If I were to fly off at incredible speed in a spaceship and return to Earth, less time would pass for me than it would for everyone I left behind. And it turns out that moving clocks must tick slower than stationary ones. Einstein realised that the consequence of the speed of light being absolute is that space and time itself cannot be. Now, if the speed of light is universal (in the vacuum of empty space), we must measure it to be the same – 299,792,458 metres per second – however fast we ourselves are moving. For me to travel back in time and set in motion events that prevent my birth is to put the effect (me) before the cause (my birth). It is a serious theoretical thorn in the side of time-travelling protagonists. This gives us “causality” – the law that effects must always come after their causes. The speed of light is the fastest any signal can travel, putting a fundamental limit on how soon we can know what is going on elsewhere in the universe. Einstein’s intellectual leap was to suppose that the exchange rate from a time to a distance in spacetime is universal – and it is the speed of light.

As physicists Brian Cox and Jeff Forshaw write in their book Why does E=mc²?, time and distance “can be interchanged using something that has the currency of a speed”. Mathematically, our statement is equivalent to saying that Birmingham is about 125 miles from London. What we mean is that the journey takes that long at an average speed of 50 miles per hour. As odd as this might sound, we happily answer “about two and half hours”, when someone asks how far Birmingham is from London. Our inability to access the “now” of someone far away is at the heart of Albert Einstein’s theories of space and time.Įinstein told us that space and time are parts of one thing – spacetime – and that we should be as willing to think about distances in time as we are distances in space. The signals carrying their voices and images travel incomprehensibly fast, but it still takes a finite time for those signals to reach us. But this is something we can never actually know. We take for granted the ability to call our friends and family wherever they are in the world to find out what they are up to right now. Let’s start with our everyday experience. So where does this leave us? We cannot build a time machine today, but could we in the future? But he was also right that asking whether time travel is possible is a “very serious question” that can still be approached scientifically.Īrguing that our current understanding cannot rule it out, Hawking, it seems, was cautiously optimistic. “If one made a research grant application to work on time travel it would be dismissed immediately,” writes the physicist Stephen Hawking in his posthumous book Brief Answers to the Big Questions.

The time machine book