Dictionary tag: P-Q-R-S-T

• Qualitative analysis in physics focuses on the inherent properties or characteristics of the thing being studied, rather than on numerical values or precise measurements.
• Qualitative analysis involves making observations, interpreting patterns and relationships, and drawing conclusions based on these observations, rather than relying solely on numerical data or statistical analysis.
• Qualitative analysis might involve making observations and drawing inferences about the behaviour of a system or object based on its properties, such as its shape, colour, texture, or motion.
• Qualitative analysis can be used in many different fields, including social sciences, natural sciences, and humanities, to gain a deeper understanding of complex phenomena and systems.

• In physics, the term quantitative refers to a measurable quantity or physical property that can be expressed numerically.
• Quantitative analysis involves mathematical and statistical methods to obtain precise measurements of physical phenomena and to analyse and interpret the resulting data.
• Quantitative analysis is a fundamental part of the scientific method in physics, as it allows researchers to test hypotheses and make predictions using empirical evidence.
• Empirical evidence refers to data collected through direct observation or experimentation rather than through theoretical reasoning alone.
• The use of quantitative methods also enables physicists to develop mathematical models that can be used to describe and predict the behaviour of complex physical systems.

• QED is an integral part of the Standard Model of particle physics, a theoretical framework that encompasses the fundamental particles and their interactions. It constitutes the electromagnetic sector of the Standard Model, working alongside the weak nuclear force and the strong nuclear force.
• As a quantum field theory, QED describes particle and field behaviour in terms of probabilities and wave functions by accounting for the principles of quantum mechanics that distinguish it from classical physics.
• QED’s primary focus is on the electromagnetic force, encompassing interactions involving charged particles (such as electrons and protons) and photons (particles of light).
• In QED, interactions between charged particles are described in terms of the exchange of virtual photons. These are not “real” photons in the sense that they cannot be observed directly but are theoretical mathematical constructs that represent the invisible intermediary particles in an interaction.
• QED calculations frequently employ Feynman diagrams, graphical representations of particle interactions that offer a visual means to comprehend and compute the probabilities of likely scenarios.
• In QED, electromagnetic fields are not considered fundamental entities. Instead, they emerge from the interactions of photons. It is photons, elementary particles of light, That serve as the carriers of the electromagnetic force and generate the electromagnetic fields.

• QED explains a wide range of phenomena related to light, including:
  • Why light travels at a constant speed in a vacuum.
  • Why light is absorbed and emitted by matter (such as electrons) in discrete units or ‘quanta’ (photons).
  • Why light undergoes scattering when it interacts with matter.
  • How light causes interference and diffraction at the sub-atomic scale.
  • The photoelectric effect is the emission of electrons from a material’s surface when it is illuminated by light.
  • The Compton effect is the scattering of light by electrons.

QED’s perspective on Electromagnetism

• Quantum electrodynamics represents a more complete and accurate theory of electromagnetism than had been developed previously. It was formulated in the 1940s by a team of physicists including Richard Feynman, Julian Schwinger, and Tomonaga Shin’ichiro, all of whom were awarded the Nobel Prize in Physics in 1965 for their contributions to this theory.
• QED is based on the idea that when electric and magnetic fields are measured, the range of energy values they can hold corresponds directly with the energy of the photons they produce. Quantized fields refer to this range of specific and discrete values. This is in contrast to classical fields, which can theoretically take on any energy value.
• To understand this, imagine a ruler marked in centimetres. The ruler measures the field, and the centimetre marks represent the different values that the field can take. In classical physics, the fields can theoretically take any value. However, in quantum field theory, the values must align exactly with the marks because each one corresponds to a quantum of photon energy.
• Taking this a step further, consider an electron moving around an atom at the moment it emits a photon of light. The electron can only exist at certain energy levels, and these correspond to the orbits the electron follows around the nucleus. The electron will be in one orbit or another when it emits the photon, and the energy level of the photon corresponds to the energy carried by the electron in that particular orbit. For example, if an electron transitions from the third orbit to the second orbit from the nucleus, it will emit a photon with an energy equal to the difference in energy carried by the electron in the third and second orbits. The resulting discrete range of energy levels provides the quantized energy values.

• A quantum field is a physical field that can be described by a quantum operator. It is the fundamental entity that gives rise to elementary particles.
• The word “physical” in this context means that the field is a real entity not just a mathematical abstraction. For example, the electromagnetic field can be measured using a variety of devices, such as electric and magnetic field detectors.
• “Quantum operators” are used to represent physical observables sung variables to represent factors such as position, momentum, and energy.
• In the context of quantum field theory, quantum operators are used to describe the creation and annihilation of elementary particles. For example, the creation operator for the photon is a quantum operator that can be used to describe the quantum state of a new photon.

Properties of quantum fields

• Quantum fields are the building blocks of the universe and play a crucial role in understanding the behaviour of elementary particles and their interactions.
• Quantum fields are continuous and extend throughout space and time and can be described by inferred values at each point within spacetime.
• Quantum fields can be excited or de-excited, which corresponds to the creation or annihilation of elementary particles.
• Quantum fields are subject to the laws of quantum mechanics, which means that they are probabilistic in nature.
• Quantized, signifying that they can only have certain specific and discrete values at each point of measurement. This is because the energy of a quantum field is quantized.
• Dynamic, means they can fluctuate over time. These fluctuations are responsible for the creation and annihilation of particles.
• Interacting, means that quantum fields interact with each other.
• Here are some examples of quantum fields:
  • Electromagnetic field
  • Gravitational field
  • Strong nuclear force field
  • Weak nuclear force field.

• Quantum fields are thought to be the underlying reality of all particles and forces. Quantum field theory has developed the Standard Model to describe all the known fundamental particles and force carriers as fields.
• Quantum field theory uses mathematical formulas to represent things that are often too small or transient to observe. The exact behaviour of forces, particles and waves can often only be inferred and so must be described in terms of a mathematical probability of different events and outcomes.
• Quantum fields serve as the comprehensive framework that encompasses the entirety of matter-energy and can be thought of as a dynamic medium that permeates all of space. It is a unified theory of spacetime and matter in which particles are simply localized excitations of this medium. This means that when a particle appears, it is a localized disturbance in a quantum field. When a particle is destroyed, the disturbance in the quantum field disappears.
• In quantum field theory, a quantum field is an entity whose fundamental element is a “quantum” of energy. A quantum of energy is the smallest possible unit-quantity that can be contained within a field. This is the smallest possible unit that can be used to describe the behaviour of the quantum field in question.
• So, a quantum field can be thought of as a field that may be made up of a single quantum of vibrating matter-energy – a particle. However, a quantum field can include more than one unit. The number of particles in a quantum field is determined by the state of the field so can be between zero to an infinity of quanta.
• A field containing a single particle and a field containing groups of particles are related in the sense that they are both manifestations of the same underlying quantum field. The difference is that the field containing a single quantum is a localized disturbance, while the field containing groups of particles is more spread out (diffuse).
• The electromagnetic field (EM field) is an example of a quantum field. Disturbances in the EM field create photons. Photons are responsible for all forms of electromagnetic radiation from radio waves through visible light to gamma rays. It is also responsible for the force of electromagnetism, which is one of the four fundamental forces of nature.

Quantum fields & fundamental forces

• There is a quantum field corresponding with each fundamental force:
  • The electromagnetic force is mediated by the exchange of photons between charged particles. The EM field is the underlying reality that allows this exchange to take place.
    • Using the term mediated in this context is a way of explaining how a force can act at a distance. It is a way of saying that the force is not transmitted directly from one object to another. Instead, it is transmitted through a medium and that medium is the electromagnetic field.
    • A simple example of this is when two charged particles interact with each other and exchange photons. The photons carry momentum and energy, which can cause the charged particles to accelerate or change direction.
• The weak nuclear field is the field associated with the weak nuclear force, one of the four fundamental forces of nature. It is responsible for radioactive decay and other processes that involve the nucleus of an atom.
• The strong nuclear field is the field associated with the strong nuclear force, one of the four fundamental forces of nature. It is responsible for binding protons and neutrons together in the nucleus of an atom.
• The graviton field is the field that is theorized to be responsible for gravity, one of the four fundamental forces of nature. The graviton is a hypothetical particle that is thought to be the carrier of the gravitational force.
• Meanwhile, The Higgs field is the field that is responsible for the Higgs boson, which is a fundamental particle discovered in 2012. The Higgs boson gives mass to other particles, such as electrons and quarks.

Clouds of gas and beams of light

• A cloud of gas made up of countless vibrating subatomic particles, or a beam of light in which all units are free to interact are both quantum fields. The particles in a cloud of gas are held together by the electromagnetic force, while the photons in a beam of light are the force carriers of the electromagnetic force.
• The matter and energy of a particle or group of particles are manifestations of the underlying quantum field. When a particle is created, it is a localized disturbance in the field. When a particle is destroyed, the disturbance in the field disappears.
• Quantum fields are said to persist, even when particles or energy are absent. Classical physics fails to comprehensively explain this. In quantum mechanics (quantum physics), particles lack a definite location until observed. This doesn’t imply electrons can manifest anywhere at random. Instead, they are generated and annihilated in alignment with the principles of quantum mechanics.

Properties of quantum fields

• Quantum fields have:
  • Continuous Nature: Quantum fields extend continuously throughout space and time, imbuing every point in the cosmos with their presence. This means that they are fields of energy that permeate all spacetime. Quantum fields can be described by mathematical statements used to describe the behaviour of the particles and forces within them. This is possible even though quantum fields do not contain simple objects with identifiable locations or masses.
  • Quantized Energy: Quantum fields exhibit quantization, meaning they can only assume specific and discrete values of energy. This property underpins the discrete nature of particles and their interactions within the quantum realm.
  • Dynamic Fluctuations: Quantum fields are inherently dynamic, subject to fluctuations and oscillations over time. These fluctuations give rise to the spontaneous creation and annihilation of particles, contributing to the intricate tapestry of matter and energy in the universe.
  • Interactions: Quantum fields possess the capacity to interact with one another, giving rise to the fundamental forces that govern the cosmos.These interactions are responsible for the forces that we experience in the world, such as gravity, electromagnetism, and the strong and weak nuclear forces.

• An unresolved idea to grapple with is that quantum fields do not denote bounded spatial and temporal regions where forces act. In this sense, quantum fields are the fundamental entities that make up the universe and may play a fundamental role in the structure of space-time. Evidence is emerging that everything we see and experience is a manifestation of quantum fields. So, the energy and matter that make up the universe, the forces that govern its behaviour, and even the laws of physics themselves are all emanations of quantum fields.

• Particle Physics, a sub-field of quantum mechanics, focuses on experimental investigations into the fundamental particles composing matter and the forces acting between them. Research in particle physics, often conducted at facilities like the Large Hadron Collider at CERN, has confirmed the dual nature of light (both wave and particle) and fundamental particles like the photon.
• Quantum Field Theory (QFT), another aspect of quantum mechanics, describes particles as excitations of underlying quantum fields. The Standard Model is a QFT theory that describes the three fundamental forces of nature (the electromagnetic force, the weak nuclear force, and the strong nuclear force).
• Quantum Electrodynamics (QED) is a Quantum Field Theory that describes the interaction of light with charged particles. It describes how electromagnetic interactions work at the quantum level. QED has accounted for properties of light such as the photoelectric effect and Compton scattering.