Barrow-Tipler Argument against Extraterrestial Life:

Note: The Drake Equation has been proposed with different variables by dissenting astronomers. The one I use below is in my estimation, the most popular version.


In Chapter 9 of their book The Anthropic Cosmological Principle, physicists John Barrow and Frank Tipler propose an argument they claim shows that Extra-Terrestrial Intelligent life does not exist anywhere in our Galaxy.


In sections 9.1 and 9.2 they spend some time setting up their refutation.Not much in these sections does anything to advance this negative thesis.Actually these sections seem to be trying to illustrate how human life could travel the cosmos and colonize alien worlds.


Finally in section, 9.3 they get to the heart of their argument.

The equation that Barrow & Tipler use to prove there can't be more than one intelligent species in this Galaxy is:



fp is the probability of star systems with planets

neis the number of habitable planets in a star system that has planets

flis the probability that life evolves on a habitable planet

fiis the probability that intelligent self-aware life evolves on a habitable planet

fcis the probability that intelligent life will attempt interstellar communication within 5 billion years of evolving on a habitable planet.

Ng number of star in our galaxy


It's called the Drake equation. Its variables are sometimes named differently, but these are the usual ones. As you can see this is a numerical estimate equation based on probabilities and when we estimate the component probabilities times the Ng (number stars in our galaxy), it will give the number of planets where there might be alien species. To support Barrow and Tipler's thesis, if ne the number of planets in our galaxy is small (as of yet unknown), and Ng the number stars in our galaxy is large (and they are), then the probabilities surrounding these numbers must be small. At least that is what is inferred. Please note this is inference, not implication, in the logical deductive method. It will be important later, as I make conclusions about this weak, very weak anti-alien life argument. One estimate they give is 1/100,000,000,000. One in a hundred billion? If this is a fair estimate, I would most certainly agree that the likelihood of other intelligent life in this Galaxy is very small, and thus we ARE number one! So, the Drake equation is an effective prediction of alien life, if its component probabilities are accurate. But, we can't make accurate estimates of these probabilities being Earthbound. We don't know where a planet with species like ours or any other exist. Or where such a planet (or planets) might come into being. Thus, these probabilities are guesses. The predictable numbers we can estimate are the number of stars and planets, but all the rest are guesses.


The other thing to notice is the equation sets a limit on how long it would take intelligent species to go looking for others like it. This is important because we know that the universe has a definite age. By deduction, so do the galaxies. This means intelligent life other than us must evolve, and go searching for others like it within a strict period of time. This is crucial. In fact, the whole argument hinges on this point. They show if we take fc's normal distribution as being peaked at 6 billion years and a standard deviation of 1 billion years, this implies just one intelligent species. Of course if the normal distribution is larger, the likelihood is greater, but we won't split hairs on this point.


In addition, there is an assumption, which this probability must refute: the Principle of Mediocrity.That is, we are nothing special in the Galaxy, that in all the star systems throughout this Galaxy a similar set of circumstances could have led to the formation of life like ours. There are an estimated 500 billion (this estimate varies widely, I am using the largest one) stars in this Galaxy. If p turns out to be small, then we can negate the Principle of Mediocrity and say, we ARE something special and life did not evolve in numerous star systems in this Galaxy.


The problem with the Drake equation is the fact that fp and ne are experimental and estimates differ.The total number could be much larger depending on these values.They admit this.


Now, lets put this probabilistic argument aside.It contains variables, which are not reliably determined and thus possibly inaccurate.It would work well, if we restrict it to our Solar System. The real problem with what Barrow and Tipler are arguing is they are rightly showing that if extraterrestrial species exist, then they have not shown themselves to us.But, this doesnít prove they donít exist.We just have to think of dolphins on this planet, (whom biologists believe to have intelligence near equivalent to our own) to know that intelligent life can exist, but be incapable of traveling or otherwise communicating with species outside their worlds.That is one GIANT flaw to their argument.They are assuming that extraterrestrial life (ETI) will have both the means and an environment that will be conducive to the desire to communicate with alien species.We cannot assume that self-aware, intelligent life will take a physical form that allows it to produce devices that can signal or reach us.In fact, there could be any number of real impediments to ETIís not contacting or reaching us.And I donít mean their killing themselves off.Believing that they have the answer to those whom say, no evidence of ETI is not proof that ETIs do not exist, they counter that this is exactly what it is!If aliens exist, why donít we see them?They shouldíve come to our solar system by now?Again, believing they have found the right point of attack, they go on to show how intelligent species would have an incentive to explore and colonize the Galaxy.The incentive could be a dying world, or shrinking resources.†† They forget that the physical formation of life in a distant star system could be a major reason why it has not contacted us.An ETI could exist, but it could still not have the means to contact us, due to its physical structure.In this case, the ETI would exist, but not have been detected by us, or we by them.This is an important point.Barrow and Tipler donít seem to be excluding life in general from the Galaxy.Since they have precisely framed their own argument with probabilistic notions, it is easy to say: Canít there be non-intelligent life in the Galaxy?Do you think that there are any planets with single-celled or microbial life on them?And there is nothing in this argument to preclude the possibility.It is quite easy to imagine a scenario where simple forms of organic life could exist on a planet, and be held in that state for billions of years, never reaching our level of development.Still this would amount to alien life existing in our Galaxy.

Another fallacious argument about astronomical observations was put forth in the 19th century. It bears striking similarity in method to the Barrow-Tipler conjecture. It used the Scientific Method to make this conclusion. Click below to see how.

Obler's Paradox


Conclusions and Remarks

Barrow and Tipler are not making a sound inductive argument for the non-existence of ETI in our Galaxy.They are making a speculative argument for this non-existence.They make an obvious error in believing that not having evidence of ETI in our Solar System implies no ETI in the Galaxy.Using the probabilistic theory of Drake and Sagan, they believe a valid conclusion is made as to why no ETI exists in the Galaxy.This is the mistake. I quote:


It is important to note that the above argument uses the observed evidential fact that the ETI are not present in our solar system; the situation is not the one implied by the epigram to this section, Ďabsence of evidence is not evidence of absenceí.Rather, the evidence is that ETI are absent from our Solar System, and from this observed fact (and other astrophysical observations and theories) it is inferred as a logical consequence that ETI are absent from the Galaxy.


What the above quote is saying in different terms is we donít see any aliens in our Solar System, so we propose a theory why that is the case.We form a probabilistic model that should predict not just whether there are life forms in our Solar System, but the whole Galaxy.That model tells us it is not very likely.We conclude not only are there no aliens in our Solar System but that none exist in the whole Galaxy.The error in this conclusion is: the model is speculative and contingent upon large samples.They try to gloss over this problem with some additional constraints, but I believe them to be insufficient.So, their conclusion canít apply to the entire Galaxy.That is a mistake.


It can apply with ease to our Solar System.We know Ng and ne in our Solar System.So, yes they are right, there is one intelligent species in our Solar System, e.g. us.This is not such an obvious conclusion.There are many whom have posited alien visitation to Earth.Others claim that life forms have existed here in the past and created the ancient ruins of Egypt or those in South America.The Drake equation tells us this isnít so, and was not so in the past.And here is where Barrow and Tipler err.Their conclusion should be reduced to one about the Solar System not the Galaxy.


Both these physicists believe so strongly in their conclusion, because the feel theyíve used the Scientific Method to arrive at it.Both they and their critics agree, we donít have much evidence of ETI, here on Earth.However, unlike those that simply go on looking for it, Barrow and Tipler use a model to account for this lack of alien life.If the model agrees with reality and tells us no alien life, then we conclude no alien life.The problem is if the model is wrong or too broadly applied (as it is in this case), you still arrive at a false conclusion.


However, the argument Barrow and Tipler make for the scarcity of ETI in the Galaxy is not completely without merit.I believe that they are close to correct. Though, their method is weak, they still are on the right track.To be more exact, I speculate that there may be at most 1 perhaps 2 other species like us in the entire Galaxy.It does not take a very sophisticated string of inductions to see this. Here is a conservative estimation based on the Drake equation.


Lets take Ng as=the number of stars in our Galaxy


Assume Ng=500 billion stars.


If fp is only 1%, then we have Star systems with planets, Nss=5.0*109*.01=5,000,000,000.


If the probability of these systems having Earthlike planets is .01, then we have, Nearthlike= 5.0*109*.01=50,000,000.


If again the probability of these Earthlike planets having life on them is .01, we have, Nel= 5.0*107 *.01=500,000.


Taking the probability of intelligent and communicative life being on this set of planets at .01, we have, Nlc= 5.0*105 *.01=5,000.


We now have 5000 planets in this Galaxy that might have some form of life, that is close to ours, and can possibly communicate with us.While this may seem like few to the uninitiated, this is way, way too many! Believe it or not, itís way, way too many!Remember the Earth is a plenum with life. There are 6 billion people, trillions of insects, many millions of mammalian animals and microbial life that exceeds named numbers.If we consider life historically, then the numbers are even larger.We create detectable emissions that reach far out into space.If weíve got 5000 sources of this kind of radiation, then weíd be detecting all kinds of life through radio telescopes and other signal collecting devices. Lets add one more probability to this estimation; the probability of the 5,000 planets with intelligent and communicative life having used nuclear weapons.After all, if there are several planets with this destructive force, a probability of their use is a real possibility.This time, based on our Earthly experience (meaning we used nuclear weapons) I will up the probability to 5%.


The number of planets of that engage in nuclear destruction is Nnd=5.0*102*.05=250.


If Nnd is 250 worlds, then the original estimate is even more unlikely.The radioactive fallout of that many nuclear conflagrations, would almost certainly be detected by the low Earth orbit Hubble telescope even across several thousand of light years of space. So, even if we conservatively estimate ETI, we are bound to speculate that the number must be exceedingly small.


  I have noticed since writing this review, that the Barrow-Tipler argument is in fact a prime example of a well-known logical fallacy called Denying the Antecedent. This fallacy is symbolically written as


  If A then B.



  Therefore ~A.


  Applying this to our case we have:


  If there is alien life in the Galaxy, then we will detect it.


  Alien life has not been detected in our Galaxy.


  Therefore there is no alien life in the Galaxy.


  This is a simplified form of the fallacy, in most cases it has a major and minor premise, nevertheless it exhibits the fallacy. Why is this argument fallacious? If the consequent of a categorical argument is not fufilled it does not invalidate the premise! For instance: If it doesn't rain today, I will jog for 3 hours. I did not jog for 3 hours It rained today (applying the algebra of a negative times a negative makes a positive for the major premise). Now, clearly my not jogging for 3 hours couldn't cause it TO RAIN. The error of this reasoning is eye-rolling clear, right? The consequent of the antecedent doesn't EVER invalidate it, if the consequent isn't fulfilled. Here, we find these scientists mixing two very different forms of logic: hypothetical and categorical. In hypothetical logic we propose an explanation, then find evidence to support it. It is inductive in nature and contingent, i.e. it can change and be reworked depending on the evidence supporting it. Categorical logic, on the other hand is deductive and deals with valid forms of deducing conclusions. It is immutable and akin to (in fact the progenitor of) mathematical logic. Arguments are said to be reduced from proposition to conclusion. Its principle model is syllogism from which other models are derived. Barrow and Tipler are mixing these two different forms of inquiry. They want to take an inductive argrument and make it categorical; it doesn't work. This is why, I can't agree there are NO alien species in our galaxy. Only a categorical argument can come to an absolute conclusion. However, a hypothetical argument can come to a general conclusion. Which is why I assent that if there are alien species in the galaxy, the number is small. This inductive conclusion is sound and in my opinion likely.




Ken Wais 10/27/03

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