A century of X-ray crystallography


BaalChatzaf

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In 1913 the Braggs father and son extended the work of the German physicist von Laue who bombarded various substances with X-rays and photographed the patter of X-rays reflect and diffracted from the material. He saw an intricate and regular pattern of dots, but he could not fully deduce the make up off the substance. The Braggs father and son figured out how to deduce the atomic structure of some crystals from the diffraction pattern produced by X-ray bombardment. For this the two received a Nobel Award a few years later.

A nice summary of the technique is given in the video at http://www.theguardian.com/science/video/2013/oct/09/100-years-x-ray-crystallography-video-animation.

This technique illustrates a mode of reasoning almost always used in the physical sciences. It is a variety of induction called abduction which is inference of the likely cause of a given set of effects. In a way it is "backward logic" which goes from effect to cause. It was first studied by C.S.Peirce one of the most important philosophers of science. Please see

http://plato.stanford.edu/entries/abduction/peirce.html

and

http://en.wikipedia.org/wiki/Abductive_reasoning

While abduction is not guaranteed to produce a true result each and every time, if very often does which is why it is one of the primary reasoning tools of physical science.

The discovery of the three dimensional structure of various crystals including salt NaCl was remarkable at the time since the atomic hypothesis was not full accept by all physicists (in particular Ernst Mach). The success of X-ray diffraction probes along with Einstein's landmark paper on Brownian Motion (1905) nailed down the Atomic Hypothesis which is so well supported by experiment that it is accepted as -fact- even though no one has ever seen an atom (they are too small to be seen by the human eye).

In the study of non-deductive forms of inference, particularly enumerative inductions (which leads to the false conclusion that all swans are whte) the field of abduction and abductive inference is surprisingly under studied.

Ba'al Chatzaf

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I believe abduction is a big missing from The Logical Leap.

Yes. In -The Logical Leap- correct hypotheses are nearly portrayed as axiomatic or self evidence (which they are not). Aristotle took, position that observation would lead to general principle which are a priori true. Observation was regard as a way of finding undoubted and axomatic truths. We now know this is not the case. Even a theory which is well supported by experiment is not bullet proof. There is always the possiblility that some new fact may be discovered that will falsify the theoory.

That is how classical Maxwellian electro dynamics was shown not be totally correct. The discovery and explanation of the photoelctric effect forced the quantization of electrodhynamics. By the way, Einstein's paper on the photoelectric effect (which put quantum theory solidly on the map) was the paper for which he won the Nobel Prize in 1921. The explanation of the photo electric effect by quantized bundles of light energy is again a wonderful example of abduction.

Ba'al Chatzaf

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