That's a really great question, because you've stopped to wonder if two things you've heard really fit together.
The Second Law of Thermodynamics says that the total entropy of everything always increases. That means something like that everything gets more jumbled up, like when you shake up a batch of coins that were all heads and end up with an unpredictable mess of about half heads and half tails. Entropy is a way of keeping track of how many arrangements things might have- thereís lots of ways of getting half heads and half tails, but only one way to get all heads. So in our example the thing thatís like entropy naturally increases.
Thereís a reason why people say that means things slow down. If you have two big things that are moving at different velocities, they can dump some of their kinetic energy by coming to some average speed. (Friction is the name for the forces which make them do this.) The extra energy goes into all sorts of tiny little wigglings of atoms in the materials. There are a lot more ways for that energy to make billions of little atoms move around than there are ways to make two big things move as blocks, so entropy has increased. However, this sort of process canít make a beam of light go slower- it doesnít change the particular laws governing individual little particles. If the speed of light has changed over time (which is not what most physicists think) the reason would not be the Second Law, at least not directly.
So the part about light slowing down turns out not to happen. Unfortunately, the part about everything running down looks like it does happen.
If you want to learn more about entropy, we have a course on the Web called Physics 113
which tells a good deal about it.
(republished on 07/17/06)