> Which would suggest any solution with a positive time direction can have a negative time direction, just as easily. Is this widely assumed to be true, or at l

> Which would suggest any solution with a positive time direction can have a negative time direction, just as easily. Is this widely assumed to be true, or at least physically meaningful? It's widely assumed to be true and not at all physically meaningful. If you sub x -> -x then that's just as good a solution as well, i.e. just as you can count x as running from west to east or east to west and the results will be the same, you can also count time as increasing away from the big bang or as decreasing away from the big bang and all your calculations will be the same.