Past and Future

[One of a series in my attempt to understand the latest dust up between Bee and Lumo]

It seems clear that the (evolutionary) reason we have brains is to be able to predict the future, or at least to organize our response to the present, which amounts to the same thing. Prediction and control of the future is the full time business of essentially all of our institutions, including religion, state, and family. Science has proven a rather useful tool in that regard, and physics is one of the shiniest pennies in that coin purse – physics can predict the wavelength of certain radiations of excited atoms with phenomenal accuracy, and the evolution of the planets for tens of thousands of years.

The principle which permits prediction is causality, the notion that the past determines the future. In classical physics, it is held that if we knew the past in sufficient detail, and could do the arithmetic, we could compute the detailed future. In practice, of course, that could only be done in particularly simple physical situations, like the motion of planets around the Sun. More complex problems, like the motion of party guests around a punchbowl, were beyond our skill and knowledge. Quantum mechanics kicks a key causal leg out from under us. Even perfect knowledge and perfect computation are not enough to truly predict the future, since an irreducible probabilistic quantum uncertainty will remain.


A powerful kind of causality still exists in quantum mechanics however. The quantum state function evolves according to the Schrödinger equation, and that evolution is strictly deterministic. The irreducible quantum uncertainty only enters the picture when we actually try to measure that quantum state.

Semi-classical quantum gravity seems to offer a more serious challenge to predictivity however. The problem occurs because matter entering a black hole carries information with it, but when a black hole evaporates, it seems that information can't come out with it. Wikipedia has what I consider an excellent discussion of problem here: http://en.wikipedia.org/wiki/Black_hole_information_paradox

The always useful John Baez has this: http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/info_loss.html

More on proposed cures later.

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