BLACK HOLE: THE MOVIE
Scientists focus on the invisible monsters of the universe, as
After their success in producing the first ‘photograph’ of a black hole, the same scientists will now try to make a movie of the black hole at the centre of our own galaxy.
In 2019, we saw the first still image of a black hole, captured using a huge telescope network. Now the same scientists aim to record video footage of the supermassive black hole at the centre of our own Milky Way. Such a film might just deliver the data required for a huge revolution in physics.
In photographic terms, the image was not very impressive – a blurred circle of yellow and orange light with a round dark shadow at the centre. But it caused a sensation when it was published on 10 April 2019, because this was the first ever image taken of a black hole. Or to be more precise, it was an image of what surrounds a black hole, since the holes themselves are invisible, their immense gravity sucking in everything around them and preventing even light from escaping. Nevertheless, the image included such clear contours of the black hole that the astronomers behind the achievement could claim that they had for the first time “seen what we thought was impossible to see”.
Black holes remain one of the most studied astronomical phenomena, both because they are mysterious and because, according to astrophysicists, they have been vital to the development of the universe, the formation of galaxies, stars, and planets – and to people on Earth. Astronomers are seeking more information about what happens close to black holes as their immense gravity sets the time and space of the surrounding universe into a spin. And that’s why hundreds of scientists spent years observing outer space through a global network of telescopes to piece together a blurry yellow and orange circle around a dark shadow.
But those astronomers were only getting started. Now they are aiming the telescopes at the black hole located at the centre of our Milky Way – this time not to take a snapshot, but to record a movie which can show whether the laws of physics as we understand them still hold together when they come under extreme pressure.
Surroundings reveal holes
In short, a black hole is an extremely large mass compressed into a very small area.
Moving inwards, the gravity becomes ever stronger, and at the centre there is so much mass in such a tiny space that the force is immense – so immense that it bends space and time around it. Such a point is known as a singularity, a concept introduced by Albert Einstein in 1915 when he published his general relativity theory, which remains our best description of the nature of gravity.
Einstein himself doubted that singularities would really exist, but today, they are astrophysicists’ only explanation of the phenomena we can observe around black holes. The density in the holes is so extreme that scientists study their effects to put our current understanding of gravity under similarly extreme pressure.
Around a black hole, dust and gas swirl in the so-called growth disk. The inner edge of the disc is gradually sucked into the hole by gravity, and when the matter comes close enough, it passes the event horizon, which might be physics’ most fascinating boundary: if an object (or light) passes it, it can never come out again. The event horizon is therefore the limit of our ability to see into a black hole, and also the boundary of our direct knowledge about them.
So astronomers study black holes by observing what happens around them. The gravity of black holes makes stars orbit them, and the masses of the holes can be calculated based on the masses and orbits of stars. Closer to the black holes, intense radiation is emitted, which we can measure from Earth. The radiation derives from extremely hot gases. Close to the event horizon, the field of gravity is so intense that the radiation’s individual constituents, photons, are locked in orbits around the hole. But the orbits are unstable, and photons can either fall into the black hole or are flung away. Some of them are flung in our
SHEP DOELEMAN, ASTROPHYSICIST
We are delighted to report that we have seen what we thought was impossible to see!”
direction, arriving here millions of years later – assuming they encounter no obstacles on the way, and avoid being either diverted by other massive objects or absorbed by water vapour in Earth’s atmosphere.
This radiation from black holes does not reach Earth in the form of visible light, but as radio waves. Astronomers detect the waves using a global network of radio telescopes known as the Event Horizon Telescope (EHT), which can measure radiation down to a wavelength of 1.3mm. In April 2017 the EHT radio telescopes were used to measure radiation from the black hole of M87* and, over a period of seven days, the EHT collected 5 petabytes of data about the hole (in information terms, that’s equivalent to the total production of selfies by 40,000 people in their lifetimes). Such vast quantities of data are still faster to move physically than to transmit via the internet, so hard drives of data were flown to two data centres, where the observations could be combined. The scientists translated the radio waves into
visible colours – yellow pixels representing the most intense radiation, then red a little weaker, and black for the places in which no radiation at all was measured.
Using this method, the EHT scientists constructed the image that was to become so famous a few years later. The image was particularly groundbreaking because it shows us the event horizon for the very first time. We can see it (left) as the round dark disc surrounded by yellow and orange light. The EHT astronomers have calculated that the event horizon of M87* has a diameter of around 39.2 billion kilometres.
Milky Way black hole is a child
As for weight, M87* weighs some 6.5 billion times the mass of our Sun, making it a supermassive black hole. The black hole at the centre of our Milky Way, around which the entire galaxy rotates, is Sagittarius A*. It is also supermassive, though M87* is around 1600 times heavier and around 2000 times further away. But these differences of mass and distance mean that, as seen from Earth, there is little apparent difference between the dimensions of the two holes.
However, there is another important difference – that of speed. The M87* hole is a friendly subject for photography because the radiation from the hole remains almost unchanged over many hours, the gases swirling very slowly around the hole, enabling astronomers to take ‘photos’ of it with a relatively slow ‘shutter speed’. Its gases travel slowly because they are far away from the point around which the black hole’s mass is located – its centre of mass.
With Sagittarius A* the opposite is the case − the gases are closer to the centre of mass, so travel faster around the hole. Scientists compare M87* to an adult sitting still to have his portrait taken, whereas Sagittarius A* is like a three-year-old child in constant motion. The solution is to record a movie, instead of taking a photo.
The scientists will make adjustments to the EHT network’s telescopes, enabling them to record radio energy with a shorter wavelength – 0.87mm instead of the previous 1.3mm. They aim to combine recordings from additional telescopes located in Greenland, France and Arizona, a total of 11 telescopes from which the recordings must be combined. All this should improve the sharpness of the final images by 30-50%.
Finally, the scientists will use a computer program by the name of ‘StarWarps’ to
combine the images into a coherent film. The StarWarps program can analyse a series of images and calculate intermediate computer-generated images, so that what might otherwise look like a slide show can be presented as fluid video.
Black holes provide the answers
Such a film of the black hole at the centre of our own Milky Way could help us answer several questions. Scientists would like to know more about how the magnetic fileids of black holes are able to ‘push’ matter in the growth disc. According to the theory, gravity from a black hole and its rotation distorts a magnetic field to affect charged particles around the hole. Some of the particles fall into the hole while others are flung far away, but scientists do not know how much matter goes each way. Video of the activity surrounding Sagittarius A* might provide scientists with an answer. The particles can also end up in ‘jets’, which have been found near to many black holes, consisting of charged particles that pour out from the inner edge of the growth disc.
Another question that the video of Sagittarius A* might answer is how these supermassive black holes originated. Supermassive black holes exist at the centres of most galaxies, and were formed at the same time as the galaxies. But some supermassive black holes of up to 30 billion solar masses are further away than current theories should allow. One of these black holes is designated J2157, and the light we receive from J2157 was emitted when the universe was only 1.2 billion years old. According to current theories, black holes of more than 20 billion solar masses should not have existed so early in the history of the universe. Video of Sagittarius A* would give scientists a hint as to how the Milky Way’s dark centre formed, and so how ‘forbidden’ black holes are born.
EHT astronomers also plan to film other black holes. They dream of creating a catalogue that will allow scientists to compare supermassive black holes of different ages, so they can see how they develop.
Today, the EHT telescopes are located across the world’s surface, combining to collect radiation with a huge ‘receiver’. But a network of satellite-based telescopes would give the EHT an even larger diameter, while also recording radiation that gets absorbed in Earth’s atmosphere before it reaches ground-based telescopes. Scientists from the Netherlands’ Radboud University calculate that the two improvements would allow an image resolution up to five times greater than that of the existing EHT.
A high-resolution video catalogue is only the beginning of what we might obtain if the EHT were able to expand into space. The scientists’ hope to provide evidence towards the much-vaunted ‘theory of everything’ that might unite two fundamental theories of physics. Einstein’s relativity theory explains the universe on a large scale, and is still the best explanation of how black holes behave. But it has always been incompatible with quantum mechanics, which explain the tiniest of particles. Astrophysicists have already named the theory that might unite the two as ‘quantum gravity’. And the proof of this theory – which would be the biggest revolution in physics for more than 100 years – could exist around black holes.
As one of the EHT scientists, Canadian astrophysicist Avery Broderick, says: “We are looking at the universe with new eyes, more deeply and more sharply than ever before. The effects of quantum gravity might be observable at these scales. If they are, then this might be the point when, all of a sudden, the puzzle pieces click into place.”
AVERY BRODERICK, ASTROPHYSICIST
The quantum gravity theory problem remains unsolved, and black holes are one of the places in which we can look for an answer.”