We have been talking a lot about “models,” so you may be wondering how a “model” differs from what we call a “theory.” The answer can be a bit subtle, because the word “theory” has a somewhat different meaning in science than it does in everyday life, and even in science it is not always used correctly. But here’s the basic idea:
In science, a theory is a model that:
(1) explains a wide variety of observations in terms of just a few general principles; and
(2) has been extensively tested and verified in a variety of different ways.
Famous examples of scientific theories include Isaac Newton’s theory of gravity, Charles Darwin’s theory of evolution, and Albert Einstein’s theory of relativity.
Note that this scientific definition of “theory” differs from the way we commonly use the term “theory” in everyday life, where we might be referring to mere speculation or hypothesis. For example, someone might say, “I have a new theory about why people enjoy the beach.” Without the support of a broad range of evidence that others have tested and confirmed, this “theory” is really only a guess. In contrast (and as we’ll discuss in more detail in the next section), Newton’s theory of gravity qualifies as a scientific theory because it uses simple physical principles to explain a great many observations and experiments.
The scientific definition of a theory leads to two more important ideas that you should always keep in mind about scientific theories:
- Because scientific theories are based on models, and models must be falsifiable, we can never prove a theory to be true beyond all doubt. Our earlier discussion of the rotating Earth is a good example: since the future can’t be known, we can’t be absolutely sure that we’ll never observe a celestial object to rise in the west and set in the east. However, we are so confident that the Earth is really rotating that we accept this model as true, even without absolute certainty. In essence, you might think of this like a legal standard: we accept that Earth is rotating beyond a reasonable doubt.
- The many observational and/or experimental tests that a scientific theory has already passed cannot be undone or ignored. Therefore, if a theory fails some future test, any replacement theory must not only explain the new test but also the results of all the tests that the theory previously passed. In most cases, this means that a theory cannot be proven “wrong”; it can only be shown to have been incomplete or just part of the full story.
Discussion
“Only a Theory”
Some people have argued that the theory of evolution is “only a theory.” Discuss how this statement is using the term “theory.” Is it using it in an everyday sense, or a scientific sense? Do you think the same people would also say that gravity is “only a theory?” Defend your opinions.
This discussion is optional, but students will likely be aware of this type of “only a theory” commentary and may be interested to investigate what it means. The last part about gravity will help pave the way for the more detailed discussion of gravity in Section 3.4. Because the debate over this topic can become contentious, you might wish to do the discussion as a full class so that you can keep control over the level of emotion that comes into play. The discussion is a matter of opinion, but be aware that most scientists would say that evolution and gravity are on equally strong ground, because both have been extensively tested and verified.
Connections—Language Arts
Differing Meanings in Science and Everyday Life
The term “theory” is just one of many terms that are used with different meaning in science than in everyday life. The following table summarizes a few of the most common of these terms.
| Term | Everyday Meaning | Scientific Meaning | Example of Use in Science |
|---|---|---|---|
| model | something you build, like a model airplane | a representation of nature, sometimes using mathematics or computer simulations, that is intended to explain or predict observed phenomena | A model of planetary motion can be used to calculate exactly where planets should appear in our sky. |
| hypothesis | a guess or assumption of almost any type | a model that has been proposed to explain some observations but that has not yet been rigorously confirmed | Scientists hypothesize that the Moon was formed by a giant impact, but there is not enough evidence to be fully confident in this model. |
| theory | speculation | a particularly powerful model that has been so extensively tested and verified that we have extremely high confidence in its validity | Einstein’s theory of relativity successfully explains a broad range of natural phenomena and has passed a great many tests of its validity. |
| bias | distortion, political motive | tendency toward a particular result | Current techniques for detecting extrasolar planets are biased toward detecting large planets. |
| critical | really important; involving criticism, often negative | right on the edge, near a boundary | A boiling point is a “critical value” because above that temperature, a liquid will boil away. |
| deviation | strangeness or unacceptable behavior | change or difference | The recent deviation in global temperatures from their long-term average implies that something is heating the planet. |
| enhance/enrich | improve | increase or add more, but not necessarily to make something “better” | “Enhanced colors” means colors that have been brightened. “Enriched with iron” means containing more iron. |
| error | mistake | range of uncertainty | The “margin of error” tells us how closely measured values are likely to reflect true values. |
| feedback | a response | a self-regulating (negative feedback) or self-reinforcing (positive feedback) cycle | Gravity can provide positive feedback to a forming planet: Adding mass leads to stronger gravity, which leads to more added mass, and so on. |
| state (as a noun) | a place or location | a description of current condition | The Sun is in a state of balance, so it shines steadily. |
| uncertainty | ignorance | a range of possible values around some central value | The measured age of our solar system is 4.55 billion years with an uncertainty of 0.02 billion years. |
| values | ethics, monetary value | numbers or quantities | The speed of light has a measured value of 300,000 km/s. |
Source: Adapted from a table published by Richard Somerville and Susan Joy Hassol in Physics Today.
For this connections box, you might wish to spend a little time discussing each of the terms and their different meanings in science and everyday life, asking students to think up additional examples for each case.