A physics professor once told me that Einstein's theory of special relativity proved that traveling back in time is possible, and occurs when a person is in motion. If this is true, am I actually traveling back in time when I run? Would that make my running shoes a primitive sort of time machine? - Running out of Space-Time
I believe that you and I had the same physics professor. I share your pain. Unfortunately, I was also cursed with Sci-Fi loving Star Trekkie friends, whose single argument supporting "time travel" was that Captain Kirk and Superman had exposed its simplicity on several occasions through various cinematic sequels. They assured me that a government conspiracy to suppress the truth was the only barrier towards returning to the distant past and the sixth grade homecoming prom where I should have asked the then homely (now supermodel) Claudia Schiffer for a dance.
What Einstein said was really very simple: a) the speed of light is the same for everyone, and b) the laws of physics are the same for everyone sitting still or moving at a constant, i.e. unchanging, speed.
You and I, sitting here by our computers at rest in our comfortable chairs, are spinning around the axis of the good planet Earth at about a thousand miles per hour. The Earth (with us sitting comfortably upon it) is twirling around the Sun at sixty thousand miles per hour; the Sun and our entire solar system is hurling through space at hundred fifty miles per second around the galactic center of the Milky Way which, itself is expanding at forty miles per second per megaparsec!
From the perspective of a truly stationary object in the Universe, you and I are moving very, very fast! The fact is: we are already time traveling into the future at exactly one second, per second! If you could somehow jump off the planet and put on the brakes to remain stationary, the Universe would cruise past you, and you could be said to have fallen "out" of time.
Of course, were that to happen you would likely be hit by a passing asteroid, comet, or alien spaceship... making your glorious adventure in space-time a painfully short lived event.
A more practical means of time travel would be for you to get up and go out for a run! As you moved away from your desk at a steady nine minute per mile pace, Einstein's special theory would apply and we could talk about time travel without the silly smirks and finger pointing from the uninformed.
What does it mean to time travel back in time? When we leave our comfortable chairs to run a hearty five miler (from our point of view) and arrive back at our computers at an earlier (from the comfy chairs point of view) time, we can be said to have traveled back in time.
This happens because time, as measured by you (the runner), technically slows down compared to that of your stationary desk. Upon your return home, you will find that you have aged a minuscule bit less than that of your comfy chair, but not by much.
Should you improve your nine minute per mile pace significantly (and, by "significantly" I mean, approaching 186,000 miles per second) you would reach a point where you would finish your run before you left that comfy chair of yours!
Since our running shoes are the devices by which we execute this "time travel", they can be considered, in a sense, "time machines" for they are the tools with which we best produce the relativistic motion.
Just watch out for the asteroids.
Run long and taper.