Physics & Space

Physical principles and deep time: mechanics, electric and magnetic fields, circuits, thermal physics, relativity, Earth history, and the scale of the observable universe.

Topics

RelativityHow space and time change when motion, light speed, and reference frames are taken seriously.2 Earth History & GeologyDeep time, changing continents, mass extinctions, dating evidence, and the chemical history of Earth.13 Cosmology & ScaleThe size and observable structure of the universe, kept in human-scale comparisons.1 Physical PrinciplesPhysical ideas that explain everyday phenomena and broad constraints on how systems behave.5 Classical MechanicsHow motion, forces, energy, momentum, rotation, oscillation, and fluids fit into one coherent mechanics toolkit — with calculus bridges where they reveal the deeper structure.10 Electricity & MagnetismFrom charge and field geometry through potential, circuits, magnetic forces and induction, with derivations and energy checks.8 Thermal PhysicsFrom microscopic motion and temperature to heat, work, entropy and the limits of thermal machines.3

All Entries

Relativity

Special RelativitySpecial relativity starts from two principles:
Time DilationA clock moving relative to an inertial observer accumulates less proper time between two events than a clock at rest with that observer. For constant relative speed,

Earth History & Geology

Geological Time OverviewEarth history is 4.54 billion years long, but the part most familiar from fossils occupies only a thin recent slice. The Phanerozoic Eon — abundant visible animal life — begins only 538.8 million years ago, roughly the last 12% of Earth's history.
How We Know the Age of EarthEarth is about 4.54 billion years old. The important part is not memorizing the number; it is understanding why we trust it.
Radiometric DatingRadiometric dating turns radioactive decay into a clock. If a parent isotope decays with constant decay constant λ, then
Early Earth: What the Rocks Actually Tell UsA wet planet is not yet a living planet. Before asking how life began, ask a narrower question: what environments were actually available, and how do we know? This is where origin-of-life research meets geology rather than a cinematic reconstruction of a steaming ocean.
Prebiotic Chemistry: Ingredients Are Not Yet a SystemLife uses carbon compounds, but “organic” does not mean “made by an organism.” The deeper problem is how nonliving chemistry could repeatedly make useful products faster than dilution, decomposition and side reactions removed them.
RNA and the Problem of Making CopiesA sequence can store information without being able to reproduce it. RNA is interesting because it can potentially join two roles that modern cells distribute among different molecules: a sequence that can be copied and a folded structure that can catalyze chemistry.
Protocells, Metabolism and Competing Origin HypothesesA catalyst dispersed in an ocean may help every nearby competitor. A useful reaction inside a compartment can preferentially help the compartment containing it. A boundary changes who benefits from chemistry, making it a potential bridge from molecular reactions to reproducing lineages.
Earliest Life: Fossils, Inference and the Road to OxygenThe oldest surviving evidence of life supplies a minimum antiquity, not a birthday. If a convincing fossil is 3.48 billion years old, life was present by then; it may have begun much earlier. Nor does finding life in one habitat prove that life originated there.
Great Oxidation EventFor much of early Earth history, the atmosphere contained almost no free oxygen. Photosynthetic microbes were producing oxygen long before it accumulated in the air, because oxygen was rapidly consumed by sinks — reduced volcanic gases, minerals, and especially dissolved iron in the oceans.
Cambrian ExplosionThe Cambrian explosion is the interval early in the Cambrian when the animal fossil record suddenly becomes much richer and many major body-plan lineages become recognizable. The current international boundary for the start of the Cambrian is 538.8 ± 0.6 million years ago.
The Great DyingThe Permian–Triassic extinction, about 251.9 million years ago, was the most severe known mass extinction. Exact percentages depend on how the fossil record is analyzed, but estimates commonly put marine species loss above 80%, with catastrophic disruption on land as well.
Pangaea and Continental DriftPangaea was the supercontinent that assembled by the late Paleozoic and contained most of Earth's continental land. It began breaking apart roughly 200 million years ago, eventually producing today's Atlantic Ocean and continental arrangement.
End-Cretaceous ExtinctionAbout 66 million years ago, the Cretaceous ended in a mass extinction that eliminated all non-avian dinosaurs along with ammonites and many marine plankton groups. Roughly three-quarters of species are estimated to have disappeared.

Cosmology & Scale

The Scale of the Observable UniverseThe universe is about 13.8 billion years old, but the radius of the observable universe today is roughly 46 billion light-years, giving a diameter near 93 billion light-years.

Physical Principles

EntropyEntropy is often introduced as “disorder,” but that metaphor fails surprisingly quickly. The more useful statistical idea is multiplicity: how many microscopic arrangements correspond to the same macroscopic state.
Why Ice Is SlipperyIce is slippery because the contact between a solid slider and ice is not simply dry solid-on-solid friction. Near the melting point, the surface has enhanced molecular mobility, and sliding can create or thicken a thin lubricating layer through frictional heating and interfacial disorder.
Galileo Thermometer: Temperature Read Through BuoyancyA Galileo thermometer looks like a sealed glass column containing several floating glass bulbs, each carrying a temperature tag.
Double Pendulum: Two Simple Joints, Nonlinear MotionA simple pendulum already has nonlinear dynamics at large angles.
Chaos: Sensitive Dependence Without RandomnessIn ordinary speech, "chaos" means disorder.

Classical Mechanics

Classical Mechanics: The MapPhysics becomes much easier once the formulas stop looking like unrelated tricks.
Kinematics: Position, Velocity, and AccelerationKinematics describes motion without yet asking what causes it.
Forces and Newton's LawsKinematics says how motion changes. Dynamics asks why.
Work, Energy, and PowerForce is a local description: what interaction acts right now?
Linear Momentum, Impulse, and CollisionsMomentum becomes especially powerful when forces are large, complicated, and short-lived — collisions, explosions, catches, impacts.
Torque and Rotational DynamicsA force can change an object's translational motion. But for an extended object, where the force acts also matters.
Rotational Energy and Angular MomentumRotation has close analogues of the energy and momentum ideas used for translation.
Oscillations and Simple Harmonic MotionMany systems displaced from a stable equilibrium experience a force pushing them back toward it.
Fluids: Pressure, Buoyancy, and FlowA fluid cannot sustain a static shear stress the way a rigid solid can. It deforms and flows.
From AP Physics 1 to Physics C Mechanics: What Calculus AddsAP Physics 1 and AP Physics C: Mechanics describe largely the same classical-mechanics world.

Electricity & Magnetism

Electric Fields: Charge, Superposition, and CancellationWhy can a neutral object respond to electricity, and why does zero total charge not imply zero field? The answer requires separating sources, fields, and the charge experiencing a force.
Electric Flux and Gauss’s Law: When Symmetry Does the WorkGauss's law is always true for electric fields, but it is not always a shortcut for finding them. Its power comes from combining a global accounting rule with enough symmetry to determine a local field.
Electric Potential: Turning a Field into an Energy LandscapeVoltage is not a stock of charge or a force. It is potential-energy difference per unit charge. This lets us translate the local force description into the energy accounting already familiar from mechanics.
Current, Resistance, and Power: Where Electrical Energy GoesClosing a circuit does not send a fresh batch of electrons from a battery to a distant lamp at light speed. Mobile charges are already present in the wires. Changes in electromagnetic fields propagate through the circuit, establishing a drift motion whose speed is usually much smaller than the signal speed.
DC Circuits: Kirchhoff’s Laws and Loaded SourcesA circuit drawing is a claim about connections, not a picture of physical wire positions. Two components are in parallel only if both ends connect to the same two nodes. They are in series only when the same current must pass through both, with no branching junction between them.
Capacitors and RC Circuits: Energy Storage Takes TimeA capacitor stores energy by separating opposite charges, creating an electric field between conductors. In the ideal two-terminal model the plates hold +Q and -Q; the device can remain neutral overall while storing energy.
Magnetic Fields and Forces: Bending Motion Without Doing WorkElectric fields can push a stationary charge. Magnetic force on a point charge depends on its motion. The combined classical Lorentz force is
Electromagnetic Induction: Flux Change, Lenz’s Law, and EnergyA stationary magnet near a stationary wire does not continually generate current. What matters is a change in magnetic flux through a circuit, or motion that produces the equivalent electromechanical effect.

Thermal Physics

Temperature, Heat, and the Ideal-Gas BridgeTemperature, heat, and internal energy answer different questions. Their separation is what allows thermal physics to extend mechanics rather than replace it.
The First Law: Why the Path MattersEnergy conservation becomes thermodynamics when we specify a system boundary and distinguish energy transfer as heat from energy transfer as work. For a closed system with negligible changes in bulk kinetic and gravitational potential energy, use
Entropy and Heat Engines: Why Conserved Energy Is Not All UsableThe statistical entropy entry explains why macrostates with many compatible microstates are overwhelmingly favored. Here the aim is operational: calculate entropy changes and use them to decide whether an energy-conserving machine is possible.