Issue 84, 1 July 2012 – 43rd Apollo Anniversary Year
On Certainty and the Limits to Scientific Knowledge
Stephen Ashworth, Oxford, UK
The Doomsday argument versus the human future
The Doomsday argument was first posed by astrophysicist Brandon Carter in 1983, and subsequently promoted by the philosopher John A. Leslie and the astrophysicist J. Richard Gott III. Nick Bostrom has also published several papers on the subject (for example here, here and here).
In Gott’s 1993 paper published in Nature, he used the Copernican principle and arguments from probability theory to conclude that the human species is not likely to colonise the Galaxy, and is likely to die out without leaving any intelligent successor behind it after a few million years (“Implications of the Copernican principle for our future prospects”, Nature 363 (27 May 1993), p.315-319). It is this version of the Doomsday argument which I shall mainly address here.
Gott’s argument recently received exposure and discussion on the Centauri Dreams blog, under the title “Space Exploration: A Closing Window?” (9 May 2012). My thanks to Paul Gilster at Centauri Dreams for raising this thorny issue again.
Obviously, Gott’s reasoning, and the Doomsday argument generally, flies directly in the face of the optimistic scenario proposed by space colonisation advocates, in which human civilisation occupies the Solar System, achieves long-term sustainability at much higher population and power levels than those of today, and ultimately sends its descendants to spread throughout the Galaxy.
So how does Gott reach his pessimistic conclusions? Although his paper has a scattering of equations, his basic argument can be understood without recourse to mathematics: “The Copernican revolution taught us that it was a mistake to assume, without sufficient reason, that we occupy a privileged position in the universe” (p.315). Therefore, “Knowing only that you are an intelligent observer, you should consider yourself picked at random from the set of all intelligent observers (past, present and future) any one of whom you could have been” (p.316).
If, Gott argues, some phenomenon lasts a finite period of time, and if observations of it are made at random times between its beginning and its end, then such observations will usually come near the middle of the timespan and only rarely close to its beginning or end. From the point of view of the average observed point, the future duration of that phenomenon is not wildly different from its past duration: from most points of view, long-established phenomena tend to have long future lifetimes ahead of them; newly established ones tend to disappear just as quickly.
Again, by the same logic, if a small population of some new species appears, expands to a population bulge and then declines back towards extinction, most randomly chosen individuals from that species will find themselves in the population bulge, not in the earliest or last years of the existence of that species.
And if a terrestrial species were to develop spaceflight and it and its descendants colonise a large number of worlds throughout the Galaxy, the majority of individuals randomly picked from that intelligent lineage would belong to the colonist majority rather than the original terrestrial minority.
Gott is thus 95% confident that the human species will continue to endure for longer than 5,000 years but less than 7.8 million years, and equally confident that the total number of human beings yet to be born will be more than 1.8 billion but less than 2.7 trillion. If these ranges were eventually to turn out not to bracket the true value, then we here and now would find ourselves among either the first 2.5% or the last 2.5% of all human beings, which, according to Gott, is not a reasonable expectation. And galactic colonisation is not credible because of the extraordinarily untypical position it would put us in, living on Earth here and now, prior to the great colonial expansion.
Gott further claims that his arguments apply both to present-day humans and at the same time to any hypothetical post-human intelligent successor species (whether biological or robotic) which may arise in the future. Intelligence itself, in his view, is a product with a “best before” date on it, even if that date is a flexible one.
Problems with the Doomsday argument
Clearly, as a space colonisation advocate myself, I see problems with Gott’s arguments. While my attitude is certainly biased in favour of an infinite future for humanity, it may be that it is precisely this bias which enables me to find flaws in this use of the Copernican principle to which Gott himself (as also Carter, Leslie, Bostrom et alia) is blind, as he is not looking for them.
On the other hand, it may be that my natural bias is prompting me to invent non-existent flaws. I urge readers to make their own minds up, and to let me know if they find errors in what follows in this article.
The Doomsday argument is in my view entirely spurious, for two important reasons. Firstly, it is based on a subtle confusion between a single-choice experiment and a multiple-choice one.
Suppose that human history unfolds as follows: for a million years or so (depending upon where one draws the arbitrary dividing-line between human and pre-human) there is a tiny human population of around a million individuals at any one time. Then there is a burst of population growth from say AD 1 to AD 4000, stabilising again at around a quadrillion individuals (1015) living in high-tech colonies around the Solar System, who enjoy a sustainable low-growth mode of life for the next several billion years.
In such a hypothetical scenario, the vast majority of human and post-human lives will be lived in an extraterrestrial colony with a heritage of industrial civilisation running to at least millions of years. The fossil-fuel age and the time of the first space flights will be as remote from a typical representative of our lineage as the Cambrian explosion of animal life is from us today. If individuals are selected at random from the entire span of the history of civilisation, almost all of those selected will share these characteristics. On the other hand, the proportion of the population living before and during that brief period of growth is vanishingly small.
If we further allow a subsequent burst of expansion on a galactic scale, the disparity between these proportions is increased even further.
However, we do not in fact have the privilege of selecting people at random from the entire span – both past and future – of the history of human and human-derived intelligent life in the universe. Because of the nature of time and consciousness, such an experiment is in practice intrinsically impossible.
If we nevertheless try to imagine such an experiment, what we can see is the fact that somebody somewhere on the timeline of our species must of necessity find themselves in an unusual position. By hypothesis, these people are intelligent observers, and therefore capable of reasoning about probability and the Copernican principle.
So let us reformulate the problem this way: our position here and now on the timeline of human history may be typical of the majority position (as seen in retrospect), or it may be an unusual minority position. Which is it?
When the question is phrased in that way, Gott’s reasoning collapses. He assumes the Copernican principle, that, seen in retrospect, we will prove to have been typical observers. But this is precisely the point at issue: does the Copernican principle in fact apply to us here and now, or does it not? Are we in fact typical observers or unusual ones, when seen from a later time? Given the fact that abrupt changes in human lifestyles do happen, both kinds of observer must necessarily exist. We could be the one, or the other. Which are we? Gott can only assume that we are typical, because the majority of viewpoints randomly chosen – not by Gott himself but by a hypothetical far future observer – will be typical, but the key question is whether we belong to that majority or not, and Gott can only address this question by assuming the answer.
But surely is it not more probable that we will turn out to have been typical than untypical? Given that human population has been on an exponential growth curve for the last several centuries, and that its growth must eventually reach physical limits, then the graph of population over time looks like this schematic sketch:
We are asking: surely is it not more probable that we find ourselves, here and now, at point B rather than point A on the curve? When people are randomly selected from the span of human history, most of them come from somewhere near B, and people who find themselves near point A or earlier are unusual. But, as stated above, this is emphatically not an experiment which any given observer can perform!
We do not know how things will develop in the future. The one thing we do know is that the outcome of human civilisation depends upon real physical, biological, technological and cultural factors which are hard to predict and which are completely ignored by any probabilistic argument.
An observer at point A can only see the selection of human lives in his or her present and past, to the left of that point on the graph. An observer at point B sees a different selection of lives: those seen by the observer at A, plus all those living in the interval between A and B (people whose existence, at time A, could only be hypothetical).
We currently find ourselves in a population bulge, in full agreement with Gott’s reasoning. But both points A and B satisfy this criterion, so that reasoning cannot be used to distinguish between them.
Suppose we are in fact at point A, with further massive population growth ahead of us. After a period of decades or centuries, our descendants at point B will re-examine the question and will find that they, too, are in a population bulge, again consistent with Gott’s argument. We here and now will be revealed, in retrospect, to have been living at a unique moment in time, during the first 2.5% or 1% or 0.000001% of human history or population numbers. But we will no longer exist: we will be historical characters, just as Pythagoras or Imhotep are historical figures for us, living in what we here and now regard as the cradle of civilisation.
Our presence in a population bulge today does not prove that the inhabitants of ancient Sumer, Egypt or China at the dawn of civilisation did not exist, nor that they were incapable of reasoning about probability theory. Nor can our existence be invalidated by whoever comes after us.
Doomsday theorists would disagree: if the population bulge is in our future, then point A is anomalous, they would say. But if “probability” is to be meaningful, it must predict the result of some experiment which can be carried out in practice. The debate depends upon using thought experiments involving placing balls in urns or people in different rooms (in Bostrom’s papers) whose practical implementation is easy to visualise, and then carrying the results of those experiments over to human history. But without the capability of time travel, the hypothetical future population bulge is by definition unobservable, and what was easy in the simple thought experiment becomes a practical impossibility.
In his paper, Gott attempts to support his argument by referring to events which fell within his 95% probability expectation: the fall of the Berlin Wall and of the Soviet Union, the continuation of publication of the journal Nature. But the more proofs of his correctness he adduces, the more he undermines his own argument: he is confirming his 95% expectation using a class of repeatable events. As we have seen, the nature of time and consciousness make the selection of this particular moment in history an intrinsically unrepeatable experiment. We can only repeat the experiment by asking our descendants to do so, by which time our own viewpoint here and now no longer exists.
In any case: probability arguments taken on their own cannot decide whether any one particular event is one with a high or a low probability, given that we know that both definitely exist. The Copernican principle cannot itself decide whether a particular phenomenon is subject to the Copernican principle.
If it were otherwise, it would be possible to prove that, since luxury liners do not usually suffer shipwreck on their maiden voyages, the Titanic did not sink in April 1912; or, since most American presidents do not have recent African racial heritage, Barack Obama did not win the 2008 presidential election. One can only apply statistical arguments to cases like these by taking a list of a number of ocean liners or a number of American presidents.
By taking a probabilistic view on our present-day position in history, therefore, Gott is attempting to use a technique which is only valid when applied to multiple-choice experiments to draw conclusions about an intrinsically single-choice one. All that can be said is that a statistical result applied to an individual case is either right or wrong, and we do not know which until we move beyond probabilistic thinking and examine our actual physical circumstances.
Sufficient reason for privilege
If we are to recognise that we do indeed occupy an unusual, privileged position in history, Gott requires us to find “sufficient reason” in favour of that scenario. Fair enough. Do we then have any evidence in favour of the hypothesis that our position might in fact be privileged?
Taking Darwinism in its original form, there are no privileged observers: each species mutates by infinitesimal steps into other species, there are no dividing-lines, and no viewpoint is significantly different from any other viewpoint.
In reality, evolution is not like that, and the modern “punctuated equilibrium” model of evolution is a better approximation to the truth. The evolution of life has a hierarchical pattern, in which long periods of relative stability are punctuated by occasional brief revolutionary phases in which more complex forms of life emerge. The pattern is that of a series of logistic curves, with plateaus at levels of increasing complexity in time, very much like an ascending staircase with broad steps and curved risers between the steps – however repugnant such a pattern may be to the popular anti-progressive ideology in biology.
For an unknown period of time, there was no life in the universe, so far as can be ascertained by science. When life appeared, it consisted only of procaryotic, bacterial, single-celled organisms. Later there was a transition to an ecology combining procaryotic and eucaryotic (with a complex internal structure) cells. In both cases the period of transition was of unknown length, but cannot have been long relative to the more stable periods before and after it.
Another revolutionary phase took place between around say 700 million and 550 million years ago, culminating in the well-known “Cambrian explosion” of macroscopic multicellular life. Again following this there was a relatively long period of several hundred million years of consolidation and more gradual change of multi-cellular animals before the next major biological innovation, that of practical intelligence leading to the use of technology, appeared between about 2 million years ago and the present. This led directly, about 300 years ago, to a new burst of growth in the capabilities of living creatures, one in which muscle power and the easily accessible but diffuse energy sources of wind and running water were suddenly augmented by the harnessing of fossil fuels to drive machinery.
It is reasonable to ask whether future major changes are possible. The relevant facts are that almost all the matter and energy in the universe which could be used to support life do not do so, and the reason they do not do so is because they are only accessible through the use of technology. Technology is essential to amplify the muscle strength and intellectual capacities of any biologically evolved species sufficiently to make those resources accessible and usable.
If a technological species were to evolve to take advantage of these hitherto untapped resources, it would necessarily have to go through a brief transition period of rapid growth in its capabilities leading to the invention of spaceflight, and the mastery of long-term sources of industrial energy based on the more direct harnessing of nuclear fusion than is possible through exploiting plant life (living and fossilised) alone. Its population and resource consumption would have to follow logistic curves.
Human civilisation at the present day possesses all the characteristics which one might expect of a species going through such a transition: unprecedented growth in indicators such as population, economic activity, technology and so on, increasing industrialisation of all human needs, and at the same time increasing problems adjusting to the products of growth, and consequently increasing adaptation of our way of life to those products.
We have no way of predicting whether our own civilisation and species will complete the transition, or whether it will fall back onto a pre-industrial level and eventually die out without intelligent descendants. It has been suggested that certain species of ostrich-like dinosaurs (ornithomimids, troodontids, dromaeosaurids and the like) could, given suitable environmental stimuli and selection pressures, have evolved rational intelligence, technology and spaceflight 100 million years before humans appeared. They did not, and eventually lost their chance. We have progressed towards an interplanetary and interstellar order of life much further than they did, but our development in this direction could also come to an end before that new order of life is established.
But that is not the point at issue. The questions at issue here are:
Clearly the answer to all four of these questions is: yes.
There therefore exists circumstantial evidence that human civilisation is currently going through an unusual period of growth, that a sustainable outcome at higher population levels than today is in prospect, and that we are therefore observers with what will be seen in retrospect to be a privileged view of an unusual time in history.
This evidence is the second reason I referred to above, why the Doomsday argument is spurious: it fails to take into account the observable facts concerning our current situation.
Does this evidence constitute sufficient reason to overturn the Copernican principle as applied to ourselves as observers of human history? I would argue that it should.
Let me do so by referring to a single event: the landing of Apollo 11 on the Moon in July 1969, and the safe return of the three astronauts to Earth. A planned voyage between one astronomical body and another had, so far as one can tell, never been made before (unplanned voyages, by bacteria riding rocks randomly blasted from one planetary surface to another by asteroid impact, differ in that they cannot be carried out in a large-scale, systematic fashion, nor do they greatly extend the capabilities of those bacteria to convert matter and energy into more bacteria).
Yet this single first complete space journey from Earth to Moon and back demonstrated a new biological capability which, extended to its logical conclusion, could spread living creatures in a large-scale, systematic manner to almost any location in astronomical space, utilising in the process vastly greater reserves of matter and energy than those available at the surface of any and all Earth-like planets. Such a capability is utterly beyond the powers of any and all pre-technological life-forms.
The voyage of Apollo 11 therefore represented the potential expansion of the geographical range of our species, with consequent extensions to our resource base and opportunities for growth and diversification, by a factor of up to at least 1013 (from tens of thousands of kilometres on Earth alone to tens of thousands of light-years in the Galaxy). Such an event is necessarily unique in the history of our civilisation, our species, our Solar System and arguably our Galaxy, or at least a large fraction thereof. Yet this event occurred in my own lifetime (I’m showing my age here)!
The first ever journey of multicellular life forms between one astronomical body and another is therefore a highly significant event, but one which can only occur once in the multi-billion-year lifetime of any one given biosphere. I am therefore driven to conclude that I personally live at an extremely unusual – in fact, unique – moment in the history of humanity and of terrestrial life itself.
Gott’s entire argument hinges on the assumption that we here and now will be seen in retrospect to have been typical observers. But this cannot be assumed, because it is not yet known! In order to make his argument, this assumption first needs to be established independently, and I hope I have shown that the evidence of our eyes demonstrates the opposite conclusion. Gott’s thesis therefore collapses into a specimen of circular logic, whose author merely succeeds in proving what he originally assumed: by assuming the hypothesis that we are typical, he deduces that we are not unusual, but the issue is whether that hypothesis is itself valid.
The long-term outlook for our civilisation, our species and ultimately our descendant species therefore remains unconstrained by theoretical arguments from the Copernican principle. That principle states that we are not in any special position, unless we actually are found to be in a special position! The circumstantial evidence shows us that we are in just such a special position.
Beyond the Copernican principle
There is a fundamental evolutionary point at issue in all this debate. Gott, as we have seen, argued that long-lived phenomena tend to remain long-lived, and short-lived ones to have an equally short future expectation. This type of reasoning compels him to dismiss the logistic curve as a pattern of sustainable growth: it is “not allowed because if you find yourself on the rising exponential [...] your good luck should not be more spectacular than 2.5%” (p.316).
What he is saying amounts to this: we here and now cannot find ourselves in an extremely privileged position relative to the rest of humanity past and future; therefore the entire phenomenon of intelligent life (producing rational observers such as ourselves) cannot become a permanent feature of the universe, because if it did there would be so many of our descendants living so far into the future that we here and now would find ourselves in an extremely privileged position.
This line of reasoning contradicts the known pattern of evolution of life. For a long time there was no life, then single-celled life appeared and was a permanent feature of the universe thereafter. Similarly with multicellular life. At each period of innovation, a hypothetical intelligent bacterium or hypothetical intelligent trilobite might have argued that single-celled or multicellular life would not last long – and would have been proved wrong.
Gott is therefore claiming that intelligence cannot grow to pervade the universe in the same way that bacterial life and later multicellular life grew to pervade the surface of Earth. The grounds for his claim are that if intelligence became a permanent feature in the universe from this point on, it would create privileged observers who find themselves among the first 2.5% or 1% or 0.0001% of all intelligent observers. Gott asserts that we cannot be those observers, because we must (by hypothesis) be typical.
I hope it is now clear that Gott’s claim is completely spurious. By assuming at the outset that we are typical, he claims to prove that we are not special. In doing so, he contradicts the known pattern of the evolution of life, in which new innovations appear at widely separated intervals, they extend the complexity and diversity of life, and once they have appeared and established themselves (perhaps after several excursions towards that innovation which did not take root; witness dolphin intelligence and our extinct hominid cousins) they persist indefinitely thereafter.
It is much more reasonable to consider that intelligence may follow a similar pattern to that of other biological innovations, and thus persist for the lifetime of the universe once established. That makes our own viewpoint an extremely special one. This need be no surprise, as it must be clear that somebody must enjoy that view, and we have no reason to disqualify ourselves from being that somebody, and every reason to demonstrate from our actual observed circumstances that we are.
Our proper business here and now is to make damn sure that human civilisation and its descendants stay established!
Science pushed too far
The Doomsday argument is part of a wider late 20th-century phenomenon, in which scientific arguments are pushed beyond their range of validity in order to create the impression that science knows more than it actually does. Here are the other examples of this phenomenon currently known to me.
(1) Alien life. More than 40 years of close-up spacecraft observations of Earth’s nearest neighbour planets Mars and Venus – hosting the most Earth-like conditions known beyond our own – have failed so far to either confirm or rule out the existence of any living organisms indigenous to those worlds. But science wants to be able to state definitively whether extraterrestrial life exists, and whether extraterrestrial intelligent life and civilisations analogous to our own exist. Using the same Copernican principle, it is pronounced that such life and civilisations do definitely exist.
But since most civilisations analogous to our own would be older than ours and would have left some traces which could be detected by astronomers, and since astronomers have not in fact found any such traces, this has given rise to the so-called Fermi paradox: if those extra-terrestrial intelligences exist, why have we not detected them?
A paradox is found when two lines of argument of equal validity lead to a contradiction. But there is no paradox concerning extraterrestrial intelligence, because the two sides are of greatly unequal strength.
Our lack of evidence regarding alien life of any kind is a fact, and a fact which constrains the abundance of any given type of alien life, and constrains our theories of the origin of life. But the existence of extraterrestrial civilisations is pure speculation, based on the Copernican principle, which may or may not be valid in this case.
Science does not yet know what proportion of planets host any kind of indigenous life – even within our own Solar System, let alone further afield! It cannot place any probabilities on the evolution of multi-cellular life or of rational intelligence except by means of guesswork based on the single example we have to hand of ourselves. It is not yet even known for certain whether life itself originally evolved on Earth, or in space.
Given that the stelliferous universe is of finite age, it is logically necessary that at least one civilisation during the lifetime of the universe must be unique for some period of time, and therefore logically necessary that the Copernican principle must break down in at least one case.
If we ask whether we are or are not that unique civilisation, it is impossible to answer. Science overreaches itself, makes a guess at the answer, and is then confronted with a paradox. But we do not know the answer, and it is better to admit our ignorance than to claim we know when we do not.
In 1979, Freeman Dyson wrote:
“I do not believe we yet know enough about stars, planets, life and mind to give us a firm basis for deciding whether the presence of intelligence in the universe is probable or improbable. Many biologists and chemists have concluded from inadequate evidence that the development of intelligent life should be a frequent occurrence in our galaxy. Having examined their evidence and heard their arguments, I consider it just as likely that no intelligent species other than our own has ever existed. The question can only be answered by observation.” (Freeman J. Dyson, Disturbing the Universe, Harper & Row, 1979, p.216.)
His words were an accurate summary of our state of knowledge about extraterrestrial intelligence when they were written, and they remain true today.
(2) Climate change. This popular modern cause célèbre again shows scientific methods being forced to overreach themselves. For years we have been told that industrial emissions of carbon dioxide are driving the global mean temperature up towards levels which will trigger disastrous consequences for civilisation. Now it transpires that the computer models used have grossly exaggerated the warming trend, and that in fact over the past 15 years (1997 to 2011) there has been no net warming at all, a situation which was not predicted. (See report by the Global Warming Policy Foundation.)
We are therefore not able at present to provide accurate forecasts of the global climate, with or without anthropogenic effects, and it is better to admit our ignorance than to make unsubstantiated claims which later turn out to be false.
(3) Galaxies and dark matter. In 1846 calculations using Newton’s law of gravity pinpointed the existence of a hitherto unknown planet, and when astronomers at the Berlin observatory pointed their telescope towards that location, they discovered Neptune.
In the late 20th century, it was discovered that galaxies appeared to be rotating too fast to hold together. More matter was needed, not at the centre of a galaxy, but distributed throughout its volume, in order to reproduce the observed motions of its stars. But no such matter has been observed.
Astronomers resolved the riddle by inventing “dark matter”, as if giving it a name was sufficient to prove its existence. But the fact is that we have never observed such dark matter, we do not know what it might be, and we cannot at present explain the rotation of galaxies, including our own. The triumph of the discovery of Neptune has not been repeated.
Given such a gaping hole in our understanding, efforts by some astronomers to predict the long-term fate of the universe are wildly premature, however entertaining it may be to speculate on this topic.
(4) Consciousness. This is a popular object of study by scientists, witness Daniel Dennett’s book Consciousness Explained. But it must be clear that consciousness cannot be the subject of scientific studies at present for the simple reason that no scientific instrument yet invented can actually detect the presence or absence of consciousness. Certainly science can study psychology and brain function, because one science is content with a subject’s introspective account of what they are experiencing, while the other has a physical object to analyse. But consciousness as a phenomenon in its own right is beyond the current reach of science.
Consider a thought experiment: you and I are confronted by a third entity, perhaps a human being, or a computer, or an extraterrestrial being, or a non-human animal (dolphin, octopus, bacterium – take your pick). You argue (for whatever reason) that this being is conscious, that he/she/it has the same quality of self-awareness that we usually mean when we talk of consciousness. I argue (for whatever reason) that this is a zombie, which acts like we do and talks like we do, but with no internal experience of its own, reacting to external and internal events in a purely mechanical fashion.
The only scientific resolution would be to observe that being’s mind and discover there either the presence or absence (or partial presence) of consciousness. An observation is needed in order to settle the dispute. But for that a special instrument would be required, and a suitable one is not yet on the market. So we are reduced to hand-waving and speculation; bogus “explanations” of consciousness in terms of observable cellular activity in the brain whose correlation with the subjective mind is questionable. (And which depend upon treating the subject matter of which an individual is conscious as if it were the same as the phenomenon itself: my brain serotonin level falls, so I feel sad, but the fact that I consciously observe a state of sadness, and feel emotionally involved in it, does not elucidate for one moment the nature of the subjective feeling itself, nor does it explain what happens to those perceptions and feelings when that brain dies.)
However, our conscious experience of life is central to our existence as human personalities, and science feels that it must show that it fully understands consciousness, when in fact it does not.
(5) The Doomsday argument (as discussed in detail above). The ultimate kind of knowledge is knowledge of the future, and science has made a great deal of legitimate progress in this direction, from predicting eclipses to modelling the airworthiness of different aerodynamic shapes. But the future course of complex phenomena is not predetermined by simple scientific laws. The investigation of chaos and the butterfly effect even suggests to my mind that in some cases prediction may be intrinsically impossible, because measuring all relevant variables to sufficient accuracy is intrinsically impossible, and because the system responds to perturbations in a non-linear fashion.
Human civilisation is a complex phenomenon – perhaps the most complex one known, encompassing as it does a myriad of other complex phenomena such as the human brain, terrestrial and solar climate, technology, economics and politics. Yet it is one in which we have an intense personal interest, especially if we are bringing children into the world. Science therefore wants to cobble together some way of predicting its future.
This it does using probabilistic arguments, as has been seen above. But those arguments break down when applied to an individual case, if it is not yet known whether that case is typical or unusual relative to the particular assumptions employed. Since human civilisation is just such a unique case, the supposed knowledge of the future which those arguments offer is completely spurious.
In all five cases listed here, scientific arguments are being used to create an impression of certainty where no certainty in fact exists.
Why is this happening? In the intellectual battle over the source of ultimate authority which has been raging since the start of the Enlightenment period, science has found itself in competition with established religion, which prided itself on being able to give definitive answers to all the most pressing questions about human life, albeit with zero genuine reliability.
Science has thus experienced the temptation to overreach itself, to provide certainty on difficult questions in order to compete effectively with religion and establish its superiority as the ultimate source of knowledge. Given that people crave certainty, this was perhaps inevitable.
Harmless speculation? Maybe. But look at the corrosive effect of the Doomsday argument: we are doomed to extinction in the end, so why bother to strive to give future generations a better life now? And look at the damaging way the climate change controversy has impacted politics, presenting a supposedly compelling case that the only future for human civilisation is one of global environmentalist tyranny, poverty, and destruction of a large fraction of the material progress won so painfully since the industrial revolution began. The anti-capitalist, anti-globalisation movement appears to regard it, not as a reason to accelerate our growth into the Solar System, but as an argument for shutting off that avenue of growth altogether.
When the majority of people mature to the point that they are able to accept uncertainty about difficult yet important questions, then science will be able to be more honest about what it can and cannot tell us about ourselves and our universe. Until then, the most reliable prediction that can be made is that further examples of scientific extrapolation, producing phoney claims of certainty about one or another phenomenon of intense public interest, may continue to be seen in the future.