Saving the Phenomenon and Ockham’s Razor

I ran across an interesting article this morning discussing deductive reasoning and comparing diagnosis to detective work, part of the appeal which medicine has for me.  In particular, the discussion of Ockham’s Razor was particularly enlightening, since I had never heard a satisfactory explanation of what the term “razor” applied to:

The term razor is used to highlight shaving away unnecessary assumptions to get to [the] simplest explanation.

I was first introduced to this principle as an undergraduate during an introductory astrophysics course.  The professor introduced it as part of a discussion on how the Greeks had attempted to explain some apparently bizarre celestial motion, notably the retrograde motion of Mars and Venus.  Ockham’s Razor is commonly stated :

When competing hypotheses are equal in other respects, the principle recommends selection of the hypothesis that introduces the fewest assumptions and postulates the fewest entities while still sufficiently answering the question (source: Wikipedia).

I’ve heard lots of different articulations of this principle, also known as the Principle of Parsimony, and they are more or less about the same as this one.  However, it’s often taught in science classes somewhat differently, with heavy emphasis being placed upon the idea that the simplest hypothesis tends to be the correct one.  This is how most students tend to remember it.

I want to highlight a potential flaw in that understanding by returning to the idea of saving the phenomenon.  To “save the phenomenon” is to reason in a manner that incorporates all of the observable aspects of the problem.  What this basically means is that any hypothesis, regardless of how simple or complex, must be congruent with all the available observations or data.  I’ve often seen individuals appeal to Ockham’s principle as support for a hypothesis, without recognizing that their hypothesis doesn’t explain, or flatly contradicts, some of the data.  Politicians do this with impunity.

A prime example of this is the theory of classical mechanics applied to the hydrogen atom.  Which is a simpler explanation?

  1. An electron moves in a well-defined circular orbit around a proton.
  2. An electron cannot be said to have a specific position in the space around the proton but is instead described by a probability function, where the probability of finding it at a given position is dependent upon the radial distance from the proton.

The simplest explanation is that the electron moves in a nice circular orbit around the proton.  However, we know that a charged particle moving in a circle is accelerating and thus radiates energy.  Energy conservation then implies that the orbital radius of such an electron would decrease with the atom continually losing energy until the electron collides with the proton.  One can show that, if this process were to occur, it should occur exceedingly rapidly and that classically speaking, an isolated hydrogen atom should be radiating energy as the electron spirals into the nucleus.  This doesn’t happen.

So, we have two possible explanations for how an electron moves around a proton in a hydrogen atom and an observation: hydrogen atoms do not radiate energy.  Here’s where the misapplication of Occam’s principle occurs.  If one were to only consider the complexity of the explanations, #1 is clearly superior to #2.  But, it doesn’t successfully account for our observation, which is to say that it doesn’t “save the phenomenon” and therefore shouldn’t necessarily be given a greater weight than #2.

I’m not going to get into the physics behind it, but our current understanding of the atom leads to the conclusion that #2 is correct and that #1 is not.  Electrons aren’t point particles that orbit the nucleus of an atom like planets orbiting the sun.  But, it’s interesting to realize that physicists, aware of the problems behind #1, chose to deal with them by simply waving their hands around and explaining that the lack of radiation from the atom was due to the fact that “the electrons just didn’t do that”.  “Just because” wasn’t really a scientific answer, but it formed the basis for the Bohr model of the atom for years until wave mechanics became accepted and is still how most undergraduate science students think about atoms.


2 Responses

  1. I loved this short post (as a physics student)! I just took my second quantum mechanics course as an undergrad. I believe Ockham’s razor is certainly powerful but at least in my experience as a physics major, there are really so many counterintuitive things in our universe! e.g. special relativity (when talking about Galilean relative motion)

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