I had a fascinating breakfast meeting with Rocky Kolb while he was in Berkeley recently giving a talk. Rocky formerly led the particle astrophysics group at Fermilab and is now chair of astronomy and astrophysics at the University of Chicago. He’s also a great public speaker and a bit of a celebrity: Rocky was Dr. December in the Stud Muffins of Science Calendar, circa 1996. One of the things we talked about over eggs and pancakes was the dual personality of physics. More than any other scientific discipline, physics depends on two varieties of scientist: the theorist and the experimentalist. Rocky is a theorist, but probably the most famous theoretical physicist was Einstein. His relativity theories, written early in the last century, kept the experimentalists who build observatories and particle accelerators busy for decades trying to confirm his theoretical predictions. But experimentalists like nothing better than coming up with observations about the universe, matter, or the inner workings of particles that catch theorists with their mathematical pants down. A sign about the doorway of condensed matter physicist Sid Nagel reads: Here is where theories come to die. Rocky’s take: “We’re smarter, better looking and generally taller.” But the shorter ones do occasionally exact their revenge. It happened in 1999 when Saul Perlmutter from the Lawrence Berkeley Lab announced that, billions of years after the big bang, the universe was actually accelerating rather than slowing down and collapsing under the weight of gravity. No theorist had predicted this finding and collectively they’ve been scratching their heads ever since trying to explain it. “This is the first time in 3000 years of cosmology that theorists are playing catch-up to the experimentalists. In the past there were more theories than observations could confirm or refute, but now there are more observations that the theories can not explain, ” Rocky told me. It’s certainly an interesting time to be a cosmologist and a sideline observer.