- Observation:
- A good scientist is observant and notices thing in the world around him/herself. (S)he sees, hears, or in some other way notices what’s going on in the world and becomes curious about what’s happening. This can and does include reading and studying what others have done in the past because scientific knowledge is cumulative. In physics, when Newton came up with his Theory of Motion, he based his hypothesis on the work of Copernicus, Kepler, and Galileo as well as his own, newer observations. Darwin not only observed and took notes during his voyage, but he also studied the practice of artificial selection and read the works of other naturalists to form his Theory of Evolution.
For centuries, people based their beliefs on their interpretations of what they saw going on in the world around them without testing their ideas to determine the validity of these theories — in other words, they didn’t use the scientific method to arrive at answers to their questions. Rather, their conclusions were based on untested observations.
Among these ideas, since at least the time of Aristotle (4th Century BC), people (including scientists) believed that simple living organisms could come into being by spontaneous generation. This was the idea that non-living objects can give rise to living organisms. It was common “knowledge” that simple organisms like worms, beetles, frogs, amd salamanders could come from dust, mud, etc., and food left out, quickly “swarmed” with life. For example:
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From this came a number of interesting recipes, such as: |
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With the development and refinement of the microscope in the 1600s, people began seeing all sorts of new life forms such as yeast and other fungi, bacteria, and various protists. No one knew from where these organisms came, but people figured out they were associated with things like spoiled broth. This seemed to add new evidence to the idea of spontaneous generation — it seemed perfectly logical that these minute organisms should arise spontaneously. When Jean Baptiste Lamarck proposed his theory of evolution, to reconcile his ideas with Aristotle’s Scala naturae, he proposed that as creatures strive for greater perfection, thus move up the “ladder,” new organisms arise by spontaneous generation to fill the vacated places on the lower rungs. Observations: It was known that soup that was exposed to the air spoiled — bacteria grew in it. Some people claimed that there was a “life force” present in the molecules of all inorganic matter, including air and the oxygen in it, that could cause spontaneous generation to occur, thus accounting for the presence of bacteria in spoiled soups. Even when briefly-boiled soup was poured into “clean” flasks with cork lids, microorganisms still grew there. Containers of soup that had been boiled for one hour, and then were sealed, remained sterile. Boiling for only a few minutes was not enough to sterilize the soup. |
- Question:
- The scientist then raises a question about what (s)he sees going on. The question raised must have a “simple,” concrete answer that can be obtained by performing an experiment. For example, “How many students came to school today?” could be answered by counting the students present on campus, but “Why did you come to school today?” couldn’t really be answered by doing an experiment.
- Question: Where do the flies at the butcher shop really come from? Does rotting meat turn into or produce the flies?
- Question: Is there indeed a “life force” present in air (or oxygen) that can cause bacteria to develop by spontaneous generation? Is there a means of allowing air to enter a container, thus any life force, if such does exist, but not the bacteria that are present in that air?
- Hypothesis:
- This is a tentative answer to the question: a testable explanation for what was observed. The scientist tries to explain what caused what was observed.
- Hypothesis: Rotten meat does not turn into flies. Only flies can make more flies.
- Hypothesis: There is no such life force in air, and a container of sterilized broth will remain sterile, even if exposed to the air, as long as bacteria cannot enter the flask.
- In a cause and effect relationship, what you observe is the effect, and hypotheses are possible causes. A generalization based on inductive reasoning is not a hypothesis. An hypothesis is not an observation, rather, a tentative explanation for the observation. For example, I might observe the effect that my throat is sore. Then I might form hypotheses as to the cause of that sore throat, including a bacterial infection, a viral infection, or screaming too much at a ball game.
- Hypotheses reflect past experience with similar questions (“educated propositions” about cause) and the work of others. Hypotheses are based on previous knowledge, facts, and general principles. Your answer to the question of what caused the observed effect will be based on your previous knowledge of what causes similar effects in similar situations. For example, I know that colds are contagious, I don’t know anyone with a cold, I was at the ball game yesterday, and I was doing a lot of yelling while I was there, so I think that caused my sore throat.
- Multiple hypotheses should be proposed whenever possible. One should think of alternative causes that could explain the observation (the correct one may not even be one that was thought of!) For example, maybe somebody sitting near me at the ball game had a sore throat and passed it on to me.
- Hypotheses should be testable by experimentation and deductive reasoning. For example, throat culture and other tests yielded no signs of a bacterial or viral infection, I have no fever or other signs/symptoms, and the doctor says my vocal cords are “swollen” in a way that would indicate overuse.
- Hypotheses can be proven wrong/incorrect, but can never be proven or confirmed with absolute certainty. It is impossible to test all given conditions, and someone with more knowledge, sometime in the future, may find a condition under which the hypothesis does not hold true.
- Prediction:
- Next, the experimenter uses deductive reasoning to test the hypothesis.
- Prediction: If meat cannot turn into flies, rotting meat in a sealed (fly-proof) container should not produce flies or maggots.
- Prediction: If there is no life force, broth in swan-neck flasks should remain sterile, even if exposed to air, because any bacteria in the air will settle on the walls of the initial portion of the neck. Broth in flasks plugged with cotton should remain sterile because the cotton is able to filter bacteria out of the air.
- Inductive reasoning goes from a set of specific observations to general conclusions: I observed cells in x, y, and z organisms, therefore all animals have cells.
- Deductive reasoning flows from general to specific. From general premises, a scientist would extrapolate to specific results: if all organisms have cells and humans are organisms, then humans should have cells. This is a prediction about a specific case based on the general premises.
- Generally, in the scientific method, if a particular hypothesis/premise is true and “X” experiment is done, then one should expect (prediction) a certain result. This involves the use of “if-then” logic. For example, if my hypothesis that my throat is sore because I did too much screaming at the ball game is true and if a doctor examines my vocal cords, then (s)he should be able to observe that they are inflamed, and as the inflammation heals, the sore throat should go away.
- A prediction is the expected results if the hypothesis and other underlying assumptions and principles are true and an experiment is done to test that hypothesis. For example, in physics if Newton’s Theory of Motion is true and certain “unexplained” measurements and calculations pointing to the possibility of another planet are correct, then if I point my telescope to the specific position that I can calculate mathematically, I should be able to discover/observe that new planet. Indeed, that is the way in which Neptune was discovered in 1846.
- Testing:
- Then, the scientist performs the
experiment to see if the predicted results are obtained. If the
expected results are obtained, that supports (but does not prove)
the hypothesis.
In science when testing, when doing the experiment, it must be a controlled experiment. The scientist must contrast an “experimental group” with a “control group”. The two groups are treated EXACTLY alike except for the ONE variable being tested. Sometimes several experimental groups may be used. For example, in an experiment to test the effects of day length on plant flowering, one could compare normal, natural day length (the control group) to several variations (the experimental groups).
When doing an experiment, replication is important. Everything should be tried several times on several subjects. For example, in the experiment just mentioned, a student scientist would have at least three plants in the control group and each of the experimental groups, while a “real” researcher would probably have several dozen. If a scientist had only one plant in each group, and one of the plants died, there probably would be no way of determining if the cause of death was related to the experiment being conducted.
The experimenter gathers actual, quantitative data from the subjects. For example, it’s not enough to say, “I’m going to see how the dog reacts in this situation.” Rather, in that experiment, the scientist might have a list of certain behaviors, and record how often each of the dogs tested exhibits each of those pre-defined behavior patterns. Data for each of the groups are then averaged and compared statistically. It’s not enough to say that the average for group “X” was one thing and the average for group “Y” was another, so they were different or not. The scientist must also calculate the standard deviation or some other statistical analysis to document that any difference is statistically significant.
- Testing: Wide-mouth jars each
containing a piece of meat were subjected to several variations of
“openness” while all other variables were kept the same.
control group — These jars of meat were set out without lids so the meat would be exposed to whatever it might be in the butcher shop.
experimental group(s) — One group of jars were sealed with lids, and another group of jars had gauze placed over them.
replication — Several jars were included in each group. - Data: Presence or absence of flies and maggots seen in each jar was recorded. In the control group of jars, flies were seen entering the jars. Later, maggots, then more flies were seen on the meat. In the gauze-covered jars, no flies were seen in the jars, but were observed around and on the gauze, and later a few maggots were seen on the meat. In the sealed jars, no maggots or flies were ever seen on the meat.
- Conclusion(s): Only flies can make more flies. In the uncovered jars, flies entered and laid eggs on the meat. Maggots hatched from these eggs and grew into more adult flies. Adult flies laid eggs on the gauze on the gauze-covered jars. These eggs or the maggots from them dropped through the gauze onto the meat. In the sealed jars, no flies, maggots, nor eggs could enter, thus none were seen in those jars. Maggots arose only where flies were able to lay eggs. This experiment disproved the idea of spontaneous generation for larger organisms.
- Testing: Broth was boiled in
various-shaped flasks to sterilize it. As the broth and air in the
containers cooled, fresh room air was drawn into the containers.
None of the flasks were sealed — all were exposed to the outside
air in one way or another.
control group — Some flasks opened straight up, so not only air, but any bacteria present in that air, could get into them.
experimental group(s) — Some flasks had long, S-shaped necks (swan-neck flasks) and others were “closed” with cotton plugs. This allowed air to enter these flasks, but the long, swan neck or the cotton balls filtered out any bacteria present in that air. The long necks were subsequently broken off some of the swan-neck flasks.
replication — Several flasks were used in each of the groups.
- Data: Broth in flasks with necks opening straight up spoiled (as evidenced by a bad odor, cloudiness in previously clear broth, and microscopic examination of the broth confirming the presence of bacteria), while broth in swan-neck flasks did not, even though fresh air could get it. Broth in flasks with cotton plugs did not spoil, even though air could get through the cotton. If the neck of a swan-neck flask was broken off short, allowing bacteria to enter, then the broth became contaminated.
- Conclusion(s): There is no such life force in air, and organisms do not arise by spontaneous generation in this manner. To quote Louis Pasteur, “Life is a germ, and a germ is Life. Never will the doctrine of spontaneous generation recover from the mortal blow of this simple experiment.”
Research is cumulative and progressive. Scientists build on the work of previous researchers, and one important part of any good research is to first do a literature review to find out what previous research has already been done in the field. Science is a process — new things are being discovered and old, long-held theories are modified or replaced with better ones as more data/knowledge is accumulated. For example, the idea that the sun is at the center of our solar system replaced the idea that the earth was at the center of the universe, and the idea that ulcers are caused by stress has been replaced by the idea that ulcers are caused by bacterial infection. Scientists are human, too, and so these major changes are often controversial and accompanied by violent debate!
A theory is a generalization based on many observations and experiments; a well-tested, verified hypothesis that fits existing data and explains how processes or events are thought to occur. It is a basis for predicting future events or discoveries. Theories may be modified as new information is gained. This definition of a theory is in sharp contrast to colloquial usage, where people say something is “just a theory,” thereby intending to imply a great deal of uncertainty.
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