In 1927, two physicists, C. D. Ellis and W. A. Wooster, set out to measure the energy given off by Radium E decaying into Polonium. The experiment was simple: place the most pure form of RaE available at the time into a calorimeter and measure the output. Beta decay was well understood at the time: each RaE atom naturally decays into one electron and one proton. The electron is emitted at a high velocity and the proton is recaptured by the atom to become a Polonium atom. The half life of this process is five days, meaning, it takes 5 days for half of any amount of RaE to transform into Polonium.
Electrons in the innermost part of the RaE sample collide into other atoms on their journey to the surface. Since the number of atoms in the sample is also known, Ellis and Wooster only had to measure the heat given off by the Radium E sample to discover the amount of energy emitted in the process of decay. From experimental results, they calculated that each RaE atom naturally emits 0.36 MeV: exactly equivalent to the energy of one electron.
It is important to remember that Ellis and Wooster were not interested in confirming or refuting special relativity. They did not use Einstein’s equations in their calculations. They were only interested in discovering the total amount of energy generated in the experiment. Once the experiment was performed, they moved on to other research.
During the next few years, other physicists carried out numerous related experiments, more or less confirming Ellis and Wooster’s initial findings. Several of the physicists performing similar experiments used a mass spectrograph to measure the velocity of the Radium E emitted electrons allowing them to apply Einstein’s special relativity equation to calculate the total energy. In 1931, Viennese physicist Wolfgang Pauli, a strong proponent of Einstein’s latest theory, compared these later studies to the original Ellis and Wooster experiment and noticed a discrepancy. From Einstein’s equations, Pauli saw that each Radium atom should emit 1.16 MeV: almost 3 times what was measured by Ellis and Wooster’s experiment.
Believing whole-heartedly in special relativity’s equations, Pauli could only assume that 0.8 MeV was real and had to be accounted for in order to agree with Einstein’s theory. In December 1930, Pauli, wrote a letter to Hans Geiger and Lise Meitner suggesting a new “massless”, “chargeless” particle for explaining the discrepancy which carried away energy without detection. Pauli died soon after. A few years later, a contemporary, Enrico Fermi, tried to publish Pauli’s theory of the new particle which Fermi named the “neutrino” in the English magazine, Nature. It was rejected as being too speculative and fantastic to publish.
Postulating an “invisible” particle which magically carries away energy without a trace is quite a tale to tell in the land of physics. After all, no other particle in the universe is so much “nothing” with exception of the photon (which has momentum but no mass or charge and which is also continuously debated). Yet during the earlier part of the 20th century, physicists were abuzz with the fantastic stories of Einstein’s relativistic world where time, space, and mass flow and change as readily as waves in the ocean. The universe turned out to be an even stranger place than anyone had imagined yet there were many experiments which confirmed Einstein’s predictions. So why not the neutrino?