5 The Incredible Quantum Mechanics
83
Quantum Mechanics, and other branches of modern physics, to explore
hypotheses in a logical fashion. Leucippus and Democritus, two philosophers
from the 5th Century BCE, were probably the first to suggest that if matter
is broken down as outlined above, eventually one will arrive at small particles that are indivisible [3]. The name “atoms” was coined from the Greek
“atomos”, meaning indivisible.
Democritus held the belief that atoms moved freely in a void, colliding
with each other, and sometimes sticking together. It was a remarkably accurate picture of the modern viewpoint, considering its antiquity. However, the
atomic theory was rejected by Aristotle, certainly the most influential philosopher of Ancient Greece (whom we met in Chap. 1), and so was disregarded
for two millennia, up until the nineteenth century. Paradoxically, science is
sometimes best served by a healthy disrespect for authority.
Further development of the concept of atoms had to await the work of
John Dalton (1766–1844), an English chemist with a Quaker background.
His atomic theory, which underpins modern chemistry, held that all matter
is composed of atoms, which are indivisible and indestructible. The simplest
materials, known as elements, are comprised of identical atoms. More complex
materials, or compounds, are made up from combinations of different types
of atoms. Chemical reactions proceed by rearranging the atoms of different
compounds: each new arrangement corresponds to a new compound with
chemical properties differing from those of its antecedents.
Dalton’s theory was successful at explaining much of chemistry, in particular the fact that elements combine in particular ratios to form compounds.
Ludwig Boltzmann in the late 1800s expanded Dalton’s ideas to explain the
temperature and pressure of a gas as a consequence of the motion of the
constituent molecules. However, in the eyes of many 19th Century physicists,
the atomic theory suffered from one overriding disadvantage: the atoms are
so small that surely they must be completely unobservable. As we remarked
in Chap. 3, a physical assumption must be falsifiable: if a quantity cannot be
observed, it is outside the scope of physics.
This fundamentally philosophical disagreement on the nature of matter
was resolved by Albert Einstein in the first of his ground-breaking papers of
1905 [4]. In 1827, a botanist, Robert Brown, had observed that tiny particles
in a fluid, such as air or water, exhibited a jerky, random motion. This Brownian motion, named after its discoverer, was suspected of being the result
of collisions of the fluid’s molecules against the particles. If a coloured dye
were added to water, the molecular collisions produced a slow diffusion of
the colour throughout the liquid. Einstein studied this problem mathematically, and found that the diffusion rate could be related to the size of the
83
Quantum Mechanics, and other branches of modern physics, to explore
hypotheses in a logical fashion. Leucippus and Democritus, two philosophers
from the 5th Century BCE, were probably the first to suggest that if matter
is broken down as outlined above, eventually one will arrive at small particles that are indivisible [3]. The name “atoms” was coined from the Greek
“atomos”, meaning indivisible.
Democritus held the belief that atoms moved freely in a void, colliding
with each other, and sometimes sticking together. It was a remarkably accurate picture of the modern viewpoint, considering its antiquity. However, the
atomic theory was rejected by Aristotle, certainly the most influential philosopher of Ancient Greece (whom we met in Chap. 1), and so was disregarded
for two millennia, up until the nineteenth century. Paradoxically, science is
sometimes best served by a healthy disrespect for authority.
Further development of the concept of atoms had to await the work of
John Dalton (1766–1844), an English chemist with a Quaker background.
His atomic theory, which underpins modern chemistry, held that all matter
is composed of atoms, which are indivisible and indestructible. The simplest
materials, known as elements, are comprised of identical atoms. More complex
materials, or compounds, are made up from combinations of different types
of atoms. Chemical reactions proceed by rearranging the atoms of different
compounds: each new arrangement corresponds to a new compound with
chemical properties differing from those of its antecedents.
Dalton’s theory was successful at explaining much of chemistry, in particular the fact that elements combine in particular ratios to form compounds.
Ludwig Boltzmann in the late 1800s expanded Dalton’s ideas to explain the
temperature and pressure of a gas as a consequence of the motion of the
constituent molecules. However, in the eyes of many 19th Century physicists,
the atomic theory suffered from one overriding disadvantage: the atoms are
so small that surely they must be completely unobservable. As we remarked
in Chap. 3, a physical assumption must be falsifiable: if a quantity cannot be
observed, it is outside the scope of physics.
This fundamentally philosophical disagreement on the nature of matter
was resolved by Albert Einstein in the first of his ground-breaking papers of
1905 [4]. In 1827, a botanist, Robert Brown, had observed that tiny particles
in a fluid, such as air or water, exhibited a jerky, random motion. This Brownian motion, named after its discoverer, was suspected of being the result
of collisions of the fluid’s molecules against the particles. If a coloured dye
were added to water, the molecular collisions produced a slow diffusion of
the colour throughout the liquid. Einstein studied this problem mathematically, and found that the diffusion rate could be related to the size of the