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The curve formed by an ideal heavy uniform string hanging freely from two points of support. The lowest point A (Fig. 1) is the vertex. The portion AP is an equilibrium under the horizontal tension H at A, the tension F directed along the tangent at…

Cameras in mobile phones or smartphones that can capture images and videos.The cellphone, the world’s favorite must-have gadget, has been with us for nearly a half-century, while experiencing continuous and often phenomenal improvements. Much of the…

A thermodynamic law, also known as Gay-Lussac's law, which states that at constant pressure the volume of a fixed mass or quantity of gas varies directly with the absolute temperature. As such, Charles' law, V = kT, describes how gases expand when…

An experiment at the Large Hadron Collider (LHC), the world's largest particle accelerator, has revealed the first evidence for a phenomenon known as charge-parity (CP) violation in the decays of subatomic particles called charm quarks. The results…

The force that holds atoms together in molecules and solids. Chemical bonds are very strong. To break one bond in each molecule in a mole of material typically requires an energy of many tens of kilocalories.It is convenient to classify chemical…

The process of producing the illusion of a moving picture. Cinematography includes two phases: taking the picture with a camera and showing the picture with a projector. The camera captures the action by taking a series of still pictures at regular…

The curve that is the locus of points in a plane with equal distance from a fixed point. One of the most basic geometric figures, a circle is perfectly round. In elementary mathematics, "circle" often refers to the finite portion of the plane…

Well-defined addition compounds formed by inclusion of molecules in cavities existing in crystal lattices or present in large molecules. The constituents are bound in definite ratios, but these are not necessarily integral. The components are not…

Any interaction between particles, aggregates of particles, or rigid bodies in which they come near enough to exert a mutual influence, generally with exchange of energy. Broadly speaking, interactions can occur between particles that are simply…

The principle stating that energy cannot be created or destroyed, although it can be changed from one form to another. The law of conservation of energy has been established by a multitude of meticulous measurements of gains and losses of all known…

Charged, energetic particles, mostly hydrogen and helium nuclei, that travel at nearly the speed of light through space and bombard Earth from all directions. Cosmic rays are generated in astrophysical environments, ranging from the Sun locally to…

The science and technology of phenomena and processes at low temperatures, defined generally and arbitrarily as below 150°C (123 K or −238°F). Numerous novel and interesting phenomena occur at cryogenic temperatures. These include liquefaction (Fig.…

Superposition is a confounding yet foundational principle of quantum physics in which a quantum system—for example, a group of subatomic particles or atoms—can exist in multiple quantum states simultaneously. For example, consider a coin that is…

The mass of an object divided by its volume. Density is conventionally represented by the Greek letter rho: ρ. The most common units for density are g/cm3 or specific gravity, which is dimensionless. The modern SI units of kg/m3 are increasingly…

A condition in space in the vicinity of an electrically charged body such that the forces due to the charge are detectable. An electric field (or electrostatic field) exists in a region if an electric charge at rest in the region experiences a force…

A material that blocks the flow of electric current across it. Insulators are to be distinguished from electrolytes, which are electronic insulators but ionic conductors. Electrolyte Ionic crystalsElectric insulators are used to insulate electric…

A range of physical phenomena that involve electric charges, their motions, and their effects. In conventional usage, the term “electricity” often refers to the energy that powers lighting, appliances, and other electronic devices in homes and…

A burst of electromagnetic energy. In a military context—and the subject of this article—an electromagnetic pulse (EMP) is a transient electromagnetic signal produced by a nuclear explosion in or above Earth's atmosphere (Fig. 1). Though not…

Electromagnetic field fluctuations that form quantized traveling waves and the manner in which they are created. Electromagnetic radiation is also referred to as electromagnetic waves, photons, and light (Fig. 1). Electromagnetic radiation consists…

The interaction among all electrically charged objects, objects with magnetic moments, and the electromagnetic field. The electromagnetic interaction is one of the four fundamental interactions of the universe. This interaction encompasses all…

A fundamental force representing unification of the electromagnetic and weak nuclear interactions. Until the early 1970s, the four fundamental forces — strong nuclear, electromagnetic, weak nuclear, and gravity — could only be described as distinct…

Fundamental, irreducible units that constitute the matter of the universe and express the forces of nature. The standard model of particle physics, which emerged in the 1970s, classifies and describes the 17 known elementary particles in terms of…

The energy changes that accompany biochemical reactions. Energy metabolism, or bioenergetics, is concerned with the chemical reactions required for energy transformations within living cells. Animals, plants, and bacteria require energy to sustain…

In a breakthrough for astronomy and physics, the Event Horizon Telescope (EHT) has obtained a direct image of a black hole. The first black hole captured in this manner is a supermassive one at the heart of a nearby galaxy, Messier 87. Further study…