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The Difference Between a Theory and a Law

The Difference Between a Theory and a Law

When discussing scientific principles, there are two frequently confused terms. A scientific THEORY and a scientific LAW. They describe two very different things.

Laws explain HOW something happens.

A LAW describes the nature of a phenomenon and what happens when certain conditions are present. Laws are established after repeated and verified observations. Laws have no exceptions when framed, and they're rarely revised because they are universally observable.

EXAMPLE: Sir Isaac Newton's Universal Law of Gravitation.

The law states that any two objects, regardless of their mass, exert gravitational force toward one another. The formula most physics students learn is: F = G × [(m1m2)/r2]. Using this law, we can chart the moon's course or calculate what's necessary to put a satellite in orbit. (Notice the law allows you to calculate how something will turn out, but offers no insight into why the law works.)

Theories explain WHY something happens.

A THEORY describes the cause of a phenomenon and why something is happening. The word confuses the general public. When an average person has an idea they want to test, they call it a theory. Scientists call that idea a hypothesis, an educated guess about a way to explain a natural phenomenon.

Same word, two different definitions.

For a scientist, once there is enough evidence, a theory takes shape. A theory for a scientist is something that's proven through experiments and observational studies.

EXAMPLE: Charles Darwin and Alfred Russel Wallace's Theory of Evolution by Natural Selection.

This theory describes how living things change over time. Evolution can be documented when an organism is born with a mutation that allows it to better adapt to its environment, help it survive longer and generally have more offspring. If that mutation becomes dominant, then the species is said to have evolved. (Notice the theory explains why something is happening, but does not predict what will happen.)

The reason it's crucial to understand the difference is so that you don't reveal your ignorance when talking about scientific principles.

For example, people will challenge Darwin and Wallace's Theory of Evolution because it's "ONLY A THEORY." They ignore the definition of scientific theory that it doesn't become accepted until extensive testing, experimentation and observation. That "theory" has been strengthened by over 100 years of additional research, making it a cornerstone of biology. No amount of additional information will ever turn it into a law, because it can never predict HOW those changes will manifest.

A theory does not become a law once there is more evidence. They are describing two different things. However, both theories and laws CAN change when new evidence comes to light.

To overthrow a scientific theory, three crucial steps must be followed:

Reproduce all the successes of the leading theory: A new theory must reproduce the successful predictions of the prevailing theory. In other words, it must demonstrate that it doesn't fail where the existing theory succeeds.

Succeed where the prior theory does not: The new theory must resolve any conflicts or gaps between the current theory's predictions and the observed reality. It should provide better explanations for phenomena the current theory fails to address.

Make new, testable predictions that differ from those of the original theory: The new theory should offer new predictions that can be tested and confirmed through observations or experiments. These predictions should differ from the ones made by the prevailing theory, allowing for a clear distinction between the two theories.

Once these three steps are accomplished, a scientific revolution can occur, as has happened in the past with the transitions from Newtonian mechanics to Einstein's General Relativity, or from the ray theory of light to the wave-particle duality of light. Ultimately, scientific theories are only as strong as their ability to explain the natural phenomena observed in the universe.

(These three steps are from Ethan Siegel with his article "The 3 key steps to overthrowing a scientific theory" published on April 5, 2023 in Big Think.)

What’s the difference between a scientific law and theory? - Matt Anticole

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Updated 5/8/2022
Updated 4/15/2023