Over the past few weeks I have had a number of discussions with creationists about the evidence for evolution. A number of times the question of how one can test and falsify the theory has been put to me to which I cited the predictive ability of the theory. However, this was not accepted as a means through which one could test the theory. This is unfortunate because everyone should understand the central role that predictive capabilities play in judging theories in science.
In order to be successful a scientific theory must not only be able to explain all of the observed experimental facts but it must also continue to explain new ones as they are uncovered. Part and parcel with this is that fact that if a theory predicts a set of results that are not supported by future experiments then that theory is deemed defective. Sometimes the modifications will be minor but sometimes the whole theory will be discarded only to be replaced by a radically new idea. A wonderful example of this can be seen in the early 1900’s with the introduction of quantum mechanics to the field of physics.
In the beginning of the 1900’s physicists were trying to understand the phenomena of blackbody radiation. A blackbody is an object that absorbs all of the incident light falling on it, absorbing its energy, only to later emit it in the form of thermal radiation. To picture a classical example, think of a large hollow box with a tiny pinhole poked in one side. Light falling on the body will enter through the hole but become trapped as the random reflections off the cavity’s walls redirect it from the opening and prevent it from escaping. In reality, some light will ultimately escape back out but this is an ideal model and its unlikely to occur. While trapped, the energy of the light waves is transfered to the walls of the cavity and emitted as heat.
A physicist named Rayleigh calculated the energy spectrum of thermal radiation emitted from such a body using the classical electromagnetic theory of Maxwell. The result was found to agree with experiment in the infrared (low frequency) portion of the spectrum but diverged rapidly from the experimentally observed spectrum in the ultraviolet (high frequency) portion of the spectrum. This was a spectacular failure on the part of the highly successful electromagnetic theory, which earned it a dramatic but appropriate name; “the ultraviolet catastrophe”. Clearly there was a problem with the theory and it needed modification. In 1901 Max Planck discovered just what was needed and in doing so, began what became a revolution in our understanding of the natural world.
Planck proposed that the energy of the EM radiation trapped inside the blackbody did come in a continuous values but rather discreet increments. This was a radical idea at the time. It’s analogous to saying that you do not move through a room continuously but rather in small intervals like a meter at a time – something like watching a movie frame-by-frame with the missing parts being completely undefined. Just you appearing at points discretely spaced, teleporting rather than assuming the positions in between. This idea was completely counter intuitive but, once made, and Rayleigh’s calculation repeated, the resulting theoretical spectrum agreed perfectly with the observed experimental data.
So, what does this example mean for evolution? Biologists have constructed a complete scientific theory and agreed upon a philogyny for life. The theory of evolution predicts a nested hierarchy of forms which cannot be violated if the theory is correct. This is a prediction. If someone were to find a form which does not fall into this hierarchy, for example, a mammal with plant cellular tissue, then the theory as it stands would be falsified. Evolution would not be able to explain such a form and would therefore have to be modified or rethought. And, depending on how badly such a discovered life form violated the predictions, a radical change in thinking analogous to the introduction of quantum mechanics would be required.
Scientific theories make predictions about future experiments and in order for a theory to be considered valid those predictions have to be correct. The moment a theory begins to diverge from the observables it must be discarded. If someone wishes to discredit the theory of evolution then this a means by which they can do it.
