Black Holes Explained
People regard black holes as troubling, because they are the enemy you cannot defeat. They always win in the end. It's no different than the guy in the black cloak with the scythe—it's Death triumphant. We now believe that in the long run most of the mass in the universe is going to end up being eaten by a black hole. Thee and me and all that we know will probably be swallowed by one of these dragons in the long run. And if you don't think that's disturbing, well, maybe you've been watching too many horror movies.
Well, what it is really is a region of space and time where the properties are very different from those we encounter in the space and time around us. It has lots of weird effects that it can exhibit—and then those we think we can understand as physicists. But right at the heart of a black hole is the center nugget, which we call the singularity, and there we don't know what goes on. There I can't answer your question, I'm afraid.
A black hole is one of these folds in space that is so deep that things spiral down into it and they can't get out. Whether that hole leads someplace else or is completely self-contained, and whether that matter and energy and information is going to come back into our universe someday—these are subjects of argumentation. But the best way to look at it is that the fabric has been pulled in such a way that if you stick your finger down in there, you ain't getting it back.
Well, a black hole to first order is just a very large concentration of mass. Anything can be a black hole—it's just a matter of getting mass to be concentrated in a small enough volume. So let's take the Earth. If you just push all of the Earth mass into a region roughly of one centimeter across, well, then it'll be a black hole. The concentration of mass means that light cannot escape anymore, and then it becomes black in the sense that anything which is inside a black hole cannot communicate with the world outside, that is us.
We don't know! We'd like to know, but we don't actually know. The real problem is that we can't make any measurements inside the event horizon. Which leaves everything inside to the realm of theory. [But] a black hole is a region of space where the pull of gravity is so immense that not even light can escape it. That's sort of the simple definition. So that means that we can introduce the concept of an event horizon around the black hole. That's the last point at which we can get any information or light can escape from the pull of the black hole. You can also think of a black hole as an object where the pull of gravity got so strong that there's no other force that can oppose the pull of gravity, and the object collapses to an infinitely small volume.
What's a black hole? It's this monstrous, mysterious thing that, I don't know, eats everything. That's not true... [laughs] It's a region of space—well, blub, blub, blub—how should we do this? [sigh] I don't know what a black hole is. [laughs] It's a region of space where the gravity is so strong that not even light can escape. Imagine a waterfall falling over a cliff. It's like that, except it's space itself that's falling over the cliff, and there's a place where the space starts moving faster than light, and then light can't make its way out of that space. What's a black hole? Take a mass and compress it very, very prodigiously. As you do so, the gravity of its surface becomes exceedingly strong, and there comes a point where it's so strong that not even light can escape.
When something falls into a black hole, it is no longer accessible to our world. The only memory that our world has of that object after it falls into the black hole is that the black hole gets slightly bigger. But if you were to jump into the black hole, you would no longer be who you are now. The protons and neutrons and electrons that make up who you are would cease to exist. The only thing that would remain is essentially the mass of your person after you fell in. The black hole would grow in mass by your mass, but your identity would cease to exist.
A black hole is a concentration of mass that is so concentrated that the gravity of the matter has crushed the mass into a point. It has become a black hole, because there is nothing that we know of in nature that can withstand that gravitational pressure. Nothing can counteract the gravity and prevent it from collapsing completely to a point in space. What we mean by a point is the subject of current research. If we simply refer to Einstein's general theory of relativity, we mean by point a mathematical point, what's called a singularity, where all the mass is concentrated. But there are recent attempts to merge that description with a quantum mechanical description, where quantum mechanics says that nothing is located precisely at a point; there's always a fluidity to the definition of a point. It still eludes us, because people haven't successfully been able to merge quantum mechanics and general relativity. But that is a subject of current research that I think will produce results in coming years.
A black hole is one of the most interesting and unusual ideas in all of science. It's enigmatic, because it represents the laws of physics in the most extreme environments that we can imagine. It's outside of our experience. We don't make black holes, at least yet, here on Earth, so we don't really have all the words for describing them. One way you can think about it is that it's an incredibly simple object. Looking from outside, there are really only a very limited number of properties that describe a black hole. It has a total mass. The event horizon has a certain size, or there is a certain curvature associated with the hole. We know that it can have a spin, just like the Earth has a spin. And in principle it can also have an electric charge. So you can think of it as a fundamental particle which has a mass, a spin, and a charge. And that's it.
When a black hole forms by the implosion of a star, which is how most black holes probably form, the matter implodes, and as it gets more and more compact its mass generates the warping of space and time around it; its spin generates the whirling motion of space around it. But the matter continues to shrink smaller and smaller and smaller, shrinks down to the very center where it gets destroyed in a singularity, a region of infinite warped space and time. And it's gone. When it's gone there is nothing left except the warped space and the warped time. So the common idea that a black hole is just made of very compacted matter—it's wrong. It is just simply wrong. It may have been created from very compacted matter, but the matter is gone. It's been completely destroyed; it no longer exists. And all that's left behind is the warped space and the warped time, and this little nugget of a singularity at the center of the black hole that we don't understand.
Neil de Grasse Tyson
On Earth the old adage "What goes up, must come down"—that works to a point. It turns out there is a speed with which you can toss something from Earth's surface where it will never come back ever again, and that's about seven miles per second. Different objects throughout the universe have different speeds for which this is true. So imagine a place where the gravity is so strong that that escape velocity starts rivaling the speed of light itself. Imagine turning on a flashlight—the light would go up and it would never leave. It would curve and come back down, just the way a tossed ball on Earth, if not traveling fast enough, it goes up, curves, and comes back down. In fact, the black hole, according to Einstein's general theory of relativity, has curved the very fabric of space and time, preventing anything from ever coming out at all. And so there's no better hole that you could possibly imagine than a black hole.
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