Black holes are amongst the most enigmatic objects in the universe. Contrary to their name, black holes are anything but empty space. They are regions of spacetime that exhibit such Gargantuan gravitational effects that nothing, not even light, can escape from within it. Around a black hole, there is a mathematically defined surface called an event horizon that marks the point of no return. This means that we cannot see anything beyond the horizon and within the region - hence the name black hole. These black holes arise from the largest explosions that take place in space, supernovae. While black hole formation is one of two possibilities (the other being a neutron star) after a supernova, it is what occurs for stars three times the mass of the sun. These “explosions” can briefly outshine entire galaxies and radiate more energy than our sun will in its entire lifetime. Everything that remains of the star is crushed down into an incredibly small, dense object. It is a great amount of matter packed into a very small area - think of a star ten times more massive than the Sun squeezed into a sphere approximately the diameter of New York City. They are also called “black” because they absorb all the light that hits the horizon, reflecting nothing, making it almost an ideal blackbody.
These black holes are actually an exact solution of Einstein’s incredibly significant field equations. In 1916, Karl Schwarzschild developed the first modern solution of general relativity that described the gravitational field of a point mass and a spherical mass. This solution also described rather strange behaviour at a particular point, since some of the terms in the field equations became infinite. This point is now called the Schwarzschild Radius, which defines the radius of the event horizon (the “point of no return”).
Subrahmanyan Chandrasekhar, an Indian-American astrophysicist also noted a peculiar solution, namely The Chandrasekhar limit. In 1931, he used special relativity to calculate, that a non-rotating body of electron-degenerate matter above a certain limiting mass has no stable solutions. Put simply, it is the maximum mass of a stable white dwarf star. The currently accepted value of the Chandrasekhar limit is about 1.4 times the mass of the Sun.
White dwarfs resist gravitational collapse primarily through electron degeneracy pressure (unlike main sequence stars, which resist collapse through thermal pressure). The Chandrasekhar limit is the mass above which electron degeneracy pressure in the star’s core is insufficient to balance the star’s own gravitational self-attraction. Consequently, a white dwarf with a mass greater than the limit is subject to further gravitational collapse, evolving into a different type of stellar remnant, such as a neutron star or black hole. Those with masses under the limit remain stable as white dwarfs.
In recent years, many experiments and observatories have painted a new picture of black holes, that are, to many, the most fascinating objects in the cosmos. Black holes were at the heart of the most significant scientific discovery of the century yet; gravitational waves. Gravitational waves are disturbances in the fabric of spacetime, generated by accelerated masses, that propagate as waves outward from their source at the speed of light. They were proposed by French mathematician Henri Poincaré in 1905 and predicted by Einstein in his general theory of relativity eleven years later. For decades, physicists have hoped they could “listen in” on violent astrophysical events by detecting their emission of gravitational waves. The existence of these waves, which can also be described as oscillating distortions in the geometry of spacetime, were strongly conjectured by physicists, however they were never directly detected. But that all changed in February 2016, nearly 100 years after their prediction, when the LIGO collaboration announced the first direct detection of gravitational waves. This detection also represented the first observation of a black hole merger, an incredibly beautiful event, as seen in a video published by LIGO, “Two Black Holes Merge into One”. Kip Thorne, who won a Nobel Prize for his work on gravitational waves and his contributions to LIGO, said “This opens a new window onto the Universe.” The detection of these waves, which arose from black holes, is a truly phenomenal achievement of modern science for many reasons.
First of all, detecting two colliding black holes is thrilling by itself — no one knew for sure if black holes actually merged together to create even more-massive black holes, but now there’s physical proof. But what is truly monumental about this detection is that it gives humanity the ability to see the universe in a totally new way.
Further, some theories require the presence of additional spatial dimensions to account for the enigma in the cosmos, but University of Chicago astronomers found no evidence for extra spatial dimensions to the universe based on the gravitational wave data. Yet another implication of this discovery; our universe is three-dimensional and theoretical frameworks which require additional dimensions are inconsistent with this observation. This discovery is truly a landmark achievement for all of the reasons listed above and many more. Our perception of reality as we know it could change! Exciting prospects lie ahead.
The ability to directly detect gravitational waves gives us insight into nearly 94% of the universe, dark matter/ dark energy. Even though they make up the majority of our universe, we do not know much about them and have not been able to identify a dark matter particle. Nearly 90 years ago, we discovered that our universe is not static, but in a constant state of expansion. But how does this occur? What is the source of energy for expansion? The “how” of expansion is still an open question and physicists attribute it to mysterious dark energy, although we do not possess a clear understanding of what it is and why it’s so abundant. But that can soon change with the detection of these waves, as it gives us greater insight into understanding them and is a portal to an entirely new realm of information.
While gravitational waves pave the way to a “new realm of information”, their source, black holes, could potentially delete the universe. This is what’s known as the Black Hole Information Paradox. Black holes will take aeons to become extremely tiny and finally disappear, leaving behind just radiation. But in the process of disappearing, black holes may delete information altogether; this is a major problem. “Information” can be understood as a property of the arrangement of particles. It’s what leads to everything unique in the universe. Different arrangements of particles lead to different objects; carbon atoms in a particular arrangement can be graphene and, in another arrangement, a diamond. The basic building blocks of everything in the universe is the same, and what distinguishes one from the other is their information; their arrangement. Without information, everything would be homogenous. As per quantum mechanics however, information is indestructible; it can change form and become difficult to read but can never be lost. Hypothetically, one could view the entire history of the universe if they can measure every single atom, particle and wave of radiation in the universe since that enables one to see and track every bit of information in the universe.
Black holes however destroy information, which is a serious problem. Many solutions exist to resolve this paradox; however, they violate the foundation of modern physics. The most bizarre solution is that information is irretrievably lost forever. This would mean that all our current theories would have to be dismissed since it violates unitarity and energy conservation and also because it would mean that information can be destroyed. A less bizarre solution is that information is hidden. It exists, but we have no way of accessing it; instead of disappearing altogether, the black hole may split into a baby universe containing all information. The optimistic solution is that information is neither lost nor hidden but stored in a remnant which can either be extremely tiny or extremely large. While this preserves the framework upon which physics operates, it also requires the remnant to have infinite internal states, which while possible are very difficult to realise.