Abstract
A recent debate has ensued over the claim in Ref. 1 that systems with
internal degrees of freedom undergo a universal, gravity-induced, type of
decoherence that explains their quantum-to-classical transition. Such
decoherence is supposed to arise from the different gravitational redshifts
experienced by such systems when placed in a superposition of two wave packets
at different heights in a gravitational field. Here we investigate some aspects
of the discussion with the aid of simple examples. In particular, we first
resolve an apparent conflict between the reported results and the equivalence
principle by noting that the static and free fall descriptions focus on states
associated with different hypersurfaces. Next, we emphasize that predictions
regarding the observability of interference become relevant only in the context
of concrete experimental settings. As a result, we caution against hasty claims
of universal validity. Finally, we dispute the claim that, at least in the
scenarios discussed in Ref. 1, gravitation is responsible for the reported
results and we question the alleged ability of decoherence to explain the
quantum-to-classical transition. In consequence, we argue against the
extraordinary assertion in Ref. 1 that gravity can account for the emergence of
classicality.
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