🚀 go-pugleaf

For some background on topics relevant to the great superluminal debate
for the non-experts listening in:
Isham 2. Section 5.5 on "Decoherence" p.84 NATO LECTURES (1992)
"The idea of decoherence ... growing steadily .. an increasing awareness of
the inadequacy of the traditional Copenhagen interpretation of quantum
theory, especially the posited dualism between the classical and quantum
worlds, the emphasis on measurement as a primary interpretive category, and
the associated invocation of 'reduction of the state vector' as a
descriptive process that lies outside the deterministic evolution afforded
by the Schrodinger equation.  COnsiderations of this type have been
enhanced by attempts to develop physical devices (e.g. SQUIDS) that are of
macroscopic size but which are nevertheless expected to exhibit genuine
quantum properties.

.... the quantum state of the very early universe could have been
responsible for the large-scale properties of the universe we see around us
today.  But the universe itself is the ultimate closed system, and there
can be no external observer to make measurements.... the notion of state-
vector reduction is an anathema to most people who work in quantum
cosmology.

.... The goal of decoherence is to show that there are many situations in
which the replacement of the pure state" [off-diagonal coherence terms in
the density matrix] "by the mixed state" [no off-diagonal coherence terms]
"can be understood as a viable consequence of the theory itself without the
need to invoke measurement acts as a primary concept.... no actual quantum
system is really isolated:there is always some environment E to which it
couples,,,, the state for the composite system S+E ... is thooroughly
entangled.  However, if one is interested only in the properties of the
system S, the relevant state of the reduced density-matrix .. obtained by
summing (i.e., tracing out) the environment states... if the environment
states are approximately orthogonal ... the system behaves 'as if' a state
reduction has taken place ... This is what is meant by decoherence. p.86

.. the traced-out density matrix satisfies a master equation rather than
being induced by a unitary evolution... the environment 'continuously
measures' the system, and hence gives rise to a continuous process of state
reduction.  This, it is claimed is the reason why Schrodinger's cat is in
fact always seen to be either dead or alive, never a superposition of the
two.  Numerical studies have shown that the reduction of real physical
systems can take place very quickly.  For example, the gas molecules
surrounding a piece of equipment in the laboratory will serve very well,
and even the background 3^oK radiation is sufficient to produced the
desired effect in a very short time: a typical figure for a macroscopic
object is 10^-23 sec,,,     ... The essential requirement is that the
operator concerned should commute with the interaction Hamiltonian
describing the system-environment coupling ..."