Time Travel Is Impossible

The Enrichment Center regrets to inform you that this next test is impossible. Make no attempt to solve it.
— GLaDOS, Portal, Test chamber 09

I've been thinking about it lately, and I've come to the conclusion that time travel is impossible. How so? Well, picture a time machine built into a land vehicle, like the DeLorean from Back to the Future. It achieved it's temporal displacement by accelerating to 88 miles per hour and activating its flux capacitor. It then winds up in the same location it departed from, but at a pre-programmed time. It moved temporally, but maintained it's position according to an X-Y-Z coordinate system. In the movie this system is coterminous with the earth's latitude, longitude, and altitude coordinate system, so if the DeLorean left Hill Valley in 1985, it would arrive in Hill Valley in 1955. The story basically treated the earth as a stationary object. Actually, the earth is a moving object.

The earth rotates on its axis.
The earth orbits the sun.
The sun (and by extension the whole solar system) orbits the center of the Milky Way galaxy.
The galaxy is moving away from the center of an ever-expanding universe.

Therefore, if there was an actual device capable of traveling through time, built into land-based vehicle, that vehicle would need to possess space traveling capabilities, because a trip leaving Twin Pines Mall on October 26, 1985, traveling to November 5, 1955 would not arrive in Old Peabody's farm, as it does in he movie. It would arrive on November 5, 1955, but it would be at a point in space where the earth will be at the time it departed. If you had some kind of space suit, a sufficient supply of oxygen, food, and water, and a waste removal system, you might be able to catch up with the Earth, but then again, you'd return to Earth thirty years older, which would more or less defeat the purpose of time travel. If you are already in a spaceship, you shouldn't have much problem with that. Though don't ask me about the effects of superlumic travel near stars (à la Star Trek 4: The Voyage Home). Once we master superlumic space flight, then we'll talk.

All right, this next test may involve trace amounts of time travel. So, word of advice: if you meet yourself on the testing track, DON'T MAKE EYE CONTACT. Lab boys tell me that'll wipe out time. Entirely. Forward and backward. So, do both of yourselves a favor and just let that handsome devil go about his business.
– Cave Johnson, Portal 2, Chapter 6.

Another reason why I believe time travel is impossible are the potential for paradoxes. Here's a situation. You make a big mistake and experience the effects of said mistake. You invent a time machine, travel back in time, prevent yourself from making the mistake. However, because you no longer make the mistake, your need for a time machine no longer exists, and you therefore don't invent a time machine, therefore the time machine you invented could not exist.

Such paradoxes have plagued time travelers in science fiction movies. The most notable example is in Back to the Future. I will refer you to that movie for a full account, but it suffices me to say that Marty McFly almost removed himself from existence by pushing George McFly (his father) out of the way of the car driven by his maternal grandfather.

If time travel were more commonplace, there would probably be so many alterations to what we would consider "reality" that it could literally destroy the universe. There is a storyline in Back to the Future: Part 2 that demonstrates this. It involves a sports almanac, gambling, and of course, time travel. Again, I will refer you to the film for the full account.

If people were able to travel through time, correcting mistakes, preventing things from happening, changing the course of history, we would be living in a very different place.

What if there were a way to sandbox time travel? What if we could make it so that we can have a fully immersive experience in a time other than our own, without any of the history-altering effects? Movies only go so far, and there are inaccuracies in places which supposedly reenact old times. I guess that it won't be available to us until we have technology like the holodeck found in Star Trek: The Next Generation.