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gwern 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: gwern 18 April 2013 05:43:38PM *  9 points [-]

From your reference:

Fermi was not misleading Szilard. It was easy to estimate the explosive force of a quantity of uranium, as Fermi would do standing at his office window overlooking Manhattan, if fission proceeded automatically from mere assembly of the material; even journalists had managed that simple calculation. But such obviously was not the case for uranium in its natural form, or the substance would long ago have ceased to exist on earth. However energetically interesting a reaction, fission by itself was merely a laboratory curiosity. Only if it released secondary neutrons, and those in sufficient quantity to initiate and sustain a chain reaction, would it serve for anything more. "Nothing known then," writes Herbert Anderson, Fermi's young partner in experiment, "guaranteed the emission of neutrons. Neutron emission had to be observed experimentally and measured quantitatively." No such work had yet been done. It was, in fact, the new work Fermi had proposed to Anderson immediately upon returning from Washington. Which meant to Fermi that talk of developing fission into a weapon of war was absurdly premature.

Many years later Szilard succinctly summed up the difference between his position and Fermi's. "From the very beginning the line was drawn," he said. ". . . Fermi thought that the conservative thing was to play down the possibility that [a chain reaction] may happen, and I thought the conservative thing was to assume that it would happen and take all the necessary precautions."

(Even if he had an elegant technical argument, doesn't mean he would be right. Heisenberg had a short elegant argument for why the uranium critical mass would be 1 ton, but it was actually more like 10 pounds.)

Comment author: private_messaging 18 April 2013 08:36:42PM *  4 points [-]

There's something else. Most nuclei that will fission when irradiated with high energy neutrons (such as the ones from the neutron sources of the time) actually will not sustain chain reaction! That's the distinction between "fissile" and "fissionable". More here .

edit: Curiously enough, U238 can fission with neutrons produced by it's own fission, it just that most neutrons slow down before they fission any nucleus, and U238 can only be fissioned by fast neutrons. Had U235 had a bit shorter half life, or had evolution taken longer to make us, or had Sun formed later in the cloud, or the like, bench-top fission would still have been discovered (using neutrons from radium&lithium and U238) but we wouldn't have bomb anywhere near 1945 . This is quite interesting because of it's potential impact on Fermi's paradox. Nukes could be a lot harder to make.

Comment author: Luke_A_Somers 18 April 2013 09:45:27PM *  1 point [-]

Most don't, that's true. It only takes one.

There are at least four materials capable of sustaining a fission chain reaction, and any change to nuclear physics that is barely large enough to take those away would replace them with others. We are not even particularly near the boundary of it being possible.

Comment author: private_messaging 18 April 2013 09:52:05PM *  3 points [-]

1: When you only know that one fissions to begin with, you can't use that.

2: You need something naturally abundant enough, and (correct me if I am wrong) there's only one, it's U-235, with an outstandingly long half life for a fissile isotope, of 700 millions years, which is still not very long (other stuff doesn't hit even a million years). I wouldn't be surprised in the slightest if small adjustments to fundamental constants can change it to 2 and other small changes, 0.

edit: basically. No U-235, no bomb until you can bootstrap some sort of breeder using a particle accelerator, like, after decades and decades and decades of engineering, when all countries know the principle, but its too expensive.

Comment author: Luke_A_Somers 18 April 2013 10:09:17PM 0 points [-]

But they made Pu-239 in time for WW2. Did that rely on reactor-grade U-235? Even if they did use reactor-grade U-235 to make it, could they have just stuck it near some other neutron emitter?

Comment author: private_messaging 18 April 2013 10:16:25PM *  4 points [-]

Yes, Fermi engineered the nuclear reactor (and very carefully too, with very well thought out safety system for it, just in case there would be a positive feedback of some kind), using natural uranium and graphite. Other neutron emitters would be very very very expensive. edit: e.g. an accelerator would need ridiculous amounts of electrical energy. The lab emitter used radium in combination with lithium, beryllium, or some other light nuclei, which isn't a viable route either.

Comment author: Eliezer_Yudkowsky 18 April 2013 05:55:14PM 1 point [-]

It's wise to consider how non-hindsight might have been harder. It's even wiser to consider, for each training example, what general heuristics might've helped anyway.

But such obviously was not the case for uranium in its natural form, or the substance would long ago have ceased to exist on earth.

But there ought to be some unstable elements that hadn't fizzed by themselves in natural aggregations and purities, and many such, and these might be manipulated by humans. If something doesn't happen naturally, are you in a situation where you're likely to be learning about a randomly placed lower bound that's probably randomly far above you, or in a case where you're learning about a nearby lower bound that probably has some things right above it?

However energetically interesting a reaction, fission by itself was merely a laboratory curiosity.

Sounds like an absurdity heuristic; this is a bad general lesson to learn. "Laboratory curiosity" foresooth.

Only if it released secondary neutrons, and those in sufficient quantity to initiate and sustain a chain reaction, would it serve for anything more.

Which it did. So why should one have been confident that they didn't...?

"Nothing known then," writes Herbert Anderson, Fermi's young partner in experiment, "guaranteed the emission of neutrons. Neutron emission had to be observed experimentally and measured quantitatively."

The good old confusion between negative information and positive information of falsehood, perhaps?

Again, trying to avoid hindsight bias is not best done by inventing new cynical contrarian ideas that serve to steer your mind in the opposite direction of each training example. It would be better to look for truths that are hard to see, and not plausible falsehoods that by golly you ought to have believed. "It would have been just as easy to think Y as X, given Z" is a powerful argument against an alleged heuristic Z that supposedly could've told you X. "But it would have been perfectly rational to think Y!" is not how you want to train yourself.

Comment author: private_messaging 19 April 2013 04:29:02PM *  8 points [-]

But there ought to be some unstable elements that hadn't fizzed by themselves in natural aggregations and purities, and many such, and these might be manipulated by humans. If something doesn't happen naturally, are you in a situation where you're likely to be learning about a randomly placed lower bound that's probably randomly far above you, or in a case where you're learning about a nearby lower bound that probably has some things right above it?

This doesn't actually work...

There's only 3 isotopes to choose from. Th232 , U238 , U235 . Evidence that fission occurs probably came from U238 being fissioned by fast neutrons (or could just as well have). You can't make a bomb out of U238 , though, because it doesn't get fissioned by slow neutrons, and neutrons slow down quite rapidly, before they fission it enough. You need a nucleus so unstable, that it fissions when it captures a neutron. It must also fission immediately - if it fissions with a delay (if the mechanism of fissioning is that it captures the neutron, transmutes into something unstable that fissions later. Because neutrons do not leave tracks you don't immediately know that this is not what is going on).

There's precisely one naturally abundant isotope that you can use, it is U235 . Forget about plutonium, it'd be very expensive to make any without a reactor. Without naturally abundant U235 , no bomb anywhere near 1945 . It'd be something akin to an antimatter bomb - you need to make the material in a particle accelerator, which is ridiculously inefficient. (One could maybe make some plutonium in the particle accelerator, then use that plutonium in a breeder reactor to kick-start breeder economy, but the energy requirements for production of the seed plutonium are still utterly insane)

There's very little U235 because it has half life of 700 millions years. It is still ~4400 times the half life of the next most stable isotope that you could blow up, though (U233). Which has ~7x the half life of the next stablest (Pu239) , which has 3.2 x the half life of the next stablest (Am-243) . Which suggests to me that it's like "this cylinder will land with it's axis horizontal" prediction for something that turns out to be a coin rather than a pencil. (Frankly I do not understand why we even have any U235 at all. Could be some really weird anthropic reason that we don't know enough to deduce)

edit: doh, a correction. Neptunium-237 , albeit never used in bombs (critical mass 60kg - ish), can maybe be made into one, no doubt with great difficulty due to the size. It has half life of 2 million years. So the sequence of relative half-life becomes 3.2, 7, 12.5, 350

"But it would have been perfectly rational to think Y!" is not how you want to train yourself.

Well of course, but when 2 dice rolled sixes, you can't go on how it was irrational of Fermi to think the probability of such is 1/36 before anyone ever looked at the dice.

Comment author: CarlShulman 24 April 2013 02:24:03AM 6 points [-]

On the other hand, if you want a general heuristic that could've led Fermi to do better, I would suggest reasoning that previous-historical experimental proof of a chain reaction would not be strongly be expected even in worlds where it was possible, and that to discover a chain reaction to be impossible would imply learning some new fact of physical science which was not already known.

It's wise to consider how non-hindsight might have been harder. It's even wiser to consider, for each training example, what general heuristics might've helped anyway.

I think the comments have done a good job showing that learning that a chain reaction was possible would have also implied new facts of physical science, e.g. about neutron emission of the available isotopes, so the heuristic in the OP doesn't help much.

Comment author: private_messaging 24 April 2013 08:17:46PM 4 points [-]

Yes. Essentially, "they were still the default projection from what was already known." was dramatically untrue at the time of 10% assessment, when it was not known that any neutrons are produced in fission, or any similar processes. And once it became true, Enrico Fermi did rapidly do a very difficult calculation of the neutron multiplication factor, and concluded that self sustaining chain reaction is possible.

The imaginary world where people of Enrico Fermi calibre are unable to follow simple steps due to the extreme conclusions, is a shared fantasy of many, many crackpots.

Comment author: lukeprog 10 May 2013 06:24:23PM 1 point [-]

For others' reference: this begins on page 280 of The Making of the Atomic Bomb.