Fysikum, through Svante Jonsell, participates in the international collaboration ALPHA where atoms of antimatter are created and then investigated with high (and still improving) precision. The experiment is located at the European laboratory for particle physics CERN outside Geneva. This is the only place where it is possible to get antiparticles in sufficient quantity. Our goal is to subject our understanding of Physics to the toughest tests possible. We have chosen to study antihydrogen because hydrogen, its matter companion, is the most accurately investigated system there is. Some properties of hydrogen have been measured with 15 digits accuracy. If our understanding of antimatter is correct, antimatter should look exactly the same. So even the tiniest difference, even if it is only in the last decimal, would completely shatter everything we thought we knew.

ALPHA Experiment at Cern
ALPHA Experiment at Cern - Photo © CERN


Our latest results, published in Nature on February 19, concerns measurements of small energy differences, so-called fine structure and Lamb shift in antihydrogen. Fine structure is well-described by an equation derived by Paul Dirac in 1928, when he combined quantum mechanics with Einstein’s special theory of relativity. Another consequence of Dirac’s equation is that antiparticles, which at the time had not been observed, must exist. On the other hand, the even smaller energy difference observed by Willis Lamb in 1947 (earning him the Nobel Prize in 1955) cannot be explained by Dirac’s equation. This was a completely unexpected experimental result that strongly contributed to inspiring the development of a more complete theory, called quantum electrodynamics (QED).

During the 20:th century, more and more precise measurements on ordinary hydrogen was a driving force behind our understanding of fundamental physics. Given this history it is natural to continue along the same path, and use measurements on antihydrogen to test if our current understanding of antimatter really is correct. So far, our understanding has passed all test. This is true also for our latest study. Our measurements agree with ordinary hydrogen within experimental uncertainties of 2% for fine structure and 11% for the Lamb shift. So, the work continues, both by making our measurements more accurate and by looking at different properties. For example, both ALPHA and several other collaborations plan to check if antihydrogen falls towards the Earth with the same acceleration as ordinary hydrogen.

A news item (in Swedish) from radio (about 2.15 in)

IFL Science: Antihydrogen transitions measured for the first time and they are a lot like hydrogen

Investigation of the fine structure of antihydrogen Nature 2020, vol. 578, s. 375-380
News and views (commentary in Nature) : https://www.nature.com/articles/d41586-020-00384-y