The idea that loads on the Earth’s surface may be regionally rather than locally supported can be traced back to Gilbert and Barrell’s work at the turn of the twentieth century. Their work was carried out at a time when the U.S. Coast and Geodetic Survey promoted the use of local, rather than regional, models of isostasy. The geodesists, with the exception of Putnam, showed that regional models of isostasy were not required to explain geodetic data.

While Gilbert perhaps was influenced by the geodesists, Barrell was not. Barrell challenged the geodesists’ conclusions concerning local models of isostasy, invoking instead the idea that topography was supported regionally by the lateral strength of the lithosphere. Following Barrell’s death, there was a vacuum, and the new ideas of regional isostasy gradually gave way again to local models of isostasy, which continued to be championed by the geodesists, notably Hayford, Bowie and Heiskanen

The history thereafter is a punctuated one with “bursts” of activity followed by long periods of quiet. The work centred on the activities of a few individuals: Vening Meinesz, Gunn and Walcott. Vening Meinesz and Gunn were contemporaries but there is little evidence of any contact between them.

Abstract: In this chapter, I discuss an alternative perspective on interpreting the results of joint and constrained inversions of geophysical data. Typically such inversions are performed based on inductive reasoning (i.e. we fit a limited set of observations and conclude that the resulting model is representative of the Earth). While this has seen many successes, it is less useful when, for example, the specified relationship between different physical parameters is violated in parts of the inversion domain. I argue that in these cases a hypothesis testing perspective can help to learn more about the properties of the Earth. I present joint and constrained inversion examples that show how we can use violations of the assumptions specified in the inversion to study the subsurface. In particular I focus on the combination of gravity and magnetic data with seismic constraints in the western United States. There I see that high velocity structures in the crust are associated with relatively low density anomalies, a possible indication of the presence of melt in a strong rock matrix. The concepts, however, can be applied to other types of data and other regions and offer an extra dimension of analysis to interpret the results of geophysical inversion algorithms.

One important area where radio astronomers confirmed theoretical predictions was in tests of General Relativity. Radio interferometer measurements made during the 1970s were able to confirm Einstein’s prediction of the gravitational bending of light to an accuracy better than 1 percent, or an order of magnitude better than the controversial classical optical tests made during the time of a solar eclipse. In 1965, MIT Professor Irwin Shapiro suggested and subsequently confirmed a new fourth test of General Relativity resulting from the excess delay of the reflected radar signal from a planet as the signal passes close to the Sun. Radio observations have also found Einstein’s “gravitational lensing” by which a massive cluster of galaxies can form multiple radio images of a background galaxy or quasar. Observations of small periodic deviations in the time of arrival of pulsar pulses at the Arecibo Observatory led Princeton University graduate student Russell Hulse and his supervisor Joe Taylor to the 1993 Nobel Prize in Physics for the first experimental evidence for the predicted existence of gravitational radiation.

Gravity has an irresistible grip on our curiosity and is able to drive our imagination to completely different theoretical spaces. This very fact alone sets gravity aside from all other types of physical interactions we know. Indeed, gravity is the only physical interaction of which we have a conscious experience and this awareness is with us every second of our life. In this book we set out to try to address the question: '…what is gravity and how does gravity actually work?'. This book is meant as a guide in a journey that will take us from our basic understanding of gravity, the one that is deeply coded in our brains even at an instinctive level, to the more physically detailed and yet incorrect description provided by Newton’s theory of gravity. The journey will then lead us to the mathematically beautiful and physically profound description that Einstein has proposed with his 'general theory of relativity', and that is elegantly embodied in his field equations.

The view of physics that is most generally accepted at the moment is that one can divide the discussion of the universe into two parts. First, there is the question of the local laws satisfied by the various physical fields. These are usually expressed in the form of differential equations. Secondly, there is the problem of the boundary conditions for these equations, and the global nature of their solutions. This involves thinking about the edge of spacetime in some sense. These two parts may not be independent. Indeed it has been held that the local laws are determined by the large scale structure of the universe. This view is generally connected with the name of Mach, and has more recently been developed by Dirac (1938), Sciama (1953), Dicke (1964), Hoyle and Narlikar (1964), and others. We shall adopt a less ambitious approach: we shall take the local physical laws that have been experimentally determined, and shall see what these laws imply about the large scale structure of the universe.

This chapter is a broad outline of the timeline of the Big Bang and the formation of the universe and solar system. It discusses the relationship between energy and matter, and how elements are formed from simpler elements due to the application of energy and gravity. This chapter emphasizes the large scales of time and space, and also how energy and matter are distributed across the vastness of universe. Finally, it discusses our solar system and similarities to other distant solar systems with planets in the ‘habitable zones’, and the potential for life outside Earth.

This chapter guides the reader through three related enigmas of modern physics. The first is a mystery of quantum mechanics. Important aspects of quantum mechanics are still not truly understood, although competing theories have been proposed, including the Many-Worlds approach. The second enigma is the emergence of spacetime, and especially the way it interacts with gravity. Rather than following the traditional methodology of ‘quantising’ classical theories, the author proposes an alternative approach and instead seeks gravity within quantum mechanics. The chapter concludes with a discussion of the mystery of the arrow of time: what distinguishes the past from the future. Together, these three mysteries of modern physics serve as an important reminder of the endurance of enigmas in the very foundations of scholarly fields. Re-examining founding principles can provide a constructive alternative means of investigating mysteries, not only in modern science but also across other disciplines.

This chapter examines how a cyberattack that has consequences similar to a kinetic or physical attack – causing serious loss of life or physical damage – could be encompassed within the crimes that may be prosecuted before the International Criminal Court (“ICC”). While it is a very limited subset of cyber operations that might fall within the ambit of the ICC’s Rome Statute, the chapter explains when and how a cyberattack could constitute genocide, crimes against humanity, war crimes, or the crime of aggression. The chapter additionally acknowledges limitations as to which attacks would be encompassed, given, particularly, the ICC’s gravity threshold as well as the hurdle of proving attribution by admissible evidence that could meet the requirement of proof beyond a reasonable doubt. Notwithstanding such limitations, increased awareness of the largely previously overlooked potential of the Rome Statute to cover certain cyberattacks could potentially contribute to deterring such crimes and to reaching the goal of a state of “cyber peace.”

This piece takes the reader on a virtual trip through the Earth System, using visualisations as a key tool to understand our home planet and our evolving knowledge about it. It begins and ends with trips between the Earth’s surface and space, experiencing the Earth System from above. In between, however, we take an historical tour of the planet, tracking the evolution of humanity’s growing understanding of the Earth and how it functions as a system. The tour wanders through the evolving scientific landscape, beginning with the famous ‘Blue Marble’ image of Earth taken by the Apollo 11spacecraft in 1972, continuing with James Lovelock’s Gaia hypothesis and John Schellnhuber’s second Copernican revolution, and finishing with Paul Crutzen’s confronting assertion that the Earth System is now in the Anthropocene, leaving humanity’s comfort zone of the Holocene as a receding memory.

Geophysical data from Dawn’s mission revealed complex and divergent internal structure evolutionary paths for Vesta and Ceres. Dawn’s data indicated that Vesta has a differentiated internal structure with uncompensated topography and Ceres is partially differentiated with compensated topography. Vesta experienced a magma ocean state, leading to effective early shape relaxation. Vesta’s current non-hydrostatic shape is dominated by Rheasilvia and Veneneia impact basins, formed when Vesta was too rigid to relax. However, northern terrains still reflect its pre-impact, closer-to-hydrostatic shape. Ceres incorporated abundant volatile material upon its accretion and subsequently underwent ice–rock fractionation. Observed surface aqueous alteration indicates extensive past hydrothermal circulation that facilitated efficient heat transfer and preserved Ceres’ interior in a relatively cool state. Lower viscosities at depth allowed isostatic compensation of Ceres’ long-wavelength topography. The high inferred abundance of water ice, hydrated salts, and/or clathrate phases suggest previous globally significant regions of solute-rich fluids that froze from the surface inward, leading to the vertical density gradient inferred from Dawn’s Second Extended Mission (XM2) high-resolution gravity data. This, coupled with thermal modeling, indicated that Ceres could have brine reservoirs, at least regionally, which were likely mobilized by the Occator crater-forming impact, leading to long-lived brine extrusion and faculae formation.

As an appendix, we can look briefly at the central ideas of General Relativity (though we are limited, since much of the maths is beyond our scope). We prepare the ground with a number of thought experiments, and then discuss, in outline, the geometrical ideas we have to use. We can get a sense of what Einstein's equation is doing, and we look at some solutions of Einstein's equation (including the Schwarzschild metric), describing possible spacetimes.

Motion in curved spacetime, classical equations in covariant form, tidal forces, Einstein’s field equation in empty space, and weak (linearized) gravitation.

It has long been recognized that astronomy was a catalyst of the Scientific Revolution, spurring on deeply consequential speculation about the nature of the cosmos and its physical principles. Yet the history of celestial physics is far richer than was thought a generation ago, and there is much to be learned about the origins of the field, particularly in the sixteenth century, when humanist activity brought forth a dazzling array of philosophical possibility—from reconsiderations of Aristotle and Islamicate commentary to the revival of Platonic, Epicurean, and Stoic worldviews. Celestial physics offered some of the most heated arguments for or against the Aristotelian cosmos, with controversial attempts to account for astronomical observation by integrating various causal innovations. This chapter will focus on a number of themes that mark celestial physics and cosmological speculation in the sixteenth and early seventeenth centuries: the order of the celestial bodies and their nature, the relationship between celestial and terrestrial things, the question of celestial animism or vitalism, and the status of the divine in celestial nature.

The application of gravity gradient measurements to exploration has been growing over the past 20 years. The ability of tensor gradiometry instruments to greatly improve signal/noise when deployed on mobile platforms has transformed the usefulness of this technology. Airborne and marine Full Tensor Gradiometry (FTG) surveys have become an increasingly common part of the exploration and production toolkit. The ability of the modern instruments to provide high-resolution, spatial accuracy and very good signal/noise data has made this technology a more common part of integrated exploration and production management. The technology has a distinct cost advantage over seismic data acquisition and as such can deliver a competitive solution for imaging problems in some circumstances. There are now numerous published examples of effective use of FTG in the oil industry. The development of better instruments such as integration of direct contemporaneous measurement of conventional gravity is encouraging more interest in the technology. The potential for extending the use of FTG to reservoir monitoring and carbon dioxide sequestration assurance is likely to increase the popularity of the technology in future.

The gravity and magnetic survey methods have been in use since the early days of geophysical prospecting for petroleum. They find most application in frontier exploration. In that context, regional and global datasets are often available to assist with early evaluations.

The design and execution of modern, targeted surveys has been transformed as a result of advances in instruments and the advent of satellite navigation. Imaging and interpretive techniques have been transformed by modern computer-based approaches. The potential field methods are extremely cost-effective at delineation of basins and determining structural controls on those basins, especially delineating normal faulting within rift basins. Magnetic surveys yield depth to basement and delineate any igneous rocks present. Such surveys therefore enable early decisions about cost-effective placement of seismic surveys and other intensive follow-ups.

In more mature exploration, gravity and gravity gradient data combine well with seismic data in distinguishing between alternate interpretations, thereby removing ambiguities. High-resolution magnetic data offer an effective means of fault connection in conjunction with regional seismic coverage, if shales or mudstones are present.

In a production environment, gravity logging is the most sensitive density log available, and 4D-gravity finds application in gas production and also water-flood monitoring.

Modelling of stresses that influence glacially triggered faulting has progressed substantially in the last decades with more complex models and improved modelling techniques, incorporation of a variety of relevant processes, better constraints of ice-loading history, higher model resolution and 3D geometries. Some recent developments are collected in this section to portray the scope and variability of numerical modelling relevant to glacially triggered faulting. These range from modelling of the general in situ stress field to studies on the stress field induced by glacial loading and unloading.

An appropriate estimation of the ambient background stress field is crucial for determining the effect of additional ice loading (or unloading) on pre-stressed faults. Contributions from local and far-field stress sources (topography, tectonics) need to be reconciled with in situ measurements from boreholes and fault-plane solutions from earthquakes. We describe the different types of stresses in glaciated regions with a focus on Scandinavia together with the techniques used to incorporate stresses into numerical models.