Time travel thread, Hey ho let's go!

If you move forward in time have you not sent the rest of the universe (everything except for the object moving forward in time) back in time (relative to you) as there is not exo-universal time reference that the two positions can be measured against.

(google 'twins paradox' for better explanations, but below i my attempt:)

Ignoring gravity (because i don't know much about general relativity), then although there isn't any unique absolute time reference frame, there are an infinite number of consistent time reference frames corresponding to all possible velocities less than the speed of light, any of which could be used to measure the time difference between any two events. Although different frames wouldn't agree on exactly how much time passed between the events, they would all agree on whether you can get from one event to the other at the speed of light or slower (i.e. whether the interval between the events is 'timelike' or 'spacelike') and if you can (the interval is timelike) they'd all agree which event happened first. (If the interval is spacelike, then different reference frames can see the events in either order or simultaneous.)

The idea behind one-way time-travel to the future is, given two events separated by a timelike interval, there are lots of courses (sequences of motion) that could be taken to get from the first to the second, and you can calculate how much time elapses along each course by cutting the course into segments each moving along at a constant velocity, measuring the time that elapses during that segment in the reference frame that's moving along at the same velocity, and adding up all the bits. If you do this you find that most time elapses on the course that goes directly from the first event to the second at a constant velocity. The results for all other possible courses are smaller, and in the extreme case of courses made up entirely of segments of motion at the speed of light, no time passes at all between the two events. In special relativity, a straight line is the longest 'distance' (time) between two points.

So basically, there is no going back, only going forwards slower or faster.

Sorry. I'm aware that this is a rather lame re-hash of things i've seen explained much better elsewhere. I'm much better at explaining this face to face with diagrams.

Ie: if you wanted to send something back in time, you need only (and I use 'only' in a purely hypothetical sense!) send everything except for the object forward in time.

?

If you could send everything else forwards, you still wouldn't actually be able to go back, but you could effectively slot 'extra' time into your life - if you arrive at a deadline with 10 hours of work still to do, you could send everything else 10 hours into the future, spend 10 hours working, and still meet the deadline as the deadline is defined relative to everything else's view.