What is a law in science?
The one thing a scientific law doesn't explain is why the phenomenon exists or what causes it.
In general, a scientific law is the description of an observed phenomenon. It doesn't explain why the phenomenon exists or what causes it. The explanation for a phenomenon is called a scientific theory. It is a misconception that theories turn into laws with enough research.
"In science, laws are a starting place," said Peter Coppinger, an associate professor of biology and biomedical engineering at the Rose-Hulman Institute of Technology in India. "From there, scientists can then ask the questions, 'Why and how?'"
Difference between a scientific theory and a scientific law
Many people think that if scientists find evidence that supports a hypothesis, the hypothesis is upgraded to a theory, and if the theory is found to be correct, it is upgraded to a law. That is not how it works, though. Facts, theories and laws — as well as hypotheses — are separate elements of the scientific method. Though they may evolve, they aren't upgraded to something else.
"Hypotheses, theories and laws are rather like apples, oranges and kumquats: One cannot grow into another, no matter how much fertilizer and water are offered," according to the University of California, Berkeley. A hypothesis is a potential explanation of a narrow phenomenon; a scientific theory is an in-depth explanation that applies to a wide range of phenomena. A law is a statement about an observed phenomenon or a unifying concept, according to Kennesaw State University.
"There are four major concepts in science: facts, hypotheses, laws and theories," Coppinger told Live Science.
Though scientific laws and theories are supported by a large body of empirical evidence that is accepted by the majority of scientists within that area of scientific study, and help to unify that body of data, they are not the same thing.
"Laws are descriptions — often mathematical descriptions — of natural phenomena for example, Newton's Law of Gravity or Mendel's Law of Independent Assortment. These laws simply describe the observation. Not how or why they work," Coppinger said.
Coppinger pointed out that the law of gravity was discovered by Isaac Newton in the 17th century. This law mathematically describes how two different bodies in the universe interact with each other. However, Newton's law doesn't explain what gravity is or how it works. It wasn't until three centuries later, when Albert Einstein developed the theory of Relativity, that scientists began to understand what gravity is and how it works.
"Newton's law is useful to scientists in that astrophysicists can use this centuries-old law to land robots on Mars. But it doesn't explain how gravity works, or what it is. Similarly, Mendel's Law of Independent Assortment describes how different traits are passed from parent to offspring, not how or why it happens," Coppinger said. Gregor Mendel discovered that two different genetic traits would appear independently of each other in different offspring. "Yet, Mendel knew nothing of DNA or chromosomes. It wasn't until a century later that scientists discovered DNA and chromosomes — the biochemical explanation of Mendel's laws. It was only then that scientists, such as T.H. Morgan, working with fruit flies, explained the Law of Independent Assortment using the theory of chromosomal inheritance. Still today, this is the universally accepted explanation (theory) for Mendel's Law," Coppinger said.
The difference between scientific laws and scientific facts is a bit harder to define, though the definition is important. Facts are simple, one-off observations that have been shown to be true. Laws are generalized observations about a relationship between two or more things in the natural world based on a variety of facts and empirical evidence, often framed as a mathematical statement, according to NASA.
For example, "Apples fall down from this apple tree" is considered a fact because it is a simple statement that can be proven. "The strength of gravity between any two objects (like an apple and the Earth) depends on the masses of the objects and the distance between them" is a law because it describes the behavior of two objects in a certain circumstance. If the circumstance changes, then the implications of the law would change. For example, if the apple and the Earth shrank to a subatomic size, they would behave differently.
Scientific laws and mathematics
Many scientific laws can be boiled down to a mathematical equation. For example, Newton's Law of Universal Gravitation states:
Fg = G (m1 ∙ m2) / d2
Fg is the force of gravity; G is the universal gravitational constant, which can be measured; m1 and m2 are the masses of the two objects, and d is the distance between them, according to The Ohio State University.
Scientific laws are also often governed by the mathematics of probability. "With large numbers, probability always works. The house always wins," said Sylvia Wassertheil-Smoller, a professor at Albert Einstein College of Medicine in New York. "We can calculate the probability of an event and we can determine how certain we are of our estimate, but there is always a trade-off between precision and certainty. This is known as the confidence interval. For example, we can be 95% certain that what we are trying to estimate lies within a certain range or we can be more certain, say 99% certain, that it lies within a wider range. Just like in life in general, we must accept that there is a trade-off."
Do laws change?
Just because an idea becomes a law doesn't mean that it can't be changed through scientific research in the future. The use of the word "law" by laymen and scientists differs. When most people talk about a law, they mean something that is absolute. A scientific law is much more flexible. It can have exceptions, be proven wrong or evolve over time, according to the University of California, Berkeley.
"A good scientist is one who always asks the question, 'How can I show myself wrong?'" Coppinger said. "In regards to the Law of Gravity or the Law of Independent Assortment, continual testing and observations have 'tweaked' these laws. Exceptions have been found. For example, Newton's Law of Gravity breaks down when looking at the quantum (subatomic) level. Mendel's Law of Independent Assortment breaks down when traits are "linked" on the same chromosome."
Examples of scientific laws
- The law of conservation of energy, which says that the total energy in an isolated system remains constant. In other words, energy cannot be created or destroyed, according to Britannica.
- The laws of thermodynamics, which deal with the relationships between heat and other forms of energy
- Newton's universal law of gravitation, which says that any two objects exert a gravitational force upon each other, according to the University of Winnipeg
- Hubble's law of cosmic expansion, which defines a relationship between a galaxy's distance and how fast it's moving away from us, according to astrophysicist Neta A. Bahcall
- The Archimedes Principle, which states that the buoyant force on an object submerged in a fluid is equal to the weight of the fluid displaced by that object.
- This resource from the New South Wales Education Standards Authority has an in-depth explanation of scientific theories and laws.
- Find out why a theory can’t evolve into a law in this article from Indiana Public Media.
- Watch a video about the difference between a scientific law and a scientific theory from TEDEd.
University of California, Berkeley, "Misconceptions about science." https://undsci.berkeley.edu/teaching/misconceptions.php
NASA IMAGE Education Center, "Teacher's Guide: Theories, Hypothesis, Laws, Facts & Beliefs." https://www.nasa.gov/pdf/371711main_SMII_Problem23.pdf
The Ohio State University, "Lecture 18: The Apple and the Moon: Newtonian Gravity." https://www.astronomy.ohio-state.edu/pogge.1/Ast161/Unit4/gravity.html
Encyclopedia Britannica, "Conservation of energy." November 16, 2021. https://www.britannica.com/science/conservation-of-energy
University of Winnipeg, "Newton's Law of Gravitation." 1997. https://theory.uwinnipeg.ca/physics/circ/node7.html
Neta A. Bahcall, "Hubble's Law and the expanding universe," Proceedings of the National Academy of Sciences, Volume 112, March 2015, https://doi.org/10.1073/pnas.1424299112
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