Blackbodies and White Lies

Max Planck, 1933

Max Planck, 1933

By Matthew Vannette, Associate Professor of Physics,                                                                                                                                                 Saginaw Valley State University

In the late 19th and into the early 20th centuries, physics had a problem.  The way scientists understood the world at that time could not explain why hot objects, like iron in a blacksmith's forge, glow the precise way they do. Such glowing objects are called blackbodies, and the light they emit is blackbody radiation. The spectrum (how bright the light is at each color) of a glowing body shows a bright peak at middle wavelengths, and gets dimmer at very long, infrared wavelengths and the shorter ultraviolet to X-ray wavelengths.  The particular wavelength where the peak is observed depends on the temperature of the object - higher temperature means a shorter wavelength for the peak.  At the time, Rayleigh's* analysis, using the accepted -- and very successful -- model of light as an electromagnetic wave, predicted that the spectrum should get continually brighter as the wavelength gets shorter, with very short wavelengths being infinitely bright, irrespective of the object's temperature.  Since brighter light means more energy, an infinitely bright light at any wavelength implies that every object gives off infinite energy. 

Rayleigh's result was so wrong it is termed the "ultraviolet catastrophe."  Then, in 1900 a young German physicist named Max Planck settled the matter by introducing energy quantization, the first step toward quantum mechanics.  This was an entirely new way of thinking about things, and it straight-forwardly prevents the infinite energy Rayleigh's model predicts.  Physics was saved.  The idea was so radical that even Planck felt it had no physical basis and that someone smarter would come along and correct it.  But, it solved the problem.

This is the story we tell physics students about the development of quantum mechanics and modern physics.  It has a nice feel to it.  Very scientific method-y, if you will.  And it's a lie.  Planck was not solving the ultraviolet catastrophe known to the rest of the physics community.  Planck's first paper on the subject was published in January of 1900 (though not read at a conference until October of that same year), and he was motivated by a small discrepancy in the long wavelength limit.  Rayleigh's was not published until July 1900.  It just so happens that Planck's work provided a good model over the entire spectrum.  Unless Planck had worked out the ultraviolet catastrophe himself, he could not have been trying to correct for it. And if he had worked it out, for some reason, he chose not to publish. Perhaps he refused to present a model that gave such bad predictions. A core tenet of science is that if the model does not match the data, it cannot be correct, except in a very limited sense.

Understanding the motivations of a researcher is very important. It can reveal subconscious biases that may have led to inadvertent mistakes or omissions. If a particular researcher, then, has the weight of authority, those mistakes and biases can become part of our culture. Even for scientists, it is important to know our history so that we can examine our intellectual forebears honestly. Many years ago, a mentor of mine at Boston College, Andrzej Herczynski said that Einstein was an ordinary genius -- well beyond what our normal minds can expect to achieve -- but Planck was a transcendental genius. We can appreciate Planck's contribution more fully when we realize that he solved a problem well before the rest of the physics community knew there was a problem to solve, and scientists can have a greater understanding for how research and theories are developed.

*British physicist, Lord Rayleigh, John William Strutt