# Story about a mathematician, a dinner party, and the three-body problem

I remember dimly hearing a story, coincidentally also at a dinner party, and I was trying recently to track the details down with no success. I was hoping someone here might have also heard this story and could help track down a source.

The story bears some resemblance to the story about "turtles all the way down". In this story, told as an historical account about a particular mathematician's motivation for studying the three-body problem, the mathematician, perhaps Lagrange, perhaps d'Alembert, perhaps Poincaré, is over at a dinner party where he is the only mathematician. An older woman, perhaps a dean's wife, asks him if he knew about the mysterious planet sharing an orbit with the earth but always diametrically opposed from it, blocked from view by the sun. He tries to convince her that this is preposterous, and that we would know if there was such a planet by the effect on our own orbit. He can't seem to convince her, but--even worse--when he thinks about it harder at home after dinner he can't seem to quite convince himself either. This inspires the mathematician to study the three-body problem in full depth and to show rigorously that the kind of setup described by the woman would be unstable.

It is very possible that this account is apocryphal. Nevertheless, I would like to identify a source where it was told, or where it is attributed. Is there such a source?

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I've never heard this story before, but it was Poincare that worked out the fact that the solutions to the three body problem are chaotic. –  Jerry Schirmer Aug 11 at 21:33
It sounds like you (Yoav) are talking specifically about lagrangian points. I haven't heard the story though. –  NowIGetToLearnWhatAHeadIs Aug 11 at 21:43
This question appears to be off-topic because it is about a story rather than actual physics. –  ACuriousMind Aug 11 at 21:47
I think it's on-topic. I think the term story here could be rewritten as historical account of the motivation of the $N$-body problem. (Initially I thought story meant a joke of some sort until I read further.) –  BMS Aug 11 at 21:49
@BMS: The question, as written, is just asking about anything related to the story. It could become on-topic if narrowed to something like "Who discovered the instability of this solution" or something like that, but currently, the main focus of the question is unclear. –  ACuriousMind Aug 11 at 21:52