Force of Earth's gravity on antimatter is measured for the first time

A team of researchers led by the Antihydrogen Laser Physics Apparatus (ALPHA) collaboration at CERN in Geneva, Switzerland has recently investigated experimentally the behavior of antimatter, showing that, much like regular matter, antimatter is subject to gravity’s pull. 

Thus, the notion of antimatter exhibiting antigravity properties, which has already been argued to be theoretically improbable, has finally been dispelled by this study. 

Gravitational acceleration

According to the experts, the gravitational acceleration of antihydrogen is strikingly close to that of regular matter on Earth, nearly 9.8 meters per second per second. 

“It surely accelerates downwards, and it’s within about one standard deviation of accelerating at the normal rate,” said co-author Joel Fajans, a physicist at the University of California, Berkeley who, alongside theorist Jonathan Wurtele (the current study’s senior author), first proposed the experiment over ten years ago. 

“The bottom line is that there’s no free lunch, and we’re not going to be able to levitate using antimatter.”

Theory of relativity 

These findings are unlikely to astonish the majority of physicists. Albert Einstein’s theory of general relativity, formulated before the identification of antimatter in 1932, does not discriminate between forms of matter, indicating that both antimatter and matter should exhibit identical responses to gravitational forces. 

Every component of regular matter, such as protons, neutrons, and electrons, possesses counterparts bearing the opposite electrical charge, and interaction with their regular matter counterparts results in mutual annihilation.

“The opposite result would have had big implications; it would be inconsistent with the weak equivalence principle of Einstein’s general theory of relativity,” explained Wurtele. 

“This experiment is the first time that a direct measurement of the force of gravity on neutral antimatter has been made. It’s another step in developing the field of neutral antimatter science.”

Fajans pointed out that there is no prevailing physical theory suggesting that gravity should repel antimatter. In fact, some physicists have argued that, if gravity were repulsive to antimatter, it could facilitate the creation of a perpetual motion machine – a concept deemed theoretically unfeasible.

Cosmic mysteries

Despite this, the hypothesis that gravity might interact differently with matter and antimatter had sparked interest, as it offered a potential explanation for several cosmic mysteries. 

For instance, it might have accounted for the spatial segregation of matter and antimatter in the early cosmos, explaining the observed scarcity of antimatter in our observable universe, particularly since most theories postulate that the Big Bang should have given rise to equivalent quantities of matter and antimatter.

According to Fajans, although there have been many indirect experiments suggesting that antimatter gravitates normally, they have been relatively subtle, failing to definitively prove this point.

Measuring gravity

“Why not do the obvious experiment and drop a piece of antimatter, a sort of leaning tower of Pisa experiment? You know, the experiment that Galileo didn’t actually do – it was apocryphal – where he supposedly dropped a lead ball and a wooden ball from the top of the tower and proved that they both reached the ground at the same time,” he said.

“The real problem is that the gravitational force is incredibly weak compared to electrical forces. So far, it has proved impossible to directly measure gravity with a drop-style measurement with a charged particle, like a bare positron, because any stray electric field will deflect the particle much more than gravity will.”

Forces of nature

Indeed, the gravitational force is the most feeble among the four known forces of nature. Although it dominates the evolution of the universe due to its universal impact over vast distances, on a minuscule piece of antimatter, its effect is negligible. 

An electrical field of 1 volt/meter exerts a force on an antiproton that is approximately 40 trillion times greater than force of gravity exerted on it by the Earth.

A new approach 

To overcome these experimental limitations, the scientists devised a new methodological approach. By 2010, significant quantities of antihydrogen atoms were being trapped by the ALPHA team, prompting Wurtele in 2011 to persuade Fajans that, given antihydrogen’s charge neutrality, it remains unaffected by electric fields, thus warranting an exploration of the possibility of a gravity measurement. 

Although Fajans was initially skeptical, he eventually agreed to conduct simulations to test Wurtele’s idea. Further contributions from UC Berkeley lecturer Andrew Charman and postdoctoral fellow Andrey Zhmoginov led to an analysis concluding that antihydrogen experiences no more than about 100 times the acceleration due to Earth’s gravity compared to ordinary matter.

Despite its modest beginnings, this analysis encouraged the ALPHA team to devise a more accurate experiment. 

In 2016, supported by various international sources, including the National Science Foundation, the Department of Energy, the Canadian government, and the Danish brewer Carlsberg, they began to construct a new experiment, ALPHA-g, which conducted its first measurements in 2022.

Gravity does not repel antimatter 

The subsequent results are derived from simulations and statistical evaluation of the observations made in the previous year, estimating the gravitational constant for antimatter at 0.75 ± 0.13 ± 0.16 g, or, accounting for both statistical and systematic errors, 0.75 ± 0.29 g, which aligns with the expected 1 g within error bars. This experiment revealed that the change of gravity being repulsive for antimatter is so small as to be meaningless.

In the proposed ALPHA-g project by Wurtele and Fajans, the objective was to trap approximately 100 antihydrogen atoms within a magnetic bottle measuring 25 centimeters in length. 

The ALPHA apparatus has the capability to confine antihydrogen atoms which possess a temperature just below half a degree above absolute zero, or 0.5 Kelvin. 

Gravity’s pull

Surprisingly, even at such low temperatures, the antiatoms are in motion at an average velocity of 100 meters per second, repetitively colliding with the powerful magnetic fields situated at the bottle’s extremities, with the magnetic dipole moment of an antihydrogen atom being repelled by the pinched 10,000 Gauss magnetic fields at each end of the bottle.

In a scenario where the bottle is positioned vertically, gravity causes the descending atoms to speed up, while the ascending ones slow down. 

When the magnetic fields at both ends are equivalent or balanced, the atoms moving downward generally possess more energy. 

Consequently, they have a higher likelihood of breaking through the magnetic mirror, colliding with the container, annihilating in a flash of light and producing three to five pions. The detection of these pions aids in discerning the direction in which the antiatom escaped, be it upwards or downwards.

“The balancing allows us to ignore the fact that the antiatoms are all of different energies. The lowest energy ones escape last, but they’re still subject to the balance, and the effect of gravity is enhanced for all antiatoms,” he explained.

Gradually reducing the mirror magnetic fields enables the escape of all atoms. If antimatter exhibits behavior identical to normal matter, a majority of the antiatoms – around 80 percent – are anticipated to escape through the bottom rather than the top.

Powerful experimental tool

Additionally, the experimental design of ALPHA permits alterations to the strength of the bottom magnetic mirror relative to the top one. This modification influences the energy of each antiatom, potentially counteracting or overpowering gravitational effects, which results in an equal or greater number of antiatoms exiting through the top as opposed to the bottom. 

“This gives us a powerful experimental knob that allows us, basically, to believe the experiment actually worked because we can prove to ourselves that we can control the experiment in a predictable manner,” Fajan said.

Challenges of uncertainty 

Due to several uncertainties, the experiment’s outcomes necessitated statistical analysis. The investigators faced challenges including uncertainty in the quantity of trapped antihydrogen atoms, detection of every annihilation, potential presence of unidentified magnetic fields influencing the trajectories of antiatoms, and accurate measurement of the magnetic field within the bottle. 

“ALPHA’s computer code simulating the experiment could be subtly wrong because we don’t know the precise initial conditions of the antihydrogen atoms, it could be wrong because our magnetic fields aren’t correct, and it could be wrong for some unknown unknown,” Wurtele explained. “Nonetheless, the control provided by adjusting the balance knob lets us explore the extent of any discrepancies, giving us confidence that our result is correct.”

Future research 

Anticipating future enhancements to both the ALPHA-g apparatus and the associated computer codes, the experts are optimistic about a hundredfold increase in the instrument’s sensitivity. 

Despite the seemingly ordinary outcome, the importance of the experiment lies in its role as a critical examination of general relativity, a theory that has successfully withstood all previous tests. 

“If you walk down the halls of this department and ask the physicists, they would all say that this result is not the least bit surprising. That’s the reality. But most of them will also say that the experiment had to be done because you never can be sure,” said Wurtele.

Physics is an experimental science. You don’t want to be the kind of ignorant that you don’t do an experiment that explores possibly new physics because you thought you knew the answer, and then it ends up being something different.” 

The study is published in the journal Nature.

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