Every single second of every single day, you're bombarded with trillions of billions of subatomic particles that rain down from the depths of space. They blow you through the force of a cosmic hurricane, exploding almost at the speed of light. They are coming from all over the sky, every moment of the day and night. They penetrate the earth's magnetic field and our protective atmosphere like so much butter.
Still, the hair on the top of the head is not even ruffled.
Small Neutral [19659005Questipiccoliproiettilisichiamanoneutriniuntermineconiatonel1934dalbrillantefisicoEnricoFermiLaparolaèvagamenteitalianaper”
;piccoloneutro”elaloroesistenzaèstataipotizzataperspiegareunareazionenuclearemoltocuriosa[The Biggest Unsolved Mysteries in Physics]
Sometimes the elements feel a bit '… unstable. And if they are left alone for too long, they disintegrate and turn into something else, something a little lighter on the periodic table. Also, a small electron would pop out. But in the 20s, accurate and detailed observations of those decays found tiny and insignificant discrepancies. The total energy at the beginning of the process was slightly higher than the energy that came out. Mathematics did not add up. Odd.
Thus, some physicists have invented a brand new particle on the whole fabric. Something to take away the missing energy. Something small, something light, something without charge. Something that could go unnoticed by their detectors.
A little neutral. A neutrino.
It took another couple of decades to confirm their existence – that's how they are slippery, cunning and subtle. But in 1956, neutrinos joined the growing family of known, measured, confirmed particles.
And then things got strange.
The trouble began to ferment with the discovery of the muon, which coincidentally happened about the same time when the idea of the neutrino was starting to gain ground: the years ". The muon is almost exactly like an electron. Same charge Same spin. But it is different in a crucial way: it is heavier, over 200 times more massive than its brother, the electron.
Muons participate in their particular type of reactions, but do not tend to last long. Because of their impressive size, they are very unstable and quickly decay into smaller bits of rain ("quickly" here means within a microsecond or two).
This is all right, so why do muons find themselves in neutrino history?
Physicists noticed that the decay reactions that suggested the existence of the neutrino always had a pop-up of electrons and never a muon. In other reactions, the muons would emit, and not the electrons. To explain these results, they reasoned that neutrinos are always associated with electrons in these decay reactions (and not in any other type of neutrino), while the electron, the muon must mate with a type of neutrino still unknown. After all, the electron-friendly neutrino would not be able to explain the observations from muon events. [Wacky Physics: The Coolest Little Particles in Nature]
And so the hunt continued. And go. And go. It was only in 1962 that physicists finally succeeded in blocking the second type of neutrino. Originally it was nicknamed the "neutretto", but the more rational heads prevailed with the scheme of calling it muon neutrino, since it was always associated with the reactions with the muon.
The Way of the Tao
Okay, I know two confirmed neutrinos. Has nature reserved us more? In 1975, researchers at the Stanford Linear Accelerator Center bravely ran through monotonous data mountains to reveal the existence of an even heavier brother to the agile electron and the powerful muon: the gigantic tau, which enormous 3,500 times the mass of the electron. This is a great particle!
So immediately the question became: if there is a family of three particles, the electron, the muon and the tau … could there be a third neutrino, to be combined with this new creature?
Perhaps, maybe not. Perhaps there are only the two neutrinos. Maybe there are four. Maybe 17. Nature has not exactly met our expectations before, so no reason to start now.
Jumping over many gruesome details, over the decades, physicists became convinced by using a variety of experiments and observations that a third neutrino should exist. But it was not until the millennium, in 2000, that a specially designed experiment at Fermilab (called the DONUT experiment humorously, for the direct observation of the NU Tau, and no, I'm not inventing it) in the end obtained quite confirmed sightings to rightfully claim a detection.
Chasing after ghosts
So why do we worry so much about neutrinos? Why have we been pursuing them for over 70 years, from before the Second World War to the modern era? Why have generations of scientists been so fascinated by these small, neutral ones?
The reason is that neutrinos continue to live outside our expectations. For a long time we were not even sure that they existed. For a long time, we were convinced that they were completely massless, until the experiments discovered they had to have mass. Exactly "how much" remains a modern problem. And neutrinos have this annoying habit of changing characters as they travel. It is true, while a neutrino travels in flight, he can change the masks between the three flavors.
There may still be a further neutrino out there who does not participate in normal interactions – something known as the sterile neutrino, which physicists are looking for
. In other words, neutrinos continually challenge everything we know about physics. And if there's one thing we need, both in the past and in the future, it's a great challenge.
Paul M. Sutter is an astrophysicist at The Ohio State University conductor of  Ask Spaceman and Space Radio and author of Your Place in the Universe .
Originally published on  Live Science .