Anil Ananthaswamy has written a very nice piece for New Scientist about semiclassical gravity. It deals with recent attempts (to which I have taken part) in trying to make sense of a theory in which gravity is fundamentally classical.


The article is a bit too kind and likely gives me a more central place in this adventure than I actually deserve. Nonetheless, the article contains the right amount of qualifiers and lets the skeptics speak. I understand them all too well: our approach is clearly not without flaws. It is more a counterexample to pessimistic views about semiclassical gravity than a believable proposal for a theory of everything. And I would not be surprised if it were to be falsified in the near future. But as Lajos is quoted saying at the end: “we must explore”.

Around the end of the article, Carlo Rovelli says he gives gravity 99% chance of being quantum. There, I think he is being a bit overconfident about the path he and his collaborators are pursuing, although his skepticism about our own work is again warranted.  Are the reasons why we think gravity should be quantum so strong? I am not sure, after all we know very little about gravity (see this recent essay). If gravity is not semiclassical in the way we have proposed, it could be in many others. Fortunately this question is answerable and will not require a particle accelerator the size of the Milky Way. If gravity is not quantum, this proposed experiment (which I had advertised here) will see it. Meanwhile, we have to remain open.

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