From: Hal Finney (hal@finney.org)
Date: Wed Mar 27 2002 - 13:07:12 MST
Amara writes:
> Would you accept entropy being *conserved* in quantum mechanics ..?
As I understand it, this is only true while a system is evolving in
isolation, maintaining quantum coherence and not interacting with the
environment or an observer. When a system is measured we get the "wave
function collapse" which can involve a loss of state function information,
as the state function is projected onto a measurement basis vector.
This process is irreversible, in contrast to the "smooth" evolution when
the system is not being observed, which is reversible.
Of course in a larger sense, information is conserved in both quantum
and classical mechanics. Classical mechanics is reversible as well,
at the microscopic level. The size of the state volume is unchanged in
phase space as the system evolves. What happens is that macroscopic
information turns into microscopic information, which is heat. So we
interpret this as a loss of information.
Likewise with QM if we go to a many worlds formulation then there
is no state function collapse and all systems are always reversible.
However since we are unable to perceive the other Everett branches there
is a loss of information as far as we are concerned.
Hal
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