Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Semiclassical limit problems with concurrent use of several clocks in
  quantum cosmology   We revisit a recent proposal for a definition of time in quantum cosmology,
to investigate the effects of having more than one possible type of clock "at
the same time". We use as test tube an extension of Einstein gravity with a
massless scalar field in which the gravitational coupling $G_N$ is only a
constant on-shell, mimicking the procedure for $\Lambda$ in unimodular gravity.
Hence we have two "simultaneous" clocks in the theory: a scalar field clock,
and the conjugate of $G_N$. We find that attempts to use two coherent clocks
concurrently are disastrous for recovering the classical limit. The Heisenberg
relations, instead of being saturated, are always realized abundantly above
their bound, with strong quantum effects expected at least in parts of the
trajectory. Semi-classical states always result from situations where we
effectively impose a single clock, either by making the other clock a failed
clock (i.e. by choosing a state where its conjugate constant is infinitely
sharp), or by choosing a basis of constants where all clocks but one are
redundant, i.e. motion or change in phase space does not occur with the passing
of their "times". We show how this conclusion generalizes to fluids with any
equation of state. It also applies to systems where "sub-clocks" of the same
type could be used, for example in mixtures of different fluids with the same
equation of state.

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The categories of this paper are: hep-ph
12
The categories of this paper are: nucl-th
16
The categories of this paper are: stat
2
The categories of this paper are: math
21
The categories of this paper are: adap-org
9
The categories of this paper are: nlin
13
The categories of this paper are: quant-ph
31
The categories of this paper are: ao-sci
6
The categories of this paper are: astro-ph
8
The categories of this paper are: hep-ex
24
The categories of this paper are: dg-ga
25
The categories of this paper are: patt-sol
28
The categories of this paper are: alg-geom
26
The categories of this paper are: bayes-an
7
The categories of this paper are: hep-th
36
The categories of this paper are: cmp-lg