Black holes are without a doubt one of the most awesome and awe-inspiring phenomena in the Universe. There are also one of the most mysterious, seeing as how scientists are unable to study them in the conventional sense. So elusive are they that astronomers and astrophysicists have only been studying them for about half a century.
In fact, scientists first stumbled onto the possible existence of black holes theoretically, thanks to Albert Einstein and his theories about gravity. It was not for several decades that their presence was confirmed thanks to the invention of telescopes that could discern objects billions of light years away.
And yet, scientists have still not managed to capture a direct image of a black hole. Much like extrasolar planets and the Milky Way Galaxy, every image you've ever seen of a black hole is an illustration based on what scientists think they look like.
So how do we find them? How do we study them? When the first images of a black hole were unveiled for the first time on April 10, 2019, how did we even know it was are there? All these questions require a bit of a retrospective, as well as a recap of some fundamental principles.
What are Black Holes?
Put simply, black holes are what result when sufficiently-massive stars undergo gravitational collapse at the end of their life cycle. Long after the star has exhausted the last of its hydrogen fuel and expanded to several times its standard size (what is known as the Red Giant Branch phase), it will blow off its outer layers in a spectacular explosion known as a supernova.
In the case of lower-mass stars, this explosion will leave behind a superdense stellar remnant known as a neutron star (aka. white dwarf). But where more massive stars are concerned, the collapse and explosion will leave behind a compact mass that is capable of deforming spacetime around it.
The gravitational field of a black hole is so strong that nothing - not even subatomic particles or electromagnetic radiation (i.e., light) - can escape it. The outer boundary of the black hole - the point from which there is no return - is known as the Event Horizon.
It is this boundary where a collapsing star recedes to; at which point, time stands still, and the collapsing object can collapse no more. Beyond this point, the gravitational force of a black hole is the same as an object of comparable mass and matter and energy can still be observed.
But within the Event Horizon, nothing can escape, and nothing can be observed. Anything that passes within this boundary (matter or energy) will be compressed matter infinitely dense region of spacetime known as a singularity.
Speaking of which, scientists also theorize that this is what lies at the center of a black hole. Otherwise known as a gravitational singularity, it is in this region that the spacetime curvature becomes infinite. In other words, it is within a singularity that the normal laws of physics become indistinguishable from each other, and time and space cease to have any meaning.
Black holes also range in size. Whereas your more massive stars created "stellar black holes", which can range from ten to 100 Solar masses, there are also larger ones that are the result of mergers. These mergers produce gravitational waves, which General Relativity also predicted, that cause spacetime to ripple.
Scientists only recently began to be able to detect these waves thanks to facilities like the Laser Interferometer Gravitational wave Observatory (LIGO) - which consists of two facilities located in Hanford, Washington, and Livingston, Louisiana; the Virgo observatory near the city of Pisa, Italy; and the soon-to-be-completed Kamioka Gravitational Wave Detector (KAGRA) in Japan.
This merger process is believed to have created the supermassive black holes (SMBH) that exist at the center of most (if not all) spiral and elliptical galaxies. And when galactic mergers occur, these SMBHs also come together and become even larger!
The closest SMBH is known as Sagittarius A*, which is located about 26,000 light years from our Solar System at the center of our galaxy, near the border of the Sagittarius and Scorpius constellations. This SMBH has a mass that is equivalent to roughly 4 million Suns and is one of the few black holes close enough for astronomers to observe the flow of matter nearby.
Classification of Black Holes:
Black holes are characterized based on three parameters - mass, rotation, and charge. Based on these characteristics, scientists have identified four different types of black holes. First, you have Primordial Black Holes (PBH), which are less than a tenth of a millimeter in diameter and have about as much mass as planet Earth.
These are a purely hypothetical class of black holes that are believed to have formed through patches of overdensity shortly after the Big Bang. According to a theory put forth by Stephen Hawking in 1972, these black holes could be responsible for the Universe's "missing mass" (aka. Dark Matter) - thought this theory has been recently tested with negative results.
Second, there are Stellar Mass Black Holes, which range in mass from about 4 to 15 Solar masses and are the result of a massive star undergoing core-collapse at the end of its lifespan. Third, you have Intermediate Mass Black Holes of a few thousand solar masses and are thought to believe in star clusters.
Lastly, you have the aforementioned Supermassive Black Holes (SMBHs), which exist at the centers of most large galaxies and range from being in the millions to billions of Solar Masses (depending on the galaxy's size). Alternatively, black holes can be classified by the properties of rotation and charge, from which three categories emerge.
First, there's a Schwarzschild Black Hole (aka. a "static black hole") which does not rotate, has no electric charge, and is characterized solely by its mass. There's also a Kerr Black Hole is a more realistic scenario, which is a rotating black hole with no electrical charge.
Last, there is a Charged Black Hole, which can either be charged and non-rotating black hole (aka. Reissner-Nordstrom black hole) and a charged, rotating black hole (a Kerr-Newman black hole).
History of Study:
The existence of black holes was predicted by Einstein's Theory of General Relativity, which states that the curvature of spacetime becomes distorted in the presence of gravitational fields. In time, astronomers and scientists would expand on his field equations, which would lead to the theory of black holes.
The first was Karl Schwarzschild (1873-1916), a German astronomer who used Einstein's theory of General Relativity to determine that matter compressed to a point of singularity would be enclosed by a spherical region of space from which nothing could escape - i.e., the Event Horizon.
Schwarzschild is also credited for determining the radius at which compressed matter would form a black hole shortly before his death in 1916. This is known as the Schwarzschild radius (or gravitational radius), which describes a point where the mass of a sphere is so compressed that the escape velocity from the surface would equal the speed of light.
This was followed in 1931 by Indian-American astrophysicist Subrahmanyan Chandrasekhar calculating the maximum mass a white dwarf/neutron star can have before collapsing into a black hole. This is known as the Chandrasekhar Limit, which he determined was about 1.4 solar masses.
In that same year, physicist and radio astronomy Karl Jansky - considered by many to be the "father of radio astronomy" - discovered a radio signal coming from the center of the Milky Way in the direction of the constellation of Sagittarius. This radio source would later be determined to be the SMBH known as Sagittarius A*.
In 1939, Robert Oppenheimer and others concurred with Chandrasekhar’s analysis and theorized that within the boundary of the Schwarzschild radius was a bubble in which time stopped. To the outside observer, the star would appear frozen in time at the instant of collapse, but an observer trapped within the Event Horizon would have an entirely different perspective.
By the 1960s, the "Golden Age of General Relativity" began, which was characterized by General Relativity and black holes becoming mainstream subjects of research - rather than theoretical curiosities. Fundamental discoveries included the discovery of pulsars by Jocelyn Bell Burnell in 1967, which were shown to be rapidly rotating neutron stars by 1969.
It was also during the 1960s that the term "black hole" was officially coined by physicist Robert H. Dicke, who reportedly compared the phenomenon to the Black Hole of Calcutta, a notorious prison in India from which no one was said to have returned.
It was also during this time that more general solutions to theoretical issues arising from black holes were found. These included mathematical solutions for rotating black holes, rotating and electrically-charged black holes, and stationary black holes.
By the 1970s, the work of Stephen Hawking and other theoretical astrophysicists led to the formulation of black hole thermodynamics. Much like regular thermodynamics, these laws outlined the relationship between mass and energy, area and entropy, and surface gravity and temperature.
By 1974, Hawking showed that quantum field theory predicts that black holes radiate like a black body where temperatures are proportional to the surface gravity of the black hole. This phenomena where black holes emit radiation in the form of exotic particles have come to be known as "Hawking radiation."
This theory gave rise to the "Black Hole Information Paradox." In accordance with the classical theory of General Relativity, once a black hole is created, it will never disappear, and anything that passes into it will see its quantum information preserved forever.
However, Hawking's theory predicted that black holes will slowly lose mass by emitting radiation over time and eventually evaporate - though this would happen on incredibly long timescales for even single-Solar mass black holes. To date, all attempts at detecting Hawking radiation have failed to produce verifiable results.
In 1974, astronomers at the National Radio Astronomy Observatory (NRAO) confirmed the existence of Sagittarius A*, the name of which was coined by a member of the discovery team (Robert Brown) in a 1982 study describing the discovery.
How Do We Detect Black Holes?:
Simply put, we don't. Since black holes do not reflect any energy and nothing (not even light) can escape them, they are for all intents and purposes invisible. However, for many decades, scientists have been able to infer their presence based on the influence they exert on the surrounding Universe.
These include the gravitational influence black holes have on nearby stars and celestial objects, which is made evident by the motions of nearby objects that orbit them. For instance, since 1995, astronomers have tracked the motions of 90 stars orbiting Sagittarius A*.
Based on their orbits, astronomers were able to infer that Sagittarius A* had a mass of at least 2.6 million Solar masses, which they later refined to 4.3 million within a volume of space measuring less than 0.002 light years in diameter. One of these stars, called S2, has since completed a full orbit and its motions have been used to test General Relativity.
There is also the high-energy phenomena associated with black holes, such as high-energy emissions in the ultraviolet, X-ray, and gamma-ray wavelengths and relativistic jets. Essentially, when matter falls into orbit around a black hole, it forms an accretion disk around the black hole.
The powerful gravitational pull of the black hole imparts energy into this disk, causing it to spin rapidly and become heated by friction. This causes matter in the disk to emit energy in the form of electromagnetic radiation in multiple wavelengths.
Some accretion disks have become so bright incredibly bright that they outshine the billions of stars their galaxy hosts. Galaxies that have particularly bright disks are known as active galactic nucleus (AGN) galaxies, where their centers are much brighter than the rest of the galaxy.
Rapidly spinning SMBHs are also known to emit energy in the form of relativistic jets. This is what happens when hot, energized gas is focused by magnetic field lines and shoots out from the poles, and at velocities that are a fraction of the speed of light.
Studying these jets not only lets astronomers discern the presence of a black hole, the way they change direction reveals things about the rotation of the black holes themselves (like the orientation and size of their rotating disks). Because the jets are so large, they are also relatively easy to spot in the cosmos.
In fact, astronomers have observed these jets coming from the centers of several massive galaxies, which is an indication of an SMBH. These jets also allow astronomers to identify which galaxies have an AGN and which do not.
The technologies that allow for this include highly sensitive instruments and telescopes that are capable of taking images of our Universe in the visible and non-visible parts of the spectrum. These include optical, infrared, ultraviolet, radio, X-ray, and gamma-ray instruments and space-based telescopes.
Some examples of note include the Hubble Space Telescope, which has provided remarkable and high-resolution images of our Universe, some of which were used to determine the presence of black holes. Then there's the Spitzer Space Telescope, NASA's premier infrared space telescope.
Then you have the Galaxy Evolution Explorer (GALEX), which observes the Universe in the ultraviolet end of the spectrum; the Arecibo Radio Observatory and the Karl G. Jansky Very Large Array (VLA), which conduct radio astronomy; and the Chandra X-ray Observatory, XMM Newton X-ray Observatory, the Fermi Gamma-ray Space Telescope and the Neil Gehrels Swift Observatory.
As noted, black holes are undetectable in visible light, which makes located them with conventional optics very difficult. This necessitates that astronomers look for the influence a black hole's powerful gravity has on the surrounding cosmic environment and the energy that this releases.
Naturally, this requires large telescopes equipped with sophisticated optics and instruments, not to mention plenty of computing power to process the images. In addition, atmospheric distortion is an issue, which either requires that telescopes come with adaptive optics or are placed in orbit.
Another method is known as interferometry, where two or more sources of light are merged to create an interference pattern which is then measured and analyzed. These patterns contain vital information about the object or phenomenon being studied and can achieve a level of precision that would be impossible otherwise.
The only problem is, similar phenomena have been observed around other types of compact objects - such as neutron stars, pulsars, and white dwarfs. As a result, astronomers need to observe accretion disks, energy sources and nearby objects closely to calculate the mass of the object affecting them.
In short, to find and study black holes, you need sophisticated instruments, proven methods, and a lot of hard work. Luckily, next-generation instruments are becoming operational that are making the job easier. One of which is the Event Horizon Telescope (EHT).
The Event Horizon Telescope:
The EHT is an international project that takes advantage of recent advancements in astronomy to create a massive "virtual telescope." This involves combining data from a global network of radio antennas and several very-long-baseline interferometry (VLBI) stations around the world.
The EHT aims to observe the immediate environment around Sagittarius A* as well as the even larger SMBH at the center of Messier 87 (aka. Virgo A). This supergiant elliptical galaxy is many times the size of the Milky Way and is located about 54 million light-years away from Earth in the Virgo constellation.
The EHT will gather light from these SMBHs by relying on the dozens of observatories that are participating in the project. Once this light is collected, the data will be combined and processed using imaging algorithms that will fill in the missing gaps in the data, thus allowing the project team to reconstruct a picture of the black hole's event horizon.
By linking together radio dishes across the globe, astronomers have been able to create an Earth-sized interferometer capable of measuring the size of the SMBHs emission regions. The project also takes advantage of a key millimeter- and submillimeter-wavelength facilities at high altitude sites.
The project began collecting light in 2006 and has added several observatories since it first began. Today, it links ten radio telescopes and the respective institutions that operate them, which include the:
James Clerk Maxwell Telescope (JCMT) at the Mauna Kea Observatory (CSO) in Hawaii
Large Millimeter Telescope Alfonso Serrano (LMT) on Volcán Sierra Negra, near Veracruz, Mexico
Combined Array for Research in Millimeter-wave Astronomy (CARMA) in eastern California
Kitt Peak National Observatory's (KPNO) two radio telescopes, located just south of Tucson, Arizona
Arizona Radio Observatory's (ARO) Submillimeter Telescope (SMT) in southern Arizona
European Southern Observatory's (ESO) Atacama Large Millimeter/submillimeter Array (ALMA) in northern Chile
In the coming years, two more arrays will be added: the Greenland Telescope, which is jointly operated by the Smithsonian Astrophysical Observatory and the Academia Sinica Institute of Astronomy and Astrophysics; and IRAM's Northern Extended Millimeter Array (NOEMA) in southern France.
The data collected by participating observatories is then uploaded to hard drives and transported by plane to the MIT Haystack Observatory in Massachusetts, USA, and the Max Planck Institute for Radio Astronomy, Bonn, Germany. Once there, the data is cross-correlated and analyzed by 800 computers that are connected through a 40 Gbit/s network.
While the first image of Sagittarius A* was expected to be produced in April 2017, this was delayed due to the South Pole Telescope being closed during winter (April to October). This delayed the data shipment until Dec. 2017 which also delayed the processing. The first image is now scheduled for release on April 10th, 2019.
In addition to being the first image of an event horizon, this image and others like it will also test Einstein's Theory of General Relativity (GR) under the most extreme circumstances. So far, most attempts to measure gravity's effect on the curvature of spacetime has involved smaller objects like the Sun and Earth (one exception being observations of S2's orbit).
But with superior images of Sagittarius A* and M87's SMBH, the observed effects of GR will be incredibly profound. Other anticipated results include a greater understanding of how matter forms disks around black holes and accretes onto them, which is what allows them to grow.
This is necessary since scientists do not yet understand how matter manages to escape the debris disk and cross the event horizon of a black hole. Over time, it is understood that as matter in accretion disks loses energy, it will fall into the black hole's event horizon.
But since black holes are such compact masses, the matter would need to give up a lot of energy to fall all the way in. In addition, it is unknown why matter in a debris disk experiences such friction when it is so dilute. Ergo, some other physical force must be responsible for causing matter to heat up in debris disks and accrete onto black holes.
Currently, the leading hypothesis is that rotating magnetic fields create some special type of turbulence that causes atoms to emit energy in a way consistent with friction. Until now, scientists have not been able to test this theory experimentally; but with the EHT, they finally will!
In addition, scientists hope to learn why Sagittarius A* is relatively dim when compared to SMBHs observed in other galaxies. A better understanding of the mechanisms that power debris disks and cause SMBHs to grow will go a long way towards answering this question.
With the first image of Sagittarius A*'s event horizon and "shadow" - which was presented in the early morning hours on Wednesday, April. 10th - scientists are well on their way to accomplishing that goal. Here is how Dr. Erin Macdonald (the host of the online series "Dr. Erin Explains the Universe") summarized the accomplishment:
"The main scientific achievement from this discovery today is that we are finally seeing the event horizon of a black hole. This is the moment right before the escape velocity is so great, due to the gravitational pull of the black hole, that not even light can escape. This was imaged using a world-wide "telescope" - eight radio telescopes combined their imagery to be able to see a resolution and wavelength good enough to capture this image.
"This discovery seems to meet expectations established by Einstein's equations for General Relativity established over 100 years ago. His equations laid a foundation for theoretical phenomena such as black holes and gravitational waves. In just over 100 years, humanity took these equations and relentlessly pursued observations of these, to great success.
"Not only is it a wonderful scientific achievement, but a reminder that it took the work of the whole world to achieve this image. The study of space continues to unite the globe and is a great demonstration of what humanity can achieve when we work together."
In the coming years, the international team behind the EHT plans to mount observation campaigns of ever-increasing resolution and sensitivity. In so doing, they hope to be able to overcome the barriers that prevent us from directly observing one of the most powerful and fascinating phenomena in the Universe.