Yorkshire Philosophical Society

Event Information

Listening to the Universe with Gravitational Waves

Dr Hannah Middleton, Institute for Gravitational Wave Astronomy, University of Birmingham

Gravitational wave observations are opening up new ways to learn about our Universe. In this talk we will explore the past, present, and future of gravitational wave astronomy. In 2015, the first observation of a gravitational wave signal from the collision and merger of two black holes a billion light years away marked the beginnings of a new form of astronomy. Since then, gravitational wave observatories (like LIGO, Virgo,and KAGRA) have continued to find more and more signals. This year, new evidence was announced from pulsar timing array experiments which may be seeing the hints of gravitational waves from the biggest black holes in our Universe. And the future is bright with gravitational wave observatories heading to space.

7pm in the Tempest Anderson Lecture Theatre in the Yorkshire Museum

All welcome to this free event; although donations are welcome.

Joint event with the Institute of Physics. 

Member’s report

Gravitational waves were first theorized by Albert Einstein. They are created during events such as Neutron stars, supermassive black hole mergers, or collisions between two black holes that are billion times bigger than our Sun. These collisions are so powerful that they create distortions in spacetime, known as gravitational waves. They stretch and squash, travel at the speed of light and are small so we need high masses and high accelerations of astronomical objects to make them large enough for us to detect.

Before gravitational wave observatories were established there was evidence for gravitational waves from 2 Neutron stars orbiting each other discovered in 1974 and known as the Hulse-Taylor binary star (B1913+16). Their separation decreases due to gravitational wave emission exactly as General Relativity predicts. This won the 1993 Nobel prize.

There is a wide spectrum of gravitational waves. A frequency of 100 Hz (milliseconds) can be observed with ground based experiments, frequencies of 10 to the power of – 2 (hours) observable with a space base and frequencies of 10 to the power of -8 (years) observable with a Pulsar timing array. The first Pulsars were discovered in 1967 by Jocelyn Bell Burnell and they are effectively radio lighthouses and providing observatories on a galactic scale. In June this year pulsar timing array groups throughout the world announced evidence for gravitational waves in their data.

Ground based gravitational wave detectors are used for searching for black holes and are based on a laser interferometer. This consists of a laser aimed at a beam splitter creating two laser beams aimed at mirrors which are then reflected back to the beam splitter and directed on to a photodiode as a combined beam; this can detect tiny differences in the position of the mirrors caused by gravitational waves. Hannah had such a device in the lecture on a small scale to demonstrate the effect. It was clear that it has to be isolated from external influences as any vibrations caused distortion.

There are several Laser Interferometer Gravitational wave Observatories (LIGO) throughout the world with a great deal of effort taken to eliminate unwanted interference including mirrors mounted on glass wires and optics held in vacuums, the distances between beam splitter and mirrors being measured in kilometres, LIGO Livingston (USA) has arms 4km long containing steel vacuum tubes within concrete enclosures protecting them.

In Autumn 2015 2 LIGOs were ready to begin observing and on 14th September just before 10 am there was the first detection, about a billion light years away when 2 black holes spiralled together each 30 times the mass of our sun. LIGO detected gravitational waves from the final 0.2 seconds as they merged into a bigger black hole. Gravitational wave frequencies are similar to audio frequencies and can be converted into sound.  Since then there have been about 90 observations during a further 2 observing runs using world wide LIGOs, observing run 4 started in May 2023.

For mid frequency waves a Laser Interferometer Space Antenna (LISA) is planned for 2030 which will consist of 3 satellites with lasers 2.5 million kilometres apart and presenting many technical challenges.

Jon Coulson

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