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turchin comments on Doomsday Argument Map - Less Wrong Discussion

6 Post author: turchin 14 September 2015 03:04PM

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Comment author: Avo 14 September 2015 09:04:27PM -2 points [-]

I think it is an excellent idea to try DA logic in other domains.

Example: A a biologist prepares a petri dish with some nutrients, and implants a small colony of bacteria. The question is: will this colony grow exponentially under these conditions? According to DA logic (with a reference class of all bacteria that will ever live in that petri dish), the biologist does not need to bother doing the experiment, since it is very unlikely that the colony will grow exponentially, because then the current bacteria would be atypical.

To the best of my knowledge, this sort of DA logic is never used by scientists to analyze experiments of this sort (or to decide which experiments to perform). I believe this casts severe doubt on the validity of DA.

Comment author: turchin 14 September 2015 09:11:44PM 3 points [-]

Your example is not DA-style, because observer is not self-sampled here. Observer is not bacteria in this experiment. (By the way, most from all existing bacteria is not starters of exponential colonies)

I may suggest another experiment: you tell me your age and I will tell your life expectancy based on DA-logic.

Comment author: Avo comment score below threshold [+] (7 children)

Comment author: turchin 14 September 2015 09:57:09PM 2 points [-]

(I did not downvote you)

Look, we could replace "self sampling" with "random". Random bacteria (from all existing bacteria on Earth) will not start exponential growth. Infinitely small subset of all bacteria will start it. There is no difference between prediction of statistic and biologist in this case. DA is statistical argument. It just say that most of random bacteria will not start exponential growth. The same may be true for young civilizations: most of them will not start exponential growth in the universe. But some may be.

Comment author: Avo 14 September 2015 10:24:54PM -2 points [-]

So do you agree with me that, in the experiment I described (a biologist sets up a petri dish with a specific set of initial conditions, and wants to find out if a small bacteria colony will grow exponentially under those conditions), DA logic cannot be applied (by either the biologist or the bacterium) to judge the probable outcome?

Comment author: turchin 14 September 2015 10:44:56PM 2 points [-]

DA should be applied to the situation where we know our position in the set, but do not know any other evidence. Of course if we have another source of information about the set size it could overweight DA-logic. If in this experiment the substrate is designed to support bacterial growth, it have is very strong posteriory evidence for future exponential growth.

But if you put random bacteria on random substrate it most likely will not grow. In this case DA works. DA here says that 1 bacteria most likely will have only several off springs, and it is true for most random bacteria on random substrates.

So, will DA works here or not depends of details of the experiment with you did not provide.

Comment author: Avo 14 September 2015 10:57:32PM -1 points [-]

OK, let me rephrase the question.

The biologist has never heard of DA. He sets up the initial conditions in such a way that his expectation (based on all his prior knowledge of biology) is that the probability of exponential growth is 50%.

Now the biologist is informed of DA. Should his probability estimate change?

Comment author: turchin 14 September 2015 11:12:44PM 2 points [-]

Probably, not, as he has a lot of information about the subject. DA is helpful in case if you don't have any other information about the subject. Also DA is statistical argument thereby it could not be disproved by counterexample. It is always possible to construct a situation where it will not work. Like some molecules in the air are not moving, despite the fact that median velocity is very high.

It may be used in such problems as bus waiting problem (variant of Laplace sunrise problem). If last bus was 5 minutes ago, want is the probability that it will come in next 1 millisecond, next 5 minutes? next 1 year?

Comment author: Avo 15 September 2015 06:14:20AM -1 points [-]

At least some DA proponents claim that there should always be a change in the probability estimate, so I am pleased to see that you agree that there are situations where DA conveys no new information.

Comment author: turchin 15 September 2015 08:25:58AM 2 points [-]

The situation would change, if I were Adam, first man in the world, and a priory will be able to start exponential human growth with P= 50 per cent probability (or think so). After finding that I am Adam, I would have to update this probability to lower. The way I update depends of sampling method - SSA or SIA but both result in early doom. SSA says that I am in short world. SSI said that my apriory estimation of universal distribution of short and long civilization may be wrong. It would be especially clear if apriory P would not 50 percent, but say 90 per cent.

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