The story of quantum mechanics really begins at the end of the 1800s.

A time when scientists thought that the properties of light were completely understood.

Their understanding, one which is still fully relevant today,

was that visible light is a manifestation of electromagnetic waves.

Light has been shown in experiments to be a wave

corresponding to the changing electric and magnetic fields.

Maxwell had introduced his equations describing the behavior of light waves.

There was one problem that remained in that wave description of light,

and it really troubled physicists.

The standard description of light waves did not

correctly describe the light emitted from hot objects.

An emission known as blackbody radiation.

We know that if we turn on a stove,

the element will become hot and glow red.

But when we consider light using the physical laws laid out in the 1800s,

we would instead predict that our stove would emit more blue light than red,

and even more ultraviolet light than blue.

The theory of light created in the 1800s

predicted the wrong spectrum of colors from blackbody emitters.

Theory predicted that the frequency of

light increases as the light becomes more and more intense,

contributing more and more energy to the spectrum.

This is clearly wrong as it results in the stove emitting an infinite amount of energy.

When you turn on your stove,

it does become hot and the energy emitted in turn heats your food.

However, the energy emitted from your stove is not infinite.

Otherwise, turning it on would destroy the planet.

The problem of supposedly infinite energy being emitted from

blackbodies came to be known as the Ultraviolet Catastrophe.

The ultraviolet catastrophe was a huge problem for theoreticians of the era,

and its resolution became the turning point in the history of physics.

In 1899 Max Planck was commissioned to study the efficiency of incandescent light bulbs,

which shine due to their heat.

That same year, he proposed a theory that resolved the problem of

the infinite energy emitted by black bodies by introducing a new idea.

Electromagnetic waves can transport only a special amount of energy called a quantum,

instead of any arbitrary amount of energy.

This was the key to explaining why the energy emitted by

a light bulb or any other hot object is not infinite.

The new equations Planck developed with

a mathematical foundation to resolving the ultraviolet catastrophe.

They correctly describe the color and energy emission properties of a blackbody.

This new concept of a quantized packet of light energy,

was called the photon.

To accompany this concept,

Planck also introduced a new physical constant.

Which is now named after him called, Planck's constant.

The letter h is often used for Planck's constant,

and that is equal to 6.6 times 10 to the minus 34 Joules seconds.

The minus 34 tells you that Planck's constant is a very small number.

It's hard to wrap your head around how small this number actually is,

or what it means,

but we'll investigate its implications in more detail soon.

Planck's introduction of the tiny constant h was

mainly a trick to make the maths work properly for blackbody radiation.

He didn't suggest that it had any true physical meaning.

However, Albert Einstein realized that Planck's

constant does imply a new physical reality,

that light can be thought of as a particle.

In a brilliant paper published in 1905,

Einstein showed that particles of light or photons come in units that are called quanta,

where a specific amount of energy is related to each color of light.

If we have light of a certain color,

we know its wavelength and its frequency.

Einstein introduced a simple equation for the energy of a photon of that color.

The energy of a photon is equal to h,

which is Planck's constant, times frequency.

This radical idea led to Einstein receiving

the Nobel Prize for his work in physics in 1922.