The abruptness of nuclear weapons

Nuclear weapons seem like the marquee example of rapid technological change after crossing a critical threshold.

Looking at the numbers, it seems to me like:

  • During WWII, and probably for several years after the war, the cost /​ TNT equivalent for manufacturing nuclear weapons was comparable to the cost of conventional explosives, (AI impacts estimates a manufacturing cost of $25M/​each)

  • Amortizing out the cost of the Manhattan project, dropping all nuclear weapons produced in WWII would be cost-competitive with traditional firebombing (which this thesis estimates at 5k GBP (=$10k?) /​ death, vs. ~100k deaths per nuclear weapon) and by 1950, when stockpiles had gown to >100 weapons, was an order of magnitude cheaper. (Nuclear weapons are much easier to deliver, and at that point the development cost was comparable to manufacturing cost).

Separately, it seems like a 4 year lead in nuclear weapons would represent a decisive strategic advantage, which is much shorter than any other technology. My best guess is that a 2 year lead wouldn’t do it, but I’d love to hear an assessment of the situation from someone who understands the relevant history/​technology better than I do.

So my understanding is: it takes about 4 years to make nuclear weapons and another 4 years for them to substantially overtake conventional explosives (against a 20 year doubling time for the broader economy). Having a 4 year lead corresponds to a decisive strategic advantage.

Does that understanding seem roughly right? What’s most wrong or suspect? I don’t expect want to do a detailed investigation since this is pretty tangential to my interests, but the example is in the back of my mind slightly influencing my views about AI, and so I’d like it to be roughly accurate or tagged as inaccurate. Likely errors: (a) you can get a decisive strategic advantage with a smaller lead, (b) cost-effectiveness improved more rapidly after the war than I’m imagining, or (c) those numbers are totally wrong for one reason or another.

I think the arguments for a nuclear discontinuity are really strong, much stronger than any other technology. Physics fundamentally has a discrete list of kinds of potential energy, which have different characteristic densities, with a huge gap between chemical and nuclear energy densities. And the dynamics of war are quite sensitive to energy density (nuclear power doesn’t seem to have been a major discontinuity). And the dynamics of nuclear chain reactions predictably make it hard for nuclear weapons to be “worse” in any way other than being more expensive (you can’t really make them cheaper by making them weaker or less reliable). So the continuous progress narrative isn’t making a strong prediction about this case.

(Of course, progress in nuclear weapons involves large-scale manufacturing. Today the economy grows at roughly the same rate as in 1945, but information technology can change much more rapidly.)

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