The coldest place in the universe is without a doubt in a laboratory. And unless there’s an alien civilization doing experiments like they’ve been doing at Kyoto University, the coldest fermions in the universe are here on Earth.
The coldest known natural location would be the Boomerang Nebula, a protoplanetary nebula whose temperature is just one degree above absolute zero , even colder than the cosmic microwave background . This nebula is located about 5,000 light years from Earth and although the name “protoplanetary” suggests it, it is not the cloud that surrounds a star after its birth, but after its death. The central star of this nebula is a red giant that is gradually shedding its outer layers , forming this nebula. This nebula is almost two degrees cooler than the intergalactic medium .
But even so, this temperature is extremely high compared to the many experiments that have been done on Earth seeking to bring different particles to the lowest possible temperature, such as the one that has brought these fermions to just one billionth of a degree above from absolute zero . Fermions, although they have a seemingly exotic name, are the most familiar particles that exist. Your whole body, your home and the planet are made up of them. The electrons, protons, and neutrons that make up our atoms are in fact fermions .
Fundamental particles, and those created by combining several of these, can be classified into two types: bosons and fermions . The difference between these two types of particles is given by their spin . This purely quantum property is related to the angular momentum of the particles . This is why it is often explained, erroneously, that it has to do with the rotation of the particles on themselves. This is not so, because the fundamental particles, such as electrons, quarks and photons are, as far as we know, punctual: they have no size . Therefore they cannot rotate on themselves , since there is no structure that can really rotate.
Well, the fermions will have half-integer spin and the bosons will have integer spin. That is, the spin of a boson can take values like 0, 1, 2, etc, while the spin of a fermion will take values like ½, 3/2, 5/2, etc. This distinction, which might seem to be of only theoretical interest, is key to explaining how our universe works. The fermions will follow the Pauli exclusion principle , a quantum mechanical principle that tells us that only one fermion can occupy a given quantum state. This is the reason, or part of it, that the electrons are arranged in different orbitals around the atomic nuclei, since two electrons cannot occupy the same state and these will gradually fill up.
In addition, fermions form the matter of which the universe is composed , since our atoms are composed of combinations of quarks (which give rise to protons and neutrons), surrounded by electrons. Also other more exotic particles, such as neutrinos, muons or strange or charm quarks are fermions .
Bosons, however, do not constitute matter, since they can occupy a specific state at the same time, preventing them from forming large-scale structures. The bosons will play the role of transmitting the fundamental interactions between the fermions. Thus, the photon, the gluon or the W and Z particles are bosons and are responsible for mediating the electromagnetic, strong nuclear and weak nuclear interactions, respectively.
The Japanese university team has used lasers to cool ytterbium (Yb) atoms to this temperature. The absolute zero of temperature has a certain parallelism with the speed of light and is that it is a value for a certain magnitude that, according to our theories, is unattainable . But not only that, but as we get closer to said value, the laws of physics change completely .
In the case of the speed of light, it is the theory of special relativity , by the German physicist Albert Einstein, which explains how this change occurs. When we approach absolute zero, the temperature at which all movement of the atoms or particles that make up a substance ceases, the laws of quantum physics are no longer restricted to the microscopic world and macroscopic manifestations arise. A phenomenon associated with temperatures close to absolute zero is that of superconductivity (although more recently it has been possible to reproduce this phenomenon at higher temperatures).
This team has studied precisely this, observing the magnetic properties of the material and studying how its electrons show collective behaviors, which cannot be explained as innumerable independent particles, but rather as a whole with new properties.
These investigations can lead us to understand what ultimately makes a material a conductor, an insulator, or exhibit any other property. With the around 300,000 atoms they have arranged in a highly symmetric crystal lattice. In addition, this research is promoting the development of the necessary tools to study these systems in even more detail , with the hope of being able to create materials with the properties that we want, à la carte.
Shintaro Taie, Eduardo Ibarra-García-Padilla, Naoki Nishizawa, Yosuke Takasu, Yoshihito Kuno, Hao-Tian Wei, Richard T. Scalettar, Kaden RA Hazzard, Yoshiro Takahashi. Observation of antiferromagnetic correlations in an ultracold SU(N) Hubbard model. Nature Physics , 2022; DOI: 10.1038/s41567-022-01725-6