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. 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 [1]. 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" [2]. 

 

 

      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.

      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. 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 [3]. 

 

      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.

 

      So what is to be concluded from all of this? Is interstellar flight indeed

      as "improbable" as the nay-sayers 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"

                                   [4].

 

      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. But the galaxy is ten billion years old, with second-generation

      stars up to nine billion years old. Thus, the time needed to colonize the

      galaxy is much shorter than the age of the galaxy. 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 argued, with good reason, that such concepts could be realized

      with current technology in just a few decades.

 

      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 such 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.  Another possibility is that

      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 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," [5].

 

      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" [6]. 

 

      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. Neither technical

      feasibility, nor energetics, economics, and social factors are likely to

      prevent interstellar travel or slow the colonization of the galaxy.  The

      probabilities appear to be heavily in favor of aliens turning up on our

      doorstep, if there were any aliens to do so. 

 

 

 

                                                                               

      Works Cited:

 

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

     

      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, Oktober 1996.

 

       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.

 

      Mallove, E. F., and Matloff, G. L., "The Starflight Handbook," Wiley

      Science Editions, John Wiley and Sons, Inc., New York, 1989.