A Distinctive Family of Errors Part I: Don’t Get Burned

By Phil Pennington

The author is a member of O4R and a physicist who has worked at Hughes Semiconductors and on the physics faculty of PSU. He has a special interest in problems of education and the misunderstanding of physics.

A sunbather works on her sun tan every evening after work because the days are hot and the skies are clear. A skier on Mt. Hood slathers on sunscreen on a warm December 24 because a Portland weather forecaster warned him the temperature was going to get up to 70 degrees so get out that sunblock. But the sunbather never seems to get tan, and the skier doesn’t notice that even those who didn’t use sunblock didn’t get sunburned, even after skiing all day in the sun.

     The sunbather, the skier, and the weather forecaster are seeing the obvious, but missing something of substance. Their reasoning is incomplete. The sunbather won’t reliably get that tan; the skier won’t reliably avoid sunburn and the weather forecaster will continue to give bad advice ... until each looks at the world in certain mind-stretching ways. They need to extend their perception a bit beyond the limits dealt us by evolution. And, they need to sort out a tangle of potential influences and identify those which are important and those which are not. This task is more difficult than it might seem and errors are frequently made.

     The sunbather’s problem is a good starting place for exploring several routes through the rocky terrain between pseudoscience and science. It is terrain that has pitfalls that make some of even the simplest science difficult, and some of the silliest pseudoscience so seductive.

     Getting sunburned or tanned correlates highly with air temperature. But that’s because when it’s warm we remove clothing and expose more skin to the sunlight. Contrary to a common belief, temperature has nothing to do with the ability of sunlight to burn or tan. The “actinicity” of sunlight comes from its ultraviolet content. Temperature is irrelevant.

     Temperature is obvious to us; it is something we are directly able to perceive. Some animals have the ability to perceive ultraviolet; however, we are evolutionarily deprived on this count. Ultraviolet lies just outside the limits of human vision. We too easily, then, let the obvious (temperature) guide our thinking.

     The widespread belief that higher temperature contributes to actinicity of sunlight feeds on the common error of seeing correlation as causation. (Attune yourself to this error and you will see it being made frequently). However, two things may correlate simply because both are caused by something else, something of which we might not be aware. Clearly, distinguishing between correlation and causation straddles the edge of human comprehension. When we confuse correlation and causation we misinterpret the interplay of multiple influences.

     If we had UV color vision, we would see a great shift in the color of sunlight every day as the sun traverses the sky. When the sun gets much lower than 45 degrees, the UV is largely gone, shifting the color toward longer wavelengths, red to us.

     Rayleigh scattering, well understood by the end of the 19th century, explains why there is a color shift as the sun gets lower in the sky: The amount of scattering decreases by the fourth-power of the wavelength. As a consequence, UV with half the wavelength of, say, blue light is scattered 16 times as strongly as is the blue light. Scattering selectively removes shorter wavelengths. Furthermore, scattering becomes more important as the sun gets lower in the sky: Not only does the light have a greater distance to travel through air to reach the surface, its path becomes more complex because of multiple scattering, increasing the odds of being lost to outer space.

     Ultraviolet is not a single wavelength, nor is it two or three clearly distinct wavelengths - such as might be interpreted from the terms UVA, UVB, etc. It is a continuous range of wavelengths. Tanning is accomplished by longer UV wavelengths, as well as some visible blue and violet light. Burning is accomplished by a range that peaks at somewhat shorter wavelengths. The very dangerous wavelengths - those that break chemical bonds in DNA and that ozone absorbs - not scatters - are shorter yet. Common misconception oversimplifies UV into a single, undifferentiated entity. Listen carefully to news coverage of ultraviolet phenomena and you will hear severe oversimplification.

     So, sun angle is the most readily available observation to us for predicting the actinicity of sunlight. When the sun is overhead in a cloudless sky, both burning and tanning are at a maximum. With the sun at 45 degrees, burning is attenuated; tanning is also, but much less so. Below 45 to 50 degrees both tanning and burning are very slight. A good rule of thumb is: if your shadow on a horizontal surface is much longer than you are tall, the sunlight is not actinic.

     Within the family of oversimplifications are the errors of omission. They occur when we are confronted with interactions of multiple potential influences. The sunbather fails to identify what is relevant, what is not, and how the many potential influences might interact. The sunbather’s problem is science in microcosm.

     Errors occur when multiple influences are at work and only one at a time is seen. When one influence is focused upon, all the others disappear from mental view: The interactions get lost in this conceptual tunnel vision. The problem might be imagined as that of discovering “The Cause.” Find The Cause and you can forget the rest.

     Such oversimplification has many manifestations: For example, it leads to the union of one false, one true statement, of the fascinating form: “It’s not the width of a corn field, that produces the area, it’s the length.” “Unsafe cars don’t kill people; bad drivers kill people,” says the car manufacturer. “It’s not the weather, its the economy that’s been troubling farmers,” says the economist looking for business. “It’s not the voltage that is dangerous, it’s the current,” pontificates the electrical theory disadvantaged. “This wasn’t a domestic abuse trial, it was a murder trial,” declares a Simpson trial jurist.

     A little more subtle, but common, are misunderstandings from concepts so oversimplified they sidesteps the essence. For example, “The uncertainty principle states that you can’t simultaneously measure both position and momentum,” starts the popular argument of the modern day relativist. It leads him to the ultimate self-deception: asserting that mere wishing is sufficient to achieve one’s goals. Something essential is overlooked. The uncertainty principle assumes that both position and momentum are measured simultaneously; those two parameters are merely different components of an inseverable multicomponent whole - for most, probably all, useful applications of the principle.

     Difficulties within physics are common and notorious. But they are often difficulties of simple logic found everywhere. Because physics deals with the simplest aspect of things, the errors were recognized early. More complex things await comparable insight.

     Overcome oversimplification and we become impressively empowered, users of knowledge. It’s how we got to the moon. Overlook oversimplification and we get lost in an unseen logic-space populated by such pitfalls as shell game con artists, state lotteries, disastrous ecological surprises, unsolvable social complexities ... and sunburn.