Question: “If your car could travel at the speed of light, would your headlights work?” by Harry Gething from Back To The Battle

What is your first reaction when you read this question? I’m sure a large amount of you who are only familiar with classical mechanics would have thought that they would work, but the light would just travel at the same speed as the car that is moving.  Those of you that are familiar with what we do here would realise for it to warrant a post, it just couldn’t be that simple!  So what is happening?  Well, for this, we have to bring in a new type of physics, relativity.

The Theory of Relativity was a revolutionary theory created by Albert Einstein at the beginning of the 20th century. Einstein’s genius was half in his ability to form theoretical experiments that the physics of the time could not explain and half in his ability to come up with solutions with entirely new methods. Around Einstein’s time, a shocking discovery was made. It seemed that, after a few pieces of experimental evidence, light moved at the same speed relative to you no matter what speed you moved at. Think of it as if someone is running along side your car whilst you drive a long. To someone standing still, that person appears to be moving fast, however, if you were to look at them, in fact they look not to be moving at all. In relativity, that person would appear to be moving fast for someone standing still and for you! It is a very strange phenomenon and it led to a number of discovery’s by Einstein, some of which we will be going in to later.

Anyway, just reading that paragraph, what can we assume would happen? Well, from the point of view of me standing on the side of the road, both your car and the light rays are moving at the speed of light. This means that the light rays can’t move away from the car and therefore I wouldn’t see the headlights. However, what about from your point of view? Relative to you sitting in it, the car isn’t moving, but we learnt that the light is going at the speed of light relative to you. So, you would see the headlights stretch away from you and the car! See, relativity leads to some strange results, and actually, we haven’t even  used all of the effects of relativity yet, so it is going to get stranger still.

You see, Einstein realized something strange about relativity, you can’t tell which object is actually moving, no matter where you are observing from and this leads to some very strange effects. For you in the car, the light would indeed stretch away, however, everything else around you would stretch in very weird ways. This video with Carl Sagan explains all these effects very well (despite the slightly dark ending):

From the point of view of me, watching you, you and the car would shrink in the direction of travel. So what does this mean overall? Well…I haven’t been completely honest with you guys here. You see, the experiment is broken.

It isn’t possible for a car to travel at the speed of light. Not that it is difficult, it is simply impossible. So what happens in an impossible situation? We don’t really know. We can look at all the supposed effects but really, it just gets very strange overall. For example, for you in the car, time shouldn’t move, it would just stop. So you couldn’t perceive the light leaving the car, because you wouldn’t be able to perceive anything at all! Anyway, at the end of the day, if you are in a car moving at the speed light, in the time it takes for you to turn on the headlights, you probably would have hit whatever it was you were trying to avoid!

We have posted before on Relativity: The Irregularity of Time and Gravity

We hope you’ve enjoyed this post! If you did then please check out our last two posts:

Spacetime, Extra Dimensions and a TARDIS: Is it possible for an box to be bigger on the inside?  - In the TV show, Doctor Who, the Doctor’s famous Tardis is bigger on the inside than out, appearing from the outside to be a simple police box. This is science fiction of course, but could any real objects have this effect under some circumstances.

Let There Be Light - In 1861, James Clerk Maxwell released a paper in which he created four equations. These equations described the relationship between electricity, magnetism and light. So what were they, and what does each one mean?