Popular

Stephen Hawking once wrote to a 5 Live listener to settle an argument and it’s out of this world

John Plunkett. Updated March 16th, 2020

We might occasionally have had one of these debates with a friend, but we probably wouldn’t have thought to write to Stephen Hawking.

And if we had, we probably wouldn’t have expected a reply.

And yet…

And here it is (it’s the final paragraph we like best).

And just in case that’s tricky to read …

Thanks for your letter of March 15th, and what turns out to be a very fundamental question in physics.

With one provision I have to say that your friend is right – the proviso being that it is impossible for any particle with mass to travel at the speed of light as it would take an infinite amount of energy to accelerate it to that speed. Photons of light are OK as they have no mass.

Let me give a slightly different example to try and explain what’s happening. Take the two cars again, but this time they travel in opposite directions, each at a speed very close to the speed of light. So apparently they are travelling apart at nearly twice the speed of light. The strange fact is that if a man in the back seat of one car shines a torch towards the other car, the light will not only reach the other car, but it will be measured to be travelling at exactly the speed of light. So there seems to be a big problem in that a light beam travelling at the speed of light can catch up with something travelling nearly twice as fast.

In general it was found that the speed of light is always the same no matter how fast you are moving. This goes against all logic, but finally Einstein took it fact value and developed his special theory of relativity. This says that in order to compensate for the speed of light to be constant, something else has to give (ie distance or time) so that the laws of physics still hold true eg distance = speed x time. And so this led to the ideas of space and time dilation, the fact that depending on how fast you are travelling you age at different rates.

Well that’s the fun answer to your question, but maybe you just wanted a yes or no! Just to clarify, yes the car in front will see the headlights.

Thanks again for the letter.

Brillant.

Source