Physics and Our Universe: How It All Works

Physics is at the heart of our understanding of physical reality. Principles of physics apply universally—from the behavior of the infinitesimally tiny quarks that comprise the protons and neutrons of the atomic nucleus to the ordering of matter into galaxies, galaxy clusters, and superclusters at the largest scales imaginable. Physics also lays the groundwork for the other sciences, especially chemistry and biology. Nevertheless, emergent properties in complex systems mean that physics alone cannot provide a complete and comprehensible description of chemical and biological phenomena.

Physics is mathematical, but its verbal language is just as important. Physicists give everyday words precise meanings, refining common usage or sometimes conveying altogether new concepts. Theories in physics provide the overarching conceptual framework for understanding vast realms of physical phenomena. The mathematics side of physics provides concise statements of physical principles that would be cumbersome to articulate using natural language. Mathematics expresses not only the numbers of physics, but also the relationships between physical quantities.

Without motion, the universe would be frozen in an instant of time. Motion is ubiquitous, from the swarming of electrons within an atom to the gravitational dance of distant galaxies. Fundamentally, motion is about changes in position. We quantify motion by first defining position and then introducing velocity as the rate of change of position. Velocity, too, can change, and its rate of change defines acceleration. Graphs of position, velocity, and acceleration versus time show how the 3 fundamental concepts of motion are related.

Knowing an object’s initial position and velocity, and its subsequent acceleration, allows us to predict precisely its future position at any given time. This is the idea behind Newton’s clockwork universe theory. In general, predicting the future position of an object presents a mathematical challenge that can be solved with calculus. But in the special case where acceleration doesn’t change, a set of straightforward algebraic equations describes motion. Gravity near Earth’s surface provides an important case of motion with constant acceleration.

We live in a 3-dimensional world; most significant cases of motion necessarily involve at least 2 and often all 3 dimensions. The richness of multiple dimensions requires a new mathematical tool for quantifying motion: the vector. A vector is a quantity that has both size—its magnitude—and direction. As in 1 dimension, the description of motion with constant acceleration in multiple dimensions is straightforward. Near Earth’s surface, the result is so-called projectile motion, which provides a good approximation to the motions of objects.

Motion on a circular path is an important case of 2-dimensional accelerated motion. Even when speed is constant, the motion is nevertheless accelerated because the direction of motion is changing. A simple expression gives the acceleration in circular motion in terms of the speed and the radius of the circle and shows that the direction of the acceleration is centripetal. Even when speed is changing, description of circular motion is straightforward because centripetal acceleration and acceleration along the direction of motion are considered independently.

Over 2000 years ago, Aristotle incorrectly claimed that a force, a push or pull, was needed to sustain motion. Galileo and Newton got it right: They recognized that uniform motion—straight-line motion at constant speed—is a natural state that requires no explanation. What does require explanation is any change in motion. That’s what forces do: they cause change in motion, which can be in speed, direction, or both. Newton summarized the role of force with his 3 laws of motion.

Newton’s second law provides the link between force and acceleration. One important force is gravity, which, near Earth’s surface, results in a force called weight that is proportional to an object’s mass. Because weight and mass are proportional, all objects experience the same gravitational acceleration. If you weigh yourself in an elevator, your apparent weight will be greater than normal as the elevator accelerates upward and less than normal when it accelerates downward.

Newton’s third law is among the most widely misunderstood aspects of classical physics—in part because it’s generally described in terms of the archaic phrase “action and reaction.” Actually, the third law describes forces between interacting objects, asserting that the force one object exerts on another is equal and opposite the force that the second object exerts on the first. It does not stand as a separate feature of classical mechanics but rather goes hand-in-hand with the second law to provide a consistent description of motion.

Force and acceleration are both vectors; they have direction as well as magnitude. Newton’s second law shows that acceleration and force are proportional, which means that an object’s acceleration—but not necessarily its velocity—is in the direction of the net force. Applying this idea to 2-dimensional motion resolves many seeming puzzles of the physical world. This lecture explores applications of Newton’s laws in multidimensional motion and explores the common forces, including normal forces, tension forces, and friction.

In physics, work results when a force acts on an object that moves, provided the force isn’t at right angles to the object’s motion. Energy is a central concept in physics; kinetic energy and potential energy are among the types of energy used in physics. One of the most important principles in physics is conservation of energy, which in this context states that as long as the only forces acting are conservative, then an object’s total energy—the sum of its kinetic and potential energy—is unchanged.

The law of conservation of energy shows that, when only conservative forces act on an object, the object’s total mechanical energy—the sum of its kinetic and potential energy—is unchanged. Conservation of energy provides a “shortcut” to understanding motion, allowing us to predict aspects of complicated motion without getting into intractable mathematical details. Sometimes we’re concerned not so much with energy as with the rate at which we do work, consume energy, or transform energy from one kind to another—power describes this rate.

Newton used his gravitational theory to explain the orbital motion of the planets and even foresaw the possibility of artificial satellites. Newton developed calculus to solve the problem of planetary motion, showing that the orbits of planets must be ellipses. The special case of circular orbits doesn’t require calculus and yields insights into orbital energy and spacecraft maneuvers. Consideration of gravitational energy also shows that it takes a finite speed—the so-called escape speed—for an object to break its bond with Earth or any other gravitating body.

Real objects consist of many particles. In principle, studying many- particle systems is complicated by the need to consider all the forces acting among the particles, as well as any forces applied from outside the system. But Newton’s laws enable us to describe the overall motion of a complex system as though it consisted of a single particle located at a special point called the center of mass. Newton’s third law, which describes the conservation of momentum, lets us analyze intense interactions called collisions.

Rotational motion can be characterized in terms of quantities analogous to those describing ordinary motion: position, velocity, acceleration—which become angular position, angular velocity, and angular acceleration; force, which becomes torque; and mass, which becomes rotational inertia. Together, rotational analogs of acceleration, force, and mass obey a law analogous to Newton’s second law. This, in turn, leads to the concept of angular momentum and the all-important conservation of angular momentum, which explains some surprising and seemingly counter-intuitive phenomena involving rotating objects.

Skyscrapers, bridges, and other structures are in static equilibrium—a state characterized by having no tendency to accelerate or rotate. Thus, objects in static equilibrium must be subject to zero net force and zero net torque. It’s the job of engineers to design structures so these conditions are met, thus ensuring that structures will be in static equilibrium. But even that’s not enough: equilibrium should be stable, meaning that small disturbances that disrupt the system’s state of equilibrium will result in the system returning to equilibrium.

Start a new section in which you apply Newtonian mechanics to more complex motions. In this lecture, study oscillations, a universal phenomenon in systems displaced from equilibrium. A special case is simple harmonic motion, exhibited by springs, pendulums, and even molecules.

Investigate waves, which transport energy but not matter. When two waves coexist at the same point, they interfere, resulting in useful and surprising applications. Also examine the Doppler effect, and see what happens when an object moves through a medium faster than the wave speed in that medium.

Fluid is matter in a liquid or gaseous state. In this lecture, study the characteristics of fluids at rest. Learn why water pressure increases with depth, and air pressure decreases with height. Greater pressure with depth causes buoyancy, which applies to balloons as well as boats and icebergs.

Explore fluids in motion. Energy conservation requires low pressure where fluid velocity is high, and vice versa. This relation between pressure and velocity results in many practical and sometimes counterintuitive phenomena, collectively called the Bernoulli effect—explaining why baseballs curve and how airplane speedometers work.

Beginning a new section, learn that heat is a flow of energy driven by a temperature difference. Temperature can be measured with various techniques but is most usefully quantified on the Kelvin scale. Investigate heat capacity and specific heat, and solve problems in heating a house and cooling a nuclear reactor.

Analyze heat flow, which involves three important heat-transfer mechanisms: conduction, which results from direct molecular contact; convection, involving the bulk motion of a fluid; and radiation, which transfers energy by electromagnetic waves. Study examples of heat flow in buildings and in the sun's interior.

Heat flow into a substance usually raises its temperature. But it can have other effects, including thermal expansion and changes between solid, liquid, and gaseous forms—collectively called phase changes. Investigate these phenomena, starting with an experiment in which Professor Wolfson pours liquid nitrogen onto a balloon filled with air.

Delve into the deep link between thermodynamics, which looks at heat on the macroscopic scale, and statistical mechanics, which views it on the molecular level. Your starting point is the ideal gas law, which approximates the behavior of many gases, showing how temperature, pressure, and volume are connected by a simple formula.

The first law of thermodynamics relates the internal energy of a system to the exchange of heat and mechanical work. Focus on isothermal (constant temperature) and adiabatic (no heat flow) processes, and see how they apply to diesel engines and the atmosphere.

Turn to an idea that has been compared to a work of Shakespeare: the second law of thermodynamics. According to the second law, entropy, a measure of disorder, always increases in a closed system. Order can only increase at the cost of even greater entropy elsewhere in the system.

The second law puts limits on the efficiency of heat engines and shows that humankind's energy use could be better planned. Learn why it makes sense to exploit low-entropy, high-quality energy for uses such as transportation, motors, and electronics, while using high-entropy random thermal energy for heating.

Embark on a new section of the course, devoted to electromagnetism. Begin by investigating electric charge, which is a fundamental property of matter. Coulomb's law states that the electric force depends on the product of the charges and inversely on the square of the distance between them.

On of the most important ideas in physics is the field, which maps the presence and magnitude of a force at different points in space. Explore the concept of the electric field, and learn how Gauss's law describes the field lines emerging from an enclosed charge.

Jolt your understanding of electric potential difference, or voltage. A volt is one joule of work or energy per coulomb of charge. Survey the characteristics of voltage—from batteries, to Van de Graaff generators, to thunderstorms, which discharge lightning across a potential difference of millions of volts.

Study stored electric potential energy in fuels such as gasoline, where the molecular bonds represent an enormous amount of energy ready to be released. Also look at a ubiquitous electronic component called the capacitor, which stores an electric charge, and discover that all electric fields represent stored energy.

Learn the definition of the unit of electric current, called the ampere, and how Ohm's law relates the current in common conductors to the voltage across the conductor and the conductor's resistance. Apply Ohm's law to a hard-starting car, and survey tips for handling electricity safely.

All electric circuits need an energy source, such as a battery. Learn what happens inside a battery, and analyze simple circuits in series and in parallel, involving one or more resistors. When capacitors are incorporated into circuits, they store electric energy and introduce time dependence into the circuit's behavior.

In this introduction to magnetism, discover that magnetic phenomena are really about electricity, since magnetism involves moving electric charge. Learn the right-hand rule for the direction of magnetic force. Also investigate how a current-carrying wire in a magnetic field is the principle behind electric motors.

No matter how many times you break a magnet apart, each piece has a north and south pole. Why? Search for the origin of magnetism and learn how magnetic field lines differ from those of an electric field, and why Earth has a magnetic field.

Probe one of the most fascinating phenomena in all of physics, electromagnetic induction, which shows the direct relationship between electric and magnetic fields. In a demonstration with moving magnets, see how the relative motion of a magnet and an electric conductor induces current in the conductor.

Survey some of the technologies that exploit electromagnetic induction: the electric generators that supply nearly all the world's electrical energy, transformers that step voltage up or down for different uses, airport metal detectors, microphones, electric guitars, and induction stovetops, among many other applications.

Study the phenomenon of self-inductance in a solenoid coil, finding that the magnetic field within the coil is a repository of magnetic energy, analogous to the electric energy stored in a capacitor. Close by comparing the complementary aspects of electricity and magnetism.

Direct current (DC) is electric current that flows in one direction; alternating current (AC) flows back and forth. Learn how capacitors and inductors respond to AC by alternately storing and releasing energy. Combining a capacitor and inductor in a circuit provides the electrical analog of simple harmonic motion introduced in Lecture 17.

Explore the remarkable insight of physicist James Clerk Maxwell in the 1860s that changing electric fields give rise to magnetic fields in the same way that changing magnetic fields produce electric fields. Together, these changing fields result in electromagnetic waves, one component of which is visible light.

Starting a new section of the course, discover that light often behaves as rays, which change direction at boundaries between materials. Investigate reflection and refraction, answering such questions as, why doesn't a dust mote block data on a CD? How do mirrors work? And why do diamonds sparkle?.

See how curving a mirror or a piece of glass bends parallel light rays to a focal point, allowing formation of images. Learn how images can be enlarged or reduced, and the difference between virtual and real images. Use your knowledge of optics to solve problems in vision correction.

Returning to themes from Lecture 18 on waves, discover that when light interacts with objects comparable in size to its wavelength, then its wave nature becomes obvious. Examine interference and diffraction, and see how these effects open the door to certain investigations, while hindering others.

Embark on the final section of the course, which covers the revolutionary theories that superseded classical physics. Why did classical physics need to be replaced? Discover that by the late 19th century, inexplicable cracks were beginning to appear in its explanatory power.

Review the famous Michelson-Morley experiment, which was designed to detect the motion of Earth relative to a conjectured "ether wind" that supposedly pervaded all of space. The failure to detect any such motion revealed a deep-seated contradiction at the heart of physics.

Discover the startling consequences of Einstein's principle of relativity—that the laws of physics are the same for all observers in uniform motion. One result is that the speed of light is the same for all observers, no matter what their relative motion—an idea that overturns the concept of simultaneity.

Einstein's special theory of relativity upends traditional notions of space and time. Solve the simple formulas that show the reality of time dilation and length contraction. Conclude by examining the twins paradox, discovering why one twin who travels to a star and then returns ages more slowly than the twin back on Earth.

In relativity theory, contrary to popular views, reality is what's not relative—that is, what doesn't depend on one's frame of reference. See how space and time constitute one such pair, merging into a four-dimensional space-time. Mass and energy similarly join, related by Einstein's famous E = mc2.

Special relativity is limited to reference frames in uniform motion. Following Einstein, make the leap to a more general theory that encompasses accelerated frames of reference and necessarily includes gravity. According to Einstein's general theory of relativity, gravity is not a force but the geometrical structure of spacetime.

Begin your study of the ideas that revolutionized physics at the atomic scale: quantum theory. The word "quantum" comes from Max Planck's proposal in 1900 that the atomic vibrations that produce light must be quantized—that is, they occur only with certain discrete energies.

Apply what you've learned so far to work out the details of Niels Bohr's model of the atom, which patches one of the cracks in classical physics from Lecture 44. Although it explains the energies of photons emitted by simple atoms, Bohr's model has serious limitations.

In the 1920s physicists established that light and matter display both wave- and particle-like behavior. Probe the nature of this apparent contradiction and the meaning of Werner Heisenberg's famous uncertainty principle, which introduces a fundamental indeterminacy into physics.

In 1926 Erwin Schrödinger developed an equation that underlies much of our modern quantum-mechanical description of physical reality. Solve a simple problem with the Schrödinger equation. Then learn how the merger of quantum mechanics and special relativity led to the discovery of antimatter.

Drawing on what you now know about quantum mechanics, analyze how atoms work, discovering that the electron is not a point particle but behaves like a probability cloud. Investigate the exclusion principle, and learn how quantum mechanics explains the periodic table of elements and the principle behind lasers.

See how atoms join to make molecules and solids, and how this leads to the quantum effects that underlie semiconductor electronics. Also probe the behavior of matter in ultradense white dwarfs and neutron stars, and learn how a quantum-mechanical pairing of electrons at low temperatures produces superconductivity.

In the first of two lectures on nuclear physics, study the atomic nucleus, which consists of positively charged protons and electrically neutral neutrons, held together by the strong nuclear force. Many combinations of protons and neutrons are unstable; such nuclei are radioactive and decay with characteristic half lives.

Investigate nuclear fission, in which a heavy, unstable nucleus breaks apart; and nuclear fusion, where light nuclei are joined. In both, the released energy is millions of times greater than the energy from chemical reactions and comes from the conversion of nuclear binding energy to kinetic energy.

By 1960 a myriad of seeming elementary particles had been discovered. Survey the standard model that restored order to this subatomic chaos, describing a universe whose fundamental particles include six quarks; the electron and two heavier cousins; elusive neutrinos; and force-carrying particles such as the photon.

Trace the discoveries that led astronomers to conclude that the universe began some 14 billion years ago in a big bang. Detailed measurements of the cosmic microwave background and other observations point to an initial period of tremendous inflation, followed by slow expansion and an as-yet inexplicable accelerating phase.

Having covered the remarkable discoveries in physics, turn to the great gap in our current knowledge, namely the nature of the dark matter and dark energy that constitute more than 95% of the universe. Close with a look at other mysteries that physicists are now working to solve.