Why does time only ever move in one direction? We talk to philosophers of physics Jeremy Butterfield and David Wallace, as well as the eminent Roger Penrose about the puzzle time poses to physicists and what it has to do with the Big Bang and the second law of thermodynamics.

Everyone knows what time is. We can practically feel it ticking away, marching on in the same direction with horrifying regularity. Time has enslaved the Western world and become our most precious commodity. Turn it over to the physicists however, and it begins to morph, twist and even crumble away. So what is time exactly?

This podcast featuring Paul Davies, a theoretical physicist and cosmologist at Arizona State University and Director of BEYOND: Centre for Fundamental Concepts in Science, explores this difficult question and accompanies our What is time article.

Did the Big Bang mark the beginning of time? Not if we live in a bubble multiverse!

On this, the third day of advent, we explore some strange consequences of general relativity on our concept of time.

Atomic clocks are the best time keepers, but need to be kept incredibly cold. Paradoxically, the cooling is achieved by shining light on them.

Time and space form the stage on which events unfold. But what if the stage itself becomes part of the action? We take a brief tour through the history of space and time and see how ideas have changed since the time of Isaac Newton.

Fundamental physics says time is symmetric - so why does time move forwards for us in a block universe?

Is time real? Are we just puppets living out a future already written? Marina Cortês explains why she thinks time is fundamental and that we don't live in a block universe.

Marina Cortês is one of a growing number of physicists who believe time is fundamental. We ask her about the alternatives theories to the block universe, where time comes first.

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?

Why today is 24 hours and 1 second long.