Black Hole Introduction
It is said that fact is sometimes stranger than fiction, and nowhere is that more true than in the case of black holes. Black holes are stranger than anything dreamed up by science-fiction writers, but they are firmly matters of science fact. The scientific com munity was slow to realize that massive stars could collapse in on themselves, under their own gravity, and to consider how the objects left behind would behave. Albert Einstein even wrote a paper in 1939 claiming that stars could not collapse under gravity because matter could not be compressed beyond a certain point. Many scientists shared Einstein's gut feeling. The rincipal exception was the American scientist John Wheeler, who in many ways is the hero of the black hole story. In his work in the 1950s and 1960s, he emphasized that many stars would eventually collapse, and pointed out the problems that possibility posed for theoretical physics. He also foresaw many of the properties of the objects which collapsed stars become - that is, black holes.
During most of the life of a normal star, over many billions of years, it will support itself against its own gravity by thermal pressure, caused by nuclear processes which convert hydrogen into helium. Eventually, however, the star will exhaust its nuclear fuel. The star will now contract. In some cases, it may be able to support itself as white dwarf' star. However, Subrahmanyan a ' Chandrasekhar showed in 1930 that the maximum mass of a white dwarf star is about 1.4 times that of the sun. A similar maximum mass was calculated by Soviet physicist Lev Landau for a star made entirely of neutrons.
What Is A Black Hole?
When John Wheeler introduced the term 'black hole in 1967, it replaced the earlier name frozen star'. Wheeler's coinage emphasized that the remnants of collapsed stars are of interest in their own right, independently of how they were formed. The new name caught on quickly. It suggested something dark and mysterious. But the French, being French, saw a more risqué meaning. For years, they resisted the name trou noir, claiming it was obscene. But that was a bit like trying to stand against le weekend, and other Franglais. In the end, they had to give in. Who can resist a name that is such a winner?
From the outside, you can't tell what is inside a black hole. You can throw television sets, diamond rings, or even your worst enemies into a black hole, and all the black hole will remember is the total mass, the state of rotation and the electric charge. John Wheeler is known for expressing this principle as a black hole has no hair'. To the French, this just confirmed their suspicions.
A black hole has a boundary, called the event horizon. This is where gravity is just strong enough to drag light back and prevent it escaping. Because nothing can travel faster than light, everything else will get dragged back also. Falling through the event horizon is a bit like going over Niagara Falls in a canoe. If you are above the falls, you can get away if you paddle fast enough, but once you are over the edge, you are lost. There's no way back. As you get nearer the falls, the current gets faster. This means it pulls harder on the front of the canoe than the back. There's a danger that the canoe will be pulled apart. It is the same with black holes. If you fall towards a black hole feet first, gravity will pull harder on your feet than your head, because they are nearer the black hole. The result is you will be stretched out longways, and squashed in sideways. If the black hole has a mass of a few times our sun's you will be torn apart and made into spaghetti before you reach the horizon. However, if you fall towards a much larger black hole, with a mass of a million times the sun's, you'll reach the horizon without difficulty. So, if you want to explore the inside of a black hole, make sure you choose a big one. There is a black hole with a mass of about four million times that of the sun at the centre of our Milky Way galaxy.
What Is Inside A Black Hole?
If determinism, the predictability of the universe, breaks down with black holes, it could break down in other situations. Even worse, if determinism breaks down, we can't be sure of our past history either. The history books and our memories could just be illusions. It is the past that tells us who we are; without it, we lose our identity.
It was therefore very important to determine whether information really was lost in black holes or whether, in principle, it could be recovered. Many scientists felt that information should not be lost, but no one could suggest a mechanism by which it could be preserved. The arguments went on for years. Finally, I found what I think is the answer. It depends on the idea of Richard Feynman that instead of one single history there are many different possible histories, each with its own probability. In this case, there are two kinds of history. In one, there is a black hole, into which particles can fall; in the other, there is no black hole.
The point is that from the outside, one can't be certain whether there is a black hole or not. So there is always a chance that there isn't a black hole. This possibility is enough to preserve the information, but the information is not returned in a very useful form. It is like burning an encyclopaedia. Information is not lost you keep all the smoke and ashes, but it is difficult to read. The scientist Kip Thorne and I had a bet with another physicist, John Preskill, that information would be lost in black holes.
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