Wikipedia under “Neutron cross section” lists U-235 as having a capture cross-section of 60 and a fission cross section of 300, while U-238 has a capture cross-section of 2. This is for thermal neutrons (the cross-section depends on the neutron speed).
I’m surprised. I guess CP-1 could’ve been, in effect, mostly empty space filled with U-235 dust. And I’ll go ahead and agree that if all non-particle-accelerator pathways to chain reactions bottlenecked through U-235 then Fermi may have been correct to say 10% (though it is still not totally clear why 10% would’ve been a better estimate than 2% or 50%, but I’m not Fermi). This would then form only the second case I can think of offhand where erroneous scientific pessimism was not in defiance of laws or evidence already known. (The other one is Kelvin’s careful calculation that the Sun was probably around 60 million years old, which was wrong, but because of new physics—albeit plausibly in a situation where new physics could’ve rightly been expected, and where there was evidence from geology. Everything else I can think of offhand is “You can’t have a train going at 35mph, people will suffocate!” or “You can’t build nanomachines!” so you can see why my priors made me suspicious of Fermi.)
I wonder how low is the probability of obtaining at least 1 such sufficiently stable fissile isotope, if you change fundamental physical constants a little, preserving stars and life (and not making Earth blow up). It may be very low, actually, seeing it as U235 does have unusually long half life for a fissile isotope.
Wikipedia under “Neutron cross section” lists U-235 as having a capture cross-section of 60 and a fission cross section of 300, while U-238 has a capture cross-section of 2. This is for thermal neutrons (the cross-section depends on the neutron speed).
I’m surprised. I guess CP-1 could’ve been, in effect, mostly empty space filled with U-235 dust. And I’ll go ahead and agree that if all non-particle-accelerator pathways to chain reactions bottlenecked through U-235 then Fermi may have been correct to say 10% (though it is still not totally clear why 10% would’ve been a better estimate than 2% or 50%, but I’m not Fermi). This would then form only the second case I can think of offhand where erroneous scientific pessimism was not in defiance of laws or evidence already known. (The other one is Kelvin’s careful calculation that the Sun was probably around 60 million years old, which was wrong, but because of new physics—albeit plausibly in a situation where new physics could’ve rightly been expected, and where there was evidence from geology. Everything else I can think of offhand is “You can’t have a train going at 35mph, people will suffocate!” or “You can’t build nanomachines!” so you can see why my priors made me suspicious of Fermi.)
I wonder how low is the probability of obtaining at least 1 such sufficiently stable fissile isotope, if you change fundamental physical constants a little, preserving stars and life (and not making Earth blow up). It may be very low, actually, seeing it as U235 does have unusually long half life for a fissile isotope.
EDIT: Scratch that, your post is the right response.
I’ve currently got a Discussion post running to figure out how much this generalizes.