With only three minutes of airtime between us (including the host’s comments), neither Aczel nor I got to say much about the probability for extraterrestrial intelligent life. (We both did agree that the probability for intelligent life on Earth is nearly, but not quite, one.) Since the “debate,” however, I have received several questions from listeners and others asking for my response to Aczel’s book, Probability 1.
In a nutshell, Aczel’s argument is that astronomer Frank Drake’s equation for the number of planets containing extraterrestrial intelligent life must be equal to or greater than 1. He reasons that even though several factors in the Drake equation may have very small values, all it takes is one factor in the equation to possess an infinite value for the outcome to be one or greater. Aczel has a valid point. An extremely tiny fraction multiplied by infinity equals infinity. However, zero multiplied by infinity equals zero. Therefore, if any factor in the Drake equation has a zero value, then no matter how large the values the other factors may possess, the probability of the universe containing another planet with intelligent life would be zero.
Human knowledge about the universe is not complete, nor will it ever be complete. Thus, nothing in science can be assigned with absolute certainty—either a zero value or an infinite value. To settle the debate between Aczel and me one must use realistic values (or at least possibly realistic values) for the different factors in the Drake equation.
The big numbers that Aczel appeals to in the Drake equation are the number of planets that may exist in the observable universe and how much larger the actual universe may be compared to the observable universe. In my 90-second part of the radio debate I conceded that the observable universe may contain as many as 1022 (ten billion trillion) planets. This number is certainly an upper limit since it is based on the assumption that all the other galaxies in the observable universe are just as planet-rich as the Milky Way Galaxy. In truth, well over 90 percent of the galaxies in the observable universe manifest conditions that would make them planet-poor compared to the Milky Way Galaxy, and astronomers see no galaxies that would be significantly richer in planets than the Milky Way Galaxy.
Owing to the time it takes light to travel from distant galaxies to astronomers’ telescopes, researchers observe the universe of the past, not of the present. Thus, because the universe has been continuously expanding since the cosmic creation event, the actual universe (that is, the universe of today) must be much larger than the observable universe. How much larger depends on what kind of geometry and dimensionality one chooses to assign to the universe. Aczel appeals to dimensionality speculations that undergird the multiverse hypotheses. That is, he presumes that in addition to the universe in which we live there may exist a very large or even an infinite number of other universes.
No physical or testable evidence exists, however, for any of the multiverse speculations. According to the principles of general relativity, such evidence is not even potentially possible for physical observers, like human beings, who are confined to the space-time surface of the universe.
Even the theoretical case for the multiverse is thin. Some physicists have pointed out that in certain string theory formulations there might exist as many as 10500 different possible string theory solutions to the universe. The possibility for that many theoretical solutions has led some cosmologists to speculate that for every possible solution there may exist an actual universe.
The combination of 1022 times 10500 gives Aczel a very large number indeed. His enormous number, however, must be multiplied by the probability that a randomly selected planet would manifest the necessary characteristics that would permit intelligent life to exist and develop a globally distributed civilization. That result must be multiplied again by the probability that an origin of primitive life would occur on such a planet, and that result multiplied by the probability that primitive life would evolve into advanced life capable of launching a global civilization. There are other small-valued factors to be considered as well, but these three are adequate to make the point that the probability for extraterrestrial intelligent life is much less than 1.
On the Reasons To Believe website we document that the probability a randomly selected planet would possess all the characteristics intelligent life requires is less than 10-304. A recent update that will be published with my next book, Hidden Purposes: Why the Universe Is the Way It Is, puts that probability at 10-1054. In the book I wrote with Fuz Rana, Origins of Life, we describe a calculation performed by biophysicist Harold Morowitz in which he showed that if one were to break all the chemical bonds in an E. coli bacterium, the probability that it would reassemble under ideal natural conditions (in which no foreign elements or chemicals would invade and in which none of the necessary elements or chemicals would leave) would be no greater than 10-100,000,000,000.1 In another book I wrote with Fuz, Who Was Adam?, we describe calculations done by evolutionary biologist Francisco Ayala and by astrophysicists John Barrow, Brandon Carter, and Frank Tipler for the probability that a bacterium would evolve under ideal natural conditions—given the presumption that the mechanisms for natural biological evolution are both effective and rapid. They determine that probability to be no more than 10-24,000,000.2
The bottom line is that rather than the probability for extraterrestrial intelligent life being 1 as Aczel claims, very conservatively from a naturalistic perspective it is much less than 10500 + 22 -1054 -100,000,000,000 -24,000,000. That is, it is less than 10-100,024,000,532. In longhand notation it would be 0.00 … 001 with 100,024,000,531 zeros between the decimal point and the 1. That longhand notation of the probability would fill over 20,000 complete Bibles. In other words, the probability is as close to zero as any scientifically determined probability has ever been.
The probability, of course, could be much higher if God supernaturally intervenes. For a spectrum of Christian interpretations on the probability of supernaturally created extraterrestrial life, see an article by three Christian astronomers published in Facts & Faith.3