In Goethe’s ballad “Erlkönig,” immortalized in Schubert’s fevered 1815 setting, a dying boy riding through the night sees a spectral king beckoning from the darkness. His father calms him: “Mein Sohn, es ist ein Nebelstreif”—my son, it is only a wisp of fog. In the poem, the father’s reassurance proves tragically wrong. In the quantum world, however, his words acquire an uncanny new meaning.

For four decades, physicists have been haunted by their own apparition: a particle that scatters off a magnetic monopole and seems to vanish from the theory entirely, its outgoing state missing from the books. A team of physicists from Ghent University, the University of Cambridge and the University of Oxford now shows, in a paper published in Nature Physics, where such particles go.

Throwing quantum wave packets at a so-called “duality defect”—an interface stitching together two quantum worlds that are secretly descriptions of the same physics—they find that the particle is never reflected: it always passes through, with 100% probability, and reemerges as precisely what the father promised. A wisp of fog: faint, smeared out, trailing an invisible thread of quantum mist back to the mirror’s edge.

At the International Conference on High Energy Physics in Brazil, the BESIII Collaboration report that, after 15 years of sustained research, it identified the dominant constituent of the X(2370) as a pseudoscalar glueball with spin-parity quantum numbers of 0⁻⁺

The strong interaction force tightly binds quarks inside protons and neutrons. Its mediator is called the gluon, just as the photon mediates the electromagnetic interaction. Notably, however, gluons can attract each other and form an entirely new particle—a bound state called a glueball.

The glueball is an important prediction of quantum chromodynamics, the theory that describes the strong interaction, and is also the only type of particle in nature composed entirely of force mediators. No particle of this kind has ever been observed experimentally, and its existence constitutes a crucial test of quantum chromodynamics.

The matter surrounding us consists of atoms that obey the laws of quantum mechanics. At normal temperatures these often agree with classical conceptions, and a gas under these conditions behaves rather like a swarm of billiard balls bouncing against one another and the containing walls. When the temperature is lowered and the speed of the atoms is reduced, however, their properties will be increasingly dominated by the principles of quantum mechanics. The atoms rotate round their axes – they have spin – and this movement is described by a spin quantum number, which has to be an integer – a whole number – or a half-integer. Particles that have integer spin are called bosons, while those with half-integer spin are called fermions. Bosons show strong “social” behaviour and at low temperatures strive to gather in one and the same quantum state, the one with the lowest energy. Fermions on the other hand avoid one another. They cannot appear in exactly the same quantum state, so that states of higher energy must also be used. The arrangement of the elements in the periodic system may be understood on the basis of the fact that the electrons in the atomic shells are fermions.

As early as 1924 the Indian physicist S. N. Bose carried out a statistical calculation for the kind of particles which have since come to bear his name, bosons, and more specifically light particles later termed photons. Bose presented an alternative derivation for the radiation law earlier found by Planck. Bose sent his work to A. Einstein, who realised its importance. He translated it to German and had it published. Einstein rapidly extended the theory to cover Bose particles with mass and he himself published two articles in quick succession, predicting that when a given number of particles approach each other sufficiently closely and move sufficiently slowly they will together convert to the lowest energy state: what we now term Bose-Einstein condensation (BEC) occurs.

Ever since publication of this pioneering work, physicists have wished to be able to achieve this new fundamental state of matter, which was expected to have many interesting and useful properties. Seventy years were to pass before this year’s laureates, Eric A. Cornell, Wolfgang Ketterle and Carl E. Wieman, using very advanced methods, finally managed to do this in 1995. The state was achieved in alkali atom gases, in which the phenomenon can be studied in a very pure manner. Nowhere else in the universe can one find the extreme conditions which BEC in dilute gases represents. Manifestations of Bose-Einstein condensation have earlier been observed in more complicated systems: condensation of paired electrons in superconductors (loss of all electrical resistance) and suprafluidity (loss of internal friction in fluids). Here, too, low temperatures are required. Research in these areas has been rewarded with several Nobel Prizes. As opposed to alkali-atom vapours these quantum-mechanical systems are not simple since the condensation phenomenon concerns only a part of the systems and the strong interactions involved tend to hide the BEC phenomenon.