On the implications of Time Travel

Almost every good work of Time Travel science fiction addresses the issue which is sometimes called the "Grandfather Paradox" and most of them conclude that going back in time and changing the past must be either

1. impossible
2. extremely difficult
3. extremely dangerous

or more than one of the above.

This goes back a long way. In 1933, American author Nathaniel Schachner published a short story titled "Ancestral Voices" in Astounding Stories. In it, a time traveller goes back 5,000 years, accidentally kills one of his own direct ancestors in a brawl, and immediately fades out of existence.

The modern form of the "Grandfather Paradox" was first formulated ten years later by René Barjavel (1943): Barjavel was the first to distil this concept into the exact family dynamic we reference today, presenting a character who travels back to deliberately eliminate his grandfather before his parents are conceived.

There are three opposing theories about what happens if you succeed in going back in time and killing your grandfather - one side gives Schachner's answer, that the paradox you created is resolved by your immediate removal from existence, a second theory is that, because you had to go back to a point before you killed your ancestor, you pre-existed that action and cannot be erased by it. Some version of this theory hold that those who have erased their origins in this way become invulnerable to further changes in the timeline. The third theory is that because it violates the laws of cause and effect, you can't do it, something will always stop you.

Science fiction writers have used various different language for the theory that trying to change the past, except where that change does not alter anything you already know - is difficult or dangerous.

Let me explain that exception. Suppose there is a patch of ground which nobody has ever excavated. But you want there to be a bag of gold hidden under it.

So you tell people - "I bet you there is a bag of gold hidden under this ground."

Then you build a time machine, go back a hundred years, and bury a bag of gold where you said it would be.

So when they dig up the field, they find it. They ask you "How did you know" and you answer "I put it there."

Did you change the past? Yes. Did you break the laws of causality? No. The bag of gold was there because you put it there and you did not do anything to create a paradox, such as check the field first and find it empty.

But be very careful. One famous time travel story, "Experiment" by Fredric Brown, has the protagonist do a trick of the same kind. After they have obtained the item which travelled in time, in my example the bag of gold, in Brown's story a brass cube, but before sending it back it time, some fool suggests that it would be an even more interesting experiment to see what happens if they don't send it back.

When the protagonist even more foolishly agrees, they don't find out what happens, and there is no violation of causality, because the entire universe quietly disappears.

Science fiction in general, and both Doctor Who and Star Trek in particular, are replete with examples where someone goes back in time, either intending to change something or thinking they have accidentally changed something, and then find that what they did was always meant to happen after all.

I'm not going to spoil the story by saying much about the plot as there will probably be people reading this who intend to watch Season 4 of "Star Trek: Strange New Words" and have not yet done so, but during Season 4, Episode 1, "Valles Marineris" the Enterprise is accidentally sent back 65 million years in time and her crew have to make agonising decisions about what protecting the timeline does and does not mean.

One of the most entertaining if bizarrely titled short stories about the forces which might make it difficult to change the past is Larry Niven’s 1977 short story "Rotating Cylinders and the Possibility of Global Causality Violation" which introduces the concept that the universe inherently resists time travel by unleashing catastrophic local disasters (like a nearby star going nova) to destroy anyone who might even try to build a time machine before it can violate causality.

You can find that story, by the way, in his collections Convergent Series by Larry Niven or N-Space: by Larry Niven.

Does it seem strange that the Universe might appear to resist attempts to change the timeline?

Not to me, it doesn't.

In fact if time travel is theoretically possible, I think the appearance that the unierse does this would be almost inevitable.

Stop to think for a moment. Knowing what humans are like, it is highly likely that It's almost inevitable that if time travel is possible, and changing the past is possible, and if time travel is then discovered without any obvious constraints on changing the past, someone's going to try to do it. And then someone else is going to try to change that new past. And then it will be changed again.

And the timeline will keep changing until it arrives at a stable situation in which the past is not changed. That means that either nobody can change it, or nobody wants to. It seems to me that this might happen in one of four ways:

• Every species which might be capable of time travel is rendered extinct.
• Every species capable of discovering time travel who do not have a strong aversion to the idea of changing the past is rendered extinct.
• Time travel is not in fact discovered or the theory is discovered but nobody attempts to do it.
• Time travel is only discovered, and remains controlled, by people who are determined not to let anyone change the past and very effectively enforce such a prohibition.

Now, ask yourself what such a stable timeline is going to look like?

It seems to me almost certain that it will look like either

1. The universe in Larry Niven's short story referenced above, in which anyone who tried to build a time machine gets wiped out first;
2. The universe in Steve White's Jason Thanou (5 book series) time travel stories, in which the people who have time travel believes there is something that they call "The observer effect" which means that if you try to change any known fact about the past, something, quite possibly something extremely nasty, will happen to stop you from doing it.
3. Or there will be some very powerful and effective Time Police or Time Patrol dedicated to preventing any idealistic idiot, malign idiot, or incompetent idiot from changing the past and probably using extremely ruthless enforcement to do it.

Examples of such Time police forces in fiction include:

• The Time Patrol from Poul Anderson's eponymous book
• The "Eternity" organisation from The End of Eternity by Isaac Asimov
• The Time Police from Jodi Taylor's Chronicles of St. Mary's (15 book series) and The Time Police (6 book series)
• The Temporal Corps from Simon Hawke's "Time wars" series - Amazon doesn't appear to have a series page for this, the first one is The Ivanhoe Gambit by Simon Hawke, book 1 of 12 in the Time Wars series
• The "Anti-Revision League" or ARC, from David Drake and Janet Morris's two ARC Riders books.

There are a handful of novels which involve moving people or information in time which which address the issues in this essay in an intelligent way without requiring the universe to impose such terrifying penalties.

An example of a story where people are aware of the consequences but try to change time anyway is the brilliant Thrice Upon a Time by James P Hogan in which sending a message back in time - and thereby erasing the current timeline - is the only way to prevent the destruction of planet Earth.

If everyone is about to die, replacing the present timeline with one in which the Earth survives might be the one situation where the alternative is so catastrophic that changing time is worth attempting.

For all that I love and enjoy reading Time Travel stories I hope that they remain fictional. Because if time travel is actually possible, we dare not allow anyone to experiment with trying to change the past.  

Because the people in the "Jason Thanou" books who believe in an "observer effect" which will do something, perhaps something very nasty to stop them, are probably right. "Something very nasty" possibly including, as in Niven's short story, the sun going nova or other events which might render the human race extinct.

So if it ever appears that someone is experimenting with Time Travel and might possibly succeed, the world community is going to have to set up that powerful and ruthless Time Police force.  And I don't like what the existence of such a force might do to our society.

Oh and by the way - if you've been experimenting with time travel, don't be too surprised if the Time Police or the ARC Riders turn out to be real, and come and smash your door down!

Professor X