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Re: starship-design: FTL idea
>- - - - - - - 8< - - - - - - - cute here - - - - - - - 8< - - - - - - -
>For instance, let's say a particular gizmo, "The Foo Radio", can
>transmit (and receive) a morse code message at 1.8c (relative to
>the frame of reference of the gizmo).
>What you need to send a message backwards in time is two
>space ships travelling very fast apart from each other (for
>instance .9c). Both have Foo Radios pointed at each
>other. Space ship A has his Foo Radio set up to simply
>echo whatever he receives from his Foo Radio receiver.
I guess that should be ship B.
>For simplicity, assume I am talking about ship A's frame of
>reference (and a clock on ship A) unless specified otherwise.
>Let's say ship A decides to send a message to ship B when
>ship B is .9 light hours away; the time is 10:00. Ship
>B will receive the message at 11:00 when ship B is 1.8
>light hours away. Ship B echoes the message back towards
>ship A at 1.8c in _its_ frame of reference. You need to
>do a Lorentz transformation to figure out where/when this
>message goes in A's frame of reference. The result is
>that in A's frame of reference, ship B's Foo Radio beam
>moves at 1.42 c _backwards_ in time!
How nice it would have been if you'd not left out the calculations for this
transform... Now I still don't see why it goes backwards in time.
Ship B echoes the message back towards ship A at 1.8c
in _its_ frame of reference.
That means it is moving at only 1.03053c in the frame where B moves with 0.9c
So for A the beam has approaches with:
(I haven't the faintest idea how you get 1.42)