Lesson 2: The origin of the scientific method
In this lesson, you learn about the scientific method and what makes evidence from this method different from other types of evidence, and why you should regard it as the most trustworthy way of learning about the world.
The scientific method is old. One classic example of the essence of the scientific method and how it differs from other ways of learning about the world is Halley's work on comets.
Example: Halley's comet
Once every 76 years, the comet that is now known as "Halley's comet" passes close to the Earth and can be seen with the naked eye. Halley's comet has been observed since at least the year 240 BC, but it is only since 1704 that we know that this is the same comet that returns again and again. Since Aristotle, scholars believed that comets were disturbances in the Earth's atmosphere. In 1577 the Danish astronomer Tycho Brahe found that comets passed behind the moon, and could therefore not be part of the atmosphere.
In 1704, Edmund Halley used the methods published by Newton to calculate the orbits of comets that had been observed in the centuries before. He found that the comets that had been seen in 1531 and 1607 had the same orbit as a comet that had been seen in 1682. Based on this observation, Halley formulated a hypothesis:
"I observed that the comets from 1531, 1607, and 1682 have the same orbit. Based on this observation, I believe that this is in fact the same comet, that moves in ellipses around the sun and returns once every 76 years. I predict that this comet will be seen again in 1758."
Or, in his own words: "Hence I dare venture to foretell, that it will return again in the year 1758."
Halley died before he could see his prediction verified. The comet returned on schedule, in 1758 to the amazement of the scientific community, the public and the British The Gentlemen's magazine, which wrote: "By its appearance at this time, the truth of the Newtonian Theory of the Solar System is demonstrated to the conviction of the whole world, and the credit of the astronomers is fully established and raised far above all the wit and sneers of ignorant men."
Question: Would Halley's prediction have been as impressive if he had predicted: "I predict that this comet will be seen again some time between 1740 and 1770?" Would Halley's prediction have been as impressive if we saw comets almost every year?
The observation of the comet at the predicted time provided fairly strong evidence for his hypothesis, because it was very specific. Predicting the next observation of a comet and getting it right by pure luck would have been very unlikely.
The scientific method
The scientific method has five basic steps, plus one feedback step:
- Make an observation
- Ask a question
- Form a hypothesis, or testable explanation
- Make a prediction based on the hypothesis
- Test the prediction
- Iterate: use the results to make new hypotheses or predictions
What is an experiment?
An experiment is a procedure designed to test a hypothesis as part of the scientific method.
There are different types of experiments:
Uncontrolled experiments
An uncontrolled experiment involves making a prediction or forming a hypothesis and then gathering data by observing a system. The variables are not controlled in a natural experiment. A historical example is Edmund's Halley's hypothesis about the orbits of comets described above.
Controlled experiments
In a controlled experiment, you compare a treatment group with a control group, to test the effect of a treatment on an outcome. Ideally, the two groups are the same except for the change in treatment. A historical example is the Salk Polio vaccine trial, which treated 600,000 children with either the new vaccine or placebo. A/B tests and rollouts are controlled experiments.