On 6/7/2016 4:21 PM, Bruce Kellett wrote:
On 8/06/2016 2:35 am, smitra wrote:
On 07-06-2016 11:03, Bruce Kellett wrote:
That is false. As I explained earlier in the post, decoherence into
the warm thermal environment will always result in IR photons. These
escape at the velocity of light and can never be captured to be
returned -- this is an _in principle_ limitation on the possibility of
reversibility. Reversibility is possible only in very special and
tightly controlled situations. In general, the formation of distinct
worlds is irreversible in principle.
This has nothing to do with reversibility. Reversibility has to do
with whether the information about the initial state is present in
the final state, not whether you can imagine a situation where you
cannot actually reverse the time evolution in practice. Note that
infrared photons escaping is not going to cause information loss at
the fundamental level. It may look that way when doing practical
computations by treating the system under consideration in an
effective way using density matrices where you trace out the
environmental degrees of freedom.
I suppose it depends on what you want "reversibility" to do for you.
If your concern is with unitary evolution and the conservation of
information, the the evolution is "reversible" in that if you replace
t with -t in your equations, you will get another possible physical
process. But if your concern is with recovering the coherence of a
quantum state that has fully decohered (by unitary evolution) then you
face a different problem. There is no physical procedure that replaces
t with -t -- that is a purely mathematical operation that has no
parallel in the physical world. So the loss of IR photons at the speed
of light absolutely excludes the possibility of recovering the
original coherent state. Thus decohered separate worlds can never
recombine or interact in any way -- generic quantum processes are not
reversible.
And besides that, if you could actually reverse the evolution you and
everyone else would not even notice it. In fact maybe it is reversed.
Or more to the point t increases with entropy - it makes no sense to
"reverse" it, it's just a coordinate label in the equation. The t that
is measured by a clock only goes one way.
Brent
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