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Dark Matter  

Timothy Sumner

Dark matter is one of the most fundamental and perplexing issues of modern physics. Its presence is deduced from a straightforward application of Newton’s theory of gravity to astronomical systems whose dynamical motion should be simple to understand. The success of Newton’s theory in describing the behavior of the solar system was one of the greatest achievements of the 18th century. Its subsequent use to deduce the presence of a previously unknown planet, Neptune, discovered in 1846, was the first demonstration of how minor departures from its predictions indicated additional mass. The expectation in the early 20th century, as astronomical observations allowed more distance and larger celestial systems to be studied, was that galaxies and collections of galaxies should behave like larger solar systems, albeit more complicated. However, the reality was quite different. It is not a minor discrepancy, as led to the discovery of Neptune, but it is extreme. The stars at the edges of galaxies are not behaving at all like Pluto at the edge of the solar system. Instead of having a slower orbital speed, as expected and shown by Pluto, they have the same speed as those much further in. If Newton’s law is to be retained, there must be much more mass in the galaxy than can be seen, and it must be distributed out to large distances, beyond the visible extent of the galaxy. This unseen mass is called “dark matter,” and its presence was becoming widely accepted by the 1970s. Subsequently, many other types of astrophysical observations covering many other types of object were made that came to the same conclusions. The ultimate realization was that the universe itself requires dark matter to explain how it developed the structures within it observed today. The current consensus is that one-fourth of the universe is dark matter, whereas only 1/20th is normal matter. This leaves the majority in some other form, and therein lies another mystery—“dark energy.” The modern form of Newton’s laws is general relativity, due to Albert Einstein. This offers no help in solving the problem of dark matter because most of the systems involved are nonrelativistic and the solutions to the general theory of relativity (GR) reproduce Newtonian behavior. However, it would not be right to avoid mentioning the possibility of modifying Newton’s laws (and hence GR) in such a way as to change the nonrelativistic behavior to explain the way galaxies behave, but without changing the solar system dynamics. Although this is a minority concept, it is nonetheless surviving within the scientific community as an idea. Understanding the nature of dark matter is one of the most intensely competitive research areas, and the solution will be of profound importance to astrophysics, cosmology, and fundamental physics. There is thus a huge “industry” of direct detection experiments predicated on the premise that there is a new particle species yet to be found, and which pervades the universe. There are also experiments searching for evidence of the decay of the particles via their annihilation products, and, finally, there are intense searches for newly formed unknown particles in collider experiments.

Article

Electromagnetism and Electrodynamics in the 19th Century  

Chen-Pang Yeang

Electromagnetism and electrodynamics—studies of electricity, magnetism, and their interactions—are viewed as a pillar of classical physics. In the 1820s and 1830s, Ampère founded electrodynamics as the science of mechanical forces associated with electric currents, and Faraday discovered electromagnetic induction. By the mid-19th century, Neumann, Weber, and others in Germany had established an electrical science that integrated precision measurements with a unified theory based on mathematical potential or forces between electrical corpuscles. Meanwhile, based on Faraday’s findings in electrolysis, dielectrics, diamagnetism, and magneto-optic rotation, Faraday and Thomson in Britain explored a theory of the electromagnetic field. In the 1850s and 1860s, Maxwell further developed the Faraday–Thomson field theory, introduced the displacement current, and predicted the existence of electromagnetic waves. Helmholtz’s reworking of these Maxwellian insights led to Hertz’s discovery of electric waves in 1887.

Article

The Emergence of Modern Cosmology  

Helge Kragh

The term modern cosmology primarily refers to the developments concerned with the expansion of the universe, its origin billions of years ago, and the concept of dark matter. Similar to the history of any other area of science, the history of cosmology is rich in wrong theories and false trials. According to the simplest version of Brandon Carter’s anthropic principle, carbon-based life could not have originated in a universe evolving just slightly differently from the one observed. The present debate concerning the anthropic principle and its consequences is in some ways strikingly similar to the cosmological controversy of the past between the steady-state theory and relativistic evolution theories.

Article

The Evolution of Public Funding of Science in the United States From World War II to the Present  

Kei Koizumi

Large-scale U.S. government support of scientific research began in World War II with physics, and rapidly expanded in the postwar era to contribute strongly to the United States’ emergence as the world’s leading scientific and economic superpower in the latter half of the 20th century. Vannevar Bush, who directed President Franklin Roosevelt’s World War II science efforts, in the closing days of the War advocated forcefully for U.S. government funding of scientific research to continue even in peacetime to support three important government missions of national security, health, and the economy. He also argued forcefully for the importance of basic research supported by the federal government but steered and guided by the scientific community. This vision guided an expanding role for the U.S. government in supporting research not only at government laboratories but also in non-government institutions, especially universities. Although internationally comparable data are difficult to obtain, the U.S. government appears to be the single largest national funder of physics research. The U.S. government support of physics research comes from many different federal departments and agencies. Federal agencies also invest in experimental development based on research discoveries of physics. The Department of Energy’s (DOE) Office of Science is by far the dominant supporter of physics research in the United States, and DOE’s national laboratories are the dominant performers of U.S. government-supported physics research. Since the 1970s, U.S. government support of physics research has been stagnant with the greatest growth in U.S. government research support having shifted since the 1990s to the life sciences and computer sciences.

Article

From the Interpretation of Quantum Mechanics to Quantum Technologies  

Olival Freire Junior

Quantum mechanics emerged laden with issues and doubts about its foundations and interpretation. However, nobody in the 1920s and 1930s dared to conjecture that research on such issues would open the doors to developments so huge as to require the term second quantum revolution to describe them. On the one hand, the new theory saw its scope of applications widen in various domains including atoms, molecules, light, the interaction between light and matter, relativistic effects, field quantization, nuclear physics, and solid state and particle physics. On the other hand, there were debates on alternative interpretations, the status of statistical predictions, the completeness of the theory, the underlying logic, mathematical structures, the understanding of measurements, and the transition from the quantum to the classical description. Until the early 1960s, there seemed to be a coexistence between these two orders of issues, without any interaction between them. From the late 1960s on, however, this landscape underwent dramatic changes. The main factor of change was Bell’s theorem, which implied a conflict between quantum mechanics predictions for certain systems that are spatially separated and the assumption of local realism. Experimental tests of this theorem led to the corroboration of quantum predictions and the understanding of quantum entanglement as a physical feature, a result that justified the 2022 Nobel Prize. Another theoretical breakthrough was the understanding and calculation of the interaction of a quantum system with its environment, leading to the transition from pure to mixed states, a feature now known as decoherence. Entanglement and decoherence both resulted from the dialogue between research on the foundations and quantum predictions. In addition, research on quantum optics and quantum gravity benefitted debates on the foundations. From the early 1980s on, another major change occurred, now in terms of experimental techniques, allowing physicists to manipulate single quantum systems and taking the thought experiments of the founders of quantum mechanics into the labs. Lastly, the insight that quantum systems may be used in computing opened the doors to the first quantum algorithms. Altogether, these developments have produced a new field of research, quantum information, which has quantum computers as its holy grail. The term second quantum revolution distinguishes these new achievements from the first spin-offs of quantum mechanics, for example, transistors, electronic microscopes, magnetic resonance imaging, and lasers. Nowadays the applications of this second revolution have gone beyond computing to include sensors and metrology, for instance, and thus are better labeled as quantum technologies.

Article

Philosophical Issues in Thermal Physics  

Wayne C. Myrvold

Thermodynamics gives rise to a number of conceptual issues that have been explored by both physicists and philosophers. One source of contention is the nature of thermodynamics itself. Is it what physicists these days would call a resource theory, that is, a theory about how agents with limited means of manipulating a physical system can exploit its physical properties to achieve desired ends, or is it a theory of the basic properties of matter, independent of considerations of manipulation and control? Another source of contention is the relation between thermodynamics and statistical mechanics. It has been recognized since the 1870s that the laws of thermodynamics, as originally conceived, cannot be strictly correct. Because of fluctuations at the molecular level, processes forbidden by the original version second law of thermodynamics are continually occurring. The original version of the second law is to be replaced with a probabilistic version, according to which large-scale violations of the original second law are not impossible but merely highly improbable, and small-scale violations unpredictable, unable to be harnessed to systematically produce useful work. The introduction of probability talk raises the question of how we should conceive of probabilities in the context of deterministic physical laws.

Article

Physics-to-Technology Partnerships in the Semiconductor Industry  

Robert Doering

The development of physics over the past few centuries has increasingly enabled the development of numerous technologies that have revolutionized society. In the 17th century, Newton built on the results of Galileo and Descartes to start the quantitative science of mechanics. The fields of thermodynamics and electromagnetism were developed more gradually in the 18th and 19th centuries. Of the big physics breakthroughs in the 20th century, quantum mechanics has most clearly led to the widest range of new technologies. New scientific discovery and its conversion to technology, enabling new products, is typically a complex process. From an industry perspective, it is addressed through various R&D strategies, particularly those focused on optimization of return on investment (ROI) and the associated risk management. The evolution of such strategies has been driven by many diverse factors and related trends, including international markets, government policies, and scientific breakthroughs. As a result, many technology-creation initiatives have been based on various types of partnerships between industry, academia, and/or governments. Specific strategies guiding such partnerships are best understood in terms of how they have been developed and implemented within a particular industry. As a consequence, it is useful to consider case studies of strategic R&D partnerships involving the semiconductor industry, which provides a number of instructive examples illustrating strategies that have been successful over decades. There is a large quantity of literature on this subject, in books, journal articles, and online.

Article

Solar Cycle  

Lidia van Driel-Gesztelyi and Mathew J. Owens

The Sun’s magnetic field drives the solar wind and produces space weather. It also acts as the prototype for an understanding of other stars and their planetary environments. Plasma motions in the solar interior provide the dynamo action that generates the solar magnetic field. At the solar surface, this is evident as an approximately 11-year cycle in the number and position of visible sunspots. This solar cycle is manifest in virtually all observable solar parameters, from the occurrence of the smallest detected magnetic features on the Sun to the size of the bubble in interstellar space that is carved out by the solar wind. Moderate to severe space-weather effects show a strong solar cycle variation. However, it is a matter of debate whether extreme space-weather follows from the 11-year cycle. Each 11-year solar cycle is actually only half of a solar magnetic “Hale” cycle, with the configuration of the Sun’s large-scale magnetic field taking approximately 22 years to repeat. At the start of a new solar cycle, sunspots emerge at mid-latitude regions with an orientation that opposes the dominant large-scale field, leading to an erosion of the polar fields. As the cycle progresses, sunspots emerge at lower latitudes. Around solar maximum, the polar field polarity reverses, but the sunspot orientation remains the same, leading to a build-up of polar field strength that peaks at the start of the next cycle. Similar magnetic cyclicity has recently been inferred at other stars.