0% in my opinion. Time travel into the past creates impossible paradoxes. In the classic example, if you went into the past and killed your grandfather, you prevent your own birth and therefore would’t be able to kill him in the first place. And Stephen Hawking has said if time travel is possible, where are all the tourists from the future? He also demonstated in one of his books (a brief history of time maybe?) that using time travel you could make a bomb that could produce infite energy, destroying the universe. It uses a wormhole that copies a single photon infinite numbers of time in an instant. Time doesn’t really have a backward and a forward anyway, it’s just a way to measure change.