Gravity, one of the four fundamental physical forces that hold the universe as we know it together (along with electromagnetism, the weak nuclear force and the strong nuclear force), is something that we witness and feel the effects of everyday. At the highest level, it’s the force responsible for the visible structure of our world, the Solar System, the Milky Way galaxy and the entire cosmos as far as we can see with our telescopes.
Perhaps nowhere is the full power of gravity so well represented as it is in the form of what we have come to call black holes. German physicist Karl Schwarzschild took Einstein’s Theory of General Relativity and first used it to formulate the mathematics that would come to describe black holes in 1916. It wasn’t until 1958 when American physicist David Finkelstein would give us the modern conception of a black hole as an actual “black hole” in space — a spot where gravitational pull was so great that no matter could escape from it, even particles of light.
Einstein’s Field Equations (EFE) for general relativity can be shown as:
Rμν is the Ricci curvature tensor, R is the scalar curvature, gμν is the metric tensor, G is Newton’s gravitational constant, and Tμν is the stress–energy tensor. The tensors describe the geometry of objects in spacetime.
A black hole can be shown mathematically in the equation for the Schwarzschild radius:
Here, Rs is the Schwarzschild radius, G is the universal gravity constant, M is the mass of the star, and c is the speed of light (186,000 miles per second).
The nearest black hole to us may be the A star in the binary system V616 Mon, nearly 2800 light years away from us.
Our own sun (“Sol”)is not nearly massive enough to ever become a black hole. In about 5 billion years, Sol will run out of its hydrogen fuel, first slowly expanding into a cooler red giant which will envelop the 4 inner planets, before collapsing into a dim white dwarf and eventually “dissolving” into a nebula.
Stars that are about 20+ times more massive than Sol have another afterlife opportunity, however: transforming into a black hole. As with the smaller stars, these giant ones also use up their hydrogen over cosmological time by fusing it into helium. When the hydrogen is gone, the stars fuse helium, and on and on, up through heavier elements like iron, until fusion no longer generates enough energy to support the star’s outer layers. Those layers collapse inward and create the massive stellar explosion known as a supernova. One star every 50 years or so goes supernova in the Milky Way galaxy. In the Orion constellation, the star known as Betelgeuse is thought to be a good candidate for going supernova. At roughly 430 light years away, it will be too far away to cause Earthly life any harm — we would need to be within about 50 light years to worry about radiation — but it will become the brightest object in our night sky aside from the Moon.
After this tremendous explosion, Einstein’s equations predict that if at least 3 times the mass of our sun remain, the gravitational force will be enough for it to collapse back together and create a singularity: an infinitely tiny point in space with infinite density. This is then a black hole.
Anything with mass, including light, will be pulled toward the black hole as it nears. If the mass gets close enough, it will spiral down into the black hole, eventually adding to its mass and growing it. This “point of no return” is called the event horizon.
Although you wouldn’t notice anything particular as you fell into a black hole, someone watching you from a distance, would never see you cross the event horizon. Instead, you would appear to slow down, and hover just outside. You would get dimmer and dimmer, and redder and redder, until you were effectively lost from sight. As far as the outside world is concerned, you would be lost forever.
The Chandra X-Ray Observatory space telescope, in orbit 65,000 miles above Earth since 1999, has found evidence of many black holes in the middle of our galaxy. A proliferation of black holes at our galactic center has been a theory for a long time, and it is also thought that the center of almost every spiral and elliptical galaxy contains what is known as a supermassive black hole — a singularity that could be equal to billions of solar masses. The location of this supermassive black hole corresponds to a compact source of astronomical radio waves called Sagittarius A*.
Though black holes appear to be empty volumes of space, they are definitely not. They represent some of the most gargantuan collections of mass in the universe, and this “absence” they create is just an illusion. Not even light can escape the event horizon due the the immense gravitational pull. However, Sir Stephen Hawking used quantum physics in the 1970s to show that black holes do not simply eat everything without consequence: they also generate what has come to be called “Hawking radiation”.
Various groups of physicists in the latter quarter of the 20th century have proposed some potential ways to collect this Hawking radiation, involving reaching nearly to the event horizon with “buckets” or lowering superstrong strings into the radiation field. The limits of modern materials science renders all of these ideas purely theoretical, but with time and enough R&D we may be able to develop tools that can survive both the gravitational force of black holes and the extreme heat of Hawking radiation.
One cosmic artifact that is actually generated by black hole activity is the powerful quasar. These awesome energy-generating events are created at the center of active galaxies where a supermassive black hole rapidly eats up immense quantities of matter.
Aside from simply harnessing the energy of a black hole, we might also be able to create miniature black holes one day to use as a power source:
…there could be much smaller mini black holes. These might have formed in the very early universe, if it had been chaotic and irregular. A black hole of the mass of a mountain, would give off x-rays and gamma rays, at a rate of about ten million Megawatts, enough to power the world’s electricity supply. It wouldn’t be easy however, to harness a mini black hole. You couldn’t keep it in a power station, because it would drop through the floor, and end up at the center of the Earth. About the only way, would be to have the black hole in orbit around the Earth.
Using black holes as a way to travel across vast distances could be one other way we might make use of these phenomena. The idea of the Einstein-Rosen bridge, or “wormhole”, states that two points in spacetime, though they may be separated by many light years, could be connected by a structure between them. The wormhole is actually based on a solution of the Einstein Field Equations, and as such falls in line with General Relativity and is theoretically possible.
While we have been able to find actual evidence of the existence of black holes, proving that wormholes exist using current technology is wholly impossible. It may be that the only way to ever prove such a potentially transcendent idea will be to attempt to traverse through one such suspected structure in a vessel designed to survive the rigors of passing into a black hole, whether that is a probe or a crewed vessel. However, even if the wormhole actually exists where we want it to, the test vessel might find itself hundreds or thousands of light years away, leaving any confirmation of the mission’s success to some hopeful scientists many generations into the future.
Increasing our basic understanding of black holes, and the workings of the universe in general, is the best we can hope for our own present generations. Maybe one day we will finally discover answers to some of our greatest astronomical mysteries. Keeping the Earth, and humanity, around until then should be a priority.
Thank you for reading and sharing!