Science Test Topic 2 – Free

Hypotheses, Theories, and Laws

One misconception about scientific ideas and explanations is that they can be proven true or false.  For example, imagine that you set up an experiment to test how the amount of water affects the height that grass can grow.  Your explanation is that more water would cause grass to grow taller.  You conduct the experiment and find that the grass grew taller with larger amounts of water.  While it would be tempting to say that this experiment proved your explanation true, it is possible that if you tried even larger amounts of water, they might cause the grass to die!  Alternatively, if you had tested large amounts of water in your experiment, and found that they caused your grass to die, you might conclude, incorrectly that the experiment proved your explanation (more water=taller plants) false.

However, explanations that have been supported by evidence from many experiments, in many settings, at many different times are considered stronger explanations.  They are stronger because they have more predictive power.  The predictive power of an explanation is how well it can predict results of further experiments.  For example, if many experiments with different types of plants (grass, shrubs, trees, weeds) had supported the explanation that increasing the amount of water plants receive (up to a reasonable level) would result in taller plants, we would say that this explanation has predictive power.  We can predict, with some confidence, that experiments in which scientists apply more water to plants will result in taller plants.

In the above examples, you can see that science ideas and explanations can be specific to a single, narrow experiment (such as your experiment with the grass), or can be broader and apply to many experiments.  Some of the science explanations were tentative and untested, and some had been tested many times.  All of these explanations have a place in science.  Different types of scientific explanations are described below:

  • Hypotheses: Hypotheses (the plural of the word hypothesis) are testable, proposed explanations for a single, narrow phenomenon. While inferences are also proposed explanations, hypotheses differ because they are  For example, scientists observed that alpine butterflies exhibit characteristics intermediate between two species that live at lower elevations. Based on these observations, the scientists inferred that this species of alpine butterfly evolved as the result of hybridization between the two other species living at lower elevations.  The testable hypothesis from this inference is that the alpine butterflies would show DNA segments in common with both species of lower elevation butterflies.  Additionally, most hypotheses are written as “if/then” statements, such as if I test the DNA of three species of butterflies, then the alpine butterfly will have DNA segments in common with both of the butterfly species found at lower elevations.
  • Theories: Theories, on the other hand, are broader, well-tested explanations for a wide range of phenomena.  Theories often are formed from repeated testing of a single hypothesis, or a generalization made from the results of many related hypotheses.  As a result, theories have predictive power.  For example, the theory of evolution by natural selection broadly applies to all populations with some form of inheritance, variation, and differential reproductive success — whether that population is composed of alpine butterflies, fruit flies on a tropical island, or a new form of life discovered on Mars. This theory helps us understand a wide range of observations (from the rise of antibiotic-resistant bacteria to the physical match between pollinators and their preferred flowers), makes predictions in new situations (e.g., that treating AIDS patients with a cocktail of medications should slow the evolution of the virus), and has proven itself time and time again in thousands of experiments and observational studies.
  • Scientific Laws: A scientific law is actually not an explanation, but rather a summary of observations that is true, without existing exceptions. Scientific laws, like theories, have predictive power.  Unlike theories, scientific laws do not explain why or how something happens.  For example, the law of gravity allows one to predict, with a high degree of certainty, what will happen when you drop an object within earth’s gravitational pull.  However, the law of gravity does not explain how or why gravity works.  As scientific laws describe, rather than explain, they are considered very stable, and unlikely to change.

What is important to understand about these different scientific ideas and explanations is that regardless of how broadly they can be applied, or how many times they have been tested, no scientific idea is ever considered “proven true.”  With better technology and methods, and the ever-increasing body of scientific knowledge, it is possible that even well-tested and established theories could change (though theories are less likely to change than hypotheses). However, it can help one to differentiate between hypotheses, theories, and laws to consider them on a continuum from less to more certain, and from narrow to broad.

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While this placing of hypotheses, theories, and laws on a continuum can be helpful, it should not mislead you into thinking that as a hypothesis is tested more and more, it becomes a theory and then a law. A law is not a more definite, well-tested theory.  Laws summarize and describe observations, and no existing exceptions.  Theories and hypotheses explain phenomena.

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