Scientific knowledge is constantly evolving. Our understanding of the world changes with each new discovery.
Some are only small changes - such as finding a new species of mushrooms. Others are significant changes - finding out that your mushroom contains anti-cancer compounds.
But every now and then, a major change - a paradigm shift - alters our understanding of the world. They require textbooks to be re-written and lead to the development of whole new areas of inquiry.
One of the major changes in our understanding of the world began in the early 1800s when John Dalton revitalized the concept of the atom. He argued that matter came in discrete units. You could have a 1- to-1 or a 2-to-1 ratio of oxygen with carbon but not a 1.5-to-1 or a 1.423-to-1 ratio.
Molecules were made of whole numbers of atoms.
This concept ushered in the atomic age. Chemists applied these ideas to molecular studies, yielding great advances in chemistry and other disciplines ever since.
But what exactly are atoms remained an unanswered and often debated question with a number of schools of thought.
Did atoms really exist? If so, what were they made of? If not, why did the theory work?
In the 1890s, J.J. Thompson and Henri Becquerel independently determined atoms were not, in fact, the smallest particles of matter.
Thompson discovered the electron and worked out its mass-to-charge ratio. He had no way of independently measuring either property but he suspected he was looking at something smaller than an atom in size.
Becquerel was studying the strange effects of certain minerals on photographic paper when exposed to sunlight. Through a happy accident, he discovered that the effects had nothing to do with sunlight but were an intrinsic property of the mineral itself. He had discovered alpha particles which are the nucleus of helium atoms.
This led to Ernest Rutherford, Hans Geiger and Ernest Marsden conducting an experiment in 1908 to explore the inside of the atom. Their work gave us the Solar System model in which the light, small electrons circle the massive, large nucleus - much like the planets circling the Sun.
This was a huge paradigm shift. Atoms were real and had inner structure. More to the point, the inner core of an atom could be probed.
The discipline of atomic physics was born.
But as with all good science, the results generated more questions than they answered. How did the nucleus with nothing but positive charges hold together? Where were the electrons moving? Why didn't the electrons generate X-rays? How did the whole thing work?
Over the past 100 years, tens of thousands of physicists have devoted their professional lives to understanding atoms and quantum physics. Along the way, they have discovered a zoo of particles consisting of muons, kaons, and positrons among others. They have come to understand fission and fusion. They have developed explanations for atomic structure built on the standard model.
One of the most important components of the standard model is the neutrino. The history of neutrinos dates back to the very early days of atomic physics.
In 1914, James Chadwick was examining the beta spectrum decay of a radioactive element and observed that it was continuous. This was in contrast to the alpha or gamma spectrum and seemed to imply a missing particle or a breakdown in energy conservation.
The latter would have caused all sorts of problems for physicists. In 1930, Wolfgang Pauli postulated a solution to the problem - an electrically neutral, weakly interacting, spin 1/2 fermion with a mass similar to an electron. He called the particle a "neutron."
When Chadwick discovered the more massive neutron in the nucleus in 1932, which is almost the mass of an electron larger than the proton, Pauli's particle was renamed the "neutrino" meaning "little neutron." It was a particle with no mass but still carried the spin, solving a great many problems for physics.
Unambiguous proof for neutrinos came in 1956 and everything seemed settled until physicists started to measure neutrinos coming from the Sun. This was a herculean task as neutrinos don't react with matter very well. Indeed, a neutrino could pass through the Earth and not interact with a single atom. Billions and billions of neutrinos do just this every second.
However, once physicists had worked out how to detect neutrinos, another problem arose. There aren't enough. This puzzle was eventually solved by Takaaki Kajita at the Super-Kamiokande detector in Japan and Arthur B. McDonald, using the Sudbury Neutrino Observatory (SNO).
Neutrinos could change identity. They are not massless but carry away an incredibly small mass and this allows for three different flavours of neutrinos to occur.
The discovery has changed our understanding of the universe and for their results, Kajita and McDonald have been awarded the 2015 Nobel Prize for Physics.
