Great Ideas in Physics
Idea 1: Cause and effect
The Causality Principle has played an important role in the development of the
theory of knowledge. It states that all real events necessarily have a cause. The principle indicates the existence of a logical relationship between two events, the cause and the effect, and an order between them: the cause always precedes the effect. An important property of the principle is that it entails predictability. If a force is applied on a body then it will cause the body to accelerate. Without a force, there cannot be an acceleration.
Idea 2: Action at a distance
Action at a distance is the concept that an object can be affected or influenced
without being physically in contact by another object. It is the nonlocal interaction of objects that are separated in space. Some forces can act between two objects without physical contact. We call these non-contact forces. Action at a distance explains the operation of non contact forces.
Aristotelian physics holds that every motion requires a conjoined mover. Thus according to Aristotle, action can never occur at a distance, but needs a medium enveloping the body. Although natural motions like free fall and magnetic attraction were recognized in the post-Aristotelian period, the rise of the corpuscularian philosophy again banned unmediated actions at a distance. Cartesian physical theory postulated a ‘subtle matter’ to fill space and provide the medium for force and motion. Its successor, the aether, was postulated in order to provide a medium for transmitting forces and causal influences between objects that are not in direct contact.
“Action at a distance” was introduced in the context of early theories of gravity and electromagnetism. Coulomb's law and Newton's law of universal gravitation describe non cntact forces acting at a distance without an intervening medium. The gravitational force of earth acts on moon at a distance, a bar magnet can pick up a steel paperclip from a distance. Efforts to account for action at a distance in the theory of electromagnetism led to the development of the concept of an electric field which mediated interactions between currents and charges across empty space. According to field theory, we account for the Coulomb (electrostatic) interaction between charged particles through the fact that charges produce around themselves an electric field, whose influence can be felt by other charges as a force.
Idea 3: Newton’s laws of motion
In 1687, Issac Newton in his seminal work "Philosophiæ Naturalis Principia Mathematica" (Mathematical Principles of Natural Philosophy) formalized the description of how massive bodies move under the influence of external forces, and presented his three laws of motion.
In formulating his three laws, Newton simplified the treatment of massive bodies by considering them to be mathematical points with no size or rotation. This allowed him to ignore factors such as friction, air resistance, temperature, material properties, etc., and concentrate on phenomena that can be described solely in terms of mass, length and time.
Thus Newton developed a special kind of relation between abstract mathematical constructs and the physical systems that we observe in the world around us by means of experiment and critical observation. The heart of the radical Newtonian style is the construction on the mind of a mathematical system that has some important features in common with the physical world; this system is then modified when the deductions and conclusions drawn from it are tested against the physical universe.
Newton's laws were verified by experiment and observation for over 200 years, and they are excellent approximations at the scales and speeds of everyday life. Newton's laws of motion, together with his law of universal gravitation and the mathematical techniques of calculus, provided for the first time a unified quantitative explanation for a wide range of physical phenomena.
Idea 4: Newton’s universal law of gravitation
In 1687 English physicist Sir Isaac Newton (1642-1727) published a law of universal
gravitation in his important and influential work Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy). Newton's law of universal gravitation states that bodies with mass attract each other with a force that varies directly as the product of their masses and inversely as the square of the distance between them. It is more precisely expressed with the equation F = Gm1m2/r2, where m1 and m2 are masses, r is the distance between them, and G is universal gravitation constant.
Newton discovered the relationship between the motion of the Moon and the motion of a body falling freely on Earth. By his dynamical and gravitational theories, he explained Kepler’s laws and established the modern quantitative science of gravitation. Newton assumed the existence of an attractive force between all massive bodies, one that does not require bodily contact and that acts at a distance
Gravity is what holds the planets in orbit around the sun and what keeps the moon in orbit around Earth. The gravitational pull of the moon pulls the seas towards it, causing the ocean tides. Gravity creates stars and planets by pulling together the material from which they are made.
Newton’s universal law of gravitation showed that the same laws of physics that govern the fall of an apple also govern the motions of the moon and planets. It began the search for laws of nature that were precise, simple to express, and applied equally to all parts of the universe.
Idea5: Conservation of momentum
The idea of momentum, was developed by the French scientist and philosopher René Descartes, before Newton. By “momentum”, he meant “amount of motion”. Descartes also tried to introduce the general principle of conservation of momentum in collisions. He believed that the total “quantity of motion” in the universe is conserved. But the first correct statement of conservation of momentum was given by English mathematician John Wallis, in 1670, in his Mechanica sive De Motu, Tractatus Geometricus.
However, it turns out that conservation of momentum can be deduced from Newton’s laws, as outlined in Newton’s Philosophiæ Naturalis Principia Mathematica(1687). In the absense of external forces the total momentum of a system remains unchanged. The momentum of an object is that object’s mass multiplied by its velocity. The law of conservation of momentum states that the total momentum of all the objects in the universe never changes.
The law of conservation of momentum helps us understand collisions between objects. The total momentum before a collision equals the total momentum after a collision. We can use this fact to understand how the velocities of objects before a collision relate to velocities after a collision.
Idea 6: Conservation of energy
Though the conservation of energy is of fundamental importance in physics. The law of conservation of energy states that energy can never be created or destroyed; it can only be transferred and converted into different forms. This is one of the most important unifying principles of physics. We constantly experience its effects in our day to day lives. Isaac Newton’s Principia was published in 1687, marking the start of quantitative theoretical physics, but conservation of energy was not established until the 1840s, over 150 years later.
In the 1600s, greats scientists such as Christiaan Huygens and Gottfried Wilhelm Leibniz recognized that something besides momentum seemed to be conserved in the collisions of moving bodies; Leibniz called it the “vis viva” (living force) of the system. Émilie du Châtelet (1706 – 1749) first proposed and tested the hypothesis of the conservation of total energy, as distinct from momentum. In 1798, Count Rumford (Benjamin Thompson) performed measurements of the frictional heat generated in boring cannons, and developed the idea that heat is a form of kinetic energy. The mechanical equivalence principle was first stated in its modern form by the German surgeon Julius Robert von Mayer in 1842. Meanwhile, in 1843, James Prescott Joule independently discovered the mechanical equivalent in a series of experiments. Thus it was firmly established that heat is a form of energy and the total energy is conserved.
The law of conservation of energy is important wherever energy is involved. At power plants, chemical and mechanical energy are transformed into electrical energy. Kitchen appliances transform electrical energy into thermal and mechanical energy. Automobile engines transform chemical energy into thermal and mechanical energy. In each of these cases, the total energy remains conserved.
Idea 7: Unification of Electricity and Magnetism
Scottish Physicist James Clerk Maxwell unified electricity and magnetism, predicted the existence of electromagnetic waves and identified light as an electromagnetic wave of oscillating electric and magnetic fields moving with a speed c (in vacuum) (1865)
In his revolutionary paper titled “A Dynamical Theory of the Electromagnetic Field,” Published in June 1865 in the Philosophical Transactions of the Royal Society, Maxwell described how electricity and magnetism are closely connected. The paper shows how their combined action creates electromagnetic waves. Maxwell further demonstrated that these waves can propagate through any medium, including perfectly empty space. He proved that if the waves travel through pure vacuum, they must move at the speed of light. Finally, he made the giant leap that light, in all its forms, is electromagnetic waves of different frequencies.
Electromagnetism was the first true unification of forces. Its formulation showed that nature offers deep mathematical connections between varied phenomena. Today physicists are searching for a “theory of everything” that unites all of the natural forces: gravitation and the strong and weak nuclear forces, along with electromagnetism.
Idea 8: Einstein’s theory of relativity
In 1905, Albert Einstein proposed the most revolutionary theoretical innovation in physics, the special theory of relativity. Einstein showed that the laws of physics are the same in all inertial reference frames, and that the speed of light in a vacuum was independent of the motion of all observers. The special theory of relativity introduced a new framework for all of physics and proposed new concepts of space and time. Einstein showed that space, time, and mass are not absolute. Instead, all three vary depending on speed. Because speeds are always measured relative to a frame of reference, Einstein’s theory is typically referred to as a theory of relativity.
The most famous equation in physics, E = mc2, comes from Einstein’s theory of relativity. This equation states that energy equals mass multiplied by the speed of light squared. This means that mass and energy are equivalent and interconvertable.
Einstein spent 10 years trying to include acceleration in the theory and published his
theory of general relativity in 1915. When Einstein tried to apply accelerating masses to his special theory, he realised objects with mass must somehow influence the surrounding dimensions (spacetime) in such a way that the object seems to act as if it can pull on other masses. It's as if matter weighs down the fabric of spacetime it is sitting in, creating a 'curve' that causes other nearby matter to slide towards it. Thus, massive objects cause a distortion in space-time, which is felt as gravity.
The mathematical equations of Einstein's general theory of relativity, tested time and time again, are currently the most accurate way to predict gravitational interactions, replacing those developed by Isaac Newton several centuries prior.
Idea 9: The quantum theory
Quantum theory is the theoretical basis of modern physics that explains the nature and
behavior of matter and energy on the atomic and subatomic level. In a pioneering act, Max Planck in 1900 presented his quantum theory to the German Physical Society. Planck explained the black body radiation by assuming that the atomic oscillators emit energy in the form of small packets called quanta of energy. Further Albert Einstein explained photoelectric effect by assuming that light consists of quanta or packets of energy called photons. The three themes of quantum theory are; the quantization of energy, the probabilistic behavior of energy quanta, and the wave–particle nature of matter.
The foundations of quantum mechanics were established during the first half of the 20th century by Max Planck, Niels Bohr, Werner Heisenberg, Louis de Broglie, Arthur Compton, Albert Einstein, Erwin Schrödinger, Max Born, John von Neumann, Paul Dirac, Enrico Fermi, Wolfgang Pauli, Satyendra Nath Bose, Arnold Sommerfeld, and others. The Copenhagen interpretation of Niels Bohr became widely accepted.
Idea10: The standard model
The Standard Model is a theory in particle physics, which describes and predicts all the fundamental particles that constitute matter. It’s a unified theory that includes three of the four known forces in Nature: electromagnetic force, weak nuclear force, and the strong nuclear force. The current formulation was finalized in the mid-1970s. The Standard Model has been verified with the announcement on July 4, 2012, that evidence for the Higgs boson was obtained at the Large Hadron Collider (LHC). The LHC is the world's largest and most powerful particle collider,run by CERN.
The standard model is at the frontier of scientists’ understanding of fundamental physics. Much of the research in basic physics over the last 50 years has been centred around the standard model.
