Invitation to a problem

Black hole are perhaps the most fascinating and mysterious objects in the Universe. Many are born from the most violent events in the Universe - supernova explosions. Anything coming close to a black hole gets sucked in, and then crushed to infinite density at the center. Particles falling into the hole rub against each other and get very hot, resulting in an amazingly bright object called a quasar. A quasar is no bigger than the sun, yet it can be a billion times brighter.

But there is another aspect to black holes, which will be the focus of this website. Imagine a hole that is sitting in empty space, with no matter around to feed it and make a quasar. In 1974 Hawking found a remarkable fact: even left to itself, the hole will radiate very, very softly. This radiation is so soft because it is a an effect of quantum mechanics, a theory that is usually applied only to the microworld of atoms.

No experiment today can detect a radiation this soft. But this gentle process caused a tremendous storm in the world of theoretical physics. The reason was that this radiation process would end, Hawking argued, in a violation of the principles of quantum mechanics.

For years people have struggled with this conclusion. One thing is clear: when we do understand the resolution of this puzzle, we would have learnt something deep about the nature of space, time and gravity.

In these pages we will explain the problem, which has come to be known as the "black hole information paradox". We will explain some of the twists and turns in this saga. Interesting ideas have been proposed, but later found to be wrong. We will discuss a solution which is suggested by computations in string theory: that quantum physics radically changes the structure of the hole, resulting in an object called a fuzzball. We will also outline other ideas that have been proposed for solving the problem: remnants, non-causal evolution, nonlocal interactions, wormholes, etc.

To start reading these pages, all you should need are (i) a recollection of high school physics (ii) an abundance of enthusiasm.

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