In every day use, the word “theory” is synonymous with an “idea” that someone has about something. It doesn’t require any evidence behind it in order for it to be called a theory, and we’ve often heard some fairly outrageous ones that don’t seem to make much sense. In science, in order for something to be called a theory, there has to be a lot of evidence behind it. These theories are not only explanations; they are explanations that encompass everything we know about that subject. Because you can’t have an explanation without something to explain, theories are built from facts. They are more than facts.

So what is a fact? Like the word theory, this also has a slightly different definition in science than it does in every day use. Non-scientifically, a fact is defined as something that exists. Existence itself is a little vague for science to use; just ask Kierkegaard. Science specifies a fact as a repeatedly confirmed observation practically accepted as “true.” Think about that for a second. It is something that has been observed multiple times and confirmed.

So what are some examples of a fact?

• Fact: The sun rose today.
• Fact: When I let go of an object, it falls to the earth. 
• Fact: When I leave food out on the counter for a long period of time, it becomes spoiled.
• Fact: When I flip a light switch, the light comes on.

Is it a fact that the sun will rise tomorrow? Is it a fact that if I drop an object, it will fall to the earth? Is it a fact that if I leave food out, it will always become spoiled? Is it a fact that every time I flip a light switch that the light will come on? To all of these, the answer is “no.” That is because all of these are guesses about what will happen in the future – predictions. Facts are not predictions. They have no bearing on events to come, only on events we’ve already seen. Facts are historical. That’s it. 

Some people think that predictions are less significant than facts, but that doesn’t make any sense. We base predictions on facts that we’ve witnessed. Therefore, predictions are as grounded in “reality” as facts. Think of it this way: if a person only relied on facts, then they would never be able to predict what would happen next because they could never extrapolate into the future. Basing your life merely on facts is a pretty limited way of living. Predictions based on facts allow us to operate, to make decisions and judgment calls. In science, these predictions are called hypotheses, and scientists use hypotheses to make predictions about what will happen in different controlled circumstances.

So let’s say that you’ve witnessed objects falling your entire life (facts). You feel relatively comfortable with the idea that if you were to drop something, it would then fall (hypothesis). You are able to rely on the fact that objects fall. But then let’s say that you are in outer space far from any large planet or star, and you drop an object. Does it fall? No. It floats. Suddenly, this “fact” of objects falling seems to not be a fact at all! So is it still a fact? If so, then how is it that it floats in space? The two facts contradict!

That’s because facts change based on the circumstances. Facts are only true when and where they’ve been observed. Remember, facts don’t predict behavior. It is a fact that objects have fallen on earth in the past, but it is not a fact that they fall everywhere else or that they will continue to fall on earth tomorrow. Facts are the most limited type of information we have because they don’t tell us anything new. Hypotheses are actually stronger pieces of information because they allow us to do something with facts. 

We know that objects have fallen on earth, and we are pretty sure that they will continue to fall on earth. We also know that at at least one point in space, they float instead of falling. So how can we tell what they are going to do and when? What if we are in orbit around earth? What if we drop something over water? How can we determine how something is going to behave?

We use laws. Scientific laws describe behavior. We look at all the little facts that have accumulated, we look at all the hypotheses that have been tested (those disproved and supported), and we create some type of model that then describes what is going on and what will go on. In the case of objects falling, we have the law of gravity. Like most scientific laws, it involves a formula that we can then use to accurately predict what will happen. Laws describe what will happen in different circumstances.

Using laws, we were able to predict what would happen when Neil Armstrong walked on the moon even though it had never happened before. We created probes that travel through space and rovers that explore on Mars. We can design vessels that can withstand intense pressure of deep sea exploration. We place satellites in orbit, and we program those satellites to allow us to communicate via cell phones. Laws are tools that scientists use.

So far, we can describe what has happened (facts), we can make guesses about what will happen (hypotheses), and we can describe what will happen (laws), but we don’t know why these things happen. That is what a scientific theory is. Remember, theories are explanations. And in science, these theories take into account all of the facts that we’ve witnessed, all of the hypotheses that have been tested, and all of the laws that we’ve discovered. Theories are all-encompassing explanations.

In the case of gravity, not only is it a law, but it is also a theory – the Geometric Theory of Gravitation, also known as the Theory of General Relativity. This theory explains why objects float in space and fall on Earth. It explains why mass and distance are important in the equation above. The law only tells us that they are important; theories explain why they are important. In the case of gravity, it is because mass bends space-time.

Imagine space like one big sheet with objects sitting on it. The more mass an object has, the more it will cause the sheet to bend. Let’s say you have one big object in the center with smaller objects around it. Depending on how close the smaller object is to the big object and just how “big” those smaller objects are, the big object will draw the smaller objects around it. You can see this if you’ve ever been on a trampoline or an air mattress with someone. Their weight will cause the surface to bend deeper, and you can get pulled into them. The same thing works with gravity.

The theory of relativity is backed by a lot of evidence. Einstein first proposed it in 1915, and since then, it has explained anomalies in Mercury’s orbit, unified Newton’s laws with special relativity (what happens when objects are traveling at really high speeds), been supported by gravitational lensing (how objects with really high gravity bend light – for example, why light can’t escape a black hole), and been supported by “gravitational red-shifting” (when you move from an area of strong gravity to an area of weaker gravity, time starts moving faster resulting in a “red-shift” in light).

A scientific theory isn’t just someone’s idea. Theories incorporate everything we know including simple observed facts, confirmed hypotheses, and well-established laws. Theories are incredibly well-supported. As we make new discoveries by testing new hypotheses, we add to the collective understanding of theories, but the knowledge that we have right now is not about to suddenly disappear. When we say that “science is always changing,” it isn’t because the data we’ve gathered for centuries are suddenly invalid; it’s because we are constantly adding to our current level of knowledge, and we understand that tomorrow may reveal something new.

The process of science isn’t unlike restoring ancient artwork obscured by dirt and grime. We slowly work on different parts of the work, revealing bit by bit as we go along. As we work, we start to draw conclusions about what we’ve uncovered; one part may look like it could be a hand, and another part may look like an eye. As we uncover more, we may realize that our initial conclusions may not be accurate; the hand and the eye may turn out to be something else entirely. We understand that something new may completely change our ultimate conclusions, but what we’ve already uncovered isn’t about to disappear. We just see it in a new context.

Evolution is a theory, yes. But it is not “just” a theory. It is not “just” anything. The evidence for evolution is overwhelming and has accumulated from a variety of different fields. Paleontology gives us the fossil record which uses millions of fossils to documents the graduate change of species over a very long period of time (we’ll talk more about this later); biogeography combined with plate tectonics (also a theory, by the way) provides support for the distribution and diversification of species; comparative anatomy allows us to compare the morphological evolution of species through studying vestigial, homologous, and analagous structures (for example, humans and bats have the same bones in their arms even though they have adapted different uses); embryology provides evidence for the evolution of basic cellular processes; genetics shows us how closely related organisms share more DNA than those that are more distant; and biochemistry provides evidence of evolution through studying metabolic processes. We have witnessed entire speciation events in our lifetimes both in the wild and in the lab.

Not only do we have all of this evidence pointing toward evolution, but we actively use evolution in a number of ways. We use it in medicine and agriculture as we combat ever-resistant bacteria and pests. We use it in computer programing to create better and more efficient programs. We use it when we selectively breed domestic animals both for agriculture and for pets (for example, creating specialized breeds of dogs with particular traits). We use it when we manage trophy game populations. The principles behind evolution are so important that they have become the foundation of modern medicine.

Dismissing evolution as “just a theory” not only shows a misunderstanding of what a theory is but it also dismisses an enormous amount of evidence. It dismisses the work of thousands of scientists and the entire foundation of biological science. Theodosius Dobzhansky said, “Nothing in biology makes sense except in the light of evolution,” [1] and dismissing the principles of evolution while reaping its benefits is tantamount to basking in the warmth of daylight while rejecting the sun.