I wonder what the effect, then would be of a FTL transmission that could only
work over tens of millions, or several billion lightyears; but non functional
any closer then that? Should we listen for messages eminating from the
In a message dated 5/31/00 10:13:18 PM Pacific Daylight Time, firstname.lastname@example.org
<< According to relativity theory, if you can communicate faster than light
in an arbitrary inertial reference frame, you can communicate backwards
The reason is because of the relativity of simultaneity. Two events
which happen at the same time in one reference frame will happen at
different times in another frame at motion with respect to the first.
Someone in a spaceship zooming past the earth could use his FTL
communicator to send a message from the earth to the moon which
arrived 1.2 seconds before it was sent, by the earth's reference frame.
By his frame, the moon's reception of the signal is after the earth's
transmission, with the signal transferred FTL. But by the earth's frame,
the reception is at an earlier time than the transmission.
An Earth-Moon based FTL system could then get it back to the Earth in
less than 1.2 seconds, making it arrive before it was sent.
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