A scientific theory is a description of the natural world that scientists have proven through rigorous testing. As understood within the scientific community, a theory explains how nature behaves under specific conditions. Theories tend to be as broad as their supporting scientific evidence will permit. They seek to serve as a definitive explanation of some aspect of the natural world.

A theory begins as a hypothesis: a proposed explanation for a natural phenomenon. In order to turn a hypothesis into a proven theory, researchers design science experiments to challenge their ideas under the conditions of the natural world. By adhering to the scientific method and working with careful attention to detail, scientists can eventually accumulate enough evidence to prove their hypothesis, thus making it a theory with predictive power.

Many famous scientific theories have shaped our understanding of the natural world as we know it.

1. The Big Bang Theory: The Big Bang Theory claims that the universe started as a small singularity 13.8 billion years ago and expanded suddenly.

2. The Heliocentric Theory: Nicolaus Copernicus’s theory demonstrates that Earth travels around the Sun in our solar system.

3. The Theory of General Relativity: Albert Einstein’s theory claims that massive objects (like the Earth) cause a distortion in space-time, which is experienced as gravity. This theory actually supplanted one of the most famous scientific laws, Newton’s Law of Universal Gravitation.

4. The Theory of Evolution by Natural Selection: Charles Darwin’s theory—most succinctly summarized as “survival of the fittest”—explains how gradual changes in populations of organisms over time leads to the emergence of traits that allow those organisms to survive.

Like theories, scientific laws describe phenomena that the scientific community has found to be provably true. Generally, laws describe what will happen in a given situation as demonstrable by a mathematical equation, whereas theories describe how the phenomenon happens. Scientific laws develop from scientific discoveries and rigorously tested hypotheses, and new theories generally uphold and expand laws—though neither is ever held to be unimpeachably true.

The laws that anchor the world’s scientific knowledge include:

1. Newton’s Law of Universal Gravitation: Sir Isaac Newton’s 1687 law of gravity describes the attractive forces between all forms of matter. This theory of gravity establishes a bedrock for many subsequent theories, as the force of gravity impacts nearly all physical relationships in the universe.

2. Newton’s Laws of Motion: First published in 1687, this set of three laws describes the role that competing forces play on an object at motion or at rest.

3. Boyle’s Law: Alternately known as Boyle-Mariotte Law or Mariotte’s Law, this describes the relationship between gas volume and gas pressure. Physicists Robert Boyle and Edme Mariotte discovered the law independently in 1662 and 1676, respectively.

4. The Laws of Thermodynamics: This set of four laws concerns thermodynamic work, entropy, heat, temperature, and other forces pertaining to the transfer of energy.

Scientific laws differ from theories in that they tend to describe a narrower set of conditions. A scientific law might explain the relationship between two specific forces or between two changing substances in a chemical reaction. Theories are typically more expansive, and they focus on the how and why of natural phenomena.

Both scientific laws and theories are considered scientific fact. However, theories and laws can be disproven when new evidence emerges. Certain accepted truths of Newtonian physics were partially disproven by Albert Einstein’s theory of relativity. The work of Louis Pasteur disproved prior theories of disease in animals. If thorough scientific research upends a previously held belief, scientists must find new hypotheses that better describe how nature works.