Cosmic Ripples

by Nicholas Mee on July 19, 2017

Michael Faraday commemorated on a £20 note.

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.

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 LIGO facility in Livingston, Louisiana. Credit: Caltech/MIT/LIGO Lab.

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 1600 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 light detector. 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 light detector, as shown in the figure below. The sensitivity of LIGO is extraordinary, as it must be if there is any chance of detecting gravitational waves.

Schematic diagram of the LIGO gravitational wave detector. Credit: Science News.


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.

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 gravitational 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.

The first ever gravitational wave signal detected by the LIGO observatories. Credit: Caltech/MIT/LIGO Lab.

Much information has been extracted from the brief signal detected by LIGO. It came from an event 1.3 billion light years away and 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. Despite the small amplitude of gravitational waves they carry vast amounts of energy distributed over enormous regions of space.

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, a more precise determination of the source of gravitational wave signals will be possible. (Detectors are currently being constructed or undergoing upgrades in Germany, Italy and Japan. There are also plans to build a detector in India.)

The GOTO Observatory

Earlier this month a new wide-field telescope was inaugurated on the Roque de Los Muchachos in the clear skies high above La Palma in the Canary Islands. It is known as the Gravitational-wave Optical Transient Observer (GOTO) and will seek the optical counterpart to any gravitational wave events detected by LIGO and the VIRGO detector in Italy. This will provide further valuable insights into their origin.

The GOTO observatory on the Roque de Los Muchachos in La Palma. Credit: Gravitational-wave Optical Transient Observer (GOTO) project.

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


Further Information

You can find out more about LIGO here: https://www.ligo.caltech.edu/

For more information about the GOTO observatory see: https://goto-observatory.org/

There is a lot more information about black holes and general relativity in my book Gravity: Cracking the Cosmic Code. www.virtualimage.co.uk/html/gravity.html

This article was partially based on text from The Physical World: An Inspirational Tour of Fundamental Physics by Nicholas Manton and Nicholas Mee.

 

 

{ 6 comments… read them below or add one }

Bob July 20, 2017 at 6:41 pm

Thanks for passing this along. It was very interesting.
Question:
Photons are the carrier of electromagnetic radiation, through space, is that correct? Is it electrons or photons that carry current through a conductor?

Reply

Nicholas Mee July 20, 2017 at 7:16 pm

Dear Bob
Photons are the fundamental particles that electromagnetic radiation is composed of. They are the quantized vibrations of the electromagnetic field. All charged particles, which includes electrons, protons, quarks and various other particles, are affected by the electromagnetic force. In the quantum theory of electromagnetism, known as QED, the electromagnetic force is produced by the exchange of photons between charged particles. But photons are not electrically charged themselves. An electrical current is a flow of electric charge, usually this is carried by electrons.

Reply

Charles Ivie July 21, 2017 at 1:06 pm

Actually, gravitational astronomy had a few false starts years ago and I participated in one of them. In the early 70’s Joseph Weber constructed what he hoped would be a functioning gravitational wave detector.

https://en.wikipedia.org/wiki/Joseph_Weber

And when he detected a signal that appeared to be coming from the direction of Sagittarius A, the center of the Milky way galaxy he was convinced that it worked. When word of this reached CalTech it was realized that a gravitational event that could be detected with Weber’s equipment was very likely to be accompanied by an electromagnetic signal that could be detected with more “conventional” astronomical instruments. Unfortunately, Sag A is obscured by interstellar dust clouds that cannot be penetrated by optical telescopes. I was working as a radio astronomer at the Table Mountain Observatory near Wrightwood in California and we had been observing Sag A intermittently for a year or so. We had a radio telescope that operated in the 8mm band (36 GHz) and had observed several electromagnetic events coming from the same region of the sky. However, none of our observations coincided with Weber’s detection’s. So we coordinated with Weber to synchronize our observations with his and set up a campaign that lasted for several weeks. A number of events were detected by both Weber and us but none of them could be coordinated. It was suggested (by me) that because we didn’t know the time dispersion differential between EM waves and gravitational waves we couldn’t be sure our correlation window was large enough. Special relativity predicts that gravitational waves propagate at the speed of light but radio waves are subject to the refractive index of the media through which they pass and as a result travel somewhat slower than c. The implications of this for our observations were unknown. We enlarged our window to the entire observation opportunity but still could find no correlations with Weber’s detection’s.
Now we believe that his detected events were the result of noise but it was exciting while it lasted.

Reply

Charles Ivie July 21, 2017 at 3:15 pm

My mistake. The propagation velocity of gravitational waves is predicted by general relativity not special relativity. Although it is consistent with special relativity.

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Angelo Laudisi July 21, 2017 at 1:44 pm

Very Interesting and informational; does this mesh with the standing wave theory and is this a step towards uniting Classical and Quantum mechanics? Could this help explain
the inflation of the universe at an accelerated rate? Force apart and expand rather than attract when colliding gravitational waves fail to cancel out each other?

Reply

Nicholas Mee July 21, 2017 at 2:03 pm

Dear Angelo
The gravitational waves that have been discovered are a purely classical phenomenon, as predicted by general relativity. They won’t having any bearing on the quest for a quantum theory of gravity. There are plans to increase the sensitivity of LIGO and there are also plans for a space-based gravitational wave detector called LISA, but that is some way in the future. Eventually, these more sensitive detectors might shed some light on cosmological questions such as inflation and the accelerating expansion of the universe.

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