Scientific “Placeholders”

January 30, 2011

One of the puzzles currently engaging the attention of scientists working at the Large Hadron Collider [LHC] in Switzerland is the nature of the dark matter that is postulated to account for something like 80% of the matter in the universe.  It is called dark, because it apparently does not interact with electromagnetic radiation (such as light), and was proposed as an explanation for experimental observations that seem to show that there is too much gravity to be accounted for by ordinary, visible matter.  (The “amount” of gravity can be inferred by examining the dynamic behavior of space objects, such as rotating galaxies.)   But, so far, we do not have any direct evidence of its existence, or real understanding of its properties.

The “Nobel Intent” blog at Ars Technica has an interesting article that tries to put the issue of dark matter in some historical scientific context.  There are some people who are troubled by the idea of assuming the existence of something that cannot be detected.

The comments appear like clockwork every time there’s a discussion of the Universe’s dark side, for both dark matter and dark energy. At least some readers seem positively incensed by the idea that scientists can happily accept the existence of a particle (or particles) that have never been observed and a mysterious repulsive force. “They’re just there to make the equations work!” goes a typical complaint.

As the article points out, this is hardly the first time that a scientific theory or explanation has been accepted, even though it did not “fill in” all the blanks for the phenomenon concerned. Sometimes a conceptual “placeholder” has to stand in for the parts that we haven’t yet understood.

Charles Darwin’s theory of evolution by means of natural selection, cited in the article,  is an obvious example.  In The Origin of Species, Darwin gave a thorough and convincing account of how heritable traits that improved reproductive success could become pervasive in a population.  But Darwin offered no explanation of the mechanism  by which traits could be inherited; Mendel made a careful study of the effects of heredity, but similarly had no explanation of how heredity worked.  It was only in the mid-20th century, with the discovery of DNA, that biologists could show how traits were inherited in the first place.

Another example, not in the article, is Newton’s development of the laws of gravity.  His work gave us tools for computing gravity’s effects that are accurate enough to send a spacecraft to Mars; but Newton really had no idea how gravity worked.  The beginning of that understanding had to wait for Einstein’s theory of General Relativity, which describes gravity as affecting the fabric of space-time.  And we are still trying to reconcile General Relativity with the Standard Model of quantum physics.

There are examples in other fields, too.  Aspirin, for example, was used in medicine for decades before the mechanism by which it works was elucidated in 1971.

As the article points out, even those “placeholders” that turn out to be really wrong are not useless.  The case of phlogiston is instructive.  Phlogiston (from the Attic Greek φλογιστόν, “burning”) was a combustible element presumed to be contained in all materials that would burn, being released during combustion.  So materials that burned in air,  like wood or oil, were supposed to be rich in phlogiston; when they were burned, they became “dephlogisticated”, leaving the pure material (sometimes called the calx).  The released phlogiston was absorbed by the air; that a fire in a small enclosed volume of air went out after a short time showed that the air could not absorb any more phlogiston – it was said to be completely “phlogisticated”.  The theory, which in some ways was almost the inverse of the correct explanation, oxidation, was not dropped  until the experiments of Lavoisier demonstrated that the mass of all combustion products, including gases, were greater than the initial mass of fuel.  Nonetheless, the theory of phlogiston did lead scientists to theorize (correctly) that combustion, metabolism, and corrosion (rusting) had something in common.

The whole point of all this, really, is that, like most forms of human endeavor, science often proceeds along a somewhat meandering path, rather than in a straight line from premise to complete explanation.  Sometimes even small gaps in our knowledge can be frustrating; yet it is gratifying that, on the whole, we keep making progress.

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