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private_messaging comments on Being Half-Rational About Pascal's Wager is Even Worse - Less Wrong

18 Post author: Eliezer_Yudkowsky 18 April 2013 05:20AM

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Comment author: private_messaging 18 April 2013 07:58:15PM *  6 points [-]

Ohh, the elephant in the room that I somehow neglected to mention. (It is hard to argue against silly ideas, I suspect for a reason similar to why it is very hard / impossible to truly reflect on how you visually tell apart cats and dogs)

There's a lot of nuclei that can fission, but can't sustain a chain reaction! Because they do not produce high enough energy neutrons, or they capture neutrons too often and fission too rarely, and so on. And the neutron source of the time (radium plus lithium, or radium plus beryllium, or something like that), it produced a lot of high energy neutrons.

It would be quite interesting if someone far more obsessive compulsive than me would go over the table of isotopes and see if about 1 in 10 isotopes that can fission when irradiated with radium-lithium or radium-beryllium neutron source produce enough neutrons of high enough energy. Because if it is close to 1 in 10, and I think it is (on appropriate, i.e. logarithmic, scale), then the evidence that one isotope can fission, will only get you to 1 in 10 chance that it makes neutrons that can fission it.

Comment author: endoself 18 April 2013 10:40:55PM 2 points [-]

Szilard was proposing the idea of fission chain reactions in general. Of course he would be less confident if asked about a specific isotope, but he's still right that the idea is important if he gets the isotope wrong. Anyway, the fact that he discusses uranium specifically shows that the evidence available to him points toward uranium and that this sort of reference class is not using all the evidence that they had at the time.

Comment author: private_messaging 18 April 2013 11:31:26PM *  5 points [-]

and that this sort of reference class is not using all the evidence that they had at the time.

You're making it sound like you have a half of the periodic table on the table. You don't. There's U-238, U-235, Th-232, and that's it . Forget plutonium, you won't be making any significant amount of that in 1945 without a nuclear reactor. Of them the evidence for fission would be coming, actually, from U238 fissioning by fast neutrons, and U238 can't sustain chain reaction because too many of the the neutrons slow down before they fission anything, and slow neutrons get captured rather than cause fission.

U235 is the only naturally abundant fissile isotope, and it has a half life of 700 million years, which is 4400 times longer than the half life of the second most stable fissile isotope (U-233) and 30 000 longer than that of the third most stable isotope (that's it. The factor of 4400 difference, then the factor of less than 7 , and so on). That's how much U235 is a fluke. One can legitimately wonder if our universe is fine tuned for U235 to be so stable.

edit: note, confusing terminology here: "fissile" means capable of supporting a chain reaction, not merely those capable of fissioning when whacked with a high energy neutron.

edit2: and note that the nucleus must be able to capture a slow neutron and then fission due to capturing it, not due to being whammed by it's kinetic energy, contrary to what you might have been imagining, because neutrons lose kinetic energy rather quickly, before sufficient chance at causing a fission. It must be very unstable, yet, it must be very stable.