Black Holes: A Very Short Introduction by Katherine Blundell

Author:Katherine Blundell [Blundell, Katherine]
Language: eng
Format: epub
ISBN: 9780199602667
Publisher: OUP Oxford
Published: 2015-10-27T00:00:00+00:00

Chapter 5

Entropy and thermodynamics of black holes

You are what you eat

It is often said that you are what you eat. Thus if your diet is purely junk food and chocolate, then your complexion, not to mention your physical and mental well-being, will be rather different than if you subsist on a healthy diet of salad and Mediterranean food. However, it seems that black holes are not fussy eaters. Whether they are hoovering up a vast expanse of interstellar dust or a cubic light-year of fried eggs, their mass will similarly increase inexorably. In fact, after a black hole has finished its sumptuous meal, you have no way of telling what it was eating, only how much it has consumed (although you could tell if what it ate had charge or angular momentum). You only know the quantity of its diet, not about the quality. The ‘no-hair theorem’ described in Chapter 2 says that the black hole is only characterized by a very few parameters (mass, charge, and angular momentum), and thus we cannot talk about what the black hole is made of.

This lack of knowledge about the nature of what has been sucked in by a black hole may seem like a trivial observation, but it is actually rather profound. Information about a black hole’s lunch menu has been fundamentally lost. Any matter which has fallen into the black hole has surrendered its identity. We can’t perform measurements on that matter, or discern any details about it.

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