In this busy time why not put your feet up and learn some maths in just a few minutes with this year's Plus advent calendar. Now what's behind today's door?

Can the very act of observing something change what's being observed? This series of articles and videos explores some basic questions about the role of the observers in physics.

Traditionally, observers play no special role in physics. Like bird watchers in a perfect hide, we observe the outcome of experiments, or gaze at the stars through our telescopes, taking no part in the action. Modern physics, however, tells a different story ... find out more with these articles and videos.

We all know what we mean when we say that something has happened — well, all except for theoretical physicists. That's because the notion of a "happening" becomes fuzzy when you go down to the foundations of the science. Find out more with these articles and videos.

Physics tells us that we live in a block universe, containing all of the past and all of the future. What does this perspective mean for our understanding of time, events, and free will?

Five favourite problems inspired by Leonhard Euler, one of the greatest mathematicians of all time.

Current physical theories suggest that beautiful symmetries underly the fundamental particles and forces of nature. We describe those symmetries using an analogy from economics, and even rediscover the famous Higgs boson.

Schrödinger's cat has come to symbolise everything that is strange about the famously strange theory of quantum mechanics. Here's an accessible introduction to one of the weirdest, yet most successful, physical theories we have.