A single antihydrogen atom, trapped in a magnetic bottle and cooled to near absolute zero, is about to be dropped to see which way it falls. This matters because the universe is made of matter, but it should have been made of equal parts matter and antimatter. That imbalance is one of the biggest unsolved puzzles in fundamental physics. The researchers are testing whether antimatter obeys the same laws as matter—specifically, whether it absorbs light at the same frequencies and falls under gravity at the same rate. Any difference would shatter the Standard Model of particle physics. This is fundamental science with no immediate practical application. The project’s impact is conceptual: it will either confirm a cornerstone of modern physics or force a rewrite of it. If antimatter falls differently, the implications for our understanding of gravity and quantum theory would be profound. Past fundamental research on hydrogen spectroscopy, for example, led directly to the development of atomic clocks that now underpin GPS navigation. A deeper understanding of antimatter could eventually enable similar technologies—antimatter-based imaging or propulsion—but that remains speculative. For now, the goal is simply to measure whether antihydrogen behaves like hydrogen, to fourteen decimal places.
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The observation that the particulate Universe is currently comprised mostly of matter seems unequivocal, as does the assertion that at its birth, the Universe contained equal amounts of matter and antimatter. Just why this imbalance, or asymmetry, has evolved is currently not understood, and indeed it is one of the central questions of physics beyond what is known as the Standard Model. The conventional approach to experimentally explore symmetry in fundamental physics is to study particle collisions at ever-higher kinetic energies, in an effort to reproduce conditions further back towards the beginning of the Universe (the Big Bang). It is becoming increasingly clear, though, that such investigations can be complemented and enriched via small scale experiments, for instance in setting limits on particle electric dipole moments, with novel dark matter searches and, as here, in precision comparisons of the properties of matter and antimatter. We have chosen to bring the powerful toolbox developed via the physics of atom trapping and cooling and atomic spectroscopy to bear on this problem. In short, we create, capture and then cool antihydrogen atoms before studying their properties and behaviour. In one set of experiments (ALPHA-III, which will be devoted to spectroscopic investigations) we intend to systematically probe the transition between the ground state of the anti-atom and its first excited state using a technique known as two-photon Doppler-free spectroscopy. We hope to determine its frequency with a precision similar to that currently achieved for the hydrogen atom, for which it is known to a staggering 14 decimal places. This will deliver a very direct test of symmetry. We have already measured the same transition in antihydrogen to 12 decimal places and we are now aiming for the hydrogen precision. Additionally, we intend to determine fundamental constants in anti-atoms, such as the anti-Rydberg constant and the antiproton charge radius, by combining the ground-to-first excited state work with spectroscopic measurements of additional transitions. In our second major experimental avenue, so-called ALPHA-g, we will analyse the trajectories of antihydrogen atoms as they leave a purpose-built atom trap whose magnetic fields have been carefully tailored to enhance experimental sensitivity to the gravitational behaviour of the anti-atom. We expect to make the first determination of the acceleration of antimatter due to gravity. Eventually we hope to extract the value of g for antihydrogen to an accuracy of 1% or better. Interest in the behaviour of gravity on (anti-)atomic systems stems in part from another puzzle of modern physics, namely that our theory of gravity (Einstein's General Relativity) is incompatible with currently accepted quantum field theories. And whilst the equivalence principle dictates that all objects, irrespective of their content (e.g., in this context independently of whether they are comprised of matter or antimatter), should fall with the same acceleration towards the Earth, testing the (classical) theory of gravity on quantum objects is of fundamental interest. Electrically neutral antimatter-systems are preferable, since they are immune to the influence of electric fields, which can swamp the effects of gravity for charged particles, and antihydrogen is particularly suitable, since it can now be trapped and cooled. Thus, our two-pronged attack on symmetry and gravity by exploring the physics of antihydrogen promises the development of new insights into nature. Our ability to pin down the properties and behaviour of anti-objects is unprecedented, and we aim to further develop this with the work set out in this proposal. Any difference between matter and antimatter, however small, will have profound consequences for our understanding of physics and the laws of nature.
Christopher Isaac (Co-Investigator)Dirk Van Der Werf (Co-Investigator)Mike Charlton (Co-Investigator)Niels Madsen (Principal Investigator)Stefan Eriksson (Co-Investigator)
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