Back in the day, when Einstein was grappling with a general theory of relativity, which in turn gave rise to curvature of space, he also envisioned a case where something could, through gravitational force, shrink down into an immensely massive area whose gravity would pull in even light photons. Since light could not pass close to it, it would appear to be a black region in space, hence the name “black hole.”
For decades and decades, it was theoretical. No one had observed (knowingly) a black hole. Now, of course, it is believed that virtually every galaxy has a black hole at its core. As with so many inanimate things, humans love to give them anthropomorphic characteristics. So, we see headlines talking about black holes eating large stars for lunch. To me, this is so silly. A black hole began as one or more stars that eventually ran out of fusion energy to counteract relentless gravitational compression. The only thing driving a black hole’s behavior is its mass and the proximity of nearby stars and gases. It simply obeys the law of gravity.
To humans, gravity seems less dramatic than electromagnetic and nuclear forces. Sure, when we jump up, we come back down. But put two tennis balls six inches apart and there appears to be no overt forces or motions associated with them. Take a very strong rare-earth magnet and put a steel ball six inches away, and, wham, the ball gets pulled to and slams into the magnet. It’s all very dramatic. Unbalance the number of positively and negatively charged entities on an insulation’s surface, and, wham, an electric spark can be made to jump, just as between clouds, or between clouds and Earth, during a thunderstorm.
Yet, these forces – gravity and electromagnetic – obey the same basic inverse-distance-squared relationship. When separated by a distance, d, they have a force, f, between them. When separated by 2d, the force is now one-fourth of what it was. Move them 4d apart, and the force becomes one-sixteenth of what it was at a distance, d. This is all a consequence of the spherical force field and force magnitudes. Relatively speaking, gravity is a much weaker force than electromagnetics, or weak or strong nuclear forces. Yet, it is the tie that binds our universe together, so to speak.
Black holes are simply an extreme condition where the distances between masses grows so small that the forces become immense. The opposite case is where distances become so great that gravity has little influence. Thus, at the extreme distances of the outer part of the universe, dark energy force far outweighs gravitational force, and the galaxies and stars expand space/time at an accelerating rate. With nothing to slow that acceleration, that expansion could occur at light speed so that the outer reaches of the universe become invisible to areas nearer its center.
All these phenomena are incredibly interesting, and the forces and distances involved are immense. Yet, the space/time scope takes millions or billions of years to reach some kind of end state. There is talk about the Milky Way and Andromeda galaxies eventually colliding. It is based on speeds and directions as computed at present. But the encounter will take billions of years to occur and if a humanoid was around to witness it, the humanoid would notice nothing special.
Within our individual lifetimes we see things grow and change but the really consequential universal growth and change happens way too slowly for an individual to note it. Yet, by looking at our sky, making the measurements we are able to make, and referring to the physical laws we believe accurately describe the interaction of matter, we are able to project backwards and conjure up a Big Bang event, and project forward (somewhat) and imagine a future end state. What an amazing gift our intelligence bestows upon us to be able to do that. And, even though none of us will be here to witness these prognostications, it is such a wonder to conceive of them based on something tangible – math and science – rather than mythology.