The ETH and  the Likelihood of Interstellar Travel

by Jean van Gemert

If we at once admit the foolishness of these perennially suggested "impediments" to star flight, we will be on our way to understanding that interstellar space does not need a bridge too far. Interstellar travel may still be in its infancy, but adulthood is fast approaching, and our descendants will someday see childhood's end - Dr. Eugene Mallove and Dr. Gregory Matloff, The Starflight Handbook, 1989.

The (un)likelihood of extraterrestrial visitation is probably one of the most debated aspects of the Extraterrestrial Hypothesis, the answer being an essential component to the validity of the ETH. After all, the assumed unlikeliness of interstellar travel has become the cornerstone of those who resist the ETH as an explanation for UFOs. So, does extraterrestrial visitation necessarily require all sorts of "unlikely" science, or is it possible to accomplish interstellar travel using conventional wisdom? 

Can they get here?

Opinions on the practicality of interstellar travel diverge, but the negative and positive opinions are seen to stem primarily from the differences in background of those people doing the studies. SETI researchers think that the degree of dispersion of stars throughout the galaxy, combined with the limitations of interstellar travel as we understand general relativity, effectively preclude the feasibility of extraterrestrial visitation, thus believing that any extraterrestrial intelligence would only be transmitting their love and good wishes to us. The other group, largely composed of physicists and engineers involved in propulsion research, tends to believe that interstellar travel is difficult, but not a barrier, or not difficult at all once technology progresses [Mallove and Matloff, 1989; Forward, 1986; Crawford, 1990]. Not surprisingly, the latter choice appears to be the most defensible. 

Quite a number of clever designs have appeared in print, describing various methods of getting mankind to the stars. These include such projects as the star probe Daedalus, a robot interstellar vehicle designed by members of the British Interplanetary Society which uses nuclear fusion power, or interstellar ramjets which scoop up their fuel between the stars. Physicist Robert Forward, one of the leading experts on space travel, has also proposed an entirely different method of interstellar propulsion, using photon pressure to accelerate a vehicle to a significant fraction of the speed of light in a few years [Forward, 1984]. Such ships would appear as huge sails, using the output of space-based orbital power platforms (Beamed Power Propulsion) for acceleration, which would also eliminate the need for an on board energy supply [Mallove and Matloff, 1989; Crawford, 1990]. Hence, much less mass would need to be accelerated. The important point here, as astronomer Ian Crawford notes, is that we "can already identify technological solutions to the problem of interstellar travel that are consistent with the laws of physics as we currently understand them. We do not need new physics" [Crawford op. cit., 1990]. 

Another factor relevant to interstellar flight is that of relativistic time dilation. Any object traveling close to the speed of light will be subjected to effects predicted by Einstein's Special Theory of Relativity. That is, an observer on board a spaceship traveling close to C, would observe that the passage of time on earth has speeded up, and the passage of time on the spaceship relative to an observer on earth would appear to have slowed down. For example, a one-way trip to Alpha Centauri would take, assuming a constant acceleration of 1g up to a high relativistic speed during the first half of the flight and a constant deceleration of 1g during the second half, only 3 years spaceship time, while 6 years will have passed outside the spaceship. 

Moreover, recent ideas on speculative space propulsion may just bring us the breakthrough we've all been waiting for, making use of yet undiscovered "loopholes" in physical laws, that would allow fast transit between widely separated points in space-time [Alcubierre, 1994; Visser, 1989; Crawford, 1995]. It might even be possible to extract large quantities of energy from the zero point field (the vacuum) itself. If this can be done in a practical way, then the energy available to a space traveler could be essentially unlimited, eliminating the need for an on board fuel supply [Froning, 1986]. 

 Too Expensive?

Although it simply is impossible to precisely now how expensive interstellar travel would be for a civilization about which no pertinent data is available to make an estimate, we can still make some educated predictions. Interstellar travel appears not to be expensive for an advanced economy whose productivity has grown steadily for millennia. Therefore, alien contact by visitation is likely once these advanced economies implement interstellar propulsion technologies at insignificant costs relative to their wealth and capital stocks which have grown steadily for millennia. Similarly, an interstellar transportation system may seem expensive from our perspective, but, then, so would a 747 to the Wright brothers [Jones op. cit., 1995]. 

So what is to be concluded from all of this? Is interstellar flight indeed as "improbable" as the naysayers claim? Indeed, only if we grant them their negative and self-defeating assumptions. And with that in mind, we quote Ian Crawford from the October 1996 issue of the New Scientist, in which he neatly outlined the current situation with regard to the feasibility of interstellar travel: 

"It seems unlikely that interstellar spaceflight is impossible. Even today, we can envisage propulsion strategies which might make it possible to reach between 10 and 20 per cent of the speed of light, permitting travel between nearby stars in a few decades. Any civilization with this technology would be able to colonize every planetary system in the Galaxy in about 10 million years, which is only one-thousandth of the age of the Galaxy" [Crawford, 1996].
Where are they?

Computer simulation and mathematical modeling of attempts to colonize the galaxy show that this could be accomplished in no more than a few million years [Hart, 1975; Jones, 1976; Papagiannis, 1978]. But the galaxy is ten billion years old, with second-generation (metal-rich) stars up to nine billion years old. Thus, the time needed to colonize the galaxy is much shorter than the age of the galaxy [1]. Moreover, in 1974, physicist O'Neill published his research on space settlements, describing large artificial structures capable of holding vast numbers of people [O'Neill, 1974]. O'Neill argued, with good reason, that such concepts could be realized with current technology in just a few decades. Frank Tipler, Hart, and quite a number of other scientists noted the relevancy of O'Neill's designs with regard to the Fermi debate, suggesting that these habitats, equipped with propulsion, could also be used to colonize other star systems. 

The consequences should be clear. There is no need to invent fantastic propulsion systems such as "warp" or "hyper drives." Current available technology would make it possible, in principle, to colonize the galaxy. And this, coupled with earlier calculations on expansion rates, suggested that if any extraterrestrial civilizations exist at all, they should have visited our solar system by now. But there is no evidence of such visitation to Earth. Enrico Fermi called this the "Where are they?" question - which is now known as the Fermi Paradox. 

Hart and other human-centrists as physicist Frank Tipler believe it proves us to be the only intelligent civilization in the galaxy, while SETI researchers tend to see the Fermi Paradox as "proof" that interstellar flight is entirely impossible. Other possibilities that have been advanced are: 

  • Extraterrestrial civilizations are short-lived. If the lifetime of an extraterrestrial civilization is only 50,000 years on average, nobody would live long enough to colonize the whole galaxy.
  • Most advanced civilizations are concerned with "more important" matters and have not developed any interest in space exploration.
  • Earth is a colony because the the entire galaxy was colonized by one civilization long ago. Only they now have a sense of benign paternalism to any developing civilization. This is also known as the "Zoo Hypothesis," [Ball, 1973].
The problem with the first and second hypothesis, is that one would have to assume it to be true for every extraterrestrial civilization. It may very well be true in some cases, but hardly, without any exception at all, in a million (which is a modest estimate of the number of advanced extraterrestrial civilizations in our galaxy). Remember, only one civilization is needed to colonize the entire galaxy! 

It has also been suggested that extraterrestrial civilizations simply haven't had enough time to find us yet. However, this appears unlikely since it is quite possible to narrow down considerably the number of stellar systems one would have to search to find life-bearing planets, by making use of methodical search techniques based on known remote sensing capabilities such as interferometry. Moreover, as Hart argued, "the consideration that ETs have not yet had time to find earth is discounted by calculations that show that another intelligent species in the Galaxy would have found earth if their space exploration efforts began at least two million years ago" [Hart, 1975]. 

This author favors hypothesis three, that there is a "galactic club," an established network of old advanced civilizations, and that Earth is under a certain "quarantine." Thus, in my opinion there simply is no Fermi Paradox. And the only reason Fermi's paradox has remained a paradox to most scientists is because they have failed to recognize the evidence that there may be extraterrestrials in our own solar system. 


Assessment of the feasibility of interstellar travel indicates that it should be easily accomplished by an advanced society. Arguments, such as that they would not have had enough time to find us yet because of the number of stars to visit, are seen to be implausible [Hart, 1975; Jones 1976, 1995; Hoerner, 1995]. Neither technical feasibility, nor energetics, economics, and social factors are likely to prevent interstellar travel or slow the colonization of the galaxy [Papagiannis op. cit., 1980]. The probabilities appear to be heavily in favor of aliens turning up on our doorstep, which I suspect they may already have. 

References and Footnotes: It appears reasonable that within about 1 to 2 million years of attaining space technology, a civilization could have spread throughout the galaxy. Given a practical average speed of about 0.2 c, one can cross the galaxy in less than half a million years. It is a fairly serious problem that if species evolve to a technological level and are around for any considerable length of time, then where are they? The galaxy should be a fairly busy place by now. Likewise, if we suppose a race sent out only one expedition every 100,000 years from a given stellar system to colonize another stellar system, in another 100,000 years, each of these would send out another expedition. Therefore, the number of space colonies would double every 100,000 years. At this rate, in only million years, 1000 stellar systems would be colonized. In 2 million years, a million systems would be colonized. In 3 million years, a billion systems. And in less than 4 million years, they could theoretically fill the entire galaxy. 
  • Alcubierre, Miguel, "The Warp Drive: Hyper-fast Travel Within General Relativity," Classical Quantum Gravity, Vol. 11, 1994, pp. 73-77.

  • Ball, J. A., "The Zoo Hypothesis," Icarus, Vol. 19, 1973, pp. 347-349.

  • Crawford, Ian A., "Some Thoughts on the Implications of Faster-Than-Light Interstellar Space Travel," Quarterly Journal of the Royal Astronomical Society, Vol. 36, 1995, pp. 205-218.

  • Crawford, Ian A., "Interstellar Travel: A Review for Astronomers," Quarterly Journal of the Royal Astronomical Society, Vol. 31, 1990, pp. 377-400.

  • Crawford, Ian A., "Where are all the extraterrestrials?," New Scientist, October 1996.

  • Forward, R. L., "Feasibility of Interstellar Travel," Journal of the British Interplanetary Society, Vol. 39, 1986, pp. 379-384.

  • Forward, R. L., "Roundtrip Interstellar Travel Using Laser-Pushed Lightsails," Journal of Spacecraft and Rockets, Vol. 21, 1984, pp. 187-195.

  • Froning, H. D., "Use of Vacuum Energies for Interstellar Space Flight," Journal of the British Interplanetary Society, Vol. 39, 1986, pp. 410-415.

  • Hart, M., "An Explanation for the Absence of Extraterrestrials on Earth," Quarterly Journal of the Royal Astronomical Society," Vol. 16, 1975, pp. 128-35.

  • Hoerner, S., "The Likelihood of Interstellar Colonization and the Absence of its Evidence," in: Extraterrestrials: Where are They?, Cambridge University Press, 1995.

  • Jones, E. M., "Estimation of Expansion Timescales," in: Extraterrestrials: Where are They?, Cambridge University Press, 1995.

  • Jones, E. M., "Where is Everybody?," Physics Today, August 1985, pp. 11-13.

  • Jones, E. M., "Colonization of the Galaxy," Icarus, Vol. 28, 1976, pp. 421-22.

  • Mallove, E. F., and Matloff, G. L., "The Starflight Handbook," Wiley Science Editions, John Wiley and Sons, Inc., New York, 1989.

  • O'Neill, G. K., "The Colonization of Space," Physics Today, Vol. 27, September, 1974, pp. 32-40.

  • Papagiannis, M. D., "Strategies for the search for life in the universe," Dordrecht, D. Reidel Publishing, 1980.

  • Papagiannis, M. D., "Could we be The Only Advanced Technological Civilization in Our galaxy?," in: Origin of Life, Japan Scientific Societies Press, 1978.

  • Tipler, Frank, "Extraterrestrial Intelligent Beings Do Not Exist," Physics Today, April 1981, pp. 70-71.

  • Visser, Matt, "Traversable wormholes: Some simple examples," Physical Review D, May 1989, S. 3182.

NICAP Home Page