I once co-taught a class for Splash on “Time Travel for Fun and Profit”, about the different ways that fictional universes handle time travel. It’s a bit like writing about magic, or faster-than-light space travel: since none of those things work in the real world, the author can make up whatever rules they like… as long as the rules are superficially plausible and internally consistent. And the dirty little secret is that even that rule isn’t ironclad; as with so many things in fiction, you can get away with nearly anything as long as the audience is entertained enough not to notice the inconsistencies until they get up to go to the fridge.
In other words: the only way to do it wrong is to do it in a way that breaks suspension of disbelief.
Which brings me to Legends of Tomorrow.
Look, the time travel on The Flash doesn’t always make sense, but it’s mostly okay if you accept that this is a universe where you can go back and change the timeline, and it’s not the focus of the show. Legends of Tomorrow puts time travel front and center and then blatantly lets the rules be whatever the plot needs them to be right now. This show holds the dubious distinction of having time travel mechanics handwavier than those of Doctor Who, and Doctor Who once described time as, and I quote, “a big ball of wibbly-wobbly, timey-wimey stuff”.
There are… many things that can be said about Steven Moffat as a writer and showrunner, but one thing I always give him credit for is pulling off plots based on loopy time travel mechanics. “Blink” is the classic one, but a disproportionate number of Doctor Who episodes where Moffat has a writing credit are particularly timey-wimey, including recent high point “Heaven Sent”. (The two-part mini-episode “Space”/”Time” is a good example in miniature.) He’s good at playing with concepts like stable time loops, jumping ahead in the timestream while someone else takes the slow path, etc. There are some kludges (“you can’t change that, it’s a fixed point in time”), but at least there’s a sense that Moffat and the other Who writers know what they’re doing.
Whereas on Legends of Tomorrow, I don’t get the sense that the writers have really wrapped their heads around time travel. For example, there’s a concept that’s come up more than once, that changes to the timestream don’t “stick” immediately and some time has to pass before they become permanent… and it goes completely unacknowledged that the concept of “time passing” for the timestream makes no sense. You cannot give your heroes a time machine and then put them on a clock; if you can go back in time now and fix what you broke, you can just as easily do it (subjectively) later, because, hello, time machine. I twitched every time Rip brought up this idea.
It all comes back to suspension of disbelief. Plot holes are acceptable if they’re only noticeable after you’ve thought it over.* But when I’m watching the episode and shouting “That makes no sense!” at the screen, it becomes a problem.
*One that Legends of Tomorrow more or less gets away with: recently, the heroes contemplated killing a future dictator at the age of 14. They didn’t do it because of moral qualms about killing a kid, but it’s implied that going through with it wouldn’t have broken the timestream. A mere two episodes later, the team drops off their younger selves at Last Refuge, an isolated spot where future Time Masters are taken as children. So… it never occurred to Rip that he could send Per Degaton to the Evil Baby Orphanage?