The Weak Equivalence Principle Tested with Antihydrogen Atoms

Monday - October 16 2023, 13:10 UTC - 1 year ago

In Sept. 28, 2023, an ALPHA-g team reported the gravitational acceleration results of antimatter, which are within 25% of normal gravity on Earth, thus confirming the predictions of the Weak Equivalence Principle. The team of 69 members from 21 institutions in 6 countries spent a year and 50 million shots to capture 20 antihydrogen atoms.

Einstein’s general theory of relativity from 1915 remains the most successful description of gravitation. From the 1919 solar eclipse to the observation of gravitational waves, the theory has passed many crucial experimental tests. However, the evolving concepts of dark matter and dark energy illustrate that there is much to be learned about the gravitating content of the universe. Singularities in the general theory of relativity and the lack of a quantum theory of gravity suggest that our picture is incomplete. It is thus prudent to explore gravity in exotic physical systems.

Antimatter was unknown to Einstein in 1915. Dirac’s theory appeared in 1928; the positron was observed in 1932. There has since been much speculation about gravity and antimatter. The theoretical consensus is that any laboratory mass must be attracted by the Earth, although some authors have considered the cosmological consequences if antimatter should be repelled by matter. In the general theory of relativity, the weak equivalence principle (WEP) requires that all masses react identically to gravity, independent of their internal structure. Here we show that antihydrogen atoms, released from magnetic confinement in the ALPHA-g apparatus, behave in a way consistent with gravitational attraction to the Earth. Repulsive ‘antigravity’ is ruled out in this case. This experiment paves the way for precision studies of the magnitude of the gravitational acceleration between anti-atoms and the Earth to test the WEP.

Weak Equivalence Principle .

The weak equivalence principle (WEP) has recently been tested for matter in Earth’s orbit with a precision of order 10^−15. Antimatter has hitherto resisted direct ballistic tests of the WEP due to the lack of a stable, electrically neutral, test particle. Electromagnetic forces on charged antiparticles make direct measurements in the Earth’s gravitational field extremely challenging. The gravitational force on a proton at the Earth’s surface is equivalent to that for an electric field of about 10^−7 V m−1. The situation with magnetic fields is even more dire: a cryogenic antiproton at 10 K would experience gravity-level forces in a magnetic field of order 10^−10 T. Controlling stray fields to this level to unmask gravity is daunting. Experiments have, however, shown that confined, oscillating, charged antimatter particles behave as expected when considered as clocks in a gravitational field. The abilities to produce and confine antihydrogen now allow us to employ stable, neutral anti-atoms in dynamic experiments where gravity should play a role.

The experimental results were reported in the Sept. 28 issue of the journal Nature by a team representing the Antihydrogen Laser Physics Apparatus (ALPHA) collaboration at the European Center for Nuclear Research (CERN) in Geneva, Switzerland. The gravitational acceleration of antimatter that the team comes up with is close to that for normal matter on Earth: 1 g, or 9.8 meters per second per second (32 feet per second per second). It was found to be within about 25% (one standard deviation) of normal gravity. By directly measuring the motion of antihydrogen atoms, they tested the prediction of the weak equivalence principle. With great accuracy, the ALPHA-g researchers have confirmed that exotic antimatter does obey the same gravity laws which normal matter on Earth follows.