🚧 Draft — not final. This page was auto-generated as day-one scaffolding for the project and still needs a human pass before it reads as finished. The blanks and placeholders below are intentional — someone who runs this project should fill them in and delete this notice.

Notes — special relativity

One concept per section, in my own words. The rule of this project: the note isn't done until it can be explained to someone without the diagram in front of me. If a section still needs the formula to make sense, it isn't understood yet.

Spacetime diagrams (the spine)

Space across, time up. A stationary thing traces a vertical line; a moving thing tilts. Light always draws the same 45° line — that constancy is the one stubborn fact everything else bends around. (Fill in my own words after the minutephysics course in sources/.)

The two postulates

1. The laws of physics are the same for everyone moving steadily.
2. The speed of light is the same for everyone — no matter how fast they move.

The second one sounds harmless and detonates the rest: if light's speed can't change, then time and space must.

The invariant interval (work by hand until it's not a formula)

Different observers disagree about the time between two events, and about the distance — but they agree on one combination:

s² = (c·Δt)² − (Δx)²

• Derive it once from the light-clock picture, no notes
• Check it survives a change of observer (the whole point of "invariant")

Explain-back test: why "invariant" is the word that matters here — what would break if it weren't.

Next

Time dilation and length contraction as consequences of the interval, not separate rules to memorise. Then the long climb: equivalence principle → geodesics → "matter curves spacetime."