quantum electrodynamics
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What is quantum electrodynamics?
Quantum electrodynamics (QED) is the quantum field theory describing interactions of charged particles with the electromagnetic field. QED mathematically describes all interactions of light with matter and those of charged particles with each other.
QED rests on the idea that charged particles interact by emitting and absorbing photons, which transmit electromagnetic forces. The theory was refined in the late 1940s by Richard P. Feynman, Julian S. Schwinger, and Tomonaga Shin'ichirÅ. Experiments dealing with the properties of subatomic particles, known as muons, have demonstrated the theory's accuracy to nine significant digits, making QED one of the most successful physical theories.
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How does quantum electrodynamics describe the behavior of light and matter?
Quantum electrodynamics (QED) is the quantum field theory describing interactions of charged particles with the electromagnetic field. QED mathematically describes all interactions of light with matter and charged particles with each other and is a relativistic theory that incorporates Albert Einsteinās theory of special relativity into each of its equations.
In QED, charged particles interact by emitting and absorbing photons, which transmit electromagnetic forces. The electric and magnetic fields described in Maxwellās equations are quantized, and photons appear as excitations of those quantized fields. Two identical charged particles electrically repel one another because they exchange virtual photons.
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What are photons in quantum electrodynamics?
In quantum electrodynamics (QED), photons are the force carriers of the electromagnetic field. They mediate interactions between charged particles.
Here's what you should know about photons in QED:
- Photons are energy packets of electromagnetic radiation.
- They have no electric charge or rest mass, possessing one unit of spin.
- Photons can be emitted or absorbed by charged particles. When charged particles interact, they exchange virtual photons, which cannot be directly detected because their existence violates the conservation of energy and momentum.
- Real photons can be emitted by charged particles and observed as light.
- QED describes the interactions of light with matter and charged particles, serving as a foundation for understanding other fundamental forces.
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How does quantum electrodynamics explain the interaction between electrons and photons?
Quantum electrodynamics (QED) describes interactions of charged particles with the electromagnetic field. It posits that charged particles interact by emitting and absorbing photons, which transmit electromagnetic forces. These photons are "virtual," meaning they can't be directly observed because their existence defies energy and momentum conservation.
QED utilizes Feynman diagrams to graphically represent particle interactions. These diagrams depict how particles exchange virtual photons, influencing their speed and direction. Each diagram includes lines that represent particles and vertices that represent interactions.
QED is a perturbation theory where the complexity of the process decreases with each exchange of virtual photons. The more virtual photons exchanged, the smaller the probability of it occurring. Despite its mathematical complexities, QED is a successful theory, serving as a model for other quantum field theories.
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What are Feynman diagrams and how are they used in quantum electrodynamics?
Richard Feynman invented Feynman diagrams in the 1940s and 50s as a visual method to represent interactions between elementary particles. These diagrams are now used to depict all types of particle interactions.
Feynman diagrams are two-dimensional representations of particle interactions where the axes represent space and time. Straight lines represent fermions (matter particles like electrons), and wavy lines represent bosons (force-carrier particles like photons). The point where lines connect is called a vertex, which represents the emission or absorption of a field particle.
Physicists use Feynman diagrams to calculate the probability of a process, such as electron-electron scattering, in quantum electrodynamics (QED). These calculations include all the lines and vertices in the diagram. The results of these calculations have been extraordinarily accurate when compared to experimental measurements.
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How has quantum electrodynamics helped us understand and predict the results of experiments with atoms and light?
Quantum electrodynamics (QED) is the quantum field theory that explains how charged particles interact with the electromagnetic field. It mathematically describes all interactions of light with matter and of charged particles with each other.
QED helps predict experimental results by resting on the idea that charged particles interact by emitting and absorbing photons that transmit electromagnetic forces. Richard P. Feynman, Julian S. Schwinger, and Tomonaga Shin'ichirÅ refined QED in the late 1940s.
QED is a relativistic theory, meaning it integrates Albert Einstein's theory of special relativity into its equations. Because atoms and molecules are primarily electromagnetic, atomic physics serves as a testing ground for QED. The theory's predictions have been confirmed with remarkable accuracy in experiments, such as those involving the magnetic moment of muons.
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quantum electrodynamics (QED), quantum field theory of the interactions of charged particles with the electromagnetic field. It describes mathematically not only all interactions of light with matter but also those of charged particles with one another. QED is a relativistic theory in that Albert Einsteinās theory of special relativity is built into each of its equations. Because the behaviour of atoms and molecules is primarily electromagnetic in nature, all of atomic physics can be considered a test laboratory for the theory. Some of the most precise tests of QED have been experiments dealing with the properties of subatomic particles known as muons. The magnetic moment of this type of particle has been shown to agree with the theory to nine significant digits. Agreement of such high accuracy makes QED one of the most successful physical theories so far devised.
In 1928 the English physicist P.A.M. Dirac laid the foundations for QED with his discovery of a wave equation that described the motion and spin of electrons and incorporated both quantum mechanics and the theory of special relativity. The QED theory was refined and fully developed in the late 1940s by Richard P. Feynman, Julian S. Schwinger, and Tomonaga ShināichirÅ, independently of one another. QED rests on the idea that charged particles (e.g., electrons and positrons) interact by emitting and absorbing photons, the particles that transmit electromagnetic forces. These photons are āvirtualā; that is, they cannot be seen or detected in any way because their existence violates the conservation of energy and momentum. The photon exchange is merely the āforceā of the interaction, because interacting particles change their speed and direction of travel as they release or absorb the energy of a photon. Photons also can be emitted in a free state, in which case they may be observed as light or other forms of electromagnetic radiation.
The interaction of two charged particles occurs in a series of processes of increasing complexity. In the simplest, only one virtual photon is involved; in a second-order process, there are two; and so forth. The processes correspond to all the possible ways in which the particles can interact by the exchange of virtual photons, and each of them can be represented graphically by means of the so-called Feynman diagrams. Besides furnishing an intuitive picture of the process being considered, this type of diagram prescribes precisely how to calculate the variable involved. Each subatomic process becomes computationally more difficult than the previous one, and there are an infinite number of processes. The QED theory, however, states that the more complex the processāthat is, the greater the number of virtual photons exchanged in the processāthe smaller the probability of its occurrence. For each level of complexity, the contribution of the process decreases by an amount given by Ī±2āwhere Ī± is a dimensionless quantity called the fine-structure constant, with a numerical value equal to (1/137). Thus, after a few levels the contribution is negligible. In a more-fundamental way the factor Ī± serves as a measure of the strength of the electromagnetic interaction. It equals e2/4ĻĪµo[planck]c, where e is the electron charge, [planck] is Planckās constant divided by 2Ļ, c is the speed of light, and Īµo is the permittivity of free space.
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QED is often called a perturbation theory because of the smallness of the fine-structure constant and the resultant decreasing size of higher-order contributions. This relative simplicity and the success of QED have made it a model for other quantum field theories. Finally, the picture of electromagnetic interactions as the exchange of virtual particles has been carried over to the theories of the other fundamental interactions of matter, the strong force, the weak force, and the gravitational force. See also gauge theory.