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A point particle (ideal particle〔H.C. Ohanian, J.T. Markert (2007), p. 3〕 or point-like particle, often spelled pointlike particle) is an idealization of particles heavily used in physics. Its defining feature is that it lacks spatial extension: being zero-dimensional, it does not take up space.〔F.E. Udwadia, R.E. Kalaba (2007), p. 1〕 A point particle is an appropriate representation of any object whose size, shape, and structure is irrelevant in a given context. For example, from far enough away, an object of any shape will look and behave as a point-like object. In the theory of gravity, physicists often discuss a point mass, meaning a point particle with a nonzero mass and no other properties or structure. Likewise, in electromagnetism, physicists discuss a point charge, a point particle with a nonzero charge.〔R. Snieder (2001), pp. 196–198〕 Sometimes, due to specific combinations of properties, extended objects behave as point-like even in their immediate vicinity. For example, spherical objects interacting in 3-dimensional space whose interactions are described by the inverse square law behave in such a way as if all their matter were concentrated in their centers of mass. In Newtonian gravitation and classical electromagnetism, for example, the respective fields outside of a spherical object are identical to those of a point particle of equal charge/mass located at the center of the sphere.〔I. Newton, I.B Cohen, A. Whitmann (1999), p. 956 (Proposition 75, Theorem 35)〕〔I. Newton, A. Motte, J. Machin (1729), p. 270–271〕 In quantum mechanics, the concept of a point particle is complicated by the Heisenberg uncertainty principle, because even an elementary particle, with no internal structure, occupies a nonzero volume. For example, the atomic orbit of an electron in the hydrogen atom occupies a volume of ~10−30 m3. There is nevertheless a distinction between elementary particles such as electrons or quarks, which have no known internal structure, versus composite particles such as protons, which do have internal structure: A proton is made of three quarks. Elementary particles are sometimes called "point particles", but this is in a different sense than discussed above. For more details see elementary particle. ==Point mass== Point mass (pointlike mass) is the concept of either matter that is infinitely small, or an object which can be thought of as infinitely small. In terms of size this concept is similar to that of point particles. However unlike point particles, point mass can only apply to an object that is infinitely small. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Point particle」の詳細全文を読む スポンサード リンク
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