starship-design: Re: Spacetime, Shmacetime

```To Ken,

>>So the stationary twin thinks his brother lived slower or equal all the
>>time.
>>But the moving twin thinks his brother first lived slower, than started
>>living hyperfast, and finally slower again. However his brother lived
>>hyperfast more than he lived slower, so the net result is that his brother
>>lived faster.
>
>That's correct.  I had a sign error in my thinking; things only go
>backwards in time when you accelerate AWAY from them, not toward them as is
>
>The Twin Paradox can be easily modified so that (far away) objects DO seem
>to go backwards in time.  After the rocket-ship twin turns around toward
>Earth, have him turn around a second time, back in the direction that he
>was going in the first place.  During this process, time on Earth (instead
>of zipping forward very fast) zips BACKWARDS very fast.  It's a reversable
>process: you can accelerate one way to make time go forward, and the other
>way to make time go backwards.  No paradox, though, because by the time you
>get back to Earth time has always gone forwards.  Unless, of course, you
>have FTL.

I think you are wrong again, time doesn't zip backward very fast, it slows
down even more but still is going forwards.

>This comes directly out of the Lorentz transformation of time:
>
>t' = gamma *(t - vx).
>
>If you're at t=0, and you're looking at a far-away object (say x=1), A
>space time event that seems like it's "now" (x=1, t=0) is transformed to a
>t' = gamma * (-v).  If v is negative (i.e. you're accelerating away from x)
>then t' is now a positive quantity.  But what does that mean?  It means
>that an event that was once "now", is now happening at a positive time
>coordinate; in the future!  So something that has "already" happened, now
>hasn't happened yet! (In your reference frame).  You've just "sent" the
>far-away object back in time.  Fortunately you can't do this for close-up
>objects!

Hmmm, it seems that now *you* are confusing real and unreal timedilation.
The story right above is about what you see, not about what happens.

Try to follow how  t' changes when t increases. You'll see that regardless
how the sign of v is, t' will increase. (Note |v|<1 )

Timothy

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