Time Dilation And Time Travel

For astronauts who accelerate until they approach light speed, time is dilated or stretched so that, e.g., a second within the ship begins and ends simultaneously with a century or more outside the ship. Like time travel, time dilation is one way to survive into the far future without aging too much in the process. See Poul Anderson's Tau Zero. For most ideas that we discuss here, there is usually an example in Anderson's works.

Now imagine that time dilation happens not to a spaceship crew speeding through space but to an individual who remains stationary, seated or standing, on the Earth's surface. We are talking about HG Wells' Time Traveler and Poul Anderson's Jack Havig. These characters are called "time travelers" although mere time dilation does not qualify them for that description.

There are two differences between the accelerating astronauts and these "time travelers." First, the time travelers are not accelerating. If they did, then they would soon pass escape velocity and then rapidly leave Earth and even the Solar System. Secondly, the time travelers can reverse the direction of their dilation. They can experience not only 1901-2000 in a second but also 2000-1901 in a second. It is this reversal ability that makes them "time travelers." The Time Traveler enduring for a second while everything around him endures for a century or more is clearly living and moving much more slowly than anyone else and he acknowledges that the slowest snail passes him much too swiftly for him to see it. Why then does he claim to experience rapid motion and acceleration so that the Time Machine swings around and falls over when he halts it abruptly?