Quantum Mechanics: Physics

• Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force
• It is one of the most fundamental scientific disciplines, and its main goal is to understand how the universe behaves

Quantum Mechanics

• Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles
• It is also referred to as quantum physics or quantum theory
• It is the foundation of all quantum physics, which includes quantum chemistry, quantum field theory, quantum technology, and quantum information science
• Classical physics, the physics existing before quantum mechanics, describes nature at ordinary scale
• Most classical physics theories can be derived from quantum mechanics as an approximation valid at large scale
• Quantum mechanics differs from classical physics in that energy, momentum, angular momentum, and other quantities of a bound system are restricted to discrete values (quantization)
• Objects have characteristics of both particles and waves (wave-particle duality)
• There are limits to how accurately the value of a physical quantity can be predicted prior to measuring it, given a complete set of initial conditions (the uncertainty principle)
• Quantum mechanics arose gradually, from theories to explain observations which could not be reconciled with classical physics
• Max Planck's solution in 1900 to the black-body radiation problem, and Albert Einstein's 1905 paper on the photoelectric effect
• Early quantum theory was significantly re-conceived in the mid-1920s by Erwin Schrödinger, Werner Heisenberg, Max Born and others
• The Copenhagen interpretation of Niels Bohr became widely accepted
• Quantum mechanics was further unified and formalized in the development of quantum field theory (QFT) in the 1940s
• QFT is relativistic quantum mechanics, applicable to systems with large numbers of particles and high energies
• Quantum mechanics has had enormous success in explaining many features of our universe, with regard to the behavior and interactions of matter and energy at the scale of atoms and subatomic particles
• Quantum mechanics is often the only theory that can reveal the individual behaviors of the subatomic constituents of matter
• Approximations are needed for practical quantum mechanical problems
• The structure of the periodic table of the elements, and the behavior of chemicals are explained by quantum mechanics
• Quantum mechanics is essential to the workings of many modern technologies, such as lasers, transistors, semiconductors, medical and research imaging, and electron microscopy
• Applications include specialized uses such as quantum computing and quantum cryptography
• Many macroscopic properties of a system are a direct consequence of quantum mechanical behavior of its parts

Electromagnetism

• Electromagnetism is a fundamental interaction of nature that causes electric and magnetic fields
• Electromagnetism is the force that acts between electrically charged particles
• It includes electrostatic force acting between stationary charged particles
• It includes the combined effect of electric and magnetic forces acting between moving charged particles
• Electromagnetic force is one of the four known fundamental forces
• The other fundamental forces are the strong force, the weak force, and the gravitational force
• Electromagnetism plays a major role in determining the internal properties of most objects encountered in daily life
• Ordinary matter takes its form as a result of intermolecular forces between individual atoms and molecules in matter, and are a manifestation of the electromagnetic force
• Electromagnetism can be described by classical electrodynamics, which is a relativistic field theory
• Quantum electrodynamics (QED) provides the quantum mechanical description of electromagnetism
• Electromagnetic phenomena are defined as the physical interactions among electric charges, magnetic moments, and electromagnetic fields
• Electromagnetism can be thought of as a combination of electricity and magnetism, which are two distinct but closely related phenomena
• Moving electric charges create a magnetic field, and changing magnetic fields induce electric fields
• A changing electric field generates a magnetic field and vice versa, these effects lead to electromagnetic waves
• The classical description of electromagnetism is Maxwell's equations, which describe the behavior of electric and magnetic fields and their interaction with matter
• Electromagnetic waves are disturbances in electric and magnetic fields that propagate through space, carrying energy and momentum
• Examples include radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays
• Electromagnetism is responsible for many phenomena, including light, radio waves, electricity, and magnetism
• Electromagnetic forces govern the interactions between atoms and molecules, and are responsible for the chemical properties of matter
• Electromagnetism is essential for many technologies, including electric power generation and distribution, radio communication, and medical imaging

Thermodynamics

• Thermodynamics is a branch of physics that deals with heat, work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation
• It defines macroscopic variables (also known as thermodynamic variables)
• Examples of thermodynamic variables: temperature, entropy, internal energy, and pressure
• Thermodynamics describes the average behavior of very large numbers of microscopic constituents
• Thermodynamics is applicable to a wide variety of topics in science and engineering, such as engines, phase transitions, chemical reactions, transport phenomena, and even black holes
• Thermodynamics is based on a set of four laws: the zeroth law, the first law, the second law, and the third law
• The zeroth law states that if two systems are each in thermal equilibrium with a third system, then they are in thermal equilibrium with each other
• The first law states that energy is conserved
• The second law states that the entropy of an isolated system can only increase or remain constant
• The third law states that the entropy of a system approaches a constant value as the temperature approaches absolute zero
• Thermodynamics studies the relationships between macroscopic properties of a system, such as temperature, pressure, and volume
• It does not deal with the microscopic details of the atoms and molecules that make up the system
• Thermodynamics is used to predict the direction of spontaneous processes, such as the flow of heat from a hot object to a cold object
• It establishes the limits of efficiency for engines and other devices
• Equilibrium thermodynamics deals with systems in equilibrium
• Non-equilibrium thermodynamics deals with systems that are not in equilibrium
• Statistical thermodynamics bridges the gap between the microscopic and macroscopic worlds, by using statistical mechanics to derive the laws of thermodynamics from the behavior of the microscopic constituents of the system
• Applications include power generation, refrigeration, chemical processes, and materials science

Classical Mechanics

• Classical mechanics is a branch of physics concerned with the motion of bodies under the influence of forces
• It is also known as Newtonian mechanics
• Classical mechanics describes the motion of macroscopic objects, from projectiles to parts of machinery, and astronomical objects, such as spacecraft, planets, stars, and galaxies
• It provides accurate results when studying large objects that are not extremely massive and speeds that do not approach the speed of light
• When the objects being examined become very small, it is necessary to use quantum mechanics
• When speeds approach the speed of light, it is necessary to use relativistic mechanics
• Classical mechanics is based on a set of laws formulated by Isaac Newton in the 17th century
• Newton's laws of motion:
  • A body remains in its state of rest or uniform motion in a straight line unless acted upon by a force
  • The acceleration of a body is directly proportional to the force acting on it and inversely proportional to its mass
  • For every action, there is an equal and opposite reaction
• Classical mechanics provides a complete and accurate description of the motion of macroscopic objects under a wide range of conditions
• Classical mechanics is used to solve a wide variety of problems, such as predicting the trajectory of a projectile, calculating the motion of a pendulum, and designing bridges and other structures