Alcubierre warp drive

[lwcamp]

Originally Posted by acrosome

And what's this stuff about using it as a time machine?

Jargon time: In relativity, an "event" is a spatial location at a given time. Two events are said to have a space-like separation if light does not have time to travel between them (meaning entering and exiting FTL, such as via a warp bubble, always has a space-like separation). If light has just enough time to get between them, the separation, or "interval" is said to be light-like, and if light has more than enough time to get between them the interval is said to be time-like.

Now the details. All observers can agree that for events with a time-like separation, that one event occurred before the other. This gives you an unambiguous past and future. However, the same is not true for a space-like separation. If one observer thinks that in his reference frame one event happened before the other, another observer will think they happened in the opposite order and a third observer will think they happened simultaneously. By choosing your frame of reference you can choose what order space-like events happen in your coordinate system.

So, start at Sol and warp to Sirius. Entering and exiting warp are events with a space-like separation. In your frame of reference exiting warp is either simultaneous or after entering warp - not a problem yet. Now when you are at Sirius, change your velocity by a lot so you are in a new reference frame (perhaps using the hovering method described earlier). Get into a reference frame where exiting warp happened a lot earlier than entering warp. Now warp back to Sol. You arrive before you left. If you want to be contrary you could try to cause a paradox, such as trying to ambush yourself and blow your other-self up before he leaves so you could never get back to blow yourself up.