I have another piece this week on the FirstPrinciples.org Hub. If you’d like to know who they are, I say a bit about my impressions of them in my post on the last piece I had there. They’re still finding their niche, so there may be shifts in the kind of content they cover over time, but for now they’ve given me an opportunity to cover a few topics that are off the beaten path.
This time, the piece is what we in the journalism biz call an “explainer”. Instead of interviewing people about cutting-edge science, I wrote a piece to explain an older idea. It’s an idea that’s pretty cool, in a way I think a lot of people can actually understand: a black hole puzzle that might explain why gravity is the weakest force. It’s an idea that’s had an enormous influence, both in the string theory world where it originated and on people speculating more broadly about the rules of quantum gravity. If you want to learn more, read the piece!
Since I didn’t interview anyone for this piece, I don’t have the same sort of “bonus content” I sometimes give. Instead of interviewing, I brushed up on the topic, and the best resource I found was this review article written by Dan Harlow, Ben Heidenreich, Matthew Reece, and Tom Rudelius. It gave me a much better idea of the subtleties: how many different ways there are to interpret the original conjecture, and how different attempts to build on it reflect on different facets and highlight different implications. If you are a physicist curious what the whole thing is about, I recommend reading that review: while I try to give a flavor of some of the subtleties, a piece for a broad audience can only do so much.
4 gravitons 
Adding nothing of real consequence to the discussion, but the weak anthropic principle and the multiverse would indicate that gravity must be as it is if we are observing it, otherwise the universe would be either just dust or black holes with no life. There may be no other reason for its weakness.
Of course the multiverse may not be true, but then it would be an incredible stroke of luck for life to exist in it. While say G could say vary by several orders of magnitude for life (there is a nice paper by Fred West, Physics Reports, on the fine tuning parameters), G could be in principle anywhere from + infinity to – infinity, so the chance that it would randomly be in an inhabitable range is zero. Even if one could come up with a new axiom as to why it is in a particular range it just shifts the question, why this axiom out of all possible others?
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Yeah, you can get the weakness of gravity from anthropics alone. I don’t think that quite gets you the conclusion of the weak gravity conjecture, since you could always have even weaker forces that don’t happen to be relevant for life. But it does mean it’s less relevant for the pop physics framing “why is gravity weak?” (though a bit more relevant for the slightly different formulation, “how is gravity weak?”)
Regardless, the framing with axioms isn’t really the right way to think about it. The conjecture is that it’s impossible to have a consistent theory of gravity at all without gravity in some sense being the weakest force. Consistency is not an optional axiom!
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