# General concepts

Experiment. Even very young children perform rudimentary experiments in order to learn about the world.

An experiment is an orderly procedure carried out with the goal of verifying, refuting, or establishing the validity of a hypothesis. Controlled experiments provide insight into cause-and-effect by demonstrating what outcome occurs when a particular factor is manipulated. Controlled experiments vary greatly in their goal and scale, but always rely on repeatable procedure and logical analysis of the results. Theoretical physics. Theoretical physics is a branch of physics which employs mathematical models and abstractions of physical objects and systems to rationalize, explain and predict natural phenomena. This is in contrast to experimental physics, which uses experimental tools to probe these phenomena. The advancement of science depends in general on the interplay between experimental studies and theory.

In some cases, theoretical physics adheres to standards of mathematical rigor while giving little weight to experiments and observations. Units of measurement. For example, length is a physical quantity.

The metre is a unit of length that represents a definite predetermined length. When we say 10 metres (or 10 m), we actually mean 10 times the definite predetermined length called "metre". State of matter. Historically, the distinction is made based on qualitative differences in properties.

Matter in the solid state maintains a fixed volume and shape, with component particles (atoms, molecules or ions) close together and fixed into place. Matter in the liquid state maintains a fixed volume, but has a variable shape that adapts to fit its container. Physical quantity. A physical quantity (or "physical magnitude") is a physical property of a phenomenon, body, or substance, that can be quantified by measurement.[1] Extensive and intensive quantities An extensive quantity is equal to the sum of that quantity for all of its constituent subsystems; examples include volume, mass, and electric charge.

For instance, if an object has mass m1 and another has mass m2 then a system simply comprising those two objects will have a mass of m1 + m2. An intensive quantity is independent of the extent of the system; quantities such as temperature, pressure, and density are examples. Observation. Observation is the active acquisition of information from a primary source.

In living beings, observation employs the senses. In science, observation can also involve the recording of data via the use of instruments. Physical system. Complexity in physical systems The complexity of a physical system is equal to the probability of it being in a particular state vector.

If one considers a classical Newtonian ball situation with a number of perfectly moving physical bodies bouncing off the walls of a container, the system-state probability does not change over time. Light. The Sun is Earth's primary source of light. About 44% of the sun's electromagnetic radiation that reaches the ground is in the visible light range.

Visible light (commonly referred to simply as light) is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight.[1] Visible light is usually defined as having a wavelength in the range of 400 nanometres (nm), or 400×10−9 m, to 700 nanometres – between the infrared, with longer wavelengths and the ultraviolet, with shorter wavelengths.[2][3] These numbers do not represent the absolute limits of human vision, but the approximate range within which most people can see reasonably well under most circumstances.

Various sources define visible light as narrowly as 420 to 680[4][5] to as broadly as 380 to 800 nm.[6][7] Under ideal laboratory conditions, people can see infrared up to at least 1050 nm,[8] children and young adults ultraviolet down to about 310 to 313 nm.[9][10][11] Gravitation. Gravitation, or gravity, is a natural phenomenon by which all physical bodies attract each other.

It is most commonly recognized and experienced as the agent that gives weight to physical objects, and causes physical objects to fall toward the ground when dropped from a height. During the grand unification epoch, gravity separated from the electronuclear force. Gravity is the weakest of the four fundamental forces, and appears to have unlimited range (unlike the strong or weak force).