downward pull of the Earth’s gravity.
“In physics, you don’t really know something until you observe it,” said Prof. Jeffrey Hangst in a news release from CERN, the European Organization for Nuclear Research, on Wednesday.
“This is the first direct experiment to actually observe a gravitational effect on the motion of antimatter. It’s a milestone in the study of antimatter, which still mystifies us due to its apparent absence in the universe.”
Most physicists had expected antimatter to fall based on Einstein’s general theory of relativity, which this experiment confirms once again. However, there were a few that theorized antimatter would experience “repulsive gravity” and fall up.
Based on the results of the new study, “we can rule out the existence of repulsive gravity of magnitude 1g [the force of gravity that we normally experience] between the Earth and antimatter,” the scientists wrote.
How the researchers dropped antimatter
Many of us have heard the story (which may or may not have actually happened) of the Italian astronomer Galileo’s 16th-century experiment, in which he dropped two spheres, one heavier than the other, from the tower of Pisa. Both landed at the same time, showing that the Earth’s gravity acts the same way on different masses.
Dropping antimatter isn’t as easy.
First of all, it annihilates on contact with matter, which is why so little of it exists on Earth.
Over a decade ago, the ALPHA collaboration figured out how to create and trap anti-hydrogen atoms using magnetic fields to keep it away from the sides of a container. That allowed them to keep dozens of them in existence for up to half an hour and precisely measure properties such as their “atomic fingerprints” or their charge — so far identical to their matter counterparts, hydrogen atoms.
But Makoto Fujiwara, senior scientist at TRIUMF in Vancouver and spokesperson for the Canadian scientists in ALPHA, said from the beginning that measuring the effect of gravity on antimatter was their “dream.”
The problem is that gravity is a very weak force compared to other forces on objects like atoms with a very small mass, Fujiwara said. “So it’s been really very difficult to measure any effect directly of gravity acting on antimatter.”
The researchers had to develop a huge new experimental setup with very precisely controlled magnets and a new detector.
“The whole thing is about eight metres [tall],” Fujiwara said. “So it’s quite a tower if you stand on the bottom.”
The setup allowed the atoms to be suspended in a vertical cylinder using the magnetic field. The scientists could maintain the field keeping the atoms away from the sides of the cylinder, but gradually weaken the magnetic field at the top and bottom so they would eventually feel the effects of gravity.
What the researchers saw
Detectors at the top and bottom could detect whether the anti-atoms fell up or down, annihilating on either end of the cylinder — showing results in real time.
“You can actually see in an experimental display whether each anti-atom came out of the bottom or came out of the top,” Fujiwara recalled. “So it was really so exciting when we did the first experiment that actually anti-hydrogen was falling down — and this is the first time that humankind was able to observe directly the effect of gravity on antimatter.”
The researchers repeated the experiment more than a dozen times, trying to get as precise a measurement of gravity as possible.
So far, it looks like the force of the Earth’s gravity is about the same on matter as antimatter.
What this means for physics
Einstein’s general theory of relativity had predicted this result, so it’s what most physicists had expected.
But a few had suggested antimatter might experience repulsive gravity towards matter. This was considered a possible explanation for why we observe so little antimatter in the universe — it might be pushed to distant parts of the universe, and that might be one explanation for why the universe is expanding more quickly. (The conventional explanation is that it’s caused by something mysterious called dark energy.)
Fujiwara called the theory “very speculative” and “not mainstream,” but said some theorists had been studying it for many decades. “Unfortunately, these people now have to go back to the drawing board, because we for the first time experimentally ruled out that.”
Massimo Villata, an astrophysicist at the Observatory of Turin in Italy, had published work on that theory, but stopped working on it in 2015.
“I had stopped precisely because I was waiting for the results of the CERN experiments,” he said in an email. “These solve a doubt I’ve always had.”
Villata’s theory says there are two kinds of antimatter, but only one has repulsive gravity. He says the CERN experiments confirm his doubt — that the kind with repulsive gravity can’t exist on Earth, except perhaps in trace amounts. However, he said it doesn’t rule out the fact that it could exist in space.
“In my opinion ALPHA is the best antimatter experiment in the world (I would say that they deserve the Nobel prize),” he wrote, “and there is no reasonable doubt in their experimental result that hydrogen and anti-hydrogen fall in the same way in the gravitational field of the Earth.”
However, he also thinks it doesn’t rule out the existence of repulsive gravity in other scenarios.
In an analysis piece accompanying the new study in Nature, Anna Soter, a physics professor at ETH Zurich, suggested the new study is just the latest step in the exploration of gravity that (so the story goes) started with an apple falling on English scientist Sir Isaac Newton’s head in the 17th century. She noted the gravity experiments performed so far on antimatter have involved types of particles called “hadrons” and still need to be done on other kinds of particles.
“Such atoms would provide Newton’s orchard with ever more exotic varieties of apple, providing scope for discovering previously unknown physics beyond gravity.”
Fujiwara acknowledged that the most recent measurement is just the beginning.
Next, the ALPHA collaboration is going to try cooling and slowing down the anti-hydrogen with a laser the Canadian members of the team developed a few years ago. That should allow for even more precise measurements.
But for a moment, they’re basking in the current results.
Fujiwara said, “Creating and holding and dropping antimatter — and seeing it drop — was really a dream for the physicists in the field for many, many years. So it’s a big deal.”