What's in a theory?

My objective with this blog, apart from having a place where I can ramble about cosmology, is to make science understandable. But if we want to understand each other, we first need to make sure we are speaking the same language, and that often doesn't seem to be the case. I'm not referring to all the scientific jargon that we scientists like to throw around, I'm talking about those words that people use everyday but which have a very different meaning when used in a scientific context. The best example of this is the word theory.

Every time someone misuses the word 'theory',
Schrödinger puts another cat in a box. Maybe.

I'm sure you've seen the situation before: a scientist explains a scientific theory, and someone replies with 'but it's only a theory'. At this point the scientist will cringe, shake their head, and decide it's not worth pursuing the issue. But it is worth pursuing.

The word theory is not a bad word, in fact it's one of the best words we can have in science. Here's the definition:

"Theory (n): A set of statements or principles devised to explain a group of facts or phenomena, especially one that has been repeatedly tested or is widely accepted and can be used to make predictions about natural phenomena." -  Collins English Dictionary – Complete and Unabridged, 12th Edition 2014

Now that we know the definition, let's do some science!
We start with a fact. This is just an observation about the world, that we are very sure is true: it's something we can all observe and agree on. Well, probably all of us; there's always someone who likes to deny the facts ('What do you mean the sky is blue? It looks decidedly green to me'). Let's use an example: I put a coffee cup on the desk, I leave the room for a few minutes, and when I come back the coffee cup is on the floor broken (don't worry; it was empty, no coffee was wasted in this experiment!). Fact: my cup is broken on the floor. So we ask a question: why is my cup on the floor in pieces?

Next we move on to a hypothesis. A hypothesis provides a possible answer to our question regarding the observed fact. We can come up with several different hypotheses to try to answer this question. For our broken cup situation, we could have a few possible answers:

1. Someone pushed the cup off the table
2. There was an earthquake/natural phenomena and the cup fell
3. The table is wonky and things tend to fall off
4. The cat pushed the cup off the table
5. The cup thought it was a bird and decided to try to fly.

We use other connected facts to eliminate some of the bad hypotheses. In our example we can use the fact that cups don't think they are birds to discard number five. We can also use the knowledge that there is no one else in the house to discard number one. Number two seems unlikely, as I would have felt the earthquake from the other room. So now we have to do an experiment to test the remaining hypotheses.

For our example, I set another cup in the same place, and I move away to observe. Will the cup fall off because the table is wonky? After several minutes, the cup hasn't fallen. So it seems we can discard the wonky-table-hypothesis. It doesn't take long, however, for a cute cat to jump on the table and knock off the cup, which falls on the strategically placed pillow I set out for our experiment. Maybe we should have left the cat in the box. All the evidence seems to support the fourth hypothesis: the cat pushed the cup off the table. Hypothesis confirmed.

Another very important concept in science are laws. Laws can generally be written as mathematical expressions, and they describe what happens, based on a series of repeated experiments. Newton's law of gravity can tell us how quickly the cup fell off the table. The conservation of mass law tells us that if we add up the masses of all the pieces, we should find the mass of the original cup.

And now we put it all together. We use multiple confirmed hypotheses, based on countless experiments, repeated time and time again, with all the laws describing the experiments, and we group it all together to form a theory. A theory contains laws, confirmed hypotheses and facts, and most importantly, a theory can make predictions about what other facts we can observe. It looks something like this:

And this is how we do science. If a theory makes predictions which don't fit the facts, the theory will be modified. If we find a fact that contradicts the theory, the theory will be modified. Science is constantly changing, and theories are being tested constantly. A theory is the best thing we can have in science, it's telling us 'I am the best model to explain the observed facts, I can make predictions, nothing we have found yet contradicts me, and I have passed every single test you have thrown at me'.

It's not 'just a theory'. It's human beings looking at the universe and saying 'I see what you did there, and I am able to understand it'. And that's awesome.

Still not convinced? Maybe Joe Hanson from It's Okay To Be Smart can help you out: