A very slight change in some nuclear properties would have been enough to make the fission chain reaction impossible. If the alpha-decay energy of 235U, the only naturally occurring fissile nuclide, were just 0.1 MeV higher than it actually is so little would remain in the Earth that extracting a critical mass would be close to impossible. Moreover, even with a sufficient abundance of 235U, if its scission point were shifted inwards by at most 2 fm not enough secondary neutrons would be emitted for a chain reaction to be possible. Speculating within the framework of the multiverse concept, we ask whether there could be a universe similar to ours in which intelligent life can emerge but in which the fission chain reaction is impossible.

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