How do a scientific law and scientific theory compare?

In general, both a scientific law and a scientific theory are accepted to be true by the scientific community as a whole. Both are used to make predictions of events and both are used to advance technology.

Some laws, such as the law of gravity, can also be theories when taken more generally. The law of gravity is expressed as a mathematical equation, and is assumed to be true throughout all of time and the universe. Without this assumption, we can do no science based on gravity’s effects, but from the law, we derived the theory of gravity which describes how gravity works, what causes it, and how it behaves.

The biggest difference between a law and a theory is that a theory is much more complex and dynamic. A law describes a single action, whereas a theory explains an entire group of related phenomena. A law is an assumption that forms the foundation of the scientific method, and a theory is the end result of that same process.

These two ideas can be compared to a slingshot and a car. A scientific law is like a slingshot. A slingshot has only one moving part, the rubber band. If you put a rock in it and draw it back, the rock will fly out at a predictable speed, depending on how far you draw it back.

A car has many moving parts, all working together to transport you from point A to point B. A car is a complex piece of machinery. Sometimes, improvements are made to one or more parts. Like a new set of spark plugs that can withstand heat better than the old set, but the overall function of the car is still the same.

A theory is like that car. Components can be changed or improved without changing the overall truth of the theory as a whole. Some scientific theories include the theory of evolution, the theory of relativity, the atomic theory, and the quantum theory. All of these theories are well documented and supported beyond reasonable doubt. Yet scientists continue to tinker with the component hypotheses of each theory in an attempt to make them more concise, or to make them more all-encompassing. Theories can be tweaked, but they are seldom entirely replaced.

A theory is developed only through the scientific method, meaning it is the final result of a series of rigorous processes. Theories do not become laws. Scientific laws must exist prior to the start of using the scientific method because, as stated earlier, laws are the foundation for all science.