The scene captured by Siegfried Sassoon in his poem Attack is a terrifying one.

In the purple light of dawn, tanks creep forward through the Flanders mud into no man's landThe unoccupied ground between two armies. The phrase is particularly associated with trench warfare in WW1, but dates back to the Domesday Book, when it meant land that belonged to nobody.. Weighed down by equipment, the British soldiers prepare to leave their trenches and face the enemy fire, their faces masked by fear - "while time ticks blank and busy on their wrists".

Sassoon recognised that though we think of time as something definite, following scientific rules that everyone has to fit in with, our own experience of it is highly subjective. A soldier who is about to risk his life and a civilian getting lazily out of bed will feel it passing in completely different ways.

The lockdown has focused many people's minds on this phenomenon. The author Ian McEwan was inspired to give a talk on BBC radio about how time "slows in an emergency, speeds up as we age, or lengthens as we wait boiling with impatience for an overdue train".

McEwan argued that when one day is much like another, "time compresses and collapses on itself". An ex-convict who had spent 20 years in jail claimed that his sentence "went in a flash".

Yet while a lockdown week can go very quickly, a day within it can feel very long. To McEwan, this is a wonderful opportunity for introspectionExamining one's own thoughts and feelings.: to experience "the miraculous fact of your own consciousness - or catch a glimpse of yourself that was partially obscured by the daily round".

In particular, when you cannot make plans for the future, you tend to think about the past, and how you travelled through time "by tiny stepping stones from your five-year-old self to the person you are now".

This sense that time is flexible overlaps with Einstein's theory of relativity.

Einstein argued that the speed at which time passes is not the same throughout the Universe: it depends on where you are and how fast you are travelling. A 15-year-old who left Earth on a spaceship tomorrow and travelled for five years at just under the speed of light would be aged 20 when he got back - but would find that the twin he had left behind was 51 years old.

There is, McEwan argues, an upside to the present crisis. "My hope is that we can take from this extended tragedy a memory and lesson in timelessness and stillness."

Some say that time is an artificial construct, dreamed up by humans for their convenience. It is only comparatively recently that we have used clocks to organise ourselves; dividing the year up into days of equal length, regardless of when the sun rises and sets, would make no sense to animals. For them, life is governed by the seasons, so it is circular rather than linear.

Others argue that time existed long before we did. It is essentially a measure of change, and the Universe has been changing ever since it came into being with the Big Bang. The Earth's orbit around the Sun is entirely predictable so, by basing our calendar on it, we are merely recognising an inescapable fact. Time may be relative but, if it stopped, life would stop too.