Luke_A_Somers 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: 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: gwern comment score below threshold [+] (5 children)

Comment author: private_messaging 24 April 2013 09:23:48PM * 3 points [-]

What do you think he should/could have done about it? Try to prevent it's creation? Try to speed up it's creation? It's 10% on the fission releasing secondary neutrons, I'd think. Successful bomb requires other conditions to be met.

Easier to illustrate on example of hypothetical hafnium isomer initiated bullet sized thermonuclear bombs. What is the probability those are possible? That those will be made in next 5 years? Are you buying stocks on basis of that probability?

Comment author: gwern 24 April 2013 10:00:00PM 1 point [-]

He could have done either, yes. He was a famous scientist, his Nobel in 1938 massively increased his credibility, his work was cited in the Einstein-Szilard letter which triggered the American bomb project and the first meeting of the bomb committee was to fund more work by Fermi. If Fermi could not have either slowed down or sped up the development of the bomb, it's hard to think who could have!

Are you buying stocks on basis of that probability?

I wasn't aware I assigned 10% either to their possibility or to the possibility of a critical but as-yet unknown requirement for their possibility.

Comment author: private_messaging 25 April 2013 04:14:56AM * 1 point [-]

He could have done either, yes.

Precisely. Now, which he should have done on basis of 10% possibility? There's a non-negligible cost as well, and 'business as usual' still leads to update from 10% to much larger probability, just a bit later, and arguably counter balanced by potential returns of other research done on the way. It's really easy to misestimate expected utility differences here by re-use of hindsight.

I wasn't aware I assigned 10% either to their possibility or to the possibility of a critical but as-yet unknown requirement for their possibility, a bit later but somewhat counterbalanced by various probabilities of discovering something else of interest.

Suppose you did assign 10% probability to hafnium triggering. You still have rest of the mini-nuke as a big conjunction, which is far clearer than for the nuke because for the nuke there's all this pop sci that makes it sound a lot easier than it actually is, plus having cherry picked the nuke that was actually made in time as an example, a lot of bad learning can be done.

Comment author: gwern 26 April 2013 04:20:19AM 1 point [-]

Now, which he should have done on basis of 10% possibility?

I'm not sure what would be the best tack to take; knowing everything I do about the subsequent course of history, I think delaying the bomb program as much as possible would have been best, but I don't know if that was the best decision one would've made at the time.

and 'business as usual' still leads to update from 10% to much larger probability, just a bit later

Secrecy was already a major issue; business as usual could well lead to a fatal leak of information to the Germans who still were probably the first or second greatest physics establishment in the world and who Fermi must have known were also working on nuclear issues and were aware of the possibility of nuclear explosives; relevant papers were being kept secret and classified, etc. No, anyone at the time knew perfectly well that this was not a harmless issue with no consequences or actions which could be taken.

Well, anyone at the time if they weren't pulling a Fermi and essentially acting as if the 10% possibility didn't exist... Whether one decided to retard or advance the bomb, it seems highly unlikely to me that acting exactly as if he hadn't figured ~10% would just happen to turn out to be the right decision! Listen, I'm not the one claiming Fermi ignored the possibility, it's right there in the reference:

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."

I don't criticize him for any sort of Pascal's wager, because 10% is a really effing big probability. Moving on...

Suppose you did assign 10% probability to hafnium triggering. You still have rest of the mini-nuke as a big conjunction

Suppose I did assign 10%. Then you cannot defend my inertia and ostrich attitude at 10% because the actual probability is lower! So make up your mind, is Fermi right to put his head in the sand at 10% or is the actual probability lower and he should be doing something else? Or are you arguing that 10% is a magical threshold at and below which head-in-the-sand is always the right response?

I think that is taking anti-Pascal-mugging way too far: if a doctor told me I had a 10% chance of dying this year of a rare disease, I would hysterically shove all the money I had at the doctor to make it go away and start drawing up my will and drafting letters for my family, I wouldn't go 'hm, this is very interesting, perhaps I should apply for more government money grants to study this fascinating possibility.'

Comment author: private_messaging 26 April 2013 04:55:53AM * 3 points [-]

I'm not sure what would be the best tack to take; knowing everything I do about the subsequent course of history, I think delaying the bomb program as much as possible would have been best, but I don't know if that was the best decision one would've made at the time.

This is another issue with wagers on speculative propositions, its unclear what actually should be done, besides secrecy which they'd already apply to any results involving secondary neutrons upon obtaining said results. edit: though, French obtained and published these results anyway. Which leaves the issue of carbon's neutron absorption cross-section, which Fermi measured as late as 1940 , after secondary neutrons (and thus well after the quote, at the point when self sustaining chain reaction was much less speculative).

Sure, the expected |utility| is huge, but expected utility is only huge when you can make predictions of what's best with good accuracy.

Suppose I did assign 10%. Then you cannot defend my inertia and ostrich attitude at 10% because the actual probability is lower!

And how'd you know that, I wonder? You have gamma ray emitting nuclear isomer that you can maybe trigger with x-rays. You have another nuclear isomer that can be triggered like this. It's not like fission's unknown-at-the-time secondary neutrons which are maybe released promptly (or maybe stay in the decay products which then undergo beta decay).

By the way, right here is your problem. "actual probability", which doesn't even meaningfully exist in such cases. You guys flip between subjectivist probability that lets you assign arbitrary numbers to the end of the world, and some intuitive notion of actual probability which is under which acting upon probabilities is the sanest thing.

if a doctor told me I had a 10% chance of dying this year of a rare disease

But the doctor did not tell that. Secondary neutrons do not necessarily imply the self sustaining chain reaction, which doesn't necessarily imply the bomb, which doesn't necessarily imply any effect on the course of the war! (Specifically the war that these European scientists cared about). This makes me wonder what exactly they were even talking about - maybe Szilard was worried they'd irradiate themselves fatally or blow themselves up doing experiments, and Fermi was like "Nuts!".

edit: also some background on quote is necessary - exact date down to the day, as the conclusions were moving really quick at that point.

edit2: a timeline: http://oznucforum.customer.netspace.net.au/ANFINFO8.pdf

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

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