What is time? Does it have a beginning? Is it possible to travel through time? And what does it mean to say that time is relative? The following Plus articles and podcasts introduce you to the concept of time and to some of the riddles it poses for physicists.

For an introduction, read

• What is time?, which sheds some light on the idea that time is relative, as well as on one of the biggest questions in physics — why time has a direction. It also explores the intriguing idea that time doesn't exist. A version of this article is also available as a podcast.

For a little more detail, read

• What's so special about special relativity? which takes a more detailed look at Einstein's special theory of relativity and shows, using some straight-forward calculations, why your perception of time depends on how fast you're moving.

• What happened before the Big Bang? asks whether the Big Bang really marks the beginning of time, or whether there was something before it. The answers are mind-boggling!

• Is time travel allowed? shows that, in theory, the answer is yes. At least as far as we know.

• The puzzle of time takes a closer look at the "arrow of time" (the question of why it has a direction) with the help of some friendly philosophers of physics and the eminent Roger Penrose. You can also listen to our interview with them in a podcast.