What do we mean when we say that something "has happened"? It's quite hard to put your finger on it even in ordinary life, but for physicists the question of what constitutes an "event" is even more subtle. And it's intimately linked to another conundrum: what or who counts as an observer who witnesses events, and what is the role of observers in physics.
In the first video below physicist Anthony Aguirre briefly explores the difficulties with the notions of events and observers, and explains why the Foundational Questions Institute has organised a whole conference to investigate them. In the second video physicist Sean Carroll goes into a little more detail, looking at the mysterious quantum wave function, explaining why thinking about these things matters, and why our intuition needs to become more quantum.
To find out more about some basic features of quantum mechanics, see this article. And to find out more about the notion of time in physics (or lack of it), see this article. We'll be bringing you more reports from the conference in the near future.