1 What is matter?
What is matter made of? This question has been puzzling humans throughout history, and it’s been a significant driver of scientific discovery. Around 2500 years ago, Greek philosophers speculated that matter consists of atoms (small indivisible objects – from the Greek word ‘atomos’ meaning uncuttable). Direct evidence for the existence of atoms was only obtained much later, with innovations in physics in the early 20th century finally providing us with a detailed understanding of the nature of atoms.
However, this is not the end of the story. It was quickly realised that atoms themselves are comprised of two parts: a small atomic nucleus, which carries a positive electric charge and most of the mass of the atom, surrounded by a cloud of negatively charged electrons. The nucleus itself is then comprised of two types of particles: positively charged protons, and neutrons which carry no electric charge.
These discoveries completely changed our picture of what constitutes matter. They provide us with explanations for the regularities observed in the periodic table of elements, as well as for properties such as radioactive decay and chemical bonding.
This might have been the end of the quest for the smallest constituents of matter. However, physicists kept discovering ‘new’ particles – which were not constituents of atoms – in cosmic rays and high-energy experiments. With each new particle, significant questions arose about how these discoveries fit together, and how many more were yet to be unearthed.
Physicists found themselves in a situation similar to one a century before, with the periodic table of elements. Those patterns were eventually explained by the structure of atoms. Here too, researchers observed striking patterns in the properties of newly discovered particles, which pointed to the presence of an underlying pattern. By assuming that these patterns would continue into unexplored areas, they predicted the existence of particles with certain properties. Their predictions turned out to be correct!
The pattern suggested that hadrons – meaning protons, neutrons and some similar particles – are in fact composite particles themselves, with a substructure consisting of quarks. These quarks are particles with intriguing properties that are very different from anything seen before. For instance, it is not possible to observe any single quark on its own. So how can we know that quarks exist, even though nobody has ever ‘seen’ one? The quark substructure of hadrons is inferred indirectly from analysing data from particle accelerator experiments.
Given the story so far, you may now be wondering: have we found the elementary constituents of matter here? Or will quarks be discovered to consist of even smaller particles? Can we ever possibly know whether we’ve reached the end of this quest?
Before delving deeper into the physics of elementary particles, let’s consider symmetry, which is a key plank in our current understanding of the universe’s elementary particles and their interactions.