What on Earth is a Boson?

by Nicholas Mee on October 20, 2012

The fundamental particles that physicists study fall into two distinct categories that behave in very different ways. The distinction between these two types of particles, known to physicists as bosons and fermions, is critical to the structure of the world around us. The particles from which matter is composed are fermions. These particles include electrons, protons, neutrons and quarks. Bosons are the particles that are exchanged between other particles to produce forces, such as electromagnetism and the weak and strong nuclear forces. These particles include photons – the fundamental constituents of light – the W and Z bosons, whose exchange produces the weak force, and, of course, the Higgs boson.

Satyendra Nath Bose

Bosons are named after the Indian physicist Satyendra Nath Bose. Bose was born in 1894 in Kolkata in West Bengal.

http://www-history.mcs.st-andrews.ac.uk/Biographies/Bose.html

In 1924, while at the University of Dhaka, Bose showed how the infant quantum theory could be used to derive Planck’s formula for the amount of light emitted as the temperature of an object changes. However, Bose’s original paper was rejected for publication, so he wrote to Einstein who immediately saw its significance. Einstein translated the paper into German and submitted it to the German journal Zeitschrift fur Physik on Bose’s behalf.

Bose–Einstein condensate. By cooling rubidium atoms, which are bosons, to less than 170 billionths of a degree above absolute zero researchers at JILA produced the first Bose-Einstein condensate in 1995. The graphic represents three successive snap shots showing the atoms clustering into the same state. Red indicates low density, yellow and green indicate intermediate densities and high density is indicated by blue to white areas. JILA is jointly operated by NIST and the University of Colorado at Boulder.

Quantum Identity

Bose’s argument was based on the fact that light is formed of particles known as photons. This built on Einstein’s work from 1905 in which Einstein explained an experimental result known as the photoelectric effect by treating light as being composed of photons. The critical idea introduced by Bose was that photons are indistinguishable particles – they are all identical. This is one of the most significant features of quantum mechanics. It means that it is not possible by any manner whatever to distinguish one photon from another. In classical physics we think of particles as akin to billiard balls. We can follow the trajectory of particle A as it collides with particle B and it is always possible to track which is which. But this is not possible in quantum theory, even in principle, and this produces profound differences when considering the statistics of large collections of particles such as photons. It means that there is an enhanced probability that these particles will all be found in the same state as their fellow particles, and this leads to collective behaviour that cannot be explained using classical physics. The behaviour of bosons is fundamental to understanding phenomena, such as lasers, superconductivity and superfluidity, as well as the recently discovered Bose-Einstein condensates.

http://en.wikipedia.org/wiki/Bose%E2%80%93Einstein_condensate

Antisocial Particles

The behaviour of fermions is quite different. Whereas bosons are the ultimate copy-cats, liking nothing more than to be in the same state as their neighbours, fermions are the most stubborn and antisocial individuals – they cannot exist in the same state as their colleagues – they obey the exclusion principle. This is fundamental to the organisation of electrons in atoms and lies at the heart of the subject of chemistry. (It plays a similar role in nuclear physics, by determining how neutrons and protons are arranged in the nucleus of an atom.)

Fermions are named after the Italian physicist Enrico Fermi.

http://www.fnal.gov/pub/about/whatis/enricofermi.html

Spin

What is it, you might ask, that makes these two categories of particle behave in such different ways? The answer is very surprising – it is the rate at which they spin! But that is another story.

 

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