Science has been coming to grips with the craziness of quantum mechanics for the last century or so, and one of the first guys to help people start wrapping their heads around it (or throwing up their hands completely) was Niels Bohr, born 129 years ago in Copenhagen on October 7, 1885. Bohr rubbed shoulders with the titans of early 20th-century physics, having friendly arguments with Einstein and critiquing papers by Werner Heisenberg. He made lasting contributions to physics like the Bohr atomic model, that describes the behavior of electrons around a nucleus, but his principle of complementarity is what will keep you up at night if you let it.

To put it bluntly, quantum mechanics (the study of reality at a really small level) is insane. The tiniest constituents of matter and energy act nothing like the macro-bodies (chairs, jet engines, Simon Cowell, etc.) that they construct. You'd send quantum mechanics home for being drunk if the existence of everything didn't depend on it. Bohr determined that quantum mechanics described "stuff" (for lack of a better term) that sometimes acts like a good ol' solid particle, and at other times acts like a misty, spread-out wave. It gets weirder: Light is a perfect example of this wave-particle duality, and can even demonstrate both properties in the same observation! It's enough to make your macroscopic head explode, but Bohr's theory is completely consistent with every observation made in the funky quantum world. His idea that objects can take on properties that seem contradictory was both a scientific and philosophical problem, and he argued for it in both domains. Combined with contemporary work like Heisenberg's uncertainty principle, Bohr's ideas gave us the beginnings of a road map to start charting the quantum realm.

Back here in the non-quantum world, he did some pretty cool stuff too. Half-Jewish on his mother's side, he helped refugees from Nazi-occupied countries find temporary jobs in Denmark in the 1930s, before fleeing to Sweden ahead of an imminent arrest by the Germans in 1943. He eventually consulted with nuclear experts in Britain and America, including the men working on the Manhattan Project. He claims to have had no role in developing the atomic bomb, but Robert Oppenheimer credited him with an important technical contribution.

After the war, Bohr helped develop the research organization CERN, which is still working to unlock the secrets of reality with the Large Hadron Collider. He died of heart failure in 1962. Bohr was the recipient of several awards during his life, including the 1922 Nobel prize in physics, but he also has an asteroid and a lunar crater named after him, as well as the synthetic element bohrium. To get an element named after you, you might be a great scientist, philosopher, or citizen…or you might be all of them at once. That's a complementarity principle anyone can understand.

Via http://www.nbi.ku.dk/english/namely_names/2011/anniversary_of_the_niels_bohr_archive/Bohr-200.jpg