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# 1.21 Theories

Science is based on observations of the natural world. To qualify as scientific evidence, observations must be quantitative and repeatable. Scientists then look for patterns in their data and might form a hypothesis to explain those patterns. The tools of this process are logic and mathematics. Astronomers build scientific theories to increase their knowledge about the universe. A theory defines the "gold standard" in science; it is a conceptual framework that is broad and very well tested.

It is important to recognize the distinction between a hypothesis and a theory. A hypothesis is a proposed explanation for a set of observations. The observations need not be very extensive, and the hypothesis need not cover a wide range of physical phenomena. Hypotheses are the building blocks of science. On the other hand, a theory must explain a large number of observations and unify them under a single coherent idea. A theory is like a completed building. Like a well-built building, a good theory will stand the test of time and will be an elegant logical construction.

You will also encounter the word model used referring to the progress of science. In general, you can treat model as being synonymous with hypothesis. A model is a mathematical representation of a set of observations. Just like a toy model, it may be a simplified and idealized version of the real world. When scientists "model" their data — the word model is used here as a verb — they are using a hypothesis to make a mathematical description of their data and seeing how well that mathematical description fits.

A 1610 portrait of Johannes Kepler: a German mathematician and astronomer. Click here for original source URL.

For example, Johannes Kepler studied the data describing the orbits of the planets in the solar system. He found that the orbits were not circular, as had been previously thought, and hypothesized that the orbits were elliptical. Isaac Newton used Kepler’s hypothesis to build his theory of gravity. Newton’s law of gravity neatly encompassed all three of the rules of planetary motion discovered by Kepler. Newton's theory explained not only the planetary orbits but also the orbits of comets and asteroids. In fact, it explained the motions of any two orbiting objects. A theory is broader and more generally applicable than a hypothesis. In biology, the theory of natural selection explains the rich variety of species — flowers and fungi and flamingos — in terms of evolutionary adaptation to a constantly changing environment. A successful scientific theory shows us the unity in nature.

Newton's first and second laws, in the original Latin, from a 1687 edition. . Click here for original source URL.

Beyond the level of a theory, there are a few ideas that are referred to as "laws of nature." What makes a theory strong enough to be a law of nature? There is no rigid rule, but in general a law of nature is a physical idea that has stood the test of time. In other words, it is a theory that has been successfully tested over and over again. Newton's laws of motion match the motions of all mechanical objects and are used in the design of all machines. The laws of thermodynamics were devised over 200 years ago, and these descriptions of heat and energy formed the basis of the Industrial Revolution. Such ideas have been tested so thoroughly that scientists think it is very unlikely that they could be wrong.

Scientists must always be aware of the limitations of their theories. Every set of data may have more than one possible explanation. This is why there are rival hypotheses and why scientists do not always agree with each other. Even the most accurate observation has some error or uncertainty attached to it. If there are no perfect measurements, then there can be no perfect tests of a theory. Also, the data that scientists work with is always limited in some way. New observations may be made which disagree with a theory. With rival explanations to choose from, and with limited and imperfect data to work with, how do we know if we have a "good" theory? A consensus is usually reached after many years of observation and testing.

A good theory must be testable. In the 18th century, scientists proposed that migrating birds followed invisible elastic threads when they traveled long distances. This is not a very fruitful hypothesis. If the threads are undetectable, the idea cannot be tested. More recently, researchers proposed that birds navigate by a mixture of visual clues and by sensing magnetic fields. This idea was supported the fact that migrating birds (and other animals) have high concentrations of the mineral magnetite in their brains. Magnetite allows birds to orient themselves by detecting variations in the Earth's magnetic field. Through testing, this hypothesis became an accepted theory, even though bird navigation is complex and still the subject of active research.

Venus is the bright light in the sky in the picture. People who are unfamiliar with the night sky often are surprised with its brightness, and can confuse it for a UFO. Click here for original source URL.

A good theory must make reliable predictions. Many people believe that UFOs are alien spacecraft that the U.S. government is keeping secret from the public. The government certainly keeps many military issues secret, but there is no evidence that this includes captured aliens. This idea is not very useful since it makes no testable predictions of when future sightings might occur. On the other hand, scientists believe that many UFO sightings represent well-understood astronomical phenomena. This leads to the confirmed prediction that UFO sightings peak around these events, such as meteor showers and times when Venus is particularly bright. Newton used his theory of gravity to accurately predict the return of Halley’s comet. We can see in this one example the power of the scientific method. The appearance of a comet had been viewed throughout history as a mystical event, the source of fear and superstition. Newton showed that the appearance of a comet is a regular and predictable event.

A good theory should explain a wide range of phenomena. Sometimes a theory is overturned when new data shows it to be a poor description of the natural world. Other times, we replace an old theory with a better one that explains more observations. At the beginning of this century, scientists found that the simple view of atoms as hard billiard balls was inadequate. The quantum theory provided a better description of nature on the subatomic scale. At about the same time, Einstein came up with his theory of relativity, which described situations of strong gravity better than Newton’s theory. The older theories are not useless since we can understand the behavior of a gas perfectly well without using quantum theory and the Apollo spacecraft were sent accurately to the Moon using only Newton’s law of gravity. But the new theories provide a more complete view of nature.

The bare bones of the scientific method are observations and the testing of a hypothesis. In practice science can be very complex, but here you have the guidelines that motivate the whole enterprise. Science is a logical and systematic endeavor, but we should not ignore the role of sudden insight, luck, and serendipity in scientific discovery. There are many examples of the human side of science — stories of dedication and inspiration — in the history of astronomy.