I recently re-read an article by Muller (https://arxiv.org/pdf/1606.07975.pdf) about the flow of time, and the possibility of time reversal given sufficient energy dissipation (basically during black hole evaporation, he concludes). Although the paper is on arXiv and not peer reviewed, Muller has written a book on the topic. I posted quite a long thread in the Quantum Physics forum, but there were still a number of unresolved issues... Regardless of whether you buy Muller's argument or not - I have a fundamental question about the relationship between GR and QM. Muller defines the expansion of time just like the expansion of space (actually due the Hubble constant in fact) - i.e., as a continuous process in time. His bound on energy dissipation is also continuous (6e11 Watts or some such). Now, my body dissipates, maybe 200 Watts, on average... So, similar to what I posted before (and thanks to @PeterDonis and @vanhees71 for chiming in - would love to hear from Arnold Neumaier ): in a much more mundane sense, let's consider the emission of a single photon from an excited hydrogen atom... If photon emission is instantaneous, then one could say that the power output (energy dissipation rate) is infinite (finite energy photon emitted in zero time). I don't think this is what Muller is getting at... but in any quantized system (e.g., a blackbody radiator), the energy emission (power output) will vary, just by virtue of energy coming out in 'chunks'. So if Muller's correct, did my electron also go back in time? I had two thoughts: 1) Photon emission is a continuous time process, then a bound on the emission rate could be true bound. No clear response on this from the community so far (@vanhees71 was looking at this...) 2) As @PeterDonis said, GR and QM can't be reconciled in this case - but this still leaves me with the question: every so often does some benign even (a energetic photon emitted in a femtosecond) actually cause time reversal in Muller's model? In the second case, it would seem that Muller's idea could be tested without looking at black holes, but he specifically states that small evaporating black holes are the only objects that radiate sufficiently. I'm totally confused about how to square the idea of continuous radiation output with the nature of a quantized systems. As @PeterDonis pointed out, he's not aware of any other physical systems (QM) that require an emission rate in continuous time... but then this is the clash between GR and QM exactly. Any insights at all would be most helpful or even places to look. The best current answer is the QM one, that a specific number of quantum events must occur in a specific time - but this does not address the issue of my lonely electron at all ... Thanks all.