Expert: James Gort - 9/4/2010

Question
If two beams of light are aimed at one another, is the closing speed twice the speed of light?

Answer
Hi Stuart,

If two beams of light are aimed at each other, their closing speed would be the speed of light!  I know it sounds strange, but that's a prediction of Einstein's Special Theory of Relativity, and it's been tested to be accurate.

Another way of looking at it is to imagine that you're on a very fast train.  That imaginary train can get close to the speed of light.  Now, if you shine a flashlight while you're on the train and carefully measure the speed of the beam, you'll see it's exactly "c" (the speed of light).  It doesn't change because the train (and your flashlight) is moving at almost the velocity of light.  This was similar to the "Michaelson-Morley" experiment, which looked at trying to measure differences in the speed of light due to the earth's orbital motion.  They couldn't detect any difference.

Now, say your friend was at the train station while your train sped past.  Your friend also measured the speed of the light beam from your flashlight (which was on the train).  Your friend would also measure "c" as the speed of the light beam!  It turns out that the speed of light (in vacuum) is always a constant, regardless of the frame of reference.

That's why Relativity seems a little strange, compared to our everyday experiences.  Hope that helps.

Prof. James Gort