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Text 5

Black Holes

A century ago astronomers believed the universe was a little bigger than our own Milky Way galaxy of stars, and was completely stable and unchanging. But over the first few decades of the twentieth century, that view was profoundly shaken.

First of all powerful telescopes revealed that there are countless galaxies beyond the Milky Way; then, in 1920s, the great American astronomer sir Edwin Hubble discovered that all these galaxies are speeding away from us, which means that, far from being stable, the universe must be expanding at a phenomenal rate.

Meanwhile, two great concepts quantum physics and Einstein theory of relativity turned the classical physics on its head. These extraordinary ideas only gradually made a general impact on the world of science, but a number of scientists immediately saw some implications. In 1917, Russian astronomer Aleksandr Friedmann inferred this expansion from Einsteins relativity. Einstein actually disagreed with Friedmann, and was shocked when Hubbles discovery proved Friedmann right.

As it happens, this was the second time Einsteins own interpretation of relativity had been challenged, again rightly as it turned out. A year earlier, in 1916, the German astronomer Karl Schwarzschild concluded that, as the star contracts, its gravity grows so powerful that nothing, not even light can escape. It becomes a black hole in space. Such a hole was later found to be centered on a minute point called singularity, where time and all forces become one. The size to which a star must shrink before it becomes a black hole is called, appropriately, the Schwarzschild radius and is about 3 km for a star the size of our sun.

Over the next half a century, scientists began mathematically to wind back the clock of the expanding universe, and they realized that, although it is now big, it once must have been very small. The theory was that it burst into existence and swelled out about 13 billion years ago that came to be called the big bang. The Big Bang Theory soon became firmly established, though understanding of the process involved was shaky.

Black holes, however, remained controversial. After all, they could not, be definition, be seen. Some scientists argued that they could not exist at all because they depended on the star collapsing perfectly symmetrically, which, they argued, was highly unlikely.

Significantly, while Einsteins relativity had played a key role in both the big bang and Black holes theory, the other revolutionary idea, quantum physics, seemed almost to have been sidelined as irrelevant to cosmology. This was because it apparently refers only to the minute, subatomic level, and not to the scale of the universe.

It was Stephen Hawkings brilliant insight that brought the big bang and black holes relativity and quantum physics all together to give an extraordinary theoretical picture of cosmic forces at work. First of all, as a young graduate, Hawking realized that the big bang might be a black hole in reverse, expanding from a singularity. This gave cosmologists the mathematical tool to develop the fuller picture of the origins of the universe.

Then, in the early 1970s, Hawking realized that quantum effects might apply to the event horizon or rim of black holes. If they did, he argued, they would make a black hole glow faintly, and so perhaps be detectable after all, making this hitherto theoretical idea a reality. Thai glow came to be called Hawking radiation. Even more importantly, by bringing quantum physics into the study of black holes, Hawking had drawn it into the whole cosmological filed and so opened the way to an all-embracing physical theory of the universe. It is this that Hawking and his colleagues are looking at the moment.



Date: 2016-04-22; view: 87

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