- The theory of relativity: that the laws of physics are the same for all observers
- The theory of evolution by natural selection: that the observed changes in species occur due to selection of well adapted specimens over less well adapted specimens.
- Big Bang theory: that the universe began as an infinitely small point that underwent expansion to form the universe as we know it today.
Define scientific theory for the students. A good definition could be like, "A scientific theory is a general statement intending to explain nature that is confirmed by all available evidence such that it can be used to predict new, as yet unobserved phenomena." Importantly, a theory is worth very little if it doesn't correctly predict all known evidence. Very importantly, theories are subject to changes as new evidence becomes available. Most theories that you will discuss in a high school science class are well-confirmed and are unlikely to be revised in any significant sense. Theories that are less confirmed are abundant in fields like theoretical physics and cosmology, such as String Theory.Ask the students if they can think of some more theories in the context of this definition. You will likely have to help. Consider suggesting:
- The germ theory of illness: that some illnesses are caused by microscopic organisms.
- The heliocentric solar system: that the planets in our solar system orbit around the sun.
- The round Earth theory: that the Earth is round, and not flat, or some other shape.
- The atomic theory of matter: that all matter is made up of atoms.
Ask the students why they don't think of these as theories. Students tend to not think of these as theories because they consider them facts. Indeed, they are facts, but they still fit into the definition of theory because they intend to explain.5
Define a scientific fact. A fact is an objective, verifiable observation. It is the same everywhere. It can be, and has been, verified many times. For example, we know that the germ theory of illness is a fact because we can take bacteria from someone suffering from an illness, look at that bacteria under a microscope, and then inject that bacteria into another individual-who will then get that same illness. We know that the Earth is round because we can travel due west and eventually end up where we started from.6
Ask how a theory can be transformed into a fact. Most theories cannot ever be transformed into fact. No amount of facts supporting such theories makes this so. Remember, a theory is a general statement intended to explain facts. For example, the ancient peoples noticed peculiar points of light that wandered amongst their background. We now know these to be the planets. They wandered because they, like the Earth, were orbiting around the sun, each at different speeds, different distances from the Sun. Nicolaus Copernicus is generally considered to be the first to propose this, and support his theory with hard evidence, but ancient cultures stumbled upon this through speculation. We now consider this a fact because we have sent many craft to these planets and can predict their motions to a very high precision. Of course, our predictions come from the theory (and the laws underlying it).Bring up evolution. Clearly, one of the most controversial theories of the day is the theory of evolution, but saying solely "the theory of evolution" is a misnomer. The theory part of the statement is "by natural selection." That is, does evolution occur because of natural selection or by some other method? Evolution is, in fact, a fact. It is constantly observed in nature; it is constantly observed in the lab. In truth (to not say fact so many times), scientists considered evolution a fact before Darwin proposed his theory. Consider Lamarckian evolution, which predates Darwin's theory. This theory also proposed that all warm-blooded animals (a more specific statement than Darwin's) arose from a common ancestor, but did not propose the correct method. In Lamarckian evolution, an animal can pass on characteristics acquired during its lifetime (also known as "use and disuse"). As such, it would predict that giraffes obtained long necks by stretching to reach higher leaves. One can easily propose an experiment that would test this. If one were to cut off the tails of mice at their birth and breed such mice with each other, eventually, mice would be born with no tails. This is not the case, and, thus, Lamarck's theory is now obsolete because it does not account for all known facts. We now know, as another fact, that only DNA (and the components of sperm and egg cells) can be passed to offspring. Thus, changes in somatic cells cannot affect future generations. Define a scientific law. This is a more complicated concept and can tend to confuse students. A law can also be used to make predictions, but a law is less general in the actual sense. They tend to be more mathematical in nature and are usually derived from first principles, that is simple statements about mathematical (ideal/not actual) systems and their behaviors. Some scientific laws include:
- Newton's Law of heating and cooling: the change in temperature of two bodies in thermal contact is proportional to their difference in temperature.
- Newton's Laws of motion: statements about how large objects made of atoms behave when moving at low speeds relative to each other.
- The Laws of Thermodynamics: statements about entropy, temperature, and thermal equilibrium.
- Ohm's Law: the voltage across a purely resistive element is equal to the current through the element times its resistance.