Try out the interactive below!

According to Einstein's theory of relativity, the speed of light is the fastest speed that any object can travel in this universe. It is the universal speed limit. This may seem odd. Why would our universe have a speed limit and why does light always travel at the speed limit?

This oddity of the universe doesn't seem so odd once you think of mass1 as resistance to acceleration.2 Since light has no mass there's nothing to resist its acceleration - nothing to slow it down. Without anything to slow it down, it's always going as fast as possible, which just happens to be about 670 million miles per hour in our universe. Since light always travels at this maximum speed we call it "The speed of light".

If it seems counterintuitive to think of mass as resistance to acceleration, imagine that we are in outer space3 and we have a ball with a lot of mass like a bowling ball. We push on this bowling ball with force F (nine Newtons in the interactive below) for two seconds until it reaches its final speed, Vbowlingball.

Now imagine we have a ball with less mass, like a basketball. We push on the basketball with the same force for two seconds until it reaches its final speed, Vbasketball. Because this ball has less mass, intuitively its final speed is greater than that of the bowling ball.

Vbasketball > Vbowlingball

Now imagine that we have a completely massless ball. Because this ball has no mass there is nothing to resist the force of our push so the ball zooms off as fast as universally possible. In our universe as fast as possible just happens to be 670 million miles per hour, or what physicists call the constant C.4

See for yourself in the interactive below. Vary the mass of the ball and see what happens to its final speed.

Effect of Mass on Speed Demo