Antimatter Falls Down

This shouldn't be a surprise to anyone, but experiments have now confirmed, at the ALPHA Experiment at CERN, that anti-matter does indeed fall down, which is to say that its mass-energy has the same gravitational "charge" as all other mass-energy, rather than "falling up" and having a repulsive gravitational reaction to ordinary mass-energy. A link to the peer reviewed paper in Nature, and it abstract can be found here. The abstract states:

Einstein’s general theory of relativity (GR), 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 GR theory 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 GR, 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.

The WEP has recently been tested for matter in Earth 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 Vm-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.