Text: The power of spin Editorial: August 1999 http://physicsweb.org/article/world/12/8/1/1 Spin is one of the most fundamental and mysterious phenomena in physics. All particles possess an amount of intrinsic angular momentum or "spin" that is completely separate from any angular momentum they might possess as a result of their motion. The effects of spin can be seen throughout physics and chemistry - in the energy levels of nuclei, in the electronic structure of atoms and molecules, in the periodic table, in magnetism and, most fundamentally, in the division of particles into fermions and bosons. And, as several articles in this issue report, the spin of atoms plays a crucial role in the behaviour of atomic gases at nanokelvin temperatures. This role can be traced back to the spins of the basic constituents of matter. Most fundamental particles have a spin of h/4 pi, where h is the Planck constant : any particle with this spin (or a spin of 3h/4 pi, 5h/4 pi and so on) is a fermion. All the constituents of matter in the Standard Model of particle physics - electrons, neutrinos and quarks, and their antiparticles - are fermions. Any particle with zero spin (or with a spin of h/2 pi, h/pi and so on) is a boson. The best known boson is the photon. Indeed, all the particles that carry forces in the Standard Model - photons, W and Z particles, gluons and the so-far hypothetical graviton - are bosons. Composite particles like protons and neutrons can also be divided into bosons and fermions, as can composite systems like atoms and molecules. Neutrons and protons are fermions because they contain an odd number of quarks. Similarly, if the total number of neutrons, protons and electrons in an atom is odd, then the atom is a fermion: if this total is even then the atom is a boson. For example, lithium-6 atoms are fermions, while lithium-7 atoms are bosons. Collections of bosons and fermions behave in completely different ways because they obey different statistics. Fermions obey Fermi-Dirac statistics, which means that two (or more) identical fermions cannot be in the quantum state. There is, however, no limit to the number of identical bosons that can occupy a given quantum state. In the 1920s Einstein showed that if the number of bosons in a given state could be increased beyond a certain value, then a phenomenon known as Bose-Einstein condensation would take place. A Bose condensate is formed when the de Broglie wavelength of the particles exceeds the interparticle separation, which requires exceedingly low temperatures. This new state of matter is described by a single quantum wavefunction and exhibits many unusual properties (see Bose condensates make quantum leaps and bounds). Twenty or so groups around the world have produced a Bose condensate since the first one was created in 1995 (see the Georgia Southern University BEC homepage). And four groups have now used Bose condensates to produce highly coherent beams of atoms in which all the atoms have the same energy and direction, just like the photons in a laser beam (see Atom lasers). These so-called atom lasers could be used in a variety of fundamental physics experiments and in other applications in science and technology. The race is now on to create a quantum-degenerate gas of fermions (see ultracold fermion race is on). Groups in the US have recently set new records for the lowest temperature in a gas of fermionic atoms, and the longest trapping time with lasers. Although fermions are nothing like bosons, they look set to be an equally exciting source of new physics in the next few years. Traffic in a spin It is tempting to try to link Bose-Einstein condensation with the article on the physics of traffic flow. Indeed at least one respected journal has published a paper that makes such a link, and the different phases of traffic are often compared with the solid, liquid and gas phases of matter. However, recent experimental results have shown that current theoretical models are not capable of explaining many of the phenomena observed in real traffic data. And for the safety of all concerned, it is to be hoped that drivers behave as fermions and not as bosons!

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