# Why can't you go faster than the speed of light?

Why is this the limit?
17 November 2023

## Question

What law of physics dictates that you cannot go faster than light?

## Answer

Thanks to the University of Nottingham's Tony Padilla for the answer!

The speed of light is the limit because, the faster you move, the more energy you need to move you. And, in our current understanding of the universe, you would need unlimited energy to move at the speed of light. Because of this, as Paul on the forum says, ‘the speed of light can be approached asymptotically but never reached.’ So where does this idea come from? Here to talk us through is Professor of Physics at the University of Nottingham and author of 'Fantastic Numbers and Where to Find Them', Tony Padilla...

Tony - Well, it all follows from the laws of electromagnetism, or more specifically the idea that you and your mate should agree on those laws, even if one of you is running around at really high speeds. You see, when Maxwell wrote down his equations for electromagnetism, he realised they emitted a really sort of cool solution in the form of a wave, like an electromagnetic wave. And what he noticed was that this wave actually propagated at a speed that was really, really close to the speed of light. So Maxwell made this, this sort of intellectual leap that maybe light was an electromagnetic wave, and his laws, his equations were mathematically really elegant, absolutely delicious. Fast forward 40 years and Einstein starts thinking about what it would be like to actually sort of run after a wave at one of these electromagnetic waves, what it would be like to actually sort of catch up with light. And he realised that if you could catch up with light, then the laws of electromagnetism would have to look very different. If you could catch up with an electromagnetic wave, then it would be like that wave wasn't moving at all. It wasn't propagating at all. And that's a very different set of electromagnetic laws compared to what Maxwell wrote down. And Einstein didn't like the idea that these laws should be changing at all. They were far too elegant, they were far too beautiful. And so he declared that they shouldn't change. It shouldn't matter how fast you are running Maxwell's laws should be the same for everybody. It doesn't matter whether you're running towards a ray of light or away from it, you should measure the wave as moving at exactly the same speed. It's always that speed. And that's why we all agreed that the speed of light is the speed of light. It's just one speed. It doesn't matter whether you're running towards the light or away from it, you still measure the same speed of roughly 300 million metres per second. So this was a huge inter intellectual leap, of course, by Einstein. And it had massive implications for what we mean by time. But that's a completely other story...