CPT Theorem

(redirected from CPT symmetry)
Also found in: Wikipedia.

CPT theorem

A fundamental ingredient in quantum field theories, which dictates that all interactions in nature, all the force laws, are unchanged (invariant) on being subjected to the combined operations of particle-antiparticle interchange (socalled charge conjugation, C), reflection of the coordinate system through the origin (parity, P), and reversal of time, T. In other words, the CPT operator commutes with the hamiltonian. The operations may be performed in any order; TCP, TPC, and so forth, are entirely equivalent. If an interaction is not invariant under any one of the operations, its effect must be compensated by the other two, either singly or combined, in order to satisfy the requirements of the theorem. See Quantum field theory

The CPT theorem appears implicitly in work by J. Schwinger in 1951 to prove the connection between spin and statistics. Subsequently, G. Lüders and W. Pauli derived more explicit proofs, and it is sometimes known as the Lüders-Pauli theorem. The proof is based on little more than the validity of special relativity and local interactions of the fields. The theorem is intrinsic in the structure of all the successful field theories. See Quantum statistics, Relativity, Spin (quantum mechanics)

CPT assumed paramount importance in 1957, with the discovery that the weak interactions were not invariant under the parity operation. Almost immediately afterward, it was found that the failure of P was attended by a compensating failure of C invariance. Initially, it appeared that CP invariance was preserved and, with the application of the CPT theorem, invariance under time reversal. Then, in 1964 an unmistakable violation of CP was discovered in the system of neutral K mesons. See Parity (quantum mechanics)

One question immediately posed by the failure of parity and charge conjugation invariance is why, as one example, the π+ and π- mesons, which decay through the weak interactions, have the same lifetime and the same mass. It turns out that the equality of particle-antiparticle masses and lifetimes is a consequence of CPT invariance and not C invariance alone. See Elementary particle, Meson, Symmetry laws (physics)

McGraw-Hill Concise Encyclopedia of Physics. © 2002 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

CPT Theorem


a theorem of quantum field theory stating that the equations of the theory are invariant under the CPT transformation—that is, they do not change form if the three transformations of charge conjugation (C), space inversion (P), and time reversal (T) are performed simultaneously. In charge conjugation, particles are replaced by the corresponding antiparticles. Space inversion, or reflection, involves the replacement of the space coordinates r by – r. Time reversal means the replacement of the time coordinate t by —t.

The CPT theorem was formulated and proved by the German physicist G. Lüders between 1952 and 1954 and by the Swiss physicist W. Pauli in 1955. The theorem follows from the fundamental principles of quantum field theory. If some process occurs in nature, the CPT theorem implies that there can also occur in nature, with the same probability, the process obtained by replacing the particles in the original process with the corresponding antiparticles, by changing the signs of the projections of the particles’ spins, and by interchanging the initial and final states of the original process.

It follows, in particular, from the CPT theorem that (1) a particle and the corresponding antiparticle have equal masses and equal lifetimes, (2) the electric charges of a particle and the corresponding antiparticle differ only in sign, as do the magnetic moments, (3) the interaction of a particle and of the antiparticle with a gravitational field is identical (that is, there is no “antigravitation”), and (4) in those cases where the interaction of particles in the final state is negligible, the energy spectra and angular distributions of the decay products are the same for particles and anti-particles, and the projections of the spins are of opposite sign.

Not a single instance of violation of the CPT theorem has been observed. The equality of the masses of a particle and the corresponding antiparticle has been tested for the kaons K0 and K̅0 with an accuracy of approximately 10–15, which exceeds by ten orders of magnitude the best accuracy achieved for the masses of other particles: ~10–5 for the electron (e) and positron (e+), ~10–4 for the muons µ and µ+, and ~10–3 for the kaons K and K+. The equality of the lifetimes of particles and antiparticles has been verified with an accuracy not exceeding 10–3. The equality of the magnetic moments has been tested with an accuracy of ~10–6 for µ and µ+ and ~10–5 for e and e+. The accuracy of the comparison of spectra and polarization in the decays of particles and antiparticles apparently does not exceed 10–2.

Until 1956 the laws of nature were believed to be symmetric under each of the C, P, and T transformations performed separately. The discovery in 1956 of violations of P and C invariance and the discovery in 1964 of a violation of CP invariance (seeCOMBINED INVERSION) had little effect on the theoretical apparatus of physics, which was able to make allowance for these discoveries in a natural manner, without violating the fundamental principles of the theory. In contrast to the violation of P, C, and CP invariance, the detection of a violation of CPT invariance would require changes in the foundations of quantum field theory. The violation of the CPT theorem would “break” the connection between particles and antiparticles. Within the framework of traditional quantum field theory, the bases of the CPT theorem—such as relativistic invariance, the localization of interactions, and the relation between spin and statistics—are such that it cannot yet be seen how even one of them could be sacrificed without radically altering the entire theory. The same statement can be made with regard to axiomatic quantum field theory. The experimental search for effects reflecting a violation of CPT invariance is thus all the more interesting.


Lapidus, L. I. “Sledstviia CPT-invariantnosti i eksperiment.” Uspekhi fizicheskikh nauk, 1968, vol. 95, issue 4.
Fainberg, V. la. “Teoreticheskie osnovy CPT-invariantnosti.” Uspekhi fizicheskikh nauk, 1968, vol. 95, issue 3.


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.

CPT theorem

[¦sē¦pē¦tē ′thir·əm]
(particle physics)
A theorem which states that a Lorentz invariant field theory is invariant to the product of charge conjugation C, space inversion P, and time reversal T.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
The second measurement, testing the CPT symmetry using the Gammasphere detector which covered almost a full solid angle, did not therefore rely on external magnetic field.
Scientists believe that measuring the magnetic moments of antiparticles and comparing them to those of the corresponding particles could reveal violations in CPT symmetry.
Apart from carefully drawing parallels and correspondences between the Torah's inner dimension and modern physics, in these lectures, Rabbi Ginsburgh proposes new directions for scientific research into important areas such as a unified field theory, CPT symmetry, the relationship between acceleration and gravitation, and the possibility of uncovering additional dimensions in physical reality, demonstrating how the Torah's depth can be used to fertilize science and further our understanding of nature.
CP violation does imply that time symmetry must be violated to preserve CPT symmetry. But CP violation occurs only in a very small number of specific interactions.
In this regards, it is worth to remark here that there were some attempts in literature to generalise the notion of symmetries in Quantum Mechanics, for instance by introducing CPT symmetry, chiral symmetry etc.
Concerned with antiproton beams with kinetic energies of order keV or less, the papers delve into such topics as CPT symmetry and gravitation, the structure of exotic nuclei, atomic collisions, and atom physics in general, with particular focus on recent advances in manipulating large numbers of ultra-slow antiprotons and synthesizing antihydrogen atoms.
The aim of Romalis' project, "A Test of CPT Symmetry Using a New [K-.sup.3] He Self-Compensating Magnetometer," is to perform a high-precision test of combined charge conjugation, parity inversion, and time reversal (CPT) invariance and local Lorentz invariance by comparing the Larmor precession frequencies of potassium (K) and helium 3 ([He.sup.3]) atoms in the same cell as a function of time, i.e., the daily rotation of the Earth about its axis and the movement of the Earth relative to the cosmic microwave background radiation.
Physicists now speak of CPT symmetry. Cronin and Fitch shared the Nobel Prize for physics in 1980.
The noncommutative extensions are of particular interesting since the CPT symmetry can be broken in both ways [29-31] and has been extensively studied [29-32].
CPT symmetry is a central tenet of the (https://home.cern/about/physics/standard-model) Standard Model of particle physics , which describes how three of the four known fundamental forces work.
If particle interactions are thought of as a movie, CPT symmetry requires that whatever physics occurs during the show must be the same when the film is run forward or backward (time), viewed through a mirror (parity) and when replacing each particle by its antiparticle (charge).
Pospelov and Romalis [5] tell us that the breaking of Lorentz symmetry enables the CPT symmetry, which combines charge conjugation (C), parity (P), and time-reversal (T) symmetries, to be violated.