Are there parallel universes? Universes in which, rather than reading this article, you are still asleep; in which you are happier, unhappier, richer, poorer, or even dead? The answer is "possibly". It's a controversial claim but one that has won more and more followers over the last few decades.
Most of us know what we mean when we say that something has happened. For theoretical physicists, however, this isn't an easy question. Sean Carroll of the California Institute of Technology explains why it's hard to define events and what to do about it.
Quantum mechanics seems to suggest that passive observation is impossible: the very act of looking at something can change what's being looked at. In this video interview, we talk to David Wallace and Adrian Kent to find out more.