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

As much as we would sometimes like to, particularly those of us who would like to return to more normal times before COVID-19, we cannot go back in time. For us, time marches relentlessly on. Which makes it so surprising that, according to the equations that govern fundamental physics, time doesn't only have to move forward, it could move backwards too. "In the mathematics everything is so beautiful and symmetric," says Marina Cortês, a cosmologist from the Royal Observatory, Edinburgh. "The fundamental equations [of physics] are reversible with time."

You can think of this as watching a film playing forwards or in reverse. Watching a video of some events, such as spilling a glass of milk, it's easy to tell when the film is running forwards in time (and the mess ensues) and when it's running backwards (when all the milk leaps back into the glass). But this event is the culmination of many physical processes each governed by different equations: such as those governing the gravity and the force applied to the glass and its contents, and the fluid dynamics governing the behaviour of the milk itself.

But at a fundamental level of physics, for example at the level of Newton's laws of motion that you learnt in high school, things start to look a little different. Suppose you move an object by applying a force. Then by Newton's second law of motion, an equal force applied in the opposite direction will reverse the motion. If you were watching a film of the object's motion, say a ball gently rolling across a floor, you wouldn't be able to tell if the film was playing forwards or backwards. Fundamental physics has no preference for time moving forwards.

Symmetry of time is one of the underlying arguments for the block time. Block time is all of time – the past, present and future – existing in a four-dimensional block of spacetime known as a block universe. If we live in a block universe, our perception of time as only ever moving forward at a steady rate is just an illusion, a "mathematical artefact of the physics". This might sound surprising but this counterintuitive concept of block time is a direct result of one of the most successful theories of physics: Einstein's general theory of relativity, which we explored in previous advent doors and is accepted by the scientific community as the most accurate description of the true nature of the Universe at the cosmological scale. You can read more about block universes and block time, and watch our interview with Marina Cortês, in our package of articles Time in a block universe.

This year's advent calendar was inspired by our work on the documentary series, Universe Unravelled, which explores the work done by researchers at the Stephen Hawking Centre for Theoretical Cosmology and is available on discovery+. Return to the 2020 Plus Advent Calendar.