All the Myriad Ways
Saw a lovely game this morning: Chronotron. The premise is that your robotic alter-ego’s time machine is broken; without skip chips, you can only make short-range jumps. So you have to collect all the lost skip chips to travel time freely—that is, you have to fetch the doohickey from each level and bring it back to the starting point in order to progress to the next level. The clever gimmick that drives the game is that you can use your time machine to replay a level while your past selves retrace your earlier steps.
So, to give you a very basic example: the chip is sealed behind a heavy door, where you can’t reach it. An obvious switch opens the door…but as long as you’re holding the switch down, you can’t go get the chip. The solution is to go hold down the switch for a while, holding the door open, then return to your time machine. On returning to the level, your new beta-self can walk to the door while your alpha-self goes and holds the switch, just like you did earlier. The door swings open, your beta-self gets the chip and returns to the time machine. After a while, your alpha-self, bored with standing on the switch, returns to the time machine, just like you did. A screen is complete when the skip chip and all your incarnations return to the time machine.
The time travel replay isn’t perfect; it’s entirely possible to engineer events to contradict earlier ones. For example, your alpha-self might wait twenty seconds, then go and pick up a crate. If you then drop the crate and return to the time machine, your beta-self can rush in, snatch the crate, and carry it off, so that your alpha-self finds no crate to pick up when he arrives where the crate was. Instead, he stands helplessly, pushing the space bar to lift and drop a crate that doesn’t exist before returning to the time machine. I can see that recording and replaying this kind of instruction list, independent of objects, is much easier than recording the entire sequence of events would be, but requiring the player to avoid such paradoxes would be more satisfying. Instead, the game only recognizes a paradox if you screw things up to the point where a past self fails to get to the time machine when he’s supposed to, at which point a warning message pops up and you’re forced to replay the level.
It’s possible to screw things up—and rest assured, you will do so—to the point where the level is insoluble, but the penalty is mild: you can restart the level, or you may choose to rewind only to the start of your last incarnation, a welcome time-saver.
I’m only a little way in: the seventh level out of forty, and I can already see how this is going to go. Soon, the levels will contain switches that affect multiple obstacles at once, and so need to be activated and deactivated at the appropriate times, instead of merely activated once to provide access. Soon thereafter, active elements will require timing your incarnations and their switches, instead of merely doing everything in the right sequence. When enough elements must be activated, it will become fiendishly difficult to estimate the necessary delays. And then, my past selves will start sabotaging my current self by doing things at the wrong time.
Fans of Doctor Who and similar time-travel adventures might enjoy grappling with the threat of time paradoxes themselves, but there’s a more obvious market in fans of the brilliant if frustrating game Lemmings, which required you to herd the suicidally oblivious lemmings by assigning them to build staircases and dig tunnels, hoping to create a safe path to the exit point before too many of them marched into various deathtraps. Chronotron is very similar, except that in Chronotron, if those stupid robots drifting around on auto-pilot ruin your plans, you have no one to blame but yourself. After all, they’re only doing exactly what you told them to—in fact, what you yourself did not two minutes earlier.