Science uses controlled experiments to test models.

One way or another, scientific experiments are mainly about control.

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In short…

Experiments are used to collect data under controlled conditions.

An experiment is a question which science poses to Nature, and a measurement is the recording of Nature’s answer.

Max Planck, German physicist, 1947

What is an experiment?

Experiments are investigations in which environmental conditions are controlled and data are collected.

What happens in a controlled experiment

In a controlled experiment, most conditions are constrained so that they do not change. One or two other conditions are then changed and measured by the experimenter. These are known as the independent variables. These changes cause some other property or condition, the dependent variable, to change in response. It, too, is measured. The data are analysed to try to find relationships between the independent and dependent variables.

Why controlled experiments are used

All scientific models must be tested against the real world (see Scientific models are tested against the real world). Experiments are useful for testing models in the physical sciences, where conditions can be easily controlled. This is not the case in astronomy or geology because many of the things of interest are either too far away or happened in the past. And in branches of science that study living things, there are ethical questions in carrying out experiments. When experiments are not possible, science must use other methods of testing (see Science uses randomised controlled trials to test models; Science uses observation to test models).

Examples

• In 1660 Robert Boyle varied the pressure on a trapped sample of gas, and measured the resulting variations in its volume. All other variables – such as amount of gas and temperature – were held constant. Boyle showed that the volume was inversely related to the pressure.
• In 1858, Louis Pasteur used flasks with long, S-shaped necks to falsify the theory of spontaneous generation. He heated broth in the flasks to sterilize it. After it cooled, be broke the necks off some of the flasks. Before long, the broth in those flasks became contaminated because microbes could enter. The broth in the control flasks did not.
• Here’s a passage from Richard Dawkins’ book The Magic of Reality, describing Mendel’s pioneering experiments in genetics:

Back in the nineteenth century, an Austrian monk called Gregor Mendel did experiments in his monastery garden, breeding peas in large quantities. He counted the numbers of plants that had flowers of various colours, or that had peas that were wrinkly or smooth, as the generations went by. Mendel never saw or touched a gene. All he saw were peas and flowers, and he could use his eyes to count different types. He invented a model, which involved what we would now call genes (though Mendel didn’t call them that), and he calculated that, if his model were correct, in a particular breeding experiment there ought to be three times as many smooth peas as wrinkly ones. And that is what he found when he counted them.  Leaving aside the details, the point is that Mendel’s ‘genes’ were an invention of his imagination: he couldn’t see them with his eyes, not even with a microscope. But he could see smooth and wrinkled peas, and by counting them he found indirect evidence that his model of heredity was a good representation of something in the real world.

• In 1909, Ernest Rutherford decided to check on the accepted ‘plum  pudding’ model of the atom. Two of his colleagues, Hans Geiger and Ernest Marsden fired α-particles at a sheet of gold foil and measured their deflections. Rutherford realized that the results falsified the old model and devised a new one in which atoms have a heavy, positively-charged nucleus.
• In 1980, James Randi and Dick Smith conducted experiments to test the ability of dowsers to find water or metal beneath the earth. They found that the dowsers’ claims were not supported.
• Homeopathic models claim that water can retain a ‘memory’ of substances once dissolved in it. In 2005, an experiment showed that water loses its ‘memory’ of any regularities in its molecular structure within 50 millionths of a nanosecond, showing that the model does not fit reality.

Max Planck’s quote is from ‘The Meaning and Limits of Exact Science’ (1947), in Scientific Autobiography and Other Papers, trans. Frank Gaynor (1950), 110
The Lenski experiment image by Brian Baer and Neerja Hajela is from Wikimedia Commons. Lenski’s experiment is ongoing, and has provided much evidence supporting the biological evolutionary model.
Richard Dawkins’ quote is from The Magic of Reality (2011), p 16 – 17.

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Updated: 2014/02/15