Scientific Hypothesis, Theories and Laws
The principles and theories of science have been established through repeated experimentation and observation and have been refereed through peer review before general acceptance by the scientific community. Acceptance does not imply rigidity or constraint, or denote dogma. Instead, as new data become available, previous scientific explanations are revised and improved, or rejected and replaced. Science is a way of making sense of the world, with internally-consistent methods and principles that are well described. There is a progression from a hypothesis to a theory using testable, scientific laws. Only a few scientific facts are natural laws and many hypotheses are tested to generate a theory. Find out how scientific hypotheses, theories and laws describe the natural world.
What is a hypothesis?
A hypothesis is an idea or proposition that can be tested by observations or experiments, about the natural world. In order to be considered scientific, hypotheses are subject to scientific evaluation and must be falsifiable, which means that they are worded in such a way that they can be proven to be incorrect.
Example: When Gregor Mendel in 1865 studied the pattern of single trait inheritance of garden peas he formed a hypothesis on the manner of how these traits were inherited. The hypothesis he formed based on his observations included the following:
- In the organism there is a pair of factors that controls the appearance of a given characteristic.
- The organism inherits these factors from its parents, one from each.
- Each is transmitted from generation to generation as a discrete, unchanging unit.
- When the gametes are formed, the factors separate and are distributed as units to each gamete. (This statement is also known as Mendel's rule of segregation.)
- If an organism has two unlike factors for a characteristic, one may be expressed to the total exclusion of the other.
What is a scientific theory?
To scientists, a theory is a coherent explanation for a large number of facts and observations about the natural world.
A theory is:
- Internally consistent and compatible with the evidence
- Firmly grounded in and based upon evidence
- Tested against a wide range of phenomena
- Demonstrably effective in problem-solving
In popular use, a theory is often assumed to imply mere speculation, but in science, something is not called a theory until it has been confirmed over many independent experiments. Theories are more certain than hypotheses, but less certain than laws. The procedures and processes for testing a theory are well-defined within each scientific discipline.
Example: Between 1856 and 1863 Mendel cultivated and tested some 28,000 pea plants which brought forth two theories of how character traits are inherited. Ironically, when Mendel's paper was published on 1866, it had little impact. It wasn't until the early 20th century that the enormity of his ideas was realized.
What is a scientific law?
A scientific law is a description of a natural phenomenon or principle that invariably holds true under specific conditions and will occur under certain circumstances.
Example: In the early 20th century, after repeated tests and rejection of all competing theories Mendel's Laws of Heredity were accepted by the general scientific community.
- The law of segregation, which states that the alleles governing a trait are separated during the creation of gametes (meiosis).
- The law of independent assortment, which states that the genes controlling different traits are distributed separately from each other during meiosis.
Lots of historical and scientific information about Gregor Mendel and his work can be found at the Mendel Museum.
Example 2: In the late 17th Century, Nicholas Steno established some natural laws relating to geology.
- The law of original horizontality - this states that when sediments are deposited in water, they will sink through it and deposit as horizontal layers as the result of gravity (unless acted on by other forces).
- The law of stratigraphic succession - this states that in a given sequence of sediments, the oldest will be at the bottom and the youngest at the top. It does not provide ages for those sediments, simply the sequence of their deposition.
For more on Nicholas Steno, try this book - a most interesting read:
Cutler, Alan (2003). The seashell on the mountain top. Heinemann, London, p. 228
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A scientific hypothesis is a tentative, testable explanation for a phenomenon in the natural world. It's the initial building block in the scientific method. Many describe it as an "educated guess" based on prior knowledge and observation. While this is true, a hypothesis is more informed than a guess. While an "educated guess" suggests a random prediction based on a person's expertise, developing a hypothesis requires active observation and background research.
Hypothesis basics
The basic idea of a hypothesis is that there is no predetermined outcome. For a solution to be termed a scientific hypothesis, it has to be an idea that can be supported or refuted through carefully crafted experimentation or observation. This concept, called falsifiability and testability, was advanced in the mid-20th century by Austrian-British philosopher Karl Popper in his famous book "The Logic of Scientific Discovery" (Routledge, 1959).
A key function of a hypothesis is to derive predictions about the results of future experiments and then perform those experiments to see whether they support the predictions.
A hypothesis is usually written in the form of an if-then statement, which gives a possibility (if) and explains what may happen because of the possibility (then). The statement could also include "may," according to California State University, Bakersfield (opens in new tab).
Here are some examples of hypothesis statements:
- If garlic repels fleas, then a dog that is given garlic every day will not get fleas.
- If sugar causes cavities, then people who eat a lot of candy may be more prone to cavities.
- If ultraviolet light can damage the eyes, then maybe this light can cause blindness.
What makes a hypothesis testable?
A useful hypothesis should be testable and falsifiable. That means that it should be possible to prove it wrong. A theory that can't be proved wrong is nonscientific, according to Karl Popper's 1963 book "Conjectures and Refutations (opens in new tab)."
An example of an untestable statement is, "Dogs are better than cats." That's because the definition of "better" is vague and subjective. However, an untestable statement can be reworded to make it testable. For example, the previous statement could be changed to this: "Owning a dog is associated with higher levels of physical fitness than owning a cat." With this statement, the researcher can take measures of physical fitness from dog and cat owners and compare the two.
Types of scientific hypotheses
The null hypothesis might be: There will be no difference in spin speed between a turbine with 6 versus 8 blades. (Image credit: SDI Productions/Getty Images)
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In an experiment, researchers generally state their hypotheses in two ways. The null hypothesis predicts that there will be no relationship between the variables tested, or no difference between the experimental groups. The alternative hypothesis predicts the opposite: that there will be a difference between the experimental groups. This is usually the hypothesis scientists are most interested in, according to the University of Miami (opens in new tab).
For example, a null hypothesis might state, "There will be no difference in the rate of muscle growth between people who take a protein supplement and people who don't." The alternative hypothesis would state, "There will be a difference in the rate of muscle growth between people who take a protein supplement and people who don't."
If the results of the experiment show a relationship between the variables, then the null hypothesis has been rejected in favor of the alternative hypothesis, according to the book "Research Methods in Psychology (opens in new tab)" (BCcampus, 2015).
There are other ways to describe an alternative hypothesis. The alternative hypothesis above does not specify a direction of the effect, only that there will be a difference between the two groups. That type of prediction is called a two-tailed hypothesis. If a hypothesis specifies a certain direction — for example, that people who take a protein supplement will gain more muscle than people who don't — it is called a one-tailed hypothesis, according to William M. K. Trochim (opens in new tab), a professor of Policy Analysis and Management at Cornell University.
Sometimes, errors take place during an experiment. These errors can happen in one of two ways. A type I error is when the null hypothesis is rejected when it is true. This is also known as a false positive. A type II error occurs when the null hypothesis is not rejected when it is false. This is also known as a false negative, according to the University of California, Berkeley (opens in new tab).
A hypothesis can be rejected or modified, but it can never be proved correct 100% of the time. For example, a scientist can form a hypothesis stating that if a certain type of tomato has a gene for red pigment, that type of tomato will be red. During research, the scientist then finds that each tomato of this type is red. Though the findings confirm the hypothesis, there may be a tomato of that type somewhere in the world that isn't red. Thus, the hypothesis is true, but it may not be true 100% of the time.
Scientific theory vs. scientific hypothesis
The best hypotheses are simple. They deal with a relatively narrow set of phenomena. But theories are broader; they generally combine multiple hypotheses into a general explanation for a wide range of phenomena, according to the University of California, Berkeley (opens in new tab). For example, a hypothesis might state, "If animals adapt to suit their environments, then birds that live on islands with lots of seeds to eat will have differently shaped beaks than birds that live on islands with lots of insects to eat." After testing many hypotheses like these, Charles Darwin formulated an overarching theory: the theory of evolution by natural selection.
"Theories are the ways that we make sense of what we observe in the natural world," Tanner said. "Theories are structures of ideas that explain and interpret facts."
Additional resources
- Read more about writing a hypothesis, from the American Medical Writers Association. (opens in new tab)
- Find out why a hypothesis isn't always necessary in science, from The American Biology Teacher. (opens in new tab)
- Learn about null and alternative hypotheses, from Prof. Essa on YouTube (opens in new tab).
Bibliography
Encyclopedia Britannica. Scientific Hypothesis. Jan. 13, 2022. //www.britannica.com/science/scientific-hypothesis
Karl Popper, "The Logic of Scientific Discovery," Routledge, 1959.
California State University, Bakersfield, "Formatting a testable hypothesis." //www.csub.edu/~ddodenhoff/Bio100/Bio100sp04/formattingahypothesis.htm
Karl Popper, "Conjectures and Refutations," Routledge, 1963.
Price, P., Jhangiani, R., & Chiang, I., "Research Methods of Psychology — 2nd Canadian Edition," BCcampus, 2015.
University of Miami, "The Scientific Method" //www.bio.miami.edu/dana/161/evolution/161app1_scimethod.pdf
William M.K. Trochim, "Research Methods Knowledge Base," //conjointly.com/kb/hypotheses-explained/
University of California, Berkeley, "Multiple Hypothesis Testing and False Discovery Rate" //www.stat.berkeley.edu/~hhuang/STAT141/Lecture-FDR.pdf
University of California, Berkeley, "Science at multiple levels" //undsci.berkeley.edu/article/0_0_0/howscienceworks_19
Alina Bradford is a contributing writer for Live Science. Over the past 16 years, Alina has covered everything from Ebola to androids while writing health, science and tech articles for major publications. She has multiple health, safety and lifesaving certifications from Oklahoma State University. Alina's goal in life is to try as many experiences as possible. To date, she has been a volunteer firefighter, a dispatcher, substitute teacher, artist, janitor, children's book author, pizza maker, event coordinator and much more.