In Brief
Knowing the difference between a hypothesis, a theory, and a law is essential for the understanding of science, but for laypeople, sometimes these definitions can be unclear as the same terms are used differently in a colloquial context.

Defining Science

When reading scientific articles (and many other articles on Futurism), you’re likely to come across the terms “hypothesis,” “theory,” and “law.” In the scientific community, these words have very specific definitions. For laypeople, sometimes these definitions can be unclear as the same terms are used differently in a colloquial context.

So, what does it mean when you call something a hypothesis, a theory, or a law?

Hypothesis

A hypothesis is a reasonable guess based on what you know or observe, and hypotheses are proven and disproven all of the time.

Hypotheses play a strong role in the scientific method, the process that acts as the foundation of scientific exploration. After formulating a question, a scientist must create a hypothesis, a potential answer to the question. They then make a testable prediction, test, and analyze the data. Even then, a hypothesis needs to be tested and retested many times before it is generally accepted in the scientific community as being true.

Example: You observe that, upon waking up each morning, your trashcan is overturned and trash is spread around the yard. You form a hypothesis that raccoons are responsible. Through testing, the results will either support or refute your hypothesis.

Theory

A scientific theory consists of one or more hypotheses that have been supported through repeated testing. Theories are one of the pinnacles of science and are widely accepted in the scientific community as being true. A theory must never be shown to be wrong; if it is, the theory is disproven. Theories can also evolve. This doesn’t mean the old theory was wrong. It just wasn’t complete.

The evolution from Newtonian physics to general relativity is an example of creating a more complete theory.

When Sir Isaac Newton discovered the theory of gravity and wrote laws that explained the motions of objects, he was not wrong about how the world worked, but he wasn’t fully right either. Einstein later discovered the theories of special and general relativity, and they create a more complete theory of gravity. In fact, when you stay far below the speed of light, many of the equations in general and special relativity give you Newton’s results. NASA, for the record, uses Newton’s equations when planning missions for their spacecraft.

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So, what happens when you have two theories that contradict each other, such as the Steady State and Big Bang theories (the former says the universe’s density doesn’t change over time and has no beginning or end, while the latter claims the universe is becoming increasingly less dense and started at some point in time with a big bang).

In this case, scientists made observations, hypotheses, and testable predictions to figure out which theory was right. For example, one scientist might observe that the universe is expanding, hypothesize that it had a beginning, and test their hypothesis by doing the math. Eventually, either one theory is overturned completely, as was the case with the Steady State and Big Bang theories, or the correct aspects of each theory are combined to form a new theory.

In either case, the theories then need to withstand the rigors of testing and retesting. After a theory proves itself over time, it is accepted by the scientific community as being correct. In many cases, these theories form the foundation upon which other theories are built. An example are Einstein’s theories of general and special relativity. These theories lay the foundation for many, many other theories and equations (such as Hubble’s law and the Schwarzschild radius).

If relativity were ever overturned, it would cause a ripple effect throughout many other areas of science. However, since the math always works out in the case of relativity, the likelihood of that happening is very, very small. Relativity will most likely prove to be a smaller piece in a lager, more complete theory scientists hypothesize about called Grand Unification, but that is a post for another time.

Law

Scientific laws are short, sweet, and always true. They’re often expressed in a single statement and generally rely on a concise mathematical equation.

Laws are accepted as being universal and are the cornerstones of science. They must never be wrong (that is why there are many theories and few laws). If a law were ever to be shown false, any science built on that law would also be wrong.

Examples of scientific laws (also called “laws of nature”) include the laws of thermodynamics, Boyle’s law of gasses, the laws of gravitation.

Theories Versus Laws

A law isn’t better than a theory, or vice versa. They’re just different, and in the end, all that matters is that they’re used correctly.

A law is used to describe an action under certain circumstances. For example, evolution is a law — the law tells us that it happens but doesn’t describe how or why.

A theory describes how and why something happens. For example, evolution by natural selection is a theory. It provides a host of descriptions for various mechanisms and describes the method by which evolution works.

Another example is Einstein’s famous equation E=mc^2. The equation is a law that describes the action of energy being converted to mass. The theories of special and general relativity, on the other hand, show how and why something with mass is unable to travel at the speed of light.

Hopefully, this has helped expand your understanding of what it means when scientists call something a hypothesis, a theory, or a law.