Just a Theory. It’s been the catch cry of pseudo-scientists and the ideologically driven who attempt to undermine the public’s perception of scientific evidence and push their own pet alternative. To give them credit, it has worked, in the mind of the general public a scientific concept labeled “Just a Theory” instantly becomes less convincing and a target of misunderstanding. Like any other word, theory has multiple meanings. You might use the word to describe the feeling you have that a particular person is responsible for you missing cookies, or why you didn’t get the raise you were hoping for. While suitable for everyday conversation this is not what is meant when scientists use the word to describe an aspect of our knowledge of the natural world.
So, what is this thing called a Theory? A Theory is more than just a conjecture or a feeling, more than something you came up with after a night out with the boys. It is a description of a particular phenomenon or group of related phenomena that is built upon multiple lines of evidence and can be tested by making predictions and verifying those predictions through experiment and observation. A Theory is not a fact, it is so much more, a theory takes facts and weaves them into a coherent whole that explains why the facts exist and makes it possible for us to point the way towards facts that may yet be discovered.
Far from a poorly substantiated claim, for a Theory to survive it must be capable of withstanding the criticism of the entire scientific community that it relates to, which means that it must out compete other explanations by being more parsimonious, explaining more facts and making more numerous and more accurate predictions. A new theory may not necessarily replace an old one but just provide a more complete understanding of the world, for example Relativity does not so much make Newton’s Theory obsolete as provide a richer explanation and more accurate predictions. Other Theories have in contrast been completely discredited, such as those listed below.
If a Theory manages to pass the tests thrown at it then it becomes accepted as true by the scientific community at large. This then filters down to the rest of the population and in most cases becomes part of common sense. Germs cause disease? of course! The Earth goes around the Sun? everyone knows that! Mammals evolved from a group of reptiles known as Therapsids? uh, ok. The speed of light is constant in all frames of reference leading to time dilation effects and contraction of a body along the direction of movement with respect to the observer? er, sure. Well you get the point. There are Theories that almost no-one doubts the validity of and accept as true as a matter of course. There are one or two that we have more problems with, either because they are so far outside our way of thinking that we just can’t understand them or because we feel that we have such a firm grasp on how the world is that they just seem intuitively wrong.
It is sometimes offered as a criticism of science that it can never know everything and that it’s conclusions are always tentative. I reply first, that it is not the business of science to know everything, only those things that are amenable to the methods of science and second, that this is the strength of science not it’s weakness. A point of view that can never change in light of new information is doomed to stagnate and become irrelevant. Theories don’t give us the Truth, with a capital T, they provide the nearest approximations to the truth that we can currently know.
Examples of widely accepted Theories: