Quantum Physics For Dummies: What Is It? (The Ultraviolet Catastrophe)

Physics divides the universe into three "reality ranges", and currently we have a different set of laws that apply to each range.  The physics of everyday, human experience that is taught in your basic high school physics class is called "Classical Physics".  These are all the laws generalized by Newtonian mechanics and Maxwell’s laws of electromagnetism.  Then there is the physics of cosmic scale (the very big) and high velocities and energies, which falls under the realm of "General Relativity", championed by Einstein.  Finally, there is the atomic scale (the very small) of low velocities and energies, which is governed by "Quantum Mechanics", worked out by scientists such as Planck, Bohr, and Schroedinger. 
By far the least well known and understood by the general public (and scientists for that matter) is Quantum Mechanics.  Quantum Mechanics actually has its roots a few years before that of Relativity theory, with its birth generally considered to be in 1900. Einstein did not develop Special Relativity until 1905 and General Relativity until a decade after that.  And so, Quantum Mechanics was science's first monumental leap from the safety and security of the Classical Physics that humanity believed ruled reality for the past several centuries.

The Ultraviolet Catastrophe refers to that first time, in 1900, that the laws of classical physics were found to grossly disagree with reality.  In the mid-to-late 1800's, the primary focus of applied science was a race between nations to build the best electricity industry.  German companies sought to design a more efficient light bulb than their British and American competitors.  Naturally, this led to the study of materials called "blackbodies", which are perfect absorbers and emitters of radiation (light).  While the concept of a perfect blackbody is largely theoretical, scientists wanted a material that closely approached these properties in order to try to attain a bulb that emitted as much light as possible with minimal energy losses to heat.  But during their study of blackbodies, scientists happened upon an eerie conundrum.  It seemed that the laws of Classical Physics were predicting entirely different properties for blackbodies than were actually being observed.






The classical theory predicted that a blackbody could emit radiation with infinite intensity, while actual experiment showed that there was a defined maximum to the intensity, where after it decreased to zero.  This was the so-called Ultraviolet Catastrophe, because the classical predictions deviated in the ultraviolet wavelength region.  To solve the problem, Max Planck rewrote a fundamental equation of classical statistical mechanics called the Law of Equipartition of Energy with a revolutionary interpretation of energy.  Instead of treating energy as continuous (where a system can take on any value of energy), he treated it as discrete, or "quantized" (where a system can only take on certain values of energy).  Sure enough, Planck's theory of quantized energy matched experiment almost perfectly.  Thus was born one of the fundamental tenants of Quantum Mechanics, which states that energy (including light) exists in packets that can only take on certain allowable values which are integer multiples of Planck's constant (h = 6.626x10^-34).

The extreme smallness of the value of Planck's constant is the main reason why quantum effects are not visible to us on the scale of everyday life.  If they were, we might notice that cars could only go at speeds that were multiples of 10 mph, and would instantaneously jump from 10 mph to 20 mph without going at any speed in between.  This does in fact happen, but at speed intervals so infinitesimal that the quantization is impossible to detect. Does this mean that everything you ever learned about how the world works is wrong? Not entirely.  Remarkably, Newton's classical law F = ma can actually be derived from the Schroedinger equation of Quantum Mechanics in the limit of high energy and large mass. This means that Classical Physics is actually a special case of Quantum Mechanics. In fact, the physical effects we see every day on the macro scale are just the average of quantum mechanical effects occurring on the atomic scale. Nonetheless, the quantum reality tells us many fascinating things about our universe that the classical reality does not, most of which are nothing short of mind boggling.