Atoms of antimatter and matter are perfect mirror images, even when weird quantum phenomena come into play.

The energy levels of antihydrogen atoms — the antimatter opposites of hydrogen atoms — are altered by a quantum effect called the Lamb shift, just as hydrogen atoms are, physicists report February 19 in Nature.

Hydrogen atoms can exist in several states of higher and lower energy, known as energy levels. Some subtle quantum effects slightly alter those energy levels. One such tweak — the Lamb shift — surprised physicists when it was reported in hydrogen atoms in 1947. That discovery helped scientists form the theory of quantum electrodynamics, which describes how light interacts with electrically charged particles. The Lamb shift results from flighty particles that, according to quantum electrodynamics, appear and disappear constantly, even in empty space (SN: 12/9/16).

Now, the Lamb shift has been spotted in antihydrogen atoms too. The energy shift is about the same size as in hydrogen atoms, report researchers with the ALPHA experiment (SN: 12/19/16). Located at the particle physics lab CERN in Geneva, ALPHA also revealed a tweak known as fine-structure splitting. That effect occurs in hydrogen, too, and results from spin-orbit coupling, an interaction between the electron’s movement within the atom and a quantum property called spin.

According to physicists’ current understanding, matter and antimatter atoms should have the same energy levels, based on a principle called charge-parity-time, or CPT, symmetry. This symmetry means that physics would remain the same if the universe were reflected in a mirror, all antiparticles swapped with particles, and if time ran backward.

So far, physicists have never discovered a case where CPT symmetry is violated. But, says physicist Jeffrey Hangst of Aarhus University in Denmark, spokesperson for the ALPHA collaboration, “you can never be sure until you actually check.”