By Steve Humphrey
If you remember, last month I started talking about black holes, regions of space-time of (almost) infinite density that contain the remnants of a collapsed star. In this column, I want to say more about these incredibly bizarre objects.
Einstein’s General Theory of Relativity predicts that a “barrier” will form around the remnant, called the “event horizon.” This represents a final limit. Anything crossing the event horizon is doomed to fall into the hole, and nothing inside can get out, not even light (which is why they are called “black” holes). If an astronaut or anything else were to fall through the event horizon, they would be “spaghettified” (a technical term). Tidal forces would force the astronaut into a longer and longer, and thinner and thinner object until even the atoms which make him up are torn asunder.
Even stranger things happen inside the event horizon. General Relativity predicts that with no equilibrium states for matter remaining, the star shrinks to a point and leaves spacetime, leaving behind what is called a “singularity.” If it is indeed a massive point in spacetime, having no volume, then gravitational forces will approach infinity closer and closer to the singularity. Physicists are not fond of infinities, so this presents a problem. Some have suggested that the “Cosmic Censorship Principle” renders singularities harmless because it says that there are no “naked” singularities, i.e., singularities that can be observed from outside the event horizon, so they can have no effect on the outside world. Others think that when a Quantum Theory of Gravity is found, a theory that unites Quantum Field Theory with General Relativity, it will show that spacetime is not continuous, but has a “foamy” structure, so that a singularity would not be point-like, but would be spread out a bit. This would circumvent the prediction of infinite forces because you can’t get arbitrarily close to the singularity. There is also an information paradox. It is widely believed that information cannot be lost, that it is conserved in any process. If an encyclopedia burns up, the information it contained is still there. It might be difficult to retrieve, but it isn’t lost. But what happens if the encyclopedia falls into a black hole? There was a famous debate between Stephen Hawking and Leonard Susskind on this issue. Hawking maintained that the information would be destroyed, while Susskind held that it wouldn’t, and would somehow still be in the surface of the event horizon, like a hologram. This is discussed in detail in Susskind’s book “The Black Hole War.” Eventually, Hawking conceded that Susskind was right, but many in the physics community remain unconvinced.
Speaking of Hawking, he and Jacob Bekenstein made the outrageous proposal that black holes could be treated as thermodynamic systems and that eventually, they would lose mass, decay and disappear. In fact, Hawking suggested that mini-black holes might have been created during the tumultuous period immediately after the Big Bang and that some of these might be in the process of decaying even now. He suggested that this decay process would accelerate as the black hole got smaller, finally exploding with a big “pop,” and this would be observable to astronomers. Unfortunately, no one has observed anything that might be a viable candidate for an exploding mini-black hole.
It has also been suggested that singularities are gateways to “wormholes” that lead to another part of space-time. These wormholes are also called “Einstein-Rosen Bridges,” discovered by Einstein and Nathan Rosen in 1935. To say that they were “discovered” is to say that they found a solution to the Einstein Field Equations that describe a wormhole connecting two widely separated regions of space-time. Anything falling into a black hole would emerge (spaghettified) in a distant part of the Universe. At one point, it was suggested that quasars (quasi-stellar objects), fantastically energetic sources of radiation seen throughout the universe, might be “white holes,” the other end of a wormhole connected to a black hole. Everything that falls into a black hole comes out of the corresponding white hole. Now it is realized that quasars involve massive black holes at the center of galaxies, and the radiation comes from the accelerating matter falling into them.
The “Time Travel Consortium,” a group of physicists led by Kip Thorne, has concocted scenarios in which something traveling through a wormhole would not only end up in a different part of space, but also a different time, perhaps in the past, which could lead to the familiar paradoxes of time travel. Suppose a bowling ball fell into a black hole, emerged at an earlier time, came back to the original black hole, and banged into itself, thus preventing it from falling into the hole in the first place. But if it didn’t go into the hole, what banged into the ball and kept it from going in?
Another oddity is that wormholes might not lead to different regions of our Universe, but different universes completely. There are many cosmological models that contain multiple universes, exist in different dimensions and do not interact. Maybe I will discuss some of these in a future column if I don’t get hit by a bowling ball first!
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.