1. Laws, Theories, and Hypotheses: Oh, My!1

2. Hypothesis
In society, the word “hypothesis” typically refers to a guess or prediction that is based on reasoning but not necessarily evidence.
In science, a hypothesis is a proposed and testable explanation or supposition based on limited evidence as a starting point for further investigation. 2

3. Hypothesis: Key Words Key Words:
further investigation/testable = the claim/statement can and will be tested
limited evidence = observations have been made, but very few 3

4. Hypothesis: Example 1
Continental drift hypothesis: Wegener noticed that coastlines seemed to fit like a puzzle and there were similar fossils on separate continents. He hypothesized that the continents had moved around in the past.
Image: Antonio Snider-Pellegrini, (CC0) 4

5. Hypothesis: Example 2
A student observes ants are avoiding a certain area of the lawn where there is clover they hypothesize that ants have an aversion to clover.
This is testable and based on limited evidence only.
Ant image: Stephen Ausmus, (CC0)
Clover image: Joe Papp, (CC BY-SA 3.0)

6. Theory
In society the word theory implies a random idea of how something works.
In science a theory is a well-substantiated explanation of some aspect of the natural world that is acquired through the scientific method and repeatedly tested and confirmed through observation and experimentation. 6

7. Theory: Key Words Key words:
well-substantiated=a large amount of evidence from many sources
explanation= tells how something works 7

8. Theory: Example Cell Theory 8
Image: Robert Hooke, (CC0) 8

9. Law In society a law is a rule that must be followed.
In science a law is a statement based on repeated experimental observations that describes some aspect of the universe. A scientific law always applies under the same conditions, and implies that there is a cause and effect relationship involving its parts. They are often expressed as mathematical formulas. 9

10. Law: Key Words Key Words:
repeated experimental observations= a large amount of evidence from many sources
describes =tells what happens
mathematical formulas= equation or expression that shows a relationship between variables (for example: Boyle's law: P1V1=P2V2 shows the direct relationship between pressure and volume of a gas.) 10

11. Law: Examples Law of Universal Gravitation
Law of Conservation of Energy
Image: Dna-Dennis, (CC BY 3.0) 11

12. Does a theory become a law?
No! Theories and laws are different aspects of science.
While they are similar:
based on tested hypotheses.
supported by a large body of empirical data.
widely accepted by the vast majority scientists within a discipline.
could be shown to be wrong or need to be adapted at some time if there is enough data to suggest so. 12

13. What is the difference?
A law describes what nature does under certain conditions, and will predict what will happen as long as those conditions are met.
A theory explains how nature works. 13

14. Important differences
Another difference is based on mathematics.
Laws are often times mathematically defined (once again, a description of how nature behaves)
Theories are often non-mathematical
This can help explain, in part, why physics and chemistry have lots of laws and biology has very few laws. The complexities of life are difficult to describe using mathematics. 14