Thanks to The Time Machine, Dr. Who and Back to the Future, it is easy to imagine what time travel might look like: people get into a machine, cool stuff happens (coolness is proportional to the special effects budget) and the machine vanishes. It then reappears in the past or the future (without all that tedious mucking about in the time between now and then).
Thanks to philosophers, science fiction writers and scientists, there are enough problems and paradoxes regarding time travel to keep thinkers pontificating until after the end of time. I will not endeavor to solve any of these problems or paradoxes here. Rather, I will add another time travel scenario to the stack.
Imagine a human research team has found a time gate on a desolate alien world. The scientists have figured out how to use the gate, at least well enough to send people back and forth through time. They also learned that the gate compensates for motion of the planet in space, thus preventing potentially fatal displacements.
As is always the case, there are nefarious beings who wish to seize the gate for their own diabolical purposes. Perhaps they want to go and change the timeline so that rather than one good Terminator movie, there are just very bad terminator movies in the new timeline. Or perhaps that want to do even worse things
Unfortunately for the good guys, the small expedition has only one trained soldier, Sergeant Vasquez, and she has limited combat gear. What they need is an army, but all they have is a time gate and one soldier.
The scientists consider using the gate to go far back in time in the hopes of recruiting aid from the original inhabitants of the world. Obvious objections are raised against this proposal, such as the fear the original inhabitants might be worse than the current foe or that the time travelers might be arrested and locked up.
Just as all seemed lost, the team historian recalled an ancient marketing slogan: “Army of One.” He realized that this marketing tool could be made into a useful reality. The time gate could be used to multiply the soldier into a true army of one. The team philosopher raised the objection that this sort of thing should not be possible, since it would require that a particular being, namely Vasquez, be multiply located: she would be in different places at the same time. That sort of madness, the philosopher pointed out, was something only metaphysical universals could pull off. One of the scientists pointed out that they had used the gate to send things back and forth in time, which resulted in just that sort of multiple location. After all, a can of soda sent back in time twenty days would be a certain distance from that same soda of twenty days ago. So, multiple location was obviously something that particulars could do—otherwise time travel would be impossible. Which it clearly was not. In this story.
The team philosopher, fuming a bit, raised the objection that this was all well and good with cans of soda, because they were not people. Having the same person multiply located would presumably do irreversible damage to most theories of personal identity. The team HR expert cleared her throat and brought up the practical matter of paychecks, benefits, insurance and other such concerns. Vasquez’s husband was caught smiling a mysterious smile, which he quickly wiped off his face when he noticed other team members noticing. The philosopher then played a final card: if we had sent Vasquez back repeatedly in time, we’d have our army of one right now. I don’t see that army. So, it can’t work. Because it didn’t.
Vasquez, a practical soldier, settled the matter. She told the head scientist to set the gate to take her well back before the expedition arrived. She would then use the gate to “meet herself” repeatedly until she had a big enough army to wipe out the invaders.
As she headed towards the gate with her gear, she said “I’ll go hide someplace so you won’t see me. Then I’ll ambush the nefarious invaders. We can sort things out afterwards.” The philosopher muttered but secretly thought it was a pretty good idea.
The team members were very worried when the nefarious invaders arrived but were very glad to see the army of Vasquez rush from hiding to shoot the hell out of them. After cleaning up the mess, one of the Vasquez asked “so what do I do now? There is an army of me and a couple of me got killed in the fight. Do I try to sort it out by going back through the gate one me at a time or what?”
The HR expert looked very worried—it had been great when the army of one showed up, but the budget would not cover the entire army. But, the expert thought, Vasquez is still technically and legally one person. She could make it work…unless Vasquez got mad enough to shoot her.