What Goes In The Black Hole Stays In The Black Hole. OK?

A Relatively Big Mystery

Reviewed by James Trefil
Sunday, September 7, 2008; Page BW08


My Battle with Stephen Hawking to Make the World Safe for Quantum Mechanics

By Leonard Susskind

Little, Brown. 470 pp. $27.99

What is it about black holes, anyway? To most scientists, a black hole is something like a duck-billed platypus in the sky: weird, unusual, esoteric and not all that connected to real life. On the other hand, people just can't seem to get enough of them. Any teacher will tell you that it's a whole lot easier to get a class interested in black holes than in DNA, even though the latter will most assuredly have a real impact on their future and the former will not.

Oh well, if you must learn about black holes, you could do a lot worse than to pick up this engagingly written book. Stanford physicist Leonard Susskind provides a marvelous introduction to the subject that is both readable and easy to understand. Or at least as easy as something involving the two great 20th-century advances in science -- relativity and quantum mechanics -- can possibly be.

You see, until the end of the 19th century, scientists who thought about the fundamental structure of the universe had concentrated on normal-sized objects moving at normal speeds (think billiard balls). We associate this kind of science with Isaac Newton. Then, in rapid succession, the 20th century brought two revolutions. The first, which dealt with objects moving near the speed of light or having very large mass, was relativity, the brainchild of Albert Einstein. The second revolution came when people starting thinking about very small objects, such as the stuff inside the atom. The resulting theory is called quantum mechanics and was developed by a small group of young scientists in the 1920s, the most familiar probably being Werner Heisenberg of uncertainty-principle fame. One note in passing: These revolutions didn't so much replace Newton as extend his reach. Like a tree, mature sciences grow by adding new material while leaving their heartwood intact.

With increasing urgency over the past 50 years, theoretical physicists have tried to tie these two great 20th-century advances together, to produce what Nobel Laureate Steven Weinberg calls "The Final Theory." So far, we have not been successful. But if you can't bring the two fields together, you would at least like to know that they don't contradict each other, that they are mutually consistent. And this is where Susskind's "war to make the world safe for quantum mechanics" comes in, because for a period of almost 20 years, it looked as if there could well be a fundamental contradiction between the basic postulates underlying the two theories. At least that's what Stephen Hawking argued, and when Hawking talks, physicists listen.

Remember that a black hole is an object so compact and so massive that nothing, not even light, can escape from it. It is, in fact, a kind of one-way gate in the universe: Stuff can fall in, but nothing can come out. Because it involves both a large mass and extremely high energy, the black hole forms a kind of nexus where both relativity and quantum mechanics come into play. Thus, if there are going to be problems joining these two fields, they are likely to turn up in the behavior of black holes.

In 1983, Hawking proved that, against all expectations, black holes are not eternal. In fact, over unimaginably long spans of time, they evaporate, more or less like a puddle of water on a sunny day. And that's when the "war" started, because if a black hole evaporates (and everyone agrees that it will), what happens to all the information that was carried by the stuff that fell in? That information might include things like the mass of the particles that fell in, their spin, their identity and all kinds of other properties. Hawking argued that this information was lost forever, that the black hole was truly a one-way street to oblivion.

The problem is that one of the basic laws of quantum mechanics is that information cannot be lost. (I should point out that in quantum mechanics the term "information" has a technical meaning, and that losing it is more of a problem than, say, losing the shopping list you need at the supermarket.) In the case of the evaporating puddle, for example, it is theoretically possible to reconstruct the puddle by looking at the air molecules above the spot where it used to be. Hawking argued, however, that with the material that evaporated from the black hole, no such thing is possible, that the information simply disappeared. Susskind's account of his reaction to this claim and of driving home from the conference where it was first presented, distractedly scribbling equations in the frost on his windshield, beautifully describes how disturbing the idea of disappearing information was to those of us steeped in the lore of the quantum.

In the end, Susskind and his colleagues were able to resolve this dilemma and, in the words of the subtitle, "make the world safe for quantum mechanics." I won't spoil the book for you by telegraphing the ending. Suffice it to say that it involves a tour through the whole arcane menagerie of modern physics -- quarks, gluons, branes, strings. And this illustrates the main problem faced by authors of this sort of book. Black hole astrophysics is about as far from everyday experience as you can get, which means that the author has to spend a lot of time bringing the reader up to speed (indeed, it takes Susskind almost 200 pages). Even when, as in this book, there is virtually no mathematics, there is an overwhelming number of strange new concepts. Consequently, I recommend digesting this book in small segments, allowing each new concept to settle in before moving on.

In the end, The Black Hole Wars is as good an introduction as you're going to find to the strange world of black hole astrophysics. Add that to the chance to ride along as real scientists resolve a fundamental issue and you have the makings of a great read. ยท

James Trefil is Clarence J. Robinson Professor of Physics at George Mason University. His most recent book is "Why Science."

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