The distinction between a scientific law and a scientific theory is frequently blurred in popular language, where the word “theory” often implies a mere guess or an unsubstantiated idea. In the context of science, however, both laws and theories represent high levels of certainty, but they serve entirely different functions in describing the natural world. This difference is clearly illustrated by the two ways we understand gravity: as a predictable force defined by a law and as a fundamental interaction explained by a theory. Laws and theories are not steps on a hierarchy, but rather two distinct kinds of scientific knowledge that work together to provide a complete understanding.
The Role of a Scientific Law
A scientific law is a descriptive statement, often expressed as a concise mathematical equation, that summarizes observed natural phenomena. Laws describe what happens in the world under a specific set of conditions, providing a framework for accurate prediction. They are generalizations based on repeated empirical observations and are considered universally applicable within their defined scope.
Laws do not attempt to explain the underlying mechanism or the why behind the phenomenon; they simply state that a particular relationship or pattern exists. The Law of Conservation of Energy, for instance, states that energy cannot be created or destroyed, only transformed, without offering a deeper explanation for this fundamental constraint.
Laws are powerful tools for scientists and engineers because they allow for reliable calculations and forecasts. Because they are so closely tied to direct observation and measurement, laws resist change unless new data shows that the previously defined conditions need to be refined.
The Role of a Scientific Theory
In contrast to a law, a scientific theory is a comprehensive, well-substantiated explanation of some major aspect of the natural world. Theories are the highest level of scientific explanation, built from a vast body of facts, laws, tested hypotheses, and confirmed observations. A theory explains why the observed phenomena and the descriptive laws occur, providing the conceptual model for the mechanism at work.
A theory’s strength lies in its ability to explain a wide range of diverse, seemingly unrelated phenomena and its capacity to make verifiable predictions about future observations. Unlike the common, everyday use of the word “theory,” a scientific theory is a robust, extensively tested structure of ideas, not a speculation.
Theories are broader in scope than laws and may incorporate multiple laws within their framework to explain a complex system. They are constantly subjected to rigorous testing, modification, and refinement as new evidence emerges, but the core explanation typically remains intact.
Gravity: Described by Law, Explained by Theory
The difference between a law and a theory is perfectly illustrated by the history of our understanding of gravity. Isaac Newton’s work provided the Law of Universal Gravitation, which is a mathematical description of the gravitational force between two masses. Newton’s Law allows for the precise calculation of the force between two objects, stating that the force is proportional to the product of their masses and inversely proportional to the square of the distance between their centers.
This law accurately predicts the motion of planets, the trajectory of projectiles, and the behavior of objects on Earth under a wide range of common conditions. However, the Law of Universal Gravitation does not explain how that attractive force is transmitted across empty space or why mass has this property. It simply provides the calculation for the force, defining the “what” of gravity.
Albert Einstein’s Theory of General Relativity, introduced in 1915, provides the comprehensive explanation for gravity, defining the “why” and “how.” This theory completely reframed gravity not as a force, but as a manifestation of the curvature of spacetime caused by mass and energy. According to General Relativity, objects like planets do not fall because of an invisible force pulling them, but because they are following the straightest possible path—a geodesic—through the curved geometry of spacetime created by a massive object like the sun.
Einstein’s theory is a conceptual model that explains phenomena Newton’s law could not, such as the slight anomaly in Mercury’s orbit and the bending of starlight around the Sun. Even though the theory is more accurate and explains the underlying mechanism, Newton’s Law is still used widely because its simpler mathematical formula provides an extremely accurate approximation for calculations in low-gravity and low-velocity environments, like most of our solar system and all of Earth-based engineering. The Law of Universal Gravitation is a highly effective predictive tool, while the Theory of General Relativity is the deeper, more accurate explanatory framework.