Unravelling the Long-Standing Mystery of Black Holes
Black holes are among the most enigmatic objects in the Universe. Stellar black holes are, to put it briefly, the remnants of massive stars that have reached the end of their life cycles and collapsed into a region of spacetime that is incredibly dense. The gravitational force is so strong that nothing - not even light - can escape its surface, and the laws of time and space become warped.
Ever since they were predicted about a century ago, astronomers and astrophysicists have struggled to learn more about these strange and mysterious objects. This has been no small feat, and has required the dedication and efforts of generations of scientists. More questions remain than answers. But what they have learned so far has already revealed much about the Universe we live in and the laws that govern it.
After theorizing their existence, scientists began to see indications of these objects by the latter half of the 20th century. By studying their effects on the space surrounding them, they were able to indirectly identify where black holes reside. Today, many astronomers believe that most galaxies have a supermassive black hole (SMBH) at their core, and that includes the Milky Way.
This year, the Nobel Prize in Physics has been awarded to three scientists who have had a particularly large impact in the field of black hole research. One half of the prize was awarded to famed British mathematical physicist Roger Penrose who - in the words of Caltech scientist Kip Thorne - “revolutionized the mathematical tools that we use to analyze the properties of spacetime.”
The other half of the prize was awarded to astronomers Andrea Ghez and Reinhard Genzel, both of whom have led groups since the 1990s which mapped the orbits of stars close to the Galactic Center. These studies led Ghez and Genzel to conclude that an extremely massive object - known as Sagittarius A* (Sgr A*) - was dictating the stars’ movements, thus providing the most compelling proof for the existence of an SMBH at the core of our galaxy.
The Nobel Prize Committee has recognized all three individuals for achievements that go back many decades and have revolutionized our understanding of the Universe. As David Haviland, the chair of the Nobel Committee for Physics, said:
“The discoveries of this year’s Laureates have broken new ground in the study of compact and supermassive objects. But these exotic objects still pose many questions that beg for answers and motivate future research. Not only questions about their inner structure, but also questions about how to test our theory of gravity under the extreme conditions in the immediate vicinity of a black hole.”
However, to really do justice to what these three individuals have accomplished requires that we acknowledge what brought us to this point in history.
A Brief History of Black Holes
In 1915, legendary physicist Albert Einstein formalized his General Theory of Relativity. This theory was the culmination of decades worth of research, which Einstein began shortly after proposing Special Relativity in 1905. It synthesized Newton’s laws of motion with electrodynamics to explain the behavior of light.
Best known for its famous equation, E=mc², Special Relativity introduced a new framework for physics by proposing new conceptualizations of space and time, and establishing that matter and energy were not discrete entities (as previously thought), but different expressions of the same reality.
Over the next ten years, Einstein sought to expand on this revolutionary theory in the hopes of reconciling it with Newton’s law of universal gravitation, which was beginning to fall short in the field of astrophysics.
Whereas Newton’s theory described gravity as a force of mutual attraction between massive objects (which is proportional to the mass of those objects), Einstein’s General Theory of Relativity asserted that gravitational effects between masses are the result of the impact they have on spacetime.
In short, Einstein argued that massive objects alter the curvature of spacetime, which dictates how matter moves in their vicinity. Along with Einstein’s field equations, this theory revolutionized our understanding of space and time (which are also two expressions of the same reality) and also predicted several astronomical/cosmological phenomena. These include:
- Perception of time slowing in a gravitational field (gravitational time dilation)
- Massive objects merge to create ripples in space-time (gravitational waves), confirmed by the LIGO observatory for the first time in 2016
- Gravitational fields will bend and magnify the light coming from a distant object (gravitational lensing)
- Black holes
In 1916, German astronomer Karl Schwarzschild published a solution to Einstein’s field equations that described the curvature of spacetime around a spherically symmetric, non-rotating mass (known as the “Schwarzschild Metric”). In short, he speculated that a sufficiently compact mass could significantly deform spacetime.
Part of this solution, which established the radius a particularly-massive object needs to reach to have this effect, is known as the Schwarzschild Radius. This equation is described mathematically as RS = 2GM/c2, where RS is the radius between the center of the black hole and its outer edge (Event Horizon), G is the gravitational constant, M is the mass of the object, and c is the speed of light.
By the 1930s, Indian-American astrophysicist Subrahmanian Chandrasekhar calculated the maximum mass a white dwarf star could possess without becoming unstable. Any white dwarf star that surpassed this limit, he argued, would collapse into a black hole. These calculations came to be known as the “Chandrasekhar Limit,” which is about 1.4 times the mass of our Sun.
By the 1960s, during what is commonly known as the “Golden Age of General Relativity,” astronomers and astrophysicists were finally able to study black holes. It was also during this time that the term “black hole” reportedly emerged. Before this, scientists used other terms - such as “dark star” or "gravitationally collapsed object” - to describe them.
According to science writer Marcia Bartusiak, the term "black hole" was first used by physicist Robert H. Dicke during his lectures at Princeton in 1960, where he compared the phenomena to the Black Hole of Calcutta - a notorious prison in 18th century India from which no one was said to return. American physicist John Wheeler helped popularize it thereafter and the term began to appear in print by 1963/64.
Since black holes are invisible in normal light and nothing can escape their surface, these studies were all indirect. Rather than observing black holes themselves, astronomers had to infer their presence based on the effects they had on the objects around them.
Nevertheless, astronomers were able to learn a great deal this way, and even began to stumble onto the first clues that hinted at the existence of supermassive black holes (SMBHs). Though not observable in visible light, radio astronomers began to notice radio sources that were thousands of times brighter than our galaxy. Astronomers soon realized that these “quasi-stellar objects” (or quasars, for short) were at the center of most massive galaxies, including our own.
Those galaxies that exhibited them were remarkably more energetic in their central regions, leading to the term “Active Galactic Nuclei” (AGN) to describe galaxies that possessed a quasar. In time, astronomers realized that these objects were too large to be stellar masses, and therefore had to be gravitationally-compact objects (black holes).
The next great leap came with the development of interferometry, where multiple telescopes capture light from an object and then combine it to create a more complete picture. This process made it possible for astronomers to detect faint objects, which includes the debris disks that commonly form around black holes.
This was how astronomers with the Event Horizon Telescope conducted the first direct observations of a black hole in history. On April 10th, 2019, the first images of the SMBH at the center of the M87 supergiant elliptical galaxy were shared with the world (see below).
Structure of Black Holes
Thanks to this extensive research, astrophysicists have a pretty good understanding of what a black hole looks like. As the image below illustrates, at the center of a black hole is what is known as a Singularity: a point where the density of matter and the curvature of spacetime become infinite. Anything that passes into this point will be crushed to the point of singularity as well, and disappear forever.
The extent of a black hole is known as its Event Horizon, which is the radius its progenitor star will retreat to after undergoing gravitational collapse. Within this radius, matter and energy cannot escape and will fall towards the singularity.
Immediately beyond this radius is the Accretion Disc, where matter and energy fall into orbit around the black hole and are accelerated to relativistic speeds (a fraction of the speed of light) by the extreme gravitational forces at work. The innermost part of the disc is known as the Photon Sphere, which is where photons (the constituent particles of light) are trapped and form a bright ring that spins with the black hole.
Some black holes also have Relativistic Jets, which are composed of particles and energy that are blasted out from the center of the black hole after it has consumed a star or other astronomical objects. Anything in these jets is accelerated to near the speed of light, and they can extend for thousands of light-years (allowing astronomers to spot them from great distances).
This is what black holes look like, at least from the outside. What happens beneath the veil of an Event Horizon is not currently known, and may never be. Unfortunately, this is one of many things that we don’t know about black holes, and what we do know tends to bend and warp the mind - much like what black holes do to the nature of reality!
An Enigma for the Ages
To borrow from an old saying, black holes are like “a riddle, wrapped in a mystery, inside of an enigma.” For starters, the name “black hole” is a bit of a misnomer, since these objects are not holes at all. What they are is essentially stars that have collapsed to the point where the gravitational force they exert overwhelms all other physical laws.
In order for objects (i.e. a rocket ship) to escape an object’s gravity, they need to achieve what is known as an “escape velocity.” On Earth, all objects are subject to a gravitational force of 9.8 m/s² (32.15 ft/s²), which means that when an object is falling, it is accelerating towards the center of our planet by an additional 9.8 meters (35.28 km/h; ~22 mph) for every second that it’s falling.
To “escape” Earth’s gravity, one needs to achieve a velocity of 11.186 km/s (40,270 km/h; 25,020 mph) or higher. On the Sun, the escape velocity is about fifty-five times higher: 617.7 km/s (~2220 km/h; 1380 mph). The speed of light, on the other hand, is a whopping 299,792,458 m/s - or about 107.9 million km/h; 670.6 million mph.
What this means is that when a sufficiently-large star collapses to its Chandrasekhar limit (see below), its gravity will become so intense that its escape velocity will be equal to the speed of light! And since gravity affects the observer’s perception of time (the stronger the gravitational field, the slower the passage of time), that means that within the Event Horizon of a black hole, time itself could be said to cease.
This is also why black holes do not continue to collapse beyond their Schwarzschild Radius. Once they’ve collapsed to this point, they become frozen in time. Hence, it’s also theorized that while any matter that passes within its Event Horizon will be pulled apart and broken down to its constituent particles, the quantum information of those particles will be preserved for all time.
However, that last part remains a point of contention. Ever since Stephen Hawking theorized that black holes could emit radiation in 1974 - which has since been confirmed and appropriately named “Hawking Radiation” - scientists have been forced to accept that black holes actually do shed mass over time. While it might take trillions of years, this would eventually cause them to evaporate, and all information contained within to be lost.
This conundrum, known as the “Black Hole Paradox,” illustrates how these objects continue to mystify us. Still, we’ve come very far in our understanding of them, thanks to the efforts of countless scientific minds. But in the end, the greatest theoretical breakthroughs were made by a handful of luminaries, which is something this year’s Nobel Prize in Physics has recognized.
A Three-Way Award
In recognition of his immense contributions, the Royal Swedish Academy of Sciences has chosen to award half of the 2020 Nobel Prize in Physics to Robert Penrose. When Penrose began studying black holes, there were several unresolved theoretical issues that were causing astronomers no shortage of headaches!
For one, a key assumption was that black holes had to be spherically symmetrical, otherwise they shouldn’t be able to collapse to a single point and create a singularity. In response, Penrose developed a theory that did away with the assumption of spherical symmetry and assumed only that the collapsing matter had a positive energy density. In order to do this, he had to invent new mathematical methods and find a way to describe the geometry of black holes.
For starters, Penrose developed the concept of a trapped surface, a closed two-dimensional surface where all light rays traveling perpendicular to the surface converge towards the future. This was contrary to what happens with a spherical surface in flat space, where light-rays traveling outwards diverge.
A consequence of this theory is that within a black hole’s Event Horizon, time and space switch roles, to the point where traveling out of a black hole would be tantamount to traveling backward in time. An even more dramatic consequence of the trapped surface is that all the matter that collapsed to form the black hole will be frozen in time.
To visualize space-time, Penrose introduced what would come to be known as the “Penrose diagram.” This technique uses conformal transformations, where points infinitely far away in space (and events in the infinite past or future) can be brought in from infinity to fit inside a framework of finite size - aka. a Penrose diagram.
As a result, Penrose demonstrated that gravitational collapse cannot be stopped after the trapped surface is formed. A few years later, Penrose and Stephen Hawking showed that similar results could also be obtained when his theories were applied to cosmological singularities - which came to be known as Penrose–Hawking singularity theorems.
By 1969, Penrose summarized these results and theorized that a singularity was present at the beginning of the Universe (at the time of the Big Bang) - which is still the subject of scholarly debate. Because of his discoveries, Penrose is credited with triggering a new era in physics and astronomy and ensuring that the term “black hole” caught on.
The other half of the Prize went to Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics (MPE) and Andrea Ghez of the University of California, Los Angeles (UCLA). Since the 1990s, they have led observation campaigns of the stars orbiting the Galactic center, to learn more about the object known as Sagittarius A*.
Whereas Genzel’s group relied on telescopes operated by the European Southern Observatory (ESO) in Chile, Ghez and her colleagues relied on the Keck Observatory in Hawaii. Thanks to their observations, the orbits of the brightest stars near the center of the Milky Way have been mapped with increasing precision.
Based on the very rapid and elliptical nature of these orbits, these measurements hinted at the presence of an extremely massive object. Their pioneering work has provided the astronomical community with the most convincing evidence yet that Sagittarius A* is an SMBH at the center of the Milky Way
Genzel and Ghez further developed methods to see through the huge clouds of obscuring dust and gas at the center of the Milky Way. This consisted of developing new instruments and refining techniques to compensate for distortions caused by the Earth’s atmosphere (known as adaptive optics).
From its humble origins as a matter of theory and speculation, the study of black holes has progressed by leaps and bounds in the space of a few generations. However, this has done little to dispel the sense of awe and wonder that black holes continue to inspire in even the most advanced scientific minds.
In the future, thanks to further advances in interferometry and with next-generation instruments like the EHT available, scientists hope to probe deeper and address the remaining mysteries of black holes. Beyond the questions of thermodynamics and information loss (the Black Hole Paradox), there are questions about the role they have played in the evolution of the Universe.
There is even the theory, as argued by physicist Lee Smolin of the University of Waterloo’s Perimeter Institute for Theoretical Physics (PITP), that black holes contain the seeds of new Universes. It’s even possible that the vicinity around black holes is where we might find super-advanced species, which would be drawn to them because of the abundant energy they release, the exotic physics they allow for, and maybe even the possibility for time travel!
- The Nobel Prize - The Nobel Prize in Physics 2020
- Penrose, R. "Gravitational collapse and space-time singularities." Physical Review Letters, Vol. 14, No 3 (1965)
- Hawking, S. W. & Penrose, R. "The singularities of gravitational collapse and cosmology." Proceeding of the Royal Society, Vol. 314, No. 1519 (1990)
- Eckart, A. & Genzel, R. "Observations of stellar proper motions near the Galactic Centre." Nature, Vol. 383 (1996)
- Nature - "Physicists who unraveled mysteries of black holes win Nobel prize," Oct. 6th, 2020
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