![]() ![]() The Earth’s escape velocity is 11.2 kilometers per second (about 25,000 m.p.h.), while the Moon’s is only 2.4 kilometers per second (about 5300 m.p.h.). The escape velocity also depends on how far you are from the planet’s center: the closer you are, the higher the escape velocity. A lighter planet would have a smaller escape velocity. The speed with which you need to throw the rock in order that it just barely escapes the planet’s gravity is called the “escape velocity.” As you would expect, the escape velocity depends on the mass of the planet: if the planet is extremely massive, then its gravity is very strong, and the escape velocity is high. If you threw the rock hard enough, though, you could make it escape the planet’s gravity entirely. Assuming you don’t throw it too hard, it will rise for a while, but eventually the acceleration due to the planet’s gravity will make it start to fall down again. You throw a rock straight up into the air. Suppose that you are standing on the surface of a planet. Since our best theory of gravity at the moment is Einstein’s general theory of relativity, we have to delve into some results of this theory to understand black holes in detail, but let’s start of slow, by thinking about gravity under fairly simple circumstances. ![]() Loosely speaking, a black hole is a region of space that has so much mass concentrated in it that there is no way for a nearby object to escape its gravitational pull. Frequently Asked Questions about Black Holes Ted Bunn Originally written ca 1995 What is a black hole? The sources there are still good, but they’re old. The section with suggestions for further reading is also quite out of date, of course. That field has advanced so much that bringing that part of this document up to date would be a huge task, and I frankly don’t want to do it. Other than minimal formatting, I haven’t updated a thing.įor what it’s worth, I think that most of it holds up quite well, except for the section on astronomical observations of black holes, which is woefully out of date. Someone just pointed out to me that the original page no longer exists, so I thought I’d repost it here in case anyone is still interested. I don’t have data on this, but it often showed up at or near the top of Google searches for “black holes”, and I regularly got email and questions about it until quite recently. It stayed up for over 20 years, and during that time it seems to have been widely read. Note: I wrote the following in the mid-1990s as a contribution to a web site at UC Berkeley devoted to cosmology education. Frequently Asked Questions about Black Holes.Frequently Asked Questions About Black Holes.This black hole would not only need to be supermassive, but completely isolated from any surrounding space material, gas, or stars as well. A person falling into a stellar-size black hole will be much closer to the black hole's center when passing through the event horizon, which results in a gravitational pull so large that they will likely immediately die as they'll be stretched into a "long, thin noodle-like shape." A person falling into a supermassive black hole, however, would safely pass through, free of noodle-like stretching, because of how far away the event horizon is from the gravity-causing center of the black hole. "Thus, someone falling into a stellar-size black hole (non-supermassive size) will get much, much closer to the black hole's center before passing the event horizon, as opposed to falling into a supermassive black hole," the two physicists write. The supermassive black hole, by way of its sheer size, has a mass that's roughly 4 million times the mass of our Sun and has an event horizon with a radius of 7.3 million miles as a result. There are two main types of black holes in the universe, according to them, and one is supermassive while the other is not. ![]() 12 Images Physicists Leo Rodriguez and Shanshan Rodriguez are both assistant professors of physics at Grinnell College and they explain how this successful trip through a black hole could happen safely in their report on The Conversation. ![]()
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