October 20, 2021

I, Science

The science magazine of Imperial College

Time is so integral to our experience of life that we could be forgiven for thinking it is a simple concept. But many physicists are intrigued by the question of what exactly time is...

clocks header

The concept of time feels so natural that we often forget just how peculiar it is. Time governs all our everyday lives. We keep track of it, waste it and wish we had more of it. It seems to go fast when we’re having fun, and drag when we’re not. But figuring out what exactly ‘time’ is remains one of the most difficult problems in physics. 

One of the biggest misconceptions is the notion of ‘now’. What we perceive as happening in the present is actually based on things that have happened in the past. This is because light does not travel instantaneously but takes time to reach us.

The Sun is about 93 million miles away from Earth (that’s about 150 thousand million metres) meaning that even though light travels at just under 300 million metres per second through space it still takes light from the Sun just over 8 minutes to reach us. That means we actually see the Sun how it was 8 minutes ago. Looking into the night sky we see the stars how they were millions or billions of years ago, as they are so far away the light has only just reached us. Even reading these words from your computer screen there is a delay of about 3 nanoseconds – so small you never notice it, but the fact is this intake of information is not instantaneous.


Everyday life is defined by hours, minutes and seconds. Scientific experiments regularly use much smaller intervals like nanoseconds, with some precise time measurements now on the attosecond (10-18) scale. This is still a relatively large division of time – the smallest unit of time in current theoretical physics is the Planck time of 10-43 seconds. It’s almost impossible to imagine how small the Planck length is (although try this). To put in in some kind of context, imagine a ‘grain’ of time being blown up to be the size of a grain of sand. If we also blew up real grain of sand in the same proportion they would  end up being 10000 trillion times the size of the Milky Way.

Einstein fundamentally altered the way we think about time. His theory was that time is a dimension just like the three spatial ones, and they are intertwined into a four-dimensional ‘spacetime’ that forms the fabric of the universe. Moving through  the time dimension is what gives us the feeling of time passing.

One of the well known laws of physics is that you can’t travel at, or faster than, the speed of light. More accurately, you can’t travel through space at the speed of light. This restriction doesn’t apply to that of time. Because space and time are together as 4D spacetime, the faster you travel through space, the slower you travel through time. This means that if you’re stationary in space, you’re travelling through time at the speed of light.

The nature of time is even stranger. It’s not just about how fast you’re moving, but also what you’re close to that influences it. This is because massive objects such as stars and planets – in fact anything with a gravitational field – warps the spacetime around it.

Because the Earth’s gravity warps spacetime, and this effect diminishes further from the planet’s centre of mass, each day our heads age around 10-11 seconds more than our feet. Live until 100 and that adds up to around 365 nanoseconds. This effect has been most notable in space missions – the Russian cosmonaut Sergei Krikalev has spent 803 days on space stations, meaning he’s around 21 milliseconds younger than he would have been had he stayed on Earth.

Black holes warp spacetime

In the 1960s Irwin Shapiro  conducted experiments that showed the Sun’s warping of spacetime causes a signal sent between Earth and Mercury to  travel at a different time to  that expected without this effect This validation of Einstein’s theory is called the Shapiro Time Delay.On earth the effect is miniscule, but out in the universe the vast mass of the stars and galaxies warp spacetime so much that time ticks all over the place.

There are even things that can warp spacetime so much they appear to not just slow down time, but stop it altogether. Black holes are phenomena that warp spacetime so much that they appear to stop time. Observing someone’s watch as they moved towards a black hole it would be seen to tick slower and slower, until when they reach its event horizon time would appear to stop completely.

Our intuition about time is that we live in the present. The past has happened and no longer exists, and the future is yet to happen. But if time is simply a dimension like those of space, as Einstein theorised, then we can come up with a completely counter-intuitive idea: that time and all events have always existed and always will. The past, the present, and the future all exist simultaneously.

Think of this as like your journey from home to work. As you travel away from your home, it still exists in space, and similarly past events still exist in time. Your work place already exists and you are simply travelling through space to it. Similarly future events already exist and you’re travelling through time to when they occur. Travelling through time is no different to travelling through space.

This idea that the future is as real as the past is poignant but doesn’t seem quite right. That’s because it’s a classical viewpoint and doesn’t take into account quantum mechanics. In the QM world nothing is certain and the future least of all. Teaming Einstein’s ideas with QM should give us a better grasp on the true nature of time.

An alternative theory is that time is not the smooth structure proposed by Einstein, but rather that it is granular. Intervals of time are like grains of sand, with the passing of time able to be thought of as like sand flowing through an hour glass.

If spacetime is grainy then it could grow grain by grain, event by event. This is not possible in Einstein’s vision, where spacetime is a continuum with all events already and always in existence. This theory is a bit strange, but does take into account the uncertainty of quantum mechanics which we know defines how the universe behaves at its smallest scale.

Time is a fundamental concept, governing not only our everyday lives but the entire universe. It’s remarkable that we still know so little about it. Will we ever understand the true nature of time? That’s uncertain. Or perhaps it isn’t.

IMAGES: Leo Reynolds, Andrew Coulter Enright and Alan Cleaver, via flickr.