Wiktor Wise
A permanent electric dipole moment (EDM) is a separation of charge along the angular-momentum axis of a particle or system. It is characterized by its dipole moment, a vector quantity, which points from the negative charge towards the positive charge. EDMs are a direct signal of T-violation and, assuming CPT symmetry, CP violation. The existence of an EDM in a nondegenerate physical system with well-defined angular momentum, such as the cesium atom in its ground state, would be evidence of a violation of parity and time-reversal invariance. While EDMs have been studied for over sixty years, they have only been observed in certain molecules and particles, such as the homonuclear molecule and the cesium atom.
| Characteristics | Values |
|---|---|
| Definition | A permanent electric dipole moment (EDM) of a particle or system is a separation of charge along its angular-momentum axis. |
| Electron's Magnetic Dipole Moment | Caused by an intrinsic property of the electron. |
| Magnetic Dipole | A closed circulation of an electric current system. |
| Simple Electric Dipole | The electric dipole moment points from the negative charge towards the positive charge. |
| Magnitude | The magnitude of a simple electric dipole is equal to the strength of each charge times the separation between the charges. |
| Magnetic Monopoles | No known magnetic monopoles in nature, but magnetic dipoles exist in the form of electron spin. |
| Magnetic Dipole Moment of a Bar Magnet | Intrinsic magnetic dipole moment of the electron. |
| Permanent Electric Dipole Moment in Molecules | Requires a breaking of parity symmetry, which conventionally relies on the presence of heteronuclear constituents. |
| Permanent Electric Dipole Moment of Strontium Monofluoride | Calculated using a relativistic coupled cluster method. |
| Permanent Electric Dipole Moment of the Cesium Atom | Interpreted as a violation of parity and time-reversal invariance. |
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What You'll Learn
- Electric dipole moments are a direct signal of T-violation and CP violation
- A permanent electric dipole moment is a separation of charge along its angular-momentum axis
- A permanent magnet has a permanent magnetic dipole moment
- A physical dipole consists of two equal and opposite point charges
- A molecule with a permanent dipole moment is called a polar molecule
Electric dipole moments are a direct signal of T-violation and CP violation
A permanent electric dipole moment (EDM) is a separation of charge along the angular-momentum axis of a particle or system. It is characterised by its dipole moment, a vector quantity. The electric dipole moment points from the negative charge towards the positive charge, with a magnitude equal to the strength of each charge multiplied by the separation between them. Electrons and some other fundamental particles can have magnetic dipole moments, but an electron's magnetic dipole moment is intrinsic and not due to a current loop.
Electric dipole moments are a direct signal of T-violation and, assuming CPT symmetry, CP violation. In the context of relativistic quantum field theory, the Hamiltonian for a single electron in an external electric field exhibits P- and T-odd behaviour. This implies that the spin changes sign under charge conjugation (C). A non-zero value for the electric dipole moment would confirm CP violation.
The study of EDMs has a long history, initially focusing on parity-symmetry violation and later shifting towards CP violation. CP violation plays a crucial role in explaining the cosmological matter-antimatter asymmetry. Experimental techniques have become increasingly sensitive, but theoretical interpretation is essential for understanding the upper limits on EDMs. Nuclear theory, atomic theory, and molecular theory provide valuable insights into the observable EDMs and the manifestation of CP-violation in these systems.
CP violation at low energy is investigated through detailed analyses of CP-odd observables related to τ-pair production. The time-reversal odd electric dipole moment (EDM) of the τ particle is identified as the source of CP violation in this context. The electromagnetic properties of τ leptons, particularly their magnetic form factors, can be studied using unpolarized or polarized electron beams. These investigations contribute to our understanding of the fundamental physical parameters of standard heavy leptons.
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A permanent electric dipole moment is a separation of charge along its angular-momentum axis
A permanent electric dipole moment (EDM) is a separation of charge along its angular-momentum axis. This phenomenon has been studied for over sixty years, initially as a signal of parity-symmetry violation. The existence of an EDM in a nondegenerate physical system with a well-defined angular momentum, such as the cesium atom in its ground state, would be evidence of a violation of parity and time-reversal invariance.
In simple terms, a dipole consists of two equal and opposite point charges, often referred to as the two poles. An electric dipole moment points from the negative charge toward the positive charge, with a magnitude determined by the strength of each charge and the separation between them.
Molecular dipoles, extensively studied by physical chemist Peter J. W. Debye, are measured in the unit "debye" in his honour. These occur when atoms within a molecule have significantly different electronegativity, resulting in one atom attracting more electrons and becoming more negative, while the other becomes more positive.
The permanent electric dipole moment of the electronic ground state of the strontium monofluoride molecule has been calculated using a relativistic coupled cluster method. This method is believed to yield accurate results for molecules where relativistic effects are significant.
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A permanent magnet has a permanent magnetic dipole moment
A permanent magnet, such as a bar magnet, owes its magnetism to the intrinsic magnetic dipole moment of the electron. A magnetic dipole is the closed circulation of an electric current system. A simple example is a single loop of wire with a constant current passing through it. A bar magnet is an example of a magnet with a permanent magnetic dipole moment.
The magnetic moment can be defined as a vector (or pseudovector) relating the aligning torque on an object from an externally applied magnetic field to the field vector itself. The magnetic moment of an object determines the magnitude of torque the object experiences in a given magnetic field. When the same magnetic field is applied, objects with larger magnetic moments experience larger torques. The strength and direction of this torque depend on the magnitude of the magnetic moment and its orientation relative to the direction of the magnetic field.
The magnetic dipole moment of a permanent magnet is related to the residual magnetism by the volume of the magnet. The dipole component of an object's magnetic field is symmetric about the direction of its magnetic dipole moment and decreases as the inverse cube. The magnetic field of a magnetic dipole is proportional to its magnetic dipole moment.
The magnetic dipole moment of an object can be calculated using the formula:
> {\\displaystyle {\\boldsymbol {\\tau }}=\\mathbf {m} \\times \\mathbf {B}}
Where τ is the torque acting on the dipole, B is the external magnetic field, and m is the magnetic moment.
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A physical dipole consists of two equal and opposite point charges
A permanent electric dipole moment (EDM) is a separation of charge along the angular-momentum axis of a particle or system. This separation of charge results in the emergence of two equal and opposite point charges, also known as a physical dipole. In simpler terms, a physical dipole consists of two poles, one carrying a positive charge and the other carrying an equal negative charge.
The concept of a permanent electric dipole moment is closely related to the idea of molecular geometry and the distribution of electrons within a molecule. In certain molecules, such as boron trifluoride, the arrangement of atoms and their electronegativity can result in a net dipole moment of zero due to the molecule's symmetry. However, when the symmetry is disrupted or in molecules with heteronuclear constituents, a permanent electric dipole moment can occur.
The presence of a permanent electric dipole moment in a molecule or particle has significant implications. For example, it serves as a direct signal of T-violation and CP violation, offering insights into the role of CP violation in nature and theory. CP violation, or the violation of parity and time-reversal invariance, is crucial in explaining the cosmological matter-antimatter asymmetry.
Additionally, the concept of a permanent electric dipole moment extends to the field of magnetism. A bar magnet, for instance, exhibits a permanent magnetic dipole moment due to the intrinsic magnetic properties of its electrons. This results in the magnet having two distinct poles, commonly referred to as the north-seeking and south-seeking poles, analogous to the positive and negative charges of a physical dipole.
The study of permanent electric dipole moments has been a subject of scientific investigation for over sixty years, with ongoing experiments and theoretical advancements contributing to our understanding of this phenomenon.
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A molecule with a permanent dipole moment is called a polar molecule
A permanent electric dipole moment (EDM) is a separation of charge along the angular-momentum axis of a particle or system. This separation of charge creates an electric field, with the dipole moment pointing from the negative charge towards the positive charge. The magnitude of the dipole moment is equal to the strength of each charge multiplied by the distance between them. EDMs have been studied for over sixty years, initially as a signal of parity-symmetry violation.
In the context of molecules, a permanent dipole moment occurs when two atoms within the molecule have significantly different electronegativities. This difference in electronegativity results in an unequal distribution of electrons, with one atom attracting more electrons and becoming more negative, while the other atom becomes more positive. This unequal charge distribution creates a permanent dipole.
Molecules with a permanent dipole moment are called polar molecules. An example of a polar molecule is strontium monofluoride, which has a permanent electric dipole moment in its electronic ground state. The presence of a permanent dipole moment in a molecule can have important implications for its chemical behaviour and interactions with other molecules.
It is important to note that not all molecules exhibit permanent dipole moments. Some molecules, such as boron trifluoride, have a symmetrical structure that results in a zero dipole moment, despite the presence of polar bonds. Additionally, molecules with identical atoms (homonuclear molecules) typically do not possess a permanent dipole moment due to their symmetrical nature. However, in certain cases, exotic homonuclear molecules with asymmetric electronic excitation can exhibit permanent dipole moments.
The study of permanent electric dipole moments has led to advancements in various fields, including physics and chemistry. For example, the concept of EDMs has been applied to understand the behaviour of atoms, such as the cesium atom, and their ground states. Furthermore, the search for EDMs in different systems has provided valuable insights into the violation of parity and time-reversal invariance, contributing to our understanding of the asymmetry between matter and antimatter in the universe.
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Frequently asked questions
A permanent electric dipole moment (EDM) is a separation of charge along the angular-momentum axis of a particle or system. It is a direct signal of T-violation and, assuming CPT symmetry, CP violation.
An electric dipole consists of two equal and opposite point charges, or two poles. The dipole moment points from the negative charge toward the positive charge.
Permanent electric dipole moments in molecules require a breaking of parity symmetry. This symmetry breaking usually relies on the presence of heteronuclear constituents. However, it has also been observed in homonuclear molecules, where the binding is based on asymmetric electronic excitation between the atoms.
Permanent electric dipole moments have been studied in various systems, including the cesium atom, strontium monofluoride molecule, and exotic homonuclear molecules consisting of rubidium (Rb) atoms.
