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One of the more popular plot lines in science fiction involves the hero going back in time, and preventing something from happening so that in the future he, or she, came from some, even more terrible, thing will not happen. For some reason, probably one best explored while watching the currents created by the ice melting in a glass of scotch, the even more terrible thing very often is the birth of some individual. The argument goes something like this: the world, as we know it, is crap. The reason it is crap can be traced directly back to this exact person. Therefore, if that exact person is never born then, ipso facto, the world will not be crap.

Now, if you were to take a poll you would most likely discover that in the private thoughts of most people, the person usually blamed for causing this scatological condition of the world is either a spouse, in-law, or, more and more frequently these days, middle management. These, of course, are the people most capable of making one’s life a living hell, and the most likely reason one would be sitting in a gloomy bar on a Thursday afternoon watching ice melt in some amber colored liquid. We understand the anger, and the need to stop these people before they can do their harm; but for time travel to be involved in a novel or a movie the person has to be doing some serious, global, genocidal crap making. They have to be a Hitler or Pohl Pot or Stalin or someone equally evil like the guy who invented conference calls. Otherwise it’s just a family squabble or workplace grievance.

The only thing wrong is that it can’t happen.

The reason it can’t happen is: the past is fixed. You can’t change it because if you did then it wouldn’t be the past. That is, whatever you did to change the past already happened to make the past you are trying to change the past that it is. Now to balance things out the future is always a crapshoot. Any conceivable variation of an event that could possibly happen has, at any moment right up until the moment happens, a more than fighting chance of occurring and you cannot be sure of what will actually happen until it has. But once it has happened it is frozen, and the only thing that can change is how it’s remembered.

For example, you pick a date—preferably one you wish had come out just a bit differently. For Thomas he, or we, could very likely pick 14 June 1981. That was one of the days he got married. In the next several years he spent a great many nights watching ice melt, and thinking. Thinking he could have gotten a flat tire. A cop could have stopped him for speeding, and then locked him up for having an expired registration. A plane could have crashed into the wedding hall. The psychology student who had started dating the girl with improbably auburn hair and the…smile could have tracked him down and beaten him unconscious for no really valid reason.

Maybe he could go back to 4 October 1979 and say, “I don’t think so,” instead of “Sure, why not?” Or perhaps he could go back to that meeting with her family on 17 April 1981 and set them straight about just what kind of nameless hell he was willing to put up with. Or on another fateful day he could have driven to the airport, flown to Zanzibar, and gotten arrested for being drunk and disorderly instead of walking up to that attractive young lady and saying, “Hi. My name is Thomas, but some people call me TDM.”

So many things could have happened.

In his blacker moments, when he remembers some of the finer agonies, he tends to daydream about fiddling with the brakes on her maternal great aunt’s car. If only he had a way of getting back to, say March of 1965, and spending five minutes in her garage, life would have been so much nicer.

But like I keep saying, even if you can go back, you can’t change what happened. You can have twenty, thirty, a hundred time travelers all doing their best to prevent something. Let say they want to stop the bombing of Hiroshima. One team of time traveling heros go back to some heavy water plants in Norway hell bent on blowing them up. Another bunch get some jobs up in Redmond, Washington with the aim of creating some mischief in the production of U235. Still others are in Los Alamos hoping to sabotage the test firing of the first atomic bomb.

Our intrepid heros work assiduously for many days, adding this, changing that, totally wrecking the other. All things that no one knew would happen until fifty years after they happened, so no prior steps could have been taken to prevent the changes. And still, the future very definitely has this past you are trying so hard to prevent.

That’s because, like it or not, each moment in the past is made up of the cumulative effects of all the stuff going on at the very instant of their happening. This includes the guys in Norway getting completely pissed the night before and sleeping two hours too long. It also includes the team sent back to keep the first team sober having an altercation with a local constable which kept them from joining the first team in time. And includes a team sent from two hundred years further in the future that all got arrested as drunk and disorderly because no one could understand their accents.

I will try to explain it a bit more cogently.

Somewhere around 20 April 1889, Adolf Hitler was born. A great many people quite rightly would like to see that not happen. So, in the year 2030 an intrepid hero volunteers to take on the job of stopping this birth. He’s seen all the old “Terminator” films, and has worked out assiduously to make sure he looks good during the nude scene. He is sent back to July, 1888, and begins making sure Herr und Frau Hitler never get a moment together to do some canoodling. But on 20 April 1889’s only appearance, in this dimension at least, baby Adolf was certainly born, which means that for one night, or afternoon, or coffee break, our intrepid hero was asleep, so to speak, at the switch.

The problem is that if you are a time traveler from the future, you have to go back to a time that has already happened; and you were there when it happened even though you only decided to go back then last Thursday because, in our dimension at least, time only happens once. (I said that already just a couple sentences back, but it bears repeating.) So, no matter how many time travelers go back to July (or August) of 1888, Adolf Hitler’s mother and father will still manage to have that one, all important connubial moment because you can’t undo what’s done.

“What,” I can imagine you asking, “has any of this have to do with Thomas? And can I have the last five minutes of my life back?”

The answers are: a lot, and no.