the universe. The belt of life hypothesis (Marochnik and Mukhin, 1988: 49–59) suggests a narrow circular zone passing between the spiral arms of the galaxy, Perseus and Sagittarius, which includes the Sun’s galactocentric orbit. Once a planet enters these arms, all life will perish due to the radiation from the exploding supernovae there. The spiral arms, so the hypothesis goes, are places of life and death. If the galactic belt hypothesis is correct, then proximity to the spiral arms limits the first and last factor in the Drake equation: the number of suitable stars will be limited to those in the belt, not the galaxy as a whole, and the length of technological civilization will be determined by the time spent between the two arms. There is, however, nothing in the belt of life hypothesis to suggest that life will appear between the galactic arms; all that can rightly be inferred from the hypothesis is that any probability of intelligent life will be restricted to a smaller area of the galaxy than is predicted by the more optimistic estimates. One cannot argue from factors which would place a limit on the number of technological civilizations to a more accurate prediction of the number of such civilizations.
Further restrictions upon the probability of intelligent life have been introduced, by Antony A. Stark’s (1988: 291–3) ‘sleeping dragon’ hypothesis, according to which the galactic centre is full of dense gases which become unstable every few million years or so. Intense bursts of Gamma-rays, X-rays and other energetic particles have been detected in the galactic centre. According to Stark, ‘Every few million years the dragon awakes, and sterilizes the galactic centre with a burst of fire’ (1988: 293). There is also a school of thought which supports the view that there is a gigantic black hole at the galactic centre. On these terms the development of civilizations will consequently be curtailed by periodic disasters, thus limiting the factor of L in regions located towards the centre of the galaxy.
According to James Annis (1999: 19), who is an astrophysicist at Fermilab near Chicago, periodic cataclysmic Gamma-ray bursts may sterilize entire galaxies, wiping out all life-forms before they have evolved sufficiently to either leave their planet or establish the ability to communicate with other planets. These Gamma-ray bursts (GRBs), he claims, are probably the most powerful explosions in the universe and are believed to be caused by collisions between either neutron stars or black holes. Annis maintains that each GRB unleashes massive radiation. If, for example, one went off near the centre of our galaxy, within seconds the Earth would be exposed to a massive wave of Gamma-rays that would exterminate almost every form of life. Even those which survived the initial zap of Gamma-rays, through shelter on the planet’s dark side, would be wiped out by indirect effects such as the destruction of the ozone layers which protect from UV radiation. So it would seem that the effect of a GRB within a galaxy is that the evolution of life is pushed back to zero.
If this is the case, then the rate at which GRBs occur might profoundly influence the factors, f i to f c in Drake’s equation, limiting the prospects of an emergent communicative network. The same limit would apply to developments
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in space travel and colonization, as a high incidence of GRBs throughout the universe would suggest that civilizations were unlikely to reach the threshold required for space travel. Apparently the rate of GRBs is one burst per galaxy every few hundred million years. Now this might allow a few civilizations to emerge and even serve as an incentive to communication and space colonization, as knowledge of future destructive episodes became apparent. However, Annis argues that the rate of GRBs was actually much higher in the past, with galaxies receiving one burst every few million years, which would almost certainly prohibit the evolution of an intergalactic civilization. Thus the reason we are not in contact with ETIs is that no civilization has been given enough time to develop that far.
This thesis has an intriguing implication: if GRBs are the main inhibiting factor on space communication and colonization, and so far they have not prevented the inhabitants of Earth from developing interstellar communication and reaching the threshold of space colonization, then other forms of intelligent life may be appearing in our galaxy and will continue to do so until we are all destroyed by the next GRB. So either we shall be in contact soon or we shall be annihilated when the sleeping dragon awakens.
Conclusion
The Drake equation is an edifice constructed from degrees of guesswork based on a range of theories which vary in evidential support. Only R* and f p are supported by observations extending beyond our solar system. The equation makes no claims on behalf of the immunity of any of its component theories and inferences drawn from them carry only a theoretical possibility. Taken together each factor in the equation amounts to a chain of probabilities. Frank J. Tipler (1980: 273) has argued that ‘the problem with the equation is that only f p – and to a lesser extent ne – is subject to experimental determination’. To measure probabilities with any degree of accuracy would require a large sample. But, argues Tipler, ‘for f e, f i and f c we have only one obvious case, the Earth’ (ibid.). An unkind critic might say that the Drake equation is nothing more than a piece of inspired guesswork; comparable to guessing the number of stray dogs in Manchester by multiplying estimates of the canine birthrate by the number of food-containing waste-bins by a guess at the average life-span of a dog. In a more favourable light the equation might be said to provide a heuristic guide whereby new theories can be appended and various blanks filled in. Consequently the numbers which make up N will vary according to the theories currently used to underpin any of the stages in the equation and, of course, the personality of the scientist. Drake (1990), optimistically, argues that N = tens of thousands of communicating intelligences across the galaxy. Others are less optimistic.
At the other extreme, pessimists have assigned a value of 1 for N. The N = 1 civilization argument is based on a version of Fermi’s Paradox (examined in Chapter 6), which maintains that if expansion into space is a consequence of
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