Physics, drawn.
Physics is taught in equations and understood in pictures, and the pictures are usually the part left out. This is a collection of essays about the second kind — one idea at a time, illustrated to the point where the argument becomes visible.
Nine fields
the divisions physics is conventionally cut into
Mechanics
Motion, force, and the quantities that refuse to change.
Waves
Oscillation, and everything that turns out to be an oscillation.
Optics
Light, and the small number of rules it obeys.
Electromagnetism
Charge, field, and the lines drawn between them.
Thermodynamics
Heat, disorder, and the one law with a direction in it.
Relativity
Space and time, drawn on the same axes.
Quantum
Where the continuous picture runs out, and what replaces it.
Fluids
Matter that will not hold a shape, and the forces that act in it anyway.
Astrophysics
Gravity read as geometry, and the laws carried where no laboratory can follow.
Start anywhere
one from each field, of 533
Mechanics
The pendulum, and the small lie that makes it simple
A pendulum's period does not depend on how far it swings. This is one of the most useful false statements in physics, and it is worth knowing exactly how false.
Waves
A wave is a shape that travels, and nothing else does
In a wave on water, no water goes anywhere. What moves is the shape — and separating the two motions is the whole of wave physics.
Optics
The bend at the boundary, and what it is really about
Light changes direction when it changes speed. Snell's law is the geometry of that statement, and it can be derived without knowing anything about light at all.
Electromagnetism
Field lines are a choice, not a discovery
Nothing in space is arranged in lines. The lines are a drawing convention — and an unusually good one, because three separate facts about the field survive the translation.
Thermodynamics
The speeds in a still room
The air in a quiet room is not still. Every molecule in it is moving at hundreds of metres per second, and temperature is a single number summarising an entire distribution.
Relativity
Two axes, one speed, and a diagram that does the arguing
Put position across and time up, insist that light travels at forty-five degrees for everyone, and nearly every result in special relativity becomes something to read off rather than derive.
Quantum
The curve that would not come down
Classical physics predicted that a warm object radiates infinite power at short wavelengths. Every step of the derivation was correct, the prediction was absurd, and closing the gap required assuming that energy comes in lumps.
Fluids
The pressure that only knows depth
A litre of water and a swimming pool press equally hard on a floor at the same depth. Pressure in a still fluid is a scalar with no direction of its own, it depends on how far down and on nothing else, and the shape of the container falls out of the arithmetic entirely.
Astrophysics
The floor that cannot be told from gravity
Seal a laboratory, take away the windows, and no experiment inside it can distinguish standing in a gravitational field from accelerating through empty space. That is not a philosophical remark — it forces light to bend, forces clocks to disagree, and has a size at which it stops being true.
Recently added
18 essays, 18 September 2026
- The half of the bend a slow body never feels — astrophysics
- The orbit special relativity cannot close — astrophysics
- The force whose sign a frequency chooses — electromagnetism
- The balloon that leans the wrong way — fluids
- The gas that cools itself into clumps — fluids
- The bounces that add up to a stop — mechanics
- The ellipse a pendulum turns by itself — mechanics
- The medium that images every point — optics
- The lens with no axis — optics
- The site that fills like an electron level — thermodynamics
- The product doping cannot move — thermodynamics
- The metal that is yellow because it is heavy — quantum
- The speed below which nothing can be made — quantum
- The right angle a fast collision closes — relativity
- The spring that becomes a light-clock — relativity
- The phases that turn a glow into pulses — waves
- The lattice no phase can bend — waves
- The wiggle faster than any wave in it — waves
All 533 essays · every field · every ladder · every object named · every figure · what is taught wrongly · search
Threads running through
themes, not chapters
What stays the same
Conservation laws, and the habit of solving a problem by refusing to look at the middle of it.
The same equation again
A pendulum, a circuit, a molecule and a bridge, all obeying one differential equation and not knowing about each other.
Approximations that lie
The small-angle assumption, the frictionless plane, the point mass — where each is fine, and precisely where it stops being.
Fields, not forces
Replacing action at a distance with something that fills space, and what that buys.
The arrow of time
Nearly every law works equally well backwards. Almost nothing else does, and the gap between those facts is thermodynamics.
Where the model stops
Every picture in physics has a domain of validity, and the interesting physics usually lives at its edge.
Made into an instrument
The point at which a principle stops explaining something and starts measuring it — a critical angle sold as a refractometer, a Doppler shift weighing a planet, a null result testing a law.
Order out of the random
Enormous numbers of unpredictable particles producing quantities you can print on a dial.
The shape decides
A number that looks as though it should depend on the forces involved and turns out to depend only on the arrangement — a falloff exponent, a capacitance, a moment of inertia, the angle a rainbow has to be.
Who is measuring
Quantities that are not properties of the thing but of the thing and an observer: which events are simultaneous, how long something is, how fast it is going and whose second is being counted.
What happens at the edge
Quantities that live on a surface rather than in a volume, and conditions imposed at an edge that decide what the whole interior may do — a skin that costs energy per unit area, charge that sits entirely on the outside of a conductor, a clamped end that permits some frequencies and forbids the rest, a wall that quantises a box, and the interface where light has to change direction.
Only some values fit
Continuous objects producing discrete answers. A string that will sound some frequencies and not others, three permitted exponents, a speed no composition can exceed — the first sightings of quantisation, in a subject that has not reached it yet.