‘Listen, two black holes are clashing!’

MiniGRAIL: first spherical gravitational wave antenna in the world

Since last week, Professor Giorgio Frossati of Leiden University’s Institute of Physics can ‘listen’ to gravitational waves. That is, if such a wave happens to come along. Gravitational waves originate from violent clashes between black holes in the universe and from instabilities in neutron stars.

MiniGRAIL is the name of the first spherical gravitational wave antenna in the world. The ball was made at the Leiden Institute of Physics (LION) of Leiden University. It is the product of years of close cooperation between Frossati’s research group and the technicians of the fine-mechanic and electronic workshop in the Institute. “A result to be proud of”, says Professor Peter Kes, LION’s scientific director.

The MiniGRAIL detector is made of copper with a pinch of aluminium (6%), has a diameter of 65 cm and weighs 1150 kilos. If a gravitational wave passes by the antenna, it will transmit a very small part of its energy to the ball. Gravity waves with a frequency of circa 3000 hertz will make the ball vibrate in all kinds of different ways.

Yet, these vibrations are very small, a billionth of a billionth part of a centimetre (10 -20 m), which makes them very difficult to measure. MiniGRAIL will have to attain a sensitivity good enough to detect these ultra-small vibrations. Astronomers predict that at the frequency and amplitude of such ultra-small vibrations various sources of gravitational waves can be measured, like clashes of black holes and instabilities in neutron stars.

In order to preclude false vibrations as much as possible, MiniGrail is built on vibration-free poles, and the ball is cooled down to ultra-low temperatures. At this moment the ball is 4 Kelvin, which is -269 degrees Celsius. This is as cold as it can get in the coldest corners of the universe. In a number of weeks the ball’s temperature will be decreased even more, to reach record depth, and then the scientific race will break loose: who in the world will be the first to measure gravitational waves?

The race will be between American teams, an Italian team and Frossati’s own team. Still, cooperation will be more important than competition. “You can never be sure you have measured a gravitational wave until you have compared notes with the other teams. Only if all of us, simultaneously, have a hit will we know that it was indeed a gravitational wave.”