Time is a constant flowing like a river, you can swim against the current but you wont get anywhere think of it like this but in a river you also get rocks and natural bits that remain fixed in place who says time can not be like this if you get latched onto one of those "rocks" you are no longer moving with the time but you can let go on either side of the rock yes? so you could theoretically travel backwards or forwards in time if you go by the theory that time is a constant flow, but then that would also suggest that you can not use a device to travel because to build it you would have to be outside of the time flow, but you cud launch off of the "rock" in the same manner u would a wall in a swimming pool kick off it and generate forward momentum and then the past would be the future for you because that is the direction you are headed. (yes i am aware this is probably a load of rubbish but aren't most theories proved to be wrong ???)
