What we still don’t know about black holes

Last year the first image of a black hole made headline news around the world. In this online lecture, leading astronomer Chris Impey explained why black holes occupy a special place in modern science and in the public mind.

He revealed why the idea of an object with gravity so strong that even light can’t escape begins with general relativity; how Stephen Hawking transformed our understanding of black holes; and why theorists still struggle to understand the singularity and what happens to information that falls into the event horizon.

We found out how astronomers plan to use black holes, large and small, to test general relativity in new ways. And why despite recently entering an exciting era with the detection of gravitational waves from merging black holes and even the first image of a black hole, these enigmatic objects are still not ready to give up all their secrets.

Chris’s inspiring and fascinating lecture, was followed by a Q&A where participants of the live event were able to ask him questions about black holes.

About the speakers:

Chris Impey is a University Distinguished Professor of Astronomy and Associate Dean of the College of Science at the University of Arizona.

He has over 180 refereed publications on observational cosmology, galaxies, and quasars, and his research has been supported by $20 million in NASA and NSF grants. He has won 11 teaching awards and has taught two online classes with over 180,000 enrolled and 2 million minutes of video lectures watched.

A prolific author, his latest book is Einstein’s Monsters: The Life and Times of Black Holes.


The event was hosted by Valerie Jamieson, New Scientist Events‘ creative director

Valerie Jamieson is creative director of New Scientist Events and hosts a programme of evening lectures, masterclasses and New Scientist Live. Previously she ran the New Scientist features desk and specialised in writing and editing features about the latest ideas in physics, astronomy and mathematics. She has PhD in experimental high-energy physics


Recorded on Thursday 30 April 2020

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 Professor Impey and my readers will forgive any mistakes and let me know what I got wrong.


It could be said that the fathers of Black Holes were Albert Einstein and Stephen Hawking

https://en.wikipedia.org/wiki/Albert_Einstein (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).

https://en.wikipedia.org/wiki/Stephen_Hawking (below right)


Stephen William Hawking CH CBE FRS FRSA (8 January 1942 – 14 March 2018) was an English theoretical physicist, cosmologist, and author who was director of research at the Centre for Theoretical Cosmology at the University of Cambridge at the time of his death. He was the Lucasian Professor of Mathematics at the University of Cambridge between 1979 and 2009.

Ideas about Black Holes actually goes back before these two eminent physicists and general relativity.


General relativity is a theory of gravitation that was developed by Albert Einstein between 1907 and 1915. According to general relativity, the observed gravitational effect between masses results from their warping of spacetime.

Early ideas


An English clergyman, John Michell, speculated about very large 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. Considered “one of the greatest unsung scientists of all time”, he was the first person known to offer the following concepts: proposed the existence of black holes; suggested that earthquakes travelled in (seismic) waves; explained how to manufacture an artificial magnet; and, recognizing that double stars were a product of mutual gravitation, he was the first to apply statistics to the study of the cosmos. He invented an apparatus to measure the mass of the Earth. He has been called the father both of seismology and of magnetometry.


It was Michell who, in a paper for the Philosophical Transactions of the Royal Society of London, read on 27 November 1783, first proposed the idea that there were such things as black holes, which he called “dark stars”. Having accepted Newton’s corpuscular theory of light, which posited that light consists of minuscule particles, he reasoned that such particles, when emanated by a star, would be slowed down by its gravitational pull, and thought that it might therefore be possible to determine the star’s mass based on the reduction in speed. This insight led in turn to the recognition that a star’s gravitational pull might be so strong that the escape velocity would exceed the speed of light. Michell calculated that this would be the case with a star more than 500 times the size of the Sun. Since light would not be able to escape such a star, it would be invisible. In his own words:

“If there should really exist in nature any bodies, whose density is not less than that of the sun, and whose diameters are more than 500 times the diameter of the sun, since their light could not arrive at us; or if there should exist any other bodies of a somewhat smaller size, which are not naturally luminous; of the existence of bodies under either of these circumstances, we could have no information from sight; yet, if any other luminous bodies should happen to revolve about them we might still perhaps from the motions of these revolving bodies infer the existence of the central ones with some degree of probability, as this might afford a clue to some of the apparent irregularities of the revolving bodies, which would not be easily explicable on any other hypothesis; but as the consequences of such a supposition are very obvious, and the consideration of them somewhat beside my present purpose, I shall not prosecute them any further.” — John Michell, 1784


He gave a talk before this renowned society in 1783 on the gravitation of stars. He used a thought experiment to explain that light would not leave the surface of a very massive star if the gravitation was sufficiently large. And he deduced: “Should such an object really exist in nature, its light could never reach us.”

More than a decade after Michell, another scientist took up this same topic: in his book published in 1796 – Exposition du Système du Monde – the French mathematician, physicist and astronomer Pierre-Simon de Laplace described the idea of massive stars from which no light could escape; this light consisted of corpuscles, very small particles, according to the generally accepted theory of Isaac Newton. Laplace called such an object corps obscur, i.e. dark body.


Stellar thoughts: in 1796, the French mathematician, physicist and astronomer Pierre-Simon de Laplace described the idea of idea of ​​heavy stars from which light could not escape. Credit: Public domain


Pierre-Simon, marquis de Laplace (23 March 1749 – 5 March 1827) was a French scholar and polymath whose work was important to the development of engineering, mathematics, statistics, physics, astronomy, and philosophy.

“A star large or dense enough could have an escape velocity equal to light (Michell 1783, LaPlace 1796).

Both Michell and LaPlace’s ideas were based on Isaac Newton’s ideas about light.


As mentioned above Isaac Newton had imagined light as being composed of tiny particles called corpuscles, which differed from our modern understanding of the photon in one crucial respect. Corpuscles were thought to have mass, which meant that gravity could slow down the speed of light.


However, things changed in 1802 when Thomas Young put forth a number of theoretical reasons supporting the wave theory of light, and he developed two enduring demonstrations to support this viewpoint. With the ripple tank he demonstrated the idea of interference in the context of water waves. With Young’s interference experiment, or double-slit experiment, he demonstrated interference in the context of light as a wave.





Young, speaking on 24 November 1803, to the Royal Society of London, began his now-classic description of the historic experiment:

“The experiments I am about to relate … may be repeated with great ease, whenever the sun shines, and without any other apparatus than is at hand to every one.”

In his subsequent paper, titled Experiments and Calculations Relative to Physical Optics (1804), Young describes an experiment in which he placed a card measuring approximately 0.85 millimetres (0.033 in) in a beam of light from a single opening in a window and observed the fringes of colour in the shadow and to the sides of the card. He observed that placing another card in front or behind the narrow strip so as to prevent the light beam from striking one of its edges caused the fringes to disappear. This supported the contention that light is composed of waves


Image of plate XXX from Thomas Young’s “Lectures”, publ. 1807, the text of lectures to London’s Royal Institution in 1802. Shows Young’s grasp of ocular anatomy, the fact that an image is upside down on the retina, notice of the “two slit” phenomenon, which supports wave theory of light… etc.



Thomas Young FRS (13 June 1773 – 10 May 1829) was a British polymath who made notable contributions to the fields of vision, light, solid mechanics, energy, physiology, language, musical harmony, and Egyptology.

Now the acceptance of light had a problem as nobody understood how a wave could be trapped by gravity. So, the idea of a “dark star” disappeared.

Welcome to Einstein’s world

Where his theory of gravity leads to black holes


When working on his theory of general relativity Einstein was struck by the “coincidence” that inertial mass (the objects resistance to a change in motion) is identical to gravitational mass (its change in motion due to gravity force).


Einstein saw that there is no way to distinguish between acceleration caused by gravity and acceleration due to any other force.

Mass-energy curves space-time


When an object is being deflected it is simply following space-time.

Space-time is the key idea of general relativity


The Hubble telescope image above shows mass bends light thousands of time during the 30 years it has been in orbit. Distant galaxies lensed by gravity by an intermediate set of galaxies.

The curved arcs are caused by the mass of the intervening galaxy bending light around it like a lens. There is a similarity to optical lensing.



The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope but it is one of the largest and most versatile, well known both as a vital research tool and as a public relations boon for astronomy. The Hubble telescope is named after astronomer Edwin Hubble and is one of NASA’s Great Observatories, along with the Compton Gamma Ray Observatory, the Chandra X-ray Observatory, and the Spitzer Space Telescope.

Gravity slows down time



General relativity theory makes various predictions about the behaviour of space and time. One of these predictions, put in everyday terms, is that the stronger the gravity, the slower the pace of time.

An ingenious experiment in 1959 used the most accurate atomic clock known to compare time measurements on the ground floor and the top floor of the physics building at Harvard University. For a clock, the experimenters used the frequency (the number of cycles per second) of gamma rays emitted by radioactive cobalt. Einstein’s theory predicts that such a cobalt clock on the ground floor, being a bit closer to Earth’s centre of gravity, should run very slightly slower than the same clock on the top floor. This is precisely what the experiments observed. Later, atomic clocks were taken up in high-flying aircraft and even on one of the Gemini space flights. In each case, the clocks farther from Earth ran a bit faster. While in 1959 it didn’t matter much if the clock at the top of the building ran faster than the clock in the basement, today that effect is highly relevant. Every smartphone or device that synchronizes with a GPS must correct for this (as we will see in the next section) since the clocks on satellites will run faster than clocks on Earth.

In November 1976, when the two Viking spacecraft were operating on the surface of Mars, the planet went behind the Sun as seen from Earth (see below). Scientists had preprogrammed Viking to send a radio wave toward Earth that would go extremely close to the outer regions of the Sun. According to general relativity, there would be a delay because the radio wave would be passing through a region where time ran more slowly. The experiment was able to confirm Einstein’s theory to within 0.1%.


Time Delays for Radio Waves near the Sun: Radio signals from the Viking lander on Mars were delayed when they passed near the Sun, where spacetime is curved relatively strongly. In this picture, spacetime is pictured as a two-dimensional rubber sheet.

Black holes extend time, and once you are inside one, time stands still.

So how do you make one?

You can’t get any information from a black hole in order to make one. The reason is that a black hole has an escape velocity equal to the speed of light so no information can escape.

However, any matter could become a black hole it you could crush it beyond its “Schwarzschild radius”


The Schwarzschild radius (sometimes historically referred to as the gravitational radius) is a physical parameter that shows up in the Schwarzschild solution to Einstein’s field equations, corresponding to the radius defining the event horizon of a Schwarzschild black hole. It is a characteristic radius associated with every quantity of mass. The Schwarzschild radius (Sch. R) was named after the German astronomer Karl Schwarzschild, who calculated this exact solution for the theory of general relativity in 1916.

The Schwarzschild radius is given as


where G is the gravitational constant, M is the object mass, and c is the speed of light.



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

So black holes could be smaller than a peanut or larger than a town

image image

To turn our Sun into a black hole you would need to crush it to the size of a small town


Black holes and star death


When a massive star exhausts its fuel and its core is more than three times the mass of our Sun then no force can resist the contraction.

Black hole basics

In the 1940s, British astronomers Hermann Bondi, Fred Hoyle, and Raymond Lyttleton investigated the possibility of a star gathering matter from the surrounding medium due to gravitational attraction.

https://en.wikipedia.org/wiki/Hermann_Bondi (below left)


Sir Hermann Bondi KCB FRS (1 November 1919 – 10 September 2005) was a British-Austrian mathematician and cosmologist.

https://en.wikipedia.org/wiki/Fred_Hoyle (above centre)

Sir Fred Hoyle FRS (24 June 1915 – 20 August 2001) was an English astronomer who formulated the theory of stellar nucleosynthesis.

https://en.wikipedia.org/wiki/Raymond_Lyttleton (above right)

Raymond Arthur Lyttleton FRS (7 May 1911 – 16 May 1995) was a British mathematician and theoretical astronomer.

Two elements define a black hole:

The event horizon; Infinite point of density and mass – the singularity


Just beyond the event horizon objects feel the gravity of the black hole, but light can still escape

Physicists don’t understand what it means to have a singularity – infinite density and mass should be impossible according to Stephen Hawking.

Do they actually exist and how can we understand them?


1964 — The first recorded use of the term “black hole”, by journalist Ann Ewing

1966 — Yakov Zel’dovich and Igor Novikov propose searching for black hole candidates among binary systems in which one star is optically bright and X-ray dark and the other optically dark but X-ray bright (the black hole candidate)

1967 — John Wheeler introduces the term “black hole” in his lecture to the American Association for the Advancement of Science

1972 — Identification of Cygnus X-1/HDE 226868 from dynamic observations as the first binary with a stellar black hole candidate


An isolated black hole is very hard to find which is why you need a binary system


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.

Welcome to Stephen Hawking’s world


Black holes are not black

Virtual particle and virtual antiparticle pairs are always being created from radiation in the vacuum of space, then turning into radiation.


Hawking realized that one of a pair could pass into the event horizon, so black holes radiate, and will eventually evaporate. The other particle is trapped in the black hole.

The particle that escapes could cause the black hole to lose some mass, have an increased temperature or lose some energy.

Because black holes radiate, they are not eternal.

Hawking radiation hasn’t been detected yet as the temperature difference is very small. A billionth of a Kelvin, which is impossible to measure.

Information paradox


What happens inside a black hole? There is a lot of entropy/disorder. Information in a particle interaction is believed, according to quantum theory, to be conserved in it somehow – forward and backward in time. The problem is that the information is lost to us. This is a big problem.

Information may be coded on the event horizon (the holographic principle) or the correlations might be destroyed (in a firewall).


The holographic principle states that the entropy of ordinary mass (not just black holes) is also proportional to surface area and not volume; that volume itself is illusory and the universe is really a hologram which is isomorphic to the information “inscribed” on the surface of its boundary.



The holographic principle allows gravity to be described using a language that does not contain gravity, thus avoiding friction with quantum mechanics


A black hole firewall is a hypothetical phenomenon where an observer falling into a black hole encounters high-energy quanta at (or near) the event horizon.

The firewall would exist at the black hole’s event horizon, and would be invisible to observers outside the event horizon. Matter passing through the event horizon into the black hole would immediately be “burned to a crisp” by an arbitrarily hot “seething maelstrom of particles” at the firewall.

Are star black holes the only type?


Primordial black holes are a hypothetical type of black hole that formed soon after the Big Bang. In the early universe, high densities and heterogeneous conditions could have led sufficiently dense regions to undergo gravitational collapse, forming black holes. The theory behind their origins was first studied in depth by Stephen Hawking in 1971. Since primordial black holes did not form from stellar gravitational collapse, their masses can be far below stellar mass (c. 2 x 1030 kg). Hawking calculated that primordial black holes could weigh as little as 10−8 kg. Unfortunately, there is no observational progress.

Massive black holes

Fifty black holes have measured masses in binary systems. They’re nearest among 10 million in the entire galaxy


One supermassive black hole equals 20 billion masses of our Sun.

Intermediate mass black holes exist in globular clusters. There is a 4 million solar mass black hole in the centre of our galaxy. Its existence has been revealed by stellar orbits.

The data shows the black hole at the centre of our galaxy exists and indicates what its mass is





Observations of the Galactic centre have revealed two paradoxes: there are far fewer old stars and many more young stars than expected based on theories. Stars in the central ~0.04 pc (1”) are on randomly distributed orbits. Just outside the central arcsecond, there are many young stars orbiting the supermassive black hole in a common plane. These stars likely formed in a massive, gaseous disk in the central parsec.

Adaptive optics teased out the motion of stars around the black hole.


Adaptive optics (AO) is a technology used to improve the performance of optical systems by reducing the effect of incoming wavefront distortions by deforming a mirror in order to compensate for the distortion. It is used in astronomical telescopes and laser communication systems to remove the effects of atmospheric distortion, in microscopy, optical fabrication and in retinal imaging systems to reduce optical aberrations. Adaptive optics works by measuring the distortions in a wavefront and compensating for them with a device that corrects those errors such as a deformable mirror or a liquid crystal array.


Zoom in On Galaxy M87 https://www.youtube.com/watch?v=-22Gv-20LuM


Star S2 has been monitored over the last two decades as it passes close to a black hole. It has been followed for two full orbits.


S2, also known as S0–2, is a star that is located close to the radio source Sagittarius A* (Sgr A*),

A relativity rosette

This is a map of how the orbit of the star changes


The S2 star is orbiting Milky Way’s supermassive black hole in the shape of a rosette, which is supported by Einstein’s general theory of relativity. The orbit is tweaked by the bending of space-time, precession of the perihelion.



Observations made with ESO’s Very Large Telescope (VLT) have revealed for the first time that a star, S2, orbiting the supermassive black hole at the centre of the Milky Way moves just as predicted by Einstein’s theory of general relativity. Most stars and planets have a non-circular orbit and therefore move closer and further away from the object they are rotating around. S2’s orbit precesses, meaning that the location of its closest point to the supermassive black hole changes with each turn, such that the next orbit is rotated with regard to the previous one, creating a rosette shape. This effect, known as Schwarzschild precession, had never before been measured for a star around a supermassive black hole. This animation shows S2’s orbit around Sagitarius A*, the supermassive black hole at the centre of the Milky Way. The precession movement is exaggerated for easier viewing. More information and download options: http://www.eso.org/public/videos/eso2…


Artist’s impression of the odd orbit of the star S2. Credit: ESO/L. Calçada



The movement of this star close to our galaxy’s supermassive black hole has proved Albert Einstein right about gravity once again. After 27 years of observation, the orbit of the star was finally nailed down precisely enough to spot a strange effect predicted by his general theory of relativity.

S2 circles the supermassive black hole at the centre of the Milky Way about once every 16 years. Since 1992, astronomers have been observing it with some of the most powerful telescopes on Earth to precisely trace its looping orbit.

“The precision we now have in measurements of the relative positions of the black hole and the star is comparable to watching a football game on the moon. Then you have to measure the size of the football to within of a few centimetres,” says Frank Eisenhauer at the Max Planck Institute for Extraterrestrial Physics in Germany.

This sort of movement is predicted by Einstein’s general theory of relativity, which dictates that the black hole should distort space-time around it, dragging the orbits of nearby stars as well.

It has been observed in our own solar system – Mercury’s orbit is also rosette-shaped rather than elliptical – but the effect is much more pronounced at the centre of the galaxy because the black hole is far more massive than the sun and thus stretches space-time in a more extreme way. Yet again, Einstein was right.


Mercury deviates from the precession predicted from Newtonian effects. This anomalous rate of precession of the perihelion of Mercury’s orbit was first recognized in 1859 as a problem in celestial mechanics.


In general relativity, this remaining precession, or change of orientation of the orbital ellipse within its orbital plane, is explained by gravitation being mediated by the curvature of spacetime. Einstein showed that general relativity agrees closely with the observed amount of perihelion shift. This was a powerful factor motivating the adoption of general relativity.

The first image of a black hole was taken in 2019.

Zooming in on M87


This movie zooms into galaxy M87 using real visible light, X-ray and radio pictures of the galaxy, its jet of high-speed particles, and the shadow of its central black hole. Credit: NASA’s Goddard Space Flight Center



The accretion disc is visible. The black blob (not circular due to distortions in the image not the hole) is the event horizon. It is 6/7 billion times more massive than our Sun. The observations were made from a network of telescope.


Seeing the event horizon

The size of the event horizon is greater than our Solar System

Image of the elements of the black hole is not made optically but using microwaves, radio interferometry with a network of radio telescopes.

Radio is used as visible light doesn’t reach us from this part of the galaxy due to dust obscuring it.

From the image they could work out the rotation speed, speed of orbit at different regions of its gas disc. This was evidence for general relativity.


Something that is predicted but not seen yet is a region near a black hole which acts as a magnifying glass for light reaching it from different directions in the universe.

If we could get better resolution from these radio telescopes, we would see a series of narrow rings around the event horizon. Each capturing light from different directions.

Each is a demagnified image is capturing an earlier time like frames of a movie capturing the history of the visible universe.

Seeing photon rings

If light passes too close it’s trapped in a photon ring. There’s a series of narrow rings, each one capturing light from all directions. Each ring is a demagnified image of the entire Universe and each subring captures an earlier time. They’re like frames of a movie, capturing the history of the visible universe, as seen from a black hole.


Unspooling cosmic history

Light that arrives directly is called n = 0



Higher orders, n = 1, n = 2 may have taken alternative paths round a black hole, orbiting once or twice.

Lensed and demagnified into a ring, photons arrive from all different directions.


Now we have an exquisite test for general relativity and hopefully further studies will tells us about the singularity

Our galaxy is not unique in having a black hole (Hubble and other telescopes have gone looking for them)

Supermassive black holes


After launch, the Hubble space telescope took spectra of the nuclei of nearby galaxies and all shows evidence of “dark masses”, or very massive black holes.

It seems that every galaxy has a central black hole, but most are not accreting mass and spend most of their time inactive.

Spectrographs across nearby galaxies show how the stars’ velocity changes as they move across the centre of the galaxy.

Blue and red show extreme blue and red shifts. The difference in the red shifts represents a velocity caused by a black hole and if it occurs in a small region there is a black hole at the centre of the galaxy.

Every galaxy that has been inspected has dark masses. A lot of these galaxies didn’t have obvious black holes because they aren’t shining brightly enough. The inference is that the black holes aren’t active or actively accreting material. They must be dark most of the time.

Black holes are only a tiny part of our Universe. 0.1 of 1% of matter is locked in black holes. They don’t dominate the Universe.

Black Hole Pie

The multiple components that compose or Universe

Current composition (as the fractions evolve with time)


Our nearest black hole is several hundred light years away

As radio telescopes improved, we got to see centres of some galaxies and visualise the black hole at their centres


Spinning black hole emits jets of relativistic plasma out of the poles. In the above right picture, which is a radiogram, we can see a galaxy shovelling plasma jets 100s of 1000s of light years into inter-galactic space.

These active black holes are spectacular and we can call them gravitational engines.


1% of all galaxies are active, putting out radiation from radio waves up to gamma rays, with more energy than any accelerator (real as in the LHC or imagined on Earth).

Quasars are the most luminous objects in the Universe, as bright as 1000 trillion suns, turning mass into energy at 40% efficiency (using E = mc2).


A quasar (also known as a quasi-stellar object abbreviated QSO) is an extremely luminous active galactic nucleus (AGN), in which a supermassive black hole with mass ranging from millions to billions of times the mass of the Sun is surrounded by a gaseous accretion disk. They put out energy out across the whole of the electromagnetic spectrum

Our Sun is only 0.4% efficient.


XTE J1650-500


XTE J1650-500 is a binary system containing a stellar-mass black hole candidate and 2000–2001 transient binary X-ray source located in the constellation Ara. In 2008, it was claimed that this black hole had a mass of 3.8±0.5 solar masses, which would have been the smallest found for any black hole; smaller than GRO 1655-40, the then known smallest of 6.3 MSun. However, this claim was subsequently retracted; the more likely mass is 5–10 solar masses.

The binary period of the black hole and its companion is 0.32 days.



It contains the mass 3-4 times of our Sun. It is one of the smallest, but already a destroyer of worlds.


How are small black holes like this compared to large black holes?

An intermediate black hole and its event horizon is about the size of a planet rather than a small town.

For example, M82 X-1, a mid-range black hole


M82 X-1 is an ultra-luminous X-ray source located in the galaxy M82. It is a candidate intermediate-mass black hole, with the exact mass estimate varying from around 100 to 1000. One of the most luminous ULXs ever known, its mass goes over the Eddington limit.


The Eddington luminosity, also referred to as the Eddington limit, is the maximum luminosity a body (such as a star) can achieve when there is balance between the force of radiation acting outward and the gravitational force acting inward. The state of balance is called hydrostatic equilibrium.



Sir Arthur Stanley Eddington OM FRS (28 December 1882 – 22 November 1944) was an English astronomer, physicist, and mathematician.

M82 X-1 is about the size of a planet with a mass of about 1000 of our Sun



Central black hole of the Phoenix Cluster



The Phoenix Cluster (SPT-CL J2344-4243) is a massive, type I galaxy cluster located at its namesake constellation, the southern constellation of Phoenix. It was initially detected in 2010 using the Sunyaev–Zel’dovich effect by the South Pole Telescope collaboration.

The central elliptical cD galaxy of this cluster hosts an active galactic nucleus, which is powered by a central supermassive black hole.


It contains the mass of 20 billion Suns


Next innovation was the detection of gravitational waves in 2015. This was a revolution in astronomy.

LIGO detected space-time ripples from a merger of two black holes a billion light years away has opened up a new window onto the Universe.





The Laser Interferometer Gravitational-Wave Observatory (LIGO) is a large-scale physics experiment and observatory to detect cosmic gravitational waves and to develop gravitational-wave observations as an astronomical tool. Two large observatories were built in the United States with the aim of detecting gravitational waves by laser interferometry. These observatories use mirrors spaced four kilometres apart which are capable of detecting a change of less than one ten-thousandth the charge diameter of a proton.

It is a very accurate physics experiment and will soon be detecting roughly one black hole merger every week.




Animations showing how space-time is changing as a ripple propagates through it.

Einstein predicted that gravitational waves would be emitted any time a mass configuration changed and they would radiate through space at the speed of light.

Detecting space-time ripples


When a gravitational wave passes by Earth, it squeezes and stretches space. LIGO can detect this squeezing and stretching. Each LIGO observatory has two “arms” that are each more than 4 kilometres long. A passing gravitational wave causes the length of the arms to change slightly. The observatory uses lasers, mirrors, and extremely sensitive instruments to detect these tiny changes.

The animation below shows how it works!






Animations show how laser light travels down 2 arms 4/5km long and bouncing back and forth – then recombined indicating a change in the length of arms caused by the passage of a gravitational wave.

1 x 1021 accurate. Detected dozens of gravitational waves caused by merging black holes.

The point of merger is when the biggest gravitational wave is detected.

We can’t see the merger. We only have gravity eyes.

Cacophony of gravitational waves at the point of merger.

When black holes merge







The LIGO orrery


Computer calculations modelling the gravitational waves LIGO has observed to date and the black holes that emitted the waves. The image shows the horizons of the black holes above the corresponding gravitational wave.


A visualization of the merging black holes that LIGO and Virgo have observed so far. The video shows numerical-relativity calculations of the black holes’ horizons and the emitted gravitational waves, during the final few orbits of the black holes as they spiral inwards, merge and ring down. Each numerical-relativity calculation is consistent with one of the observations in the LIGO-Virgo catalogue. As the horizons of the black holes spiral together and merge, the emitted gravitational waves become louder (larger amplitude) and higher pitched (higher in frequency). This movie is inspired by the Kepler Orrery.

Credits: Teresita Ramirez/Geoffrey Lovelace/SXS Collaboration/LIGO-Virgo Collaboration







When the black holes combine their masses add.

Some of the mass-energy is turned into gravitational wave energy. About an Earth’s mass of pure gravitational energy is shot into space.

LIGO is being upgraded. At the end of 2020 it is expected that a black hole merger will be detected once every week.

Death by black hole


Falling into a black hole resulting from a massive star’s would be a nasty fate: “spaghettification” at the level of muscles, bones, and tissues. The tidal forces on each part of the body would be extreme.

Tidal forces are smaller for big black holes (over 1000 solar masses) than small ones and falling into one would be survivable.

As seen from afar, time slows down asymptotically. If you were to fall in to one you would appear to be frozen in time at the event horizon.

Animation of falling in to a black hole



Making a black hole


In theory, black holes of any size could be made if matter is sufficiently compressed. Leading to speculation that lasers or particle accelerators like the LHC could make a black hole, after which it will fall to the Earth’s core and eat everything.

Governments made CERN do calculations that the LHC couldn’t make a black hole that would eat everything.

Even if we could make a tiny black hole Hawking radiation would cause it to fizz away.

The short goodbye


1012 – 1050 yr

All stellar fusion and light from stars will end after a trillion years. In the future dark, dissipating universe, energy can be extracted from the rotation energy of black holes by sending in, and retrieving, probes with suitable trajectories.

Civilisations could survive by using gravity of black holes


The long goodbye


1050 – 10100yr

Life needs a temperature difference. Normally, it is a hot sun and a cold sky, but in a future of black holes a microwave sky is a little warmer than the Hawking radiation from black holes. Civilisations can build Dyson spheres to harness feeble energy of black hole evaporation.

Massive black holes will last the longest.


A Dyson sphere is a hypothetical megastructure that completely encompasses a star and captures a large percentage of its power output (1000 watts). The concept is a thought experiment that attempts to explain how a spacefaring civilization would meet its energy requirements once those requirements exceed what can be generated from the home planet’s resources alone. Only a tiny fraction of a star’s energy emissions reaches the surface of any orbiting planet. Building structures encircling a star would enable a civilization to harvest far more energy.


3D rendering of a Dyson sphere utilizing large, orbiting panels.

The concept was popularized by Freeman Dyson in his 1960 paper “Search for Artificial Stellar Sources of Infrared Radiation.” Dyson speculated that such structures would be the logical consequence of the escalating energy needs of a technological civilization and would be a necessity for its long-term survival. He proposed that searching for such structures could lead to the detection of advanced, intelligent extraterrestrial life.



Freeman John Dyson FRS (15 December 1923 – 28 February 2020) was a British-American theoretical and mathematical physicist, mathematician and statistician known for his works in quantum field theory, astrophysics, random matrices, mathematical formulation of quantum mechanics, condensed matter physics, nuclear physics and engineering. He was professor emeritus in the Institute for Advanced Study in Princeton, a member of the Board of Visitors of Ralston College and a member of the Board of Sponsors of the Bulletin of the Atomic Scientists.

Questions and answers

1) M87 accretion disc. One side is brighter – rotating very fast towards us. Relativistic boosting

2) Are accretion discs flat?


3) M87 looks like we are viewing it from above. We would like different orientations.

4) Does the black hole shape the size of the Milky Way?

No. Its mass is tiny in comparison to the galaxy.

5) Why are jet shooting stuff out. They are outside the even horizon. Accelerated greatly.

6) Why do black holes take up so much space?

We don’t know what is inside. The event horizon is just an information barrier.

7) Supermassive black hole formation isn’t completely known. Which came first, galaxy or black hole?

Black holes are believed to start first

8) How long does a black hole merger last?

Low level gravitational waves last several years before the final death spiral. Not detected by LIGO

LIGO detects the end bit that lasts just a minute. The peak gravitational wave lasts a few seconds (after jelly wobbling effect)

9) Black holes are made up of the same proportion of normal universe matter. They won’t tell us anything about dark matter

10) Big black holes are not dark matter. Microscopic black holes are unlikely to be dark matter.

Primordial black holes might be dark matter, but probably not.

11) Do black holes have a connection with multiverse quantum mechanics.

I missed the answer to this one.

12) Real world application?


13) Wi-fi protocol originated from black hole studies


Books by Professor Impey














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