24 Examples of Theories

John Spacey, updated on May 09, 2023

A scientific theory in an explanation of nature that is based on empirical evidence that has been repeatedly tested and confirmed. This differs greatly from the everyday use of the word theory to mean a guess, hypothesis, or an unproven idea. A scientific theory is essentially a fact. As with all science, theories are open to challenges that are strongly supported by evidence. The following are common examples of theories.

Einstein’s Theory of Special Relativity Explains the relationship between space and time.	Einstein’s Theory of General Relativity Describes gravity and how matter and energy affect the curvature of spacetime.
E=mc² An equation published by Einstein in 1905 that describes the relationship between the mass of matter and energy. Predicts nuclear energy.	Chaos theory Explains how small seemingly insignificant things can completely change the future of large systems.
The Theory of Inheritance Traits are passed down from parent organisms to their offspring through genetic material.	Darwin’s Theory of Natural Selection Organisms that are best adapted to their environment are more likely to survive and reproduce.
Darwin’s Theory of Evolution Traits that are advantageous to survival and reproduction are more likely to pass to future generations such that traits in a species improve over time.	Darwin’s Theory of Common Descent All species have a common ancestor and the diversity of life on Earth changes over time.
Big Bang Theory The theory that the known universe is essentially an explosion that originated at a point and time and is still underway.	Hubble's Law Describes the expansion of the universe whereby galaxies far from us are moving away from us at ever increasing speeds. This has interesting implications as light from very distant galaxies will never reach us such that we can never communicate.
Conservation of Energy The principle that energy can neither be created nor destroyed only transformed.	Heliocentric Theory The Sun is as the center of the Solar System with the planets orbiting it.
Theory of Gravity Objects with mass are attracted to each other.	Atomic Theory All matter is made up of tiny particles called atoms.
Theory of Electromagnetism Electric and magnetic fields are two aspects of the same force.	Plate Tectonics The Earth's surface is made up of a series of large plates that are constantly moving, changing shape and sliding past each other. Explains things like earthquakes.
Anthropic Principle The universe has life and therefore is compatible with the emergence and existence of life.	Cell Theory The theory that all living organisms are composed of cells such that the cell is a basic building block of life.
Germ Theory of Disease The theory that bacteria and viruses case infectious diseases by invading and multiplying within the body.	Quantum Mechanics Describes the universe at the smallest scale as probabilistic and nondeterministic. For example, particles can exist in multiple states or locations simultaneously and their behavior can’t be predicted with certainty.
Hierarchy Theory The theory that complex systems have levels that each have their own properties and behaviors such that there is no way to describe the entire system with a single rule.	String Theory The fundamental building blocks of the universe are tiny, vibrating strings of energy.
Uncertainty Principle The principle that you can’t know the position and momentum of small particles with precision. This is an example of the unknowable.	Second Law of Thermodynamics States that the entropy of an isolated system tends to increase over time. Interpreted to mean that the disorder and complexity of the universe is always increasing.

Einstein’s Theory of Special Relativity states that the speed of light in a vacuum is a universal speed limit that can't be exceeded. Special relativity has some interesting implications such as time dilation whereby time moves slower as you move faster.

Many of the theories above may seem obvious now but were revolutionary at the time they were introduced whereby some took many years to gain widespread acceptance. Other theories are accepted by science but aren't well understood by the general public. For example, scientific determinism is a popular belief but this is heavily contradicted by Quantum Mechanics whereby the universe at its smallest scale appears spontaneous and unpredictable as opposed to a machine with a certain future.