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Re: starship-design: FTL idea
Timothy van der Linden wrote:
BTW, I wrote this FTL example once a while ago, but I didn't keep records
of my calculations.
>>- - - - - - - 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.
It's worded correctly, but confusingly. It is ship _A_'s
receiver, but of course this receiver is set up to receive
Foo messages from 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.
It's very difficult to explain why it moves backwards in time without
space-time graphs. ASCII art wouldn't suffice for this example.
> Ship B echoes the message back towards ship A at 1.8c
> in _its_ frame of reference.
Of course. As I said.
>That means it is moving at only 1.03053c in the frame where B moves with 0.9c
1.8=(.9-1.45)/(1+(.9*(-1.45)), whatever the significance of this
equation. You have to be very careful about whether you're looking
at a beam sent in the same direction or opposite to the direction
ship B is moving in.
_____ Isaac Kuo firstname.lastname@example.org http://www.csc.lsu.edu/~kuo
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