I do not know the details, but I have read that some theories allow backwards time travel using wormholes. Backwards time travel seems so impossible because of things like Grandfather's paradox and such. I had an idea and was wondering if anyone who knew more than me about the subject could share their thoughts. Let's say that we were to open up a wormhole from the present time here on Earth to many years in the past on some planet very far away. Let us say that the spatial distance that the wormhole traverses is [tex]\Delta x[/tex] and the amount of time into the past it connects to is [tex]\Delta t[/tex]. Let us also say that the distance is far enough away such that we can say [tex] \Delta x \geq c \Delta t [/tex] where c is the speed of light. We let one of our friends jump through the wormhole to this other planet and he looks back through the portal at us. Being a brilliant genius, he looks up at the sky on the unknown world, calculates the direction of earth relative to his current location and shines an extremely powerful laser towards us (not through the portal, through normal space). A few seconds later, our sensors on earth pick up his signal coming through the atmosphere which has traveled many light years over the course of many years. My point: what if backwards time travel is possible, but only to locations in space far enough away such that any action you perform in the past can only affect the future of your original time (due to the speed of light limit) i.e. it is impossible for the laser to reach earth before he went through the portal. In this way, causality can never be violated, and we can still say we traveled back in time. He has two paths to communicate with us, through space and through the wormhole, and through neither medium will he violate causality. Some might ask: what if your friend opens another wormhole such that the spatial locations of this new wormhole are the same as the previous worm hole, but this time the time travel is reversed, and should he enter that new wormhole, he would wind up on earth in the distant past. Surely then he could violate causality. I thought I remembered reading something a while ago saying that (don't quote me on this) the ability for a worm hole to exist depends on the presence of a strong gravitational field, and that the "past" end must be in an area of higher gravitational field (or was it the other way around), then the "present" end of a wormhole. If that is the case, then you can never have two wormholes with different time directions leading to the same areas of space because the gravitational field of those areas will determine which way time will flow through the wormhole. Thus you still can't violate causality. I know that discussions like this are mostly just hand-waving since it is hard to prove anything like this and it is so complicated, but I think it is a fun thought exercise. Let me know what you think.