|When you project a wormhole from a metropole world to a colony, you almost always exploit relativistic time dilation to reduce the perceived time to reach the colony from the metropole. For example, if the colony—metropole distance were 100 light years and the wormhole was projected at 99.9999% the speed of light, then it would take 100.0001 years for the wormhole mouth to reach its destination in the reference frame of the metropole and the colony. But due to relativistic time dilation, in the reference frame of the projected wormhole mouth it only takes 0.1414 years. Since you can look through the wormhole, you will see that 0.1414 years after launch, it arrives at its destination. At that point, you can go through and get to your new colony – you only need to wait about a month and a half to feel alien soil under your feet, rather than a century. In the metropole's reference frame, going through the wormhole takes you 100 light years away, and 99.8596 years into the future (going the other way, from the colony to the metropole, takes you 100 light years away and 99.8596 years into the past).|
One might think that these time warps would let you engage in all kinds of time travel. It is easy to see that the metropole—colony situation described here doesn't allow these kinds of shenanigans. For practical purposes, you only have a time machine when you can go back to the place you left at a time before you left. And you can't do that here. Go from Colony to Metropole and you go back in time 99.8596 years. Go back to Colony through the wormhole, and you go forward in time the same amount, plus any time you spent on Metropole, so you get back after you left. If you go back through flat space-time, it will always take at least 100 years since you can't go faster than the speed of light so you also get back after you left. No paradoxes for you!
However, it is easy to imagine situations where a wormhole, or a configuration of wormholes, does make a time machine. Imagine that there are two colonies, Colony A and Colony B, each 100 light years away from Metropole, and 100 light years away from each other. The wormholes to both colonies go 99.8596 years into the future when traveling from Metropole to either colony. Now Colony A sends a wormhole to Colony B. The Colony A wormhole also goes 99.8564 years into the future when going from Colony A to Colony B. This means if a traveler at Colony B went through the Colony A wormhole he would go back in time 99.8564 years. Then going from Colony A to Metropole he would go back in time another 99.8564 years. Then he could go from Metropole to Colony B and go forward in time 99.8564 years. The net result is that he ended up back where he started nearly a century before he left.
It seems that nature really doesn't like time machines. Here's why. Think about what happens when the Colony A – Colony B wormhole has gone just far enough that a light signal going through the wormholes can get back to where it left just as it is leaving. Now, since the propagating signal and the newly transmitted signal are both leaving at the same time, you have double the intensity. So this doubled intensity signal goes around and meets itself again, quadrupling its intensity. And so on. At this point, just as the configuration is on the verge of becoming a time machine, it becomes a perfect resonator for light signals, which then build up to arbitrarily high intensities until something breaks and you don't have an incipient time machine any more.
Now clever people will try to come up with ways around this - like putting a lead shield in the way of the signal's path. It turns out these tricks don't work. When you pull quantum mechanics into the picture, what get amplified are virtual fluctuations in the electromagnetic field and those can go around and anything it is possible to go around and through anything it is possible to go through. And it's not just light. All other particles behave the same way, so even if you somehow got the wormhole past the point where light would destroy it, it would be ruined by all kinds of other quantum fluctuations. You can't beat nature. And nature doesn't like time machines.
The consequence of this is that if you have closed loops in your wormhole network, it is really hard to keep time machines from forming. There are tricks you can play on a planet, but all interstellar wormhole networks form tree-like branching patterns without closed loops for just this reason.
But you can exploit this no-time-machine property. It is called a causality attack. If you are on one branch of a wormhole network and want to expand a bit but are blocked by a neighboring branch, you beef up all your wormholes and then send a wormhole to the neighboring branch. Something will break, but if it is a wormhole connection in the neighboring branch that is the weakest link, the network will break there. Now you have just stolen all the nodes (worlds) in the network that had been cut off by the break, and you can get to them using the wormhole you just sent.
In the Human expansion into the Milky Way, these sorts of attacks were common between the Americans and Chinese, Americans and Europeans, Europeans and Indians, and Indians and Chinese. This jockeying for territory was considered just the way things were done, and generally accepted back on Earth. The American president might engage in trade negotiations with the Chinese premiere the day after a causality attack stole a dozen American worlds and all their colonists for the Chinese, with little more than lodging and official complaint. The colonists, on the other hand, usually get pretty pissed off about such things.