Einstein's Gravity and the hunt for Gravitational Waves

Tuesday 18th October, 2016

Our speaker was Dr Christopher Berry from Birmingham University who describes himself on his own website as a "gravitational-wave astronomer" and so it was fitting that his talk was entitled "Einstein's gravity and the hunt for gravitational waves".

He began by saying that his main work involves analysing data from two large science experiments situated in the USA that are known as LIGO, which is short for the Laser Interferometer Gravitational-Wave Observatory. These are two observatories that sit just over 3,000 km apart on either side of the North America continent. One is in Livingston, Louisiana and the other near Richland, Washington. The idea behind having two detectors is to rule out false readings that could be mistaken for gravitational waves. Dr Berry said that there are over 200 effects that have to be ruled out before they can be certain a gravitational wave has been detected. These "false positives" include distant earthquakes, traffic and even changes in air pressure.

Each observatory consists of several buildings but the actual detector at each site consists of two 4-kilometre long "arms" at right angles to each other containing long vacuum tubes. A laser beam is split into two and is then sent down each vacuum tube before being reflected back along the arms by mirrors at the ends of the arms. If no gravitational wave disturbs the laser beams then nothing is recorded when they meet again. However, if a gravitational wave has passed through the arms, distorting their lengths, then a weak light signal will be detected.

Dr Berry explained this result by saying that gravitational waves actually distort space itself just as a pebble dropped in a pond produces a series of expanding concentric waves. Einstein predicted the existence of gravitational waves in 1916 in his General Theory of Relativity which showed that objects such as colliding black holes would send out these waves into the surrounding Universe at the speed of light.

LIGO had its first successful detection on September 14th 2015 when it registered the waves from two colliding black holes that were 36 and 29 times the mass of our Sun. This was an incredible achievement as the LIGO experiment had detected a change in length of the arms that was roughly a thousandth the width of a proton - a miniscule subatomic particle. This success was followed by another detection on Boxing Day 2015 which recorded the merger of two black holes that were roughly 14 and 8 times the mass of our Sun.

LIGO scientists are now asking for help in the form of an online Zooniverse project called Gravity Spy for "citizen scientists". The Gravity Spy website asks members of the general public to help classify sources of noise in their LIGO data called glitches. According to the website "these glitches are difficult to model using computers, can mimic true astrophysical signals, and generally make LIGO less sensitive to gravitational waves."