The search for the indivisible
As we break matter down into its components, it is easy to imagine an ultimate particle that is not made of anything else. The atom was once just such an idea. The Greek word itself means ‘indivisible’.
Today, we call particles with an internal structure atoms. The name has survived, even though our understanding of the atom itself has changed.
Atoms and the void
Ancient thinkers looked for a common foundation beneath the diversity of things. Some saw water as the fundamental substance, others air; there was also a theory of four elements. What they shared was the assumption that many phenomena could be explained through a small number of basic principles.
In the fifth century BCE, Democritus developed atomism. In this picture, the world consists of indivisible particles and the void in which they move. The properties of things depend on the particles’ shapes, their order and their relative positions.
Our senses tell us that something is warm or sweet. Atomism proposes looking for the causes of these sensations in the structure of matter, which the senses cannot directly distinguish.
Democritus could not devise an experiment that would settle the argument. Aristotle had a different picture: understanding a thing also meant understanding its form, how it changes and its purpose. These ideas cannot be treated as early versions of modern physics: they had different foundations and different ways of supporting their claims.
Later, Lucretius set out the atomist picture in his poem On the Nature of Things. Understanding nature was meant to free people from fear of the gods and of death. For him, knowledge was directly connected to how one should live.
It was only much later, with the development of chemistry, that atomic theory could be tested against measurements.
From conjecture to measurement
In the early nineteenth century, Dalton developed atomic theory on the basis of quantitative patterns in chemistry. For example, for the same mass of carbon, carbon dioxide contains twice as much oxygen as carbon monoxide. The atomic hypothesis explains this through different numbers of oxygen atoms bonded to a carbon atom.
In 1869, Mendeleev published his periodic table. As he arranged the elements, he noticed recurring properties, left spaces for elements that had not yet been discovered and later predicted the properties of some of them. The subsequent discoveries of gallium, scandium and germanium confirmed those predictions.
The table contained empty cells: an element had not yet been discovered, but its place and some of its properties had already been identified. Mendeleev’s system worked long before the atom’s electronic structure was explained.
What keeps your hand on the table
In 1897, Thomson investigated cathode rays and obtained evidence for the existence of the electron. Particles with the same properties were found regardless of the material used for the electrodes. This pointed to a shared constituent of different atoms.
Thomson proposed a model with positive charge spread throughout the atom and electrons embedded in it. Later experiments would show that charge and mass are distributed differently inside an atom.
In 1909, Geiger and Marsden, working under Rutherford, studied the scattering of alpha particles by thin foil. Most passed almost straight through, while some were sharply deflected. Those rare large deflections proved decisive: a model had to explain them too. In 1911, Rutherford explained the results by concentrating the positive charge and almost all the atom’s mass in a very small nucleus.
If an atom were enlarged to the size of a twenty-metre hall, its nucleus would be roughly 0.2 to 2 millimetres across. Electrons surround the nucleus, with their distribution described by quantum mechanics. This raises a question: why does matter with this structure resist compression?
Your hand is still resting on the table. It is supported by electromagnetic interactions and the quantum properties of matter, including the Pauli exclusion principle: two electrons cannot occupy the same quantum state. When the atoms in your hand and the table come too close, their electronic states change and the energy of the system increases. Further compression requires work; this is what we experience as resistance to pressure.
What light can tell us
In 1913, Bohr introduced an atomic model with specific allowed energy states. During transitions that absorb or emit light, the photon’s energy equals the difference between the levels. Quantum mechanics would later replace the familiar picture of orbits with a more precise description; discrete energy levels would remain.
Atoms of different elements have characteristic spectral lines. These let us identify matter we cannot hold in our hands. Light from a distant star carries information about the atoms it has interacted with. An enormous distance separates us from the star, yet its spectrum obeys the same physics we test here.
Why quarks were needed
Research into the nucleus led to the discovery of its constituents. The proton carries a positive charge. In 1932, Chadwick discovered the neutron, a particle with no electric charge and a mass close to that of the proton. Its lack of charge makes detection more difficult, but does not make the neutron invisible to every experiment: it can be detected through its interactions with matter.
The main constituents of the atom had now been found: electrons, protons and neutrons. Accelerators made it possible to study collisions at ever higher energies, and the list of particles continued to grow.
Many particles found in cosmic rays and at accelerators had similar properties. Their sheer number prompted a search for a classification and tests of whether they shared a common internal structure.
In 1961, Gell-Mann and Ne’eman independently proposed a classification of hadrons. Hadrons include the proton, neutron and many other particles that participate in the strong interaction. By comparing their charges and other quantum properties, physicists found groups with similar characteristics and places for missing particles.
In 1964, Gell-Mann and Zweig independently proposed the quark model. A concise account of a proton’s composition is two up quarks and one down quark; a neutron has one up quark and two down quarks. This is known as the valence composition. A fuller description includes gluons, which mediate the strong interaction, and quark–antiquark pairs. A proton cannot be pictured as three stationary beads inside a shell.
has been detected
Quarks and electrons are considered elementary particles: experiments have not yet found any internal structure in them. Their behaviour, however, is unlike that of small solid objects.
Particles and fields
In quantum field theory, particles are described as excitations of fields. A photon, the particle of light, is an excitation of the electromagnetic field; an electron is described through the electron field. The theory lets us calculate, for example, the probabilities of producing particular particles in collisions.
The energy, momentum and other properties of such an excitation can be measured. In general, the theory predicts the probabilities of possible outcomes. Its accuracy is tested by how closely the calculations agree with measurements.
Electrons have identical intrinsic properties. Quantum mechanics does not give them individual labels that would let us tell them apart as we might distinguish two stones that look alike. They can be in different states, but exchanging the labels of identical particles does not create a new physical situation.
With enough energy, collisions can produce new particles. Unstable particles can spontaneously decay into others. Conservation laws apply throughout all of these processes.
What composition can tell us
To understand why your hand stays on the table, we needed to know about interactions and quantum states.
Diamond and graphite are both made of carbon. In diamond, the atoms form a strong three-dimensional lattice; in graphite, they are connected in layers. This explains the difference in their properties: diamond is extremely hard, while graphite is soft.
With people, the task is more complex. To understand how we recognise a familiar voice and its intonations, we have to study hearing, memory, learning and our experience of communication.
Someone close to you says your name. A familiar intonation may bring back a conversation from long ago that you both remember. What you hear contains those few sounds and the history you share.
