Einstein’s general theory of relativity (GR), from 19151, remains the most successful description of gravitation. From the 1919 solar eclipse2 to the observation of gravitational waves3, 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 theory4 appeared in 1928; the positron was observed5 in 1932. There has since been much speculation about gravity and antimatter. The theoretical consensus is that any laboratory mass must be attracted6 by the Earth, although some authors have considered the cosmological consequences if antimatter should be repelled by matter7,8,9,10. 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 orbit11 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 12 . 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 antiproton13 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 clocks14,15,16 in a gravitational field. The abilities to produce17 and confine18 antihydrogen now allow us to employ stable, neutral anti-atoms in dynamic experiments where gravity should play a role. Early considerations19,20 and a more recent proof-of-principle experiment21 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.
E.K. Anderson, C. J. Baker, W. Bertsche, N.M. Bhatt, G. Bonomi, A. Capra, I. Carli, C. L. Cesar, M. Charlton, A. Christensen, R. Collister, A. Cridland Mathad, D. Duque Quiceno, S. Eriksson, A. Evans, N. Evetts, S. Fabbri, J. Fajans, A. Ferwerda, T. Friesen, M. C. Fujiwara, D.R. Gill, L.M. Golino, M.B.Gomes Gonçalves, P. Grandemange, P. Granum, J. S. Hangst, M. E. Hayden, D. Hodgkinson, E.D. Hunter, C.A. Isaac, A.J.U. Jimenez, M. A. Johnson, J.M. Jones, S. A. Jones, S. Jonsell, A. Khramov, N. Madsen, L. Martin, N. Massacret, D. Maxwell, J. T. K. McKenna, S. Menary, T. Momose, M. Mostamand, P. S. Mullan, J. Nauta, K. Olchanski, A.N. Oliveira, J. Peszka, A. Powell, C. Ø. Rasmussen, F. Robicheaux, R. L. Sacramento, M. Sameed, E. Sarid, J. Schoonwater, D.M. Silveira, J. Singh, G. Smith, C. So, S. Stracka, G. Stutter, T. D. Tharp, K.A. Thompson, R. I. Thompson, E. Thorpe-Woods, C. Torkzaban, M. Urioni, P. Woosaree & J. S. Wurtele.