Seeing the Invisible: Imaging a Black Hole

In early April this year, newspapers and online websites were agog with the news that astronomers had obtained the first image of a black hole known to reside in a distant galaxy called M87. Why did this image cause such a stir?
 
Fig. 1. The image of a ‘black hole’ at the heart of galaxy M87.
This image was made by combining radio waves received from dishes (antennas) distributed over the entire globe.
Credits: Provided by Event Horizon Telescope (https://www.eso.org/public/images/eso1907a/) and uploaded by BevinKacon, Wikimedia Commons. URL:
 
 
In April 2019, astronomers managed to obtain an image of a black hole for the first time (see Fig. 1). This black hole is known to reside in the heart of a distant galaxy called M87 (see Box 1).
Based on measurements of the velocities of rapidly moving stars and gas near the centre of M87, the mass of this black hole is estimated to be billions of times more than that of our sun. Similarly, the size of this black hole is calculated to be billions of kilometres — bigger than our solar system (see Box 2). However, because of its distance from us, the angular size of the expected image of the black hole itself is very small, one part in hundred million (10-8) of a degree. Despite this, astronomers chose to image it because this is the largest angle subtended by a black hole known to us.
 
The idea of a black hole
Our story begins more than 200 years ago. In 1783, John Michell, an English clergyman imagined a body from which even light could not escape. Another early proposal of what we now call a black hole came from the writings of the French scientist Pierre-Simon Laplace in 1799. Both Michell and Laplace based their calculations on the idea of escape velocity (see Box 3). 
 
A more comprehensive understanding of black holes came in 1915 from Einstein’s general theory of relativity abbreviated to GTR) based on the curved geometry of space and time. Because its mathematics was so unfamiliar, it took the physics and astronomy community nearly four decades to agree that this theory could be used to describe a black hole. GTR describes the spherical surface of a black hole as a wave of light that is unable to travel outwards because of gravity.
Does this contradict Einstein’s special theory of relativity, proposed in 1905, which suggests that all observers will see light travelling at the same speed (c = 300,000 km/s)? This apparent contradiction can be resolved since the spherical wavefront that represents the surface of the black hole only appears to be standing still when viewed from far away (see Box 4). An observer who is present at this surface will see it moving outwards at the speed of light, because she cannot stand still — she is falling in! This wavefront is called an Event Horizon. If an event occurs inside this surface, no light or message from it is sent to the outside world. To an observer standing outside, this surface is like a horizon — we don’t see beyond it. It is for this reason that the network of researchers who worked on making the image of the black hole called their collaboration the Event Horizon Telescope (or, EHT).
 
 
Today, astronomers believe that a black hole is the final state of a massive star (one that is more than 20 times heavier than our sun). According to this view, after a massive star exhausts its source of energy, it collapses to a much smaller size. Its large mass and small radius mean that the force of gravity on its surface is so strong that nothing, not even light, is able to escape it. These predictions were validated in 2016, when the LIGO observatories ‘heard’ ripples of two black holes that were each 30 times heavier than our sun. 
 
Black holes at the heart of galaxies like M87 are known to rotate. This is because a black hole is formed by collecting material, like gas or even whole stars, that orbited it before falling into it. To understand this, think of the black hole pulling in space and time like a waterfall pulls in floating objects. If we think of spacetime around a rotating black hole as a fluid, it is not only being pulled in, but also being swirled around. A particle, or even a ray of light, coming inwards moves sideways in the direction of rotation. This idea is aptly illustrated in a cartoon by C. V. Vishveshwara, a very well-known researcher in the area of GTR who also played a major role in setting up the Bengaluru planetarium (see Fig. 2).
 
Fig. 2. C. V. Vishweshwara's cartoon illustrating the behaviour of observers around a rotating black hole, and drawing a parallel with ‘Alice in Wonderland’.
Credits: This image is derived from C. V. Vishweshwara's article “Black Holes for Bedtime” in the volume
“Gravitation, Quanta and the Universe; proceedings of the Einstein Centenary Symposium held on
29th January—3rd February, 1979, in Ahmedabad, India.” Edited by A. R. Prasanna, J. V. Narlikar, and C. V.
Vishveshwara. A Halsted Press Book, published by John Wiley & Sons, New York, 1980, p154-167. Image
reproduced here courtesy Prof. Sarawathi Visweshwara.
 
 
 
Radio waves from M87
 
Many Australian, British, and US scientists who worked on radar technology during the World War II turned their attention to the study of radio waves from astronomical objects in the post-war period. This was far more challenging than using visible light to probe the universe. The main disadvantage was the fact that the wavelength of radio waves (which is measured in centimeters or meters) was much longer than that of visible light (which is about half a micrometer). This meant that not only was it not possible to determine the precise position of the source of radio waves and its finer details, there was no clue as to how far away the source was.
 
Nevertheless, this approach was used to make many outstanding discoveries. For e.g., in 1948, two Sydney based scientists — John Bolton and Gordon Stanley — found a strong source of radio waves in the constellation of Virgo. They offered the tentative proposal that this source was the same galaxy known to us as M87 even though the object was thought to be about 30 million light-years away (the modern value is 55 million). The technique they used for this discovery is called interferometry. In this technique, radio waves arriving at two (or more) radio telescopes are compared to measure the difference in arrival times of the crests and the troughs. One can then infer the direction and strength of the source from these measurements. This is similar to how we, and most other animals, determine the direction of sound waves using two ears and the appropriate hardware/software in the brain. The same principle — of receiving and accurately comparing signals at telescopes separated in space — underlies most of radio astronomy today, and is the foundation of the EHT effort (see Box 5).
 
Around the beginning of this century, some of the bolder scientists realized that radio telescopes and the techniques of radio astronomy had reached a stage where viewing the surroundings of a black hole was just about possible. Since the technology (using millimeter waves) required to do this is quite demanding (see Box 6), the EHT project required the co-operation and collaboration of eight different telescopes (see Fig. 3). Many of these observatories are located at high altitudes since the water vapour in the lower atmosphere blocks these millimetre waves.
 
Fig. 3. The eight radio telescopes contributing to the EHT and their locations.
Credits: Adapted from an image by © APEX, IRAM, G. Narayanan, J. McMahon, JCMT/JAC, S. Hostler, D. Harvey, ESO/C. Malin, Max Planck Institute for Radioastronomy.
 
Significance of the ring shaped image of a black hole
 
Unlike laboratory physicists, astrophysicists do not control the systems they study. They have to work with radiation received on earth, and images which do not reveal the finer details of the object they are studying. Therefore, astrophysicists create ‘models’. A model is a guess as to what kind of material, at what temperature, moving in what manner, will explain the limited observational information that astrophysicists have. Of course, any model has to obey the known laws of physics. Often, very elaborate mathematics and/or computer programmes are needed to make predictions that can be compared with actual observations. When the observations are limited, many different models may work. With improvements in the quality of observations (for e.g., by observing an object at different wavelengths, or making images with higher resolving power), many of these models get rejected. If all goes well, the one model that survives is generally accepted as being the most plausible one.
 
Fig. 4. Radiation originating near a nonrotating black hole.
The dashed line is called the photon sphere. Any radiation crossing it falls into the event horizon.
Rays that just miss it are bent and can reach a distant observer, forming a ring-shaped image.
No rays are seen emanating from this sphere, which lies outside the event horizon.
Credits: Rajaram Nityananda. License: CC-BY-NC.
 
It is this kind of process, over nearly half a century, that has given us a model that explains what is happening around the black hole to produce the powerful radio emissions we receive on earth. The black hole is surrounded by gas, which orbits it. The different gas streams at different radii and orbiting with different speeds results in friction. This has two consequences. One, the gas in the inner region spirals inwards into a lower orbit, in the same way that artificial satellites descend into lower orbits due to the friction of the earth’s atmosphere. Two, this friction heats up the gas. Ultimately, the energy generated is accounted for by the potential energy lost while descending to lower orbits. As a very simple example, a stone falling towards the earth picks up kinetic energy, which becomes thermal energy when the stone hits the ground. At these high temperatures, the electrons in the gas ring get separated from the nuclei of the atoms, turning the gas into an electrical conductor. This carries electrical currents which produce a magnetic field. Electrons moving in curved orbits in a magnetic field emit radio waves. The rapidly rotating gas in the centre acts like a pump, and some of the gas is flung out and away from the black hole along the magnetic field lines. This too has a parallel — many borewells have a rapidly rotating ‘centrifugal pump’ at the bottom, which gives the water enough energy to climb to the surface. 
 
While this model may seem very general, it was backed by large-scale computations to explain the energy and wavelength of the observed radiation even before the EHT work. The recent image allows astrophysicists to compare their model with actual observations to solve for some quantities which were unknown before. These include the mass of the black hole, how rapidly it is rotating, the amount of gas falling into it, and the strength of the magnetic field. Therefore, the recent image not only allows us to observe the black hole, but also learn more about its surroundings. 
 
To conclude
 
The excitement about the ring is justified. This image provides direct evidence for the existence of black holes — first offered as mere speculation more than two centuries ago. While it was worked out by astrophysicists in some detail even 50 years ago, the evidence was always indirect — calculations that were based on the assumption that a black hole seemed to agree with the observations. This was true both for the end states of massive stars and for the energy sources at the centre of galaxies. Clearly, astronomers were waiting for a more direct confirmation of the role of black holes. LIGO in 2016, and the EHT in 2019, have provided this long awaited evidence. 
 
To a physicist, black holes are a fascinating aspect of gravitation, with properties like the event horizon, and the dragging of objects by a rotating black hole. Even more fascinating, but still unsolved, is the problem of what happens to matter once it crosses the event horizon. These recent astronomical discoveries will undoubtedly result in more work — both in terms of theory and observation. We can look forward to a better understanding of some of the most unusual objects in our universe.
 
 

Rajaram Nityananda is currently teaching at the Azim Premji University, Bengaluru. Prior to this, he was at the Raman Research Institute (RRI), Bengaluru. He has been the Chief Editor of Resonance, journal of science education, for one term (~three years). Much of his research work has been theoretical — in areas of physics relating to light and to astronomy and, hence, involving mathematics and/or computation. Rajaram enjoys collaborating with students and colleagues — many of them experimenters, and many outside his own institution.

 

 
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