7. "It's Only a Theory"

Of all the great many theories in science, one in particular, far more than any other, causes vast degrees of animosity and emotive argument between its supporters and its opponents. Yet the theory in question is, within the field of science, considered as indisputable, as close to being proven fact, as any theory possibly can be; there is no serious scientist in the world who disputes it. But there are many people who do, for reasons which have nothing to do with science.
I refer, of course, to the Theory of Evolution by Natural Selection. Those who dispute it do so for religious reasons – mainly “Young Earth Creationists”, who insist, despite the overwhelming evidence to the contrary, that “the Creation” happened as described in the Bible, with God creating the world as it is today, with humans and all the animals we know already in it, just a few thousand years ago. Anyone who rejects evolution is not only rejecting a single theory; they are, in effect, rejecting almost the whole of modern science! But this isn’t the place to discuss that point; I plan to do so elsewhere on this site.
( A Christian friend of mine has pointed out that all Christians – and Jews and Muslims – are “creationists”, in the sense of believing that the Universe was created by God. However, the word is usually applied only to “Young Earth Creationists” – those who deny the existence of evolution and the age of the Earth, and insist that everything in the Bible is literally true. This is the sense in which I’m using the word here. )
Such people are fond of dismissing evolution by claiming that “it’s only a theory” – as if that somehow means that it’s no more “valid” than their own beliefs. I once had a “debate” with a fellow who was preaching in the street, inflicting his own religious beliefs on passers-by, whether they wanted to listen or not. He said that he “doesn’t believe in evolution”, and insisted belligerently that “It’s not called Darwin’s Fact of Evolution; it’s called Darwin’s Theory of Evolution!”
What this fellow and others like him are actually doing is displaying their own complete lack of comprehension of how science works, and what the word “theory” actually means! ( Or in some cases, they may be deliberately ignoring the true meaning, and preying on their audience’s lack of comprehension. ) In science, there is no such expression or concept as “only a theory”, or “just a theory”. A theory is in fact the highest, grandest status which any scientific idea or model can attain; there is no “only” or “just” about it! I shall now explain why.

7.1. What is a theory?

In everyday language, the word “theory” is often used pretty loosely, to mean an idea, a hunch or a guess – such as in TV crime dramas, when a detective has a “theory” about who committed the murder and why. But in science, it means something much more precise.
The Oxford English Dictionary gives several definitions of the word, one of which more or less corresponds to the common usage described above. But the one listed first, which is the meaning used in science, states as follows:
“A scheme or system of ideas or statements held as an explanation or account of a group of facts or phenomena; a hypothesis that has been confirmed or established by observation or experiment, and is propounded or accepted as accounting for the known facts; a statement of what are held to be the general laws, principles or causes of something known or observed.”

To put it a little more simply, I would personally define a scientific theory as follows:
“A model or explanation of an observed phenomenon or group of phenomena, which is strongly supported by the available evidence, and is generally regarded as the correct explanation, until or unless evidence is found to the contrary.”

That last phrase is vitally important. Unlike religious beliefs, which are based on dogma and the concept of “sacred truths”, which must not be challenged, a scientific theory is accepted as being correct, only so long as it continues to be supported by evidence! If evidence is ever found which can’t be explained by the theory, then the theory – or some detail of it – is proved wrong, and must be modified to account for that new evidence.
Those creationists who claim that “evolution is only a theory” often also claim something to the effect that it “has not been proved”. This again proves their ignorance of how science works; they apparently think that a theory “becomes a fact”, or a “law”, when it is proved to be correct. This is not the case! Theories and laws are two completely different things; a theory does not “become a law when it’s proved”. In fact, strictly speaking, a theory can never be proved, as I’ll explain in Section 7.2.
In science, a law is a mathematical description of a phenomenon, deduced by observation or experiment. A theory is an attempt to explain a phenomenon. To illustrate the difference, let’s consider the phenomenon of gravity.
Newton’s Law of Gravity – or more correctly, the Law of Universal Gravitation – as formulated by Sir Isaac Newton in 1687, states as follows:
“Every object attracts every other object, with a force which is directly proportional to the product of their masses, and inversely proportional to the square of their distance apart.”

It can be expressed mathematically by the equation:

F = G m1 m2 / r2

( See Section 6.5. ) According to legend, the concept first occurred to Newton when he saw an apple fall from a tree, and realised that something must cause it to fall. He deduced the law by studying the observed motions of the planets. The value of the constant of proportionality, denoted by G, was calculated by means of experiments performed by later physicists. ( Its numerical value depends on the units we use for mass, distance and force. )
So this law tells us that all objects in the Universe attract each other with some kind of mysterious force, and tells us how this force behaves. It gives us a mathematical formula, with which we can calculate the attractive force between any two objects, if we know their masses and their distance apart. But it doesn’t tell us what gravity is! Newton made no attempt to explain what causes masses to attract each other with this mysterious “force”; he couldn’t possibly have done so, because the explanation depends on concepts which were not thought of until more than two centuries later!
An explanation of gravity had to wait for the genius of Albert Einstein, who published his General Theory of Relativity in 1915. Einstein realised that gravity isn’t really a “force” at all, but is the effect of a distortion, or “bending” of space itself, caused by the presence of a mass.
So Newton’s law tells us mathematically how masses attract each other, and Einstein’s theory attempts to explain why they do so. I hope you can now see the difference!

7.2. You can’t “prove” a theory!

I’ll now elaborate on what I said above, that a theory, strictly speaking, can never be “proved”. By the strictest definition, the concept of “proof” exists only in mathematics.
In mathematics, a proof is a logical demonstration that a conclusion necessarily follows from a set of assumptions. A theorem is a principle which can be proved by reasoning from basic principles. The best known theorem is Pythagoras’ Theorem, which states that for any right-angled triangle, the square of the length of the hypotenuse is equal to the sum of the squares of the lengths of the other two sides. This can be proved to be true – the classical Greeks proved it over 2000 years ago – by assuming certain Euclidian axioms ( self-evident statements ), such as the axiom that parallel straight lines never meet. In fact, there are hundreds of different ways to prove it, some of which are very simple to follow. There is no doubt about it; it simply is true.
A mathematical principle which is generally accepted to be true, but has not been proved, is called a conjecture. A famous one is Goldbach’s Conjecture, which states that every even integer can be expressed as the sum of two prime numbers. For relatively small numbers, this can be shown to be true, simply by trying it for each even number in turn. By using computer algorithms, mathematicians have shown it to be true for every even integer up to some extremely large but arbitrary number. But there exists – so far – no purely logical proof that the conjecture is true for every conceivable even integer. If and when someone ever does discover a proof, the conjecture will be “promoted” in status, and become known as Goldbach’s Theorem.
So mathematicians use the concept of proof to distinguish between conjectures and theorems; when a conjecture is proved, it becomes a theorem. But in other sciences, there is no equivalent process; a theory is not “promoted” to any other status when it’s proved, because it never can be proved, in the way that a mathematical theorem can.
Mathematical theorems are proved by a process of pure logic. The same can’t be done with scientific theories. All we can say is that a theory has been subjected to rigorous testing, and has not been disproved; the more rigorous tests a theory passes, the more confident we can be that it’s correct.
In science, an idea which is proposed as an explanation for a phenomenon is initially called a hypothesis; this is more or less equivalent to the “everyday language” use of “theory”. The hypothesis is then tested, by means of observations and/or experiments; given a set of initial conditions, the hypothesis can be used to predict the outcome of each experiment – “If the hypothesis is true, and we start with these conditions, then such and such should happen”. The actual outcome is then compared with the predicted outcome; if it matches, then we can say the hypothesis is likely to be correct. But if it doesn’t match, then the hypothesis is wrong, and must be rejected.
A hypothesis is promoted to the status of a theory, when it is sufficiently strongly supported by the evidence of observations and/or experiments, as to become generally accepted as the most likely explanation of a phenomenon. In many cases, after that “promotion”, the theory is still subjected to ever more rigorous tests; if it continues to pass those tests, then we can be ever more confident that it’s almost certainly correct.
But “almost certainly” is the operative phrase. We can subject a theory to more and more tests, and accrue more and more evidence which supports it, until we can be sure that it’s correct, to a confidence level of 99.999… percent – but we can never be absolutely 100 percent certain that no piece of evidence will ever be found in future, which will show some small detail of the theory to be wrong. To cover every possible combination of conditions would require an infinite number of tests – which is obviously impossible! That’s why it isn’t possible to absolutely prove a theory correct – because to do so would require testing to infinity.
Conversely, though, it’s very much possible to prove a theory wrong! All that takes is to find a single piece of evidence which doesn’t fit – which can’t be explained by the theory. If a theory is wrong, then sooner or later, it will be proved wrong, by the discovery of evidence which contradicts it. And when that happens, the theory must be abandoned, or at least modified to account for the new evidence.
This is in fact part of the definition of a scientific theory – it must be testable, or as scientists say, falsifiable. That is, there must be, in principle, a way of proving it wrong, if it happens to be wrong. There are many misuses of the word “theory”, applied to ideas or assertions which most certainly don’t meet that criterion, and are therefore not true theories at all. The most obvious are so called “conspiracy theories”, such as those involving UFOs, the imaginary faking of the Apollo Moon landings, the death of Princess Diana, etc. No “conspiracy theory” is ever falsifiable, as whatever evidence the rational among us present which disproves it, the “believers” will simply claim that that evidence was itself faked, as part of the conspiracy.
The same applies, naturally, to religion. Some creationists claim that biblical creation is an “alternative theory” to evolution – but it’s no such thing! Any “explanation” of anything involving God, or anything “supernatural”, is by definition unfalsifiable, as it depends on the assumption that God performs “miracles”, and is not bound by the laws of physics! So any such assertion can’t possibly be a valid theory, in the scientific meaning of the word.
Of course, if a theory is conclusively proved to be wrong, then it must be abandoned. Richard Dawkins, in The God Delusion, tells a true story from his days as an undergraduate, which illustrates the point1. A distinguished Oxford professor of biology had long held onto his own theory about some aspect of cell structure ( which I don’t pretend to understand ), which went against the conventional wisdom. Most people in the field disagreed with his theory, but none had found any evidence which conclusively disproved it. One day, a visiting American researcher gave a presentation to the department, in which he presented some newly discovered evidence, which finally proved, beyond any doubt, that the professor’s cherished theory was wrong.
At the end of the presentation, the professor, in front of his own entire department, shook the visitor’s hand and congratulated him, saying, “You have proved that I have been wrong these last fifteen years!” That admission was met with rapturous applause.
That’s a fine example of how science is meant to work. If your pet theory is proved to be wrong, then you accept that it’s wrong, and move on – though many might do so rather more reluctantly than that professor!

7.3. “Beyond reasonable doubt”

So we have established that, while a theory can be proved wrong, it’s impossible to absolutely prove one correct. On the other hand, there are many theories which are so strongly supported, by such overwhelming amounts of evidence, that they can be considered as close to being proven as it’s possible for a theory to be, such that to all extents and purposes, common sense would treat them as “facts”.
As a comparison, think of the instruction which is given to a jury, before they retire to consider their verdict. In order to return a verdict of guilty, they must be convinced “beyond reasonable doubt” that the accused is guilty. Note the wording – “beyond reasonable doubt”. It’s rarely, if ever, possible to be certain beyond any conceivable doubt whatsoever; no matter how overwhelming the evidence against the accused, it’s usually possible to imagine some way in which the evidence might be flawed – indeed, defence lawyers are paid to do so – but what matters is whether any such doubt is sufficiently realistic as to be worthy of consideration.
For example, the most powerful forensic evidence available today is that of DNA analysis. No two people in the world have absolutely identical DNA, except for identical twins. ( They do, by definition, as they originate from a single fertilised egg which split into two, and then formed two separate embryos. ) So if a person’s DNA is found at a crime scene, there can be no doubt at all that he was there at some point. If the suspect’s DNA was found on the murder weapon, then it doesn’t absolutely prove that he committed the murder; it only proves that he touched the weapon at some point. It’s remotely possible that his DNA could have got onto the weapon by coincidence; for example, if the weapon is a knife, it’s possible that the suspect just happened to pick up the knife in a shop, then decided against buying it and put it back, before someone else did buy it, and went on to commit the murder with it. But if there is other evidence connecting the suspect to the victim – say, he was seen having a violent argument with the victim a few days before, and was then caught on CCTV close to the scene of the murder within minutes of it happening – then the probability of such a coincidence is so miniscule that no sane person would give it any credence. In such a case, the jury would indeed be convinced “beyond reasonable doubt”.
Similarly, there are many scientific theories, for which the supporting evidence is so overwhelming that we can apply the same criterion of “beyond reasonable doubt”, and regard those theories as “facts”, to all extents and purposes.
To take an extreme example; prior to the 16th Century, everyone took it for granted that the Earth was the centre of the Universe, and the Sun, Moon and planets revolved around it. After all, in our everyday experience, we don’t feel the Earth moving, and there is nothing to suggest that it does. At least, not to those with no knowledge of astronomy. When Nicolaus Copernicus first proposed that the planets, including the Earth, orbit the Sun, this was regarded as a theory, for which there was no proof. Johannes Kepler later refined the theory, and realised that the planets’ orbits are elliptical, rather than circular, and found that his model explained the observed motions of the planets in the sky with great precision. But it was still regarded as a mathematical model, for which there was no direct physical evidence.
The first direct evidence that the Earth moves was found in 1728, with the discovery of the aberration of starlight – a phenomenon which can only be reasonably explained by assuming that the Earth moves. Another 110 years later, astronomers measured the first stellar parallaxes – the apparent change in position of a nearby star against the background of more distant ones, when observed from opposite sides of the Earth’s orbit. As early as the late 17th Century, astronomers had realised that if the Earth orbited the Sun, then nearby stars must show parallaxes, and tried to measure them – but the angles were too tiny to be measured by even the best instruments of the time, and it took a century and a half of trying before anyone succeeded! This is a classic example of how a theory is verified; the theory predicts that something should happen, and then observations show that it actually does happen! Ever since, parallax measurements have been used to determine the distances of nearby stars.
Today, that the Earth orbits the Sun is regarded as an indisputable fact by any sane person! The orbital motions of the planets have been measured with such accuracy that we can send space probes to the outer planets, on journeys lasting years, and be certain that the planet will be in precisely the right place at the right time, for the probe to meet it. ( When Voyager 2 flew past Neptune, after a journey of 12 years and three billion kilometres, its estimated time of closest approach was in error by three minutes! ) By the strictest definition, we should still call heliocentrism a theory – but one which is verified beyond any slightest shred of reasonable doubt. Only a certifiable lunatic could possibly still believe that the Earth is the centre of the Universe! ( Amazingly, there are still a few people today who do believe exactly that, for reasons which have nothing to do with science, and everything to do with dogmatic religion! I myself once had a truly mind-boggling online “debate” with one of them. )
Richard Dawkins has invented the word theorum ( pronounced to rhyme with “decorum”, to distinguish it from the mathematical theorem ), which he uses to mean a theory which is as close as is possible to being an indisputable fact2.
To take a more sensible example, consider Einstein’s two theories of Special and General Relativity, published in 1905 and 1915 respectively. For the last century, these have been subjected to a multitude of ever more rigorous tests – such as flying an incredibly accurate atomic clock aboard an aircraft, and comparing it with one on the ground, to measure the tiny effect of time dilation – and have passed every test ever applied, with the observed results matching those predicted by the theory to an accuracy of better than one part in a billion. To most people, that constitutes “beyond reasonable doubt”; very few scientists now dispute the theories. Indeed, the GPS system, in which the positions of the satellites must be tracked with extreme accuracy, takes account of relativistic effects, both Special and General; at the speed of satellites in orbit, these effects are minute, but measurable. ( In my own job, I once tested the software for an aircraft landing system which utilised GPS measurements. The algorithms involved a lot of truly horrific maths, with a required accuracy of one part in 1011! )
Now let’s return to the main theme of this essay – the claim by creationists that evolution is “only a theory”. Creationists of the “Young Earth” kind – those who insist that the Bible is literally true – claim that the age of the Earth is only a few thousand years, whereas the accepted scientific view is that it’s 4.6 billion years! When the rational among us point out the overwhelming evidence in favour of the latter, the creationists’ only pathetic defence is to claim that “the scientists have got it all wrong; they have misunderstood the evidence!” So let’s see… the ages of rocks are determined by radioactive dating; the underlying physical process, radioactive decay, is so thoroughly understood that it’s utilised daily in manufacturing ( e.g. the smoke alarm in your house ) and medical procedures ( e.g radiotherapy to treat tumours ), the Voyager probes travelled across the Solar System powered by it, and millions of people routinely entrust their lives to heart pacemakers whose batteries are powered by it!
Following on from this, much of the evidence in favour of evolution comes from the fossil record. The ages of fossils are determined by measuring the ages of the rocks in which they are embedded. This is done by means of radioactive dating – a purely physical and chemical process, which is totally independent of biology. In this way, we build up a timeline of evolution, showing beyond doubt that certain creatures existed before other more complex ones. Combining this with huge amounts of evidence observed in living creatures – such as the fact that every species of mammal, from a mouse to an elephant to a bat to a whale to a human, has the exact same configuration of bones in its skeleton, strongly suggesting that they all evolved from a common ancestor – we can say, very much so, that evolution is itself one of those theories which are established “beyond reasonable doubt”. That is, what Dawkins calls a theorum.
Some creationists, in perhaps the ultimate example of clutching at straws, have even gone a step further, and claimed that “God has deliberately planted false evidence, to test our faith”! In other words, the entire Universe is a huge practical joke; their egomaniac God created it 6000 years ago, but then invested vast amounts of effort in making it look as if it’s billions of years old – just to see whether we humans will continue to believe in him, when confronted with overwhelming evidence of his non-existence! ( Some scientists have countered this stupidity by presenting the alternative “theory” of “Last Thursdayism” - that the Universe was created last Thursday, and each of us simply popped into existence, just as we are right now, with all our knowledge of history, and our memories of our lives up to now, falsely implanted in our brains – rather like the android "hosts" in TV’s Westworld. Think about it; this is no more ridiculous, and no less falsifiable! )
Think back to my analogy of the murder case. It’s possible to claim that every scientist in the world has got the evidence all wrong, and the Earth is only 6000 years old after all, by means of the “God deliberately planted false evidence” argument, or something like it – but such arguments are so ridiculous that no sane person would ever give them a moment’s serious consideration! Most modern-day Christians, and all “mainstream” churches, readily accept evolution and the age of the Earth as facts, while still believing that their God created it all in the first place.

7.4. The scientific method

One of the most ridiculous arguments I’ve heard from some creationists – and other types of reality-deniers – is along the lines of “You can’t trust scientists, because they keep changing their minds! They tell us one thing, then they tell us something else!”
To put it another way, they are actually claiming that scientists can’t be trusted, because they are willing to admit when they are wrong!!!! But anyone with the slightest understanding of science knows that that’s exactly how it works; far from being a weakness, this is its strength!
While religion is based on the concept of “sacred truths”, which must never be questioned, science is based on exactly the opposite principle. There is no such thing as a “sacred truth”; anything which is regarded as the truth, or a fact, is regarded as such only as long as it’s supported by the evidence – think back to the story about the Oxford professor. Many of the facts which are now accepted as “beyond reasonable doubt”, were very much disputed in the past, when less evidence was available.
One of the greatest failings of our education system today is the way in which children are taught science. In many – probably most – schools, “science” lessons simply consist of learning a lot of “facts”, which must be learned parrot fashion in order to pass the exams. But science is not just a collection of facts; it’s the process by which those facts are established in the first place! Most schoolchildren are taught little or nothing about that process – what we call the scientific method. As Richard Dawkins succinctly puts it, "They need to be taught how to think, not taught what to think!"
I’ve already pretty well described the scientific method, but I’ll summarise it here. When a scientist tries to explain some phenomenon which is observed in nature, he formulates a hypothesis – a tentative explanation or model, which he believes can explain the observed facts, while not violating any known scientific laws. He presents his hypothesis to other scientists in the field, either by means of presentations or seminars, or by publishing it in a journal, or both. Others then try to verify or disprove the hypothesis, by making further observations and/or performing experiments, in which the observed results are compared with those predicted by the hypothesis.
If a hypothesis fails to correctly explain the observed data, then it’s wrong, and must be either rejected or modified. If it passes all the tests, to the extent that it becomes regarded as the most likely explanation of the phenomenon, then it’s eventually promoted to the status of a theory. Even then, it may well be subjected to further and more stringent tests – for example, as improvements in technology enable measurements to be made with ever greater accuracy – and the more rigorous tests it passes, the higher the confidence level with which the theory can be assumed to be correct. In many, though not all, cases, a theory eventually reaches the status of being universally accepted as “beyond reasonable doubt”.
But if, at any time, any new piece of evidence is found, which can’t be explained by the accepted theory, then the theory – no matter how well established or indisputable it was thought to be – has to be modified to account for that evidence. That is why scientists sometimes “change their minds” – because they admit their mistakes, and accept it when their theories are proved wrong!
A good example is the story of the astronomer Johannes Kepler ( 1571-1630 ), who has been described as the world’s first astrophysicist. He lived at a time when science as we now know it was only just becoming established; he was a contemporary of Galileo, who could be said to have invented the methods of modern experimental physics. Kepler was one of the first to accept Copernicus’ theory that the planets orbited the Sun, rather than the Earth. Copernicus himself had admitted that there were discrepancies in the observed motions of the planets, which his theory couldn’t explain; Kepler set out to try to develop a model which could explain them.
Kepler had been assistant to Tycho Brahe, who was probably the greatest observational astronomer of the pre-telescopic era ( it’s a tragic irony that he died just seven years before the invention of the telescope! ); Tycho had compiled the most accurate tables to date of planetary positions, and Kepler inherited all of his data after his death. He initially devised a theory to describe the orbits of the planets, based on classical Greek geometry, which was frankly silly, and had more to do with ancient mysticism than with modern science! But when he tested the theory, by comparing its predictions of planetary motions with Tycho’s meticulous observations, he found that it didn’t work. So in the manner of a true modern scientist, he abandoned it, and went back to the drawing board.
He eventually realised that the observed motions of the planets could be best explained by assuming that their orbits were not circles, but ellipses. By a painstaking process of trial and error, experimenting for each planet in turn with ellipses of different eccentricity, he eventually came up with a model which matched Tycho’s observations with great precision. In so doing, he realised that the motions of the planets could be explained by purely mathematical principles, and discovered his famous Three Laws of Planetary Motion – which were a forerunner of Newton’s Law of Universal Gravitation.
For a fine example of how a theory is tested by comparing prediction with observation, consider again Einstein’s General Theory of Relativity, published in 1915. One of the predictions of the theory was that light would be affected by gravity, and its path bent by passing close to a mass. This effect would only be noticeable in the case of a very large mass, such as a star.
One of Einstein’s earliest supporters, Sir Arthur Eddington, realised that if a star could be observed very close to the Sun in the sky – very nearly in line with it – then it would appear to change position with respect to other stars further from the Sun, as the path of its light would be slightly bent by the Sun’s gravity. The only time that stars can be observed close to the Sun is of course during a total solar eclipse – so in 1919, Eddington travelled to Africa to observe one. During the total phase of the eclipse, he photographed, through a telescope, the eclipsed Sun and the surrounding star field. He later compared the photos with photos of the same star field, taken several months earlier when those stars were visible in the night sky – and found that the stars closest to the Sun did indeed appear to have slightly changed position, exactly as Einstein’s theory predicted.
Sometimes, there can be two or more competing theories which attempt to explain the same phenomenon, and which can each be regarded as valid theories, because no evidence has yet been found, which conclusively disproves one or the other. But “yet” is the operative word; sooner or later, some evidence will be found, which is consistent with one theory, but not with the other.
The history of astronomy provides a good example. In the 1920s, it was discovered that all galaxies, apart from the few nearest ones, which are strongly gravitationally bound, are moving away from our own, and the further away they are, the faster they are receding. It was soon realised that they are not in fact moving away from us, as such, but rather, every galaxy is moving away from every other; the Universe is in fact expanding.
The theory proposed to explain this became known as the Big Bang Theory. In this model, the Universe had a definite beginning, several billion years ago, in which space itself ( or more accurately, space-time ) came into existence in a kind of explosion, and has been expanding ever since. It was initially incredibly dense and incredibly hot, but has expanded and cooled to its present state, and continues to do so.
In 1948, Sir Fred Hoyle proposed an alternative theory, called the Steady State Theory, to explain the observed expansion of the Universe. ( It was in fact Hoyle who invented the term “Big Bang” for the theory which he opposed; he intended it to be derogatory, but it became accepted as the standard name! ) In this model, the Universe had no beginning, but had existed for an infinity of time; new matter was somehow continuously coming into existence, and as new stars and galaxies formed, older ones moved apart. For a while, each theory had its supporters, and it was impossible to say which was correct.
That all changed in 1965, with the discovery of the Cosmic Microwave Background ( CMB ) – a flux of microwave radiation of a specific wavelength, which pervades the whole of space, indicating that the Universe has an overall average temperature of just three degrees above absolute zero. This was entirely consistent with the Big Bang Theory – indeed, the theory predicted it, long before it was discovered. If the Universe had begun in an immensely hot and dense state, and had expanded and cooled ever since, then you would expect it to still have a certain very low overall temperature to this day, and to be filled with radiation indicative of that temperature. The CMB is nothing less than the remnant of the Big Bang! ( Incidentally, the two scientists who first detected the CMB were not actually looking for it to try to verify the Big Bang; they discovered it by accident, while investigating something else entirely. That’s how things sometimes happen in science. )
But there was no way that the CMB could be explained by the Steady State Theory – so Hoyle and his supporters had to accept that their theory was wrong. Today, the Big Bang Theory is another of those regarded as “beyond reasonable doubt”.

7.5. Evolution – fact and theory

That fellow who said, “It’s not called Darwin’s Fact of Evolution…” demonstrated another aspect of his ignorance. In common with many of his fellow creationists, he clearly doesn’t understand the difference between the word “evolution” itself and “the Theory of Evolution by Natural Selection”. The former refers to the phenomenon which is observed in nature; the latter attempts to explain it.
I once read a comment in an internet forum, which summed up the difference in a single sentence: “Evolution is a fact; natural selection is the theory.” In other words, that evolution happens is a fact, which is beyond dispute in the mind of any rational person; the Theory of Evolution by Natural Selection, as proposed by Charles Darwin and since modified by many others, is the theory which attempts to explain the process by which it happens.
In case this isn’t clear, let me make an equivalent statement about something which I described earlier: “Gravity is a fact; General Relativity is the theory.” That gravity exists is a fact, deduced from observation, which is absolutely beyond dispute; you experience its effects every time you drop something, or walk uphill! Newton discovered the mathematical law, which enables us to calculate the gravitational force between any two objects. But Einstein’s General Theory of Relativity is the theory which explains what causes gravity, in terms of the curvature of space.
Similarly, that evolution happens is a fact, deduced from observation. Darwin himself first deduced it by observing the similarities between many species of animals, which suggested that they must be descended from common ancestors – the closer the similarities, the more closely the species were related. During his famous voyage, everything came together in his mind in the Galapagos Islands, where he found that each small island has its own unique set of animal and plant species, adapted to the particular environment of that island. For example, there are several species of finches, each found on only one island, which are closely related to each other, but have distinct differences; in particular, each species has a distinct shape of beak, perfectly adapted to the specific food source which is available on its island.
Today, there is an overwhelming amount of evidence in favour of evolution. There is the fossil record, which shows quite clearly that more complex organisms came into existence more recently than simpler ones. It shows intermediate forms, which don’t specifically belong to one or another class of animal, but are part way between. There are even intermediate forms still living, such as the duck-billed platypus, which is part way between a reptile and a mammal. ( It has hair and suckles its young like a mammal, but lays eggs like a reptile. ) And among the animals and plants which exist today, there are many ways in which different species share common characteristics, which enable biologists to construct a “tree” of evolution, showing which species share common ancestors at different times in the past. For an excellent discussion of the evidence, I refer the reader to Richard Dawkins’ book The Greatest Show on Earth2.
If that isn’t enough, evolution has been observed to happen! We normally think of it as happening on timescales of millions of years – but with very short-lived creatures, such as insects, evolutionary changes can often occur within the timescales of human history. An example is a species of English moth, the Pepper Moth, which changed its camouflage colouring from white with black speckles to the exact opposite over just a few decades, as the silver birch trees which formed its habitat became blackened by man-made pollution during the Industrial Revolution. Then from the 1960s onward, as we humans cleaned up our act and vastly reduced our pollution, and the trees gradually returned to normal, so did the moths; they changed back to their original colouring, even more quickly!
The Greatest Show on Earth has an entire chapter entitled “Before Our Very Eyes”, devoted to examples of evolutionary changes which have been observed during human timescales. The American biologist Richard Lenski has conducted a famous experiment involving a species of bacteria, which has been running for around 30 years, corresponding to over 50000 “generations” of bacterial reproduction; distinct evolutionary changes have been observed to take place.
Returning now to the Theory of Evolution by Natural Selection - remember what I said about a theory having to be falsifiable, and that any “explanation” involving God clearly isn’t. Many creationists have tried to turn the tables, by claiming that the Theory of Evolution is itself not falsifiable, and therefore not a valid theory; they claim that there is no imaginable evidence which could prove it wrong. But of course there is!
Think again about the fossil record – and remember that the ages of rock strata are determined by radioactive dating – a purely physical and chemical process, which is completely independent of evolution or biology. If any fossil were ever to be found in the “wrong” stratum – one corresponding to a time long before that organism is known to have existed – then that would blow the entire theory out of the water, wouldn’t it? But no such anomaly has ever been found, so the theory survives scrutiny.

7.6. Evolution is not a belief!

Finally, many creationists are fond of claiming that evolution is “a belief system”, which is no more or less “valid” than their own. They claim that “belief in evolution” is itself something akin to a religion, due to arguments such as the one above, that it can’t be falsified. Needless to say, this is utter rubbish!
The prominent American creationist Ray “Banana Man” Comfort - a man whom even most of his fellow Christians consider an embarrassment - once made a video, in which he went around a university campus, asking students the somewhat loaded question, “Do you believe in evolution?” Naturally, whatever percentage answered “no” was ammunition to his cause – but in his own mind, so was the remainder! For each student who answered “yes”, he triumphantly crowed, “There, you see – evolution is a belief, just as creationism is!”
Anyone who answers that question with a simple “yes” is either being lazy and not bothering to think about how to answer it properly, or else is showing their own lack of comprehension of the scientific method! Of course, a scientific theory is not something which you either “believe in” or don’t, in the sense in which religious people “believe in” God! It’s something which is evaluated against evidence, and held to be true, only as long as it continues to be supported by the evidence.
So in case you are ever asked that same question by a creationist, here is how I would answer it:
“No, I don’t ‘believe in’ evolution, in the sense in which you ‘believe in’ God – because science isn’t based on belief; it’s based on evidence. I believe that the Theory of Evolution by Natural Selection is correct, to a confidence level of 99.9 recurring percent, because it’s so strongly supported, by such overwhelming amounts of evidence, that it can be considered proven beyond all reasonable doubt.”


1. Richard Dawkins, The God Delusion, Bantam Press, 2006.
2. Richard Dawkins, The Greatest Show on Earth, Bantam Press, 2009.

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