Physics & Space

Physical principles and deep time, currently spanning relativity, Earth history, geologic evidence, thermodynamics, 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.8 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

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 \(\lambda\), then
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 bodyplan 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 nonavian dinosaurs along with ammonites and many marine plankton groups. Roughly threequarters 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 lightyears, giving a diameter near 93 billion lightyears.

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 solidonsolid 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 shortlived — 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 classicalmechanics world.