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␈↓ ↓H␈↓␈↓βS␈↓∧ Artificial Intelligence Laboratory, STANFORD UNIVERSITY, Stanford, California 94305
␈↓ ↓H␈↓∧Telephone 415 497-4971␈↓ 
≥July 18, 1975




␈↓ ↓H␈↓Engineering and Science
␈↓ ↓H␈↓Caltech
␈↓ ↓H␈↓1201 E. California St.
␈↓ ↓H␈↓Pasadena, Calif. 91125

␈↓ ↓H␈↓To the editor:

␈↓ ↓H␈↓        Fred␈α
Hoyle␈αis␈α
indulging␈αin␈α
wishful␈αthinking␈α
in␈αconstructing␈α
a␈αmodel␈α
of␈αintelligent␈α
life␈αin␈α
the
␈↓ ↓H␈↓universe␈α
in␈α
which␈αinterstellar␈α
travel␈α
is␈α
impossible.␈α Why␈α
he␈α
wishes␈α
it␈αto␈α
be␈α
impossible␈α
is␈αbeyond
␈↓ ↓H␈↓me despite his explanation, but here are some facts:

␈↓ ↓H␈↓        1.␈α
There␈α
is␈α
no␈α
scienti≡cally␈αplausible␈α
way␈α
of␈α
travelling␈α
at␈α
speeds␈αclose␈α
to␈α
that␈α
of␈α
light␈α
-␈αlet
␈↓ ↓H␈↓alone close enough to get signi≡cant time dilation.

␈↓ ↓H␈↓        2.␈α∂Using␈α∂a␈α∂nuclear␈α∂reactor␈α∂like␈α∂those␈α∂used␈α∂today␈α∂in␈α∂submarines␈α∂to␈α∂expel␈α∂a␈α∂working␈α∞∨uid
␈↓ ↓H␈↓electrically␈α∞at␈α∞the␈α∞optimal␈α∞velocity␈α∞(energy␈α∞and␈α∞mass␈α∞∨ow␈α∞have␈α∞to␈α∞be␈α∞compromised␈α∞and␈α∞the␈α∞best
␈↓ ↓H␈↓velocity␈αvaries␈αduring␈αthe␈αjourney)␈αleads␈αto␈αtimes␈αbetween␈αseveral␈αhundred␈αand␈α
several␈αthousand
␈↓ ↓H␈↓years to nearby stars.

␈↓ ↓H␈↓        3.␈αThe␈αtime␈αrequired␈αfor␈α
the␈αjourney␈αis␈αproportional␈αto␈α
␈↓↓p␈↓¬1/3␈↓↓s␈↓¬2/3␈↓,␈αwhere␈α␈↓↓p␈↓␈αis␈αthe␈α
power␈αper
␈↓ ↓H␈↓per␈αunit␈αmass␈αof␈αthe␈αsystem␈αand␈α␈↓↓s␈↓␈αis␈αthe␈αdistance␈αto␈αbe␈αtravelled.␈α The␈αderivation␈αcan␈αbe␈αsupplied
␈↓ ↓H␈↓if␈αwanted.␈α The␈α␈↓↓p␈↓¬1/3␈↓␈αfactor␈αmeans␈αthat␈αthe␈αperformance␈αis␈αnot␈αvery␈αsensitive␈αto␈α
improvements␈αin
␈↓ ↓H␈↓the␈αpower␈αsystem.␈α
 The␈α␈↓↓s␈↓¬2/3␈↓␈αfactor␈α
means␈αthat␈αlong␈α
distances␈αare␈αcovered␈α
in␈αless␈αthan␈α
proportional
␈↓ ↓H␈↓time.

␈↓ ↓H␈↓        4. The galaxy can be occupied in less than a million years.

␈↓ ↓H␈↓        These␈αtimes␈α
look␈αvery␈αlong␈α
to␈αus␈α
-␈αonly␈αa␈α
couple␈αhundred␈αyears␈α
into␈αthe␈α
technological␈αage.
␈↓ ↓H␈↓However,␈αour␈αsolar␈α
system␈αcan␈αsupport␈α
life␈αfor␈αa␈αfew␈α
billion␈αyears,␈αand␈α
after␈αman␈αhas␈α
had␈αspace
␈↓ ↓H␈↓technology␈α∩for␈α⊃10000␈α∩years,␈α⊃surely␈α∩the␈α⊃nearer␈α∩stars␈α⊃will␈α∩be␈α⊃explored,␈α∩and␈α⊃unless␈α∩colonies␈α⊃are
␈↓ ↓H␈↓prevented from seceding by some very e≥ective dictatorship, the expansion process will start.

␈↓ ↓H␈↓        My␈αown␈αguess␈αis␈αthat␈αif␈αthe␈αbureaucratic␈αethic␈αof␈αpresent␈αliberal␈αthought␈αcontinues␈αfor␈αeven
␈↓ ↓H␈↓a␈α∞few␈α∞tens␈α∂of␈α∞years,␈α∞there␈α∂will␈α∞develop␈α∞a␈α∞very␈α∂strong␈α∞desire␈α∞among␈α∂small␈α∞groups␈α∞to␈α∂escape␈α∞into
␈↓ ↓H␈↓space from the welfare state.


␈↓ ↓H␈↓Sincerely,



␈↓ ↓H␈↓John McCarthy
␈↓ ↓H␈↓BS '48