Scientific Theories and Laws

Learning Objectives

Distinguish between a scientific hypothesis, a scientific theory, and a scientific law.

Understand that theories and laws are well-supported by evidence, but are different kinds of scientific knowledge.

Explain that scientific theories can change with new evidence.

Provide examples of major scientific theories and laws.

The Hierarchy of Scientific Knowledge

In everyday language, people might say they have a 'theory' when they really mean they have a guess or a hunch. But in science, the word theory has a much stronger and more specific meaning. It's important to understand the difference between a hypothesis, a theory, and a law.

Hypothesis

A hypothesis is a testable, proposed explanation for a specific observation. It's an educated guess that serves as a starting point for an experiment.

It is narrow in scope.

It has not yet been extensively tested.

It is often written in an 'If... then...' format.

Example: 'If I give a plant more fertilizer, then it will grow taller.'

Scientific Law

A scientific law is a description of an observed phenomenon in the natural world. It describes what happens under certain conditions, but it does not explain why it happens. Laws are often expressed as mathematical equations. They have been tested repeatedly and are considered to be factual.

Function: Describes a pattern in nature.

Answers: What happens?

Examples:

Newton's Law of Universal Gravitation: Describes the force of attraction between two objects with mass (`F = G(m1m2)/r²`). It tells you how to calculate the force, but not why gravity exists.

The Law of Conservation of Energy: States that energy cannot be created or destroyed. It describes what always happens to energy, but doesn't explain what energy fundamentally is.

Scientific Theory

A scientific theory is a broad, in-depth explanation for a wide range of observations and experimental results. A theory explains why or how things happen.

Function: Explains a pattern in nature.

Answers: Why or how does it happen?

Key Features:

It is supported by a massive body of evidence from many different experiments.

It is broad in scope and can be used to make many testable predictions.

It can incorporate and explain many different laws.

Examples:

The Theory of Plate Tectonics: Explains why we have earthquakes and volcanoes and how mountains are formed.

The Germ Theory of Disease: Explains how microorganisms cause infectious diseases.

The Theory of Evolution by Natural Selection: Explains why there is such a diversity of life on Earth and how species adapt to their environments.

Theories and Laws are Different

It is a common misconception that with enough evidence, a theory 'graduates' into a law. This is incorrect. Theories and laws are two different types of scientific knowledge. Laws describe, and theories explain. A theory will never become a law, and a law will never become a theory. Both are equally important and are supported by extensive evidence.

Theories are the cornerstones of science, but they are not absolute truth. If new evidence is discovered that contradicts a theory, the theory must be modified or even completely replaced. This ability to change is a strength of science, not a weakness.

Key Terms

**Hypothesis: A testable, educated guess or prediction about the outcome of an experiment.
**Scientific Law: A statement that describes a consistent pattern in nature, often expressed as a mathematical equation. It describes what happens but not why.
**Scientific Theory: A well-substantiated explanation of some aspect of the natural world, based on a body of facts that have been repeatedly confirmed through observation and experiment. It explains why or how something happens.