Black holes, particularly those of the supermassive variety, are often the subjects of science fiction novels. They invariably hold the key to interstellar travel, time travel or some other important plot element of the story. But, what are the facts surrounding these incredible objects? And is there any science in the science fiction?
What Are Supermassive Black Holes?
Generally, supermassive black holes are just what they sound like, really, really big black holes. Measuring in the hundreds of thousands of solar masses -- one solar mass equals the mass of the Sun -- up to billions of solar masses, these objects posses an immense power.
Found in the cores of galaxies, supermassive black holes are, at least partially, responsible for holding galaxies together. Their gravity is so immense, because of their incredible mass, that even stars hundreds of thousands of light years away are bound in their orbits.
Whenever black holes are discussed, one property that sets them apart from other "normal" objects in the Universe is their density. This is because the density at the core of normal black holes becomes infinite. Specifically, the volume approaches zero while still containing an incredible amount of mass. While this feature would still be present in the very core of a supermassive black hole, the great majority of the object would have a much lower density.
The reason for the difference is that density is defined as mass divided by volume. And while the radius of the black hole will increase linearly with mass, the volume goes as the cube of the radius (and therefore mass). The result is that the volume of the black hole increases at a much faster rate than the mass.
Unbelievably, one can calculate that the average density of supermassive black holes can actually be less then the very air we breathe. In fact, the greater the mass, the less dense the supermassive black hole is. So, would would it not only be possible to approach a supermassive black hole, one could even fall into a supermassive black hole and survive for quite some time until they began to approach the core.
How Do Supermassive Black Holes Form?
The formation of supermassive black holes is still one of the mysteries of astrophysics. Normal black holes are the core remnants left behind from the supernova explosion of a massive star. The more massive the star, the more massive the black hole left behind.
One could therefore assume that supermassive black holes are created from the collapse of a supermassive star. The problem is that no such supermassive stars have ever been detected. Moreover, physics tells us that these supermassive stars shouldn't even exist in the first place. It souldn't be stable enough to persist. (It should be noted that the most massive stars ever detected were recently reported -- nearly 300 solar masses. While they are a far cry from the types of masses that would be needed to create a supermassive black hole, they are still nearly double the mass limit once thought possible for stars to exist. So, clearly there is something that we still don't understand.)
So, if these objects aren't created in the traditional fashion of other black holes, where do they come from? There are a few models, each with their features that lack explanation. Perhaps the most common theory is that these objects were formed as black holes and other stars merged together to form an even larger black hole. Eventually the build up of mass would lead to the creation of a supermassive black hole.
While we see black holes accreting mass all the time, there is still a gaping hole in this theory. Namely, we have never observed a black hole in the "intermediate" stage. We have detected normal black holes up to about 33 solar masses, while the smallest supermassive black holes register in the hundreds of thousands of solar masses. If these objects are created through accretion, then we should also see black holes in between these two masses, in the midst of formation.
While absence of evidence is not definitive evidence of absence, it is still a problem for the theory. The leading alternative is that these objects were formed in the first moments following the Big Bang. The natural obstacle with this theory is that the specific conditions of those moments, particularly considerations of structure and mass distribution, are fuzzy at best. So while it is easy to conjecture what it might have looked like, proving it is more difficult.
There are, of course, a host of alternative theories. Their difficulties are more evident and extensive than these theories. (Some even struggle to show that a supermassive black hole would be created at all.)
Observational evidence tends to suggest that the merger theory carries the most weight. Examination of the oldest, most distant and massive supermassive black holes, quasars specifically, there is evidence that they are formed from the merger of many galaxies as the Universe has evolved. Perhaps this is evidence that in the early Universe there were more, smaller galaxies than there are today. And that the galaxies that we are familiar with today are each the result of the merger of much smaller galaxies.
If this is the case then it would also seem to supple a partial solution to the intermediate black hole problem. In either case, the answer is not clear and we may never completely understand their origin.
Science in the Science Fiction
As with any black hole object, there are properties that completely bend the mind. Stories of faster than light travel, interstellar travel and time travel pervade science fiction novels. There are even theories that black holes are gateways to alternative Universes.
So is there a shred of evidence to support any of these claims? Actually, yes. Though only under very extreme circumstances. The idea of using black holes as worm holes, connecting us with the other side of the Universe have been around for years. The possibilities have even been calculated using serious physics and general relativity.
The problem is in the "special conditions". These seem to eliminate any real possibility of using black holes for such purposes, mostly because it seems unlikely that these special conditions will ever exist. But who knows, much of the technology that we have today was also once thought impossible. So, don't give up yet.
