[Epistemic status: This is original thinking and I’m not a physicist, and it’s on a weird subject. I’m pretty confident in my general claims but the specifics are plausibly shaky.]

Reverse mathematics is a field of math where you try to answer the opposite question from usual. Normally, mathematicians start with some assumptions and try to figure out what you can prove given those assumptions. In reverse mathematics, you start with a result and try to characterize exactly what assumptions need to hold in order for the result to be true.

I’m interested in the subject which could be called “reverse physics”. In physics we try to figure out what the rules of the universe are. In reverse physics, we look at the world around us and figure out exactly which parts of physics are required for which parts of the world. In particular, I’m interested in figuring out which parts of physics are required for life to work.

The most interesting thing that I’ve gotten out of this is a pretty substantial shift in how I think about the fine-tuning of the universe for life.

From the perspective of biological life, I think that our physics seems to break pretty nicely into two different parts. Firstly, it allows chemical bonding, chemical reactions, and interactions between molecules, which is the physics that governs almost everything important that happens in biology. Secondly, it forms a nice stable universe with lots of different atoms in it where you have stars radiating energy to planets for a really long time, so that we can give the chemistry on the planets a chance to evolve into something interesting.

As far as I can tell, there are lots of ways to tweak physics so that chemistry still ends up looking basically the same. For example, a while ago I tried to analyse what happens if you teleport me into a universe without the laws of relativity (or equivalently into a universe where the speed of light is way higher), and I came away thinking that that change, while probably fatal to me, would only cause pretty minor modifications to how life needs to work. As another example, chemistry is basically the study of the quantum mechanics of electrons; chemistry is mostly unaffected if you decide that nuclei of atoms are classical particles which aren’t described by quantum mechanics. (You lose proton tunnelling, which probably changes a few things.) You can also make your nuclei have different weights than they have, which messes up current living systems but probably allows similar chemistry-based life.

(A more extreme example with jargon which I don’t know how to explain briefly: I suspect life based on chemistry also works out in a world where you have nuclei modelled as classical particles which move in a potential which comes from assuming your electrons are always in the minimum energy configuration for the current nuclei positions.)

(One of my favorite topics is how the quantum mechanics of electrons lead to the features of chemistry that we need for life. I hope to write something one day about it, but it’s pretty hard to explain briefly. I enjoy attempting to explain it in person, though.)

In contrast, the universe seems to have a bunch of constants that need to be set really precisely in order to make the whole “stable universe with lots of different kinds of atoms and very long-lived stars” thing work out, and the laws seem way less flexible than those backing quantum chemistry. If you look at Wikipedia’s list of examples of fine-tuned physical parameters, all of them except for our universe being three-dimensional are related to cosmology and don’t affect chemistry. Nuclear physics, particle physics, relativity, and cosmology stuff like dark matter all appear to be crucial to how the universe evolved into galaxies and stuff, and none of them matter much to how biological life works.

And our universe might not be that far beyond the minimum required star longevity. Earth took 4.5 billion years to get humans on it, and stars like the sun only last 10 billion years. Red dwarf stars probably support life and last a thousand times longer, which is a few orders of magnitude of comfort. But chemical reactions happen on a timescale of femtoseconds. Life took 1.4 * 10^32 femtoseconds, and red dwarves last like 3.2 * 10^35 femtoseconds, which isn’t that much leeway. (I’m unsure whether this is an appropriate comparison to make.)

Nick Tarleton points out this paper and this response to it. From the abstract of the first paper:

A universe without weak interactions is constructed that undergoes big-bang nucleosynthesis, matter domination, structure formation, and star formation. The stars in this universe are able to burn for billions of years, synthesize elements up to iron, and undergo supernova explosions, dispersing heavy elements into the interstellar medium. These definitive claims are supported by a detailed analysis where this hypothetical “Weakless Universe” is matched to our Universe by simultaneously adjusting Standard Model and cosmological parameters. For instance, chemistry and nuclear physics are essentially unchanged.

And the abstract of the second:

We point out, however, that on closer examination the proposed “weakless” universe strongly inhibits the development of life in several different ways. One of the most critical barriers is that a weakless universe is unlikely to produce enough oxygen to support life. Since oxygen is an essential element in both water, the universal solvent needed for life, and in each of the four bases forming the DNA code for known living beings, we strongly question the hypothesis that a universe without weak interactions could generate life.

I think this supports my claim that changes in physical parameters plausibly inhibit life via messing up cosmology and production of crucial elements, rather than affecting chemistry directly.

I think that people underrate the extent to which most of our universe doesn’t look like the universe around us. Here, we have matter made of atoms, where the atoms form molecules and the molecules often form liquids or gases. We have stable systems like rocks, which resist if you push back on them. Most of the universe is not like this–most of it isn’t even atoms. Most of it is just chaotic plasma and other stuff like that which I suspect is totally unsuitable for life. These phenomenons–liquids, molecules, chemical reactions–are barely present outside an extremely small temperature range which most of the mass of the universe is way too hot for.

So overall my perspective on the universe is that chemistry has a bunch of phenomena in it (eg water, methane, acids, salts) which are crucial to the small scale operation of life and also arise in a wide variety of different models of physics, and you also have the physics that leads to stars, eg nuclear physics and general relativity, which is pretty hard to tune to get extremely long-lived stars.