With the recent debate between Bill Nye and Ken Ham, there has been a resurgence of people claiming that “evolution is just a theory.” This is something that really needs to stop.
In common usage, the word “theory” means a guess. You might have a theory about who will play in the super bowl next year or what will happen later this season on American Horror Story. In this respect, your “theory” is as good as anybody else’s. This is not what we mean when we say “theory” as scientists.
In science, our level of certainty about an idea can be summed up with three words: speculation, hypothesis and theory.
A speculation is the lowest level of certainty. When we suspect that something is true but we haven’t done enough research on the topic to be confident in it, we use this word. This is like having a puzzle that is 10% completed and making a guess about what the missing pieces look like.
When a scientist forms a hypothesis, it means that they have learned everything or nearly everything that humans know about a subject, and they are confident in making a claim about something that is unknown. This is like having a puzzle that is completed but for a couple of pieces and making a claim about what the remaining pieces look like. My students will often define “hypothesis” as “an educated guess.” This is technically true, but it is not a particularly good definition because it implies a larger degree of uncertainty than there really is. While a particular hypothesis may not turn out to be correct, a scientist is confident enough in its accuracy to spend years of their life and tens of thousands of dollars determining whether or not it is true.
In scientific parlance, the word “theory” is synonymous with “fact.” It is an idea for which we have a great deal of evidence. A hypothesis can graduate to a theory once enough evidence is collected in support of it. Having a new theory is like adding a piece to a puzzle. When we know a new piece of information, it give us more insight into the things we do not yet know. It allows us to form new hypotheses that we couldn’t have before. So when someone says “x is only a theory,” they are saying “x is only a fact,” which doesn’t make much sense.
For reference, here are some ideas that are considered theories by scientists: The earth is spherical. The earth orbits the sun. General Relativity. Some diseases are caused by microorganisms. Plate tectonics. Electricity. The diversity of life on earth is the result of evolution by natural selection. Of these, the theory of evolution by natural selection is one of the most strongly supported — about as well-supported as the theory of the heliocentric solar system. It is a benchmark against which we can compare our certainty in other ideas. When something is supported as well as evolutionary theory, we know we’ve got it in the bag.
But when something is a theory, doesn’t that mean it hasn’t been proven? In a sense, yes. Outside of mathematics, scientists do not use the word “proof.” This convention is to constantly remind ourselves that we are not finished learning about things and that there is always a chance, however slim it may be, that we are wrong about something. Within mathematics, a statement can be proven within certain, carefully-established bounds. Some mathematically-proven statements are called “theorems.” Math aside, “theory” means as close to proven as we’re willing to say.
So what is a law? Isn’t a law higher than a theory? Not at all. As it is used, one could define it as a robust and generalizable observation. Unlike with a theory, there is no demand that we understand how it works. For example, Kleiber’s Law is the observation that the body size and metabolic rate of animals are related through the function y = x^3/4. There are hypotheses to explain why this is true, but these explanations are not a part of Kleiber’s law. Despite what many people think, theories do not graduate into laws once we get enough evidence for them. Rather, when we do have laws, we need to develop theories to explain them.
Whenever someone is trying to convince you that a particular scientific idea is wrong by claiming that it is “only a theory,” they are either deliberately trying to mislead you, or they are profoundly ignorant of science. These are the only two options.
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