futures Only connect An elegant demonstration that the Universe is made of quantum graphs.

Greg Egan JACEY

. M. Forster’s famous advice to ‘Only connect!’ is beginning to look super- Efluous. A theory in which the building blocks of the Universe are mathematical structures — known as graphs — that do nothing but connect has just passed its first experimental test. A graph can be drawn as a set of points, called nodes, and a set of lines joining the nodes, called edges. Details such as the length and shape of the edges are not part of the graph, though: the only thing that distin- guishes one graph from another is the con- nections between the nodes. The number of edges that meet at any given node is known as its valence. In quantum graph theory (QGT) a quan- tum state describing both the geometry of space and all the matter fields present is built up from combinations of graphs. The theory reached its current form in the work of the Javanese mathematician Kusnanto Sarumpaet, who published a series of six papers from 2035 to 2038 showing that both general relativity and the Standard Model of particle physics could be seen as approxima- Join the dots: each edge of a quantum graph carries one quantum of area. tions to QGT. Sarumpaet’s graphs have a fascinating the network that intersect it. These edges can known as ‘dopant’ nodes, in analogy with the lineage, dating back to Michael Faraday’s be thought of as quantized ‘flux lines of area’, impurities added to semiconductors — can notion of ‘lines of force’ running between and in quantum gravity area and other geo- persist under the Sarumpaet rules if they are electric charges, and William Thomson’s metric measurements they take on a discrete arranged in special patterns: closed, possibly theory of atoms as knotted ‘vortex tubes’. spectrum of possible values. It then makes knotted chains of alternating valence. These More recent ancestors are Roger Penrose’s sense to quantize the topology as well, with loops of dopant nodes, classified by their spin networks, trivalent graphs with each the nodes and edges of the network replacing symmetries and mutual interactions, match edge labelled by a half-integer, correspond- the usual idea of space as a continuum of up perfectly with the particles of the Stan- ing to a possible value of the spin of a quan- points. dard Model. tum particle. Penrose invented these net- In the first decades of the new millenni- Since the area associated with the edges of works in the early 1970s, and showed that the um, John Baez, Fotini Markopoulou, José- a quantum graph is of the order of a few set of all directions in space could be generat- Antonio Zapata and others did ground- square Planck lengths, some 10ǁ50 times the ed from simple, combinatorial principles by breaking work on the possible dynamical surface area of a hydrogen atom, it was once imagining an exchange of spin between two laws for spin networks, assigning quantum feared that QGT would remain untestable parts of a large network. amplitudes to the process of one network for centuries. However, in 2043 computer Generalizations of spin networks later evolving into another. In the 2030s, simulations identified a new class of ‘poly- appeared in certain kinds of quantum field Sarumpaet began to synthesize these results mer states’: long, open chains of dopant theory. Just as a wavefunction assigns an into a new model, based on graphs of arbi- nodes that were predicted to have energies amplitude to every possible position of a par- trary valence with unlabelled edges. and half-lives within the grasp of current ticle, a spin network embedded in a region of The geometry of three-dimensional technology to create and detect. space can be used to assign an amplitude to space arises from tetravalent graphs (pic- A search for polymer states that com- every possible configuration of a field. The tured), with the four edges emerging from menced at the Orbital Accelerator Facility in quantum states defined in this way consist of each node giving area to the faces of a ‘quan- 2049 has now yielded its first success (see J. lines of flux running along the edges of the tum tetrahedron’. Allowing graphs of higher Quant. Graphs 15, 7895–7899; 2050). If the network. valence runs the risk of producing an result can be repeated, Sarumpaet’s graphs In the 1990s, Lee Smolin and Carlo Rovel- explosion of unwanted dimensions, but will shift from being merely the most elegant li discovered an analogous result in quantum Sarumpaet found a simple dynamical law description of the Universe, to being the gravity, where spin-network states have a which always leads to the average valence most likely one. ■ simple geometric interpretation: the area of stabilizing at four. However, trivalent and Greg Egan is a science fiction writer. His latest book is any surface depends entirely on the edges of pentavalent nodes — which have come to be Teranesia (Gollancz). © 2000 Macmillan Magazines Ltd NATURE | VOL 403 | 10 FEBRUARY 2000 | www.nature.com 599