Gravity pulls antimatter down just like ordinary matter, a new experiment shows. The finding won’t shock many physicists. But it does put a damper on some offbeat theories that, in order to solve some of cosmology’s biggest mysteries, posit that gravity pushes rather than pulls on antimatter—so that the stuff is subject to “antigravity.”
“I’m not surprised,” says Alan Kostelecky, a theoretical physicist at Indiana University Bloomington. “On the other hand, this is the kind of thing people take for granted, and such things need to checked.” Gabriel Chardin, a cosmologist with CNRS, France’s national research agency, says, “It’s a beautiful experiment by outstanding people” and “a blow” to speculative theories that assume antimatter experiences antigravity—but not yet a fatal wound.
According to the so-called equivalence principle, in a gravitational field all objects fall at the same rate regardless of what they’re made of. Galileo Galilei demonstrated the principle by rolling balls of different materials down a small ramp (not, as legend has it, by dropping them from the Leaning Tower of Pisa). Starting from the principle, Albert Einstein deduced that gravity arises as massive objects warp space and time, a foundation of his 1915 general theory of relativity. Until now, however, nobody had tested whether the equivalence principle holds for matter and antimatter.
To find out, physicists working with the Antihydrogen Laser Physics Apparatus (ALPHA) at the European particle physics laboratory CERN performed an updated version of Galileo’s apocryphal drop test. First, the team used an electric field to catch antiprotons and antielectrons generated in particle collisions. They coaxed those particles to form antihydrogen atoms, which they captured with a magnetic trap built around the electric one. Crudely speaking, they then released the antiatoms, about 100 in each trial, to see whether they fell down or up.
The details of the experiment are more complicated, says Jeffrey Hangst, a physicist at Aarhus University and leader of the 71-member ALPHA team. The antihydrogen atoms are relatively hot and fast moving, and prone to fly out the top and the bottom of the tall cylindrical trap, making the effect of gravity hard to discern. What’s more, even a tiny stray magnetic field could send a disproportionate number out the top of the trap, creating a spurious antigravity signal.
To control for such effects, the ALPHA team used the magnetic fields that trap the antiatoms to shove them slightly, up or down, as they were released. The technique “gives us a knob to turn where we can add an extra force, either in the same direction or against gravity, and control the outcome,” Hangst says. Over many trials, the researchers varied the direction and strength of that extra force and compared the fraction of atoms falling out of the bottom of the trap with results from a detailed simulation. The data agreed much better with simulations in which the antihydrogen experiences ordinary gravity than those in which they experienced antigravity or no gravity at all, the researchers report today in Nature.
Quantitatively, the experiment indicates that antimatter experiences a pull from gravity that’s 75% as strong as that on ordinary matter, give or take 20%—a statistical agreement between the two. Hangst says 99.9% of physicists would have predicted the result. Still, he notes, “You have to do the experiment with an open mind.”
Did anyone seriously consider the possibility that antimatter would experience antigravity? In fact, yes. For example, in 2012 Chardin and a colleague hypothesized that the universe might contain equal amounts of matter and antimatter, with the latter subject to antigravity. That idea might seem like a nonstarter, as astronomers haven’t observed antimatter galaxies or knots of matter and antimatter explosively annihilating each other. But antigravity avoids that problem, Chardin says, and it could also do away with two of the biggest puzzles in cosmology: the mysterious dark matter whose gravity keeps the galaxies intact and the even weirder dark energy that is stretching space and accelerating the expansion of the universe.
Subject to opposite forces, matter and antimatter would separate. Matter would clump to form galaxies. Antimatter would spread as thinly as possible between the galaxies and act like dark energy. Around the galaxies, moats of empty space would open that would prevent matter-antimatter annihilations and, oddly, mimic dark matter dynamically.
The new result might seem to torpedo Chardin’s model, as it rules out antigravity equal in strength to gravity. However, Chardin notes that all his theory really requires is antimatter that’s subject to some amount of repulsion, and the result isn’t precise enough to rule that out. “Pure antigravity is excluded,” Chardin says. “Beyond that I would be careful.”
Kostelecky points out that it’s relatively easy to construct a coherent theory in which the effects of gravity on matter and antimatter differ to any degree you like. So it will be important to boost the precision of experiments like ALPHA’s, he says. “I suspect that this experiment is just the first step in a research program.” Indeed, Hangst says the ALPHA team is already aiming to sharpen its measurement by, for example, cooling the antihydrogen atoms to within a fraction of a degree of absolute zero and slowing them down before releasing them.
