Why, you might ask, do we bother with such a strange theory as relativity? It doesn't appear to affect anything in our daily lives, at least not yet; just about all it is good for is explaining odd phenomena such as the detection of muons and other things that non-specialists have never heard of before. We can talk about muon decay, since so far only relativity can explain why muons reach the earth's surface. With a half-life of about 2.2 microseconds, classical physics says they should all decay after having travelled a mere 700 meters. But muons travel at close to the speed of light; and a fair number make it to our detectors on the earth's surface.
Relativity explains why. Looked at from the frame of reference of the earth, the muon's internal time slows down relative to an outside observer, including its rate of radioactive decay. The muon itself does not notice any time dilation; however, because it is traveling so fast with respect to the earth it seems as if the distance to the surface has contracted considerably, well within the 700 meters the muon can expect to exist before it decays.
But let's face it: there aren't a whole lot a practical applications of relativity at our current level of technology. Mostly, relativity is useful for theoretical physicists to create fantastic ideas about the universe (as, for instance, Kip Thorne does in the wonderful book Black Holes and Time Warps: Einstein's Outrageous Legacy). The latest theories on black holes make for wonderful reading because they seem like science fiction -- but in the last few years evidence of several likely black holes has emerged.
(For a nice site of black hole arcana, try Erk's 'Black Hole' Pages; be sure to see the page Black Hole Information Paradox: frogs with dirty feet for a discussion of a current controversy in black hole physics, whether things can make it out of a black hole after all. )
But, somewhere in the future there should be some really useful applications. Relativity, for example, tells us that it would be possible for us to travel to the stars within our lifetimes, if we can go fast enough and don't mind coming home a few centuries later. In Black Holes and Time Warps, there is even a description of how we could use black holes to power an entire civilization. And of course, there is the almost obligatory speculation about using relativity to make time machines...
Technology just hasn't caught up with our minds yet.
Maintained by Elisabeth Adams. Feel free to send questions, comments, or corrections my way.