Click here to sign up. Download Free PDF. A short summary of this paper. Download Download PDF. Translate PDF. Stated baldly his discovery was that when the uncertainties of quantum physics the theory that describes the behaviour of atoms are taken into account, a black hole is not completely black, after all.
Rather, just like a material body such as the bar of an electric fire , heated to a certain temperature, it radiates energy, at a rate which is determined by the mass of the hole. The discovery was in the mathematical theory which underlies physics, not in the real world of experiments. Indeed the same thing may be said about black holes themselves. They originated as theoretical constructs; on the other hand, there are a few known objects in the sky which are most probably star-sized black holes, according to astronomers.
Such an object is known by its effect on nearby matter, particularly matter which is being dragged into the hole; certainly not by its predicted Hawking radiation, which turns out to be disappointingly little, its temperature being a fraction of a degree above absolute zero. Why should anyone be interested in such a patently theoretical phenomenon as this radiation, which cannot exist if black holes do not exist, and may be very difficult to detect even if they do?
The answer is, because in terms of the theory it is so unexpected; but even more, because it brings together the basic ingredients of contemporary physics in a completely new way. In order to convince you of this I must try and outline what this physics is, and how it is organized out of its ingredients. Newton's First Law, with its confident universality, sets the tone of the ingredient level of physics.
It also has great powers of resistance to refutation : if you produce a case of a body that doesn't go in a straight line, I just say that there is some force present which you haven't thought of. However, all the forces that I ever need to invoke fall into just three categories.
The first is gravity. The last part of that statement follows from the fact that all bodies fall towards the earth with the same acceleration, plus Newton's Second Law, which tells us that force equals mass times acceleration. The constant of proportionality, relating mass to force exerted, is , the universal constant of gravitation. The second category of forces consists of electricity and magnetism.
The two forces are inseparable; together they are governed by Maxwell's Equations, which involve another universal constant , a speed: , miles per second. Electromagnetism, like gravity, can permeate empty space; in fact waves of it can travel along, and their speed works out to be.
These forces obviously depend on the material. It's like chemistry, a matter of getting to know each individual substance. Thus one can think of the pressure of a gas in terms of the energy required to compress it into a smaller volume; the force of a spring in terms of the energy required to extend it.
One has a picture of all physical processes as the conversion of energy from one form into another. The most basic form is kinetic energy: in Newton's First Law it is represented by the uniform motion of a body, while the forces impressed on it represent other forms gravitational, electromagnetic or material.
Now suppose I stir a cup of tea — for the sake of argument, cold tea — and wait a minute or two for it to settle down, then the kinetic energy corresponding to the circular motion of the tea in the cup will appear to have vanished.
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Into a Black Hole Stephen Hawking. Vinod Kumawat. A short summary of this paper. Download Download PDF. Translate PDF. You may not reproduce, edit, translate, distribute, publish or host this document in any way with out the permission of Professor Hawking. This is to allow correct pronunciation and timing by a speech synthesiser. Can you hear me. It is a great pleasure for me to be back again in Chile, to celebrate the sixtieth birthday of an old friend, and esteemed colleague, Claudio Bunster, whom I have known for almost forty years.
Claudio has done so much for science in general, and for science in Chile in particular. Being in the city of Valdivia where CECs, the center he created, is located, is quite meaningful to me. It is said that fact is sometimes stranger than fiction, and nowhere is this more true than in the case of black holes. It was about a spaceship, that was sent to investigate a black hole that had been discovered. It wasn't a very good film, but it had an interesting ending.
After orbiting the black hole, one of the scientists decides, the only way to find out what is going on, is to go inside. So he gets into a space probe, and dives into the black hole. After a screen writer's depiction of Hell, he emerges into a new universe. This is an early example of the science fiction use of a black hole as a wormhole, a passage from one universe to another, or back to another location in the same universe.
Such wormholes, if they existed, would provide short cuts for Interstellar space travel, which otherwise would be pretty slow and tedious, if one had to keep to the Einstein speed limit, and stay below the speed of light.
In fact, science fiction writers should not have been taken so much by surprise. The idea behind black holes, has been around in the scientific community for more than years. He pointed out that a star that was sufficiently massive and compact, would have such a strong gravitational field that light could not escape. Any light emitted from the surface of the star, would be dragged back by the star's gravitational attraction, before it could get very far. Michell suggested that there might be a large number of stars like this.
Although we would not be able to see them, because the light from them would not reach us, we would still feel their gravitational attraction.
Such objects are what we now call black holes, because that is what they are, black voids in space. Perhaps he decided that it was a crazy idea. But a famous experiment, carried out by two Americans, Michelson and Morley in , showed that light always traveled at a speed of one hundred and eighty six thousand miles a second, no matter where it came from. How then could gravity slow down light, and make it fall back.
This was impossible, according to the then accepted ideas of space and time. In this, space and time were no longer separate and independent entities. Instead, they were just different directions in a single object called spacetime. This spacetime was not flat, but was warped and curved by the matter and energy in it. In order to understand this, considered a sheet of rubber, with a weight placed on it, to represent a star. The weight will form a depression in the rubber, and will cause the sheet near the star to be curved, rather than flat.
If one now rolls marbles on the rubber sheet, their paths will be curved, rather than being straight lines. In , a British expedition to West Africa, looked at light from distant stars, that passed near the Sun during an eclipse. They found that the images of the stars, were shifted slightly from their normal positions. This indicated that the paths of the light from the stars, had been bent by the curved spacetime near the Sun.
General Relativity was confirmed. Consider now placing heavier and heavier, and more and more concentrated weights on the rubber sheet. They will depress the sheet more and more. Eventually, at a critical weight and size, they will make a bottomless hole in the sheet, that particles can fall into, but nothing can get out of. What happens in spacetime according to General Relativity, is rather similar. A star will curve and distort the spacetime near it, more and more, the more massive and more compact the star is.
If a massive star that has burnt up its nuclear fuel, cools and shrinks below a critical size, it will quite literally make a bottomless hole in spacetime, that light can't get out of. Such objects were given the name, black holes, by the American physicist, John Wheeler, who was one of the first to recognize their importance, and the problems they pose.
The name caught on quickly. It suggested something dark and mysterious, But the French, being French, saw a more riskay meaning. For years, they resisted the name, troo noir, claiming it was obscene.
In the end, they had to give in. Some of the techniques listed in Black Holes: The Reith Lectures may require a sound knowledge of Hypnosis, users are advised to either leave those sections or must have a basic understanding of the subject before practicing them.
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