This year marks the 50th anniversary of the first production of ultracold neutrons, which could hold the secret to such mysteries as the expansion of the Universe and what dark matter is made of.

These particles were actually discovered twice in the same year — in the former Soviet Union and in the West (see V. I. Luschikov et al. J. Exp. Theor. Phys. Lett. 9, 23–26; 1969 and A. Steyerl Phys. Lett. B 29, 33–35; 1969).

To celebrate, we created a version of the traditional ‘Happy birthday to you’ song using a musical intonation system based on the properties of an ultracold neutron — its inertial and gravitational mass, Planck’s constant and the local acceleration.

Happy Birthday played using a musical intonation system based on the properties of an ultracold neutron Download MP3 Credit: Hartmut Lemmel

Our ‘song’ derives from acoustic resonant transitions (see G. Cronenberg et al. Nature Phys. 14, 1022; 2018) between different gravitational energy eigenstates. Each pair of energy levels has a unique coupling frequency: for example, the |5>→|8> transition corresponds to a frequency of 445.77 Hz, which is close to the concert pitch at 444 Hz used by many orchestras. This spectrum offers vast new musical possibilities, but our selection relies on just a modest few.