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To: mrobison
Is it just me or was Clarke not talking about time travel at all and the reporter leaped to the conclusion? I got the impression he was speaking of travelling faster than the speed of light for purposes of space exploration which is not the same thing as time travel.
127 posted on 09/30/2003 10:48:29 AM PDT by Naspino
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To: Naspino
was Clarke not talking about time travel at all

You are correct. Journalists often make this leap into the abyss when they write about things of science.

129 posted on 09/30/2003 10:50:48 AM PDT by RightWhale (Repeal the Law of the Excluded Middle)
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To: Naspino
Is it just me or was Clarke not talking about time travel at all and the reporter leaped to the conclusion? I got the impression he was speaking of travelling faster than the speed of light for purposes of space exploration which is not the same thing as time travel.

FTL implies the possibility of time travel. Here's an example I worked out for an old thread:

Just after the stroke of midnight on January 1, 3000, a tachyon beam signal, travelling at four times the speed of light, is sent from Earth towards the starship Tempus Fugitive. The message is "Ping!" At the time the message is sent, the ship is 0.8 light years from Earth, travelling at a speed of 0.8 times the speed of light.

By the time the tachyon signal reaches the starship, it is 1 light year away, as measured from the Earth. But on the Tempus Fugitive, the Earth is only 0.6 light years away (Lorentz contraction).

The date of this event is April 1, 3000, just after 6:00 AM, as measured on Earth. But in the reference frame of the starship, this event is contemporaneous with events taking place on the morning of January 30, 2999 on Earth (frame dependence of simultaneity). [Geek alert: t' = gamma*t - L*beta*gamma/c; if t=0.25 years and L=1 l.y., beta=v/c=0.8, and gamma=1/sqrt(1-beta^2)=5/3, then t'=-0.9166 years.]

The Tempus Fugitive replies with an "Ack!" upon receipt of the message. It takes .15 years for the signal to traverse that distance, but the Earth is travelling away from the starship at .8 c, so the signal takes .1875 years or 68.4 days for the signal to reach Earth. But in the starship's frame of reference, time on Earth is moving only at .6 its regular speed, so only 41 days pass there (time dilation). The return signal arrives on Earth on March 11, 2999, almost nine months before the original message was sent.

153 posted on 09/30/2003 11:15:08 AM PDT by Physicist
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