By Steve Humphrey
By now, I expect that everyone has heard of “black holes,” regions of space time where the gravitational forces are so great that nothing can escape, not even light. Back in the ‘70s, when I was in grad school, black holes were purely theoretical, an obscure prediction made by Einstein’s General Theory of Relativity, a theory of gravity which supplanted Newton’s of the late 17th century. There was one candidate object that astronomers thought might be an actual black hole; Cygnus X-1. In fact, Stephen Hawking and Kip Thorne had a bet on whether it was in fact a black hole, and it was not until 1990 that they were sure enough that it was, and Hawking paid up. In this column, I would like to shed a bit more light (pun intended) on these mysterious objects.
It is now known that black holes are littered throughout the Universe. There are super massive black holes at the center of virtually every galaxy and they can form in a variety of ways. But let us restrict ourselves to perhaps the most common source of black holes, which comes at the end of stellar evolution.
Stars, like our Sun, are in a state of equilibrium, where the outward pressure from the heat of nuclear burning is balanced by the inward force of gravity. If the burning gets hotter, the star expands, and if it cools, it contracts. The source of this heat is nuclear fusion, wherein atomic nuclei (protons and neutrons) are squeezed together to form heavier elements, releasing massive amounts of energy in the process. This fusion continues, and burns hotter and hotter, forming heavier and heavier elements, until the star is dominated by iron nuclei, after which the star runs out of fuel and begins to cool down. What happens after this depends upon the mass of the original star. Gravity begins to overwhelm any outward thermal pressure until the atoms in the star are so close together that electrostatic pressure begins to counter the gravitational force. This means that the electrons in the atoms resist each other, much like the positive poles of a magnet. If the mass of the star is low enough, it will fall into another equilibrium state, where the force keeping electrons apart is equal to the gravitational force trying to squeeze them together. It then becomes a “White Dwarf,” a stellar remnant with the mass of the Sun and the volume of the Earth. However, if the star is above about 1.4 times the mass of our Sun, gravity will overcome electron degeneracy pressure and nuclei will be pushed together, forming one giant nucleus. This contraction is halted by neutron degeneracy, much like the force due to electron resistance, but much stronger. Again, an equilibrium state is reached in which the star gives off its residual heat but settles into a cold, dark, dense sphere called a “neutron star.” A neutron star of 1.4 solar masses will have a diameter of about 10 kilometers (about six miles), a very dense thing indeed. A neutron star with the mass of Mt. Everest wouldn’t fill a tablespoon. Neutron stars spin very rapidly, like an ice skater pulling her arms in, and they emit beams of radiation from their poles. When they are aligned in just the right way, those beams sweep past the Earth creating what seems like pulses of radiation. The first “Pulsar” was discovered in 1967, and the regular period of the pulses (up to a thousand per second) made some think they were signals coming from an alien civilization.
If the star is more massive yet, gravity will keep contraction going past the resistance of neutron degeneracy. This is often cataclysmic, a very rapid collapse starting in the center will lead to an “explosion” wherein the energy from the collapse of the center will “bounce” outward, blowing the outer parts of the star away. This is called a “supernova” and can be a spectacular sight. The first confirmed sighting of a supernova occurred in 185 AD when a bright “guest star” was observed by astronomers and took about eight months to fade away.
If the remaining mass is large enough, there is nothing to counterbalance gravity, and the star will shrink to a point. The forces holding the fundamental particles apart are finitely strong, but the force of gravity is limited only by the amount of mass. Get enough mass in one place, and gravity will overwhelm any force struggling to maintain an equilibrium state. So what happens to all that material that was the star? According to the theory, it is gone, leaving only a “hole in space-time,” a point called a “singularity.” This singular point is surrounded by an “Event Horizon,” a sort of boundary through which things can fall, but through which nothing can come back out. It is a “point of no return.” The diameter of the event horizon is a function of the mass of the black hole and only changes size when matter and radiation fall through it.
In the Spring of 2019, the first photo of a black hole was published. Of course, this was not actually a picture of the black hole itself. Rather, it was a photo of the radiation given off by highly energetic, i.e. rapidly moving, matter falling into the black hole. Black holes have angular momentum, i.e. they spin, and the infalling matter revolves around the hole, moving faster and faster, and getting closer and closer until it passes through the event horizon to be lost forever. Maybe. In my next column, I will describe black holes in a bit more detail, elaborating upon their bizarre nature and some of the more speculative ideas that have arisen regarding them.
Steve Humphrey has a Ph.D. in the history and philosophy of science, with a specialty in the philosophy of physics. Questions? Comments? Suggestions? Email him at firstname.lastname@example.org.