“…We don’t allow faster-than-light neutrinos in here,” says the barman. A neutrino walks into a bar…” As reports spread of subatomic particles moving faster than light and potentially travelling through time, such gags were born. But apparently super-hasty motion is not the only strange thing about neutrinos.
What exactly are they?
With a neutral charge and nearly zero mass, neutrinos are the shadiest of particles, rarely interacting with ordinary matter and slipping through our bodies, buildings and the Earth at a rate of trillions per second.
First predicted in 1930 by Wolfgang Pauli, who won a Nobel prize for this work in 1945, they are produced in various nuclear reactions: fusion, which powers the sun; fission, harnessed by humans to make weapons and energy; and during natural radioactive decay inside the Earth.
If they are so stealthy, how do we know they are there at all?
Wily neutrinos usually avoid contact with matter, but every so often, they crash into an atom to produce a signal that allows us to observe them. Fredrick Reines first detected them in 1956, garnering himself a Nobel prize in 1995.
Most commonly, experiments use large pools of water or oil. When neutrinos interact with electrons or nuclei of those water or oil molecules, they give off a flash of light that sensors can detect.
Where are these experiments found?
These days, a lot of expense and extreme engineering go into detectors that are sunk into the ground to shield them from extraneous particles that might interfere with them. For instance, OPERA, which detected the apparently faster-than-light neutrinos beamed from CERN, lies inside the Gran Sasso mountain in Italy. This works because neutrinos shoot straight through such shields.
What’s cool about neutrinos?
Their stealth belies their potential importance. Take extra dimensions. Most particles come in two varieties: ones that spin clockwise and ones that spin anticlockwise. Neutrinos are the only particles that seem to just spin anticlockwise. Some theorists say this is evidence for extra dimensions, which could host the “missing”, right-handed neutrinos.
Unseen right-handed neutrinos may also account for mysterious dark matter. This is thought to make up 80 per cent of all matter in the universe and to stop galaxies from flying apart. The idea is that right-handed neutrinos might be much heavier than left-handed ones and so could provide the requisite gravity.
And what’s this about them coming in “flavours”?
Another strange thing about neutrinos is that they come in at least three types or “flavours” – tau, electron and muon – and can morph from one flavour to another. Recent experiments suggest there may be differences in the ways that antineutrinos and neutrinos morph, which might in turn explain how an imbalance of matter and antimatter arose in the early universe.
Do they have any practical applications?
Sort of – and more are in the works. Some physicists hope to detect neutrinos given off by secret nuclear reactors. Others dream of using them as the basis of a novel communication system that would allow messages to be transmitted to the other side of the world without wires, cables or satellites. Meanwhile the underwater ANTARES detector is doubling up as a telescope for marine life. That’s because, as well as neutrinos, it can detect the light given off by luminous organisms and bacteria.
(via New Scientist)