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Units are at the heart of science and technology. Most measurements require units to express their result. The International System of Units (SI) provides a widely used coherent framework of units. While the present SI is fit for purpose for all measurements, it has several shortcomings. Most notably, the kilogram is defined via an artifact that limits its realization to a specific place and certain times. To solve this problem and to take advantage of advances in quantum metrology made in the last 50 years, a revision of the SI is planned. This revision will modify the foundation of the SI from base units to fundamental constants of nature. While the revision presents a huge shift in philosophy, the size of the units will not change, and hence the revision will go mostly unnoticed.

Rydberg physics is the land of gentle giants--highly-excited electronic states where an electron is, on average, far from the nucleus. They are created with a lot of energy, have unusually large size and live relatively long. They are useful because they are extremely sensitive to their environment; so much so that a single photon of light can change the behaviour of not only one, but dozens or hundreds of them, over distances we can easily see under an optical microscope. Wherever Rydberg states are created--in atoms, molecules or solids--they all share common features that are exemplified by the physical description of highly-excited hydrogen atoms, with energy levels described by the Rydberg formula.

Space weather--changes in the Earth's environment that can often be traced to physical processes in the Sun--can have a profound impact on critical Earth-based infrastructures such as power grids and civil aviation. Violent eruptions on the solar surface can eject huge clouds of magnetized plasma and particle radiation, which then propagate across interplanetary space and envelop the Earth. These space weather events can drive major changes in a variety of terrestrial environments, which can disrupt, or even damage, many of the technological systems that underpin modern societies. The aim of this book is to offer an insight into our current scientific understanding of space weather, and how we can use that knowledge to mitigate the risks it poses for Earth-based technologies. It also identifies some key challenges for future space-weather research, and considers how emerging technological developments may introduce new risks that will drive continuing investigation.