Sagittarius A*

A Brief History of Black Holes

Black hole research started off as a purely theoretical exercise and slowly developed into a minefield of important findings in astrophysics and cosmology.

Black holes are among the most awe-inspiring objects in the universe. They represent a kind of cosmic duality by actively destroying as well as creating some of the largest galaxies in the cosmos. One such black hole is responsible for our Solar System’s existence and – by extension – ours.

That we share such an intimate relationship with black holes has only recently entered common knowledge. In fact, for a long time, black holes remained a mathematical oddity that few believed had any basis in reality. If scientists ushering in the era of quantum mechanics and relativity had a tough time swallowing the black hole pill, it is not entirely surprising that when John Michell and Pierre-Simon Laplace introduced the idea in the late 1700s, it simply fell flat.

Michell, a fellow of the Royal Society in London, wondered if the gravitational pull of a massive star could be powerful enough to slow particles of light emanating from it – perhaps to such an extent that they couldn’t escape it at all. Laplace, a decorated mathematician himself, arrived at the possibility of such stars independently a few years later.

We now know that light cannot be slowed down no matter how strongly gravity tugs at it. So tantalising as their proposals were, they were far too radical for their days, and black holes remained buried in obscurity, both in the universe and in our scientific texts. This changed when Albert Einstein’s general theory of relativity signalled a new and improved way of looking at the universe.

Also read: Beyond the Surface of Einstein’s Relativity Lay a Chimerical Geometry

But 10 years before the general theory was unveiled, there was the foundational special theory of relativity. And its foundations were in turn laid by mathematicians around the world, whose contributions Einstein built upon to create his overarching theory. If we understand space and time to be combined as one entity called spacetime, as special relativity requires, it’s because of the work of Georg Bernhard Riemann, Hermann Minkowski, Marcel Grossmann, Tullio Levi-Civita, Gregorio Ricci-Curbastro and David Hilbert, among others.

When he revealed his general theory 1915, Einstein had effectively found a way to marry gravity with the special version of the theory, providing an elegant way to describe the motions and machinations of massive objects in our universe. However, he wasn’t very interested in studying the particular solutions of these equations, each of which could reveal different types of such objects.

But someone else was. A few months after Einstein’s landmark publication, Karl Schwarzchild, a German physicist, provided the first exact solution of these equations. And this solution suggested the possibility of a so-called ‘mathematical singularity’. Schwarzschild had predicted that if an object of a given mass was compressed to a certain radius – i.e. made dense enough – it would continue to shrink and inevitably collapse into itself. Any mass that fell within that radius would be captured by the body. Today, that radius is known as the Schwarzschild radius and the boundary it demarcates is called the event horizon.

Einstein wasn’t convinced of this solution and used his own theory to try and disprove Schwarzschild’s results. The paper was published in 1939 but it has largely been forgotten. Schwarzschild himself dismissed his findings as having no practical relevance, claiming that the pressure gradients required to compress a body beyond its Schwarzschild radius couldn’t be realised in nature.

Enter Subrahmanyan Chandrasekhar.

When Sun-like stars run out of hydrogen to fuse, they go through a series of transformations leading up to one stage called the white dwarf. White dwarfs are extremely dense objects, nearing the sort of density required to make a black hole. Arthur Eddington, a famous figure in physics in the early 1900s, proposed that stars imploding due to their own gravity pack fundamental particles like electrons into denser and denser arrangements. This packing, he claimed, would exert a counter-pressure to gravity and prevent the white dwarf from fully collapsing into a black hole.

His proposal was challenged by Chandrasekhar, his own student, whose calculations suggested there was a limit on how massive such stars can be to sustain this delicate pressure balance. This has been called the Chandrasekhar limit. (While his work is celebrated today, Chandrasekhar’s disagreement with Eddington affected him very much.)

In 1939, Robert Oppenheimer and Hartland Snyder built on the work of Chandrasekhar, Fritz Zwicky and Lev Landau to introduce the first theory of black holes that incorporated general relativity, which had by then become the gold standard for all things cosmological. However, their paper also sank without a trace, likely because of the ominous timing: Germany had just invaded Poland.

Oppenheimer himself never returned to the subject of black holes and Einstein also didn’t put much thought to it. In fact, he remained largely disinterested in research on black holes even as every new discovery and experimental triumph hailed his name.

But in the early 1950s, the subject experienced a surge. John Wheeler, a physicist at Princeton University, was the leading relativist of that era. He is often credited with coining the term ‘black hole’ to refer to the gravitational singularity that Schwarzschild had spotted in the math. However, he is better known for having taught and supervised the work of many notable future physicists, including Richard Feynman and Kip Thorne.

Also read: Why Does the Black Hole Picture Look the Way It Does?

In the same decade, Dennis Sciama set up camp at Cambridge University as a modern practitioner of the general theory. His student Stephen Hawking and the mathematical physicist Roger Penrose made significant leaps towards understanding the microscopic structure of black holes.

For example, what made Hawking a champion of relativity and quantum mechanics was his result on black hole evaporation. He showed that black holes continuously leak electromagnetic radiation, proving that for all their gargantuan power, they (very) slowly dissipate and die. An equation from his theory was recently inscribed on a coin in the United Kingdom, a hat-tip also to how black holes have moved from the scientific fringes towards mainstream acceptance.

While these are among the most famous people to have studied black holes in one way or another, there are a few that are frequently and undeserving overlooked as well. One of the latter is Roy Kerr, who in 1963 developed an exact solution for Einstein’s equations that predicted the existence of spinning black holes. Another name is that of C. V. Vishveshwara, whose calculations were pivotal to advancing the experimental observation of gravitational waves.

Black hole research started off as a purely theoretical exercise and slowly developed into a minefield of important findings in astrophysics and cosmology. Its saga is proof that dogged mathematical pursuit with no obvious physical relevance at first can often lead to very physical results of great import.

Moreover, as objects of endless power and mystery, black holes have also been used as plot devices in popular fiction since the 1950s. Arthur C. Clarke, Brian W. Aldiss and Frederik Pohl. Star Trek and Doctor Who incorporated it, as more recently did Christopher Nolan, to great effect, in his 2014 sci-fi epic Interstellar. Countless artists have conjured images of what it would look like when a black hole swallows an entire world.

Also read: A 200-Year-Old Experiment Has Helped Us See a Black Hole’s Shadow

This aesthetic realisation is a direct consequence of scientists knowing as much as they do today about black holes, with new details often elicited from ‘Earth-sized’ telescopes and supercomputer simulations. In fact, their own findings are often sights to behold: two black holes orbiting each other on a collision course; a black hole emitting a relativistic jet billions of kilometres into space; and an accretion disk lighting up a carnival of energy and dust.

Then again, because a black hole represents one extreme of the natural universe, it would be unreasonable to expect that it itself isn’t distorting what we’re seeing of it. On this count, perhaps the most interesting image was published by a physicist named Jean-Pierre Luminet in 1979.

Luminet didn’t use an artist; instead, he created the first ever simulation on what was at the time about black holes. And his results as well as the image obtained by the Event Horizon Telescope look remarkably similar. One was created based on brute-force calculations of equations that we believe represent our classical world in the most accurate fashion. The other – recorded somewhat like our eyes record reality – used both classical and quantum mechanics 40 years later.

That the outcomes of both exercises closely resembled each other is testament to the scientific process as much as our tenacity when it comes to making sense of the universe. For over a century, we have probed a concept that has tested the limits of both nature and science, and have come away with something meaningful. It’s hard not to be romantic about that.

Ronak Gupta is doing a PhD in fluid mechanics at the University of British Columbia, Vancouver.

First Light From the M87 Black Hole: What Are We Looking At?

How does a picture of a luminous ring around a black region square with the popular idea that black holes trap everything including light?

As everyone knows by now, the Event Horizon Telescope (EHT) has imaged a black hole in the galaxy M87. While astronomers already knew that black holes were real objects in the Universe, the picture provides direct ocular proof of their existence.

But what exactly are we looking at? How does a picture of a luminous ring around a black region square with the popular idea that black holes trap everything including light?

To understand this, let us step back a bit and ask what we mean by an image. It is true that black holes do not emit light. In this, they are no different from any other non-luminous object, like the reader and writer of this article. When you take a selfie, you use a flash to illuminate yourself. Selfies taken in total darkness are not the kind of image you can put up on Instagram. With a flash, the light bounces off your body and is received by the camera lens. More technically, we can say that light is scattered by your body into the camera lens and this is what produces the image you post on Instagram.

The same is true for black holes. Imagine that we transport ourselves magically to the vicinity of an isolated non-rotating black hole – taking care not to fall in – and remembering to take a camera along. If we snap a picture of the hole, it would be sheer black, nothing to write home about. This fits with the popular belief that you cannot “see” a black hole, precisely because it is black.

Also read: Stephen Hawking, the Cosmic Bard

If you use the flash on your camera, the picture changes dramatically. Light from the flash is emitted in all directions. Imagine a line going from your camera to the centre of the black hole. Light going along this central line of sight to the hole will be absorbed and not return to your camera. This also applies to rays that make a small angle with the line of sight to the centre.

Thus we expect a central dark region in the picture, the “shadow” of the black hole. Similarly, rays that are emitted at large angles to the line of sight will go nowhere near the black hole and disappear into space.

The rays in between these extremes are more interesting. Rays going at angles closer to the line of sight will be bent by the gravitational field of the black hole. If the angle is small enough, they will bend all the way around the black hole and return to the camera! Since all rays making the same angle with the line of sight will behave the same way, we can expect to see a ring of light around the black hole. As the rays get closer and closer in, one would find an angle at which the bent ray goes twice around the black hole and returns to the camera.

Thus, we get a series of concentric rings as shown in the image.

A black hole selfie taken with a flash. Credit: Joseph Samuel

The rings pile up near the innermost one, and for still smaller angles, we see only blackness. This is how a black hole scatters light. This is what a non-rotating black hole’s selfie would look like, if black holes were into selfies! Definitely worth posting on Instagram.

Astrophysical black holes are not isolated, but accrete matter from neighbouring stars. This matter falls into the gravitational field of a black hole, and – if it has angular momentum – circles around the black hole just the way the planets circle the Sun.

The gravity of the black hole pulls the gaseous stellar matter in, causing it to accelerate to near-light – or relativistic – speeds. Friction causes the gas to heat up and radiate energy. The hot disc radiates at all frequencies across the electromagnetic spectrum and this is the source of illumination that enables us to “see'” the black hole.

Now, suppose we have a non-rotating black hole with an accretion disc in its equatorial plane and we view the hole from slightly above the plane. As we learned, we need only concern ourselves with rays of light that go from the light source (the accretion disc) to our eyes. In the absence of strong gravity and relativistic effects, we would expect to see a disc rather like the rings of Saturn. However, the relativistic speed of the swirling matter causes the radiation to be ‘beamed’: matter that appears to be coming towards us would appear to radiate more strongly and appear brighter, while receding matter appears dimmer.

Further, some of the light emitted by the disc on the far side of the black hole would be bent toward us and we would see it as apparently coming from “above” the black hole. The final picture we see looks something like in the one below.

An artist’s impression of a black hole accretion disc (based on a simulation by Jean-Pierre Luminet). The brightness on the left is due to motion towards the observer. Credit: Roshni Rebecca Samuel

Armed with the understanding gained from these idealised situations, we can begin to understand the the real image taken by the EHT. The black hole in M87 is rotating and, as a result, the picture is somewhat more complicated. But our qualitative understanding can still be brought to bear on the problem. If we trace the light that enters our eye backwards towards its source, it will go to the black hole, bend in the gravitational field and eventually end up at the source of light, in this case the accretion disc. Even if the disc is uniform, those parts that appear to be moving towards us will appear brighter due to beaming effects. We expect to find a ring of light with a dark spot in the middle representing those rays that fall into the black hole.

The EHT collaboration performed a detailed simulation of a magnetised accretion disc around a rotating black hole. The researchers found its results agreed with their image.

Now, let’s clear up a few points that we glossed over. We talked of “seeing'”, which colloquially means visible light is involved. In fact, the EHT works with radio waves whose wavelength is 1.3 mm. Eyes and cameras are replaced by a combination of telescopes spread all over Earth.

Also read: Look Behind the Low-Res Black Hole

The essential principle is still the same. The EHT uses a technique called very-long-baseline interferometry, or VLBI, to image the distant black hole with the accuracy of a microarcsecond, a feat of both technology and science. It involves spectacularly accurate time-keeping developed over the last few decades, and a fundamental understanding of the wave-nature of light developed over the last few centuries.

So, are we looking at a black hole? Or is it an accretion disc seen through the distorting gravitational lens of the black hole? It is both, actually. When you take a selfie, you are looking at light from a flash scattered by the atoms of your body. To a physicist or an astronomer, there is no fundamental difference between light scattered from a black hole or from an atom. The disc is like the flash and the black hole is the scatterer.

Take a good look at the dark patch in the middle of the image. You are staring into the desolate darkness of the event horizon. Wow!

Joseph Samuel is a professor at the Raman Research Institute, Bengaluru.

International Team of Astronomers Obtains First Direct Image of a Black Hole

An FAQ with details of all you need to know about the monster black hole in the M87 galaxy, the giant Event Horizon Telescope and why scientists are looking for pictures of black holes.

Chennai: The Event Horizon Telescope has released the first direct image of a black hole and its neighbourhood. This black hole lurks in the centre of a supergiant galaxy called M87, about 53 million lightyears from the Milky Way. This historic image shows a ring of light emitted by gas falling into the black hole.

The black hole itself is shielded by its event horizon, a boundary from within which nothing can escape. This is the dark interior in the image. Though astronomers have had solid evidence for the existence of massive black holes for many years, this historic moment marks the first time that it has been imaged.

What has the Event Horizon Telescope actually seen?

The Event Horizon Telescope (EHT) has imaged the silhouette – or shadow – of the black hole at the centre of the M87 galaxy. To create this image, astronomers combined data from eight different telescopes across the world in an observation run conducted in April 2017. The data in question was electromagnetic signals with a frequency of 230 GHz, or a corresponding wavelength of 1.3 mm. Using this, astronomers formed the image of a black hole for the first time.

The event horizon of a black hole is the ultimate boundary. Nothing from inside it can escape to the outside. The ring of fire in the EHT image is light from the gas falling into the event horizon, and its shadow is the dark hole in the centre. The exact shape of the ring is due to the way the incredible gravity of the black hole bends the light around it, and the extreme speed at which the gas is falling in. The ring is not seen to be uniformly bright because these numbers are uneven.

Is there a single Event Horizon Telescope?

A telescope large enough to ‘observe’ the shadow of the black hole in M87 would have had to be as big as Earth itself. Since that might have been a bit difficult to build, astronomers went for the next best thing. Using a technique called very-long-baseline interferometry, data from many telescopes distributed across Earth were combined with help from atomic clocks and advanced computing techniques. This way, the images obtained by all the telescopes together would be of similar quality and value as that obtained a single telescope as wide as Earth.

The flip side is of course an enormous computing cost, with thousands of terabytes of data that takes many months to refine and process.

Why are such computing costs incurred?

The South Pole Telescope illuminated by aurora australis and the Milky Way. Jupiter is brightly visible on the lower left, Saturn is located to the right of the telescope. The outside temperature is -60º C. Caption and credit: Daniel Michalik/South Pole Telescope

The EHT imaged M87 by observing electromagnetic radiation that had a frequency of 230 GHz – over 2,000-times higher than the frequencies at which radio stations on Earth transmit data. This is a pretty special frequency because it has many useful properties, or abilities. The lower frequency emitted by the gas surrounding the black hole is less bright. Higher frequency radiation is blocked by our own atmosphere. The sweet spot, therefore, is 230 GHz. And telescopes capturing such radiation require very hi-tech hardware, need to function at the limits of their performance, and require the use of high-precision atomic clocks and data-processing techniques.

Which telescopes were a part of the EHT, and why did they have to be so far apart?

The EHT was made up of eight telescopes operating together, tracking radiation at the sub-mm wavelengths. They were as far apart as Hawai’i, the mainland USA, Chile, Mexico – even the South Pole. All of them had to look at M87 together at the same time and record their data.

(Editor’s note: Some of these instruments were in turn made up of multiple smaller telescopes; e.g. ALMA in Chile is composed of 66! The full list of telescopes that contributed to the present result is as follows:

ALMA
APEX
The IRAM 30-meter telescope
The James Clerk Maxwell Telescope
The Large Millimeter Telescope Alfonso Serrano
The Submillimeter Array
The Submillimeter Telescope, and
The South Pole Telescope)

The Submillimeter Array is a set of eight telescopes in Hawai’i, Credit: Event Horizon Telescope Collaboration

The size of the ring imaged by EHT occupies an angle of about 40 micro-arcsecond in the sky – about the angle made by the thickness of a sheet of paper viewed edge-on from 100 km away. The black hole in the M87 galaxy makes this task a little less daunting because it is the largest known one of its kind in the local universe.

To image such a small region of the sky, astrophysicists needed a telescope with a very large magnification, especially so features inside the image could be captured well. In the technique used to combine data from different telescopes, the magnification is higher if the telescopes are farther apart. And the farthest apart they can be is of course the diameter of Earth itself.

Put another way, using the EHT, you could kick back on an armchair in Delhi and read a book in Kanyakumari.

How big is the black hole at the centre of M87, really?

Almost all galaxies have black holes at their centres. These can be a few million to a few billion times the mass of our Sun. Our Milky Way galaxy has a fairly small black hole about 4 million times as massive as our Sun. However, the black hole in M87 is a veritable monster, weighing 6,500-million-times as much as our Sun. Its event horizon, its outermost boundary across which nothing can escape, is about 20,000 million km wide – bigger than the entire Solar System.

A black hole does not emit any light. So how do astronomers ‘see’ it or its shadow?

Two of the Atacama Large Millimeter/submillimeter Array (ALMA) 12-metre antennas gaze at the sky at the observatory’s Array Operations Site (AOS), high on the Chajnantor plateau at an altitude of 5000 metres in the Chilean Andes. Caption and credit: Iztok Bončina/ESO, CC BY 4.0

(Editor’s note: According to Albert Einstein’s general theory of relativity, massive objects bend the spacetime continuum around them. Other objects experience this curvature as a force we call gravity.)

Matter is attracted by the gravity of a black hole but cannot fall into it easily. In fact, it first forms a swirling disk around it – like water circling a drain inside a bathtub – through which it spirals in at a very high velocity. While doing so, the matter is also heated to very high temperatures and exists in the plasma state of matter. And this hot magnetised plasma is what emits the intense radiation that we observe.

The EHT has reconstructed an image of the radiation from this surrounding gas at a frequency of 230 GHz. However, the image is not as simple as a dark hole in front of a disk of radiating gas. Since gravity near the black hole is immense, it can bend the path of light from the surrounding plasma in peculiar ways. So even light from the gas behind the black hole is bent in such a way that it reaches the eyes of an observer looking at the black hole’s front side. Such bending is called gravitational lensing, and it determines the final shape of the ring and the inner shadow that the EHT images.

Why are black holes and their images so important?

Black holes are natural laboratories in which physics theories – such as Einstein’s general theory (GR) of relativity – are tested in extreme ways. GR relates the motion of bodies due to gravity with the curvature of spacetime. It has passed every test that humans have been able to track and/or perform in our Solar System – the accuracy of GPS systems stand testament – as well as over other astronomical bodies.

Also read: A 200-Year-Old Experiment Has Helped Us See a Black Hole’s Shadow

These more local settings are collectively called weak gravity cases, where the curvature of spacetime is small. What astronomers and astrophysicists would like to do is to test the theory in strong gravity regimes, where the curvature is much higher, and see if the theory still holds up. The EHT isn’t alone here: the twin Laser Interferometer Gravitational-wave Observatories, which have been detecting and studying gravitational waves emitted by the mergers of pairs of black holes, are performing similar tests.

Why did the EHT not image the black hole in our own galaxy?

The black hole in the centre of the Milky Way galaxy is about 1,000-times less massive than the one in M87, and so is also smaller by the same factor. The reason it appears to be a bit bigger in the sky is that it is only 26,000 light years away from us.

According to scientists at the announcement, the EHT had also observed the Milky Way’s black hole. However, because its brightness varies much more rapidly, it is harder to process the data so obtained, and it will be more time before meaningful images become available.

Why weren’t Indian telescopes part of the EHT?

India does not have a telescope working in the sub-mm wavelengths. Though it does host two of the world’s largest radio telescopes – the Giant Metrewave Radio Telescope near Pune and the Ooty Radio Telescope – they operate at the centimetre and metre wavelengths. As a result, they are completely blind to the shorter sub-mm wavelengths, and would not have been able to see 230 GHz radiation that the EHT considers to be of interest.

Niruj Mohan Ramanujam is an astronomer and a member of the Public Outreach and Education Committee of the Astronomical Society of India. He tweets at @nirujmohan.

A 200-Year-Old Experiment Has Helped Us See a Black Hole’s Shadow

With one fuzzy image, history has been made.

Note: At 6:30 pm (IST) on April 10, members of the Event Horizon Telescope (EHT) published the first direct image of a black hole, specifically the shadow of its event horizon. The EHT is a globally coordinated network of telescopes using which scientists achieved this feat. The black hole in question lies at the centre of a galaxy called M87, about 53 million lightyears away (towards the constellation Virgo in the night sky).

The EHT collaboration is also observing the black hole at the centre of the Milky Way, located at a point astronomers call Sagittarius A*. The separate location and local context of the two black holes aside, the underlying principles concerning their observation are the same. They are delineated below, first published on April 9, 2019.

While the black hole at Sagittarius A* is over 20 million km wide and weighs 3.5-4 million solar masses, it is also extremely far away: 26,000 lightyears. So astrophysicists who wanted to study it had a challenge: to find a way to view something the size of an idli on the Moon’s surface from Earth. They responded by developing the EHT. And the EHT solved their problem using a technique called VLBI, described below.

On June 25, 2014, scientists announced the discovery of a trio of supermassive black holes at the centre of a galaxy 4.2 billion light years away. The find was credited to the European VLBI Network. A Space.com report said that this network “could see details 50-times finer than is possible with the Hubble Space Telescope”. How was this achieved?

VLBI stands for very-long-baseline interferometry. It is a technique used in astronomy to obtain high resolution images of the sky using a network of telescopes across the planet that can – with the aid of high-tech computing – come close to mimicking the sharpness of a hypothetical telescope nearly as large as the planet. It is commonly used to image distant cosmic radio sources, such as quasars, although it is also sometimes used to study stars.

The concept has its roots in Thomas Young’s famous double-slit experiment, which he conducted in 1801. When Young placed a screen with two extremely narrow slits in front of a light source, such as a burning candle, the shadow cast on the other side was not simply two bright patches. It was actually an alternating patchwork of bright and dull bands, as if the candle light had passed through multiple slits. This was the interference pattern. Young’s experiment was important to establish that light travels as a wave, overturning Newton’s conviction that light was composed of particles.

The interference pattern

An illustration (that doesn't appear in the book) showing the double-slit experiment with electrons instead of light, although the principles are the same. Credit: Wikimedia Commons

An illustration showing the double-slit experiment with electrons instead of light, although the principles are the same. Credit: Wikimedia Commons

When light passes through each slit, it diffracts, i.e. starts to spread out. At some point in front of the slits, the diffracted waves meet and interfere. Where crest of one wave met the crest of another, the combined wave had a higher crest than the two, and cast a bright spot on the screen. Where crest met trough, however, they cancelled each other. And where trough met trough, there was a dark band on the screen – a shadow. When the position of the slits was changed, the interference pattern also shifted.

In VLBI, the candle is replaced by a distant source of radio waves, like a black hole. The slits are replaced by radio antennae on telescopes. Since Earth is rotating, the antennae are in motion relative to the black hole, and receive the radio waves at different times. When these signals are allowed to interfere with each other, they produce an interference pattern that is processed at a central location to recreate the state of the black hole, whether visually or any other way.

Radio waves have greater wavelength than visible light. So radio telescopes have an inherently poorer angular resolution than optical telescopes of the same size. Angular resolution is defined as the ratio of an emission’s wavelength to the diameter of the telescope receiving it. Qualitatively, it denotes the smallest separation between two points that the telescope can distinguish in the image, and engineers like it to be as low as possible. For example, a 50-meter-wide radio telescope will have an angular resolution of 50/0.01 = ~41.2 arc-second. An optical telescope of the same size will have an angular resolution of 0.004 arc-second.

Also read: The DIY Experiment That Captures ‘All the Mystery of Quantum Physics’

In other words, the optical telescope will be able to view a feature 10,000-times smaller in its image than will a radio telescope of the same size. The question does arise: why don’t we simply view the black hole’s immediate surroundings in visible light then?

This is because the astronomical objects that do emit radio waves encode certain information in them that visible radiation does not carry. Additionally, radio waves of wavelength 1.3 mm – that the EHT tracks – are not absorbed or scattered by dust in the Milky Way or in Earth’s atmosphere, allowing antennas on the surface to capture them. But this in turn requires a telescope’s dish antenna to be wider than Earth.

Fortunately, astrophysicists discovered in the late 1990s that the black hole’s prodigious gravity could be bending light ‘flowing’ near it towards itself, forming a gravitational lens that magnified it by five times. In turn, this meant a telescope required to ‘look’ at Sagittarius A* would need to have a diameter of a few thousand kilometres. Believe it or not, this was much more manageable.

The baseline, and atomic clocks

The Giant Metre-wave Radio Telescope, Pune. A radio telescope’s antenna is its dish. Credit: NCRA/TIFR

Enter VLBI. Because there are multiple telescopes receiving the radio signals, the angular resolution of a so-called interferometric telescope is defined in a different way. It is no longer the ratio between the wavelength and the diameter of the telescope. Instead, it is the ratio between the wavelength and the maximum physical separation between two telescopes in the array, called the baseline. If, say, the baseline is 1,000 km, the angular resolution of an array of radio telescopes becomes 0.002 arc-second – already 20,000-times better.

However, this technique couldn’t be implemented properly until the atomic clock was invented in the 1950s. Before these advanced timekeepers existed, a single metronome had to be connected to multiple telescopes with cables, which limited the baseline to the amount of cable you had. With atomic clocks, telescopes could be placed on different continents because the clocks were kept in sync using international protocols.

Also read: How We’re Probing the Secrets of a Giant Black Hole at Our Galaxy’s Centre

All together now: a telescope receives a radio signal, a computer sticks a timestamp on it and sends it to the receiver. The receiver collates such data from different telescopes and creates the characteristic interference pattern. Using this pattern, a processor recreates the source of all the radio waves at different locations, together with the time at which each signal was received.

The @ehtelescope observations were done over a week in July 2017 when the weather over all 8 sites was good. Data was recorded and shipped to two locations for processing. They had to wait for 8 months more to get the South Pole data when winter ended! https://t.co/udXiucF7Bn

— Astronomy outreach (@asipoec) April 9, 2019

There are also many systems in between to stabilise and improve the quality of the signal, to coordinate observations between the telescopes, etc. But the basic principle is the same as in Young’s experiment two centuries ago.

VLBI itself has been around since the 1960s. At first, it could detect radio waves with a wavelength of a few centimetres, and gradually moved to lower and lower wavelengths – or higher and higher frequencies.

A black hole’s shadow

Telescopes participating in the EHT experiment are shown in blue. Credit: ESO/O. Furtak, CC BY 4.0

The EHT itself has over 30 participating telescopes spread over the North and South Americas, Europe, the Pacific Ocean and Antarctica. Because of their need to work together and their varied geographical locations, the EHT can study the Sagittarius A* site only when there are clear skies over all these telescopes at the same time. This is about one week per year – which makes each observation very precious.

It is also notable that for all of its sophistication, the EHT is not capable of producing an image of the shadow of a black hole the way Christopher Nolan and Kip Thorne did for the movie Interstellar (2014). We are likely to see a few pixelated images tomorrow put together from radio data. However, and assuming that is indeed going to be the case, it will still be a landmark achievement and a significant moment in the history of humankind.

This infographic shows a simulation of the outflow (bright red) from a black hole and the accretion disk around it, with simulated images of the three potential shapes of the event horizon’s shadow. Credit: ESO/N. Bartmann/A. Broderick/C.K. Chan/D. Psaltis/F. Ozel

It will not have been possible without the stars and other bodies that died being torn apart by the black hole. The black hole would have accrued the ‘dead’ matter around itself, accelerating them to very high velocities and twisting them around in monstrous magnetic fields. This causes frictional heating that then prompts the matter to emit high-energy radiation, such as X-rays. According to the experiment’s website, “The details of accretion mechanisms are still a very active area of research, and we hope that the images the EHT will take of the extreme environment of [Sagittarius A*] will help us understand them.”

Now, because the black hole bends light around itself, radiation from the accretion disk from behind the black hole will be visible to telescopes that are looking at its front. Finally, because the material in the accretion disk is swirling around – say, from left to right – an effect called gravitational redshift will cause light on the black hole’s left to appear brighter, and of higher frequency, than that on its right. This will make the black hole at Sagittarius A* appear like in the infographic above. And the EHT uses VLBI to capture the black hole’s shadow against this light, this light created by the sacrifice of entire stars.

With thanks to Prajval Shastri, an astrophysicist at the Indian Institute of Astrophysics, Bengaluru, for extended inputs on the article.

Some portions of the text above were originally published as a post on the author’s blog in 2014.

In Astrophysics Milestone, First Photo of Black Hole Expected

“It’s a visionary project to take the first photograph of a black hole. We are a collaboration of over 200 people internationally.”

Washington: Scientists are expected to unveil on Wednesday the first-ever photograph of a black hole, a breakthrough in astrophysics providing insight into celestial monsters with gravitational fields so intense no matter or light can escape.

The US National Science Foundation has scheduled a news conference in Washington to announce a “groundbreaking result from the Event Horizon Telescope (EHT) project,” an international partnership formed in 2012 to try to directly observe the immediate environment of a black hole.

Simultaneous news conferences are scheduled in Brussels, Santiago, Shanghai, Taipei and Tokyo.

A black hole’s event horizon, one of the most violent places in the universe, is the point of no return beyond which anything – stars, planets, gas, dust, all forms of electromagnetic radiation including light – gets sucked in irretrievably.

While scientists involved in the research declined to disclose the findings ahead of the formal announcement, they are clear about their goals.

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“It’s a visionary project to take the first photograph of a black hole. We are a collaboration of over 200 people internationally,” astrophysicist Sheperd Doeleman, director of the Event Horizon Telescope at the Center for Astrophysics, Harvard & Smithsonian, said at a March event in Texas.

The news conference is scheduled for 9 am (1300 GMT) on Wednesday.

The research will put to the test a scientific pillar – physicist Albert Einstein’s theory of general relativity, according to University of Arizona astrophysicist Dimitrios Psaltis, project scientist for the Event Horizon Telescope. That theory, put forward in 1915, was intended to explain the laws of gravity and their relation to other natural forces.

Supermassive black holes

The researchers targeted two supermassive black holes.

The first – called Sagittarius A* – is situated at the centre of our own Milky Way galaxy, possessing four million times the mass of our sun and located 26,000 light years from Earth. A light year is the distance light travels in a year, 5.9 trillion miles (9.5 trillion km).

The second – called M87 – resides at the centre of the neighbouring Virgo A galaxy, boasting a mass 3.5 billion times that of the sun and located 54 million light-years away from Earth. Streaming away from M87 at nearly the speed of light is a humongous jet of subatomic particles.

Black holes, coming in a variety of sizes, are extraordinarily dense entities formed when very massive stars collapse at the end of their life cycle. Supermassive black holes are the largest kind, devouring matter and radiation and perhaps merging with other black holes.

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Psaltis described a black hole as “an extreme warp in spacetime,” a term referring to the three dimensions of space and the one dimension of time joined into a single four-dimensional continuum.

Doeleman said the project’s researchers obtained the first data in April 2017 from a global network of telescopes. The telescopes that collected that initial data are located in the U.S. states of Arizona and Hawaii as well as Mexico, Chile, Spain and Antarctica. Since then, telescopes in France and Greenland have been added to the network.

The scientists also will be trying to detect for the first time the dynamics near the black hole as matter orbits at near light speeds before being swallowed into oblivion.

The fact that black holes do not allow light to escape makes viewing them difficult. The scientists will be looking for a ring of light – radiation and matter circling at tremendous speed at the edge of the event horizon – around a region of darkness representing the actual black hole. This is known as the black hole’s shadow or silhouette.

Einstein’s theory, if correct, should allow for an extremely accurate prediction of the size and shape of a black hole.

“The shape of the shadow will be almost a perfect circle in Einstein’s theory,” Psaltis said. “If we find it to be different than what the theory predicts, then we go back to square one and we say, ‘Clearly, something is not exactly right.'”

(Reuters)