Ripples from the Dark Side of the Universe – a bright future ahead


In this talk Sir James discussed progress in the field of gravitational wave detection, from the first days of the aluminium bar detectors to the present time, where the laser interferometer detectors Advanced LIGO and Advanced Virgo have allowed gravitational waves to be detected and are opening up a new field of gravitational multi-messenger astrophysics with a number of ground breaking discoveries. Many experimental challenges had to be overcome and new challenges are presenting themselves as we look to further enhance the performance of ground-based detectors and look to lower frequencies with the space-based detector LISA.

Sir James Hough OBE FRS FRSE FInstP FRAS (born 6 August 1945) is a Scottish physicist and an international leader in the search for gravitational waves.


The following are notes from the on-line lecture. Even though I could stop the video and go back over things there are likely to be mistakes because I haven’t heard things correctly or not understood them. I hope the Sir James and my readers will forgive any mistakes and let me know what I got wrong.

The search for gravitational waves has being going on for more than one hundred years.

Sir James began the lecture by describing the life cycle of stars.


At the moment our Sun is at the stable stage, but once it has “burnt” up all its hydrogen and helium has been produced, it becomes a red giant.

Nuclear fusion is a reaction in which two or more atomic nuclei are combined to form one or more different atomic nuclei and subatomic particles (neutrons or protons). The difference in mass between the reactants and products is manifested as either the release or the absorption of energy. This difference in mass arises due to the difference in nuclear binding energy between the nuclei before and after the reaction. Fusion is the process that powers active or main sequence stars and other high-magnitude stars, where large amounts of energy are released.

An important fusion process is the stellar nucleosynthesis that powers stars, including the Sun. In the 20th century, it was recognized that the energy released from nuclear fusion reactions accounts for the longevity of stellar heat and light. The fusion of nuclei in a star, starting from its initial hydrogen and helium abundance, provides that energy and synthesizes new nuclei. Different reaction chains are involved, depending on the mass of the star (and therefore the pressure and temperature in its core).


The proton-proton chain reaction, branch I, dominates in stars the size of the Sun or smaller.

Nuclear binding energy is the minimum energy that would be required to disassemble the nucleus of an atom into its component parts. These component parts are neutrons and protons, which are collectively called nucleons. The binding energy is always a positive number, as we need to spend energy in moving these nucleons, attracted to each other by the strong nuclear force, away from each other. The mass of an atomic nucleus is less than the sum of the individual masses of the free constituent protons and neutrons, according to Einstein’s equation E = mc2. This ‘missing mass’ is known as the mass defect, and represents the energy that was released when the nucleus was formed.

If the star is at least eight times the mass of our Sun it will turn into a red supergiant as it starts to die. During this process helium begins to fuse.

All red supergiants will exhaust the helium in their cores within one or two million years and then start to burn carbon. This continues with fusion of heavier elements until an iron core builds up, which then inevitably collapses to produce a supernova. The time from the onset of carbon fusion until the core collapse is no more than a few thousand years. In most cases, core-collapse occurs while the star is still a red supergiant, the gravitational forces inside the star become greater than the outward radiation pressure, the large remaining hydrogen-rich atmosphere is ejected, and this produces a type II supernova spectrum. The opacity of this ejected hydrogen decreases as it cools and this causes an extended delay to the drop in brightness after the initial supernova peak, the characteristic of a Type II-P supernova.

A supernova is a powerful and luminous stellar explosion. This transient astronomical event occurs during the last evolutionary stages of a massive star or when a white dwarf is triggered into runaway nuclear fusion. The original object, called the progenitor, either collapses to a neutron star or black hole, or is completely destroyed. The peak optical luminosity of a supernova can be comparable to that of an entire galaxy before fading over several weeks or months.

Theoretical studies indicate that most supernovae are triggered by one of two basic mechanisms: the sudden re-ignition of nuclear fusion in a degenerate star such as a white dwarf, or the sudden gravitational collapse of a massive star’s core. In the first class of events, the object’s temperature is raised enough to trigger runaway nuclear fusion, completely disrupting the star.

In the massive star case, the core of a massive star may undergo sudden collapse, releasing gravitational potential energy as a supernova. The brightness is caused by violent expulsion of the outer layers of the star when it turns into a supernova.

The core of the supernova collapses further and the gravitational forces are so great that protons and electrons are forced together to produce neutrons and a neutron star is formed

A neutron star is the collapsed core of a massive supergiant star, which had a total mass of between 10 and 25 solar masses, possibly more if the star was especially metal-rich.

The existence of neutron stars was proposed in 1933, but they were thought to be too faint to be detectable and little work was done on them until November 1967, when Franco Pacini pointed out that if the neutron stars were spinning and had large magnetic fields, then electromagnetic waves would be emitted. Unbeknown to him, radio astronomer Antony Hewish and his research assistant Jocelyn Bell at Cambridge were shortly to detect radio pulses from stars that are now believed to be highly magnetised, rapidly spinning neutron stars, known as pulsars. (below left)


Franco Pacini (10 May 1939 – 25 January 2012) was an Italian astrophysicist and professor at the University of Florence. He carried out research, mostly in High Energy Astrophysics, in Italy, France, United States and at the European Southern Observatory.

Antony Hewish FRS FInstP (born 11 May 1924) is a British radio astronomer who won the Nobel Prize for Physics in 1974 for his role in the discovery of pulsars. He was also awarded the Eddington Medal of the Royal Astronomical Society in 1969. (above right)

Dame Susan Jocelyn Bell Burnell DBE FRS FRSE FRAS FInstP (born 15 July 1943) is an astrophysicist from Northern Ireland who, as a postgraduate student, discovered the first radio pulsars in 1967. She was credited with “one of the most significant scientific achievements of the 20th century”. The discovery was recognised by the award of the 1974 Nobel Prize in Physics but, despite being the first person to discover the pulsars, she was not one of the recipients of the prize.

The idea of black holes is not a recent one. Even before Einstein scientists were considering the possibility of what would happen if the gravitational pull of a star was large enough that light could not escape. At the time light was considered to be made up of particles.

A dark star is a theoretical object compatible with Newtonian mechanics that, due to its large mass, has a surface escape velocity that equals or exceeds the speed of light. Whether light is affected by gravity under Newtonian mechanics is unclear but if it were accelerated the same way as projectiles, any light emitted at the surface of a dark star would be trapped by the star’s gravity, rendering it dark, hence the name. Dark stars are analogous to black holes in general relativity.

During 1783 geologist John Michell wrote a letter to Henry Cavendish outlining the expected properties of dark stars, published by The Royal Society in their 1784 volume. Michell calculated that when the escape velocity at the surface of a star was equal to or greater than lightspeed, the generated light would be gravitationally trapped so that the star would not be visible to a distant astronomer.

If the semi-diameter of a sphere of the same density as the Sun were to exceed that of the Sun in the proportion of 500 to 1, a body falling from an infinite height towards it would have acquired at its surface greater velocity than that of light, and consequently supposing light to be attracted by the same force in proportion to its vis inertiae, with other bodies, all light emitted from such a body would be made to return towards it by its own proper gravity. This assumes that gravity influences light in the same way as massive objects.

Michell’s idea for calculating the number of such “invisible” stars anticipated 20th century astronomers’ work: he suggested that since a certain proportion of double-star systems might be expected to contain at least one “dark” star, we could search for and catalogue as many double-star systems as possible, and identify cases where only a single circling star was visible. This would then provide a statistical baseline for calculating the amount of other unseen stellar matter that might exist in addition to the visible stars.

John Michell (25 December 1724 – 21 April 1793) was an English natural philosopher and clergyman who provided pioneering insights into a wide range of scientific fields including astronomy, geology, optics, and gravitation.

Henry Cavendish FRS (10 October 1731 – 24 February 1810) was an English natural philosopher, scientist, and an important experimental and theoretical chemist and physicist.

In 1915, Albert Einstein developed his theory of general relativity, having earlier shown that gravity does influence light’s motion. Only a few months later, Karl Schwarzschild found a solution to the Einstein field equations, which describes the gravitational field of a point mass and a spherical mass, in other words, a black hole (although classed as a dark star at the time). (below left)


Albert Einstein (14 March 1879 – 18 April 1955) was a German-born theoretical physicist who developed the theory of relativity, one of the two pillars of modern physics (alongside quantum mechanics). (above right)

Karl Schwarzschild (9 October 1873 – 11 May 1916) was a German physicist and astronomer.

Size of Black Holes and Neutron stars

Both of these result from the collapse of stars

Electromagnetic observations (emission of X-rays) suggest that:

The smallest black holes are five times the mass of our Sun

The heaviest neutron stars are two times the mass of our sun

So, there is a range of masses where neither exists – a mass gap.

Theory suggests that Black Holes should have a maximum mass of less that sixty times the mass of our Sun but supermassive Black Holes are believed to exist at the centre of galaxies. How do supermassive Black Holes occur?

The value of less than sixty times the mass of our Sun comes from physicists looking theoretically how Black Holes form and the calculations never quite get to the Black Hole stage. Something seems to go wrong. There is some instability that takes place due to the formation of electron-proton pairs, which is called pulsed pair instability.

Comprehensive models of the late-stage evolution of very massive stars predict that a pair-instability supernova occurs when pair production, the production of free electrons and positrons in the collision between atomic nuclei and energetic gamma rays, temporarily reduces the internal radiation pressure supporting a supermassive star’s core against gravitational collapse. This pressure drop leads to a partial collapse, which in turn causes greatly accelerated burning in a runaway thermonuclear explosion, resulting in the star being blown completely apart without leaving a stellar remnant behind.

Pair-instability supernovae can only happen in stars with a mass range from around 130 to 250 solar masses and low to moderate metallicity (low abundance of elements other than hydrogen and helium – a situation common in Population III stars).

Of course, we now have good evidence that black holes exist as a image was taken of one in 2019.


Using the Event Horizon Telescope, scientists obtained an image of the black hole at the centre of galaxy M87, outlined by emission from hot gas swirling around it under the influence of strong gravity near its event horizon. Credits: Event Horizon Telescope collaboration et al.

There is also strong evidence that supermassive Black Holes exist at the centre of galaxies including our own. But how do they form if theory suggests they can’t be that big.

Gravitational waves

One of the most exciting predictions of General Relativity is the existence of gravitational waves.

An analogy for gravitational waves is radio waves. Just as you can produce radio waves by accelerating electrons up and down a wire you can produce gravitational waves by accelerating mass up and down in a rather more complicated way.

Radio waves are a type of electromagnetic radiation with wavelengths in the electromagnetic spectrum longer than infrared light. Radio waves have frequencies as high as 300 gigahertz (GHz) to as low as 30 hertz (Hz).

Albert Einstein predicted the existence of gravitational waves in 1916. But nobody, including himself, took the idea seriously. There are lots of transformations in general relativity and physicists just thought there was something wrong with the theory and that gravitational waves could be just transformed away.

Interest resurfaced in 1957 at the Chapel Hill conference by Felix Pirani, who explained how gravitational waves would make particles move. This inspired Richard Feynman

In short, Pirani’s argument, known as the “sticky bead argument”, notes that if one takes a rod with beads then the effect of a passing gravitational wave would be to move the beads along the rod; friction would then produce heat, implying that the passing wave had done work. (below left)


Felix Arnold Edward Pirani (2 February 1928 – 31 December 2015) was a British theoretical physicist specialising in gravitational physics and general relativity. (above centre)

Richard Phillips Feynman ForMemRS (May 11, 1918 – February 15, 1988) was an American theoretical physicist, known for his work in the path integral formulation of quantum mechanics, the theory of quantum electrodynamics, the physics of the superfluidity of supercooled liquid helium, as well as his work in particle physics for which he proposed the parton model. (above right)

Joseph Weber (May 17, 1919 – September 30, 2000) was an American physicist. He gave the earliest public lecture on the principles behind the laser and the maser and developed the first gravitational wave detectors (Weber bars).

After the Chapel Hill conference, Joseph Weber started designing and building the first gravitational wave detectors now known as Weber bars. In 1969, Weber claimed to have detected the first gravitational waves, and by 1970 he was “detecting” signals regularly from the Galactic Centre; however, the frequency of detection soon raised doubts on the validity of his observations as the implied rate of energy loss of the Milky Way would drain our galaxy of energy on a timescale much shorter than its inferred age. These doubts were strengthened when, by the mid-1970s, repeated experiments from other groups building their own Weber bars across the globe failed to find any signals, and by the late 1970s general consensus was that Weber’s results were spurious.

So, Gravitation waves were predicted to be produced by a violent acceleration of mass in, for example: black hole formation and coalescences and coalescences of neutron stars. That the process was similar to that of accelerating electric charges (although this is caused by a changing dipole moment, where there are two types of charge, whereas there is only one type of mass)) producing radio waves and that gravitational waves could be described as “ripples in the curvature of spacetime”. These ripples carry information about changing gravitational fields – or fluctuating strains in space of amplitude h where h ~ DL/L. L is the stable separation between the two objects and DL is the change in that separation.


(Above left) General relativity predicts the gravitational bending of light by massive bodies. Matter changes the geometry of spacetime, this (curved) geometry being interpreted as gravity. White lines do not represent the curvature of space but instead represent the coordinate system imposed on the curved spacetime, which would be rectilinear in a flat spacetime. You can visualise spacetime as a rubber sheet where a massive object would warp the rubber sheet s it would warp spacetime. (Above right) Gravitational waves are distorting the curvature of spacetime. They are ripples on the “rubber sheet”. The ripples squeeze together and move apart.

In physics, spacetime is any mathematical model which fuses the three dimensions of space and the one dimension of time into a single four-dimensional manifold. Spacetime diagrams can be used to visualize relativistic effects, such as why different observers perceive differently where and when events occur.


(Above left) Artwork depicting gravitational waves emanating from two black holes coalescing. LIGO/T. Pyle (Above right) On August 17, astronomers detected gravitational waves and a gamma-ray burst from two colliding neutron stars. National Science Foundation/LIGO/Sonoma State University/A. Simonnet

Formation and Coalescence of Supermassive Black Hole Binaries in Supermassive Star Collapse (video above)

The Collision of Two Neutron Stars (video above)

High gravitational forces (as in black holes and neutron stars) are needed with systems that are rotating around each other. Looking end on their movement seems like they are coming together and then moving apart, coming together and moving apart. This motion is described as changing quadruple moment.

In general relativity, the quadrupole formula describes the rate at which gravitational waves are emitted from a system of masses based on the change of the (mass) quadrupole moment.

A quadrupole or quadrapole is one of a sequence of configurations of things like electric charge or current, or gravitational mass that can exist in ideal form, but it is usually just part of a multipole expansion of a more complex structure reflecting various orders of complexity.

The mass quadrupole moment is also important in general relativity because, if it changes in time, it can produce gravitational radiation, similar to the electromagnetic radiation produced by oscillating electric or magnetic dipoles and higher multipoles. However, only quadrupole and higher moments can radiate gravitationally. The mass monopole represents the total mass-energy in a system, which is conserved—thus it gives off no radiation. Similarly, the mass dipole corresponds to the centre of mass of a system and its first derivative represents momentum which is also a conserved quantity so the mass dipole also emits no radiation. The mass quadrupole, however, can change in time, and is the lowest-order contribution to gravitational radiation.

The simplest and most important example of a radiating system is a pair of mass points with equal masses orbiting each other on a circular orbit, an approximation to e.g. special case of binary black holes.


A Quadrupole gravitational wave, computed with the standard quadrupole formula for a binary orbiting at constant frequency. Shown are the wave in the XY-plane (left) and XZ-plane (right). The strongest radiation is emitted along the z-axis. (video above).

How are gravitational waves detected?

Extremely challenging experiments are needed. The theory explains that two masses separated by about a metre will only give a separation change of one thousandth of a billionth of a metre at the most if this separation change is caused by a gravitational wave.

To give some idea of how tiny this is the size of a typical atom is ~ one tenth of a billionth of a metre.

To measure the separation a very good ruler would be needed and masses which are much further apart.

When experiments were first proposed physicists weren’t aware of this which is why the initial experiments weren’t successful.

As mentioned earlier Joseph Weber produced an experiment in order to detect gravitational waves


Above right: Weber with one of his bars complete with piezoelectric detectors around the middle. Above left: The Weber Memorial Garden at the University of Maryland.


“Coincidences have been observed on gravitational-radiation detectors over a base line of about 1000 km at Argonne National Laboratory and at the University of Maryland. The probability that all of these coincidences were accidental is incredibly small. Experiments imply that electromagnetic and seismic effects can be ruled out with a high level of confidence. These data are consistent with the conclusion that the detectors are being excited by gravitational radiation.”

A Weber bar is a device used in the detection of gravitational waves first devised and constructed by physicist Joseph Weber at the University of Maryland. The device consisted of multiple aluminium cylinders, 2 meters in length and 1 meter in diameter, antennae for detecting gravitational waves.

These massive aluminium cylinders vibrated at a resonance frequency of 1660 hertz at room temperature and were designed to be set in motion by gravitational waves predicted by Weber. Because these waves were supposed to be so weak, the cylinders had to be massive and the piezoelectric sensors had to be very sensitive, capable of detecting a change in the cylinders’ lengths by about 10−16 meters.

The aluminium bars could be thought of as being made up of two halves with roughly a metre space between them. They were placed in vacuum tanks and secured with wires to prevent them reacting to surface movements.

Weber built two of these and place one at the Argonne lab in the University of Chicago and one at the University of Maryland.

The devices produced a lot of random noise but they seemed to produce a significant signal once a day. He deduced that these significant signals were produced by gravitational waves.

Around 1968, Weber collected what he concluded to be “good evidence” of the theorized phenomenon. However, his experiments were duplicated many times around the world, always with a null result. The apparatus just wasn’t sensitive enough.

Sir James outlined a story about why Weber was so convinced he had detected gravitational waves and it was to do with his experience of being in a submarine during the Second World War. When sonar detected a signal that could possibly be a torpedo you didn’t hang around to see if it actually was showing a torpedo. It was a torpedo, end of story.

One of Weber’s experiments was carried out by Sir James at the University of Glasgow.


Sir James is on the left. The apparatus consisted of two separate masses with piezoelectric crystals between them. Two were built to see if they could replicate Joseph Weber’s results.

Resonance describes the phenomenon of increased amplitude that occurs when the frequency of a periodically applied force is equal or close to a natural frequency of the system on which it acts.

Piezoelectricity is the electric charge that accumulates in certain solid materials (such as crystals, certain ceramics, and biological matter such as bone, DNA and various proteins) in response to applied mechanical stress. The word piezoelectricity means electricity resulting from pressure and latent heat.,_College_Park

A new technique for detecting gravitational waves was needed. The masses needed to be much further apart. An experiment was proposed that used laser interferometry.


‘Interferometry’ is a measurement method using the phenomenon of interference of waves (usually light, radio or sound waves). The measurements may include those of certain characteristics of the waves themselves and the materials that the waves interact with. In addition, interferometry is used to describe the techniques that use light waves for the study of changes in displacement.

By using two light beams (usually by splitting one beam into two), an interference pattern can be formed when these two beams superpose. Because the wavelength of the visible light is very short, small changes in the differences in the optical paths (distance travelled) between the two beams can be detected (as these differences will produce noticeable changes in the interference pattern). Hence, the optical interferometry has been a valuable measurement technique for more than a hundred years. Its accuracy has later been improved with the invention of lasers. Interferometer Animation

This animation illustrates how the twin observatories of LIGO work. One observatory is in Hanford, Washington, the other in Livingston, Louisiana. Each houses a large-scale interferometer, a device that uses the interference of two beams of laser light to make the most precise distance measurements in the world.

The animation begins with a simplified depiction of the LIGO instrument. A laser beam of light is generated and directed toward a beam splitter, which splits it into two separate and equal beams. The light beams then travel perpendicularly to a distant mirror, with each arm of the device being 4 kilometres in length. The mirrors reflect the light back to the beam splitter, repeating this process 200 times.

When gravitational waves pass through this device, they cause the length of the two arms to alternately stretch and squeeze by infinitesimal amounts, tremendously exaggerated here for visibility. This movement causes the light beam that hits the detector to flicker.

The second half of the animation explains the flickering, and this is where light interference comes into play. After the two beams reflect off the mirrors, they meet at the beam splitter, where the light is recombined in a process called interference. Normally, when no gravitational waves are present, the distance between the beam splitter and the mirror is precisely controlled so that the light waves are kept out of phase with each other and cancel each other out. The result is that no light hits the detectors.

But when gravitational waves pass through the system, the distance between the end mirrors and the beam splitter lengthen in one arm and at the same time shorten in the other arm in such a way that the light waves from the two arms go in and out of phase with each other. When the light waves are in phase with each other, they add together constructively and produce a bright beam that illuminates the detectors. When they are out of phase, they cancel each other out and there is no signal. Thus, the gravitational waves from a major cosmic event, like the merger of two black holes, will cause the signal to flicker, as seen here.


The interferometer can give signals of varying intensity.

By digitising and recording the specific patterns of signals that hit the LIGO detectors, researchers can then analyse what they see and compare the data to computer models of predicted gravitational wave signals. Tiny changes of light intensity are being looked for.

The effects of the gravitational waves on the LIGO instrument have been vastly exaggerated in this video to demonstrate how it works. In reality, the changes in the lengths of the instrument’s arms are only 1/1000th the size of a proton. Other characteristics of LIGO, such as the stability of its mirrors, also contribute to its ability to precisely measures distances. In fact, LIGO can be thought of as the most precise “ruler” in the world.

The mirrors in the interferometer are suspended on pendulums to reduce the effect of surface movements.

In the animation one arm can be seen to increase and the other to decrease in order to see a change in the interference pattern. This ties in very well with the fact that gravitational waves are quadrupole not dipole in nature, because a quadrupole system can be likened to a football changing into a rugby ball, then changing back into a football, and so on. That is what a gravitational wave would do if it interacted with a football.

A number of labs set up interferometers. One was in Garching in Germany. It had 30m long arms.


Another was in Glasgow with 10m long arms.


Another one was at Caltech with 40m arms. This was a prototype which actually used information from the Glasgow interferometer as one of the senior researchers at Glasgow moved to Caltech in the late 1970s.


The California Institute of Technology (Caltech) is a private research university in Pasadena, California. The university is known for its strength in science and engineering, and is one among a small group of institutes of technology in the United States which is primarily devoted to the instruction of pure and applied sciences.

These experiments worked very well. After many years of work the researchers were able to show that a good enough sensitivity could be achieved by making the arms longer allowing something significant to be discovered.

Large scale interferometers were proposed to funding agencies around the world in the mid to late 1980s.

LIGO in the USA with 4km arms

Virgo in France and Italy with 3km arms

A British project was also proposed. Unfortunately, the funds weren’t available.….1…..H/abstract


It was going to have 1km long arms. Using two convenient railway lines. You can just see them in the above right image.

Glasgow and the Garching groups decided to unite efforts to collaborate in a plan to build a large detector. It did not take long for both groups to jointly submit a plan for an underground 3km installation to be constructed in the Harz Mountains in Germany, but again their proposal was not funded.


Although reviewed positively, a shortage of funds in 1989 on both ends (the British Science and Engineering Research Council (SERC) and the Federal Ministry of Research and Technology (Bundesministerium für Forschung und Technologie BMFT)) prevented the approval. SERC thought there were better projects to spend money on.

The reason for the lack of funds for science in Germany was a consequence of the German reunification (1989–1990), as there was a need to boost the Eastern German economy.

In spite of this disheartening ruling, the new partners decided to try for a shorter detector and compensate by employing more advanced and clever techniques. A step forward was finally taken in 1994 when the University of Hanover and the State of Lower Saxony donated ground to build a 600m instrument in Ruthe, 20 km south of Hanover. Funding was provided by several German and British agencies. The construction of GEO 600 started on 4 September 1995.


GEO600 is a gravitational wave detector located near Sarstedt South of Hanover, Germany. It is designed and operated by scientists from the Max Planck Institute for Gravitational Physics, Max Planck Institute of Quantum Optics and the Leibniz Universität Hannover, along with University of Glasgow, University of Birmingham and Cardiff University in the United Kingdom, and is funded by the Max Planck Society and the Science and Technology Facilities Council (STFC). GEO600 is part of a worldwide network of gravitational wave detectors. This instrument, and its sister interferometric detectors, when operational, are some of the most sensitive gravitational wave detectors ever designed. They are designed to detect relative changes in distance of the order of 10−21, about the size of a single atom compared to the distance from the Sun to the Earth. GEO600 is capable of detecting gravitational waves in the frequency range 50 Hz to 1.5 kHz. Construction on the project began in 1995

The advantage of building it in Hanover was that it attracted funding because it was one part of communist East Germany and the German government thought that the project would be advantageous for the area.

The GEO 600 used unique material technologies to make up for its smaller size. Better detector sensitivity was required in order to measure smaller changes in position than the other detectors (LIGO was going to have 4km arms).

Because of research the gravitational wave detectors started to be built.

Two LIGO detectors were built in Hanford, Washington state and Livingston, Louisiana.

The image below is an aerial view of LIGO facility in Livingston.


The initial LIGO observatories were funded by the National Science Foundation (NSF) and were conceived, built and are operated by Caltech and MIT. They collected data from 2002 to 2010 but no gravitational waves were detected.

The Virgo interferometer is a large interferometer designed to detect gravitational waves predicted by the general theory of relativity. Virgo is a Michelson interferometer that is isolated from external disturbances: its mirrors and instrumentation are suspended and its laser beam operates in a vacuum. The instrument’s two arms are three kilometres long and located in Santo Stefano a Macerata, near the city of Pisa, Italy.

The Michelson interferometer is a common configuration for optical interferometry and was invented by Albert Abraham Michelson. Using a beam splitter, a light source is split into two arms. Each of those light beams is reflected back toward the beamsplitter which then combines their amplitudes using the superposition principle. The resulting interference pattern that is not directed back toward the source is typically directed to some type of photoelectric detector or camera. For different applications of the interferometer, the two light paths can be with different lengths or incorporate optical elements or even materials under test. (below right)

Albert Abraham Michelson FFRS HFRSE (December 19, 1852 – May 9, 1931) was an American physicist known for his work on measuring the speed of light and especially for the Michelson–Morley experiment. In 1907 he received the Nobel Prize in Physics, becoming the first American to win the Nobel Prize in a science. He was the founder and the first head of the physics department of the University of Chicago.

The image below left demonstrates a simple but typical Michelson interferometer.The bright yellow line indicates the path of the light. Photo was taken at the laboratory of physics of the Anhui Normal University.


The Japanese built an interferometer

TAMA 300 is a gravitational wave detector located at the Mitaka campus of the National Astronomical Observatory of Japan. It is a project of the gravitational wave studies group at the Institute for Cosmic Ray Research (ICRR) of the University of Tokyo. The ICRR was established in 1976 for cosmic ray studies, and is currently developing the Kamioka Gravitational Wave Detector (KAGRA).


TAMA 300 was preceded in Mitaka by a 20m prototype TAMA 20 in years 1991-1994. Later the prototype was moved underground to Kamioka mine and renamed LISM. It operated 2000-2002 and established seismic quietness of the underground location.

Construction of the TAMA project started in 1995. Data were collected from 1999 to 2004. It adopted a Fabry–Pérot Michelson interferometer (FPMI) with power recycling. It is officially known as the 300m Laser Interferometer Gravitational Wave Antenna due to having 300 meter long (optical) arms.

The goal of the project was to develop advanced techniques needed for a future kilometer sized interferometer and to detect gravitational waves that may occur by chance within the Local Group.

Observation of TAMA has been terminated, and work moved to the 100 m Cryogenic Laser Interferometer Observatory (CLIO) prototype in Kamioka mine.

As of 2020, modified TAMA 300 is used as a testbed to develop new technologies.

The Australian International Gravitational Observatory (AIGO) is a research facility located near Gingin, north of Perth in Western Australia. It is part of a worldwide effort to directly detect gravitational waves.

The current aim of the facility is to develop advanced techniques for improving the sensitivity of interferometric gravitational wave detectors such as LIGO and VIRGO. A study of operational interferometric gravitational wave detectors shows that AIGO is situated in almost the ideal location to complement existing detectors in the Northern hemisphere.

Current facilities (AIGO Stage I) consist of an L-shaped ultra-high vacuum system, measuring 80 m on each side forming an interferometer for detecting gravitational waves.


Initial observing between 2001 and 2010 saw no believable signals so sensitivity need to be improved by a factor of 10 to 15 times the values achievable at the time. These values were based on theoretical calculations carried out over this period.

The technology developed with GEO 600 could potentially help with the improvement of sensitivity in the larger detectors. In particular, the use of monolithic silica fibre-optic suspensions, which reduced thermal noise, and signal recycling mirrors.


What was needed was the mirrors of the interferometer to be hung as pendulums. The mirrors had internal modes of vibration and researchers wanted to look between the two modes of vibration (the pendulum and internal mode), which is the detection band (a couple of kHz). That is the region where they wanted to look for gravitational waves.

The peaks, as shown in the graph above, needed to be as narrow as possible so that the thermal noise caused by movement of atoms and molecules is as low as possible across the detection band.

This is why special materials were required to make these resonance peaks as narrow as possible. To do this a special type of glass was used to make the pendulum wires to hang the mirrors and the mirrors themselves. Technology was also required to join the silica fibres on to the glass mirrors, although there was the worry that this form of glass was not strong and would break easily.


Suspension testing at MIT

As part of the upgrades to LIGO (aLIGO) and based on their work, the GEO600 group were invited to join the LIGO team and bring their technology in 2008 (they had been successfully using the technology they were bringing for the last eight years)..

So, there was a strong GEO involvement in the LIGO upgrade in order to improve the sensitivity by a factor of 10 to 15 times the original value.



The GEO technology applied to LIGO were:

Silica suspensions; more sophisticated interferometry, more powerful lasers from the Hanover group (200W), plus active isolation developed in the USA, high power optics and other input from other US groups.

RAL (helping with the engineering) and Birmingham, Cardiff, Glasgow. Strathclyde and Sheffield Universities also had essential roles in this work.

Suspension testing


There were still worries about the fibres so a demonstration was carried out.

The fibres carried a mass equivalent to the mass of the mirror (40kg) in advanced LIGO (aLIGO) and you could bash the apparatus with a mallet without the fibres breaking.

The fibres could last for many years if they were kept in a vacuum and not touched. In the video (links above) it was explained that they were very strong in tension but they would break very easily if you just swiped them sideways with your finger.


The above image shows one of the evacuated tubes that carries the laser light to the mirror at the very end (and carries it back too). The building at the front contains the beam splitter.

Listening for merging binary Black Holes

The point of these experiments is to look for black holes forming or black holes merging.


This video is divided into two parts, each part showing a different numerical simulation, with brief captions that describe what is being shown. Part 1: Binary black holes orbit, lose energy because of gravitational radiation, and finally collide, forming a single black hole; gravitational waveform, spacetime curvature, and orbital trajectories are shown. Part 2: Event horizon and apparent horizons for the head-on collision of two black holes.

There are very good theories of what would happen when two Black Holes merge.

They rotate around each other, getting faster and faster as energy is radiated. They come together and merge to produce one object. This object wobbles.

The waves showing at the bottom of the image above are tracing out the gravitational waves produced. The frequency of these increases as the Black Holes merge.

After they have merged the big Black Hole wobbles for bit and then the gravitational wave signal disappears. This is what the researchers were looking for.

With the two LIGO detectors a signal was found on the 14th September 2015.


The frequency detected by both LIGO detectors saw the increased frequency.

This was an exciting discovery as it verified Einstein and Schwarzchild by making the first true gravitational observation of two Black Holes coalescing. But at the time the researchers didn’t quite know what to make of it. They were worried that it could be a mistake but on the 25th of December 2015 in the US another event was recorded and they were convinced they had detected gravitational waves.

This was also the first direct evidence that Black Holes existed. Prior to this all Black Hole work had been theoretical, or by looking at indirect evidence such as the emission of X-rays (x-rays are not coming from the Black Hole itself) or their effect on their surroundings

As gas spirals toward a black hole through a formation called an accretion disk, it heats up to roughly 10 million degrees Celsius. The temperature in the main body of the disk is roughly 2,000 times hotter than the sun and emits low-energy or “soft” X-rays. However, observations also detect “hard” X-rays which produce up to 100 times higher energy levels.

By nature, black holes do not themselves emit any electromagnetic radiation other than the hypothetical Hawking radiation, so astrophysicists searching for black holes must generally rely on indirect observations. For example, a black hole’s existence can sometimes be inferred by observing its gravitational influence upon its surroundings.

Cygnus X-1 (abbreviated Cyg X-1) is a galactic X-ray source in the constellation Cygnus and was the first such source widely accepted to be a black hole. It was discovered in 1964 during a rocket flight and is one of the strongest X-ray sources seen from Earth, producing a peak X-ray flux density of 2.3 x 10−23 Wm−2 Hz−1. It remains among the most studied astronomical objects in its class. The compact object is now estimated to have a mass about 14.8 times the mass of the Sun and has been shown to be too small to be any known kind of normal star, or other likely object besides a black hole. If so, the radius of its event horizon has 300 km “as upper bound to the linear dimension of the source region” of occasional X-ray bursts lasting only for about 1 ms.

What can we learn from the signal shape?

The images below show two massive Black Holes spiralling towards each other as they orbit each other. As the separation decreases the velocity increases (the scale is fractions of the speed of light) because the forces are very high.


On top: Estimated gravitational wave amplitude of GW150914 at the Hanford detector. Above that are the Schwarzschild horizons of both merging black holes shown as calculated numerically from the general theory of relativity. Below: The effective distance of the black holes in units of Schwarzschild radii RS and the relative velocity in units of the speed of light. [Image: LIGO / Redesign: Daniela Leitner]


The above image shows the end of the strain graph. The ringdown frequency and Q give the mass and spin of the final Black Hole. The amplitude scale factor gave the luminosity distance (a standard siren),

The ringdown part of gravitational waves in the final stage of the merger of compact objects like Black Holes. Q is the quality factor of the resonance.

Gravitational waves originating from the inspiral phase of compact binary systems, such as neutron stars or black holes, have the useful property that energy emitted as gravitational radiation comes exclusively from the orbital energy of the pair, and the resultant shrinking of their orbits is directly observable as an increase in the frequency of the emitted gravitational waves.

By observing the waveform, the chirp mass can be computed and thence the power (rate of energy emission) of the gravitational waves. Thus, such a gravitational wave source is a standard siren of known loudness

In astrophysics the chirp mass of a compact binary system determines the leading-order orbital evolution of the system as a result of energy loss from emitting gravitational waves. Because the gravitational wave frequency is determined by orbital frequency, the chirp mass also determines the frequency evolution of the gravitational wave signal emitted during a binary’s inspiral phase. In gravitational wave data analysis it is easier to measure the chirp mass than the two component masses alone.

B.P. Abbott et. al. (LIGO Scientific Collaboration and Virgo Collaboration), Observation of Gravitational Waves from a Binary Black Hole Merger, Phys. Rev. Lett. 116, 061102 (2016)

The data from the merger gave quite a lot of information


The data was able to show how far away the merger took place (about 1.3 billion light years as 1Mpc = 3.262 x 106 light years and a light year is the distance travelled by light in a year). On Earth at that time only single celled creatures were around. It was also able to give the masses of the two Black Holes (about 36 and 29 solar masses) and the resultant mass of the Black Hole (62 solar masses along with 3 solar masses worth of radiated gravitational waves).

E = mc2 with m = 3 solar masses meant a lot of energy was radiated away as gravitational waves. Looking at the energy intensity of the gravitational waves over the short time of the pulse. It exceeds all the other light output in the Universe.

The announcement

A computer simulation shows the collision of two black holes, a tremendously powerful event detected for the first time ever by the Laser Interferometer Gravitational-Wave Observatory, or LIGO. LIGO detected gravitational waves, or ripples in space and time generated as the black holes spiraled in toward each other, collided, and merged. This simulation shows how the merger would appear to our eyes if we could somehow travel in a spaceship for a closer look. It was created by solving equations from Albert Einstein’s general theory of relativity using the LIGO data.

Prof Stephen Hawking celebrates gravitational wave discovery

Front page news

Everyone was happy at the fact that the discovery made the news all over the world.



Private Eye, a satirical magazine had its own humorous take on the subject

Private Eye is a British fortnightly satirical and current affairs news magazine, founded in 1961. It is published in London and has been edited by Ian Hislop since 1986. The publication is widely recognised for its prominent criticism and lampooning of public figures. It is also known for its in-depth investigative journalism into under-reported scandals and cover-ups.

The Sounds of Spacetime at the end of 2015

Comparison of LIGO gravitational Wave Events

Natural Pitch


The best-fit models of LIGO’s gravitational-wave signals are converted into sounds. The first sound is from modelled gravitational waves detected by LIGO on Dec. 26, 2015, when two black holes merged. This is then compared to the first-ever gravitational waves detected by LIGO on Sept. 14, 2015, when two higher-mass black holes merged. This sequence is repeated. The pitch of both signals is then increased, allowing them to be heard more easily, and this sequence is also repeated.

The two events are slightly different (the September and December signals). The reason for this is that the masses were slightly different.

“LIGO, the Laser interferometer Gravitational-Wave Observatory is a collaborative project with over one thousand researchers from more than twenty countries. Together, they have realised a vision that is almost fifty years old. The 2017 Nobel Laureates have, with their enthusiasm and determination, each been invaluable to the success of LIGO.”


Rainer “Rai” Weiss (born September 29, 1932) is an American physicist, known for his contributions in gravitational physics and astrophysics. He is a professor of physics emeritus at MIT and an adjunct professor at LSU. He is best known for inventing the laser interferometric technique which is the basic operation of LIGO. He was Chair of the COBE Science Working Group.

He is a member of the Fermilab Holometer experiment, which uses a 40m laser interferometer to measure properties of space and time at quantum scale and provide Planck-precision tests of quantum holographic fluctuation.

In 2017, Weiss was awarded the Nobel Prize in Physics, along with Kip Thorne and Barry Barish, “for decisive contributions to the LIGO detector and the observation of gravitational waves”

Barry Clark Barish (born January 27, 1936) is an American experimental physicist and Nobel Laureate. He is a Linde Professor of Physics, emeritus at California Institute of Technology and a leading expert on gravitational waves.

In 2017, Barish was awarded the Nobel Prize in Physics along with Rainer Weiss and Kip Thorne “for decisive contributions to the LIGO detector and the observation of gravitational waves”.

In 2018, he joined the faculty at University of California, Riverside, becoming the university’s second Nobel Prize winner on the faculty.

Kip Stephen Thorne (born June 1, 1940) is an American theoretical physicist known for his contributions in gravitational physics and astrophysics. A longtime friend and colleague of Stephen Hawking and Carl Sagan, he was the Feynman Professor of Theoretical Physics at the California Institute of Technology (Caltech) until 2009 and is one of the world’s leading experts on the astrophysical implications of Einstein’s general theory of relativity. He continues to do scientific research and scientific consulting, most notably for the Christopher Nolan film Interstellar. Thorne was awarded the 2017 Nobel Prize in Physics along with Rainer Weiss and Barry C. Barish “for decisive contributions to the LIGO detector and the observation of gravitational waves”

Nobel Lecture, December 8, 2017

By the Autumn of 2017 ten black holes had been discovered. They had different sizes and different rates of evolution.

Two runs had taken place over a year.

Spectrograms and waveforms for the gravitational-wave transient catalogue. The insets of this image show the gravitational waveform (bottom) and time-frequency spectrogram for each confident detection in the gravitational-wave transient catalogue. The spectrogram colour indicates a measure of signal strength; the increase of signal frequency with time clearly shows the characteristic “chirp” signature of a binary inspiral. The waveforms shown at the bottom of each panel are produced from either a range of models based on Einstein’s theory (orange) or a wavelet decomposition of the observed signal (gray). [Image credit: LIGO/VIrgo/Georgia Tech/S. Ghonge & K. Jani]


During the runs they discovered something unexpected. The signal took much longer and was much smaller. They realised it wasn’t a Black Holes coalescence, but a binary neutron star coalescence.


They had discovered two neutron stars. One was small and about 0.86 solar masses and the other was large at about 2.26 solar masses.

They were much closer at 40Mpc than the first Black Hole discovered. Not only were they seen because of the gravitational waves but because a short gamma ray burst occurred. This was recorded by satellites including Fermi-GST which contained the Fermi-GBM.


The Fermi Gamma-ray Space Telescope (FGST), formerly called the Gamma-ray Large Area Space Telescope (GLAST), is a space observatory being used to perform gamma-ray astronomy observations from low Earth orbit. Its main instrument is the Large Area Telescope (LAT), with which astronomers mostly intend to perform an all-sky survey studying astrophysical and cosmological phenomena such as active galactic nuclei, pulsars, other high-energy sources and dark matter. Another instrument aboard Fermi, the Gamma-ray Burst Monitor (GBM; formerly GLAST Burst Monitor), is being used to study gamma-ray bursts.

Fermi was launched on 11 June 2008 at 16:05 UTC aboard a Delta II 7920-H rocket. The mission is a joint venture of NASA, the United States Department of Energy, and government agencies in France, Germany, Italy, Japan, and Sweden, becoming the most sensitive gamma-ray telescope on orbit, succeeding INTEGRAL. The project is a recognized CERN experiment (RE7).


It became clear from this work that short gamma ray bursts were coming from neutron stars coalescing.

This event was a very significant discovery because the signal was also observed, not just as gravitational waves and gamma rays but as X-rays, visible light and radio waves.

This led on to the study of the most extreme states of matter.


The gamma rays lagged behind the gravitational waves because gravitational waves hardly interact with anything so they get to the detectors at the speed of light, whereas gamma rays, despite being electromagnetic waves interact with any ionised media they tend to slow down.

The INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) is a space telescope for observing gamma rays of energies up to 8 MeV. It was launched by the European Space Agency (ESA) into Earth orbit in 2002, and is designed to provide imaging and spectroscopy of cosmic sources. In the MeV energy range, it is the most sensitive gamma ray observatory in space. It is sensitive to higher energy photons than X-ray instruments such as NuSTAR, the Neil Gehrels SWIFT Observatory, XMM-Newton, and lower than other gamma-ray instruments such Fermi and HESS.

The event of the neutron star coalescence was modelled.

This simulation shows the final stages of the merging of two neutron stars. The merger shown in the simulation is happening much faster in reality, within less than a hundredth of a second, and produces strong gravitational waves. This illustrates one of the possible scenarios for the merger event GW170817, detected by the LIGO-Virgo gravitational-wave network. The result of the merger could have been a neutron star or a black hole, the latter of which is shown here.

Doomed neutron stars whirl toward their demise in this animation. Gravitational waves (pale arcs) bleed away orbital energy, causing the stars to move closer together and merge. As the stars collide, some of the debris blasts away in particle jets moving at nearly the speed of light, producing a brief burst of gamma rays (magenta). In addition to the ultra-fast jets powering the gamma-rays, the merger also generates slower moving debris. An outflow driven by accretion onto the merger remnant emits rapidly fading ultraviolet light (violet). A dense cloud of hot debris stripped from the neutron stars just before the collision produces visible and infrared light (blue-white through red). The UV, optical and near-infrared glow is collectively referred to as a kilonova. Later, once the remnants of the jet directed toward us had expanded into our line of sight, X-rays (blue) were detected. This animation represents phenomena observed up to nine days after GW170817.

Neutron stars moving around each other. Going faster as they move closer together. Coalescing. A mammoth explosion. Huge amounts of energy and radiation coming off. Huge amounts of ionisation coming off.

Optical astronomers studied the wavelength of the light coming off and realised that this event was called a kilonova. This resulted in lots of elements being produced.







A kilonova (also called a macronova or r-process supernova) is a transient astronomical event that occurs in a compact binary system when two neutron stars or a neutron star and a black hole merge into each other. Kilonovae are thought to emit short gamma-ray bursts and strong electromagnetic radiation due to the radioactive decay of heavy r-process nuclei that are produced and ejected fairly isotropically during the merger process.


Artist’s impression of neutron stars merging, producing gravitational waves and resulting in a kilonova.


The elements produced in the kilonova had originally puzzled scientists, who didn’t know how they had been formed.

In stars like our Sun hydrogen fuses to form helium. In heavier stars helium can fuse into other elements.


Interior structure of a massive star just before it exhausts its nuclear fuel: High-mass stars can fuse elements heavier than carbon. As a massive star nears the end of its evolution, its interior resembles an onion. Hydrogen fusion is taking place in an outer shell, and progressively heavier elements are undergoing fusion in the higher-temperature layers closer to the centre. All of these fusion reactions generate energy and enable the star to continue shining. Iron is different. The fusion of iron requires energy, and when iron is finally created in the core, the star has only minutes to live.

The puzzle had been how heavier elements than iron could be formed. It was thought that these heavy elements would be formed in supernova explosions because they produced huge amounts of energy. However, the theory didn’t quite work.

In looking at the optical emissions from the kilonova it was realised how these heavier elements were produced.

A supernova is a powerful and luminous stellar explosion. This transient astronomical event occurs during the last evolutionary stages of a massive star or when a white dwarf is triggered into runaway nuclear fusion. The original object, called the progenitor, either collapses to a neutron star or black hole, or is completely destroyed. The peak optical luminosity of a supernova can be comparable to that of an entire galaxy before fading over several weeks or months.


SN 1994D (bright spot on the lower left), a type Ia supernova within its host galaxy, NGC 4526

Binding energy is the amount of energy released when a particular isotope is formed. Protons and neutron are held together by the strong force, which only acts over very small distances but is able to overcome the electrostatic repulsion between protons. The strength of the bonding is measured by the binding energy per nucleon where “nucleon” is a collective name for neutrons and protons (sometimes called the mass defect per nucleon). The mass defect reflects the fact that the total mass of the nucleus is less than the sum of the mass of the individual neutrons and protons that formed it. The difference in mass is equivalent to the energy released in forming the nucleus.

The general decrease in binding energy beyond iron is due to the fact that, as nuclei gets bigger, the ability of the strong force to counteract the electrostatic repulsion between protons becomes weaker.



The build-up of heavier elements in the nuclear fusion processes in stars is limited to elements below iron, since the fusion of iron would subtract energy rather than provide it. Iron-56 is abundant in stellar processes, and with a binding energy per nucleon of 8.8 MeV, it is the third most tightly bound of the nuclides. Its average binding energy per nucleon is exceeded only by 58Fe and 62Ni, the nickel isotope being the most tightly bound of the nuclides.

In order to produce elements heavier than iron a great deal of energy is required.

The coalescence of two neutron stars forming a kilonova got people quite excited.


Financial Times 17th October 2017. Sir James thought perhaps the FT was interested as this process could result in the production of gold. Although it would be rather difficult to go and fetch it as it would be scattered throughout the Universe (which would explain how it, along with other precious metals ended up on Earth).

A three runs have been carried out. The last finished in April 2020 (at the start of the Covid-19 situation).


Between each run the sensitivity was improved. The event rate increased significantly

Preparation is underway for a future upgrade to the Advanced LIGO + (A+).

February 2021 situation

There have been 39 detections published since the start of the third data run (which ran for about a year).

38 Black Hole/Black Hole coalescences published including

a) One with components thought to be too large for a stellar collapsed Black Hole (it had a mass of more than 60 solar masses) – an intermediate mass Black Hole, which was not known to exist

An intermediate-mass black hole (IMBH) is a class of black hole with mass in the range 102–105 solar masses: significantly more than stellar black holes but less than the 105–109 solar mass supermassive black holes. Several IMBH candidate objects have been discovered in our galaxy and others nearby, based on indirect gas cloud velocity and accretion disk spectra observations of various evidentiary strength.

The gravitational wave signal GW190521 detected on 21 May 2019 resulted from the merger of two black holes, weighing 85 and 65 solar masses, with the resulting black hole weighing 142 solar masses.

b) One with a major asymmetry in the masses of the two components. Most of the others discovered had two components of similar mass.

c) One with one component with a mass which should be neither a Neutron star of a Black Hole

d) One with very little aligned spin of the components

e) One with significant aligned spin

d and e gave information on how the Black Holes formed.

1 Neutron star/Neutron star binary with a resultant object having a mass which should be neither a Neutron star or a Black Hole.

~20 further triggers released to the astronomy community. This included a further potential Neutron star/Neutron Star binary and a potential new discovery of a Neutron star and Black Hole binary.

Scientists are beginning to have enough data to check whether the coalescences are consistent with Einstein’s theory of General Relativity and measure the expansion rate of the Universe. It is also allowing them to deduce how the coalescence systems may have been formed ad perhaps enabling them to work out what dark energy is.

This plot shows the masses of all compact binaries detected by LIGO/Virgo, with black holes in blue and neutron stars in orange. Also shown are stellar mass black holes (purple) and neutron stars (yellow) discovered with electromagnetic observations. Credits: LIGO/VIrgo/Northwestern Univ./Frank Elavsky

Most of the masses of the Black Holes are very similar. Coalescing together to give a heavier mass.


A lot of energy is radiated out when the coalescences take place.

One Black Hole formed had a mass of 160 solar masses. Which had to be an intermediate Black Hole. This might produce information to explain how supermassive Black Holes form. Perhaps they formed by Black Holes merging together.

There is a mass gap between 2 and 5 solar masses. What are the objects in this gap? Are they Neutron stars or other Black Holes or something exotic which hasn’t been discovered yet?


Credit: Visualization: LIGO -Virgo / Frank Elavsky, Aaron Geller / Northwestern

Strange findings

Events with masses which are not right for Neutron stars or Black Holes. Scientists don’t yet know what they are but suspect they are Black Holes. Could they be quark stars?

A quark star is a hypothetical type of compact, exotic star, where extremely high core temperature and pressure has forced nuclear particles to form quark matter, a continuous state of matter consisting of free quarks.

There is a suggestion that intermediate Black Holes result not from straight stellar collapses but from the coalescence of lighter Black Holes. This may provide evidence of how supermassive Black Holes form.

What further research needs to be done?

A network of detectors besides LIGO, Virgo and GEO, needs to be placed for good source location.

Overall sensitivity needs to be improved.

LIGO and Virgo needs to be upgraded.

KAGRA, which is an underground interferometer, is being built in a mine in Japan. It is in initial operation.


A third LIGO detector is being planned for India.

INDIGO or IndIGO (Indian Initiative in Gravitational-wave Observations) is a consortium of Indian gravitational-wave physicists. This is an initiative to set up advanced experimental facilities for a multi-institutional observatory project in gravitational-wave astronomy located near Aundha Nagnath, Hingoli District, Maharashtra.

A second-generation network is being developed to include Advanced LIGO, Advanced Virgo and the interferometers in Japan and India.

The Global Network c. 2025


This will enable better localisation so the origin of events can be determined.

Two new detectors are planned.

Einstein Telescope (ET) or Einstein Observatory, is a proposed third-generation ground-based gravitational wave detector, currently under study by some institutions in the European Union. It will be able to test Einstein’s general theory of relativity in strong field conditions and realize precision gravitational wave astronomy.

The ET is a design study project supported by the European Commission under the Framework Programme 7 (FP7). It concerns the study and the conceptual design for a new research infrastructure in the emergent field of gravitational-wave astronomy.


The Einstein Telescope will hunt for gravitational waves – tiny ripples in the fabric of space–time that are a prediction of Albert Einstein’s general theory of relativity. (Courtesy: ASPERA). It will have 10km arms and be built underground.

Cosmic Explorer is a proposed third generation ground-based gravitational wave observatory. Cosmic explorer will use the same L-shaped design as the LIGO detectors, except with ten times longer arms of 40 km each. This will significantly increase the sensitivity of the observatory allowing observation of the first black hole mergers in the Universe. In 2019 Cosmic Explorer team published a study about research needed over 2020s decade to build the observatory.

The Gravitational Wave Spectrum


The plan is to build even more detectors, including LISA, which will be a gravitational wave detector in space.


The Laser Interferometer Space Antenna (LISA) is a proposed space probe to detect and accurately measure gravitational waves—tiny ripples in the fabric of space-time—from astronomical sources. LISA would be the first dedicated space-based gravitational wave detector. It aims to measure gravitational waves directly by using laser interferometry. The LISA concept has a constellation of three spacecraft arranged in an equilateral triangle with sides 2.5 million km long, flying along an Earth-like heliocentric orbit. The distance between the satellites is precisely monitored to detect a passing gravitational wave and should be able to detect weaker gravitational waves from Black Holes as the centre of galaxies Also detecting what happens when galaxies merge.


NASA illustration of LISA, taken from NASA’s description: The three LISA spacecraft will be placed in orbits that form a triangular formation with centre 20° behind the Earth and side length 5 million km. (The figure showing the formation is not to scale.) Each spacecraft will be in an individual Earth-like orbit around the Sun. The orbits are chosen to minimize changes in the lengths of the sides of the triangle. The orbits of the three spacecraft have a relationship between inclination and eccentricity that inclines the plane of the formation by 60° with respect to the ecliptic. The nodal longitudes of the three orbits are shifted by 120° to create the triangle. The heliocentric orbit offers a particularly quiet environment, critical for the control of disturbances on the test masses defining the interferometer arms. The test masses are free-falling and shielded by the enclosing spacecraft from disturbances of the solar wind and photon pressure. The orientation of the spacecraft with respect to the Sun changes very slowly. The Sun appears moves along a cone with a 30° half angle aligned with the spacecraft’s cylindrical axis once per year, giving constant illumination. The major source of disturbance in the measurement band is the variation in the solar constant caused by the Sun’s normal modes of oscillation, amounting to less than 10 ppm in intensity. The orbital motion of the antenna sweeps its sensitivity lobes across the sky, giving an amplitude modulation dependent on a source’s angular coordinates. Similarly, the Doppler effect gives a phase modulation dependent on a source’s angular coordinates. The two effects combine to give directional information about every source. Most of the sources observable by LISA are periodic or quasi-periodic and can be observed for at least a year. The angular position accuracy depends on the signal-to-noise ratio. For the strongest sources, the direction to the source can be determined to about 1 arc minute.

The LISA project started out as a joint effort between NASA and the European Space Agency (ESA). However, in 2011, NASA announced that it would be unable to continue its LISA partnership with the European Space Agency due to funding limitations. The project is a recognized CERN experiment (RE8). A scaled down design initially known as the New Gravitational-wave Observatory (NGO) was proposed as one of three large projects in ESA’s long-term plans. In 2013, ESA selected ‘The Gravitational Universe’ as the theme for one of its three large projects in the 2030s. whereby it committed to launch a space based gravitational wave observatory.

In January 2017, LISA was proposed as the candidate mission. On June 20, 2017 the suggested mission received its clearance goal for the 2030s, and was approved as one of the main research missions of ESA.

The LISA mission is designed for direct observation of gravitational waves, which are distortions of space-time travelling at the speed of light. Passing gravitational waves alternately squeeze and stretch objects by a tiny amount. Gravitational waves are caused by energetic events in the universe and, unlike any other radiation, can pass unhindered by intervening mass. Launching LISA will add a new sense to scientists’ perception of the universe and enable them to study phenomena that are invisible in normal light.

Potential sources for signals are merging massive black holes at the centre of galaxies, massive black holes orbited by small compact objects, known as extreme mass ratio inspirals, binaries of compact stars in our Galaxy, and possibly other sources of cosmological origin, such as the very early phase of the Big Bang, and speculative astrophysical objects like cosmic strings and domain boundaries.

Questions and Answers

1) Could you explain strain in the context of gravitational waves?

The interferometer has two arms at 90o to each other. When the gravitational wave passes through one arm gets stretched and the other gets squashed, and vice versa.

The strain is the change in length of the arms divided by the original length. A-level students will have met this concept when learning about Young Modulus where strain is the change in length of the object divided by its original length.


2) Why aren’t the light waves also stretched as they travel along the arms of the interferometer?

It’s to do with relativity. In saying one arm is squashed and the other stretched we are assuming a measuring length against the wavelength of light. We define the wavelength of light not to be affected. You can do the opposite by saying the length isn’t being affected but the light is an you get the same answer. In truth both are affected by the gravitational wave. It’s simpler to measure the wavelength of light when putting data into the relativity equations.

3) The progenitor Black Holes involved in the signal detected around September 2020 were unusual in size as well as the final Black Hole, which was also unusual. What made those Black Holes unusual?

These were large Black Holes that we weren’t expecting. 60 solar masses expected to be the maximum size. May be the 80 solar mass Black Holes were made up of smaller ones, say two 40 solar masses coming together. Maybe one of those 40 solar mass Black Holes has been made up of two 20 solar mass Black Holes coming together.

What it suggests is that larger Black Holes are built up from smaller Black Holes. Smaller Black Holes coalescing together. This may explain where supermassive Black Holes (millions of solar masses) at the centre of galaxies formed. Just starting with small Black Holes joining together. One coalescence after another, and so on. At the moment nobody knows exactly how the Black Holes at the centres of galaxies formed or how they got there, but the current work is indicating it could simply be small Black Holes coalescing to form larger Black Holes, which in turn coalesce with other Black Holes. More measurements are needed.

Due to the pulse pair instability large Solar mass objects can be formed that include 160 solar masses.

A pulsational pair-instability supernova is a supernova impostor event that generally occurs in stars at around 100 to 130 solar mass (M☉), as opposed to a typical pair-instability supernova which occurs in stars of 130 to 250 M☉. Like pair-instability supernovae, pulsational pair-instability supernovae are caused by draining of a star’s energy in the production of electron-positron pairs but, whereas a pair-instability supernova completely disrupts the star in a massive supernova, the star’s pulsational pair-instability eruption sheds 10–25 M☉. This generally shrinks it down to a mass of less than 100 M☉, too small for electron-positron pair creation, where it then undergoes a core-collapse supernova or hypernova.

4) I believe a picture of a supermassive Black Hole has been taken?

Yes, one was taken about a year and a half ago

A supermassive black hole (SMBH or sometimes SBH) is the largest type of black hole, with mass on the order of millions to billions of times the mass of the Sun (M☉). Black holes are a class of astronomical objects that have undergone gravitational collapse, leaving behind spheroidal regions of space from which nothing can escape, not even light. Observational evidence indicates that almost every large galaxy has a supermassive black hole at the galaxy’s centre. The Milky Way has a supermassive black hole in its Galactic Centre, which corresponds to the location of Sagittarius A*. Accretion of interstellar gas onto supermassive black holes is the process responsible for powering active galactic nuclei and quasars.


This is the first direct image (April 2019) taken of a supermassive black hole, located at the galactic core of Messier 87. It shows a heated accretion ring orbiting the object at a mean separation of 350 AU, or ten times larger than the orbit of Neptune around the Sun. The dark centre is the event horizon and its shadow.

It’s very difficult to actually get any information from the picture. You can see the accretion disc around the outside because as material breaks up electromagnetic radiation is emitted.

An accretion disk is a structure (often a circumstellar disk) formed by diffuse material in orbital motion around a massive central body. The central body is typically a star. Friction causes orbiting material in the disk to spiral inward towards the central body. Gravitational and frictional forces compress and raise the temperature of the material, causing the emission of electromagnetic radiation. The frequency range of that radiation depends on the central object’s mass. Accretion disks of young stars and protostars radiate in the infrared; those around neutron stars and black holes in the X-ray part of the spectrum. The study of oscillation modes in accretion disks is referred to as diskoseismology.

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