In sci.physics.relativity,

wrote
on Tue, 11 Mar 2008 12:08:30 -0700 (PDT)
:
Hello again,

Assume I take a spaceship to Alpha Centauri, which is 4 light years
away. And the ship goes so fast, that the travel time seems to me to
be one hour. Will it not seem to me that Alpha Centauri made a journey
that started at x=4[ly] and ended at x=0, during a time of one hour,
and therefore its speed was much faster than lightspeed? Where is my
mistake?

Ram.

Several problems, but assuming they can be met, the conclusion is
reasonably sound; this is another expostulation of the famous
Twin Paradox. However...

[1] Gamma is about 4 years / 1 hour = 35000. This yields a velocity
of (1 - 4.06 * 10^-10) c and far more energy than one can get from
even an antimatter-based drive.

[2] The acceleration would be phenomenal, squishing one to a rather
flat pancake.

[3] Radiation during transit would fry you. If one assumes 1 proton
per cc of space and a cross-section of 10 m^2 (the size of a good
sized room on Earth), one gets 2.99792458 * 10^15 protons per
light-second or 3.78 * 10^23 protons over the trip. Since the trip
takes 1 hour subjective, that's 1.051 * 10^19 protons per second,
each of them with energy 32.8 TeV. Total power: 5.53 terawatt per
square meter. Smells like...like....victory...

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