High Gravity From Black Holes
The science article in the last issue of Homeward (entitled "Black Holes in Outer Space") explained how science uses a body of ideas (in scientific terms, a mathematical model) for accurately describing nature. This body of ideas is termed 'theoretical physics', or 'physical theory' for short. The article explained further how real and genuine science chooses to use this body of ideas for explaining and when applicable, for manipulating nature, because of the very simple reason that physical theory 'works' every time it is ever used, error free (although the applications of our interpretations of it certainly don't seem to be).
To date, this body of ideas we call physical theory is absolutely failure free. This is why, among countless other applications, it - AND NO OTHER BODY OF IDEAS - is used for designing television sets, microwave ovens, radar, computers, and oh yes, for launching space craft into precise orbit and keeping them there for decade after decade. Theoretical physics makes these achievements possible and every one of these achievements would be flatly impossible without the tool of theoretical physics.
Physical theory has two sides. One side is a description of the space and time of nature. This is the theory of gravity (the subject of this article), which is the Theory of Relativity. Relativity describes how space and time 'tilt' (special relativity) and 'bend' (general relativity) as a consequence of motion and mass (effectively, 'weight'), which introduces the other physical theory that describes everything that isn't gravity, which includes, among other things, any physical entity with mass (again, mass is effectively 'weight'). This theory is the theory of energy, the Theory of Quantum Mechanics, which, to say the least, makes some remarkable (to the point of being counterintuitive) statements about nature. But, this article deals with gravity, which is the realm of relativity, not quantum mechanics, so we will have to wait to examine this other theory (for energy) in subsequent editions of Homeward and deal strictly with gravity for the moment.
The description of how (among other effects still yet to be mentioned) the pace of time slows, compared to how someone experiencing less gravity measures that same pace (of time), as a consequence of the effects of gravity - the greater the gravity, the more pronounced the slowing of time - is a direct outcome of equations and fundamental assumptions (called axioms) of relativity (the Lorentz transformations and derivations thereof). We explored this effect of time slowing with gravity at its most extreme limit by exploring the effects of gravity at the surface of what are the bodies in the universe where the effects of gravity are greatest: on black holes.
Black holes are so called because a black hole's 'weight' (meaning its mass) and the density of that weight (of that mass) is so great that a black hole's gravity prevents it from emitting any light at all, directly. In short, a black hole is a "dark star" - a star that emits no light (again, directly). In short, a black hole is a super dense and 'heavy' (massive) apparently former star that is darker than the darkest night. It is called a "hole" instead of a star though, because this kind of dark object is like a one-way "exit" from the universe around it (hence the term, 'hole').
A black hole seems as though it is formed as the result of an enormous star "burning up" (that is, exhausting) all the nuclear fuel that is constantly "holding it up", keeping it big and thus preventing its material from collapsing upon itself and concentrating into a smaller volume.
The energy of the star and its capacity to resist collapse against gravity are finite, which is to say that they are limited. This energy lasts for only so long. Eventually, this energy will become depleted to the point that the force of gravity will overcome the energy of the star to resist it. Without sufficient energy being released to counterbalance the immense gravity that is the outcome of the black hole's immense weight (immense mass), gravity overcomes energy and the star collapses inwardly, shrinking into a smaller volume.
Upon, collapsing, if a star is heavy (i.e. massive) enough, then it collapses into a black hole, in this way placing its contents in the closest proximity physically possible; concentrating matter to the absolute limit that it can be concentrated in our universe. As a consequence, black holes literally "sink" into space and time, thus "separating" from the rest of the universe, leaving behind afterward, only a one-way, gravity-powered "exit" from the universe as we know it, like the drain at the bottom of an infinitely deep (bottomless) sink.
What is left after a star collapses into a black hole is a body that is so heavy (massive) and its enormous weight (enormous mass) so concentrated that it exerts at its surface more gravitational force than any other kind of body in the universe - in fact, as much force as gravity is capable of exerting. Now, returning again to the fact that time slows as the force of gravity increases, and the force of gravity at a black hole's surface being as great as it can physically ever be in the universe means that time slows there, as much as it can ever slow in the universe - again, according to relativity's failure free description of nature: infinitely, which means time slowing to the point of 'stopping' altogether.
As a consequence of time at a black hole's surface slowing infinitely to the point of stopping altogether, a black hole's surface is always 'locked' into a single moment, a moment that 'never' has enough time to pass, and that is, from the perspective of anyone removed from the black hole's surface, a 'frozen' moment that is seemingly eternal in duration. On the other hand, if you are 'there' at the surface, where, according to the fundamental assumption of relativity the pace of time never changes, except according to the perspective of another subject to different gravity (though different motion accomplishes a different perspective), time would be passing no differently at the surface from how it would be passing anywhere else, even if a moment requires, again, according to a perspective outside, an eternity for any single moment to pass.
So, it would appear that, although it might take an eternity to approach a black hole's surface with the laws of physics remaining the same all the while, it would seem possible to make a journey toward such a surface, at least in principle - except for a single yet extremely significant detail. Entering feet first, at some point your toes would begin falling (accelerating) toward the surface faster than your ankles, which would be falling faster than your calves, which would be falling faster than your knees, and so on up your body. Each individual part of the body falling faster than that attached above it would tear one's body limb from limb, from the bottom up, one atomic particle at a time, starting at your toes and ending at you head, just as would be the case for any craft that you might be a passenger aboard at the time, no matter how well made the it might be. No material exists (or could ever be fabricated – there exists a physical limit) would be able to resist the force.
Understanding black holes is worthwhile because black holes are examples of the most extreme expression of (i.e. a limit condition for) relativity. Black holes are interesting because they capture our imagination with phenomena that (ordinarily) we can only barely imagine (unless we practice imagining altogether new concepts, as physicists do). But, interesting as it might be to study, a black hole is no place that you would ever want to even try to visit. Their overwhelming gravity makes them as unlikely place for life surviving as could ever be imagined. Besides, the nearest one is trillions upon trillions of miles away and a lifetime would not be long enough to make a dent in the journey - but you never know, because by virtue of the fact that black holes emit no light, were one approaching our solar system, we could never 'see' it coming, though we might happen to notice its arrival when our neighbor's clock started running a little slower than our own. Upon getting near however, we would begin feeling its gravity more and more, reeking widespread havoc on local planetary motion, which would spell certain doom for us, the very "discoverers" of black hole phenomena, who, as a final note, would learn the unquestionable validity of the conclusions of their physical theory, with regard to black hole dark stars.