Waves in water and magnetic monopoles in spin ice are examples of the whole being potentially more than merely the sum of its parts. “Emergence” is the term that scientists use for the appearance of new qualities when a large number of discrete elements combine to form a new unit. An example is the combination of many individual cells to form a multicellular living organism. Solid-state physicists know of numerous emergent phenomena, such as sound waves in crystals. Many of these collective phenomena do not constitute elementary particles in the usual sense; like the discrete magnetic poles in spin ice, however, they have certain properties in common with such particles. For this reason, they are also referred to as “quasiparticles”.
The magnetic monopoles are a class of quasiparticles that particularly fascinate physicists, while at the same time presenting them with major puzzles. “Quasiparticles exist that appear to be fragments of elementary particles that are normally considered to be indivisible,” says Moessner. The electron, for example, is defined by, among other things, two properties: its negative elementary charge and its magnetic moment of a certain strength. These two properties appear as inseparable as the two sides of a coin.
In solids, however, quasiparticles can in principle occur that, although they possess the magnetic moment of an electron, carry no charge (or vice-versa). In other words, they can be compared to one-sided coins. Against this background, it no longer seems absurd for spin ice to contain magnetic monopoles.
The deconstruction of the elementary particles in the solid does not stop here: at very low temperatures and in strong external magnetic fields, quasiparticles can even occur at interfaces between two parts of the solid that bear a fraction – for example a third – of the elementary charge that is recognized as being indivisible.
In complex physical systems, with their many billions of interacting atomic nuclei and electrons, laws therefore apply that cannot be derived directly from the fundamental formulae of elementary particle physics. Such findings suggest that some materials can be regarded, in a sense, as miniature worlds that effectively create their own laws of nature.
The Dresden-based scientist is not the only one with this view. As early as 1972, American physicist Philip W. Anderson warned in the journal Science of the consequences of the reductionist research philosophy then prevailing. This philosophy assumes that things can be understood by being broken down to their smallest constituent parts. This is the route that particle physicists were taking at the time.
Anderson countered that reductionism would never be able to deliver a complete explanation of macroscopic phenomena, such as solid-state materials, plants, human beings or the universe.
He concluded his paper by saying: “The whole is not only more than, but also very different from, the sum of its parts.” In order to understand the behavior of large and complex conglomerates of elementary particles, research must be conducted that is no less fundamental than the study of the smallest particles themselves.