David Nightingale: Lake Baikal And Neutrinos
Those who have traveled to Lake Baikal – and I have not – know it not only as long and thin, but also as the deepest lake in the world. Its greatest depth is a mile, and it lies between southern Siberia and Mongolia. The lake is fed by over 300 rivers and contains more fresh water than all the Great Lakes combined.
This ancient body of water is in a rift valley that is earthquake-prone, and drains into Russia’s river Yenisei, which flows north into the arctic. The lake widens at less than an inch/year, because of the gradual drift of geological plates – in a similar way that the Atlantic Ocean widens (at just over an inch/year.)
2,000 years ago, at the time of the Han dynasty, there was, besides warfare, some elementary science taking place around the lake – and science also takes place there today, to do with the subject of neutrinos.
Before we say what neutrinos are, we can comment that in that Han period, local technology included paper-making, as well as primitive earthquake detection. Their typical detector, or seismometer, was an upside-down pendulum – like a broomstick balanced on the hand. The broomstick would of course fall over if the ground suddenly moved.
The bottom of this very deep lake has been reached thanks to submersible submarines. Russia’s President Putin descended to the bottom in 2008, but far more importantly, the civilization around the lake has advanced, from the 2,000-year-old technology of earthquake detection, to the 1990s technology of neutrino detection.
We must say what a neutrino is – and here’s the rough story. Around 1930 scientists had noticed in their high energy experiments that something was very wrong. Repeatedly it had been found that a couple of bedrock physics principles were not holding – the laws of conservation of energy, and the conservation of momentum. These laws were just not being obeyed.
Any Olympic skier knows that in order to get to a certain height on the opposite slope one has to start at a suitable height because total energy has to be conserved. Likewise an ice hockey player colliding with another player knows that the total momentum of the two players before the collision has to be the same as the total momentum of the two players after the collision.
When things were going ‘wrong’ at the end of the 1920s some scientists were so desperate that they were thinking of giving up on these bedrock laws, just as these days some scientists want to give up on explaining spiral galaxies by suggesting that Newton’s laws – laws that govern the entire space program – are wrong! However, in 1930, the Austrian-Swiss theoretician Wolfgang Pauli suggested that those very basic conservation laws were still true – if there were some weightless invisible particle taking part in the collisions. His calculations demanded a tiny neutral particle traveling at almost the speed of light and weighing essentially nothing. Enrico Fermi, an Italian, soon dubbed it “neutrino” – or “little neutral one”. So imagine: invisible, no weight, interacting with nothing?
However, in 1956, they had been detected, by two Americans – Clyde Cowan and Fred Reines – in experiments which certainly earned them the Nobel Prize. They predicted, and found, that under water, very occasionally neutrinos could cause secondary radiation of blue light.
Since then, this (indirect) detection of neutrinos has been taking place not only deep down in Lake Baikal but also deeper still in the Mediterranean. The water above the apparatus is important, in order to shield from cosmic rays.
Thus, in the end, not only are the laws of physics still safe, but Lake Baikal continues to glean information from the universe.
Dr. David Nightingale is Professor Emeritus of Physics at the State University of New York at NewPaltz and is the co-author of the text, A Short Course in General Relativity.
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