1.

Here is the puzzle that physics starts with: why does anything happen at all? Drop a ball — it falls. Strike a match — it burns. A magnet pulls iron across a table without touching it. The Sun rises every morning without being wound up. These things seem obvious, but for most of human history no one could explain why they happened, only that they happened. Ancient philosophers guessed — Aristotle thought heavy objects fell faster because "heaviness" was their nature. For two thousand years, that was the best anyone could do. The puzzle of physics is the puzzle of cause and effect at the most fundamental level: what are the actual rules, and are they consistent everywhere and always? The astonishing answer — which we will spend this entire course unpacking — is yes. The same rules that make your phone screen light up also govern exploding stars a billion light-years away. That is not obvious. It is, in fact, one of the most surprising facts about the universe. Physics is the attempt to find those rules, state them precisely, and use them to predict what will happen next.

2.

Physics is the study of matter, energy, and the fundamental forces that govern their interactions. That definition sounds dry, so let's unpack every word. Matter is everything made of stuff — you, air, water, stars. Energy is the capacity to cause change — it comes in forms (kinetic, heat, light, chemical, nuclear) but is never created or destroyed, only transformed. Forces are the basic ways that matter interacts: there are exactly four of them. Gravity pulls every mass toward every other mass. Electromagnetism governs light, electricity, magnetism, and chemistry. The strong nuclear force holds atomic nuclei together. The weak nuclear force governs radioactive decay. Everything that has ever happened in the universe — from the Big Bang to the thought you are having right now — is, at the deepest level, just those four forces acting on matter and transforming energy. That is the audacious claim of physics: that reality is that clean, that unified, that mathematically understandable. Remarkably, the evidence overwhelmingly supports this claim. Physics has never once been caught being wrong about a well-tested prediction.

3.

The story of physics is one of the great intellectual adventures of humanity. It began in earnest with Galileo Galilei in the late 1500s, who did something radical: instead of arguing about how things should behave based on philosophy, he dropped balls from buildings and measured what actually happened. He discovered that heavy and light objects fall at the same rate — directly contradicting Aristotle. Then came Isaac Newton, who in 1687 published the Principia Mathematica, arguably the most important scientific book ever written. In it, he explained Galileo's falling balls, the orbit of the Moon, the tides, and the paths of comets using a single set of equations. For two centuries, Newton's framework seemed complete — the clockwork universe, perfectly predictable. Then, in 1905, a 26-year-old patent clerk named Albert Einstein published four papers that shook the foundations of everything. He showed that the speed of light is constant for all observers — and derived consequences that overturned our understanding of space, time, and mass. In 1915, he reimagined gravity itself. Simultaneously, Bohr, Heisenberg, and Schrödinger were building quantum mechanics, which revealed that at the smallest scales, the universe is irreducibly probabilistic. Physics has never stopped being surprising.

4.

The practice of physics has a distinctive rhythm. First, you observe something in the world — a pattern, an anomaly, a puzzling measurement. Then you build a model: a simplified mathematical description that captures the key features and ignores everything else. You use the model to make predictions, then you test those predictions against reality. If the predictions hold, you keep the model. If they fail, you refine or replace it. The most powerful models in physics are the ones that make accurate predictions far beyond the situations they were built to explain. Newton built his laws to describe planets and falling apples. Two centuries later, those same equations were used to calculate the trajectory of the Apollo missions to the Moon — a scenario Newton never imagined. Today, physics operates on two great frameworks. General relativity describes gravity and the large-scale structure of the universe with extraordinary accuracy. Quantum mechanics describes everything at the atomic and subatomic scale with even greater accuracy. The deep tension? These two frameworks contradict each other at their boundary. When you try to apply quantum mechanics to gravity, the mathematics produce nonsense. Physicists are still hunting for the unified theory that will reconcile them. It is the biggest open problem in fundamental science.

5.

Physics often expresses its deepest ideas in equations, and none is more foundational to classical physics than Newton’s Second Law: F equals m times a. Here, F represents force, the net push or pull acting on an object, measured in Newtons. A single Newton is the force needed to accelerate one kilogram of mass at one meter per second squared. That m stands for mass, the amount of matter in an object, measured in kilograms. More mass means greater inertia—a greater resistance to changes in motion. And a is acceleration, how quickly an object’s velocity changes each second, measured in meters per second squared. It’s crucial to remember that acceleration isn't just speed—it’s the rate of change of speed or direction. This equation reveals simple relationships. If you double the force while keeping the mass constant, the acceleration doubles—force and acceleration are directly proportional. But if you apply the same force to double the mass, the acceleration is cut in half—mass resists that change. A special case arises when the net force F is zero, which means acceleration is also zero. This implies a constant velocity, which includes remaining perfectly still, directly encapsulating Newton’s First Law. Consider a rocket with a thrust of five hundred thousand Newtons and a mass of fifty thousand kilograms. Using F equals m times a, we find its acceleration is ten meters per second squared—roughly one G, the acceleration due to Earth’s gravity. As the rocket burns fuel, its mass decreases, so the same thrust produces an even greater acceleration. Newton’s Second Law is far more than just a formula; it is a profound statement about cause and effect, connecting the forces acting on an object to its resulting motion.

6.

Several big ideas thread through all of physics and will resurface again and again in this course. The first is conservation laws: certain quantities — energy, momentum, electric charge, angular momentum — are never created or destroyed in any physical process, only transformed or transferred. Conservation laws are so powerful they can solve problems that would be impossibly complex using forces alone. The second is fields: rather than thinking of forces as "action at a distance" (how does the Sun pull Earth when there's nothing between them?), physics describes space as filled with fields — the gravitational field, the electromagnetic field — and objects interact by distorting and responding to these fields. The third is symmetry: the deepest laws of physics reflect symmetries in nature, and Noether's theorem (1915) proves that every conservation law corresponds to a symmetry. Conservation of energy corresponds to the fact that the laws of physics are the same today as yesterday. The fourth is that at the smallest scales, the deterministic clockwork of Newton breaks down entirely, replaced by the genuinely probabilistic weirdness of quantum mechanics. Understanding when to use which framework — and why — is one of the central skills this course develops.

7.

Physics isn't just theory—it builds the world around us. Take GPS satellites, for instance, orbiting high above. To work accurately, they must constantly correct for Einstein's theories of relativity, both special and general, because time passes differently for them than it does on Earth. Without those adjustments, your navigation would drift by about ten kilometers every single day. Or consider MRI machines in hospitals, which use the quantum mechanical "spin" of hydrogen nuclei and powerful magnetic fields to image your soft tissues non-invasively. Nuclear power plants convert tiny amounts of mass directly into enormous energy, following Einstein's famous equation E equals mc squared—a single kilogram of uranium fuel provides as much energy as burning three million kilograms of coal. Even your smartphone is a pocket-sized physics lab: its transistors rely on quantum tunneling, the touchscreen uses electromagnetic induction, and its GPS function needs those same relativistic corrections. Weather forecasting, a complex ballet of data and computation, solves Newton's laws of fluid dynamics on vast grids; today's five-day forecasts are as good as one-day forecasts were just forty years ago. And then there are gravitational wave detectors like LIGO, which measure distortions in spacetime smaller than one ten-thousandth the diameter of a proton, confirming a prediction Einstein made over a century ago. These examples show that physics isn't an abstract pursuit—it’s the engine of modern life, constantly revealing deeper truths and empowering incredible technologies.

8.

Physics is nowhere near finished — in fact the 21st century has revealed embarrassingly large holes in our understanding. About 27% of the universe appears to be made of "dark matter" — something that exerts gravity and holds galaxies together, but emits no light and interacts with nothing we can detect directly. Another 68% appears to be "dark energy" — a mysterious repulsive force causing the universe's expansion to accelerate. That means everything physicists have ever directly studied — every atom, every particle, every photon — accounts for only about 5% of the universe's total content. There is also the deep problem of unification: general relativity and quantum mechanics are each spectacularly accurate within their domains, but they are mathematically incompatible. When you apply quantum mechanics to gravity, the equations produce nonsense infinities. A theory of quantum gravity — perhaps string theory, perhaps loop quantum gravity, perhaps something no one has thought of yet — is the biggest open problem in fundamental physics. These are not small gaps in our knowledge. They are vast, thrilling unknowns waiting for the next generation of physicists to crack open.

9.

Physics sits at the centre of a family of related sciences, and understanding the relationships clarifies what physics is and isn't. Chemistry is physics applied to electrons and how atoms combine — every chemical reaction is, at the most fundamental level, electrons rearranging themselves according to quantum mechanical rules. Biology is chemistry that has organised itself into self-replicating, evolving systems — the physics of life. Astronomy and astrophysics apply physics to objects beyond Earth, from planets to black holes to the large-scale structure of the cosmos. Engineering takes the results of physics and uses them to build things that work. The key distinction: physics seeks the fundamental rules, the ones from which everything else can in principle be derived. Mathematicians sometimes quip that physics is "applied mathematics," though physicists would push back — mathematics is the tool, but physics is ultimately grounded in measurement and experiment. If the math doesn't match measurement, no matter how beautiful it is, it gets discarded. That empirical commitment — the willingness to let reality have the last word — is what makes physics a science rather than a philosophy.

10.

Several stubborn misconceptions about physics trip up students endlessly. First: "physics is just math." Wrong — physics is about reality. The math is the language we use to describe it, but a physicist who can't connect equations to physical intuition is lost. Second: "Newton's laws are wrong because Einstein replaced them." Also wrong — general relativity reduces to Newtonian gravity in low-speed, weak-gravity situations. Newton's laws work perfectly for building bridges, launching satellites, and describing your daily life. Einstein didn't prove Newton wrong; he revealed where Newton's approximation breaks down. Third: "quantum mechanics only matters at the atomic scale." Actually, quantum effects are essential for understanding lasers, semiconductors, MRI, photosynthesis, and the nuclear reactions powering the Sun — all technologies and phenomena that affect your daily life. Fourth: "physics is about memorising formulas." This course will prove otherwise. The goal is physical intuition — the ability to look at a situation and understand what's happening before you write down a single equation. Formulas are compressed intuition; the intuition comes first.

11.

Physicists use an extraordinary range of instruments to probe reality at every scale. At the largest scales, they use telescopes — optical, radio, X-ray, and gravitational wave detectors — to observe the universe. The LIGO detector that first detected gravitational waves in 2015 can measure a displacement smaller than one ten-thousandth the diameter of a proton across its 4-km arms. At the smallest scales, particle physicists use the Large Hadron Collider at CERN — a 27-km ring that accelerates protons to 99.9999991% of the speed of light and smashes them together, recreating conditions from the first billionth of a second after the Big Bang. In between, physicists use superconducting magnets, atomic force microscopes, ultra-cold atom traps, and laser interferometers. A crucial modern tool is computer simulation: many systems are too complex to solve analytically, so physicists write code to numerically solve the equations and model the behaviour. The line between experimental physicist, theoretical physicist, and computational physicist has become increasingly blurred — and all three roles are essential. Physics is a collective enterprise, and the best experimental results of the last century have typically required thousands of collaborating scientists.

12.

You can test one of Newton's deepest insights right now with a piece of string, a small heavy object, and your phone's stopwatch. Tie the object to one end of the string to make a pendulum. The period of a pendulum — the time for one full back-and-forth swing — depends on the string's length, but not on the mass of the hanging object and not on how far you pull it back (as long as you keep the angle small, under about 15 degrees). Use your stopwatch to time 10 full swings, then divide by 10 to get the period. Now swap to a heavier object. The period should stay the same. This baffled Galileo's contemporaries: why doesn't a heavier pendulum swing faster? The answer is that more mass means more gravitational force pulling down, but also more inertia resisting acceleration — and these effects cancel exactly. That cancellation is not trivial; it is a deep property of gravity that Einstein later explained using general relativity. With a 1-metre string, you should measure a period of almost exactly 2 seconds. Measure it and see. You are doing physics.

13.

Today we surveyed physics as a whole. Physics is the study of matter, energy, and the four fundamental forces — the deepest layer beneath all other sciences. The central claim is audacious: the universe follows mathematical rules, discoverable by experiment and reason. We traced the story from Galileo's experiments and Newton's Principia to Einstein's relativity and the quantum revolution. We met the two great frameworks — general relativity and quantum mechanics — that each work perfectly in their domain but contradict each other at their boundary. We saw how physics underlies GPS, MRI, nuclear power, and gravitational wave detection. And we confronted the thrilling open questions: dark matter, dark energy, and quantum gravity. The key skill this course builds is not formula memorisation but physical intuition — the habit of looking at the world and asking: what is really happening here, and why? That habit of mind is what makes a physicist. In the next class, we begin building the mathematical language needed to describe motion precisely — and from there, the whole edifice of classical mechanics.

Mastery quiz

1. How does this class define physics?
   • The study of living organisms and their evolution
   • The study of matter, energy, and the fundamental forces that govern their interactions
   • The study of chemical reactions between atoms
   • The study of numbers, equations, and pure mathematics
2. In Newton's Second Law F = ma, what happens to the acceleration if the same force is applied to twice the mass?
   • It doubles
   • It stays the same
   • It is cut in half
   • It becomes zero
3. According to the class, roughly what fraction of the universe's total content is made of the ordinary matter physicists have directly studied?
   • About 5%
   • About 27%
   • About 68%
   • About 95%
4. Why does a heavier object on a pendulum swing with the same period as a lighter one?
   • Heavier objects experience less gravity
   • More mass means more gravitational force pulling down, but also more inertia resisting acceleration, and these effects cancel exactly
   • The string stretches more under heavier objects, balancing the speed
   • Period depends only on how far the pendulum is pulled back