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AlanCrowe 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: AlanCrowe 18 April 2013 10:23:07PM 5 points [-]

I spot two holes.

First the elephant in the living room: The sun.

Matter usually ends up as a fusion powered, flaming hell. (If you look really closely it is not all like that; there are scattered little lumps in orbit, such as the Earth and Mars)

Second, a world view with a free parameter, adjusted to explain away vulcanism.

Before the discovery of radio-activity, the source of the Earth's internal heat was a puzzle. Kelvin had calculated that the heat from Earth's gravitational collapse, from dispersed matter to planet, was no where near enough to keep the Earth's internal fires going for the timescales which geologists were arguing for.

Enter radioactivity. But nobody actually knows the internal composition the Earth. The amount of radioactive material is a free parameter. You know how much heat you need and you infer the amount of Thorium and Uranium that "must" be there. If there is extra heat due to chain reactions you just revise the estimate downwards to suit.

Sticking to the theme of being less wrong, how does one see the elephant in the room? How does one avoid missing the existence of spontaneous nuclear fusion on a sunny day? Pass.

The vulcanism point is more promising. The structure of the error is to say that vulcanism does not count against the premise "ordinary matter is stable" because we've got vulcanism fully explained. We've worked out how much Uranium and Thorium there needs to be to explain it and we've bored holes 1000km deep and checked and found the correct amount. But wait! We haven't done the bore-hole thing, and it is hard to remember this because it is so hopelessly impractical that we are not looking forward to doing it. In this case we assume that we have dotted the i's and crossed the t's on the existing theory when we haven't.

One technique for avoiding "clever arguments" is to keep track of which things have been cross-checked and which things have only a single chain of inference and could probably be adjusted to fit with a new phenomenon. For example, there was a long time in astronomy when estimates of the distances to galaxies used Cepheid variables as a standard candle, and that was the only way of putting an absolute number on the distance. So there was room for a radical new theory that changed the size of the universe a lot, provided it mucked about with nuclear physics, putting the period/luminosity relationship into doubt (hmm, maybe not, I think it is an empirical relationship based on using parallax to get measured values from galactic Cepheid variables). Anyway along come type Ia supernovas as a second standard candle, and inter galactic distances are calculated two ways and are on a much firmer footing.

So there are things you know that you only know via one route and there is an implicit assumption that there is nothing extra that you don't know about. Things that you only know via a single route can be useless for ruling out surprising new things .

And there are things you know that you know via two routes that pretty much agree. (if they disagree then you already know that there is something you don't know). Things you know via two routes do have some power of ruling out surprising new things. The new thing has to sneak in between the error bars on the existing agreement or somehow produce a coordinated change to preserve the agreement or correctly fill the gap opened up by changing one thing and not the other.

Comment author: NancyLebovitz 20 April 2013 01:41:36AM 1 point [-]

I thought they did know that if the sun was solely dependent on chemical reactions, then it would have burned itself out more quickly than the age of the earth suggested.

Comment author: AlanCrowe 20 April 2013 12:55:16PM 1 point [-]

I was glibly assuming that Fermi would know that the sun was nuclear powered. So he would already have one example of a large scale nuclear reaction to hand. Hans Bethe won his Nobel prize for discovering this. Checking dates, This obituary dates the discovery to 1938. So the timing is a little tight.

As you say, they knew that the sun wasn't powered by chemical fires, they wouldn't burn of long enough, but perhaps I'm expecting Fermi to have assimilated new physics quicker than is humanly possible.

Comment author: AspiringRationalist 19 April 2013 08:08:08PM *  0 points [-]

Major nitpick: stars are examples of sustained nuclear fusion, not fission. The two are sustained by completely different mechanisms, so observation of nuclear fusion in stars doesn't really tell us anything about the possibility of sustained nuclear fission.

Minor nitpick: it's spelled volcanism, not vulcanism.

Comment author: AlanCrowe 20 April 2013 12:36:35PM 1 point [-]

I'm looking at the outside view argument: matter is stable so we don't expect to get anything nuclear.

But we look at the sun and see a power source with light atoms fusing to make medium weight ones. We already know about the radioactive decay of heavy atoms, and the interesting new twist is the fission of heavy atoms resulting in medium weight atoms and lots of energy. We know that it is medium weight atoms that are most stable, there is surplus energy to be had both from light atoms and heavy atoms. Can we actually do it with heavy atoms? It works elsewhere with light atoms, but that's different. We basically know that it is up for grabs and it is time to go to the laboratory and find out.

I fear that I have outed myself with my tragic spelling error. People will be able to guess that I'm a fan of Mr Spock from the planet Vulcan ;-(

Comment author: gwern 19 April 2013 08:55:30PM 0 points [-]

Major nitpick: stars are examples of sustained nuclear fusioin, not fission.

Quoted for irony.

Comment author: AspiringRationalist 20 April 2013 12:03:09AM 0 points [-]

I'm not sure if pointing out my typo was your intent there, but you caused me to notice it, so I fixed it.