Obviously science fiction does not have to stick clearly in the realm of the realistic, or even the possible. In order for science fiction to function, a certain amount of science must be invented; and while it is generally agreed upon that a piece of science fiction should not openly contradict any existing scientific research, we never expect anything particularly solid out of it, especially since research could so easily happen after the fiction is written which proves the fiction blatantly impossible. Standards are even lower, however, where time travel is concerned, since there is no scientific analogue; no real study compares to or can contradict with time travel, unlike such things as alien physiology (since there is quite a bit of information on biology in the public domain) and faster than light travel (because of that whole pesky relativity thing). But there is no actual science behind time travel, and thus nothing to tell the author of the fiction that something they are doing is wrong.

There are, however, a few basic, purely logical flaws behind pretty much any implementation of time travel you could think of; and these cannot be worked around because they are basic universal conceptual flaws, and not details of implementation making things difficult. Here are three. The first is a showstopper; the other two are not.

  1. The earth is not a stationary object.

    "Think fourth-dimensionally!", Doc tells us, and yet he acts as if only three exist. The earth, while it's kind of easy to exaggerate its importance, is really just a big, dirty rock. And moreover, it is a rock that is moving very quickly. Very, very quickly. The earth is spinning at several hundred thousand miles an hour, and orbiting around the sun at an even more ridiculous speed. And the sun is in turn orbiting through the outskirts of a galaxy which is itself moving. Everything in the universe is in motion. Meaning that pretty much any time travel science fiction ever written is pure bollocks; if you are truly simply moving in the fourth dimension through time, you will remain in the same place three-dimensionally, but the planet you were standing on will not. Thus when the DeLorean disappears and then reappears with the dog in it one minute into the future, it will not be (apparently, anyway) in the same spot it was when it disappeared; rather, it will probably be somewhere thousands of feet in the air, or possibly (fun) somewhere deep inside the earth. And when the DeLorean takes Marty back to the 80's from the old west, it would probably not be on the same railroad track; a more realistic placement would probably be somewhere outside the solar system.

    Like it or not, there is no real way of getting around this. You could BS some pseudoscience related to gravitational frames of reference, the warping of space/time, and relativity, but i'll bet you a dollar any real theoretical physicist would just laugh at it.

  2. The butterfly effect.

    (Ray Bradbury, i'm talking to you.) While there is an awful lot of science fiction out there dealing with how differences in tiny details can make exponentially larger differences as time passes-- i.e. you move a book on Benjamin Franklin's table, and when you come back the U.S. is a dictatorship-- the people writing these stories invariably forget the tiniest but most significant detail of all.

    I'm talking about air currents. Weather is unbelievably, unbelievably complex; the tiniest little change in initial conditions can have totally unforeseeable, drastic global implications. Whether or not the oft-abused dictum of a butterfly flaps its wings in China and causes a rainstorm in Denver applies, the fact remains that simply by walking around and breathing you are changing air currents, and thus eventually changing weather. Hell, just by being there you're displacing air. So if you go into the past, you will wind up changing, in increasingly drastic ways, all weather in the world after that point in time; and with time a point will come where the weather and cloud positions everywhere are totally totally different than they were before you interfered.

    And the implications of that? Well, i could talk here about a number of unbelievably important historical events whose outcome was determined by details of weather, but let's look at something far less melodramatic: sex. Think about what happens immediately after sex-- thousands of sperm in a huge race, and every single one has different genes; each one will result in a different person. Think about how tiny they are, and how little it would take to influence the outcome of that race-- all it would take is for their host to shift in his seat slightly the week beforehand (or maybe take a different way home from work because it's raining-- COUGH COUGH!!) and each of those sperm will be jumbled around quite a bit, and thus will start the race somewhere (relatively) wholly different.

    Now while obviously genetics isn't everything, and some people will do roughly the same thing despite a few different genes, it's still a pretty good bet that if you went back to 1878 and walked around in a field somewhere flapping your arms, when you came back you wouldn't recognize anybody. And just think about how much more difference you'd make by going to the time of the dinosaurs and firing a gun. Looking at things this way, it doesn't matter what precautions you take to "protect" the past from influence ("Look at how careful we are.."); you're still going to fuck everything up just by virtue of being there, so you might as well go stomp on all the butterflies you want.

    Looking at things this way, most time-travel science fiction starts to look rather silly, unless it takes the Hitchhiker's Guide to the Galaxy route and claims that the universe fits together like a jigsaw (meaning you can't change the present because your time-travelling actions in the past have already been factored in) or take the {telling you which book this is would ruin the book for you!} route of claiming that time is mildly resistant to change; big details, like killing hitler, can't be ignored, but if possible the universe will find a way to make the tiny details (like killing a butterfly) cancel out and become irrelivant in the end. (Note this same book manages to work around problem #1 above as well, by observing that moving between two arbitrary points in time and moving between two arbitrary points in space are both done the same way..)

  3. The first law of thermodynamics.

    Mass can be neither created nor destroyed. Energy can be neither created nor destroyed. This is really not so much of an issue as the other two, since it can be claimed none of these things actually happen in time travel, especially if you fudge by taking the idea of "time is a dimension" to an extreme; however time travel neatly crushes the idea that the amount of mass and energy in the universe is constant. Moving back in time would create a surplus of mass and energy for the time between the transported material's arrival in the past and the moment the transported material left, as you would have redundant material; moving forward in time would create a deficit. This probably isn't that important, and i shouldn't have brought it up. But it does desimplify things a good bit.
  1. All movement is relative, so if you say that the time-traveler would be left behind, you somehow have to pick a point of reference relative to which the Earth is moving and the victim is not.

    Since we're inventing the technology, it wouldn't be unreasonable to say that, for instance, the object that is traveling through time retains the same physical location with respect to the object that has exerts the largest amount of gravitational pull on it.

  2. Impossible to prove but tough to argue with.

    Diseases would also be a problem. You're probably carrying dozens to which people a few hundred years ago wouldn't have yet developed immunity.

  3. This was implicitly dealt with in The Terminator: matter swapping.

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