Time travel

Time travel is the concept of moving forward and backward to different points in time, much as we do through space. It also includes traveling sideways in time between parallel realities or universes.

Time travel, or space-time travel?

The classic problem with the concept of "time travel ships" in science fiction is that it invariably treats Earth as the frame of reference in space. The idea that a traveller can go into a machine that sends you to "A.D. 1865" and leave through a door into the same spot in Poughkeepsie ignores the issue that Earth is moving through space around the Sun, which is moving in the galaxy, etc. So, given space-time as four dimensions, and "time travel" referring to just "moving" along one of them, a traveller could not stay in the same place with respect to the surface of Earth, because Earth is an accelerating platform with a highly complicated trajectory! A vessel that moves "ahead" 5 seconds might materialise in the air, or inside solid rock, depending on where Earth was "before" and "after." (As seen in the 2000AD Comic, in which Mutant Bounty Hunter Johnny Alpha uses "Time Bombs" to propel an enemy several seconds into the future, during which time the planets passage through space causes the unfortunate victim to re-materialise in vacuum) If you moved "behind" a year, you'd end up in cold outer space, where Earth was a year earlier—in the same part of the Sun's orbit, yes, but where has the sun gone over that year? So, to really do what filmmakers make look so easy in films such as the Back to the Future series and The Time Machine, a time machine might have to be a very powerful spacecraft that could move you large distances and that kept track of Earth's motion through space as part of the solar system, galaxy, etc.

Related Topics:
Frame of reference - 1865

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But how can you decouple the ship from momentum? If you try to move forward in time, is your ship automatically going to be propelled by the momentum gained by riding Earth? Or does it decouple? But does not that bring back the idea of an absolute reference frame? Again, even to move one millisecond forward or backward in time, the ship would have to be far beyond anything humans can build, not to mention that the acceleration and deceleration in space-time would challenge the structural integrity not only of the vessel but also of the passengers' bodies. A theorist might even use this to argue in the style of Zeno's paradoxes, for the impossibility of time machines.

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A possible rebuttal to this criticism, of course, is the fact that cars and airplanes built by humans manage to move around the surface of the Earth with it, despite the surface itself moving with an astronomical speed. It is reasonable to assume that a time traveller experiences a combination of spatial temporal inertia that makes him move along with the Earth.

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In 1980 Robert Heinlein published a novel The Number of the Beast about a ship that lets you dial in the six (not four!) co-ordinates of space and time and it instantly moves you there—without explaining how such a device might work. The television series Seven Days also dealt with this problem; the chrononaut would pilot the time machine away from the earth's surface, and then back to it, by means of a joystick-like device.

Related Topics:
1980 - The Number of the Beast - Seven Days - Chrononaut

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