So, you discovered gravitational waves…

Last week there was a buzz around the Kelvin Building at the University of Glasgow as physicists excitedly prepared for the announcement they’d been waiting to hear for decades: “Ladies and gentlemen, we have detected gravitational waves. We did it!”

Martin Hendry is Professor of Gravitational Astrophysics and Cosmology at the University of Glasgow and part of the global team who made the discovery.  He has also been a part of Glasgow Science Festival since its inception ten years ago, sharing his passion for physics and astronomy with thousands of people. Martin answered our questions on what this discovery means and its link to Glasgow research.

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In addition to being a world-leading astronomer, Martin is a self-professed Star Wars geek. In reference to gravitational waves, he said “In Star Wars, Darth Vader tells us not to ‘underestimate the power of the dark side.’ This amazing discovery shows how right he was.” – Read More in Time Magazine.

What are gravitational waves?

Gravitational waves are ‘ripples’ in the fabric of space-time caused by some of the most powerful processes in the universe – colliding black holes, exploding stars, and even the birth of the universe itself. Albert Einstein predicted the existence of gravitational waves in 1916, derived from his general theory of relativity. Einstein’s mathematics showed that massive accelerating objects (such as neutron stars or black holes orbiting each other) would disrupt space-time in such a way that waves of distorted space would radiate from the source. These ripples travel at the speed of light through the universe, carrying information about their origins, as well as clues to the nature of gravity itself. – See more at: http://www.ligo.org/science/faq.php

How do we detect them?

We can detect using what we call a laser interferometer.  LIGO (Laser Interferometer Gravitational Wave Observatory) is the world’s largest gravitational wave observatory and one of the world’s most sophisticated physics experiments.  LIGO consists of two laser interferometers located thousands of kilometres apart, one in Livingston Louisiana and the other in Hanford Washington State.  LIGO uses the physical properties of light and of space itself to detect gravitational waves. An interferometer like LIGO consists of two perpendicular “arms” (in LIGO’s case each one is 4km long!) along which a laser beam is shone and reflected by mirrors at each end.

When a gravitational wave passes by, the stretching and squashing of space causes the arms of the interferometer alternately to lengthen and shorten, one getting longer while the other gets shorter and then vice-versa.  As the interferometers’ arms change lengths, the laser beams traveling through the arms travel different distances – which means that the two beams are no longer “in step” and what we call an interference pattern is produced. (This is why we call the LIGO instruments “interferometers”.)

Now the effect of this change in arm length is very small — for a typical passing gravitational wave we expect it to be about 1/10,000th the width of a proton! But LIGO’s interferometers are so sensitive that they can measure even such tiny amounts.

What is Glasgow’s involvement with this research?

The Institute of Gravitational Research at the University of Glasgow has been at the heart of the search for gravitational waves for decades, and pioneered some of the key technologies that have made this remarkable scientific discovery possible.  For example Glasgow led a consortium of UK institutions that played a key role – developing, constructing and installing the sensitive mirror suspensions at the heart of the LIGO detectors that were crucial to this first detection. The technology was based on our work on the earlier UK/German GEO600 detector. This turned LIGO into Advanced LIGO, arguably the most sensitive scientific instrument ever, to give us our first direct glimpse of the dark universe.

Why is the discovery of gravitational waves significant?

Overall, I believe our discovery is an astounding scientific achievement:  it provides the first direct evidence that black holes exist, that they can exist in pairs, and that those pairs can collide and merge – releasing enormous amounts of energy in the form of gravitational waves in the process, in fantastic agreement with the predictions of general relativity.

Our discovery isn’t just about checking if Einstein was right, however.  Detecting gravitational waves will help us to probe the most extreme corners of the cosmos – the event horizon of a black hole, the innermost heart of a supernova, the internal structure of a neutron star: regions that are completely inaccessible to conventional telescopes.   So the first direct detection of gravitational waves and the first observation of a binary black hole merger are remarkable achievements, but they represent only the first page of an exciting new chapter in astronomy.

The next decade will see further improvements to the Advanced LIGO detectors and extension of the global detector network to include Advanced Virgo in Italy, KAGRA in Japan, and a possible third LIGO detector in India.  This enhanced global network will significantly improve our ability to locate the positions of gravitational-wave sources on the sky and estimate more accurately their physical properties.  The new field of gravitational-wave astronomy has a very bright future!

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The first direct detection of gravitational waves. Graphic by Daniel Williams

For more information visit the LIGO FAQ page, authored by Martin.

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