Black holes and other space phenomena full#

Event horizon is not a physical barrier, it's a mathematical description of the place that defines where information is trapped forever. Although the analogy is not correct because we have to use general relativity to understand black holes, the concept is correct, nothing can leave the event horizon of a black hole. By analogy and by extrapolation, this would be a situation where nothing could escape, not even light. He recognized that there might be a mass or in particular, a very high density form of mass, where the escape velocity at the surface would naturally reach 300,000 kilometers per second, the speed of light. From the sun, the escape velocity is 600 kilometers per second. He just extrapolated from the terrestrial situation where the escape velocity for any object is 11 kilometers per second. In fact John Michell, in 1795, using purely Newtonian theory, made a prediction of black holes and their existence. Another way to think of black holes is by simple extrapolation or extension of Newton's theory. In principle, there can be sufficient mass energy density to pinch off space entirely, trapping a region of space time beyond the view of the rest of the space time and removing it from the visible universe. This is the central equivalence of Einstein's theory. The higher the mass energy density, the higher the curvature of space.

We can think of ball bearings or marbles rolled over a rubber sheet that has depressions in it caused by the mass in space. In general relativity, any mass or a combination of mass and energy, distorts the sheet which is the space-time continuum and objects traveling through that space-time, follow paths determined by the curvature of the space time. In Newtonian theory, the sheet is always flat and mass objects sit in the sheet without distorting it or changing its properties. The commonly used visualization involves the two-dimensional analogy of a flat sheet made of rubber. So we tend to use analogies in two dimensions or visualizations. It's of course difficult to visualize the curvature of space time in three dimensions when we occupy three dimensions. Mass and energy are themselves equivalent by Einstein's other insight, E equals mc squared. The central conceptual shift in general relativity is the idea that space and time are curved by a mass and energy. We aweighed test of gravity in the strong field situation or for the predictions that are unique to the general theory of relativity and do not occur in Newtonian gravity. Einstein's theory has only been tested in the weak field case, such as with its confirmation in the eclipse of the sun in 1916, but its passed all of those tests with flying colors and is considered the correct theory of gravity. So most of the solutions are for very artificial cases such as a black hole that's not spinning or a black hole that's spinning. It's very hard to do real-world problems.

Black holes and other space phenomena full#

The full description of space time in general relativity is called a metric and only a handful of metrics have been solved in the 60 or 70 years that people have been doing this research. The mathematics and the theory of general relativity are difficult enough that only a handful of very particular situations have been solved fully. Remember that for weak situations of gravity, which is most of the universe, general relativity produces the same predictions as Newton's theory, but when gravity is strong, it gives much better results and for some phenomena, they are simply not predicted or understood in terms of Newton's theory. Since that level of math is beyond the scope of this course, we'll just approach black holes in a conceptual way. To understand black holes fully, we'd have to delve into Einstein's general theory of relativity, a complex and difficult theory involving tensors and 10 coupled second-order partial differential equations. In principle, the stellar remnant must continue to collapse to a state whose properties are as bizarre as any state of matter in the universe, a black hole. What happens when the collapse remnant of a giant star at the end of its life is more than three times the mass of the sun? Under this situation, there is no force of nature that can resist the continued collapse, not electron degeneracy pressure, not neutron degeneracy pressure.