#QED #quantum_field Quantum Electrodynamics (QED) is a quantum field theory that describes the interactions between electrically charged particles and electromagnetic fields. It is a fundamental theory of physics and forms the basis for our understanding of the electromagnetic force, which encompasses phenomena such as light, electricity, and magnetism. QED combines quantum mechanics, which describes the behavior of particles on a microscopic scale, with Maxwell's equations of classical electromagnetism, which describe the behavior of electromagnetic fields. By treating the electromagnetic field as a quantum field, QED provides a consistent framework for studying the interaction of charged particles with photons, the particles of light. Here are some key aspects of Quantum Electrodynamics: 1. Photons and Electrons: QED describes electrons as point-like particles with electric charge and photons as massless particles that mediate the electromagnetic force. Electrons and positrons (antielectrons) can emit and absorb photons, resulting in the exchange of energy and momentum between charged particles. 2. Feynman Diagrams: QED calculations are often performed using Feynman diagrams, which are graphical representations of particle interactions. These diagrams depict the possible paths and exchanges of particles in a given process and provide a systematic way to calculate probabilities and amplitudes for particle interactions. 3. Virtual Particles: QED involves the concept of virtual particles, which are particles that are created and annihilated during interactions but do not directly correspond to observable particles. These virtual particles, such as virtual photons, contribute to the probabilities and energies of interactions in QED calculations. 4. Quantum Fluctuations: QED incorporates quantum fluctuations of the electromagnetic field, which give rise to uncertainty and variations in the strength and behavior of the electromagnetic force. These fluctuations influence the interactions between charged particles, leading to phenomena such as vacuum polarization and the Lamb shift. 5. Renormalization: As mentioned earlier, QED encounters infinities in certain calculations due to the self-energy of charged particles and the vacuum fluctuations. Renormalization is a crucial technique used in QED to address these infinities and obtain meaningful and finite results. It involves adjusting certain parameters, such as the electron mass and charge, to absorb the infinities. 6. Experimental Verification: QED has been incredibly successful in making predictions that match experimental measurements with remarkable precision. It has been tested through high-precision experiments such as electron scattering, measurement of the electron's magnetic moment, and the Lamb shift in atomic spectra. It is important to note that while QED successfully describes electromagnetic interactions, it is only one part of the Standard Model of particle physics, which incorporates other fundamental forces and particles. The Standard Model unifies electromagnetic interactions, weak nuclear interactions, and strong nuclear interactions into a single theoretical framework. Quantum Electrodynamics represents a cornerstone of modern theoretical physics and has paved the way for further developments in quantum field theory and our understanding of fundamental interactions in the universe. ## QED vs. QCD [[Quantum electrodynamics|Quantum Electrodynamics]] QED ([[Quantum electrodynamics|Quantum Electrodynamics]]) and QCD ([[Quantum Chromodynamics]]) are both quantum field theories and fundamental components of the Standard Model of particle physics. While they have similarities in their mathematical framework and share certain features, there are significant differences between QED and QCD due to the different nature of the forces they describe. Here are the key similarities and differences: Similarities between QED and QCD: 1. Quantum Field Theory: Both QED and QCD are quantum field theories that describe the interactions between elementary particles. They use a similar mathematical framework to describe these interactions in terms of fields and particles. 2. Gauge Theories: QED and QCD are both gauge theories, meaning they involve symmetries and transformations that leave the underlying physics invariant. These symmetries give rise to the interaction forces and determine the properties of the particles involved. 3. Renormalization: Both QED and QCD require renormalization techniques to handle infinities that arise in certain calculations. Renormalization allows for the extraction of meaningful and finite results by adjusting parameters and accounting for the dependence on energy scales. Differences between QED and QCD: 1. Forces: QED describes the electromagnetic force, which is responsible for interactions between charged particles and photons. QCD, on the other hand, describes the strong nuclear force, which binds quarks together to form hadrons (such as protons and neutrons) and mediates interactions between quarks and gluons. 2. Charge Carriers: In QED, the charge carriers are electrically charged particles, primarily electrons and positrons, which interact through the exchange of photons. In QCD, the charge carriers are quarks, which come in different flavors (up, down, charm, strange, top, and bottom) and carry a different type of charge called color charge. Gluons, the mediators of the strong force, also carry color charge. 3. Symmetry Group: The symmetry group of QED is U(1), representing a single electromagnetic interaction. QCD, however, has a more complex symmetry group called SU(3), representing interactions involving three types of color charge. This difference in symmetry leads to distinct mathematical structures and behavior of the forces. 4. Confinement: One of the unique features of QCD is confinement, which means that quarks cannot exist as free particles in isolation. Instead, they are always bound together within color-neutral composite particles (hadrons). This confinement phenomenon is not present in QED, where charged particles can exist as free particles. 5. Experimental Signatures: QED interactions are observable in various electromagnetic phenomena, such as electron scattering and atomic spectroscopy. QCD interactions, on the other hand, are predominantly observed in high-energy particle collisions and processes involving hadronic particles. The experimental signatures and detection methods for QED and QCD interactions differ significantly. In summary, while QED and QCD share similarities as quantum field theories, they describe different fundamental forces and involve different types of charge carriers. QED focuses on electromagnetic interactions between charged particles and photons, while QCD deals with strong interactions between quarks and gluons. The symmetry groups, confinement property, and experimental signatures also set QED and QCD apart. # References ```dataview Table title as Title, authors as Authors where contains(subject, "QED") or contains(subject, "quantum electrodynamics") ```