Would life (as we know it) be possible without the weak interaction? I understand why the strong interaction is important in everyday life (it holds nuclei together and also allows the fusion reactions that power the Sun) and also why the electromagnetic interaction is important (it holds atoms together, among too many other effects to mention).  But while I'm sure that the universe would be profoundly different at the macroscopic level if there were no weak interaction, I can't think of a reason why.  (Well, I guess it would be a whole lot harder to persuade the NSF to fund neutrino detection experiments...)  The weak force affects the amplitudes of scattering processes that don't involve external neutrinos via virtual effects, but are these important enough to qualitatively change the macroscopic physics?
(I guess you need the full "pre-symmetry-breaking" unified electroweak interaction in order to get the Higgs mechanism and give particles mass, but is the weak interaction still qualitatively important after symmetry breaking?)
 A: I would tentatively say no. It could be a matter of geophysics. Without weak interactions there would be no weak decay of Potassium-40. This means the interior of the Earth would be cold, and as a result there would be no tectonic activity nor would there likely be a strong geomagnetic field. Tectonic activity cycles carbon and other elements. Without the geomagnetic field the surface of the Earth would be subjected to the sort of radiation on the surface of Mars.
A: Without the weak force, the asymmetry between matter and anti matter, on which we all depend, (as we definitely do not want any anti matter near us), may not have occured in the primordial universe and therefore would not be found in today's universe.
This scenario is  an extrapolation of the work of Cronin and Fitch,who found that for electically neutral kaons, (a short lived particle) whose  decay pathways were known to be due to the weak force,there is a  suble difference in the decay rates of neutral kaons and neutral antikaons, possibly leading to the preponderance of matter we observe today.
From Wikipedia Kaons

Kaons have played a distinguished role in our understanding of fundamental conservation laws: CP violation, a phenomenon generating the observed matter–antimatter asymmetry of the universe, was discovered in the kaon system in 1964 (which was acknowledged by a Nobel Prize in 1980). Moreover, direct CP violation was also discovered in the kaon decays in the early 2000s.

A: Thanks everyone for lots of good responses - I'm going to summarize them here for future convenience.  It looks like there's no consensus on this issue, but here are some takeaways on two variants of my question:


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*If the weak interaction were to suddenly "turn off" with the universe in its current condition, then solar fusion would stop and we'd all be pretty screwed (although it's not like we'd be instantly blown to bits, like we would if, say, the strong interaction were to suddenly turn off).

*If the weak interaction had never existed in the first place, then the Universe might have been able to evolve into one closely resembling our own if the baryon/photon ratio were sufficiently low during Big Bang nucleosynthesis.  It also might still have the observed strong matter/antimatter abundance asymmetry.  But it's controversial whether these initial conditions would have led to sufficient production of heavy elements during supernovas.  And even if planets with chemical composition similar to Earth's did form, their cores would probably be much colder than Earth's, possibly leading to much less tectonic activity and/or geomagnetic radiation shielding, which could make advanced life less likely to evolve.
