EarthSky | What is time, and why does it move forward?

Posted by EarthSky Voices September 19, 2021 and
By Thomas Kitching, UCL

We think of the universe as having a timeline, a point at which it began, until now. But how much do modern cosmologists really know about time? Image via Alex Mittelmann/ Wikimedia.

A bell-shaped funnel with grid lines on it and ‘Big Bang’ written at the narrow end.

Imagine time running backwards. People would grow younger instead of older and, after a long life of gradual rejuvenation – unlearning everything they know – they would end as a twinkle in their parents’ eyes.

That’s time as represented in a novel by science fiction writer Philip K. Dick but, surprisingly, time’s direction is also an issue that cosmologists are grappling with.

While we take for granted that time has a given direction, physicists don’t: most natural laws are “time reversible” which means they would work just as well if time was defined as running backwards. So why does time always move forward? And will it always do so?

Source: EarthSky | What is time, and why does it move forward?