Conditions for graviton emission in the recombination of a delocalized mass

TL;DR

This paper investigates the conditions under which a delocalized mass can emit gravitons during recombination, revealing that entanglement enhances quadrupole moment variations and that Planck-scale masses set emission thresholds, challenging certain collapse models.

Contribution

It provides a detailed analysis of graviton emission conditions for delocalized masses, highlighting the role of entanglement and mass thresholds, and compares these with collapse model predictions.

Findings

Quadrupole moment variations are greatly enhanced if the field is entangled.

Recombination time for graviton emission scales with mass, not square root of mass.

No graviton emission occurs in collapse models where superposition collapses before emission is possible.

Abstract

In a known gedanken experiment, a delocalized mass is recombined while the gravitational field sourced by it is probed by another (distant) particle; in it, this is used to explore a possible tension between complementarity and causality in case the gravitational field entangles with the superposed locations, a proposed resolution being graviton emission from quadrupole moments. Here, we focus on the delocalized particle (forgetting about the probe and the gedanken experiment) and explore the conditions (in terms of mass, separation, and recombination time) for graviton emission. Through this, we find that the variations of quadrupole moments in the recombination are generically greatly enhanced if the field is entangled compared to if it is sourced instead by the energy momentum expectation value on the delocalized state (moment variation $\sim m \, d^2$ in the latter case, with $m$ mass, $d$ separation). In addition, we obtain the (upper) limit recombination time for graviton emission growing as $m$ in place of the naive expectation $\sqrt{m}$. In this, the Planck mass acts as threshold mass (huge, for delocalized objects): no graviton emission is possible below it, however fast the recombination occurs. If this is compared with the decay times foreseen in the collapse models of Di\'osi and Penrose (in their basic form), one finds that no (quadrupole) graviton emission from recombination is possible in them. Indeed, right when $m$ becomes large enough to allow for emission, it also becomes too large for the superposition to survive collapse long enough to recombine.