## Articles

Observers are, of course, vital in physics: we test our theories by comparing them to our observations. But in cosmology, as Jim Hartle explains, we could be one of many possible observers in the Universe and knowing which one we are is vital in testing our theories.

Play with our applets to explore the conic sections and their different definitions.

To celebrate the release of more English translations of Einstein's papers, we revisit one of his previously unknown models of the Universe.

Anything involving bunny rabbits has to be good.

A Klein bottle can't hold any liquid because it doesn't have an inside. How do you construct this strange thing and why would you want to?

What is 1-1+1-1+1-1+...? How infinite sums challenge our notion of arithmetic.