The Physics of Magnetic Monopoles - with Felix Flicker

The speaker begins by expressing gratitude and introducing the topic of the evening: the search for magnetic monopoles, or a North Pole without a South Pole, in materials known as spin ices. This quest is not about the geographical North Pole but a fundamental particle that has been theorized since the 19th century. The speaker ties the modern search to the historical work of Michael Faraday at the Royal Institution and sets the stage for an adventure in understanding these phenomena.

The speaker delves into the nature of magnets, explaining that traditionally, a magnet consists of two poles: North and South. The speaker challenges the audience to consider the possibility of isolating one pole, which is theoretically impossible based on our current understanding of magnetism. The explanation is given that all magnets are made up of magnetic domains, each with both poles, and even at the atomic level, the magnetic fields of electrons (spins) have both North and South. The speaker also demonstrates the connection between electricity and magnetism, as shown by Faraday's experiments, and suggests that magnetic monopoles could exist based on the symmetry between electric and magnetic fields.

The speaker discusses the ongoing efforts to detect magnetic monopoles, which include building detectors and smashing particles together at high energy, such as at CERN. The unique signature of a magnetic monopole passing through a coil of wire, as predicted by Faraday's laws, is a persistent current. The speaker also touches on the theoretical reasons to believe in the existence of magnetic monopoles, including the symmetry in the laws of physics, modern theories like string theory, and the work of Paul Dirac, which links the quantization of electric charge to the existence of even a single magnetic monopole.

The speaker explains the phenomenon of quantization of electric charge, where the charge on any object is always an integer multiple of the electron's charge. This quantization is a fundamental aspect of quantum mechanics. The speaker connects this to the potential existence of magnetic monopoles, as proposed by Paul Dirac, suggesting that the presence of even one magnetic monopole could explain why electric charges are quantized. Despite extensive searches, no magnetic monopoles have been found, but the theoretical reasons for their potential existence remain compelling.

The speaker shifts the focus to condensed matter physics, where emergent properties of complex systems are studied rather than fundamental particles. Using the analogy of light being described by photons in quantum theory and sound by phonons, the speaker illustrates how emergent particles can be useful in explaining phenomena without being fundamental. The concept of fractionalization is introduced, where fundamental properties break up into fractions of themselves, leading to the possibility of creating magnetic monopoles as emergent phenomena within a material.

The speaker uses the mutilated chessboard problem to demonstrate how, with enough pieces, it's possible to separate like charges (black squares) from unlike charges (white squares), even though it seems they should always come in pairs. This concept is applied to magnets, where flipping a magnet can create a region of concentrated North or South poles, which can then be separated. The speaker suggests that these emergent monopoles can only exist within the material and are not fundamental particles.

The speaker introduces dysprosium titanate, a crystal that becomes a 'spin ice' material at extremely low temperatures. In this state, the material's magnetic ions form a lattice with a structure similar to that of ice, giving rise to the name 'spin ice.' The speaker explains how, at low temperatures, the spins of the magnetic ions can behave as if they are magnetic monopoles, creating a scenario where the monopoles can be experimentally observed and potentially controlled.

The speaker describes the experimental setup used to detect magnetic monopoles in spin ice. The experiment, led by Radhika Desai, involves using a Faraday coil wrapped around the spin ice sample and connected to a superconducting quantum interference device (SQUID) to measure magnetic flux. The speaker discusses the theoretical predictions and the experimental results, which show a signal consistent with the presence of magnetic monopoles in the material.

The speaker explains the concept of noise in the context of the SQUID measurements, where different types of noise (white, pink, red, and violet) represent different distributions of frequencies. The speaker discusses how the team predicted and detected a 'pinky red noise' signal from the spin ice, indicating the presence of magnetic monopoles. This audio signal, the first of its kind, provides strong evidence for the existence of magnetic monopoles in the material.

The speaker outlines potential applications and benefits of magnetic monopoles, including the development of more efficient computation methods, which could lead to advancements in technology and reduce the environmental impact of data servers. The speaker also suggests that the experimental techniques developed for detecting magnetic monopoles could improve the efficiency of MRI scanning, making it less intrusive and less expensive. The speaker concludes by thanking the team and the audience for their contributions and attention.