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In physics, a physical body or physical object (sometimes simply called a body or object) is a collection of matter with some common attributes, most important, the spatial location. Examples of models of physical bodies include, but are not limited to a particle, several interacting smaller bodies (particles or other), and continuous media.
The common conception of physical objects includes that they have extension in the physical world, although there do exist theories of quantum physics and cosmology which may challenge[how?] this. In modern physics, "extension" is understood in terms of the spacetime: roughly speaking, it means that for a given moment of time the body has some location in the space, although not necessarily a point. A physical body as a whole is assumed to have such quantitative properties as mass, momentum, electric charge, other conserving quantities, and possibly other quantities.
A body with known composition and described in an adequate physical theory is an example of physical system.
In classical physics, mechanics, quantum physics, and cosmology
A physical body is an object which can be described by the theories of classical mechanics, or quantum mechanics, and experimented upon with physical instruments. This includes the determination of trajectory of position through space, and in some cases of orientation [disambiguation needed] in space, over a duration of time, as well as means to change these, by exerting forces.
In classical physics, a physical body is a body with mass, not only energy, is three-dimensional (extended in 3-dimensions of space), has a trajectory of position and orientation in space, and is lasting for some duration of time. It is the subject of study in an experiment and is the object referred to in a law of physics, or physical theory. It can be considered as a whole, but may be composed of a collection of smaller physical bodies, e.g. a weight, ball, proton, or planet.
For instance, the force of gravity will accelerate a body if it is not supported, thus causing a change of its position (that is, it falls freely). However, it is not necessary for there to be forces present for an object position to change - only the rate of change of the object's position, that is, its velocity, will change under the influence of forces.
But in Quantum physics and Cosmology, there is a debate as to whether some elementary particles are not bodies, but are mere points without extension in physical space within space-time, or are always extended in at least one dimension of space as in String theory or M theory.
In some branches of psychology, depending on school of thought, a physical body is a physical object with physical properties, as compared to mental objects. In (reductionistic) behaviorism, a physical body and its properties are the (only) meaningful objects of study. While in the modern day behavioral psychotherapy it is still only the means for goal oriented behavior modifications, in Body Psychotherapy it is not a means only anymore, but its felt sense is a goal of its own. In cognitive psychology, physical bodies as they occur in biology are studied in order to understand the mind, which may not be a physical body, as in functionalist schools of thought.
A physical body is an enduring object that exists throughout a particular trajectory of space and orientation over a particular duration of time, and which is extended in the world of physical space, e.g. as studied by physics. This contrasts with abstract objects such as mathematical objects which do not exist at any particular time or place. Examples are a cloud, a human body, a weight, a billiard ball, a table, or a proton. This is contrasted with abstract objects such as mental objects, which exist in the mental world, and mathematical objects. Other examples that are not physical bodies are emotions, the concept of "justice", a feeling of hatred, or the number "3". In some philosophies, like the Idealism of George Berkeley, a physical body is a mental object, but still has extension in the space of a visual field.
In new age philosophy, mysticism and religion
In some systems of mysticism, such as Theosophy, the physical body is understood as the last of several progressively denser "vehicles of consciousness". In Blavatskyian Theosophy it is called by the Vedantic name sthula sarira - "gross body" - and distinguished from the linga sarira, the "subtle body" or astral double. In C. W. Leadbeater and Alice Bailey, the physical body is distinguished from the etheric body, which serves as its "blueprint", and structures of the etheric body, such as chakras, are mirrored in the main glands and nerve ganglia of the physical body.
In some religions, and in some new age philosophies, a physical body is contrasted with the self, mind, spirit, soul, or astral projection, and sometimes with an heavenly body. It is ephemeral in time, not eternal. It may be what houses the spirit or soul, and it is what is left behind in an astral projection, or ascention into heaven. A physical body exists on earth, not in heaven, not in the astral world, nor in the aether.