About this series

In this exclusive four-part series, physicists Jeff Forshaw and Brian Cox introduce us to the biggest ideas in modern physics and cosmology. What is the nature of time? What is everything made from? What happened before the Big Bang, and how will the Universe end? We’ll delve into the deepest questions concerning the very essence of space, time, matter, and reality itself…

Time does not tick at a steady rate across the Universe – in some places it ticks faster 

this is the ‘twin paradox’, where an astronaut departs from Earth, leaving her twin brother behind. She zips around for a bit in her super-fast spaceship and then lands back on Earth a year later, only to find that many more years have passed back home, and her brother is now an old man. This is exactly the kind of  weirdness that must be true if Einstein is right – though we aren’t aware of it in our everyday lives because we can’t zip around fast enough, and so are tricked into thinking time is more constant than it actually is. The fact that a moving clock does not tick as fast as a stationary one is actually quite easy to demonstrate. First, imagine a clock made from two parallel mirrors, between which a particle of light or ‘photon’ bounces  back and forth (see ‘The key idea’, right). Imagine you have one of these little clocks in your hand, and that you can watch the particle as it goes up and down, counting the bounces as a way of measuring time. Now imagine that a  friend also has one of these clocks, but that she’s moving horizontally. From your point of view, her photon traces out two sides of a triangle as it bounces from one mirror to the other and back again, travelling further during each round trip than the photon in your clock. There’s nothing controversial in what we just said. Here comes the  weird bit. Because, according to Einstein, the light bouncing in your friend’s clock is travelling at the same speed as the light in your clock, the light in your friend’s clock must take longer to bounce between the mirrors. In other words, your friend’s clock is running slower than yours. This remarkable  conclusion might sound like a special feature of light clocks. But it isn’t… it is a feature of all clocks. To understand why, we need to introduce Einstein’s second crucial idea – an idea first introduced by Galileo Galilei in the early 1600s.

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