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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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Scientific Theory Definition and Examples

A scientific theory is a well-established explanation of some aspect of the natural world. Theories come from scientific data and multiple experiments. While it is not possible to prove a theory, a single contrary result using the scientific method can disprove it. In other words, a theory is testable and falsifiable.

Examples of Scientific Theories

There are many scientific theory in different disciplines:

• Astronomy : theory of stellar nucleosynthesis , theory of stellar evolution
• Biology : cell theory, theory of evolution, germ theory, dual inheritance theory
• Chemistry : atomic theory, Bronsted Lowry acid-base theory , kinetic molecular theory of gases , Lewis acid-base theory , molecular theory, valence bond theory
• Geology : climate change theory, plate tectonics theory
• Physics : Big Bang theory, perturbation theory, theory of relativity, quantum field theory

Criteria for a Theory

In order for an explanation of the natural world to be a theory, it meets certain criteria:

• A theory is falsifiable. At some point, a theory withstands testing and experimentation using the scientific method.
• A theory is supported by lots of independent evidence.
• A theory explains existing experimental results and predicts outcomes of new experiments at least as well as other theories.

Difference Between a Scientific Theory and Theory

Usually, a scientific theory is just called a theory. However, a theory in science means something different from the way most people use the word. For example, if frogs rain down from the sky, a person might observe the frogs and say, “I have a theory about why that happened.” While that theory might be an explanation, it is not based on multiple observations and experiments. It might not be testable and falsifiable. It’s not a scientific theory (although it could eventually become one).

Value of Disproven Theories

Even though some theories are incorrect, they often retain value.

For example, Arrhenius acid-base theory does not explain the behavior of chemicals lacking hydrogen that behave as acids. The Bronsted Lowry and Lewis theories do a better job of explaining this behavior. Yet, the Arrhenius theory predicts the behavior of most acids and is easier for people to understand.

Another example is the theory of Newtonian mechanics. The theory of relativity is much more inclusive than Newtonian mechanics, which breaks down in certain frames of reference or at speeds close to the speed of light . But, Newtonian mechanics is much simpler to understand and its equations apply to everyday behavior.

Difference Between a Scientific Theory and a Scientific Law

The scientific method leads to the formulation of both scientific theories and laws . Both theories and laws are falsifiable. Both theories and laws help with making predictions about the natural world. However, there is a key difference.

A theory explains why or how something works, while a law describes what happens without explaining it. Often, you see laws written in the form of equations or formulas.

Theories and laws are related, but theories never become laws or vice versa.

Theory vs Hypothesis

A hypothesis is a proposition that is tested via an experiment. A theory results from many, many tested hypotheses.

Theory vs Fact

Theories depend on facts, but the two words mean different things. A fact is an irrefutable piece of evidence or data. Facts never change. A theory, on the other hand, may be modified or disproven.

Difference Between a Theory and a Model

Both theories and models allow a scientist to form a hypothesis and make predictions about future outcomes. However, a theory both describes and explains, while a model only describes. For example, a model of the solar system shows the arrangement of planets and asteroids in a plane around the Sun, but it does not explain how or why they got into their positions.

• Frigg, Roman (2006). “ Scientific Representation and the Semantic View of Theories .”  Theoria . 55 (2): 183–206. 
• Halvorson, Hans (2012). “What Scientific Theories Could Not Be.”  Philosophy of Science . 79 (2): 183–206. doi: 10.1086/664745
• McComas, William F. (December 30, 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.
• National Academy of Sciences (US) (1999). Science and Creationism: A View from the National Academy of Sciences (2nd ed.). National Academies Press. doi: 10.17226/6024  ISBN 978-0-309-06406-4. 
• Suppe, Frederick (1998). “Understanding Scientific Theories: An Assessment of Developments, 1969–1998.”  Philosophy of Science . 67: S102–S115. doi: 10.1086/392812

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Understanding Science

How science REALLY works...

• Understanding Science 101
• Misconceptions
• The process of science works at many levels — from that of a single study to that of a broad investigation spanning many decades and encompassing hundreds of individual studies.
• Hypotheses are proposed explanations for a narrow set of phenomena. They are not guesses.
• Theories are powerful explanations for a wide range of phenomena. Accepted theories are not tenuous.
• Some theories are so broad and powerful that they frame whole disciplines of study and encompass many smaller hypotheses and theories.

Misconception:  Hypotheses are just guesses.

Correction:  Hypotheses are reasoned and informed explanations.  Read more about it.

Misconception:  Theories are just hunches.

Correction:  In science, theories are broad explanations. To be accepted, they must be supported by many lines of evidence.  Read more about it.

Misconception:  If evidence supports a hypothesis, it is upgraded to a theory. If the theory then garners even more support, it may be upgraded to a law.

Correction:  Hypotheses cannot become theories and theories cannot become laws. Hypotheses, theories, and laws are all scientific explanations but they differ in breadth, not in level of support. Theories apply to a broader range of phenomena than do hypotheses. The term  law  is sometimes used to refer to an idea about how observable phenomena are related.  Read more about it.

Science at multiple levels

The process of science works at multiple levels — from the small scale (e.g., a comparison of the genes of three closely related North American butterfly species) to the large scale (e.g., a half-century-long series of investigations of the idea that geographic isolation of a population can trigger speciation). The process of science works in much the same way whether embodied by an individual scientist tackling a specific problem, question, or hypothesis over the course of a few months or years, or by a community of scientists coming to agree on broad ideas over the course of decades and hundreds of individual experiments and studies. Similarly, scientific explanations come at different levels:

Hypotheses are proposed explanations for a fairly narrow set of phenomena. These reasoned explanations are not guesses — of the wild or educated variety. When scientists formulate new hypotheses, they are usually based on prior experience, scientific background knowledge, preliminary observations , and logic. For example, scientists observed that alpine butterflies exhibit characteristics intermediate between two species that live at lower elevations. Based on these observations and their understanding of speciation, the scientists hypothesized that this species of alpine butterfly evolved as the result of hybridization between the two other species living at lower elevations.

Theories , on the other hand, are broad explanations for a wide range of phenomena. They are concise (i.e., generally don’t have a long list of exceptions and special rules), coherent, systematic, predictive, and broadly applicable. In fact, theories often integrate and generalize many hypotheses. For example, the theory of natural selection broadly applies to all populations with some form of inheritance, variation, and differential reproductive success — whether that population is composed of alpine butterflies, fruit flies on a tropical island, a new form of life discovered on Mars, or even bits in a computer’s memory. This theory helps us understand a wide range of observations (including the rise of antibiotic-resistant bacteria and the physical match between pollinators and their preferred flowers), makes predictions in new situations (e.g., that treating AIDS patients with a cocktail of medications should slow the evolution of the virus), and has proven itself time and time again in thousands of experiments and observational studies.

"JUST" A THEORY?

Occasionally, scientific ideas (such as biological evolution) are written off with the putdown “it’s just a theory.” This slur is misleading and conflates two separate meanings of the word theory : In common usage, the word theory means just a hunch, but in science, a theory is a powerful explanation for a broad set of observations. To be accepted by the scientific community, a theory (in the scientific sense of the word) must be strongly supported by many different lines of evidence . So biological evolution is a theory: It is a well-supported, widely accepted, and powerful explanation for the diversity of life on Earth. But it is not “just” a theory.

Words with both technical and everyday meanings often cause confusion. Even scientists sometimes use the word theory when they really mean hypothesis or even just a hunch. Many technical fields have similar vocabulary problems — for example, both the terms work in physics and ego in psychology have specific meanings in their technical fields that differ from their common uses. However, context and a little background knowledge are usually sufficient to figure out which meaning is intended.

Over-arching theories

Some theories, which we’ll call over-arching theories , are particularly important and reflect broad understandings of a particular part of the natural world . Evolutionary theory, atomic theory, gravity, quantum theory, and plate tectonics are examples of this sort of over-arching theory. These theories have been broadly supported by multiple lines of evidence and help frame our understanding of the world around us.

Such over-arching theories encompass many subordinate theories and hypotheses, and consequently, changes to those smaller theories and hypotheses reflect a refinement (not an overthrow) of the over-arching theory. For example, when punctuated equilibrium was proposed as a mode of evolutionary change and evidence was found supporting the idea in some situations, it represented an elaborated reinforcement of evolutionary theory, not a refutation of it. Over-arching theories are so important because they help scientists choose their methods of study and mode of reasoning, connect important phenomena in new ways, and open new areas of study. For example, evolutionary theory highlighted an entirely new set of questions for exploration: How did this characteristic evolve? How are these species related to one another? How has life changed over time?

A MODEL EXPLANATION

Hypotheses and theories can be complex. For example, a particular hypothesis about meteorological interactions or nuclear reactions might be so complex that it is best described in the form of a computer program or a long mathematical equation. In such cases, the hypothesis or theory may be called a model .

Take a sidetrip

To see an example of how models of the atmosphere can shape policy, explore  Ozone depletion: Uncovering the hidden hazard of hairspray .

• Teaching resources
• You can help students understand the differences between observation and inference (e.g., between observations and the hypothesis supported by them) by regularly asking students to analyze lecture material, text, or video. Students should try to figure out which aspects of the content were directly observed and which aspects were generated by scientists trying to figure out what their observations meant.
• Forming hypotheses — scientific explanations — can be difficult for students. It is often easier for students to generate an expectation (what they think will happen or what they expect to observe) based on prior experience than to formulate a potential explanation for that phenomena. You can help students go beyond expectations to generate real, explanatory hypotheses by providing sentence stems for them to fill in: “I expect to observe A because B.” Once students have filled in this sentence you can explain that B is a hypothesis and A is the expectation generated by that hypothesis.

Benefits of science

Even theories change

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What is a scientific theory?

A scientific theory is based on careful examination of facts.

• The process
• Good theory characteristics

The difference between theories, facts and laws

Additional resources, bibliography.

A scientific theory is a structured explanation to explain a group of facts or phenomena in the natural world that often incorporates a scientific hypothesis and scientific laws . The scientific definition of a theory contrasts with the definition most people use in casual language.

"The way that scientists use the word 'theory' is a little different than how it is commonly used in the lay public," said Jaime Tanner, a professor of biology at Emerson College in Boston. "Most people use the word 'theory' to mean an idea or hunch that someone has, but in science the word 'theory' refers to the way that we interpret facts."

Related: 5 sci-fi concepts that are possible (in theory)

The process of becoming a scientific theory

Every scientific theory relies on the scientific method . A scientist may make an observation and devise a hypothesis to explain that observation, then design an experiment to test that hypothesis. If the hypothesis is shown to be incorrect, the scientist will develop a new hypothesis and begin the process again. If the hypothesis is supported by the results of the experiment, it will go on to be tested again. If the hypothesis isn't disproven or surpassed by a better explanation, the scientist may incorporate it into a larger theory that helps to explain the observed phenomenon and relates it to other phenomena, according to the Field Museum . 

A scientific theory is not the end result of the scientific method; theories can be proven or rejected, just like hypotheses . And theories are continually improved or modified as more information is gathered, so that the accuracy of the prediction becomes greater over time.

Theories are foundations for furthering scientific knowledge and for putting the information gathered to practical use. Scientists use theories to develop inventions or find a cure for a disease.

Furthermore, a scientific theory is the framework for observations and facts, Tanner said. Theories may change, or the way that they are interpreted may change, but the facts themselves don't change. Tanner likens theories to a basket in which scientists keep facts and observations that they find. The shape of that basket may change as the scientists learn more and include more facts. "For example, we have ample evidence of traits in populations becoming more or less common over time (evolution), so evolution is a fact, but the overarching theories about evolution, the way that we think all of the facts go together might change as new observations of evolution are made," Tanner told Live Science.

Characteristics of a good theory

The University of California, Berkeley , defines a theory as "a broad, natural explanation for a wide range of phenomena. Theories are concise, coherent, systematic, predictive, and broadly applicable, often integrating and generalizing many hypotheses." 

According to Columbia University emeritus professor of philosophy Philip Kitcher, a good scientific theory has three characteristics. First, it has unity, which means it consists of a limited number of problem-solving strategies that can be applied to a wide range of scientific circumstances. Second, a good scientific theory leads to new questions and new areas of research. This means that a theory doesn't need to explain everything in order to be useful. And finally, a good theory is formed from a number of hypotheses that can be tested independently from the theory itself.

Any scientific theory must be based on a careful and rational examination of the facts. Facts and theories are two different things. In the scientific method, there is a clear distinction between facts, which can be observed and/or measured, and theories, which are scientists' explanations and interpretations of the facts. 

Some think that theories become laws, but theories and laws have separate and distinct roles in the scientific method. A law is a description of an observed phenomenon in the natural world that holds true every time it is tested. It doesn't explain why something is true; it just states that it is true. A theory, on the other hand, explains observations that are gathered during the scientific process. So, while law and theory are part of the scientific process, they are two different aspects, according to the National Center for Science Education . 

A good example of the difference between a theory and a law is the case of Gregor Mendel . In his research, Mendel discovered that two separate 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," said Peter Coppinger, an associate professor of biology and biomedical engineering at the Rose-Hulman Institute of Technology. "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."

• When does a theory become a fact? This article from Arizona State University says you're asking the wrong question! 
• Learn the difference between the casual and scientific uses of "theory" and "law" from the cartoony stars of the Amoeba Sisters on Youtube.
• Can a scientific theory be falsified? This article from Scientific American says no. 

Kenneth Angielczyk, "What Do We Mean by "Theory" in Science?" Field Museum, March 10, 2017. https://www.fieldmuseum.org/blog/what-do-we-mean-theory-science

University of California, Berkeley, "Science at multiple levels." https://undsci.berkeley.edu/article/0_0_0/howscienceworks_19  

Philip Kitcher, "Abusing Science: The Case Against Creationism," MIT Press, 1982. 

National Center for Science Education, "Definitions of Fact, Theory, and Law in Scientific Work," March 16, 2016 https://ncse.ngo/definitions-fact-theory-and-law-scientific-work  

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Hypotheses, Theories and Laws

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• Peter Pruzan 2  

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Although we tend to speak of hypotheses, theories, and laws as though they were clearly differentiated, this is not the case; the distinctions are not clear and are primarily a matter of belief regarding how well relationships that have been conjectured are supported by evidence. But scientific theories and laws are far more than just matters of faith; they are also the product of a methodology that has developed via continual confrontation with reality. It is not an exaggeration to state that scientific theories and laws are the most reliable forms of generalization produced by rational human activity. Faith and rationality can co-exist in science in a most vital symbiosis!

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We will see later on when the focus is on the test of hypotheses that this is the so-called “alternative hypothesis”, the hypothesis we are interested in and that motivates the study. The “null hypothesis” here would be that the behaviour of the dogs and monkeys in the ashram is not different from dogs and monkeys who do not frequent the ashram.

It should be noted here that Einstein did not just formulate his theory (which, in fact was a hypothesis that later became accepted as a theory) on the basis of intuition; his formulation of relativity theory was preceded by the Michelson-Morley experiment on the velocity of light.

The term “model”, which we have met several times until now, is often used interchangeably with “theory”, although it most often refers to a representation of a theory. In the case of many of the natural sciences, this representation is most often in the form of a mathematical model, i.e. a system of equations representing the interrelationships between variables. The term “model” often refers as well to physical or pictorial representations. Examples could be a physical structure representing a geocentric model of the universe or a visual model created on a computer (and generated by a mathematical model) of the double helix of DNA with its two polynucleotide strands woven around each other and running in opposite directions.

Having just considered an example involving fossils, I am tempted to mention that what is considered by many to be one of, if not the most successful and fundamental theories in the biosciences, that of evolution, is regarded by a number of leading scientists as a research programme rather than a theory. Aside from some of the programme’s underlying partial theories (dealing with such matters as macro- and microevolution and the molecular and genetic basis for the form and function of organisms), it has not been formalized, and there exist significant controversies as to its content and form (e.g. as to whether the development of new organisms has been more or less continuous or whether revolutionary changes such as the sudden development of completely different organisms have occurred)—as well as to whether the programme’s representations of organisms’ abilities to adapt, compete, cooperate and survive in fact represent a teleological perspective on the development of organisms. See e.g. (Depew and Weber 1996) and (Ward and Brownlee 2004).

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How to Explain the Difference Between Theory, Law, and a Fact

Last Updated: December 2, 2021 References

This article was co-authored by Bess Ruff, MA . Bess Ruff is a Geography PhD student at Florida State University. She received her MA in Environmental Science and Management from the University of California, Santa Barbara in 2016. She has conducted survey work for marine spatial planning projects in the Caribbean and provided research support as a graduate fellow for the Sustainable Fisheries Group. This article has been viewed 154,601 times.

Within scientific communities, “theory,” “law,” and “fact” are technical terms which have distinct and complex meanings. Many people who do not have a scientific background—including students in introductory science classes in high school and colleges—do not have a firm understanding of the differences between these 3 terms. Many adults are also unaware of the distinctions between these 3 terms, and can benefit from a polite, conversational explanation. This article will help you understand and explain the differences between proper scientific uses for each of the three terms.

Explaining the Difference between Scientific Theory and Law

• Laws have never been refuted (hence their relatively small number) and are not explanations; they are descriptions and are often stated through relatively simple mathematical equations.
• Scientific laws, despite their formality, can change or have exceptions as scientific understandings of phenomena evolve. [2] X Research source

• As an example, the Law of Gravity has been known in the scientific community since the late 17th century. The law describes the natural phenomenon of gravity, but does not provide an explanation as to how and why gravity functions.

• A theory builds from initial hypotheses (educated guesses) and can be revised in accordance with the development of a scientific understanding of a phenomena’s cause.
• A theory is confirmed by all available evidence such that it can be used to predict new, as yet unobserved phenomena.

• For example, the scientific Theory of Natural Selection corresponds with the Law of Evolution. [5] X Research source While the law states an observed natural phenomena (life forms develop new characteristics based on external circumstances), the theory describes how and why this happens.

Explaining the Difference between Scientific Law and Fact

• While facts can be scientifically refuted or may not be consistent across time and place, they are held as true until they have been proven wrong.

• When explaining a scientific fact, start with a point of general observation.
• For example, begin your explanation by saying something like, “it is always bright outside at noon.” This is a fact in that it describes a state of nature—however, this statement may not be true in Antarctica or Greenland, where darkness lasts throughout the day in certain seasons.
• Explain how this will lead to a revision of the scientific fact: “within certain degrees of latitude, it is always bright outside at noon.”

• Facts are less formal than laws, and are not seen as an “official” definition of a phenomenon that occurs or of the reason that something happens.
• Facts are more localized and generalize less than laws. Explain that, if the Law of Evolution describes the way that species throughout the world evolve over time, a scientific fact related to evolution (and natural selection) could be: “giraffes with long necks can reach more leaves than giraffes with short necks.”

• For example, scientific theories do not develop into scientific laws. To explain the difference, focus on this distinction: laws describe phenomena, theories explain phenomena, and facts describe observations.

Explaining Scientific Theories, Laws, and Facts in the Classroom

• A theory is worth very little if it doesn't correctly predict all known evidence.
• Theories are subject to changes as new evidence becomes available. (Most theories that you will discuss in a high school science class are well-confirmed and are unlikely to be revised in any significant sense.)

• The theory of relativity: that the laws of physics are the same for all observers
• The theory of evolution by natural selection: that the observed changes in species occur due to selection of well adapted specimens over less well adapted specimens.
• Big Bang theory: that the universe began as an infinitely small point that underwent expansion to form the universe as we know it today.

• For example, we know that the germ theory of illness is a fact because we can take bacteria from someone suffering from an illness, look at that bacteria under a microscope, and then inject that bacteria into another individual, who will then get that same illness.
• We know that the Earth is round because we can travel due west and eventually end up where we started from.

• Ancient peoples noticed peculiar points of light that “wandered” among their background. (We now know these to be the planets.)
• The planets moved through the sky because they, like the Earth, were orbiting around the sun, each at different speeds, different distances from the Sun.
• Nicolaus Copernicus is generally considered to be the first to propose this theory, and supported his theory with hard evidence, but ancient cultures stumbled upon this through speculation.
• We now consider this a fact because we have sent many craft to these planets and can predict their motions to a very high precision. Of course, our predictions come from the theory (and the laws underlying that theory).

• Newton's Law of heating and cooling: the change in temperature of two bodies in thermal contact is proportional to their difference in temperature.
• Newton's Laws of motion: statements about how large objects made of atoms behave when moving at low speeds relative to each other.
• The Laws of Thermodynamics: statements about entropy, temperature, and thermal equilibrium.
• Ohm's Law: the voltage across a purely resistive element is equal to the current through the element times its resistance.

• For example, one must infer that the derived laws actually predict the facts. Accumulating all of the previous forms of knowledge, a scientist makes a general statement to explain all the evidence.
• Other scientists reaffirm the facts and use the theory to make predictions and obtain new facts.

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• ↑ http://lifehacker.com/the-difference-between-a-fact-hypothesis-theory-and-1732904200
• ↑ http://www.livescience.com/21457-what-is-a-law-in-science-definition-of-scientific-law.html
• ↑ https://ncse.com/library-resource/definitions-fact-theory-law-scientific-work
• ↑ http://futurism.com/hypothesis-theory-or-law/
• ↑ https://pseudoastro.wordpress.com/2008/12/21/terminology-what-scientists-mean-by-fact-hypothesis-theory-and-law/

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0.1: 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.

Contributions & Attributions

Marisa Alviar-Agnew  ( Sacramento City College )

Henry Agnew (UC Davis)

• Our Mission

The Law of Theories: Understanding the Science Behind Them

The facts are dead, long live the facts.

It seems simple enough: The job of science is to observe, describe, and explain the natural world through hypothesis and experimentation. A scientist will say, "I think this explanation is the reason for this observation, and I propose this experiment to test it." But the statement doesn't begin to convey the job at hand. Theories, hypotheses, laws, the scientific method -- even facts themselves -- dangle from the natural sciences like so many tree branches. How do the various parts fit together?

Let's start with scientific theory. Cambridge University's Stephen Hawking describes the essence of a theory in his best-selling book A Brief History of Time : "A theory is a good theory if it satisfies two requirements: It must accurately describe a large class of observations on the basis of a model that contains only a few arbitrary elements, and it must make definite predictions about the results of future observations." Eugenie Scott, a physical anthropologist and executive director of the National Center for Science Education, puts it more succinctly: "A theory is a construct of facts and hypotheses that attempts to explain a natural phenomenon." So, theories are neither guesses nor hunches.

Examples of good theories include Charles Darwin's theory of evolution, which explains how populations of organisms change into diverse forms over time. Combined with Gregor Mendel's experiments with garden peas, Darwinian thought laid the foundation for modern genetics and heredity. Then came Albert Einstein's theory of relativity, which describes everything from the properties of solar systems to the infinitesimal whizzing of atoms. Like many scientific breakthroughs, Einstein's theory started as nothing more than mental noodling but eventually led to technologies such as transistors and lasers (and of course the atomic bomb).

To say "good theories" implies that there are not-so-good theories, which takes us back to hypotheses and testing. Some would-be theories may not be testable. That is why science rejects theories based on supernatural claims, such as intelligent design. Established theories can be challenged by new observations that don't fit the old mold, and can be disproved or modified if observations don't agree with predictions. A classic example is geocentricism, the theory that the Sun and its planets revolve around Earth, which was slowly -- and against much resistance -- replaced by Suncentered theories postulated by Copernicus, Galileo, and Kepler.

If scientific facts (certified, repeated observations) are part of theories (explanations of facts) and some theories don't hold water, this equation implies facts are suspect, too. Pounding the table while saying "That's a fact!" implies facts are equal to capital-T truth. But scientific facts can change over time. Helen Longino, a Stanford University professor who teaches the philosophy of science, warns, "Facts are more solid than theories, but they aren't beyond question." Newton's, Galileo's, and Kepler's gravitational facts were considered universal until Einstein came along and predicted that gravity could actually stretch or shrink distances. However, in physics especially, if a set of facts supporting a theory survives skeptical onslaughts from generations of scientists, the theory becomes universally accepted, as in the case of the laws of gravity, motion, or thermodynamics.

A good theory can be tested, and must be. The theory of evolution is considered sound because years of evidence from many disciplines show that Darwin's ideas stand the test of time. Facets of evolution can be probed by new hypotheses and experimentation, adding knowledge to the theory as a whole. Such questions have focused on evolution's pace, whether new species appear gradually or more rapidly. Another has centered on whether new traits arise randomly or are selected by natural causes, as Darwin originally thought. We don't yet talk about the law of evolution, because the theory is still being refined and polished.

Theories and facts don't exist in a vacuum -- they are parts of our social fabric, influenced by community behavior. "Science is not extracultural," says Katrina Karkazis, a medical anthropologist who also works at Stanford. She points out that the power of values and opinion can cause societies to accept a scientific theory unquestionably, rather than methodically testing its strength and weaknesses. "Without healthy skepticism, science becomes dogmatic," she adds. Ironically, our fascination with technology tends to give the impression that scientific results are more real than they seem (but that's another story entirely).

Theories and facts are themselves part of an evolutionary process, popping onto the scene only to be tested by the scientific method. The fittest survive for another day and another challenge. The weak (think of phrenology) are destined for the dustbin of history, curious artifacts discarded by a community of questioning skeptics.

Christopher Thomas Scott is a contributing writer for Edutopia.

A theory thesaurus.

Hypothesis: A suggested explanation or proposal for a phenomenon

Scientific facts: Objective observations that can be confirmed, verified, and repeated

Scientific theory: A framework of facts and hypotheses that attempt to explain a natural phenomenon

Scientific method: Techniques for investigating phenomena, based on gathering measurable, observable evidence

Dogma: Unquestioned beliefs and facts; thought as anathemic to science

Laws: Agreed-on collections of facts, such as Newton's three laws of motion

Thwarted Theories

Lamarckism: A predecessor to evolution that claimed that an organism can acquire characteristics during its lifetime and pass them on to offspring

Phlogiston: An obsolete theory claiming that combustion causes metal to rust

Aether: An invisible space-filing substance thought to be responsible for electric, magnetic, and gravitational properties; the theory was superseded by modern physics

Geocentrism: A theory that the Sun and its planets revolve around the Earth, replaced by heliocentrism, a Sun-centered theory

Untestable (Pseudoscience)

Astrology: The claim that the orientation of heavenly bodies can predict personality and human affairs

Intelligent design: The belief that aspects of living things are best explained by intelligent or supernatural causes, not natural selection

Phrenology: A pseudoscience that asserts that the shape of the skull can explain personality traits such as criminality

The flat Earth: A medieval belief, based on aesthetic grounds, that Earth is flat

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.

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Theory and Fact

One source of confusion about the status of the science or theory of evolution stems from the difference between the "everyday" meaning of the word "theory" and the scientific meaning the word.

Below we list some common misconceptions about the term "theory" and describe a classroom activity that can help students rethink their understanding of this term.

Misconception 1 "Evolution is 'just a theory'".

Misconception 2 "Theories become facts when they are well supported and/or proven."

There are three important misconceptions propagated in the above statements. The first statement implies that a theory should be interpreted as just a guess or a hunch, whereas in science, the term theory is used very differently. The second statement implies that theories become facts, in some sort of linear progression. In science, theories never become facts. Rather, theories explain facts. The third misconception is that scientific research provides proof in the sense of attaining the absolute truth. Scientific knowledge is always tentative and subject to revision should new evidence come to light.

Classroom Activity

“Fact-Hypothesis-Theory Word Jumble”

• Provide students with some examples of a theory, fact, hypothesis, and law.
• Discuss each example with students, focusing on whether the statement is based on evidence and under what conditions the statement is true.
• Ask students to organize these statements in some type of relative order, from that which they most readily accept to that which they consider most tentative.

Fact: In science, an observation that has been repeatedly confirmed and for all practical purposes is accepted as “true”. Truth in science, however, is never final and what is accepted as a fact today may be modified or even discarded tomorrow.

Hypothesis: A tentative statement about the natural world leading to deductions that can be tested. If the deductions are verified, the hypothesis is provisionally corroborated. If the deductions are incorrect, the original hypothesis is proved false and must be abandoned or modified. Hypotheses can be used to build more complex inferences and explanations.

Law: A descriptive generalization about how some aspect of the natural world behaves under stated circumstances.

Theory: In science, a well-substantiated explanation of some aspect of the natural world that can incorporate facts, laws, inferences, and tested hypotheses.

• Ask students to identify each of the original statements as a Fact, Hypothesis, Law, or Theory and to revisit the arrange of statements, from that which they would most readily accept to that which they consider most tentative, and make any changes deemed necessary.
• Did the order change? If so, how and why?

Recommended Resources:

Understanding Evolution and UC Museum of Paleontology

From the PBS Evolution series "Evolving Ideas: Isn’t Evolution Just a Theory?"

"Theory in Theory and Practice" (pdf) by NCSE's Glenn Branch and Louise Mead Evol Edu Outreach 1:287-289, 2008

"Evolution as Fact, Theory, and Path" by Ryan Gregory, Evo Edu Outreach 1:46-52, 2008.

"The Role of Theory in Advancing 21st Century Biology ", a special brief report from the National Academies of Science, 2007 (pdf)

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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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Conceptual Biology

Chapter 1: about science.

• 1.1 What is Life?
• 1.2 The Scientific Method
• 1.3 Science and Technology
• 1.4 Facts, Laws, and Theories
• 1.5 Working with Numbers
• 1.6 LE: What Is Biogeology?
• 1S Chapter Summary
• 1Q Chapter Questions

This video is a careful look at the definitions for scientific fact, hypothesis, law, and theory. Duration: 4:04.

How to distinguish between skepticism and denialism. Duration: 5:31.

Watch these additional videos to complete this tutorial.

Table of Videos

• 2.1 Atoms and Molecules
• 2.2 Chemical Compounds
• 2.3 Mixtures
• 2.4 Chemical Reactions
• 2.5 Types of Reactions
• 2.6 Organic Molecules
• 2.7 Macromolecules for Life
• 2.8 LE: Isotopic Dating
• 2S Chapter Summary
• 2Q Chapter Questions
• 3.1 What Is a Cell?
• 3.2 Cell Theory
• 3.3 Looking at Cells
• 3.4 The Eukaryotic Cell
• 3.5 The Cell Membrane
• 3.6 Cell Organelles
• 3.7 LE: Geologic Time
• 3S Chapter Summary
• 3Q Chapter Questions
• 4.1 Cellular Transport
• 4.2 Cell Communication
• 4.3 ATP and Chemical Reactions
• 4.4 Enzymes
• 4.5 Photosynthesis
• 4.6 Cellular Respiration
• 4.7 Glycolysis (Honors)
• 4.8 LE: Plate Tectonics
• 4S Chapter Summary
• 4Q Chapter Questions
• 5.1 What is a Gene?
• 5.2 Chromosomes
• 5.3 The Structure of DNA
• 5.4 How DNA Is Copied
• 5.5 How Proteins Are Built
• 5.6 Genetic Mutations
• 5.7 LE: History of Earth's Atmosphere
• 5S Chapter Summary
• 5Q Chapter Questions
• 6.1 How Cells Reproduce
• 6.2 Cell Division and Genetic Diversity
• 6.3 Traits and Inheritance
• 6.4 First Law of Inheritance
• 6.5 Second Law of Inheritance
• 6.6 Beyond Mendel
• 6.7 LE: History within Earth's Ice Caps
• 6S Chapter Summary
• 6Q Chapter Questions
• 7.1 The Human Genome
• 7.2 Genetic Testing
• 7.4 DNA Technologies
• 7.5 Genetically Engineered Mosquitoes
• 7.6 Genome Editing with CRISPR-Cas9
• 7.7 Concerns About DNA Technology
• 7.8 LE: Engineering Planetary Solutions
• 7S Chapter Summary
• 7Q Chapter Questions
• 8.1 The Origin of Life
• 8.2 Is There Life on Mars or Venus?
• 8.3 Charles Darwin
• 8.4 Natural Selection
• 8.5 Examples of Natural Selection
• 8.6 Adaptation
• 8.7 LE: Earth's Geologic History
• 8S Chapter Summary
• 8Q Chapter Questions
• 9.1 Mechanisms of Evolution
• 9.2 How New Species Form
• 9.3 Natural Selection in Action
• 9.4 Fossils
• 9.5 Body Structures and Genetics
• 9.6 Biogeography
• 9.7 The Evolution of Humans
• 9.8 LE: Plate Tectonics
• 9S Chapter Summary
• 9Q Chapter Questions
• 10.1 Classifying Life
• 10.2 Evolutionary Trees
• 10.3 Three Domains of Life
• 10.4 Bacteria
• 10.5 Archaea
• 10.6 Protists
• 10.7 Plants
• 10.8 LE: Life's Impact On Earth
• 10S Chapter Summary
• 10Q Chapter Questions
• 11.2 Animals Part 1
• 11.3 Animals Part 2
• 11.4 Animals Part 3
• 11.5 Viruses and Prions
• 11.6 Life Is Interconnected
• 11.7 LE: The Impact of Natural Hazards
• 11S Chapter Summary
• 11Q Chapter Questions
• 12.1 Organization of the Human Body
• 12.2 Homeostasis
• 12.3 The Brain
• 12.4 The Nervous System
• 12.5 How Neurons Fire
• 12.6 How Neurons Communicate
• 12.7 The Senses
• 12.8 LE: Cultures and the Environment
• 12S Chapter Summary
• 12Q Chapter Questions
• 13.1 Hormones
• 13.2 Reproduction
• 13.3 Development
• 13.4 The Skeleton
• 13.5 Muscles
• 13.6 LE: Modeling Climate
• 13S Chapter Summary
• 13Q Chapter Questions
• 14.1 The Circulatory System
• 14.2 The Path of Blood Flow
• 14.4 Respiration
• 14.5 Digestion
• 14.6 LE: Ocean Acidification
• 14S Chapter Summary
• 14Q Chapter Questions
• 15.1 Nutrition, Exercise, and Health
• 15.2 The Excretory System
• 15.3 The Innate Immune System
• 15.4 The Acquired Immune System
• 15.5 LE: Regenerative Agriculture
• 15S Chapter Summary
• 15Q Chapter Questions
• 16.1 Organisms and Their Environments
• 16.2 Population Growth
• 16.3 Life History
• 16.4 Human Population Growth
• 16.5 LE: Availability of Natural Resources
• 16S Chapter Summary
• 16Q Chapter Questions
• 17.1 Food Webs
• 17.2 Competition
• 17.3 Symbiosis
• 17.4 Invasive Species
• 17.5 LE: Sustainability and Biodiversity
• 17S Chapter Summary
• 17Q Chapter Questions
• 18.1 Terrestrial Biomes
• 18.2 Aquatic Biomes
• 18.3 Biogeochemical Cycles
• 18.4 Energy Flow in Ecosystems
• 18.5 Ecological Succession
• 18.6 LE: Global Climate Change
• 18S Chapter Summary
• 18Q Chapter Questions