I’ve decided to write this post so that I have something to refer to and don’t have to constantly re-define these words: Fact, Hypothesis, Theory, and Law.
This may seem silly. “Why,” you may ask, “would you have to define such simple little words?” The reason is that the colloquial use of these words by the general public is very different from their usage by scientists. And let’s really just jump to the chase here: Calling something “Just a Theory” shows both the ignorance of Cobb County, Georgia public school administrators and anyone else who tries to use that phrase to belittle a scientific conclusion.
To use math expressions, the general use of these words goes in order of importance as: Fact > Law > Theory > Hypothesis.
“Fact” in Everyday Language: A “fact” is something that is true. Whether you like it or not, “facts are stubborn things” (thank you, John Adams … or, “facts are stupid things” courtesy of Ronald Reagan). In general use, a “fact” is the strongest thing that can be said about, well, anything.
“Law” in Everyday Language: In everyday language, a “law” is generally on the same level as a fact. A law is something that is true, that generally explains or answers lots of different things. However, outside of politics, “law” is rarely used unless actually referring to something scientific.
“Theory” in Everyday Language: This is where the supposed insult to scientists comes in when you call something “just a theory.” Outside of scientific circles, a “theory” is more of a supposition. “I have a theory that my cat will meow when it hears someone at the door.” It may or may not be “true,” but it’s a supposition I have that is probably supported by at least some sort of observation. But it’s really “just a theory” and is just as likely to be shown wrong at any given time as it is to be shown right.
“Hypothesis” in Everyday Language: A “hypothesis” is sort of on the same level as a “theory,” if slightly below. To most people, they can be used interchangeably, though most will just resort to “theory” because “hypothesis” is an extra syllable longer and makes you sound like a nerd.
In science, the order of importance of these is almost reversed: Theory > Law > Hypothesis > Facts. In addition, each term has a specific, well-defined use.
“Fact” in Science: It may surprise you to know that a “fact” is generally used the same way – it is an observation – but it is very specific. For example, if I drop a ball while holding it in the air above a surface, it is a fact that it will fall to the surface. This term is usually not used, however — we resort to “observations.” For example, I observe that when the wind blows, a flag will flutter.
“Hypothesis” in Science: This is an “idea” that is formulated to explain observations (or our “facts”). In the above to examples, I might hypothesize that there is a force that pulls on the ball, counteracted when I’m holding it. Or that the wind exerts a force on the flag that causes it to flutter. The purpose of a hypothesis is to explain one or more observations in a cogent way. A good hypothesis must be testable – it must be able to make predictions about what would happen in similar situations – otherwise a hypothesis can never be verified nor refuted … and it remains “just a hypothesis.” At present, String “Theory” is really just a hypothesis.
“Law” in Science: Laws are a descriptive generalization about how some aspect of the natural world behaves under stated circumstances. For example, Kepler’s Three Laws of Planetary Motion are (1) Planets travel in ellipses with one focus being the Sun, (2) planets sweep out equal area in equal time, and (3) a planet’s period-squared is proportional to its semi-major-axis-cubed. Laws are generally made from many facts/observations and are effectively an “elevated” level from a hypothesis. Another example are the Laws of Thermodynamics. Because a Law is just a description of how something behaves and it does not explain why it behaves that way, it is usually considered to be below the level of a theory.
“Theory” in Science: A theory is really one of the pinnacles of science – what nearly everyone strives to make out of their hypotheses. A hypothesis is elevated to a theory when it has withstood all attempts to falsify it. Experiment after experiment has shown it sufficient to explain all observations that it encompasses. In other words, a “theory” has never been shown to be false, despite – usually – hundreds if not thousands of separate attempts to break it. It explains the observations with one or more mechanisms and, because it provides that mechanism, it is considered to be above the level of a Law. Examples these days are the Theory of Relativity, Quantum Mechanics, the Germ Theory of Disease, and yes, the Theory of Evolution.
I should note that theories are usually conglomerations of several different hypotheses, laws, facts, inferences, and observations. For example, while the Theory of Evolution is a theory, various mechanisms for it are generally still hypotheses, such as Natural Selection (though some may quibble with me over that).
Another good example of a Theory is the Standard Model of Particle Physics. This describes how fundamental particles and forces interact. It is based upon countless experiments and observations and it rests on solid mathematical framework. It has many different laws in its make-up (such as how particles behave, or how forces interact) as well as many observations (such as the mass of the proton, or the energy of a tau neutrino).
A third example was partially mentioned above – Kepler’s Laws of Planetary Motion. Tycho Brahe and Johannas Kepler made many detailed observations of planetary positions over the course of many years. Kepler formed a hypothesis about how planets moved based upon the data. From the hypothesis, he made predictions on where planets would be later on. When these were confirmed, his hypotheses were elevated to laws. Later, Isaac Newton came along and with his Theory of Gravity was able to provide a physics-based framework for why and how those laws worked.
Finally, it should also be noted that nothing in science is “forever.” It is always subject to further tests and observations. In many cases, people really do try to do this since that’s how you make a name for yourself. If you’re the scientist who has verified for the 123,194th time that a ball and a feather fall at the same rate in a vacuum, so what? But, if you’re the scientist that has found evidence that gravity itself is not a force emitted by an object but rather a bending of the fabric of space itself, then, well, you’d be Einstein – a household name.
(I make this note because a common argument you’ll see from creationists is that they say materialists always want to uphold the status quo.)
That’s really about all I wanted to do with this post – clarify these terms and what they actually mean in science. I’m not naïve enough to think that now suddenly this’ll clear everything up and no one will ever say something’s “just a theory” again, but at least now I’ve gone through all these terms step-by-step so that I can refer back to them when need-be.
Edited to Add: I think my post on “the final epsilon” is a relevant follow-up to this one. If you’re interested in the concept of how classical mechanics can still be a theory even though it disagrees at some level with the theory of relativity, I recommend reading it.