Bengaluru: An international team of scientists unveiled the first-ever image of the silhouette of a black hole in a press conference Wednesday. The black hole that was imaged using the Event Horizon Telescope radio telescope network is of the ‘supermassive’ type, located in the centre of the Messier 87 galaxy. It is 6.6 billion times the mass of the sun, at 53.5 million light years from Earth, and is 40 billion kilometres in diameter.
As even light doesn’t escape from a black hole, directly capturing a photo of it is impossible. Instead, what we have is an image of the glowing disc of light wrapping itself around the black hole, revealing the outline of its structure. While this isn’t a true photograph of the actual black hole, it is as close as one can get to calling it that.
The astronomers published their results in six papers in a special edition of The Astrophysical Journal Letters.
“We have taken the first picture of a black hole,” said EHT project director Sheperd S. Doeleman of the Center for Astrophysics, Harvard & Smithsonian in an accompanying press release. “This is an extraordinary scientific feat accomplished by a team of more than 200 researchers.”
The insides of a space vacuum
Black holes are often depicted as giant monstrous vacuum cleaners of the universe — they suck everything and not even light can escape. This is true when matter approaches the boundary of a black hole, called the ‘event horizon’. The radius of the event horizon is called the Schwarzschild radius, and is effectively the radius of the black hole. Once something crosses the event horizon and falls into a black hole, it cannot escape. This is because black holes are singularities: Known laws of physics break down inside them.
According to Einstein’s theory of relativity, black holes spin so fast and are so massive that they distort the space-time fabric around them, as if everything was divided by zero. These relativistic effects play out only when matter approaches a black hole physically. Should a black hole be swapped out for a star of equal mass, nothing that’s in orbit around it would change, although the physical size of the black hole would be much more compact than that of the star.
A black hole is essentially a lot of matter packed into a very small area, thus carrying an enormous amount of gravity. It is so densely compact that if a human should stand at the event horizon, the gravitational force on their feet would be millions of times greater than that felt at their heads. If this human should fall in, she will undergo an almost comically terrifying process: The difference in gravitation will slowly stretch her feet out, then her legs, then her torso, neck, and then head. This is scientifically called ‘spaghettification’ and would all happen in a matter of microseconds. And one wouldn’t know what happens afterward.
Black holes come in different ranges of mass and size. The smallest are the theorised micro black holes, with a mass approximately the same as that of the Earth’s moon, and a pin-point sized radius of 0.1 millimetre. These were primordial black holes that are thought to have existed in the moments just after the Big Bang.
Then come ‘stellar’ black holes, which are created by the collapse of stars into themselves. These have a mass of about 10 to 100 times the sun’s, and a radius of about 30 km.
Beyond this, there are ‘intermediate-mass black holes’, with masses ranging from 100 times to 100,000 times that of the sun packed into about the same radius as Earth.
Then there are the most massive kind, called the supermassive black holes (SMBH). One SMBH can have a mass of thousands to billions of times that of the sun, all held tightly together within a radius of few thousands to millions and up to billions of kilometres. Such SMBHs are not uncommon; pretty much every galaxy is thought to have one in its centre, around which everything in the galaxy revolves, much like the sun in the solar system.
Our Milky Way galaxy has an SMBH right in the centre too, around which all stars and their planetary systems revolve. It is present in the general direction between the constellations Sagittarius and Scorpio in the sky. When it was discovered, it was classified as an “exciting” source of radio waves, and thus named Sagittarius A* — excited states of atoms are denoted with an asterisk. It is commonly shortened to Sgr A*, pronounced ‘Sagittarius A star’.
Sgr A* is 25,000 light years from Earth, has a mass about 4.3 million times that of the sun, and a radius of 30 million km. For comparison, Mercury, the nearest planet to the sun, is about 46 million km away from it.
The M87 black hole
The image released Wednesday is of a much farther black hole, located inside one of the most massive galaxies near us, Messier 87. Shortened to M87, this bright galaxy can be seen in the Virgo cluster of galaxies as a nebulous cloud. It is 53.5 million light years away from us, and is known for the large jets of material it ejects, spreading out for a distance of 5,000 light years.
M87 is bright and easy to spot in radio wavelengths, and is also a popular target with both amateur and professional astronomers for being one of the largest, most massive observable galaxies.
Bang in the centre of the M87 galaxy is an SMBH that has a mass 6.6 billion times that of the sun, unofficially nicknamed M87* at the press conference by EHT Science Council chairperson Heino Falcke of Radboud University. While it is nearly 2,000 times farther away from us as Sgr A*, it is also 2,000 times more massive. Thus, both black holes appear approximately to be the same size to the EHT telescopes.
However, the distance meant that M87 also spun slower compared to Sgr A*, thus providing for more stable observation.
Sgr A* was also observed by the EHT, back in 2016, and the direct radio observations made seem to still be under processing. The EHT had started observing the M87 black hole later, in 2017.
There are other black holes of various masses scattered across our galaxy. Thankfully, the nearest black hole to us is 1,600 light years away and doesn’t concern us.
Peering at dark giants
The M87 black hole looks less like something out of Interstellar, and more like something out of another science fiction film, Arrival. The coffee-stain pattern actually perfectly depicts the black hole.
Because black holes are so elusive, owing to no electromagnetic radiation escaping from them, they have been hard to find and locate through optical telescopes. However, because they exert gravitational pull, their effect on the matter surrounding them is visible.
Several SMBHs are surrounded by an accretion disk or matter that is being pulled in towards the gargantuan black hole. This is a disk of swirling dust and gas, orbiting outside the event horizon and feeding the black hole. The disk is an extremely energetic environment with strong magnetic fields, which cause the gas in the region to heat up as it spins around the black hole on its way in. The hot accretion disk emits light (or photons) of varying wavelengths, making the whole disk glow white hot as the light dances around the black hole.
“Thus, even though a black hole itself cannot be seen, its outline or silhouette can,” explains Abhijeet Borkar, astrophysicist at the Czech Academy of Sciences. Borkar’s research is on accretion around SMBHs, mainly Sgr A*, using radio interferometry. He is not connected with the EHT findings.
Photons can actually orbit much closer to the event horizon than the accretion disk does because they do not have mass. However, their orbits are almost always unstable. They eventually either fall into the black hole or are ejected outwards at great speeds, sometimes traveling towards us, like in this case.
The photons coming from the accretion disk are still affected by the gravity of the black hole despite not falling in. This causes the light to warp and distort as it emanates from the outer regions near the black hole. Such a process is called gravitational lensing. This kind of bending of light is one of the ways the presence of gravitationally strong bodies like SMBHs can be detected.
Observing the bending of light is possible only when traveling photons are far enough away from the event horizon to escape the deathly pull of the black hole and warp around it. So the ‘shadow’ or the dark area inside the lit image is actually much bigger than the black hole itself — 2.6 times bigger to be precise.
When observed dead-on, as in the case of the M87 black hole, the accretion disk is perpendicular to our line of sight,like a mystical halo around a deity’s head, and we get a perfect outline of the black hole’s shape as light travels around it and comes to us. When observed at an angle, where the accretion disk is probably angular or even on the same plane as our line of sight, light will still travel around the black hole, illuminating the accretion disk behind the black hole and showing it in a sort of bent manner above and below, like the Saturn-like black hole from Interstellar.
Observing the entirety of the accretion disk face-on means that the M87 SMBH image shows a clean shadow in the centre. The scientists also calculated that the radius of the black hole is 20 billion km.
Light also undergoes the Doppler effect. So when photons travel towards us as they spin around a black hole, they move faster and thus appear much brighter. As they go around to the other side, they appear dimmer. This is why the coffee-mug stain is asymmetric and brighter on on the bottom as the material and light travel in our direction while revolving around the black hole.
Sometimes, active black holes even emit ‘relativistic jets’ — jets of supercharged ions expelled outwards close to the speed of light. Scientists still have no idea what causes these jets and how they behave. These jets are characteristic of the M87 galaxy, but were not observed in this session.
Event Horizon Telescope
The EHT is not one telescope but an array of eight networked telescopes spread across the globe. It is an international collaboration involving multiple countries and institutions. The purpose of the project was to obtain the first image of a black hole.
All of the telescopes combined effectively act as a single earth-sized telescope, which now has enough power to peek at the event horizon of a black hole.
The EHT telescopes are all radio telescopes, as a black hole and its surroundings cannot be optically photographed. Radio signals arrive in the form of waves and numbers to them. The telescopes looked at the M87 SMBH on four different days. All of the signals from the different telescopes are then processed using a technique called very-long-baseline interferometry (VLBI), and the numbers converted to an image.
VLBI is a type of ‘interferometry’, where multiple telescopes capture the same image, have their different images superimposed, and then information extracted from the interference pattern produced. The EHT team used two different algorithms and ended up with matching results.
“One of the main reasons we’re observing at radio is that radio VLBI is the only way we can get this angular resolution,” explains Borkar. VLBI allowed EHT to achieve an angular resolution high enough to read a newspaper in New York from a café in Paris, according to the press release.
Radio interferometry allows astronomers to see through the dust and gas cloud surrounding a black hole, and peek at the innermost stable orbit of the accretion disk, up to the point just before the abyss of the event horizon.
The telescopes contributing to this result were ALMA, APEX, the IRAM 30-metre telescope, the James Clerk Maxwell Telescope, the Large Millimeter Telescope Alfonso Serrano, the Submillimeter Array, the Submillimeter Telescope, and the South Pole Telescope. Petabytes of raw data from the telescopes were combined by highly specialised supercomputers hosted by the Max Planck Institute for Radio Astronomy and MIT Haystack Observatory.
The data generated by these telescopes was unprecedented. In one night, the EHT produced as much data as the Large Hadron Collider does in a year, according to some reports. Data collected per session, across all telescopes for one observation, was over 7 petabytes and the rates of data recording at the telescopes were over 16 gbps.
Initial findings show that the black hole spins in a clockwise direction.
Significance of the findings
This photograph is a measure of the progress in the human understanding of black holes. Just over 100 years ago, black holes were theorised to be a solution for Einstein’s field equations, a set of 10 equations in general relativity that describe gravitation in a curved space time. The definition of a black hole was then very hazy.
Large masses tend to warp space-time but if too much matter or energy was concentrated in too small an area, space-time would collapse, causing a singularity. Einstein couldn’t tell if such singularities could actually exist in reality.
“But in the 1930s, when Subrahmanyan Chandrasekhar put forward his famous paper on the Chandrasekhar Limit in white dwarfs, it became apparent that black holes might be real,” says Borkar.
Physicist Chandrasekhar showed in his paper that stars above a specific mass, 1.44 times that of the sun, would be too unstable to continue forever. In the last stages of their lives, they eventually collapse into themselves forming dense stellar remnants such as neutron stars or black holes, depending on their mass.
Further observations and future telescopes should help us slowly understand the questions posed by general relativity and the processes that occur at these scales of gravitation.
“We are seeking answers to several questions that would form the basis of our understanding of black holes,” elaborates Borkar.
“What are the effects predicted by general relativity near the black hole? Will we need something beyond general relativity to explain what we see? Do all black holes spin? What causes these powerful relativistic jets and how are they launched? We are just beginning to understand the physics of such powerful objects.”
This article has been updated with new information from the six papers released on 10 April.
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