New experiment confirms that gravity pulls antimatter down

In a recent experiment conducted by physicists at the European Center for Nuclear Research (CERN) in Geneva, Switzerland, the hope that antimatter might defy gravity and levitate has been definitively dispelled. This experiment focused on antihydrogen, composed of an anti-proton and an antielectron (positron), and its response to gravity.

The results, published in the September 28th issue of the journal Nature, conclusively demonstrate that antimatter, like ordinary matter, is subject to gravitational attraction and falls downward. The gravitational acceleration observed for antimatter is nearly identical to that of normal matter on Earth, approximately 1 g, or 9.8 meters per second per second (32 feet per second per second), within about 25% (one standard deviation) of normal gravity.

Joel Fajans, a professor of physics at UC Berkeley, one of the experiment’s pioneers, commented, “It surely accelerates downwards, and it’s within about one standard deviation of accelerating at the normal rate. The bottom line is that there’s no free lunch, and we’re not going to be able to levitate using antimatter.”

This outcome aligns with Albert Einstein’s theory of general relativity, which treats all types of matter equally in gravitational interactions. It was a widely expected result, as antimatter and matter, such as protons, neutrons, and electrons, exhibit opposite electrical charges and annihilate each other upon contact.

The experiment marks the first direct measurement of the force of gravity on neutral antimatter, advancing the field of neutral antimatter science. Although some had speculated that antimatter might behave differently under gravity, there is no theoretical basis for antimatter to have repulsive gravity. Such a scenario could lead to perpetual motion machines, which are theoretically impossible.

While the experiment didn’t yield the tantalizing prospect of antimatter defying gravity, the idea was intriguing because it could have explained cosmic mysteries, such as the apparent scarcity of antimatter in the observable universe. Most theories predict an equal production of matter and antimatter during the Big Bang, yet we observe a predominance of matter.

An artist’s conceptual rendering of antihydrogen atoms falling out the bottom of the magnetic trap of the ALPHA-g apparatus. As the antihydrogen atoms escape, they touch the chamber walls and annihilate. Most of the annihilations occur beneath the chamber, showing that gravity is pulling the antihydrogen down. The rotating magnetic field lines in the animation represent the invisible influence of the magnetic field on the antihydrogen. The magnetic field does not rotate in the actual experiment. Credit: Keyi “Onyx” Li/U.S. National Science Foundation

Gravity is incredibly weak

Joel Fajans, a physicist, highlights that previous experiments suggesting antimatter behaves under gravity have been indirect and subtle. He explains that conducting a simple “leaning tower of Pisa” experiment with antimatter isn’t feasible because gravitational forces are exceedingly weak compared to electrical forces. Even a slight electric field could deflect a charged particle like a positron far more than gravity would.

In the realm of fundamental forces, gravity is the weakest. While it plays a dominant role in the cosmos due to its universal reach, its impact on tiny pieces of antimatter is almost negligible. To put this in perspective, an electrical field of just 1 volt/meter exerts a force on an antiproton that is around 40 trillion times stronger than Earth’s gravitational pull on it.

The ALPHA collaboration at CERN proposed a novel approach. By 2010, they had managed to trap significant quantities of antihydrogen atoms. Physicist Jonathan Wurtele suggested that since antihydrogen is electrically neutral, it should be impervious to electric fields, making it suitable for gravity measurements.

Initially skeptical, Fajans eventually took the idea seriously and conducted simulations that supported Wurtele’s concept. The team, with the involvement of UC Berkeley’s Andrew Charman and Andrey Zhmoginov, reanalyzed prior data and concluded that antihydrogen’s gravitational interaction with Earth was no more than 100 times the acceleration of regular matter, whether upwards or downwards.

This modest finding spurred the ALPHA team to construct the ALPHA-g experiment in 2016. In 2022, they began conducting measurements, leading to the results published in Nature. These results suggest that the gravitational constant for antimatter is approximately 0.75 ± 0.29 g, which falls within the error bars of 1 g, indicating that gravity for antimatter is not repulsive.

UC Berkeley postdoctoral fellow Danielle Hodgkinson, right, running the ALPHA-g experiment from the control room at CERN in Switzerland. Credit: Joel Fajans, UC Berkeley

Notably, numerous UC Berkeley undergraduate physics majors participated in the experiment’s assembly and operation, offering valuable learning experiences and diversifying the field of physics.

Joel Fajans remarked, “It’s been a great opportunity for many Berkeley undergraduates. They’re fun experiments, and our students learn a lot.”

A balance

The plan for ALPHA-g, as envisioned by Wurtele and Fajans, aimed to confine approximately 100 antihydrogen atoms within a 25-centimeter-long magnetic containment vessel. An extraordinary condition was required: these antiatoms had to be kept at temperatures just half a degree above absolute zero, or 0.5 Kelvin.

Despite this extreme cold, the antihydrogen atoms remained in motion, with an average speed of around 100 meters per second. They bounced off powerful magnetic fields at the bottle’s ends, driven by the repulsion of their magnetic dipole moments by the tightly pinched 10,000 Gauss magnetic fields at either end.

When the bottle was set in a vertical orientation, the atoms moving downward were influenced by gravity, accelerating them. Conversely, those moving upward were decelerated. When the magnetic fields at both ends were perfectly balanced, the downward-moving atoms had, on average, more energy. Consequently, they were more likely to escape through the magnetic mirror, leading to their annihilation within the container, resulting in the emission of three to five pions. Detecting these pions allowed the determination of whether the antiatoms escaped upward or downward.

Fajans likened the experiment to a conventional balance used to compare very similar weights. The magnetic balance, in this case, made the relatively minute gravitational force observable in the presence of much stronger magnetic forces, akin to how a standard balance reveals the difference between 1 kilogram and 1.001 kilograms.

To further fine-tune the experiment, the researchers gradually reduced the strength of the bottom magnetic mirror so that all the antihydrogen atoms eventually escaped. If antimatter behaved like regular matter, roughly 80% of the antiatoms should exit from the bottom, validating the influence of gravity.

The experimental setup allowed ALPHA to adjust the magnetic strength differently for the top and bottom mirrors, providing a boost in energy to each antiatom that could counteract or exceed the effects of gravity. This control served as a crucial aspect, ensuring the experiment’s reliability.

Given the numerous unknown variables, the results had to be approached statistically. Uncertainties included the exact number of trapped antihydrogen atoms, the completeness of annihilation detection, the potential presence of unaccounted magnetic fields, and the accuracy of magnetic field measurements within the bottle.

An artist’s conceptual rendering of the antihydrogen atoms contained within the magnetic trap of the ALPHA-g apparatus. As the field strength at the top and bottom of the magnetic trap is reduced, the antihydrogen atoms escape, touch the chamber walls and annihilate. Most of the annihilations occur beneath the chamber, showing that gravity is pulling the antihydrogen down. Credit: Keyi “Onyx” Li/U.S. National Science Foundation

Jonathan Wurtele emphasized, “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.”

The UC Berkeley physicists anticipate significant enhancements to ALPHA-g and its computational models, potentially increasing sensitivity by a factor of 100.

Fajans acknowledged the collaborative effort, stating, “This result is a group effort, although the genesis of this project was at Berkeley. ALPHA was designed for spectroscopy of antihydrogen, not gravitational measurements of these antiatoms. Jonathan’s and my proposal was completely orthogonal to all the plans for ALPHA, and the research would likely not have happened without our work and years of lonely development.”

While some might find the null result unexciting, the experiment is a crucial test of general relativity, which, up to this point, has withstood all other challenges.

As Wurtele aptly put it, “Physics is an experimental science. You don’t want to be the kind of stupid 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 undergraduate students who actively participated in the project include Josh Clover, Haley Calderon, Mike Davis, Jason Dones, Huws Landsberger, Nicolas Kalem James McGrievy, Dalila Robledo, Sara Saib, Shawn Shin, Ethan Ward, Larry Zhao, and Dana Zimmer.

Source: University of California - Berkeley

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