Hunter has pointed out that by using a Jupiter swingby to approach the Sun and then giving a velocity boost at perihelion, a solar system escape velocity... is possible with present-day chemical rockets... [M]ost other stars should have planets (or companion stars) with characteristics sufficiently close to... the Jupiter-Sun system to use this launch strategy in reverse to slow down in the other solar system.
American physicist noted for his book The Physics of Immortality
(born February 1, 1947) is an American mathematical physicist and cosmologist, holding a joint appointment in the Departments of Mathematics and Physics at Tulane University. Tipler has written books and papers on the based on Pierre Teilhard de Chardin's religious ideas, which he claims is a mechanism for the resurrection of the dead. He is also known for his theories on the (Tipler time machine) and for the , an argument that no intelligent life exists outside of the Solar System. His work has attracted criticism, notably from systems theorist George F. R. Ellis who has argued that his theories are largely pseudoscience.
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[T]he information to manufacture a human being is contained in the genes of a single human cell. Thus if an extraterrestrial intelligent species possessed the knowledge to synthesize a living cell—and... experts assert... the human race could develop such knowledge within 30 years—they could program a von Neumann machine to synthesize a fertilized egg cell of their species. If they possessed artificial womb technology—and such... is in the beginning stages... on Earth... they could... synthesize members of their species... As suggested by Eiseley... these beings could be raised... by the robots... free to develop their own civilization...
[M]aterials [to reproduce the von Neumann machine] should be available in virtually any stellar system—including systems—in the form of meteors, asteroids, comets, and other debris from the formation of the stellar system. ...[M]aterial in asteroids are highly differentiatied; many... are largely nickel-iron, while others contain large amounts of hydrocarbons.
What one needs is a self-reproducing universal constructor... a machine capable of making any device... capable of making a copy of itself. Von Neumann has shown... that such... is theoretically possible, and... a human being is a universal constructor specialized to perform on the surface of the Earth.
I shall assume that such a species will... develop a self-replicating universal constructor with intelligence comparable to the human level—such a machine should be developed within a century, according to the experts...—and... combined with present-day rocket technology would make it possible to explore and/or colonize the Galaxy in less than 300 million years, for an initial investment less than the cost of operating a 10 MW microwave beacon for several hundred years, as proposed by SETI...
The basic idea... is straightforward and... has led other authors, such as Fermi... Dyson... Hart... Simpson... and Kuiper & Morris... to conclude that extraterrestrial beings do not exist: if they did exist and possessed the technology for interstellar communication, they would also have developed interstellar travel and thus would already be present in our solar system.