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According to Albert Einstein, Stephen Hawking, Ho Chen, and others, Time Travel is not impossible. And in an infinite universe, if something is not impossible; then not only is it possible, but it almost certainly has already happened.
But there is a rather massive leap between something being theoretically possible, and scientifically practical. Practically Impossible, we call it.
To build up an example most people can understand of this practical impossibility, we look at one of the first experiments in time travel by a physical mass. Doctor Drew Lipsky attempted to send his pocket watch five minutes into the past, an attempt to recreate numerous thought experiments in practice.
He succeeded in the translocation, but with unexpected results. The watch successfully traveled through time, but he was unable to located it for three weeks after the conclusion of both the entrance and the exit.
The reasons why are illustrated in his foundation paper “In Principia Temporalis.” To start with the Earth has a rotational velocity of point four seven kilometers per second. Meaning that over a span of five minutes, the watch remained in in the same position in the physical dimensions, but the earth spun beneath it a total of one hundred and forty one kilometers, backwards. In addition to its rotational movement, the planet also orbits its parent star at twenty nine point eight kilometers per second. So the planet Earth had moved eight thousand nine hundred forty one kilometers away from where it was at the start of the temporal colocation.
Beyond these two factors of movement, there are dozens of other similar numbers introducing huge uncertainties into temporal colocation, or “time travel.” The Earth not only has rotational velocity on its axis, but that axis is not fixed. It oscillates back and forth over a twenty one degree arc, adding a wider cone to what would otherwise be a simple line trajectory. And the Earth’s velocity is not constant in its travel around the sun; instead it moves in an ellipse, moving slower or faster at different points, and the ratios of that ellipse also change over time.
And then there is the fact that the Earth’s rotation speeds up or slows down over time as well, affected by seismic events, tidal events, and the distance of the moon and sun on its spin.
And that is just the sun-earth-moon system. It doesn’t account for other changes such as the effects of the gravity of other planets, moons, and extra-solar stars on the Earth. It also doesn’t take into account that the entire solar system is moving, as a group of collected bodies, around the galaxy at one hundred eighty seven kilometers per second, and that the galaxy itself is also moving through the greater universe at one hundred thirty kilometers per second. And all of these velocities are constantly shifting through a complex interaction of fields of gravity and energy, and so none are constant.
None of this begins to incorporate the changes in the target body; the Earth. Over a span of a few centuries the courses of rivers may change, forests may grow or burn down, and so on. Over spans of Eons, continents move, oceans dry, mountains rise and fall, and the entire crust of Earth may slip as a unit. And so even if a prospective temporal voyager could fix the planetary coordinates, it might still materialize under miles of Earth or thousands of feet in the air. Or inside of a passing being that happened to intersect the time-space coordinates inadvertently. Trans-colocation could prove to be an unpleasant mess for actuaries in the insurance industry.
At the end of Lipsky’s experiment, he located his watch, which had fallen into the gravity of the moon and was loosely orbiting there at a lunar altitude of fourteen hundred kilometers; and that was only over a time-span of five minutes! Granted it was also a blind transit with no attempt to compensate for any of the above factors, and may also have been affected by momentum shift (the watch moving at all the above listed velocities, but instantaneously freed of gravity). In a follow-up experiment with an identical watch, Lipsky attempted to calculate all knowable variables including the velocity and rotation of the Earth, the moon, the sun, and observable velocity of the solar system through the Milky Way galaxy.
Even with some of the most powerful computer systems available, working from a relatively fixed point in space-time colocation Lipsky was only able to achieve an accuracy of four hundred and six kilometers spherical. And given that thirty percent of that was beneath the solid body of the planet, and another fourty percent was in the upper atmosphere of the planet, Lipsky determined that manned time travel was a practical impossibility over all but a few minutes span.
Similar experiments returned similarly disheartening results over the years, with one notably tragic mishap occurring when a collocated vortex was opened up inadvertently between the target laboratory and the inner layers of the sun. The lab and all personnel were destroyed, and the vortex only closed when the equipment which had opened it was vaporized and shredded by nuclear fusion and gravity.
And so Time Travel, while a scientific possibility and certainty, is still practically impossible.
Which is why when you need to do it, you call in Kim Possible.