Creationists make it sound as though a 'theory' is something you dreamt up after being drunk all night.
You know a science discussion is going down hill when someone says something is "Just a theory" This doesn't only apply only to politically-charged science discussions (e.g. Evolution; GMO's; Climate Change; Vaccinations) - any science discussion should be ended immediately after someone plays the "It's Just A Theory" card. There is an aggravating and common misperception that 'theories' have not become 'facts' yet so they should not be taken seriously. To paraphrase Pauli, that idea is so bad it is not even wrong!
Theories are extremely important to scientific research so important that serious scientists would rather know a good theory than a good fact! If you want to know what I mean, read on (sorry, it is l-o-n-g)
The problem with talking about facts, theories and all that is these words have multiple meanings and they are technical terms in science (HT GooseRock!) Take the word 'fact' - it can mean the same as 'truth' or it can mean an occurrence or any actuality. The facts are clear: 'fact' can mean different things - and that's a fact! The word 'theory' means a 'hunch' or a 'guess' in common usage but in science it applies to something very specific. Before getting to the nature of scientific theories, let's talk about facts and phenomenons (I know, academics usually say 'phenomena' but using a Greek - or Latin - plural when speaking English seems a bit douchey to me). We can't just spew out some definitions and call it 'done' - we have to describe and explore what these things really are. If your friend got a puppy and wanted to know how to take care of it, you wouldn't hand them a dictionary and tell them to look up the word 'dog.' We need to know more than what some words mean - we need to understand the nature of facts and theories.
When I use the word 'fact' I am talking about an observable 'event' or 'occurrence' - it could be the shape of a bird's beak, or the rate that an apple falls. Facts are things we can all see and agree upon*. An example of this comes from the old TV show Dragnet. Detective Joe Friday, would interview witnesses and ask them for "The facts, just the facts." Clearly he wasn't accusing them of lying to him - he wasn't demanding, "I want the TRUTH" - ala Dan Kaffee in A Few Good Men. Joe wanted them to simply recount what they saw and heard - without commenting, editorializing or interpreting it. He wanted to know what he would have seen had he been there (Yes, I realize psychologists have demonstrated that human perception doesn't work this way - please forgive this benign spherical cow) IOW: Facts are Data Points. We can record them, measure them, study them, compare them and, after peering deeply enough into them, usually agree on what they are (not always, but you'll have to wait for my upcoming article on Data and why it isn't the Trump Card you thought it was ;).
When you have a string of facts, we'll call that a 'phenomenon.' It is a pattern of related events or occurrences. (Note: most science educators make a big point that these must be natural and not mystical or magical phenomenons but I've always wondered how Clark's Third Law applies to that dividing line. Nevertheless, in deference to those who taught me science, I'll stick with natural phenomenons for now - even if I don't always write the word 'natural.'). Scientists care about facts and how they fit into (natural) phenomenons but make no mistake, figuring out which facts belong to which phenomenon is no easy task. Scientists often argue more heatedly than bloggers about which facts are part of which pattern. This is a crucial (and under-appreciated) part of science: finding the patterns. A great example of this was Newton's realization that the same gravity which causes apples to fall to the ground holds the Moon in orbit around the Earth. That is why they call it Newton's Law of Universal Gravitation. Often times just seeing the connection between the facts makes the big picture clearer. The point is a fact is some thing we saw that happened - a phenomenon is a pattern of things we see happening around us.
Now, we are ready to discuss the nature of scientific theories. BTW: Some of you may wonder why some things are called 'laws' (e.g. Newton's Laws) and others are called 'theories' (e.g. the theory of relativity) ... after digging into this question, the Bad Boy Scientist believes any rules for calling some things 'theories' and others 'laws' are bullshit - it's basically how it rolls off of the tongue. Let's call this The Bad Boy's law of Scientific Taxonomy: even scientists sometimes put 'catchy' ahead of 'accurate' when naming things. Hey, they're human, too.
Organizations, like the NSTA, say a theory is something like "A description and/or an explanation of a natural phenomenon that has passed exacting testing." There are three bits to that which are essential to the nature of a theory. First, science deals with natural phenomenons: things we see happening. We covered that already.
Second, theories are descriptive or explanatory - they explain what something is or how it works. Simply stating that gravity exists is not a theory - but explaining how it works and the relationship between masses and distances ... now that's a theory. We could also say theories are models of natural phenomenons. they can be computer simulations or mathematical representations of how & what is going on - or something else. But models are used to predict what would happen under difference circumstances (like, say, after the CO2 in our atmosphere exceeds 500 ppm for a century). It is important to understand that scientific theories must be predictive, not so much in the sense of predicting the future, more like answering the question "What happens if I do this?" Theories can help us understand things outside of our direct experience. This is important because the to extend from the known to the unknown can save a lot of money (and lives) on bad bridges, buildings and airplanes.
Finally, the third aspect of a scientific theory is it is well tested - specifically, it has passed a lot of tests. Before it is well tested, we call a description and/or explanation of a natural phenomenon an "hypothesis." I don't want to get too much into the difference between theories and hypotheses except to note that there is no special 'graduation date' when a hypothesis has passed enough tests to become a full-fledged theory. There is a big, fat grey line between theories and hypotheses - and that line is all about how much confidence the community of experts has in it. After years, decades or even centuries of trying unsuccessfully to poke holes in something, after a while experts eventually admit "It's pretty good" and even start relying on it. This isn't to say that no one will ever find a hole in it - but it is "Good enough for now!" That is really what 'passing exacting testing means' - the general community of experts who really know something about it cannot find any significant flaws and accept is as Good enough for now.
At this point a lot of people smugly say, "Yeah, but what if they're wrong?" To which a scientist says "Then we'll fix it. That's the nature of science - scrutinizing our work, looking for flaws and fixing them." When scientists find flaw in some old theory and replace it with a new one, that is not a failure - that is a scientific success! Let me say that again in bold: When scientists find flaw in some old theory and replace it with a new one, that is not a failure - that is a scientific success!
This is getting long and loved ones are clamoring for my attention so let me return to my crazy assertion that scientists would rather know a good theory than a good fact. Now that we've explored the nature of some of these sciencey concepts like facts, theories and phenomenons, let's look at an example:
Say you have a friend, Jamie - who has blue eyes. You go to their house and meet Jamie's family - and they all have blue eyes. That is a fact - no sane person would argue with you (even nit pickers wouldn't call their eye color hazel ;). So you get curious and start noticing eye color in your friends and their families and see a pattern: When both parents have blue eyes, all the kids have blue eyes (unless the postman has brown eyes, but that's another blog). If either parent has non-blue eyes the kids can have blue, brown, hazel, etc - but blue eyed couples only have blue-eyed kids. That is a legit phenomenon.
So what's the theory for all this?
It explains not only the inheritance of eye color but that of other traits like mid-digital hair, the shape of ear lobes, blood types, etc. It explains why some traits are dominant and others are recessive - it explains these things so well the theory of genetics affects financial decisions, relationships and is admissible in court.
So I ask you, which would you rather know, the fact that Jamie's family all have blue eyes, or the theory of genetics?