Scientists have known about antimatter for more than 90 years, and they have learned a vast amount about the way it behaves. However, the question of how gravity influences antimatter had never been answered—until now.
Certainly, antimatter ought to fall down towards Earth the way ordinary matter does in the presence of gravity. Or as science fiction suggests, antimatter might “fall up,” but ultimately, the only way to know for sure was to actually observe what happens.
The problem is antimatter is exceptionally rare, and in the presence of matter, antimatter and matter annihilate one another. Creating the conditions to observe antimatter and gravity together are extremely challenging.
Now finally, after years of working to eliminate all the obstacles, Simon Fraser University (SFU) Distinguished Professor (emeritus) of physics Mike Hayden and a team of international researchers directly observed the way gravity influences antihydrogen atoms.
It turns out that antimatter falls down, toward the earth. It does not “fall up” into the sky.
Hayden is a senior investigator with the Antihydrogen Laser Physics Apparatus—ALPHA Project, an international collaboration of physicists dedicated to studying the properties of antihydrogen—the antimatter counterpart of the ordinary hydrogen atom. He has worked with the project for close to two decades and contributed to numerous discoveries.
The most recent ALPHA publication, Observation of the effect of gravity on the motion of antimatter, featured in Nature, was not only a radical discovery, it was also the top-mentioned article for SFU in 2023, according to Altmetric, a weighted count of trending media and social media.
The breakthrough was featured in hundreds of news articles from dozens of media outlets around the world.
We spoke with professor Hayden about his research.
Why is it so challenging to directly observe antimatter?
It turns out that “dropping” antimatter is an exceedingly difficult proposition. For starters, one needs to use neutral antimatter, so that the gravitational force isn’t entirely swamped by much stronger electrostatic forces.
Antimatter is very scarce, and neutral antimatter is much more so; it needs to be made from scratch. Then there is the problem of handling: matter and antimatter annihilate one another when they come into contact, and so you cannot just “pick up the antimatter and drop it.”
How did your research team create the conditions to observe antimatter and gravity? What was their particular role in the project?
We make our antihydrogen atoms inside a special magnetic bottle which keeps them away from the walls of the apparatus. Inside this bottle they rattle around every which way. To release them, we open both the top and the bottom of the bottle at the same time, by turning off some of the magnetic fields.
As we do this, atoms spill out; some go up and some go down. But if we make the openings at the top and the bottom the same, we see more escape through the bottom of the bottle than through the top.
This is one of the places where SFU played a key role. Our expertise is in measuring magnetic fields very precisely, using magnetic resonance. And by performing magnetic resonance measurements every single time that atoms are released, we can quantify how well the magnetic fields at the top and the bottom of the bottle are matched. Without this information, we would never be able to tease out the influence of gravity.
This research is the latest in a series of breakthroughs by ALPHA. What is the next line of inquiry now that you have solved the question of antimatter and gravity?
Our goal is to learn as much as possible about the properties of antihydrogen atoms. And one of the ways we think we can learn an enormous amount about them is through spectroscopy, measuring the frequency of radiation they emit and absorb as they transition between different energy levels. We are already hard at work on this, and hope to have more news to report soon.
During your career as a physicist, you have worked on some major discoveries. What has surprised or delighted you most about this field? Will you continue to work with ALPHA?
Hydrogen is the simplest atom, but for more than a century it has played a foundational role in our understanding of physics. It is an absolute delight to be able to study the simplest antimatter atom in the same way. On the one hand we have the opportunity to replicate and rethink some of the truly classic experiments of physics, which in of itself is a joy. On the other hand, the same experiments are the first of their kind, and are opening a new frontier for study. I am certain I will continue to work with ALPHA for years to come.
For more about Mike Hayden’s work read the SFU News story, Simon Fraser University researchers involved in major international antimatter breakthrough, and the Scholarly Impact of the Week, Antimatter matters.
SFU's Scholarly Impact of the Week series does not reflect the opinions or viewpoints of the university, but those of the scholars. The timing of articles in the series is chosen weeks or months in advance, based on a published set of criteria. Any correspondence with university or world events at the time of publication is purely coincidental.
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