Kiefer C. Quantum gravity

310 INTERPRETATION
the quantum entanglement between apparatus and environment. Under ordinary
macroscopic situations, decoherence occurs on an extremely short timescale,
giving the impression of an instantaneous collapse or a ‘quantum jump’.
Recent experiments were able to demonstrate the continuous emergence of classical
properties in mesoscopic systems (Hornberger et al. 2003; Joos et al. 2003).
Therefore, one would never ever be able to observe a weird superposition such
as Schrödinger’s cat, because the information about this superposition would
almost instantaneously be delocalized into unobservable correlations with the
environment, resulting in an apparent collapse for the cat state.
The interaction with the environment distinguishes the local basis with respect
to which classical properties (unobservability of interferences) hold. This
‘pointer basis’ must obey the condition of robustness, that is, it must keep its
classical appearance over the relevant time-scales; cf. Zurek (2003). Classical
properties are thus not intrinsic to any object, but only defined by their interaction
with other degrees of freedom. In simple (Markovian, i.e. local in time)
situations, the pointer states are given by localized Gaussian states (Diósi and
Kiefer 2000). They are, in particular, relevant for the localization of macroscopic
objects.
The ubiquitous occurrence of decoherence renders the interpretational problem
of quantum theory at present largely a ‘matter of taste’ (Zeh 1994). Provided
one adopts a realistic interpretation without additional variables, 1 the alternatives
would be to have either an Everett interpretation or the assumption of a
collapse for the total system (including the environment). The latter would have
to entail an explicit modification of quantum theory, since one would have to introduce
non-linear or stochastic terms into the Schrödinger equation in order to
achieve this goal. The Everett interpretation assumes that all components of the
full quantum state exist and are real. Decoherence produces robust macroscopic
branches, one of which corresponds to the observed world. Interferences with the
other branches are suppressed, so decoherence readily explains the observation
of an apparent collapse of the wave function, independent of whether there is a
real collapse for the total system or not. The question is thus whether one applies
Ockham’s razor 2 to the equations or the intuition (Zeh 1994): either one has to
complicate the formalism in order to have just one macroscopic branch or one
retains the linear structure of quantum theory and has to accept the existence
of ‘many worlds’.
10.1.2 Decoherence in quantum cosmology
In this subsection, we investigate the question: how can one understand the classical
appearance of global space–time variables such as the radius (scale factor)
of the universe? If decoherence is the fundamental process, we have to identify a
‘system’ and an ‘environment’. More precisely, we have to differentiate between
relevant and irrelevant variables. All degrees of freedom exist, of course, within
1 The Bohm theory would be an example for a realistic approach with additional variables.
2 ‘Pluralitas non est ponenda sine necessitate’.