Ultraviolet catastrophe

“There is nothing more to be discovered in physics now. All that remains is more and more precise measurement.”

These words are usually attributed to a renowned Scottish scientist Lord William Thompson (better recognized by the name Lord Kelvin). It is actually unknown whether he really uttered these words, but they nevertheless do absolutely reflect the viewpoints of majority of physicist who lived on the threshold of 20th century. And it was not baseless. Classical physics were tested many times over throughout the years and described reality thoroughly and unquestionably. “What? So, does it mean that we already understand how everything work in the Universe?”-probably flashed in your mind. Don’t be rush and calm down– physics is not some random nobody who can be easily trifled with. Soon, because of a new discovery, classical physics began to fall apart and it came to light how horribly wrong everyone was. In fact, classical physics was proved to be fundamentally false. The first problem which classical theories failed to explain was related to radiation of so-called “black body”.

To understand this phenomenon, it is necessary to know that all tangible bodies in the universe emit energy in the form of electromagnetic radiation (light). It means that everything touchable always glows with light. Now look at your hand and back at the screen. Wear doubtful expression and keep reading. Different bodies emit different amount of energy depending on various factors, such as body temperature and its color. Basically, the higher the temperature of a body, the higher the frequency (and also energy) of a light it emits. The thing is, most of the objects we see in everyday life emit light of lower frequency that exist in infrared spectrum, which just cannot be observed by a naked eye. Actually, night-vision devices, which spies often use in movies and games (e.g. Splinter Cell), are designed using this very principle. We can naturally see a visible light of metals when they are melted, for example, because their temperature increases dramatically.

Example of infrared vision. Areas with higher temperature glow more brightly.

Objects absorb, emit and reflect light. Usually, the light of certain frequency which was not absorbed becomes this objects visible color. In 19th century physicists tried to ascertain the spectral composition emitted to a body in relation to its temperature. To accomplish that, they used an idealized model of a body – the black body (the name has nothing to do with its actual color. It is just how physicists call it. Yeah, physicists). A black body is a hypothetical body which meats two requirements:

1. It absorbs all incoming electromagnetic radiation, regardless of frequency or angle of incidence (other bodies merely absorb a certain part of the spectrum and reflect all the leftover light).
2. It stays in thermal equilibrium with its surroundings (i.e. has the same temperatures as all the bodies in the given system).

These conditions ensure that spectrum emitted by a black body will not be affected by other factors and will be determined purely by objects temperature.

When scientists tried to apply classical Newton physics to such body, the outcoming result was such that it didn’t coincide with reality whatsoever. According to classical physics, a black body would emit the same amount of light of each frequency. However, the higher the light’s frequency the more energy the light has. A black body would therefore emit huge quantities of energy in the form of high frequency radiation – infinite, in fact.

Black curve shows what happens when classical theories are applied. It stretches to infinity in higher frequencies. Red, Green, Blue curves show what really happens, for different temperatures.

Such behavior would lead to dire consequences – classical physics thereby states that each and every body in the universe should immediately emit all of its energy from the ultraviolet spectrum. Luckily, this doesn’t happen in reality, otherwise we would not have an opportunity to exist. (Also, such behavior violates the law of conservation of energy)

This realization was a huge milestone for the evolution of modern science. Physicists were at last unwillingly forced to admit that classical theories were simply wrong. Today, we have an apt name for this huge failure of classical physics – The Ultraviolet Catastrophe. It showed that there was something deeply flawed in our understanding of physics.

The black body radiation problem’s solution was firstly proposed by a German physicist Max Plank in 1900. He came up with an idea that bodies do not emit electromagnetic radiation continuously, but via small packets called quanta. The radiation of higher frequencies is composed of larger quanta than radiation of lower frequencies. This solves black body problem – it is increasingly difficult for a black body to emit radiation of higher frequencies, as it often cannot “feed” high-frequency quanta with enough energy, and rather sticks with low-energy light.

Quantization of energy was a first step of studying a brand new world of physics. It became a basis for developing a new branch of physics – quantum mechanics. It was all possible because scientists of past were mistaken, but they accepted their mistakes and worked further in attempt to understand the reality.

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