So, you’re driving a car at half the speed of light. (Both hands on the wheel, please.) You turn on the headlights. How fast would you see this light traveling? What about a person standing by the road? Would they see the light beam moving at 1.5 times the speed of light? But that’s impossible, right? Nothing is faster than light.
Yes, it seems tricky. The problem is, our ideas about the world are based on our experiences, and we don’t have much experience going that fast. I mean, the speed of light is 3 x 108 meters per second, a number we represent with the letter c. That’s 670 million miles per hour, friend, and things start to get weird at extreme speeds.
Illustration: Rhett Allain
It turns out that both the driver and the person on the road would measure the light as traveling at the same speed, c. The motion of the light source (the car) and the relative motion of the observers make no difference. Albert Einstein predicted this in 1905, and it’s one of the two main postulates behind his theory of special relativity.
Oh, it doesn’t sound so “special” to you? Well, what he then showed is that if the speed of light is a universal constant, then time is relative. The faster you move through space, the slower you move through time. The clock on a hyper-speed spaceship would literally tick slower, and if you were in that ship, you would age more slowly than your friends back home. That’s called time dilation.
A Commonsense Example
The idea that everyone sees light traveling at the same speed seems like common sense. But let’s look at a more familiar situation, and you’ll see that it’s not how things usually work. Say you’re driving at 10 meters per second, and someone in the car takes a tennis ball and throws it forward with a speed of 20 m/s. A bystander who happens to have a radar gun measures the speed of the ball. What reading do they get ?