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*To*: starship-design@lists.uoregon.edu*Subject*: starship-design: The speed of now*From*: wharton@physics.ucla.edu (Ken Wharton)*Date*: Mon, 25 Aug 1997 13:32:22 -0700*Reply-To*: wharton@physics.ucla.edu (Ken Wharton)*Sender*: owner-starship-design

Jim writes: >Now I know that this is'nt quite the same >thing, but, could these causality violations just a result of limited >sensing ability, ie, our sensors work only a light speed, but our (ships, >torpedoes, comm systems) work FTL. Kind of like seeing an explosion >before you hear it. Help me out here. Read Gravity's Rainbow lately? Actually, these relativity issues aren't a result of only learning about things after the fact, via our light-speed information constraint. As others have pointed out, you can go back and reconstruct what ACTUALLY happened (from our perspective), and even after you do that, relavistic effects still do happen. But, in a sense, you're still right. Think about it this way: light travels at an infinite speed. If you were a photon, you would cross the entire universe in no time at all. So why does c appear finite to us? Because of the link between space and time; our idea of what is "now" and what is "1 second from now" is spatially-dependant. You can think of this as a perceptual problem, but the analogy is not between the speed of sound and light, but rather between the speed of sound and the speed of "now". It's a tough concept, but think of a 15-minute delayed signal travelling from Mars to Earth. If it arrives at Earth "now", we naturally consider "now" on Mars to be 15 minutes after the light left Mars, from which we can calculate a finite speed of light. But imagine that what we consider "now" at Mars is a confused notion, brought about by the large distance, and the real "now" on Mars is actually 15 minutes ago, when the light left in the first place. Think of ripples of "now-ness" propagating out through space, making our perception of simultaneity not reflect what is "really" one instant. In that sense, you are right: relativistic effects are only a result of our perception of a more fundamental reality. If you can manage to imagine this, that's the heart of relativity. What we consider simultaneousness is only our perception, not some absolute objective reality. The bottom line: Light appears to travel at a finite speed (from our perspective, or frame) but you could say it "actually" travels at an infinite speed. When looked at this way, the link between FTL and Time-Travel is obvious. Going FTL means travelling faster than an infinite speed. The only way to get somewhere faster than instenantously is to get there BEFORE you leave. Thus FTL = travel backwards in time. Now, from our perspective, things don't always look that way. An FTL Journey can APPEAR (to certain observers) to move forwards in time and still be faster than light, because to us light appears to be finite. But to other observers, the FTL journey will seem to go backwards in time. If you're asking what "really" happens, in a frame where "now" is not frame-dependant, the only way to be sure is to look at a value that doesn't change from frame to frame. We need to look at a measure of space-time distance that is independant of perspective. That value is D = [t ^ 2 - x ^ 2]. The distance between any two events, measured in terms of D (where t is the time-separation in seconds and x is the distance separation in light-seconds) will be identical in all frames. For light, D=0. Thus my assertion that light "really" travels infinitely fast. For STL, D>0. The time is always larger than the distance. But for FTL, D<0. You also get D<0 for local time travel; set x=0 (you don't go anywhere) and set t<0 (you go back in time). Therefore the D associated with time travel is equivalent with the D associated with FTL. It's very simple to take two FTL journeys, one out and one back, that gets you to return before you left. Just another way of thinking about it, I guess. Which, of course, was probably more confusing than illuminating... Ken

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