Physicists Attempt to Measure Gravitational Mass of Antihydrogen

A team of researchers working with CERN’s Antihydrogen Laser Physics Apparatus (ALPHA) has reported the first direct measurement of gravity’s effect on antimatter, specifically antihydrogen in free fall.

This is a schematic, cut-away diagram of the antihydrogen production and trapping region of the ALPHA apparatus, showing the relative positions of the cryogenically cooled trap electrodes, the minimum-B trap octupole and mirror magnet coils, and the annihilation detector. The trap wall is on the inner radius of the electrodes (A. E. Charman, the ALPHA Collaboration)

This is a schematic, cut-away diagram of the antihydrogen production and trapping region of the ALPHA apparatus, showing the relative positions of the cryogenically cooled trap electrodes, the minimum-B trap octupole and mirror magnet coils, and the annihilation detector. The trap wall is on the inner radius of the electrodes (A. E. Charman, the ALPHA Collaboration)

“The atoms that make up ordinary matter fall down, so do antimatter atoms fall up? Do they experience gravity the same way as ordinary atoms, or is there such a thing as antigravity?”

“These questions have long intrigued physicists,” said Dr Joel Fajans of the U.S. Department of Energy’s Lawrence Berkeley National Laboratory, co-author of a paper reporting the results in Nature Communications. “Because in the unlikely event that antimatter falls upwards, we’d have to fundamentally revise our view of physics and rethink how the Universe works.”

So far, all the evidence that gravity is the same for matter and antimatter is indirect, so the team decided to use their ongoing antihydrogen research to tackle the question directly.

The ALPHA experiment captures antiprotons and combines them with antielectons (positrons) to make antihydrogen atoms, which are stored and studied for a few seconds in a magnetic trap. Afterward, however, the trap is turned off and the atoms fall out. The researchers realized that by analyzing how antihydrogen fell out of the trap, they could determine if gravity pulled on antihydrogen differently than on hydrogen – the anomaly would be noticeable in ALPHA’s existing data on 434 anti-atoms.

The times and vertical (y) annihilation locations (green dots) of 10,000 simulated antihydrogen atoms in the decaying magnetic fields. There is a tendency for the anti-atoms to annihilate in the bottom half of the trap, as shown by the black solid line. The blue dotted line includes the effects of detector azimuthal smearing on the average; the smearing reduces the effect of gravity observed in the data. The red circles are the annihilation times and locations for 434 real anti-atoms, as measured by the particle detector (A. E. Charman, the ALPHA Collaboration)

The times and vertical (y) annihilation locations (green dots) of 10,000 simulated antihydrogen atoms in the decaying magnetic fields. There is a tendency for the anti-atoms to annihilate in the bottom half of the trap, as shown by the black solid line. The blue dotted line includes the effects of detector azimuthal smearing on the average; the smearing reduces the effect of gravity observed in the data. The red circles are the annihilation times and locations for 434 real anti-atoms, as measured by the particle detector (A. E. Charman, the ALPHA Collaboration)

Antihydrogen did not behave weirdly, so they calculated that it cannot be more than 110 times heavier than hydrogen. If antimatter is anti-gravity – and they cannot rule it out – it doesn’t accelerate upward with more than 65 Gs.

“This is the first word, not the last. We’ve taken the first steps toward a direct experimental test of questions physicists and nonphysicists have been wondering about for more than 50 years. We certainly expect antimatter to fall down, but just maybe we will be surprised,” Dr Fajans said.

“We need to do better, and we hope to do so in the next few years,” said co-author Dr Jonathan Wurtele of the Lawrence Berkeley National Laboratory. “ALPHA is being upgraded and should provide more precise data once the experiment reopens in 2014.”

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Bibliographic information: A. E. Charman and the ALPHA Collaboration. 2013. Description and first application of a new technique to measure the gravitational mass of antihydrogen. Nature Communications 4, article number: 1785; doi: 10.1038/ncomms2787