On 07-06-2016 11:03, Bruce Kellett wrote:
On 7/06/2016 6:57 pm, Bruno Marchal wrote:
On 07 Jun 2016, at 04:24, Bruce Kellett wrote:
That is just playing with words, and Deutsch's approach reduces
the concept of "separate worlds" to meaninglessness -- the concept
becomes so fluid as to become useless. One is very much better
advised to limit the idea of separate worlds to the
irreversibility following a decohered interaction.
That does not exist. In principle quantum erasure is always
possible. In practice that is quickly impossible, but reason of BIG
numbers, but the wave, or the unitary evolution, is always
reversible.
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.
Saibal
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