Most of us know what we mean when we say that something has happened. For theoretical physicists, however, the definition of what constitutes an event is more challenging, not least because notions like time and space become hazy when you go right down to the foundations of current theories. One way around this problem is to define an event without reference to time and space. Laura Mersini-Houghton of the University of North Carolina at Chapel Hill explains.

This video is part of our Stuff happens: the physics of events package. You can read more about the reasons why, when you go down to the foundations of physics, our ordinary notion of events goes out of the window, in our article Is it happening?. You can also read the article It's happened! which accompanies this video.