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Scientific Hypothesis, Model, Theory, and Law

Understanding the Difference Between Basic Scientific Terms

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Words have precise meanings in science. For example, "theory," "law," and "hypothesis" don't all mean the same thing. Outside of science, you might say something is "just a theory," meaning it's a supposition that may or may not be true. In science, however, a theory is an explanation that generally is accepted to be true. Here's a closer look at these important, commonly misused terms.

A hypothesis is an educated guess, based on observation. It's a prediction of cause and effect. Usually, a hypothesis can be supported or refuted through experimentation or more observation. A hypothesis can be disproven but not proven to be true.

Example: If you see no difference in the cleaning ability of various laundry detergents, you might hypothesize that cleaning effectiveness is not affected by which detergent you use. This hypothesis can be disproven if you observe a stain is removed by one detergent and not another. On the other hand, you cannot prove the hypothesis. Even if you never see a difference in the cleanliness of your clothes after trying 1,000 detergents, there might be one more you haven't tried that could be different.

Scientists often construct models to help explain complex concepts. These can be physical models like a model volcano or atom  or conceptual models like predictive weather algorithms. A model doesn't contain all the details of the real deal, but it should include observations known to be valid.

Example: The  Bohr model shows electrons orbiting the atomic nucleus, much the same way as the way planets revolve around the sun. In reality, the movement of electrons is complicated but the model makes it clear that protons and neutrons form a nucleus and electrons tend to move around outside the nucleus.

A scientific theory summarizes a hypothesis or group of hypotheses that have been supported with repeated testing. A theory is valid as long as there is no evidence to dispute it. Therefore, theories can be disproven. Basically, if evidence accumulates to support a hypothesis, then the hypothesis can become accepted as a good explanation of a phenomenon. One definition of a theory is to say that it's an accepted hypothesis.

Example: It is known that on June 30, 1908, in Tunguska, Siberia, there was an explosion equivalent to the detonation of about 15 million tons of TNT. Many hypotheses have been proposed for what caused the explosion. It was theorized that the explosion was caused by a natural extraterrestrial phenomenon , and was not caused by man. Is this theory a fact? No. The event is a recorded fact. Is this theory, generally accepted to be true, based on evidence to-date? Yes. Can this theory be shown to be false and be discarded? Yes.

A scientific law generalizes a body of observations. At the time it's made, no exceptions have been found to a law. Scientific laws explain things but they do not describe them. One way to tell a law and a theory apart is to ask if the description gives you the means to explain "why." The word "law" is used less and less in science, as many laws are only true under limited circumstances.

Example: Consider Newton's Law of Gravity . Newton could use this law to predict the behavior of a dropped object but he couldn't explain why it happened.

As you can see, there is no "proof" or absolute "truth" in science. The closest we get are facts, which are indisputable observations. Note, however, if you define proof as arriving at a logical conclusion, based on the evidence, then there is "proof" in science. Some work under the definition that to prove something implies it can never be wrong, which is different. If you're asked to define the terms hypothesis, theory, and law, keep in mind the definitions of proof and of these words can vary slightly depending on the scientific discipline. What's important is to realize they don't all mean the same thing and cannot be used interchangeably.

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1.6: Hypothesis, Theories, and Laws

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  Learning Objectives

• Describe the difference between hypothesis and theory as scientific terms.
• Describe the difference between a theory and scientific law.

Although many have taken science classes throughout the course of their studies, people often have incorrect or misleading ideas about some of the most important and basic principles in science. Most students have heard of hypotheses, theories, and laws, but what do these terms really mean? Prior to reading this section, consider what you have learned about these terms before. What do these terms mean to you? What do you read that contradicts or supports what you thought?

What is a Fact?

A fact is a basic statement established by experiment or observation. All facts are true under the specific conditions of the observation.

What is a Hypothesis?

One of the most common terms used in science classes is a "hypothesis". The word can have many different definitions, depending on the context in which it is being used:

• An educated guess: a scientific hypothesis provides a suggested solution based on evidence.
• Prediction: if you have ever carried out a science experiment, you probably made this type of hypothesis when you predicted the outcome of your experiment.
• Tentative or proposed explanation: hypotheses can be suggestions about why something is observed. In order for it to be scientific, however, a scientist must be able to test the explanation to see if it works and if it is able to correctly predict what will happen in a situation. For example, "if my hypothesis is correct, we should see ___ result when we perform ___ test."

A hypothesis is very tentative; it can be easily changed.

What is a Theory?

The United States National Academy of Sciences describes what a theory is as follows:

"Some scientific explanations are so well established that no new evidence is likely to alter them. The explanation becomes a scientific theory. In everyday language a theory means a hunch or speculation. Not so in science. In science, the word theory refers to a comprehensive explanation of an important feature of nature supported by facts gathered over time. Theories also allow scientists to make predictions about as yet unobserved phenomena."

"A scientific theory is a well-substantiated explanation of some aspect of the natural world, based on a body of facts that have been repeatedly confirmed through observation and experimentation. Such fact-supported theories are not "guesses" but reliable accounts of the real world. The theory of biological evolution is more than "just a theory." It is as factual an explanation of the universe as the atomic theory of matter (stating that everything is made of atoms) or the germ theory of disease (which states that many diseases are caused by germs). Our understanding of gravity is still a work in progress. But the phenomenon of gravity, like evolution, is an accepted fact.

Note some key features of theories that are important to understand from this description:

• Theories are explanations of natural phenomena. They aren't predictions (although we may use theories to make predictions). They are explanations as to why we observe something.
• Theories aren't likely to change. They have a large amount of support and are able to satisfactorily explain numerous observations. Theories can, indeed, be facts. Theories can change, but it is a long and difficult process. In order for a theory to change, there must be many observations or pieces of evidence that the theory cannot explain.
• Theories are not guesses. The phrase "just a theory" has no room in science. To be a scientific theory carries a lot of weight; it is not just one person's idea about something

Theories aren't likely to change.

What is a Law?

Scientific laws are similar to scientific theories in that they are principles that can be used to predict the behavior of the natural world. Both scientific laws and scientific theories are typically well-supported by observations and/or experimental evidence. Usually scientific laws refer to rules for how nature will behave under certain conditions, frequently written as an equation. Scientific theories are more overarching explanations of how nature works and why it exhibits certain characteristics. As a comparison, theories explain why we observe what we do and laws describe what happens.

For example, around the year 1800, Jacques Charles and other scientists were working with gases to, among other reasons, improve the design of the hot air balloon. These scientists found, after many, many tests, that certain patterns existed in the observations on gas behavior. If the temperature of the gas is increased, the volume of the gas increased. This is known as a natural law. A law is a relationship that exists between variables in a group of data. Laws describe the patterns we see in large amounts of data, but do not describe why the patterns exist.

What is a Belief?

A belief is a statement that is not scientifically provable. Beliefs may or may not be incorrect; they just are outside the realm of science to explore.

Laws vs. Theories

A common misconception is that scientific theories are rudimentary ideas that will eventually graduate into scientific laws when enough data and evidence has accumulated. A theory does not change into a scientific law with the accumulation of new or better evidence. Remember, theories are explanations and laws are patterns we see in large amounts of data, frequently written as an equation. A theory will always remain a theory; a law will always remain a law.

Video \(\PageIndex{1}\): What’s the difference between a scientific law and theory?

• A hypothesis is a tentative explanation that can be tested by further investigation.
• A theory is a well-supported explanation of observations.
• A scientific law is a statement that summarizes the relationship between variables.
• An experiment is a controlled method of testing a hypothesis.

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Marisa Alviar-Agnew  ( Sacramento City College )

Henry Agnew (UC Davis)

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Scientific Law Definition and Examples

A scientific law is a statement or mathematical equation that describes or predicts a natural phenomenon. It does not explain why or how a phenomenon occurs. Another name for a scientific law is a law of nature or law of science . All scientific laws are based on empirical evidence and the scientific method. In science, an assertion can be disproven, but never proven, so it’s possible for a scientific law to be revised or disproven by future experiments. In contrast, a mathematical theorem or identity is proven to be true.

Examples of Scientific Laws

There are laws in all scientific disciplines, although primarily they are physical laws. Here are some examples:

• Beer’s law
• Dalton’s law of partial pressures
• Ideal gas law
• Kepler’s laws of planetary motion
• Law of conservation of mass
• Law of conservation of energy
• Law of conservation of momentum
• Law of reflection
• Laws of thermodynamics
• Newton’s law of universal gravitation
• Newton’s laws of motion

Difference Between a Scientific Law and Scientific Theory

Both scientific laws and scientific theories are based in the scientific method and are falsifiable. However, the two terms have very different meanings. A law describes what happens, but does not explain it. A theory explains how or why something works.

For example, Newton’s law of universal gravitation describes what happens when two masses are a given distance apart. The law can be written as a mathematical equation [F = G(m 1 m 2 /r 2 )] and used to make predictions and calculations. However, the law does not explain how gravity works or why two masses are attracted to one another. Scientists didn’t really have an explanation for gravity until Einstein’s theory of general relativity, which continues to be revised as we understand more about the nature of spacetime.

As another example, Hubble’s law of Cosmic Expansion (velocity = Hubble constant x distance) describes the movement of galaxies away from each other. It does explain why this occurs. The Big Bang Theory is one of the theories that explains why galaxies move apart, but the theory does not offer a formula for calculating this motion.

Can a Hypothesis or Theory Become a Law?

A hypothesis , theory, and law are all parts of scientific inquiry, but one never becomes another . They are different things. A hypothesis never becomes a theory, no matter how many experiments support it, because a hypothesis is simply a prediction about how one variable responds when another is changed. A theory takes into account the results of many experiments, testing different hypotheses. A theory explains how something works. Like a theory, a law draws on the results of repeated observations and experiments. But, a law states in words or mathematical equations what happens. Laws don’t explain why.

• Barrow, John (1991). Theories of Everything: The Quest for Ultimate Explanations . ISBN 0-449-90738-4.
• Feynman, Richard (1994). The Character of Physical Law (Modern Library ed.). New York: Modern Library. ISBN 978-0-679-60127-2.
• Gould, Stephen Jay (1981). “ Evolution as Fact and Theory “. Discover . 2 (5): 34–37.
• McComas, William F. (2013). The Language of Science Education: An Expanded Glossary of Key Terms and Concepts in Science Teaching and Learning. Springer Science & Business Media. ISBN 978-94-6209-497-0.

Related Posts

Theory vs. Hypothesis vs. Law | Difference & Examples

Melissa Bialowas has taught preschool through high school for over 20 years. She specializes in math, science, gifted and talented, and special education. She has a Master's Degree in Education from Western Governor's University and a Bachelor's Degree in Sociology from Southern Methodist University. She is a certified teacher in Texas as well as a trainer and mentor throughout the United States.

Nikki has a master's degree in teaching chemistry and has taught high school chemistry, biology and astronomy.

Table of Contents

What is a hypothesis, what is a theory, what is a law, what is the difference between hypothesis, theory, and law, the scientific timeline, non-scientific meaning, incorrect use of "theory", lesson summary, can a theory become a law.

Yes, if a theory meets an extremely strict mathematical standard, it might become a law. There are very few laws in science, but they all started as a hypothesis and a theory before becoming a law.

Which comes first, theory or hypothesis?

A hypothesis will always come before a theory. If there is a research question, a hypothesis would be one of many possible answers. The theory is the correct answer that has been tested and proven many times.

Are theories valued higher than laws?

No, neither is valued higher than the other because they are different things. A theory explains why something happens, while a law explains how something happens.

Most science starts with some type of observation. "Hey that's weird." and/or, "I wonder why...." If research doesn't seem to provide an answer, scientists need to come up with possible answers. A hypothesis is a possible explanation that can be tested. This simple definition needs some further explanation. It says it must have a possible explanation. The hypothesis should apply reasoning and analysis based on the research. It cannot be something unrelated or previously proven to be incorrect. A hypothesis also must be able to be tested. It should have exactly one variable that can be controlled in an experiment or solved for using mathematical analysis of data. A hypothesis needs to be tested multiple times by different researchers following the same method before it is proven or disproven.

Examples of Good Hypotheses

• My cat will jump when it sees a cucumber.
• A plant without water will die.
• A person who gets 8 hours of sleep at night will have more energy than if they don't.
• 90% of illegal immigrants do not have health insurance.

Examples of Bad Hypotheses

• All cats will jump when they see a cucumber. - This would require testing all cats, which is not possible.
• A plant without flowers will die. - Research would have proven not all plants have flowers.
• A person who drinks 8 glasses of water a day and gets 8 hours of sleep at night will have more energy. - This is not specific enough, more energy than what or who? Additionally, there are two changing variables in this hypothesis.
• 90% of illegal immigrants are not healthy. - This is also too vague. How do you define healthy?

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• 0:01 Definitions
• 4:17 Comparing and Contrasting
• 5:43 Lesson Summary

After a hypothesis is tested, each experiment can come to a conclusion. However, each hypothesis needs to be tested multiple times by different people before there is a scientific consensus. Once a hypothesis has been documented as correct and it is supported by the scientific community, it can be considered a scientific theory . Note that this is very different from the way the word theory is used outside of science. A scientific theory is not a guess or speculation. A scientific theory has been demonstrated over and over and consistently gives the same result. If a scientific theory is ever shown to be incorrect, even once, then it is changed to include the new data.

Examples of Good Theories

• Plate Tectonics - Earth's landforms are created due to the movement of plates all over the surface.
• Gravity - All things with mass or energy are attracted to one another.
• Heliocentrism - The sun is the center of the solar system.

Examples of Disproven Theories

• Flat Earth - This is the theory that the Earth is flat. The Earth, like all planets, is a spherical shape.
• Geocentric Universe - This is the theory that the Earth is the center of the solar system. The solar system is centered around the sun.
• Blank Slate Theory - This is the theory that when a person is born, they have no built-in traits. However, some traits are inherited while others are learned.

In many aspects of life, the law is something passed by a group that must be obeyed or a person might be punished. Examples might include traffic laws or property laws. However, in science, a law is something completely different. A law explains how something is always true. It is based on mathematics and usually a single, short statement. In science, laws are universal and have a high level of precision. A scientific law never explains why something happens, only how something happens mathematically. Another word for a scientific law is a principle .

Examples of Scientific Laws

• Ohm's Law - A current through a conductor is directly proportional to the voltage.
• Biot-Savart Law - Describes the magnetic field generated by a still electric current.
• Newton's Laws of Motion - Three laws that explain mathematically how things move or don't move.

Examples of Non-Scientific Laws

• In British Columbia, Canada, it is against the law to kill bigfoot.
• In Malawi, it is illegal to "foul the air" with any bad smell.
• In Wyoming, United States, there is a law saying all public buildings must display approved art worth at least 1% of the building's cost.

A hypothesis is a researched and reasonable guess about why something happens. It needs to be tested. A scientific theory is something that answers why and it has been tested repeatedly and has so far always been true. A law is a mathematical statement that tells how something happens. A law is the easiest to differentiate of these three because it is the only one that deals with how rather than why. There are also very few laws in science. A law typically looks like a short mathematical equation.

The difference between a hypothesis and a theory is the testing. A hypothesis has not been proven, while a theory has been proven multiple times by different groups of researchers. A hypothesis is like one option on a multiple-choice test. A theory is like the correct answer key.

How do scientists move from one step to another when trying to find a new hypothesis or theory? Let's walk through the process with an example using the scientific method .

• First, the scientist makes an observation. In our example, a scientist notices people look like their pets.
• Do people pick pets that look like themselves?
• Do people change themselves to look more like their pets?
• Is this true for all types of pets or only dogs and cats?
• Does it matter if the pet is purebred or a mix?
• Next, the scientist decides to do some research. They find there are few peer-reviewed, published academic articles as well as one well documented study. All of the prior research shows a definite correlation between the appearance of dogs and their owners, but it hasn't been repeated enough to move past the hypothesis stage.
• The scientist can now form their own hypothesis. Remember, this must be a statement that can be tested and determined to either be true or false. The scientist decided on: Strangers will be able to correctly match the pictures of pets to their owners merely based on looks.
• The scientist designs an experiment testing the hypothesis.
• After a great deal of testing, it is time for the analysis. The scientist looks at the data to see if people were correct and if it is significant or not.
• Finally, the scientist states their conclusion.

The scientist can share their results and hopefully, over time, the truth about owners and their pets will be discovered. If there is enough scientific proof and consensus, there is a chance this will become scientific theory. In our example, it would be almost impossible to come up with a mathematical formula to find the answer, so it will never become a scientific law. However, if we were testing a new mathematical equation to explain something in the natural world, it might one day become a law.

All three of these words have completely different non-scientific meanings. The word theory is often used to mean a guess or an idea, rather than something that has already been proven. In philosophy, a hypothesis is a proposition without any assumption of truth. This definition of hypothesis doesn't require any prior research or even that the statement is testable. The word law is extremely common in daily life, yet extremely rare in science. In a governmental sense, the law can and does change regularly. In science, a law never changes.

If someone says, "I have a theory..." or "My theory is..." they are not using the scientific meaning of the word theory. In science, a theory already has a consensus, so it cannot belong to one person and one person did not just suddenly come up with it. It takes a long time to develop a scientific theory and it is much more than a guess or an idea. Often, these non-scientific theories are easy to identify because they are clearly just a conjecture, or opinion, and not something that could be or has been tested and proven true.

Examples of Non-Scientific Theories

• I have a theory my neighbor is not cleaning up after their pet.
• My theory is that aliens are living among us.
• Abdul's theory is that the other team cheated in order to win.

Scientists use a process called the scientific method when trying to learn new things. The scientific method includes:

• an observation
• a hypothesis
• an experiment
• analysis of the data
• a conclusion.

Each step has many sub-steps to complete them correctly. For example, to create a hypothesis, a scientist must first understand many possible explanations to what they want to learn. A hypothesis is a statement that can be tested and proven to be either correct or incorrect. Only after completing the entire scientific method can something be moved forward to becoming a scientific theory. A scientific theory requires many different research projects proving it correct so that a scientific consensus is formed about the conclusion. This theory explains why something is true in the natural or scientific world. If new evidence contradicts a theory, it can be changed. A scientific law is a short statement that proves how something is true. These are typically mathematical formulas and if new evidence contradicts a law, then it is no longer considered a law. A scientific law must always be true.

Video Transcript

Definitions.

We construct the world around us by continually making observations about what we see. An observation is a phenomenon that can be witnessed and recorded. A set of observations can be used to make a hypothesis , which is a possible explanation for the observations made, but note a hypothesis is just a possible explanation.

Sometimes, we get new evidence from an experiment or new observations that contradict our hypothesis. An experiment is a procedure carefully done to examine the validity of a hypothesis. In fact, scientists seek to test their hypotheses by making extensive observations or conducting many experiments. The idea is to prove a hypothesis by trying to disprove it first. A hypothesis can be changed or reformulated over a series of observations or experiments. Once a hypothesis holds true, it is accepted as fact.

Over the course of time, a collection of hypotheses can be used to generate either a scientific law or theory. A scientific law is a statement that summarizes a collection of observations or results from experiments. Scientific laws are always true under the same conditions and therefore can be used to make predictions.

In fact, our cartoon friend here could use some of the Laws of Inheritance to better understand why some cats are gray, while other cats are orange, black, white, or even calico! In case you've never heard of them, the Laws of Inheritance were developed by the Austrian monk Gregor Mendel to explain inheritance patterns initially observed in pea plants. Collectively, the laws explain how genes are passed from parents to their offspring.

Because Mendel's explanations were true for a variety of organisms, they became a set of laws. A scientific law may also be referred to as a principle . Other examples of scientific laws or principles include:

• The Law of Conservation of Mass
• Coulomb's Law
• Newton's Law of Universal Gravitation
• The Ideal Gas Law
• Bernoulli's Principle

A theory , or model , is also based on a set of hypotheses. Unlike a law, however, a theory describes and explains why a natural phenomenon occurs. In science, theories explain reality well and are generally accepted as truth. Theories are based on information from many different areas of study.

Our cartoon friend might find the theory of evolution quite helpful when trying to understand where cats came from or why different colored fur might be beneficial in helping cats to survive. To briefly summarize, the theory of evolution explains that organisms slowly change over time due to genetic mutations. Organisms that have beneficial mutations, like the ability to camouflage well, are more likely to survive and pass their traits on to their offspring.

Theories let scientists make testable predictions. Theories can also be changed or modified if new evidence is found! Examples of theories include:

• Cell Theory
• Theory of Relativity
• Atomic Theory
• Plate Tectonics Theory

To better understand the relationship between our three main ideas - hypothesis, law, and theory - let's compare them to an old, strong structure that has withstood the test of time: the Parthenon in Greece. The Parthenon has three fundamental parts. The foundation is the ground structure on which the entire structure is built. The columns are tall pillars built upon the foundation. Together, all of the columns support the weight of the roof, which is used for protection.

In science, our collection of hypotheses represents our foundation. Our columns are the theories and laws we have developed using our hypotheses. Our columns are all equal in strength and importance. Together, the columns support our understanding of the universe, which is the roof of our structure. As time goes on, additions or modifications are made to the structure as we discover new information or need to repair old ideas.

Comparing and Contrasting

We have established that hypotheses are essential to making scientific laws and theories. Hypotheses are based on observations and verified through experiments or more observations. Sets of hypotheses are used to generate scientific laws or theories. Both, in turn, are used to make predictions and generate experiments. If the results contradict the predictions, the theory or law is modified, and the process is started again. For scientific laws, the process rarely occurs.

Let's make a Venn diagram for scientific laws and theories to understand how they are the same and how they are different. If you've never used a Venn diagram before, they are constructed by making a circle representing each idea. Characteristics or qualities of each idea are listed inside. Circles for related ideas are then made to overlap each other. Shared characteristics are placed in the area of overlap. Unique characteristics are left outside of the overlap.

If you want a challenge, pause the video now and make your own Venn diagram for scientific laws and theories. Resume playing when you are finished to compare your work.

Theories are used to explain why natural phenomena occur. Laws are used to summarize a set of observations about natural phenomena. Both laws and theories are based on hypotheses. They can be used to make predictions, and both can be revised if necessary.

An observation is a phenomenon that can be witnessed and recorded. A hypothesis is a possible explanation for the observations made. An experiment is a procedure carefully done to examine the validity of a hypothesis. A scientific law is a statement that summarizes a collection of observations or results from experiments. A theory describes and explains why a natural phenomenon occurs.

Hypotheses are based on observations and verified through experiments or more observations. Sets of hypotheses are used to generate scientific laws or theories. Both scientific laws and theories can be used to make predictions and generate experiments. If the results contrast with the predictions, the theory or law is modified, and the process is started again.

Learning Outcomes

Upon completing this lesson, you should be able to:

• Define observation, hypothesis, experiment, scientific law and theory
• Explain the relationship between hypotheses, laws and theories

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Theories, Hypotheses, and Laws: Definitions, examples, and their roles in science

by Anthony Carpi, Ph.D., Anne E. Egger, Ph.D.

Listen to this reading

Did you know that the idea of evolution had been part of Western thought for more than 2,000 years before Charles Darwin was born? Like many theories, the theory of evolution was the result of the work of many different scientists working in different disciplines over a period of time.

A scientific theory is an explanation inferred from multiple lines of evidence for some broad aspect of the natural world and is logical, testable, and predictive.

As new evidence comes to light, or new interpretations of existing data are proposed, theories may be revised and even change; however, they are not tenuous or speculative.

A scientific hypothesis is an inferred explanation of an observation or research finding; while more exploratory in nature than a theory, it is based on existing scientific knowledge.

A scientific law is an expression of a mathematical or descriptive relationship observed in nature.

Imagine yourself shopping in a grocery store with a good friend who happens to be a chemist. Struggling to choose between the many different types of tomatoes in front of you, you pick one up, turn to your friend, and ask her if she thinks the tomato is organic . Your friend simply chuckles and replies, "Of course it's organic!" without even looking at how the fruit was grown. Why the amused reaction? Your friend is highlighting a simple difference in vocabulary. To a chemist, the term organic refers to any compound in which hydrogen is bonded to carbon. Tomatoes (like all plants) are abundant in organic compounds – thus your friend's laughter. In modern agriculture, however, organic has come to mean food items grown or raised without the use of chemical fertilizers, pesticides, or other additives.

So who is correct? You both are. Both uses of the word are correct, though they mean different things in different contexts. There are, of course, lots of words that have more than one meaning (like bat , for example), but multiple meanings can be especially confusing when two meanings convey very different ideas and are specific to one field of study.

• Scientific theories

The term theory also has two meanings, and this double meaning often leads to confusion. In common language, the term theory generally refers to speculation or a hunch or guess. You might have a theory about why your favorite sports team isn't playing well, or who ate the last cookie from the cookie jar. But these theories do not fit the scientific use of the term. In science, a theory is a well-substantiated and comprehensive set of ideas that explains a phenomenon in nature. A scientific theory is based on large amounts of data and observations that have been collected over time. Scientific theories can be tested and refined by additional research , and they allow scientists to make predictions. Though you may be correct in your hunch, your cookie jar conjecture doesn't fit this more rigorous definition.

All scientific disciplines have well-established, fundamental theories . For example, atomic theory describes the nature of matter and is supported by multiple lines of evidence from the way substances behave and react in the world around us (see our series on Atomic Theory ). Plate tectonic theory describes the large scale movement of the outer layer of the Earth and is supported by evidence from studies about earthquakes , magnetic properties of the rocks that make up the seafloor , and the distribution of volcanoes on Earth (see our series on Plate Tectonic Theory ). The theory of evolution by natural selection , which describes the mechanism by which inherited traits that affect survivability or reproductive success can cause changes in living organisms over generations , is supported by extensive studies of DNA , fossils , and other types of scientific evidence (see our Charles Darwin series for more information). Each of these major theories guides and informs modern research in those fields, integrating a broad, comprehensive set of ideas.

So how are these fundamental theories developed, and why are they considered so well supported? Let's take a closer look at some of the data and research supporting the theory of natural selection to better see how a theory develops.

Comprehension Checkpoint

• The development of a scientific theory: Evolution and natural selection

The theory of evolution by natural selection is sometimes maligned as Charles Darwin 's speculation on the origin of modern life forms. However, evolutionary theory is not speculation. While Darwin is rightly credited with first articulating the theory of natural selection, his ideas built on more than a century of scientific research that came before him, and are supported by over a century and a half of research since.

• The Fixity Notion: Linnaeus

Figure 1: Cover of the 1760 edition of Systema Naturae .

Research about the origins and diversity of life proliferated in the 18th and 19th centuries. Carolus Linnaeus , a Swedish botanist and the father of modern taxonomy (see our module Taxonomy I for more information), was a devout Christian who believed in the concept of Fixity of Species , an idea based on the biblical story of creation. The Fixity of Species concept said that each species is based on an ideal form that has not changed over time. In the early stages of his career, Linnaeus traveled extensively and collected data on the structural similarities and differences between different species of plants. Noting that some very different plants had similar structures, he began to piece together his landmark work, Systema Naturae, in 1735 (Figure 1). In Systema , Linnaeus classified organisms into related groups based on similarities in their physical features. He developed a hierarchical classification system , even drawing relationships between seemingly disparate species (for example, humans, orangutans, and chimpanzees) based on the physical similarities that he observed between these organisms. Linnaeus did not explicitly discuss change in organisms or propose a reason for his hierarchy, but by grouping organisms based on physical characteristics, he suggested that species are related, unintentionally challenging the Fixity notion that each species is created in a unique, ideal form.

• The age of Earth: Leclerc and Hutton

Also in the early 1700s, Georges-Louis Leclerc, a French naturalist, and James Hutton , a Scottish geologist, began to develop new ideas about the age of the Earth. At the time, many people thought of the Earth as 6,000 years old, based on a strict interpretation of the events detailed in the Christian Old Testament by the influential Scottish Archbishop Ussher. By observing other planets and comets in the solar system , Leclerc hypothesized that Earth began as a hot, fiery ball of molten rock, mostly consisting of iron. Using the cooling rate of iron, Leclerc calculated that Earth must therefore be at least 70,000 years old in order to have reached its present temperature.

Hutton approached the same topic from a different perspective, gathering observations of the relationships between different rock formations and the rates of modern geological processes near his home in Scotland. He recognized that the relatively slow processes of erosion and sedimentation could not create all of the exposed rock layers in only a few thousand years (see our module The Rock Cycle ). Based on his extensive collection of data (just one of his many publications ran to 2,138 pages), Hutton suggested that the Earth was far older than human history – hundreds of millions of years old.

While we now know that both Leclerc and Hutton significantly underestimated the age of the Earth (by about 4 billion years), their work shattered long-held beliefs and opened a window into research on how life can change over these very long timescales.

• Fossil studies lead to the development of a theory of evolution: Cuvier

Figure 2: Illustration of an Indian elephant jaw and a mammoth jaw from Cuvier's 1796 paper.

With the age of Earth now extended by Leclerc and Hutton, more researchers began to turn their attention to studying past life. Fossils are the main way to study past life forms, and several key studies on fossils helped in the development of a theory of evolution . In 1795, Georges Cuvier began to work at the National Museum in Paris as a naturalist and anatomist. Through his work, Cuvier became interested in fossils found near Paris, which some claimed were the remains of the elephants that Hannibal rode over the Alps when he invaded Rome in 218 BCE . In studying both the fossils and living species , Cuvier documented different patterns in the dental structure and number of teeth between the fossils and modern elephants (Figure 2) (Horner, 1843). Based on these data , Cuvier hypothesized that the fossil remains were not left by Hannibal, but were from a distinct species of animal that once roamed through Europe and had gone extinct thousands of years earlier: the mammoth. The concept of species extinction had been discussed by a few individuals before Cuvier, but it was in direct opposition to the Fixity of Species concept – if every organism were based on a perfectly adapted, ideal form, how could any cease to exist? That would suggest it was no longer ideal.

While his work provided critical evidence of extinction , a key component of evolution , Cuvier was highly critical of the idea that species could change over time. As a result of his extensive studies of animal anatomy, Cuvier had developed a holistic view of organisms , stating that the

number, direction, and shape of the bones that compose each part of an animal's body are always in a necessary relation to all the other parts, in such a way that ... one can infer the whole from any one of them ...

In other words, Cuvier viewed each part of an organism as a unique, essential component of the whole organism. If one part were to change, he believed, the organism could not survive. His skepticism about the ability of organisms to change led him to criticize the whole idea of evolution , and his prominence in France as a scientist played a large role in discouraging the acceptance of the idea in the scientific community.

• Studies of invertebrates support a theory of change in species: Lamarck

Jean Baptiste Lamarck, a contemporary of Cuvier's at the National Museum in Paris, studied invertebrates like insects and worms. As Lamarck worked through the museum's large collection of invertebrates, he was impressed by the number and variety of organisms . He became convinced that organisms could, in fact, change through time, stating that

... time and favorable conditions are the two principal means which nature has employed in giving existence to all her productions. We know that for her time has no limit, and that consequently she always has it at her disposal.

This was a radical departure from both the fixity concept and Cuvier's ideas, and it built on the long timescale that geologists had recently established. Lamarck proposed that changes that occurred during an organism 's lifetime could be passed on to their offspring, suggesting, for example, that a body builder's muscles would be inherited by their children.

As it turned out, the mechanism by which Lamarck proposed that organisms change over time was wrong, and he is now often referred to disparagingly for his "inheritance of acquired characteristics" idea. Yet despite the fact that some of his ideas were discredited, Lamarck established a support for evolutionary theory that others would build on and improve.

• Rock layers as evidence for evolution: Smith

In the early 1800s, a British geologist and canal surveyor named William Smith added another component to the accumulating evidence for evolution . Smith observed that rock layers exposed in different parts of England bore similarities to one another: These layers (or strata) were arranged in a predictable order, and each layer contained distinct groups of fossils . From this series of observations , he developed a hypothesis that specific groups of animals followed one another in a definite sequence through Earth's history, and this sequence could be seen in the rock layers. Smith's hypothesis was based on his knowledge of geological principles , including the Law of Superposition.

The Law of Superposition states that sediments are deposited in a time sequence, with the oldest sediments deposited first, or at the bottom, and newer layers deposited on top. The concept was first expressed by the Persian scientist Avicenna in the 11th century, but was popularized by the Danish scientist Nicolas Steno in the 17th century. Note that the law does not state how sediments are deposited; it simply describes the relationship between the ages of deposited sediments.

Figure 3: Engraving from William Smith's 1815 monograph on identifying strata by fossils.

Smith backed up his hypothesis with extensive drawings of fossils uncovered during his research (Figure 3), thus allowing other scientists to confirm or dispute his findings. His hypothesis has, in fact, been confirmed by many other scientists and has come to be referred to as the Law of Faunal Succession. His work was critical to the formation of evolutionary theory as it not only confirmed Cuvier's work that organisms have gone extinct , but it also showed that the appearance of life does not date to the birth of the planet. Instead, the fossil record preserves a timeline of the appearance and disappearance of different organisms in the past, and in doing so offers evidence for change in organisms over time.

• The theory of evolution by natural selection: Darwin and Wallace

It was into this world that Charles Darwin entered: Linnaeus had developed a taxonomy of organisms based on their physical relationships, Leclerc and Hutton demonstrated that there was sufficient time in Earth's history for organisms to change, Cuvier showed that species of organisms have gone extinct , Lamarck proposed that organisms change over time, and Smith established a timeline of the appearance and disappearance of different organisms in the geological record .

Figure 4: Title page of the 1859 Murray edition of the Origin of Species by Charles Darwin.

Charles Darwin collected data during his work as a naturalist on the HMS Beagle starting in 1831. He took extensive notes on the geology of the places he visited; he made a major find of fossils of extinct animals in Patagonia and identified an extinct giant ground sloth named Megatherium . He experienced an earthquake in Chile that stranded beds of living mussels above water, where they would be preserved for years to come.

Perhaps most famously, he conducted extensive studies of animals on the Galápagos Islands, noting subtle differences in species of mockingbird, tortoise, and finch that were isolated on different islands with different environmental conditions. These subtle differences made the animals highly adapted to their environments .

This broad spectrum of data led Darwin to propose an idea about how organisms change "by means of natural selection" (Figure 4). But this idea was not based only on his work, it was also based on the accumulation of evidence and ideas of many others before him. Because his proposal encompassed and explained many different lines of evidence and previous work, they formed the basis of a new and robust scientific theory regarding change in organisms – the theory of evolution by natural selection .

Darwin's ideas were grounded in evidence and data so compelling that if he had not conceived them, someone else would have. In fact, someone else did. Between 1858 and 1859, Alfred Russel Wallace , a British naturalist, wrote a series of letters to Darwin that independently proposed natural selection as the means for evolutionary change. The letters were presented to the Linnean Society of London, a prominent scientific society at the time (see our module on Scientific Institutions and Societies ). This long chain of research highlights that theories are not just the work of one individual. At the same time, however, it often takes the insight and creativity of individuals to put together all of the pieces and propose a new theory . Both Darwin and Wallace were experienced naturalists who were familiar with the work of others. While all of the work leading up to 1830 contributed to the theory of evolution , Darwin's and Wallace's theory changed the way that future research was focused by presenting a comprehensive, well-substantiated set of ideas, thus becoming a fundamental theory of biological research.

• Expanding, testing, and refining scientific theories
• Genetics and evolution: Mendel and Dobzhansky

Since Darwin and Wallace first published their ideas, extensive research has tested and expanded the theory of evolution by natural selection . Darwin had no concept of genes or DNA or the mechanism by which characteristics were inherited within a species . A contemporary of Darwin's, the Austrian monk Gregor Mendel , first presented his own landmark study, Experiments in Plant Hybridization, in 1865 in which he provided the basic patterns of genetic inheritance , describing which characteristics (and evolutionary changes) can be passed on in organisms (see our Genetics I module for more information). Still, it wasn't until much later that a "gene" was defined as the heritable unit.

In 1937, the Ukrainian born geneticist Theodosius Dobzhansky published Genetics and the Origin of Species , a seminal work in which he described genes themselves and demonstrated that it is through mutations in genes that change occurs. The work defined evolution as "a change in the frequency of an allele within a gene pool" ( Dobzhansky, 1982 ). These studies and others in the field of genetics have added to Darwin's work, expanding the scope of the theory .

• Evolution under a microscope: Lenski

More recently, Dr. Richard Lenski, a scientist at Michigan State University, isolated a single Escherichia coli bacterium in 1989 as the first step of the longest running experimental test of evolutionary theory to date – a true test meant to replicate evolution and natural selection in the lab.

After the single microbe had multiplied, Lenski isolated the offspring into 12 different strains , each in their own glucose-supplied culture, predicting that the genetic make-up of each strain would change over time to become more adapted to their specific culture as predicted by evolutionary theory . These 12 lines have been nurtured for over 40,000 bacterial generations (luckily bacterial generations are much shorter than human generations) and exposed to different selective pressures such as heat , cold, antibiotics, and infection with other microorganisms. Lenski and colleagues have studied dozens of aspects of evolutionary theory with these genetically isolated populations . In 1999, they published a paper that demonstrated that random genetic mutations were common within the populations and highly diverse across different individual bacteria . However, "pivotal" mutations that are associated with beneficial changes in the group are shared by all descendants in a population and are much rarer than random mutations, as predicted by the theory of evolution by natural selection (Papadopoulos et al., 1999).

• Punctuated equilibrium: Gould and Eldredge

While established scientific theories like evolution have a wealth of research and evidence supporting them, this does not mean that they cannot be refined as new information or new perspectives on existing data become available. For example, in 1972, biologist Stephen Jay Gould and paleontologist Niles Eldredge took a fresh look at the existing data regarding the timing by which evolutionary change takes place. Gould and Eldredge did not set out to challenge the theory of evolution; rather they used it as a guiding principle and asked more specific questions to add detail and nuance to the theory. This is true of all theories in science: they provide a framework for additional research. At the time, many biologists viewed evolution as occurring gradually, causing small incremental changes in organisms at a relatively steady rate. The idea is referred to as phyletic gradualism , and is rooted in the geological concept of uniformitarianism . After reexamining the available data, Gould and Eldredge came to a different explanation, suggesting that evolution consists of long periods of stability that are punctuated by occasional instances of dramatic change – a process they called punctuated equilibrium .

Like Darwin before them, their proposal is rooted in evidence and research on evolutionary change, and has been supported by multiple lines of evidence. In fact, punctuated equilibrium is now considered its own theory in evolutionary biology. Punctuated equilibrium is not as broad of a theory as natural selection . In science, some theories are broad and overarching of many concepts, such as the theory of evolution by natural selection; others focus on concepts at a smaller, or more targeted, scale such as punctuated equilibrium. And punctuated equilibrium does not challenge or weaken the concept of natural selection; rather, it represents a change in our understanding of the timing by which change occurs in organisms , and a theory within a theory. The theory of evolution by natural selection now includes both gradualism and punctuated equilibrium to describe the rate at which change proceeds.

• Hypotheses and laws: Other scientific concepts

One of the challenges in understanding scientific terms like theory is that there is not a precise definition even within the scientific community. Some scientists debate over whether certain proposals merit designation as a hypothesis or theory , and others mistakenly use the terms interchangeably. But there are differences in these terms. A hypothesis is a proposed explanation for an observable phenomenon. Hypotheses , just like theories , are based on observations from research . For example, LeClerc did not hypothesize that Earth had cooled from a molten ball of iron as a random guess; rather, he developed this hypothesis based on his observations of information from meteorites.

A scientist often proposes a hypothesis before research confirms it as a way of predicting the outcome of study to help better define the parameters of the research. LeClerc's hypothesis allowed him to use known parameters (the cooling rate of iron) to do additional work. A key component of a formal scientific hypothesis is that it is testable and falsifiable. For example, when Richard Lenski first isolated his 12 strains of bacteria , he likely hypothesized that random mutations would cause differences to appear within a period of time in the different strains of bacteria. But when a hypothesis is generated in science, a scientist will also make an alternative hypothesis , an explanation that explains a study if the data do not support the original hypothesis. If the different strains of bacteria in Lenski's work did not diverge over the indicated period of time, perhaps the rate of mutation was slower than first thought.

So you might ask, if theories are so well supported, do they eventually become laws? The answer is no – not because they aren't well-supported, but because theories and laws are two very different things. Laws describe phenomena, often mathematically. Theories, however, explain phenomena. For example, in 1687 Isaac Newton proposed a Theory of Gravitation, describing gravity as a force of attraction between two objects. As part of this theory, Newton developed a Law of Universal Gravitation that explains how this force operates. This law states that the force of gravity between two objects is inversely proportional to the square of the distance between those objects. Newton 's Law does not explain why this is true, but it describes how gravity functions (see our Gravity: Newtonian Relationships module for more detail). In 1916, Albert Einstein developed his theory of general relativity to explain the mechanism by which gravity has its effect. Einstein's work challenges Newton's theory, and has been found after extensive testing and research to more accurately describe the phenomenon of gravity. While Einstein's work has replaced Newton's as the dominant explanation of gravity in modern science, Newton's Law of Universal Gravitation is still used as it reasonably (and more simply) describes the force of gravity under many conditions. Similarly, the Law of Faunal Succession developed by William Smith does not explain why organisms follow each other in distinct, predictable ways in the rock layers, but it accurately describes the phenomenon.

Theories, hypotheses , and laws drive scientific progress

Theories, hypotheses , and laws are not simply important components of science, they drive scientific progress. For example, evolutionary biology now stands as a distinct field of science that focuses on the origins and descent of species . Geologists now rely on plate tectonics as a conceptual model and guiding theory when they are studying processes at work in Earth's crust . And physicists refer to atomic theory when they are predicting the existence of subatomic particles yet to be discovered. This does not mean that science is "finished," or that all of the important theories have been discovered already. Like evolution , progress in science happens both gradually and in short, dramatic bursts. Both types of progress are critical for creating a robust knowledge base with data as the foundation and scientific theories giving structure to that knowledge.

Table of Contents

• Theories, hypotheses, and laws drive scientific progress

Activate glossary term highlighting to easily identify key terms within the module. Once highlighted, you can click on these terms to view their definitions.

Activate NGSS annotations to easily identify NGSS standards within the module. Once highlighted, you can click on them to view these standards.

What is a law in science?

The one thing a scientific law doesn't explain is why the phenomenon exists or what causes it.

• Scientific theory vs. scientific law

Scientific laws and mathematics

Do laws change, examples of scientific laws, additional resources, bibliography.

In general, a scientific law is the description of an observed phenomenon. It doesn't explain why the phenomenon exists or what causes it. The explanation for a phenomenon is called a scientific theory . It is a misconception that theories turn into laws with enough research.

"In science, laws are a starting place," said Peter Coppinger, an associate professor of biology and biomedical engineering at the Rose-Hulman Institute of Technology in India. "From there, scientists can then ask the questions, 'Why and how?'" 

Difference between a scientific theory and a scientific law

Many people think that if scientists find evidence that supports a hypothesis, the hypothesis is upgraded to a theory, and if the theory is found to be correct, it is upgraded to a law. That is not how it works, though. Facts, theories and laws — as well as hypotheses — are separate elements of the scientific method . Though they may evolve, they aren't upgraded to something else.

" Hypotheses , theories and laws are rather like apples, oranges and kumquats: One cannot grow into another, no matter how much fertilizer and water are offered," according to the University of California, Berkeley . A hypothesis is a potential explanation of a narrow phenomenon; a scientific theory is an in-depth explanation that applies to a wide range of phenomena. A law is a statement about an observed phenomenon or a unifying concept, according to Kennesaw State University .

"There are four major concepts in science: facts, hypotheses, laws and theories," Coppinger told Live Science. 

Though scientific laws and theories are supported by a large body of empirical evidence that is accepted by the majority of scientists within that area of scientific study, and help to unify that body of data, they are not the same thing.

"Laws are descriptions — often mathematical descriptions — of natural phenomena for example, Newton's Law of Gravity or Mendel's Law of Independent Assortment. These laws simply describe the observation. Not how or why they work," Coppinger said.

Coppinger pointed out that the law of gravity was discovered by Isaac Newton in the 17th century. This law mathematically describes how two different bodies in the universe interact with each other. However, Newton's law doesn't explain what gravity is or how it works. It wasn't until three centuries later, when Albert Einstein developed the theory of Relativity , that scientists began to understand what gravity is and how it works. 

"Newton's law is useful to scientists in that astrophysicists can use this centuries-old law to land robots on Mars. But it doesn't explain how gravity works, or what it is. Similarly, Mendel's Law of Independent Assortment describes how different traits are passed from parent to offspring, not how or why it happens," Coppinger said. Gregor Mendel discovered that two different genetic traits would appear independently of each other in different offspring. "Yet, Mendel knew nothing of DNA or chromosomes . It wasn't until a century later that scientists discovered DNA and chromosomes — the biochemical explanation of Mendel's laws. It was only then that scientists, such as T.H. Morgan, working with fruit flies, explained the Law of Independent Assortment using the theory of chromosomal inheritance. Still today, this is the universally accepted explanation (theory) for Mendel's Law," Coppinger said.

The difference between scientific laws and scientific facts is a bit harder to define, though the definition is important. Facts are simple, one-off observations that have been shown to be true. Laws are generalized observations about a relationship between two or more things in the natural world based on a variety of facts and empirical evidence, often framed as a mathematical statement, according to NASA . 

For example, "Apples fall down from this apple tree" is considered a fact because it is a simple statement that can be proven. "The strength of gravity between any two objects (like an apple and the Earth) depends on the masses of the objects and the distance between them" is a law because it describes the behavior of two objects in a certain circumstance. If the circumstance changes, then the implications of the law would change. For example, if the apple and the Earth shrank to a subatomic size, they would behave differently.

Many scientific laws can be boiled down to a mathematical equation. For example, Newton's Law of Universal Gravitation states: 

F g = G (m 1 ∙ m 2 ) / d 2

Fg is the force of gravity; G is the universal gravitational constant, which can be measured; m1 and m2 are the masses of the two objects, and d is the distance between them, according to The Ohio State University .

Scientific laws are also often governed by the mathematics of probability. "With large numbers, probability always works. The house always wins," said Sylvia Wassertheil-Smoller, a professor at Albert Einstein College of Medicine in New York. "We can calculate the probability of an event and we can determine how certain we are of our estimate, but there is always a trade-off between precision and certainty. This is known as the confidence interval. For example, we can be 95% certain that what we are trying to estimate lies within a certain range or we can be more certain, say 99% certain, that it lies within a wider range. Just like in life in general, we must accept that there is a trade-off."

Just because an idea becomes a law doesn't mean that it can't be changed through scientific research in the future. The use of the word "law" by laymen and scientists differs. When most people talk about a law, they mean something that is absolute. A scientific law is much more flexible. It can have exceptions, be proven wrong or evolve over time, according to the University of California, Berkeley.

"A good scientist is one who always asks the question, 'How can I show myself wrong?'" Coppinger said. "In regards to the Law of Gravity or the Law of Independent Assortment, continual testing and observations have 'tweaked' these laws. Exceptions have been found. For example, Newton's Law of Gravity breaks down when looking at the quantum (subatomic) level. Mendel's Law of Independent Assortment breaks down when traits are "linked" on the same chromosome."

• The law of conservation of energy, which says that the total energy in an isolated system remains constant. In other words, energy cannot be created or destroyed, according to Britannica .
• The laws of thermodynamics , which deal with the relationships between heat and other forms of energy
• Newton's universal law of gravitation, which says that any two objects exert a gravitational force upon each other, according to the University of Winnipeg
• Hubble's law of cosmic expansion, which defines a relationship between a galaxy's distance and how fast it's moving away from us, according to astrophysicist Neta A. Bahcall
• The Archimedes Principle , which states that the buoyant force on an object submerged in a fluid is equal to the weight of the fluid displaced by that object.
• This resource from the New South Wales Education Standards Authority has an in-depth explanation of scientific theories and laws.
• Find out why a theory can’t evolve into a law in this article from Indiana Public Media .
• Watch a video about the difference between a scientific law and a scientific theory from TEDEd.

University of California, Berkeley, "​​Misconceptions about science." https://undsci.berkeley.edu/teaching/misconceptions.php

NASA IMAGE Education Center, "Teacher's Guide: Theories, Hypothesis, Laws, Facts & Beliefs." https://www.nasa.gov/pdf/371711main_SMII_Problem23.pdf  

The Ohio State University, "Lecture 18: The Apple and the Moon: Newtonian Gravity." https://www.astronomy.ohio-state.edu/pogge.1/Ast161/Unit4/gravity.html  

Encyclopedia Britannica, "Conservation of energy." November 16, 2021. https://www.britannica.com/science/conservation-of-energy  

University of Winnipeg, "Newton's Law of Gravitation." 1997. https://theory.uwinnipeg.ca/physics/circ/node7.html  

Neta A. Bahcall, "Hubble's Law and the expanding universe," Proceedings of the National Academy of Sciences, Volume 112, March 2015, https://doi.org/10.1073/pnas.1424299112  

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1.1: Scientific Laws and Theories

Chapter 1: introduction: matter and measurement, chapter 2: atoms and elements, chapter 3: molecules, compounds, and chemical equations, chapter 4: chemical quantities and aqueous reactions, chapter 5: gases, chapter 6: thermochemistry, chapter 7: electronic structure of atoms, chapter 8: periodic properties of the elements, chapter 9: chemical bonding: basic concepts, chapter 10: chemical bonding: molecular geometry and bonding theories, chapter 11: liquids, solids, and intermolecular forces, chapter 12: solutions and colloids, chapter 13: chemical kinetics, chapter 14: chemical equilibrium, chapter 15: acids and bases, chapter 16: acid-base and solubility equilibria, chapter 17: thermodynamics, chapter 18: electrochemistry, chapter 19: radioactivity and nuclear chemistry, chapter 20: transition metals and coordination complexes, chapter 21: biochemistry.

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In science, a law is a concise statement, verbal or mathematical, that summarizes an observation and states what will happen under certain conditions. It is a universally accepted statement, and must never be wrong. Otherwise, any science based upon it would be proven incorrect.

For example, in case of the phenomenon of combustion, Lavoisier validated his hypothesis through a series of experiments and then stated that the mass of an object remains conserved during combustion. This statement became one of the famous laws of chemistry, the Law of Conservation of Mass, which states that ‘Mass in an isolated system can neither be created nor destroyed’.

A scientific theory, unlike a law, is a unifying model that provides an explanation as to why and how something happens. It requires rigorous experimentation and observations conducted over a long period of time to develop a theory. 

For example, the Law of Conservation of Mass did not explain why the mass remains unchanged after combustion. An explanation for this phenomenon was put forward when John Dalton proposed the Atomic Theory. Dalton’s Theory proposed that matter is composed of small, indivisible particles called atoms. Since these particles are merely rearranged, and not created or destroyed in a chemical reaction like combustion, the total amount of mass remains the same. 

Theories are constantly tested and evolve as new observations are made. For example, Dalton’s Atomic theory was improved after scientists found that atoms are in fact further divisible into neutrons, protons, and electrons. Further revisions came with the discoveries of quarks, bosons, and so on. 

Overall, the scientific method framework leads scientists from questions and observations to laws or theory, facilitated by experimental verification of hypotheses, and any necessary modification.

Ultimately, while a hypothesis provides a limited explanation of a phenomenon, the theory provides an in-depth explanation of the observed phenomenon. A law, on the other hand, simply states the observation.  

Scientific Laws

In science, a law is defined as a concise, verbal or mathematical, statement that summarizes a vast number of experimental observations. It describes or predicts some facets of the natural world that always remain the same under the same conditions. 

Scientific Theory

A scientific theory is a unifying principle that provides a well-substantiated and testable explanation of aspects of nature and provides the reason for why things happen. Well-established theories are the pinnacle of scientific knowledge that has been developed over many years of constant experimental evaluation; they are as close to the truth as we get in science. They, too, are continuously tested and modified with newer observations obtained through advancements in science and technology. 

Thus, while a hypothesis is a proposed explanation for a particular observation, a theory is a well-tested explanation for a broad set of observations that explain a particular facet of the physical world around us. Scientific laws are statements about particular observations; they do not explain the reason involved. 

This text is adapted from Openstax, Chemistry 2e, Section 1.1: The Scientific Method.  

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The Difference Between a Scientific Hypothesis, Theory, and Law

Let’s address some common misconceptions about the basic concepts of science..

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Nobody is exempt from misunderstanding scientific concepts and/or applying them incorrectly. Statistics from the National Science Board show that Americans scored an average of 5.6 over 9 true-or-false and multiple-choice science-related questions in 2016. Because of the low number of questions, the study is better at differentiating low and medium levels of knowledge than those with higher levels of knowledge. However, the r esults weren’t much different in previous studies, suggesting that Americans generally have had the same basic levels of science literacy since the beginning of the century.

In this context, we’d like to clear up and emphasize the distinctions between scientific hypothesis, theory, and law, and why you shouldn’t use these terms interchangeably. 

Hypothesis: the core of the scientific method

The scientific method is an empirical procedure that consists of systematic observation, measurement, and experiment, and the formulation, testing, and modification of hypotheses.  It’s a process that’s meant to ensure that the collection of evidence, results, and conclusions are not biased by subjective views and can be repeated consistently by others.

Although there might be variations due to the requirements of each branch of science, the steps of the scientific method are more or less the same.

The scientific method often starts with an observation or asking a question, such as the observation of certain natural phenomena or asking why a particular phenomenon exists or why it occurs in a particular way.

Observation motivates a question and the question motivates an initial hypothesis. The initial hypothesis is a conjecture that works as a temporary answer to the question, formulated via induction on the basis of what’s been observed. 

To better understand this, let’s take the case of physician Ignaz Semmelweis. In mid-19th Century, he worked at the First Obstetrical Clinic of Vienna General Hospital, where 10% of women in labor died due to puerperal fever. Meanwhile, the Second Obstetrical Clinic had an average maternal mortality rate of 4%. Semmelweis asked himself why there was a discrepancy in mortality rates between the two clinics. 

  Through observation, he determined and eliminated a number of differences between the two clinics. Because the techniques, climate, etc., were pretty much the same in both places, he ended up concluding that it had something to do with the health workers who helped women in labor. In the Second Clinic, births were attended only by midwives, while in the First Clinic, births were often attended by medical students who also performed autopsies. Semmelweis came up with the hypothesis that medical students spread “cadaveric contamination” in the First Clinic and this was causing the puerperal fever. 

He ordered all medical students to wash their hands with chlorinated lime after performing autopsies, and the mortality rate in the First Clinic decreased by 90%. 

Semmelweis is considered one of the early pioneers of antiseptic procedures .

This story doesn’t only demonstrate the importance of the initial hypothesis, but also the importance of testing it through experiments, field studies, observational studies, or other experimental work. In fact, this is the next step in the scientific method, and it’s essential to draw conclusions. 

Theory: the Why and How of natural phenomena

A scientific theory can be defined as a series of repeatedly tested and verified hypotheses and concepts. Scientific theories are based on hypotheses that are constructed and tested using the scientific method, and which may bring together a number of facts and hypotheses.

A theory synthesizes the discovered facts about phenomena in a way that allows scientists to formulate predictions and develop new hypotheses. For example, we can hypothesize the reasons why an animal looks or acts in a certain way based on Darwin’s theory of evolution. Or we can predict that antiseptics will prevent diseases if we take into account the germ theory . The confirmation of these hypotheses and predictions reinforces the theories they’re based on.

For a theory to be valid, it must be testable, hold true for general tendencies and not only to specific cases, and it must not contradict verified pre-existing theories and laws. 

Law: the patterns of nature

In general, a scientific law is  the description of an observed phenomenon. It doesn’t explain why the phenomenon exists or what causes it. Laws can be thought of as the starting place, the point from where questions like “why” and “how” are asked.

For example, we can throw a ball under certain conditions and predict its movement by taking into account Newton’s laws of motion . These laws do not only involve several statements but also equations and formulas.  However, while Newton’s laws might mathematically describe how two bodies interact with each other, they don’t explain what gravity is, or how it works. 

Contrary to popular belief, scientific laws are not immutable. They must be universal and absolute to qualify as laws, but they can be corrected or extended to make them more accurate. For example, Euler’s laws of motion amplify Newton’s laws of motion to rigid bodies ,  and how gravity works was only understood in more detail when Albert Einstein developed the Theory of Relativity.

RECOMMENDED ARTICLES

Common misconceptions about scientific laws, theories, and hypotheses.

• There is a hierarchy between laws, theories, and hypotheses: Some people think that hypotheses “evolve” into theories and theories “evolve” into laws. While a number of verified hypotheses can be included in a theory, it’s never only one. And theories do not turn into scientific laws because they’re simply different concepts. As stated above, theories explain phenomena and laws reflect patterns. 

You don’t have to be a scientist to understand scientific terms. In the information era, scientific concepts surround us, but even if access to knowledge is easier than ever nowadays, there are still a lot of misconceptions around. It’s always better to be on the safe side and getting your facts straight. 

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ABOUT THE EDITOR

Maia Mulko Maia is a bilingual freelance writer and copywriter with a degree in Communication Studies. Although she has written for several different niches and publications, she spent most of her career writing for Descentralizar, a Spanish publication that investigates stories at the intersection of technology and society. She has also written scripts for a wide variety of science-related YouTube channels. Maia is particularly interested in space, AI, mobility, gaming, robotics, and assistive technologies. 

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The Difference Between a Fact, Hypothesis, Theory, and Law In Science

Words like “fact,” “theory,” and “law,” get thrown around a lot. When it comes to science, however, they mean something very specific; and knowing the difference between them can help you better understand the world of science as a whole.

In this fantastic video from the It’s Okay To Be Smart YouTube channel , host Joe Hanson clears up some of the confusion surrounding four very important scientific terms: fact, hypothesis, theory, and law. Knowing the difference between these words is the key to understanding news, studies, and any other information that comes from the scientific community. Here are the main takeaways:

Fact: Observations about the world around us. Example: “It’s bright outside.”

Hypothesis: A proposed explanation for a phenomenon made as a starting point for further investigation. Example: “It’s bright outside because the sun is probably out.”

Theory: A well-substantiated explanation acquired through the scientific method and repeatedly tested and confirmed through observation and experimentation. Example: “When the sun is out, it tends to make it bright outside.”

Law: A statement based on repeated experimental observations that describes some phenomenon of nature. Proof that something happens and how it happens, but not why it happens. Example: Newton’s Law of Universal Gravitation .

Essentially, this is how all science works. You probably knew some of this, or remember bits and pieces of it from grade school, but this video does a great job of explaining the entire process. When you know how something actually works, it makes it a lot easier to understand and scrutinize .

Theory vs. Hypothesis vs. Law... Explained! | YouTube

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Definition of hypothesis

Did you know.

The Difference Between Hypothesis and Theory

A hypothesis is an assumption, an idea that is proposed for the sake of argument so that it can be tested to see if it might be true.

In the scientific method, the hypothesis is constructed before any applicable research has been done, apart from a basic background review. You ask a question, read up on what has been studied before, and then form a hypothesis.

A hypothesis is usually tentative; it's an assumption or suggestion made strictly for the objective of being tested.

A theory , in contrast, is a principle that has been formed as an attempt to explain things that have already been substantiated by data. It is used in the names of a number of principles accepted in the scientific community, such as the Big Bang Theory . Because of the rigors of experimentation and control, it is understood to be more likely to be true than a hypothesis is.

In non-scientific use, however, hypothesis and theory are often used interchangeably to mean simply an idea, speculation, or hunch, with theory being the more common choice.

Since this casual use does away with the distinctions upheld by the scientific community, hypothesis and theory are prone to being wrongly interpreted even when they are encountered in scientific contexts—or at least, contexts that allude to scientific study without making the critical distinction that scientists employ when weighing hypotheses and theories.

The most common occurrence is when theory is interpreted—and sometimes even gleefully seized upon—to mean something having less truth value than other scientific principles. (The word law applies to principles so firmly established that they are almost never questioned, such as the law of gravity.)

This mistake is one of projection: since we use theory in general to mean something lightly speculated, then it's implied that scientists must be talking about the same level of uncertainty when they use theory to refer to their well-tested and reasoned principles.

The distinction has come to the forefront particularly on occasions when the content of science curricula in schools has been challenged—notably, when a school board in Georgia put stickers on textbooks stating that evolution was "a theory, not a fact, regarding the origin of living things." As Kenneth R. Miller, a cell biologist at Brown University, has said , a theory "doesn’t mean a hunch or a guess. A theory is a system of explanations that ties together a whole bunch of facts. It not only explains those facts, but predicts what you ought to find from other observations and experiments.”

While theories are never completely infallible, they form the basis of scientific reasoning because, as Miller said "to the best of our ability, we’ve tested them, and they’ve held up."

• proposition
• supposition

hypothesis , theory , law mean a formula derived by inference from scientific data that explains a principle operating in nature.

hypothesis implies insufficient evidence to provide more than a tentative explanation.

theory implies a greater range of evidence and greater likelihood of truth.

law implies a statement of order and relation in nature that has been found to be invariable under the same conditions.

Examples of hypothesis in a Sentence

These examples are programmatically compiled from various online sources to illustrate current usage of the word 'hypothesis.' Any opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Send us feedback about these examples.

Word History

Greek, from hypotithenai to put under, suppose, from hypo- + tithenai to put — more at do

1641, in the meaning defined at sense 1a

Phrases Containing hypothesis

• counter - hypothesis
• nebular hypothesis
• null hypothesis
• planetesimal hypothesis
• Whorfian hypothesis

Articles Related to hypothesis

This is the Difference Between a...

This is the Difference Between a Hypothesis and a Theory

In scientific reasoning, they're two completely different things

Dictionary Entries Near hypothesis

hypothermia

hypothesize

Cite this Entry

“Hypothesis.” Merriam-Webster.com Dictionary , Merriam-Webster, https://www.merriam-webster.com/dictionary/hypothesis. Accessed 8 Jun. 2024.

Kids Definition

Kids definition of hypothesis, medical definition, medical definition of hypothesis, more from merriam-webster on hypothesis.

Nglish: Translation of hypothesis for Spanish Speakers

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Britannica.com: Encyclopedia article about hypothesis

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• Education Resources Information Center - Understanding Hypotheses, Predictions, Laws, and Theories
• Simply Psychology - Research Hypothesis: Definition, Types, & Examples
• Cornell University - The Learning Strategies Center - Hypothesis
• Washington State University - Developing a Hypothesis
• Verywell Mind - Forming a Good Hypothesis for Scientific Research
• BCCampus Publishing - Research Methods for the Social Sciences: An Introduction - Hypotheses

hypothesis , something supposed or taken for granted, with the object of following out its consequences (Greek hypothesis , “a putting under,” the Latin equivalent being suppositio ).

In planning a course of action, one may consider various alternatives , working out each in detail. Although the word hypothesis is not typically used in this case, the procedure is virtually the same as that of an investigator of crime considering various suspects. Different methods may be used for deciding what the various alternatives may be, but what is fundamental is the consideration of a supposal as if it were true, without actually accepting it as true. One of the earliest uses of the word in this sense was in geometry . It is described by Plato in the Meno .

The most important modern use of a hypothesis is in relation to scientific investigation . A scientist is not merely concerned to accumulate such facts as can be discovered by observation: linkages must be discovered to connect those facts. An initial puzzle or problem provides the impetus , but clues must be used to ascertain which facts will help yield a solution. The best guide is a tentative hypothesis, which fits within the existing body of doctrine. It is so framed that, with its help, deductions can be made that under certain factual conditions (“initial conditions”) certain other facts would be found if the hypothesis were correct.

The concepts involved in the hypothesis need not themselves refer to observable objects. However, the initial conditions should be able to be observed or to be produced experimentally, and the deduced facts should be able to be observed. William Harvey ’s research on circulation in animals demonstrates how greatly experimental observation can be helped by a fruitful hypothesis. While a hypothesis can be partially confirmed by showing that what is deduced from it with certain initial conditions is actually found under those conditions, it cannot be completely proved in this way. What would have to be shown is that no other hypothesis would serve. Hence, in assessing the soundness of a hypothesis, stress is laid on the range and variety of facts that can be brought under its scope. Again, it is important that it should be capable of being linked systematically with hypotheses which have been found fertile in other fields.

If the predictions derived from the hypothesis are not found to be true, the hypothesis may have to be given up or modified. The fault may lie, however, in some other principle forming part of the body of accepted doctrine which has been utilized in deducing consequences from the hypothesis. It may also lie in the fact that other conditions, hitherto unobserved, are present beside the initial conditions, affecting the result. Thus the hypothesis may be kept, pending further examination of facts or some remodeling of principles. A good illustration of this is to be found in the history of the corpuscular and the undulatory hypotheses about light .

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[ hahy- poth - uh -sis , hi- ]

• a proposition, or set of propositions, set forth as an explanation for the occurrence of some specified group of phenomena, either asserted merely as a provisional conjecture to guide investigation working hypothesis or accepted as highly probable in the light of established facts.
• a proposition assumed as a premise in an argument.
• the antecedent of a conditional proposition.
• a mere assumption or guess.

/ haɪˈpɒθɪsɪs /

• a suggested explanation for a group of facts or phenomena, either accepted as a basis for further verification ( working hypothesis ) or accepted as likely to be true Compare theory
• an assumption used in an argument without its being endorsed; a supposition
• an unproved theory; a conjecture

/ hī-pŏth ′ ĭ-sĭs /

, Plural hypotheses hī-pŏth ′ ĭ-sēz′

• A statement that explains or makes generalizations about a set of facts or principles, usually forming a basis for possible experiments to confirm its viability.
• plur. hypotheses (heye- poth -uh-seez) In science, a statement of a possible explanation for some natural phenomenon. A hypothesis is tested by drawing conclusions from it; if observation and experimentation show a conclusion to be false, the hypothesis must be false. ( See scientific method and theory .)

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Derived forms.

• hyˈpothesist , noun

Other Words From

• hy·pothe·sist noun
• counter·hy·pothe·sis noun plural counterhypotheses
• subhy·pothe·sis noun plural subhypotheses

Word History and Origins

Origin of hypothesis 1

Synonym Study

Example sentences.

Each one is a set of questions we’re fascinated by and hypotheses we’re testing.

Mousa’s research hinges on the “contact hypothesis,” the idea that positive interactions among rival group members can reduce prejudices.

Do more research on it, come up with a hypothesis as to why it underperforms, and try to improve it.

Now is the time to test your hypotheses to figure out what’s changing in your customers’ worlds, and address these topics directly.

Whether computing power alone is enough to fuel continued machine learning breakthroughs is a source of debate, but it seems clear we’ll be able to test the hypothesis.

Though researchers have struggled to understand exactly what contributes to this gender difference, Dr. Rohan has one hypothesis.

The leading hypothesis for the ultimate source of the Ebola virus, and where it retreats in between outbreaks, lies in bats.

In 1996, John Paul II called the Big Bang theory “more than a hypothesis.”

To be clear: There have been no double-blind or controlled studies that conclusively confirm this hair-loss hypothesis.

The bacteria-driven-ritual hypothesis ignores the huge diversity of reasons that could push someone to perform a religious ritual.

And remember it is by our hypothesis the best possible form and arrangement of that lesson.

Taken in connection with what we know of the nebulæ, the proof of Laplace's nebular hypothesis may fairly be regarded as complete.

What has become of the letter from M. de St. Mars, said to have been discovered some years ago, confirming this last hypothesis?

To admit that there had really been any communication between the dead man and the living one is also an hypothesis.

"I consider it highly probable," asserted Aunt Maria, forgetting her Scandinavian hypothesis.

Related Words

• explanation
• interpretation
• proposition
• supposition

More About Hypothesis

What is a hypothesis .

In science, a hypothesis is a statement or proposition that attempts to explain phenomena or facts. Hypotheses are often tested to see if they are accurate.

Crafting a useful hypothesis is one of the early steps in the scientific method , which is central to every field of scientific experimentation. A useful scientific hypothesis is based on current, accepted scientific knowledge and is testable.

Outside of science, the word hypothesis is often used more loosely to mean a guess or prediction.

Why is hypothesis important?

The first records of the term hypothesis come from around 1590. It comes from the Greek term hypóthesis , meaning “basis, supposition.”

Trustworthy science involves experiments and tests. In order to have an experiment, you need to test something. In science, that something is called a hypothesis . It is important to remember that, in science, a verified hypothesis is not actually confirmed to be an absolute truth. Instead, it is accepted to be accurate according to modern knowledge. Science always allows for the possibility that new information could disprove a widely accepted hypothesis .

Related to this, scientists will usually only propose a new hypothesis when new information is discovered because there is no reason to test something that is already accepted as scientifically accurate.

Did you know … ?

It can take a long time and even the discovery of new technology to confirm that a hypothesis is accurate. Physicist Albert Einstein ’s 1916 theory of relativity contained hypotheses about space and time that have only been confirmed recently, thanks to modern technology!

What are real-life examples of hypothesis ?

While in science, hypothesis has a narrow meaning, in general use its meaning is broader.

"This study confirms the hypothesis that individuals who have been infected with COVID-19 have persistent objectively measurable cognitive deficits." (N=81,337) Ventilation subgroup show 7-point reduction in IQ https://t.co/50xrNNHC5E — Claire Lehmann (@clairlemon) July 23, 2021

Not everyone drives. They can walk, cycle, catch a train, tram etc. That’s alternatives. What’s your alternative in your hypothesis? — Barry (@Bazzaboy1982) July 27, 2021

What other words are related to hypothesis ?

• scientific method
• scientific theory

Quiz yourself!

True or False?

In science, a hypothesis must be based on current scientific information and be testable.