The gravitational self-energy of NH3 quantum superposition is E = 1.1×10 -51 J, with  = 1/(8) as proposed in ref. (see also discussion in Section 3.3). With (4), this extraordinarily tiny energy uncertainty implies an immensely long collapse time of 10 17 s or 10 9 years. (This and other collapse times estimated in this work are collected in Table .) For comparison, the decoherence time of isolated NH3 in perfect vacuum due to thermal photons at room temperature is about 10 20 s. Thus, under such conditions, the collapse would occur much before the molecule could decohere. (For this estimate, we supposed that the NH3 molecule is a small dielectric sphere of radius 1.5 Å, and followed the treatment of Joos and Zeh. ) On the other hand, supposing that the best laboratory vacuum has a number density 10 -17 smaller than standard room pressure, decoherence toward the residual gas would happen within 10 6 s, killing any sign of quantum superposition much before the wave function collapsed. (In these decoherence estimates, we neglected the recoil of the molecule caused by the environmental scattering. This is a valid approximation for photons but not suitable for a gas. Still, the difference in many orders of magnitude between collapse and decoherence times in the residual gas should be observed if recoil were considered.) Let us now consider the quantum superposition that emerges when a medium-sized molecule like C70 diffracts as a matter wave. These impressive experiments by Arndt, Zeilinger, and co-workers detected such waves using a Talbot-Lau interferometer with gold gratings with a 991 nm period of 476 nm slits.