Antigravity Does Not Exist for Antimatter

Antimatter behaves like regular matter under the influence of gravity. Antigravity doesn’t exist for antimatter.

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.

Nature – Observation of the effect of gravity on the motion of antimatter

Abstract
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.

Below is some detail from the paper:

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 from 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. Early considerations and a more recent proof-of-principle experiment in 2013 illustrated this potential. We describe here the initial results of a purpose-built experiment designed to observe the direction and the magnitude of the gravitational force on neutral antimatter.

Antihydrogen and ALPHA-g
Trapping and accumulation of antihydrogen are now routine, with up to several thousand atoms having been simultaneously stored in the ALPHA-2 device. To date, all of the measurements of the properties of antihydrogen have been performed in ALPHA magnetic traps. In 2018, the ALPHA-g machine—a vertically oriented antihydrogen trap designed to study gravitation—was constructed. The experimental strategy is conceptually simple: trap and accumulate atoms of antihydrogen; slowly release them by opening the top and bottom barrier potentials of the vertical trap; and try to discern any influence of gravity on their motion when they escape and annihilate on the material walls of the apparatus. The trapped anti-atoms are not created at rest but have a distribution of kinetic energies consistent with the trap depth of about 0.5 K (we employ temperature-equivalent energy units). Gravity is expected to be manifested as a difference in the number of annihilation events from anti-atoms escaping via the top or the bottom of the trap.

Conclusion
We have searched for evidence of the effect of gravity on the motion of particles of neutral antimatter. The best fit to our measurements yields a value of (0.75 ± 0.13 (statistical + systematic) ± 0.16 (simulation)) g for the local acceleration of antimatter towards the Earth. We conclude that the dynamic behaviour of antihydrogen atoms is consistent with the existence of an attractive gravitational force between these atoms and the Earth. From the asymptotic form of the distribution of the likelihood ratio as a function of the presumed acceleration, we estimate a probability of 2.9 × 10^−4 that a result, at least as extreme as that observed here, could occur under the assumption that gravity does not act on antihydrogen. The probability that our data are consistent with the repulsive gravity simulation is so small as to be quantitatively meaningless (less than 10^−15). Consequently, we can rule out the existence of repulsive gravity of magnitude 1g between the Earth and antimatter. The results are thus far in conformity with the predictions of General Relativity. Our results do not support cosmological models relying on repulsive matter–antimatter gravitation.

Nature – Free-falling antihydrogen reveals the effect of gravity on antimatter.

A test performed on antihydrogen atoms has shown that gravity acts on matter and antimatter in a similar way. The experimental feat is the latest in efforts to probe the crossover between theories of relativity and particle physics.