In the final years of the nineteenth century, a new problem came to light which threatened to question the very nature of physical theories since the time of Newton.

Toward the end of the nineteenth century, many physicists thought that they had just about everything figured out. With Newtonian mechanics, just about every aspect of the known universe could be understood, and with Mendelev’s periodic table of the elements, the composition of matter seemed to be under control as well.

Who could have guessed that one of the most important revolutions in the history of science would begin with a problem as simple as a red-hot piece of metal?

What is a Black-Body?

Technically speaking, a black-body is: “a hypothetical body that absorbs without reflection all of the electromagnetic radiation incident on its surface.” Or so says the dictionary.

A black-body in this sense can be thought of as being the polar opposite of a mirror. While a mirror reflects all the light that hits it, a black-body absorbs it. Just think about how hot black vinyl car seats become after absorbing the sun’s rays during the summer.

Objects that are black are chemically capable of absorbing most of the electromagnetism which strikes them, where other colors (specifically the brighter ones) don't absorb as much heat because they reflect electromagnetism rather than absorb it.

This being said, a black-body must not always remain black, and this is where the problem lies. A black-body's color is dependent on its temperature. A good example of this, as mentioned previously, is a black metal poker stuck into a fire; it begins to heat up as it absorbs the energy from the fire, then it begins to glow red, then orange, then finally white as it gets hotter and hotter.

This change in color occurs because as the temperature increases, the wavelengths of the electromagnetic radiation coming from the black-body also increase (after all, the heat absorbed by a black-body does not stay absorbed forever – it is emitted in the form of heat radiation), thus changing into visible colors with shorter wavelengths.

So far so simple.

The Black-body Problem

The problem is that the laws of classical mechanics, specifically what is called the equipartion theorem, states that black-bodies which have achieved thermodynamic equilibrium (that is, when they absorb as much energy as they radiate back out, as do black-bodies at the different colored wavelengths of light) are mathematically obligated (by classical, pre-quantum, laws) to radiate energy in the form of ultraviolet light, gamma rays and x-rays at a certain level, depending on the frequency of emitted light.

Unfortunately for classical laws, when a black-body gets hot enough, the amount of radiation which should theoretically be given off, according to the equipartition theorem, should begin to approach infinity.

In other words, a glowing body should in theory be emitting a tremendous amount of radiant energy. This means that every time an oven is opened, according to classical mechanics, we should be instantly vaporized and killed by the many deadly rays emanating from the black-body heating elements.

Obviously, this doesn’t happen.

The Ultraviolet Catastrophe

Either something is wrong with the black-bodies themselves (not very likely), or something is wrong with classical physics (very likely, as any student of the history of science should realize).

In reality, when the actual radiation emitted from a black-body was measured, it was seen not to shoot toward in infinite at the ultraviolet region of the electromagnetic scale (as the theories suggested), but rather to be highest toward the middle of the visible range of the spectrum, which seemed entirely counter-intuitive. It is for this reason that the discrepancy became known as The Ultraviolet Catastrophe.

Many attempts were made in the 19th century to try and justify this discrepancy within the narrow and fading confines of classical mechanics, but every attempt broke down in the same way – every attempt to use classical mechanics to solve the black-body problem showed the energy being the highest as the light moved into the ultraviolet spectrum.

The Fall of Classical Mechanics

When a theory fails to match observation, it is generally the theory itself which is called into question – this does not always turn out to be right, but it is usually cause to take pause, at the very least.

As this problem reared its ugly head near the end of the nineteenth century, it became one very clear-cut case where simple experimentation could easily prove wrong the mathematics of classical theories.

Even though it was clear that black-body radiation posed an insurmountable problem for classical mechanics, simply realizing that the original theory was wrong did very little to tell physicists why it happened. This is why it was such a catastrophe.

If classical laws (i.e. those of Newton and his loyal successors) couldn't account for black-body radiation, then what theory could? And what other problems might there be which also require alternate ways of thinking to account for them?

These problems led to an extremely important turning point in the history of physics – one where one might be able to find a clearly defined boundary between the “safe” world of classical mechanics and the “extremely strange” world of quantum mechanics.

It was a remarkable German physicist named Max Planck finally began to answer the black-body question, well at the tail end of the nineteenth century; and in doing so, he (with a little help from Albert Einstein) single handedly set the world on a course toward quantum physics.

References:

Einstein, A. (1905). On a Heuristic Point of View about the Creation and Conversion of Light. Annalen der Physik .

Gribbin, J. (1994). In Search of Schrodinger's Cat: Quantum Physics and Reality. New York, NY: Bantam Books.

Herbert, N. (1985). Quantum Reality. Garden City, NY: Anchor Press/Doubleday.