READ: Hypotheses, Theories, and Laws
READ: Hypotheses, Theories, and Laws
Although this cartoon pokes fun at scientific hypotheses, the concept of hypothesis is one of the most important in science. Scientific investigations discover evidence that helps science advance, and the purpose of scientific investigations generally is to test hypotheses. Finding evidence to support or disprove hypotheses is how science advances.
What is a Scientific Hypothesis?
The word hypothesis can be defined as an "educated guess." For example, it might be an educated guess about why a natural event occurs. But not all hypotheses—even those about the natural world—are scientific hypotheses. What makes a statement a scientific hypothesis rather than just an educated guess? A scientific hypothesis must meet two criteria:
- A scientific hypothesis must be testable.
- A scientific hypothesis must be falsifiable.
For a hypothesis to be testable means that it is possible to make observations that agree or disagree with it. If a hypothesis cannot be tested by making observations, it is not scientific. Consider this statement:
"There are invisible creatures all around us that we can never observe in any way."
This statement may or may not be true, but it is not a scientific hypothesis. That’s because it can’t be tested. Given the nature of the hypothesis, there are no observations a scientist could make to test whether or not it is false.
A hypothesis may be testable, but even that isn’t enough for it to be a scientific hypothesis. In addition, it must be possible to show that the hypothesis is false if it really is false. Consider this statement:
“There are other planets in the universe where life exists.”
This statement is testable. If it is true, it is at least theoretically possible to find evidence showing that it’s true. For example, a spacecraft could be sent from Earth to explore the universe and report back if it discovers an inhabited planet. If such a planet were found, it would prove the statement is true. However, the statement isn’t a scientific hypothesis. Why? If it is false, it’s not possible to show that it’s false. The spacecraft may never find an inhabited planet, but that doesn't necessarily mean there isn’t one. Given the vastness of the universe, we would never be able to check every planet for life!
Scientific Theories
This photo shows a girl and her parents. They are having a discussion. As you can see, the girl is rolling her eyes. What do you think her parents may have said that caused this reaction? Could it be they have just grounded her for some reason? That’s certainly one possibility, but without any other information to go on, it’s “just a theory.” In other words, it’s just a hunch or a guess. Theories are very important in science, but in science a theory is never a hunch or a guess. It is much more than that.
Not "Just a Theory"
The term theory is used differently in science than it is used in everyday language. A scientific theory is a broad explanation that is widely accepted because it is supported by a great deal of evidence. Because it is so well supported, a scientific theory has a very good chance of being a correct explanation for events in nature. Because it is a broad explanation, it can explain many observations and pieces of evidence. In other words, it can help connect and make sense of many phenomena in the natural world. Several examples of theories are listed below:
- Einstein’s theory of gravity
- Kinetic theory of matter - matter is made of small particles that are in constant motion.
- Cell Theory - the cell is the smallest unit of structure and function of all living organisms, all living organisms are made up of at least one cell, and living cells always come from other living cells.
- Germ Theory - microorganisms are the cause of many diseases.
Scientific Evidence and Theories
One goal of a scientist is to find answers to scientific questions. To do this, scientists first develop a hypothesis, which is a proposed explanation that tries to explain an observation. To collect evidence to support (or disprove) their hypothesis, scientists must do experiments. Evidence is:
- A direct, physical observation of something or a process over time.
- Usually something measurable or "quantifiable."
- The data resulting from an experiment.
For example, an apple falling to the ground is evidence in support of gravity. A bear skeleton in the woods would be evidence of the presence of bears.
Scientific Laws
Did you ever drive a bumper car like the one pictured here? As you drive around the track, other drivers try to bump into your car and push it out of the way. When another car bumps into yours, both cars may bounce back from the collision. The harder the two cars collide, the farther back they bounce.
It may seem like common sense that bumper cars change their motion when they collide. That’s because all objects behave this way - it’s the law! A scientific law, called Newton’s third law of motion, states that for every action there is an equal and opposite reaction. Thus, when one bumper car acts by ramming another, one or both cars react by pushing apart.
Laws vs. Theories
Newton’s third law of motion is just one of many scientific laws. A scientific law is a statement describing what always happens under certain conditions. Scientific laws state what always happen. This can be very useful. It can let you let you predict what will happen under certain circumstances. For example, Newton’s third law tells you that the harder you hit a softball with a bat, the faster and farther the ball will travel away from the bat. However, scientific laws have a basic limitation. They don’t explain why things happen. “Why” questions are answered by scientific theories, not scientific laws.
Q: You know that the sun always sets in the west. This could be expressed as a scientific law. Think of something else that always happens in nature. How could you express it as a scientific law?
A: Something else that always happens in nature is water flowing downhill rather than uphill. This could be expressed as the law, “When water flows over a hill, it always flows from a higher to a lower elevation.”
Summary
- In science, a hypothesis is an educated guess that can be tested with observations and falsified if it really is false.
- A scientific theory is a broad explanation that is widely accepted because it is supported by a great deal of evidence.
- Evidence is a direct, physical observation of something or a process.
- A scientific law is a statement describing what always happens under certain conditions.
- A scientific law states what always happens but not why it happens. Scientific theories answer “why” questions.
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