Science theories are not hunches or guesses, but all have been subjected repeated testing and verification. All scientific theories are subject to change as new evidence comes to be accepted by all scientists. Students should develop an understanding of the basic theories that are foundational in science and which guide scientific understanding. A few current scientific theories are listed below:
Atomic Theory
This theory states that the atom is the smallest unit of matter. The atom is composted of the nucleus in the middle of the atom that is composed of neutrons and protons (both of these may break down into smaller particles). The neutrons have no charge. The protons have a positive charge. Surround the nucleus are electrons that swirl around the nucleus in a large region, rather than orbiting in a fixed pattern (electron cloud). The electrons have a negative charge. Many modern scientists have contributed to our current view of the atom in which the electrons swirl around the nucleus in a large region (electron cloud), rather than orbiting in a fixed pattern. Plus modern quantum theory shows how atomic particles such as electrons may also be seen as having wavelike properties (see below).
Big Bang Theory
Big bang theory assumes that the universe began from a singular state of infinite density and expanding from an explosive moment of creation. The theory was further developed in the 1940s by George Gamow and R. A. Alpher. Fred Hoyle coined the term Big Bang. The Big Bang Theory is the dominant scientific theory about the origin of the universe. According to the big bang, the universe was created sometime between 10 billion and 20 billion years ago from a cosmic explosion that hurled matter and in all directions.
Gravity Theory
Gravitation, or gravity, is a force that attracts all objects in the universe. The most familiar of the four fundamental interactions of matter, gravitation has several characteristics that distinguish it from the other interactions: (1) It is universal; (2) It is always attractive; (3) It is a long-range interaction; and (4) It is a long-range attractive force affecting all matter.
Evolution Theory
Evolution theory says that all living things are related to one another through common ancestry from earlier forms that differed from the present forms. Exactly how evolution occurs is still a matter of debate, but that it occurs is a scientific fact. Biologists agree that all living things arose through a long history of changes shaped by physical and chemical processes that are still taking place. According to the theory, variability among individuals in a population of sexually reproducing organisms is produced by mutation and genetic recombination. The resulting genetic variability is subject to natural selection in the environment.
Cell Theory
The cell theory of life states: (1) All living material is made up of cells; (2) All cells are derived from previously existing cells; most cells arise by cell division, but in sexual organisms they may be formed by the fusion of sperm and egg; (3) A cell is the most elementary unit of life; (4) Every cell is bounded by a plasma membrane, an extremely thin skin separating it from the environment and from other cells; (5) All cells have strong biochemical similarities; and (6) Most cells are small, about 0.001 cm (0.0004 in) in length; for example, the smallest cells of the microorganism mycoplasma are 0.3 micrometers in size, whereas some giant algae cells may be several centimeters long.
The Germ Theory of Disease
French bacteriologist Louis Pasteur is considered the founder of microbiology. Pasteur argued that infectious diseases were caused by germs. The germ theory has affected our views on infectious disease, surgery, hospital management, agriculture, and industry.
Relativity Theories
Albert Einstein's theory of special relativity, published in 1905, revealed that energy and matter are different manifestations of the same phenomenon and can be transformed into each other in terms of the relationship E = mc 2 . Einstein's theory of general relativity, published in 1917, provided a powerful new way to view gravity as a warping of the four-dimensional space-time continuum by the presence of matter. If space-time is imagined as a rubber sheet, then massive objects such as stars and galaxies create deformations in space-time, just as a bowling ball sitting on a mattress creates a dent into which nearby smaller objects fall. Thus the shape of space-time determines the behavior of matter/energy. At the same time, the presence of matter/energy determines the shape of space-time.
Plate Tectonics Theory
Plate tectonics is an all-embracing theory that the Earth is divided into a number of rigid plates floating on a viscous underlayer in the mantle. Alfred L. Wegener was the first to propose in 1912 that the continents were at one time connected and had drifted apart. In 1960 when H. H. Hess suggested that new ocean floor was created at the mid-oceanic ridges and that the ocean evolved by seafloor spreading.
Quantum Theory
This theory that says that energy exists in tiny discrete units called quanta. Just as earlier theory showed how light, generally seen as a wave motion, could also in some ways be seen as composed of discrete particles (photons), quantum theory shows how atomic particles such as electrons may also be seen as having wavelike properties. Quantum theory is the basis of particle physics, modern theoretical chemistry, and the solid-state physics that describes the behavior of the silicon chips used in computers. Quantum theory and the theory of relativity together form the theoretical basis of modern physics. Later work by scientists elaborated the theory into what is called quantum mechanics (or wave mechanics).
Unified Field Theory
This theory proposes to unify the four known interactions, or forces– the strong, electromagnetic, weak, and gravitational forces– by a simple set of general laws. These four distinct forces control all the observed interactions in matter: gravitation, electromagnetism, the strong force (force that holds atomic nuclei together), and the weak force (force present in some nuclear processes).
