A recent experiment by the Antihydrogen Laser Physics Apparatus (ALPHA) collaboration at the European Center for Nuclear Research (CERN) confirms that antimatter does not levitate but falls under gravity, just like regular matter.
Researchers observed that antihydrogen, composed of an anti-proton and a positron, exhibits gravitational attraction, debunking the idea of antigravity for antimatter. The findings were published in the journal Nature on September 28, highlighting that antimatter follows the same gravitational laws as matter, closing the chapter on antigravity speculation for antimatter.
A general view of MEDICIS which is under construction during a behind the scenes tour at CERN, the World's Largest Particle Physics Laboratory on April 19, 2017 in Meyrin, Switzerland.
Gravitational Force Is Weaker Than Electrical Force
Joel Fajans, a UC Berkeley physics professor, and Jonathan Wurtele, a theoretician, initially proposed the experiment over a decade ago. Past experiments strongly suggested that antimatter behaves gravitationally like regular matter, but these tests were indirect and subtle due to the weakness of gravity compared to electrical forces, making direct measurements challenging.
Antimatter experiences minuscule gravitational effects compared to electrical forces; for instance, a 1 volt/meter electrical field has a force 40 trillion times greater on an antiproton than Earth's gravity.
The ALPHA collaboration at CERN explored the possibility of measuring antimatter's gravitational interaction. Simulations indicated the potential merit of this approach. A retrospective analysis of prior data suggested that antihydrogen experienced no more than about 100 times the acceleration of Earth's gravity in either the up or down direction compared to regular matter.
In 2016, the ALPHA team constructed a new experiment, ALPHA-g, with UC Berkeley undergraduates actively participating in its assembly and operation.
The experiment conducted its first measurements in 2022 and found that the gravitational constant for antimatter is approximately 0.75 g, with a small margin of error. This result essentially rules out the possibility of antimatter experiencing repulsive gravity, providing valuable insights into antimatter's behavior.
The experiment has offered valuable learning experiences to numerous UC Berkeley undergraduate physics majors, including those from underrepresented groups, while shedding light on fundamental questions about antimatter and gravity.
Antimatter's Interaction with Gravity During the ALPHA-g Experiment
In the ALPHA-g experiment, scientists trapped in a 25-centimeter-long magnetic container some 100 antihydrogen atoms.The ultra-low temperature of 0.5 Kelvin allowed for the confinement, even though the antihydrogen atoms moved rapidly within strong magnetic fields at each end of the bottle.
When oriented vertically, atoms moving downwards gained more energy due to gravity, making them more likely to escape through the magnetic mirror and annihilate upon hitting the container. This setup essentially acted as a balance, revealing the subtle effects of gravity amid dominant magnetic forces.
The process involved slowly reducing the magnetic mirror fields to allow all the antihydrogen atoms to escape. If antimatter behaved like regular matter, roughly 80% of the antihydrogen atoms should escape from the bottom of the container compared to the top.
The experimental setup allowed adjustments to the magnetic mirror strength, enabling precise control over the escape of antihydrogen atoms. Despite uncertainties in atom count, detection, magnetic fields, and measurements, statistical methods were applied to analyze the results.
UC Berkeley physicists hope to enhance ALPHA-g's sensitivity significantly through upcoming improvements, while the study also serves as a crucial test of general relativity, which has successfully passed previous examinations.
Although the result may not be surprising to physicists, it underscores the importance of conducting experiments to explore potential new physics and verify existing theories.
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