Let’s start by getting one thing straight: the speed of light is not constant. I am not spouting scientific heresy. I’m not quoting fringe science nor pseudo-knowledge plucked from science fiction. Instead it is a statement fundamental to physics and one agreed upon by every contemporary scientist. It is also quite possibly the single most abused piece of scientific knowledge there is. The notion that food only contains DNA once it has been genetically modified runs it a close second, I’ll admit, but loses out simply because of the sheer number of times that even the reputable media say ‘the speed of light is constant’. To make things worse, it is often associated with phrases such as ‘the speed of light is the speed limit of the universe’, when actually the two concepts are not directly related. The idea that the speed of light is constant and is the ultimate speed limit has therefore become firmly entrenched in the annals of popular knowledge.
Light travels more quickly in air than it does in water and even more quickly still in the vacuum of space. So, the speed of light does vary. The correct version of the phrase is ‘the speed of light in a vacuum is a constant’, and yes, the speed of light in a vacuum is (probably) the speed limit of the universe and is represented by the constant ‘c‘, which most people will recognise from Einstein’s famous equation E=mc2. So, the phrase ‘the speed of light is a constant’ only becomes correct once the words ‘in a vacuum’ are added. Having mentioned Einstein, it is also worth mentioning another aspect of the speed of light which seems to have become popular knowledge, which is that Einstein discovered that the speed of light was constant, and that this was part of his theory of relativity…. both of which are also incorrect. First off, it was not Einstein who discovered this, but Michelson and Morley in 1879. Perhaps more importantly, Einstein did not state that ‘the speed of light is constant’. In fact, one of the two principles postulated in his special theory of relativity, was that the speed of light “…is independent of the state of motion of the emitting body.”. In other words, the speed of light in a vacuum is constant relative to the observer.
Now, most of my attempts to explain the above to family and friends fall on fairly deaf ears that feel this is nothing but pedantry scientific snobbery. However, the addition of just those few extra words not only stops the statement being incorrect, but opens the door to one of the most counter-intuitive and marvellous aspects of scientific theory. I remember studying an OU course in physics and tackling a question similar to: “A spaceship is travelling at 80,000 metres per second towards a space station. When it is has travelled 100km past, the ship sends a radio signal (which travels at the speed of light) back to the station. How long does it take before the station receives the signal?”. I thought of this problem initially as being similar to someone throwing a ball to someone as they run past them. Which is to say that the faster the runner is travelling, the longer it will take the ball to reach the other person. Adopting this approach got me nowhere and with some frustration I chatted about it to my brother, who has a PhD in physics but also a very strict attitude towards anything even approaching ‘cheating’, who encouraged me again to look at the theory of special relativity. And then, like a light bulb going on in my brain, I finally got it. In answering the problem, one can completely ignore the speed of the space ship and simply work out how long it takes light to travel 100km. Why is this? Because unlike the ball in my initial analogy, the speed that light travels is not affected by how fast you are travelling. Throw a ball at 30kph to someone in a car travelling away at 31kph and it will never reach them, but shine a torch at them and it doesn’t matter if they are travelling at 30kph or 300,000kph. For all intents and purposes, when it comes to light, the observer is essentially stationary regardless of how fast they are actually travelling.
There is a good chance that this is likely either to sound like the type of confusing physics that bored you at school or, conversely, very obvious. It depends on which side of the epiphany you are. However, not only is it counter-intuitive that the light from a torch takes the same time to reach two ships, one travelling at 30kph and one at 300,000kph, but it also leads to the concept that if the time it takes to reach these two ships is the same, then one ship must experience time quite differently to the other. Indeed, time will pass more slowly for the faster ship than it does the slower. For me, sitting at a table in my North London flat and suddenly grasping the critical importance of the words “…is independent of the state of motion of the emitting body.” is a moment I will never forget. In a single moment the physics of relativity, of Einstein and speed, not time, being a constant opened up before me. Excluding those seemingly mundane words leads to the wholly incorrect and much abused phrase “the speed of light is a constant”. Include them and something fantastic is revealed and you will never look at the universe in quite the same way again.