It is the invisible presence that governs your world. Trailing you like an unshakable shadow, it ticks and tocks incessantly - you can sense it in your heartbeat, in the rising and setting of the sun, and in your daily rush. It brings order to our lives through the categories of past, present and future.
There is nothing with which we are so familiar with and yet when you try to pin it down you find only a relentless torrent of questions. Why does time appear to flow? What makes it different from space? What is it?
Physicists have also struggled to understand what time really is. In fact, they are not even sure it exists at all. Some researchers increasingly suspect that time is not a fundamental feature of nature, but rather an artifact of our perception. One group has recently found a way to do quantum physics without invoking time, If this approach is correct it reinforces the theory that time is an illusion, and that we may need to rethink how the universe works.
According to general relativity, time is stitched together with space to form four-dimensional space-time. The passage of time is not absolute; instead, time differs from one frame of reference to the next, and what one observer experiences as time, another might experience as a mixture of time and space.