Information Loss

[Earl]

After watching several lectures regarding Black Holes, I have a bit of a mental barrier im struggling with.

If information is irretrievable how can it not be considered lost?

[acey]

The grains of sand on a beach come from stones and shells ground to powder by water. In principle, if you had enough data, you could reconstruct the original stones and shells from the fragments (though in practice you can never have that data, and even if you did, there would be too much to work out). So the information of the original stones and shells is not lost, though in practice it's irretrievable.

If something falls into a black hole then the information appears to be completely lost, because it seems that even in principle you can't take a given black hole and work out what fell into it previously.

This is paradoxical, because a law of physics ("unitarity") says that everything should be like sand on a beach - the information should still be there in principle, even if there's no way you can actually put it all together again.

Various answers to the "paradox" have been proposed: http://en.wikipedia.org/wiki/Black_hole_information_paradox

[Earl]

I have been pondering this.

If we break things down to a Quantum level where we are unable to know Location and Velocity (IIRC) technically we never have defining information in a quantum state, even the act of measuring influences the object measured.

What is it then that makes information in its fundamental form?

[acey]

The information is the quantum wave function. The wave function doesn't offer the answer to every question (e.g. "what is the exact position and momentum of that particular electron right now?") because posing one question may exclude the possibility of simultaneously asking another. The questions "what is the position?" and "what is the momentum?" can be posed separately but not simultaneously.

[Earl]

Thanks, more reading to do