Where does string collide?

A cord held under tension between two fixed points that vibrates when plucked, bowed, or struck and radiates the result as a musical tone. Mersenne's 1636 Harmonie universelle set out the laws relating a string's pitch to its length, tension, and thickness, and Fletcher and Rossing's 1998 The Physics of Musical Instruments gave the modern account of its modes and harmonic spectrum. The referent is a physical sounding object on an instrument, described by its fundamental frequency and overtones, and it is not a sequence of symbols in a computer, a fundamental object in particle physics, or a defect stretched across the early universe.[1][2]

Unit

A taut cord fixed at both ends, described by its fundamental frequency and overtones

Recovery means

The motion decays as energy radiates away as sound and is lost to friction, returning the cord to rest

Measured by

Measured by its pitch, set by length, tension, and mass per unit length, and by the spectrum of its harmonics

A finite ordered sequence of symbols drawn from a fixed alphabet, and the basic object that formal-language theory and computing manipulate. Post's 1943 Formal reductions of the general combinatorial decision problem built its canonical systems on the rewriting of finite strings of symbols, and the same idea underlies the string as a fundamental data type in programming. The referent is an abstract sequence, counted by its length and combined by operations such as joining and matching, and it has nothing in common with a sounding cord, a vibrating fundamental object, or a cosmological defect beyond the shared word.[3]

Unit

One finite ordered sequence of symbols over a fixed alphabet, with a length equal to its symbol count

Recovery means

Not applicable

Measured by

Characterised by its length, its alphabet, and the operations defined on it such as concatenation and substring matching

A single one-dimensional object whose different states of vibration appear as the different elementary particles, proposed as a more basic entity than the point particle. Gotō's 1971 Relativistic quantum mechanics of one-dimensional mechanical continuum gave the action for such a relativistic string, developed from the earlier dual resonance model of hadron scattering, and later superstring theory built on it. The referent is a hypothesised fundamental constituent of matter at the smallest scales, defined by its action and its quantised modes, and it is distinct from a sounding cord on an instrument, a sequence of symbols in a computer, or a line-like defect in spacetime.[4]

Unit

A single one-dimensional object whose vibrational states correspond to different particles

Recovery means

Not applicable

Measured by

Studied through its action and quantised vibrational modes; no direct measurement of an individual string exists

A line-like defect in the fabric of spacetime that could have formed when the early universe cooled through a symmetry-breaking phase transition, leaving a thin, extremely dense filament. Kibble's 1976 Topology of cosmic domains and strings set out how such defects arise, and Vilenkin's 1985 Cosmic strings and domain walls reviewed their properties and possible observational traces. The referent is a hypothesised cosmological object characterised by its mass per unit length, and it is not a musical string on an instrument, a sequence of symbols in a computer, or the fundamental string of particle physics.[5][6]

Unit

A line-like defect stretched across space, characterised by its mass per unit length

Recovery means

Not applicable

Measured by

Inferred indirectly from predicted gravitational-lensing and gravitational-wave signatures; none has been confirmed observationally