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Cosmic ripples

Michael Faraday transformed our understanding of the physical world when he realised that electromagnetic forces are carried by a field permeating the whole of space. This idea was formalized by James Clerk-Maxwell who constructed a unified theory of electromagnetism in which beams of light are undulations in the electromagnetic field. Maxwell’s theory implies that visible light is just one part of the electromagnetic spectrum. Heinrich Hertz confirmed this experimentally in 1887 by generating and detecting radio waves. The invention of radio followed, along with television, radar, mobile phones, and many other applications. Electromagnetic waves are emitted whenever electrically charged objects, such as electrons, are shaken.

The gravitational field

When Einstein formulated his new theory of gravity – general relativity – he aimed to explain gravity as a theory of fields. In this he was successful. Remarkably, it turned out the appropriate field is spacetime itself.

In general relativity, spacetime is analogous to the electromagnetic field and mass is analogous to electric charge. One implication of the theory is that vigorously whirling large masses around will generate gravitational waves, and as gravity is described as the warping and curvature of spacetime, these gravitational waves are simply ripples in the fabric of space.

Schematic illustration of a binary black hole system generating gravitational waves. Copyright: Nicholas Mee. Used with permission.

Detecting electromagnetic waves is easy. We do it whenever we open our eyes, turn on the television, use Wifi, or heat a cup of tea in a microwave oven. Detecting gravitational waves is rather more difficult, because gravity is incredibly weak compared to the electromagnetic force.

We live in an environment where gravity is very important and this gives a false impression of its strength. But it takes a planet-sized amount of matter pulling together for gravity to have a significant effect, and even then it is easy to pick up metal objects with a small magnet, defying the gravitational attraction of the entire Earth.

Gravity is so weak that even shaking huge masses generates barely the tiniest gravitational ripple. Only the most violent cosmic events produce waves that could conceivably be detected, these include supernova explosions, neutron star collisions and black hole mergers. Any instrument sensitive enough to detect them must measure changes in distance between two points several kilometres apart by less than one thousandth of the diameter of a proton. Incredibly, such instruments now exist.

Detecting the ripples

In the centenary year of Einstein’s general relativity, researchers achieved their first success. It had taken decades to develop the technology to build LIGO (Laser Interferometer Gravitational-wave Observatory) consisting of two facilities 3000 km apart in the United States, at Hanford, Washington and Livingston, Louisiana. (Two well-separated detectors are required to distinguish true gravitational wave events from the inevitable local background disturbances.)

The facilities are L-shaped with two perpendicular 4 km arms housed within an ultrahigh vacuum. A laser is directed at a beam-splitter sending half the beam down each arm. The light travels 1,600 km, bouncing back and forth 400 times between two mirrors in each arm, before the two half-beams are recombined. The apparatus is designed so that the recombined half-beams completely cancel, with the peaks in the light waves of one beam meeting the troughs in the other, and no light passes to the photodetector. Whenever a passing gravitational wave ripples through the apparatus, however, the lengths of the arms alter very slightly, so the distances travelled by the half-beams changes and their phases shift (by much less than a single wavelength). There is no longer perfect cancellation and some light arrives at the photodetector. The sensitivity of LIGO is extraordinary, as it must be if there is any chance of detecting gravitational waves.

Extreme violence in the depths of space

The upgraded LIGO programme was scheduled to begin on 18 September 2015. Four days before the official start something wonderful happened. An unmistakable and identical signal was measured by the detectors in Hanford and Livingston within a few milliseconds of each other.

The first ever gravitational wave signal detected by the LIGO observatories. Still taken from a video recording the sound of two black holes colliding. Public domain via Caltech/MIT/LIGO Lab.

Researchers have studied computer models of black hole mergers and other violent cosmic processes so they can recognise the signatures of events detected by LIGO. According to the models, binary black holes produce a continuous stream of gravitational waves that drains energy from the binary system and the black holes gradually spiral together. In the final moments of inspiral the amplitude of the waves increases dramatically. Initially, the newly merged black hole is rather asymmetrical, but it rapidly settles down with a final blast of gravitational waves known as the ring-down.

Much information has been extracted from the brief signal detected by LIGO. It came from an event 1.3 billion light years away that was detonated by two merging black holes during their final inspiral and ring-down. The masses of the black holes are deduced to be 29 and 36 solar masses, and they coalesced into a rapidly spinning black hole of 62 solar masses. What is truly staggering is that during the merger process three times the mass of the sun was converted into pure energy in the form of gravitational waves.

This was the first ever detection of a binary black hole system and the most direct observation of black holes ever made. It also confirmed that gravitational waves travel at the speed of light, as expected.

The difference in arrival time at the two observatories indicates the direction towards the event, at least roughly. When further gravitational wave observatories come on line around the world, this will allow more precise determinations of the sources of gravitational waves.

GOTO Observatory

Earlier this month a new wide-field telescope was inaugurated on the Roque de Los Muchachos high above La Palma in the Canary Islands. It is known as the Gravitational-wave Optical Transient Observer (GOTO) and will seek the source of any gravitational waves detected by LIGO, providing further valuable insights into their origin.

Since 2015, two more signals have been detected by LIGO. Both are due to black hole mergers. The era of gravitational wave astronomy has begun.

A version of this post originally appeared on Quantum Waves Publishing.

Featured image credit: Binary Star, artist’s conception by NASA. Public domain via Wikimedia Commons.

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