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We study the Black Hole search problem using mobile agents in three interconnected network topologies: hypercube, torus and complete network. We do so without relying on local storage. Instead we use a less-demanding and less-expensive token mechanism. We demonstrate that the Black Hole can be located with a minimum of two (2) co-located agents performing Θ(n) moves with O(1) tokens, in each of these three topologies. Then we study the Black Hole search problem with scattered agents. We show that the optimal number of moves can be achieved with the optimal number of mobile agents using O(1) tokens. [ABSTRACT FROM AUTHOR]

Gravity is the only truly universal force. It moulded the universe and it almost certainly will overwhelmingly determine the end of the universe. Today we also know (certainly believe) that gravitation is primarily responsible for the formation of the large structures we see around us: the Earth, the solar system, the stars, and the galaxies. Gravity has fashioned the beautiful and awe-inspiring sights in the sky, which have inspired both philosophers and mystics. Without gravity the sky would have been a very boring sight. Even more exotic objects, only visible at radio, X-ray or other energies, are present in the sky and these have also been fashioned by gravity. After the formation of the solar system, gravity has played a benign role in the evolution of life in the solar system. The strong gravity of Jupiter has shielded Earth from destructive impacts by comets and asteroids, and it is reasonable to say that life on Earth would not have survived without this ‘gravity shield’. It is for this reason that there is such excitement at the discovery of large planets around other stars. Life, as we know it, may not (almost certainly does not) exist on these large planets, but without such a large planet and its gravity shield, life certainly would not survive and flourish on an inner planet if there is one. In this chapter the central role of gravity in shaping the universe and even the climate on Earth is described. FIGURES OF PLANETS The sizes and the figures of the planets have been subjects of inquiry and discussion since antiquity. The Greeks accepted the Platonic belief in perfect shapes and believed that the planets, and the Earth in particular, were perfect spheres. [ABSTRACT FROM AUTHOR]

Gravity is beautifully successful in describing the universe at macroscopic level but at microscopic level it is totally inadequate. By contrast, quantum theory is highly successful in describing matter on small scales. The detailed understanding of chemical reactions, lasers, microchips and nuclear weapons is based entirely on quantum physics. The concept of an atom originated in Greek antiquity, but it was suppressed under the baleful influence of Plato and Aristotle. In the early nineteenth century John Dalton resurrected ‘atomism’. Dalton was a Quaker and was born in 1766 in Eaglesfield, Cumberland, in England. He recognised that atoms could help to understand the data being accumulated by chemical and physical experiments. Dalton's atoms were the smallest indivisible unit of a substance and they retained their chemical properties. He maintained that chemical reactions were just the rearrangement of these basic units of matter. Dalton presented his work in a two-volume thesis called New System of Chemical Philosophy, published between 1808 and 1827. This laid the basis of modern chemistry. Dalton's atoms were not accepted enthusiastically by all chemists of the early nineteenth century. However, it was soon realised that the chemists and physicists of the day had independently accumulated data which suggested the atomic nature of matter. In particular, physicists such as Maxwell and Boltzmann maintained that the pressure exerted by gas in a container could be explained if the gas was assumed to be a collection of hard spheres, like billiard balls, which bounced off the walls of a container in accordance with Newtonian mechanics. [ABSTRACT FROM AUTHOR]

Einstein was motivated by a deep philosophical need, the quest for simplicity and unity in nature, to formulate and develop the theory of general relativity. He was not guided by a desire to confirm or interpret any particular experimental result(s) although he was aware of the need for experimental confirmation. Experiments are fundamental to modern physics: progress in physics is driven by experimental verification and no assumption can be taken seriously unless it can be tested experimentally. This is the only way to distinguish physics from metaphysics. Galileo repeatedly stressed this and his experiments in the sixteenth century were able to overthrow the 2000-year reign of the speculative laws of nature proposed by Aristotle. Today a theory without experimental verification has no value. Unfortunately general relativity, unlike its contemporary, quantum theory, does not have a secure experimental foundation. Einstein had shown that the perihelion shift of Mercury could be explained by general relativity with remarkable accuracy. He also proposed the gravitational redshift and the bending of light rays as two further tests of general relativity. Gravitational redshift was too small to be observed with the technology of the first half of the twentieth century. Also, as will be discussed later, this is really a test of the equivalence principle and not of the full theory of general relativity. The bending of light was measured in 1919 but the accuracy of the data was low and not sufficient to discriminate between general relativity and the alternative theories of gravity proposed in the 1960s. Similarly, there was considerable uncertainty, until recently, about the oblateness of the Sun which affects the perihelion shift of Mercury. Remarkably only one new test of general relativity has been proposed since the formulation of the theory by Einstein. [ABSTRACT FROM AUTHOR]

For 200 years, from the beginning of the eighteenth century to the beginning of the twentieth century, Newtonian mechanics reigned supreme. By the late nineteenth century, the few simple laws of Newtonian physics could explain, with uncanny accuracy, most of the disparate phenomena of the natural world. Everything in the heavens and on Earth appeared to obey the laws, and the mastery of these laws was bringing mankind the mastery of the environment. These laws dominated the way both scientists and laypersons thought. Newtonian mechanics has a deterministic framework for the cosmos and this was deeply satisfying to the Judeo-Christian culture of Western Europe. There were those who questioned Newton's assumptions of absolute space and time, independent of man, but anyone doubting the validity of the laws of motion or gravitation was not taken seriously by the scientific community. Newton had applied his theory of gravitation to two-body systems, such as the Sun and a planet. In the eighteenth century various attempts were made to extend it to three gravitating bodies. In 1682 Halley had claimed that the comet then observed in the sky had also appeared in 1531 and 1607; given, then, that the period of the comet was about 75 years, he predicted that it would reappear in 1758. Months before its appearance, the French mathematician Alexis Clairaut used tedious and brute-force mathematics to calculate the gravitational perturbations of Jupiter and Saturn on the otherwise elliptical orbit of Comet Halley. [ABSTRACT FROM AUTHOR]