Mining Mysterious Particles

Raymond Davis Jr and Masatoshi Koshiba, 1/2 of the prize

Stars like the Sun shine because they release vast supplies of heat and light as a result of squeezing tiny hydrogen atoms together to produce larger helium atoms. In theory, these fusion reactions also shoot out ghostly particles with almost no mass or charge called cosmic neutrinos. These particles were thought to be impossible to detect – every second billions of neutrinos from the Sun almost all pass effortlessly straight through Earth and everything on it – but Raymond Davis, Jr and Masatoshi Koshiba shared one-half of the 2002 Nobel Prize in Physics for successfully detecting the most elusive particles in the universe.

Using the principle that detecting something that rarely collides with matter requires an enormous amount of matter for it to collide with, Raymond Davis’s bold idea to trap neutrinos was to bury a tank filled with around 380,000 litres of a common chlorine-based dry-cleaning agent almost 1.5 kilometres underground in a gold mine to shield it from other forms of radiation. On the very rare occasion a neutrino hits a chlorine atom, the atom is transformed into a radioactive form of the element argon, an event that Davis could detect and count. Even then, such a grand system could trap only around 2,000 solar neutrinos over the course of 25 years.

With an underground tank that measures the tiny flashes of light given off when neutrinos collide with atoms in water, Masatoshi Koshiba showed that these neutrinos were indeed coming from the direction of the Sun. Koshiba’s tank also captured neutrinos from an exploding star 170,000 light years away, before the visible light from the explosion reached Earth. This not only provided some of the first experimental evidence that an enormous number of neutrinos are fired out in the early phases of a supernova explosion, but also showed that such a system is sensitive enough to detect similar events occurring in distant parts of the universe.

At 88 years of age, Davis became the oldest person to date to be made a Nobel Laureate, ahead of Peyton Rous (Medicine, 1966), Joseph Rotblatt (Peace, 1995) and Karl von Frisch (Medicine 1973), who were all 87 when they received their Prize.

X-ray Vision of the Universe

Riccardo Giacconi, 1/2 of the prize

To the naked eye, the vast emptiness of space might appear peaceful and serene, but in reality this is a hostile and restless environment, filled with superheated gases, intense gravity and explosive forces. The fact that we can now witness this extreme side to the universe is thanks largely to Riccardo Giacconi’s efforts to identify and explore its tell-tale signs, for which he received one-half of the 2002 Nobel Prize in Physics.

Giacconi’s work has focused on developing increasingly sophisticated ways in which to detect the X-rays that are produced when gases are heated to millions of degrees by cosmic events such as a star exploding. As our thick atmosphere prevents these cosmic X-rays from reaching Earth, Giacconi and his colleagues had to create instruments that could detect them from rockets fired into space. In a pioneering rocket experiment in 1962, the search for X-rays from the Sun being reflected by the Moon uncovered an altogether more interesting result. The mission identified cosmic X-ray sources originating from outside our Solar System, from a star 9,000 light years from Earth in the Scorpius constellation.

From this moment the field of X-ray astronomy was born, and Giacconi was instrumental in developing the first satellite and telescope to explore this previously hidden view of the universe. Chief amongst the wealth of observations obtained from these instruments is the revelation that most of the material in the Universe can be found in the hot clouds of gas and dark matter that exist between stars and galaxies. They also provided the first evidence for the existence of black holes.

Under Giacconi’s leadership, the latest X-ray telescope to launch into space, named Chandra after the Physics Laureate Subrahmanyan Chandrasekhar, is identifying more remnants of exploding stars and providing further evidence for black holes. By building up a cosmic inventory of some of its most hot and violent contents, Giacconi’s achievements in X-ray astronomy continue to change our view of the universe that surrounds us.