We think of time as linear. Tick follows tock, the inexorable marching rhythm of the universe going forward in one straight line...
The faster you travel, the slower the second hand ticks for you.
One twin is sent on a train that travels at incredible speeds. The other remains at home, moving about at a speed far slower than the first twin.
When the twin who traveled by train returns home, both twins compare the time on their clocks. Less time will have passed for the twin who took the train, relative to twin who stayed at home.
This phenomenon is known as time dilation.
It's a consequence of the nature of spacetime as described by the Theory of Relativity - one of the most celebrated works in science, Relativity was published by Albert Einstein in 1905 and describes the motion of objects relative to one another.
The thought experiment above is called the "twin paradox", and it, along with other parts of the Theory of Relativity, forever changed our understanding of space and time.
Instead, it exists to test or illustrate an idea in some way and often turns abstract thought into something that is more easily relatable. Thought experiments were common in ancient Greece and existed long before mathematics as we know it today. Taking imaginary scenarios that help us to understand a concept, they’ve been a cornerstone of philosophy and science from early recorded history right through to Galileo, Newton and Einstein. Many, such as the eponymous Schrodinger's Cat, have become famous and are often referenced outside of scientific circles and in pop culture.
Relativity revolutionised science and replaced theories that had been in place since the days of Newton, which by then were over 200 years old. Einstein's work ushered in a new age of nuclear research and astrophysics, and was fundamental to the discovery of phenomena such as black holes, neutron stars and research into gravitational waves.
Measurements have been taken from extremely precise clocks onboard jets flying at high altitudes. When they are compared to similar clocks on the ground, the clocks will show different times.
He was also an accomplished violinist, a cultural icon and a Nobel prize winner. His most important work, the General Theory of Relativity, is one of the two pillars of modern physics (the other being quantum mechanics).
German born, Einstein settled in the United States in opposition to Hitler’s rise to power. He was immensely popular in his adopted home. Treated as a celebrity, he was often accosted on the street and asked to explain his famous formula, E=Mc2. In response, he would apologise and pretend to be someone else, saying: "Always I am mistaken for Professor Einstein".
For time dilation to have a noticeable effect you’d have to be travelling close to the speed of light, which is one billion km/hr. The DART travels at around 50km/hr, so even though time is slowing down for you, the effect is incredibly tiny.