## 15 September 2008

### Doogie Howser on the Copernican principle

"You never make plans with a girl for further in the future than you've been going out." -- Barney Stinson, the Neil Patrick Harris character on How I Met Your Mother. (Neil Patrick Harris played Doogie Howser on the show by that name.)

This is an example of the "Copernican principle", which I've written about before -- if you assume that there's nothing special about now, you're equally likely to be in the second half of the relationship as the first. And if you're in the second half of the relationship, and you violate this rule, you'll have broken up before the plans happen.

Anonymous said...

That's brilliant. Another reason to wish I'd started watching HIMYM back when it started...

Mark Dominus said...

This reminds me a bit of a talk I attended back in a previous geological era. Suppose you're learning to ski, and you're not sure if you'll like it. Clearly, you shouldn't buy skis your first time out, since you might hate skiing, and then you'll be stuck with skis that you don't want to use. Clearly you should rent skis the first time.

But if you do like skiing, eventually you should save yourself the rental fees by buying skis.

But when should you stop renting and buy the skis?

In the worst case, you will buy the skis and then change your mind and quit skiing. If ski rental cost is R and ski purchase cost is P, you can minimize this worst case by buying the skis for the ceiling(P/R)'th ski trip. Then the most expensive possible outcome within a factor of 2 times as expensive as the optimal outcome.

Joel said...

The "Copernican Principle", as you've stated it, would seem to require that you not know how long your relationship has been going on, which seems unlikely.

Furthermore, it's obviously false in this context. If you've been married for (say) 50 years, then the probability you're in the second half of your relationship is approximately 100%. If you've been dating for two days, then the probability you're in the second half of your relationship is probably less than 10% (assuming, which I do, that very few relationships last 3 or 4 days).

We know lots and lots about the distribution of relationship lengths; it would make more sense to use that knowledge than to claim ignorance of it.

Anonymous said...

assuming, which I do, that very few relationships last 3 or 4 days

Clearly someone has never been to public school.

whyambee said...

This is in fact a reformulation of a simple corollary from the renewal theory, that far out, the process is invariant with respect to time reversal.