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Concepts
The ideas that run through the whole app, explained properly: why it's so, how we know, and how sure physicists are. Look for the markers around the app for the short version.
The four threads
Clocks disagreeTwo clocks that start together, then take different paths through space and time, disagree when they meet. Nothing is wrong with the clocks: time itself passes at different rates.Read →Light and 'now''Now' is local. Across distances there's no universal present — only what light has had time to bring you, and a choice of how to slice spacetime into moments.Read →The arrow of timeThe laws barely care which way time runs. The world does. The difference comes from the odds — and from how the universe began.Read →Why 3 + 1?Change the number of space or time dimensions and something essential breaks. That tells us why we're here — not why 'here' exists.Read →
Core ideas
SpacetimeSpace and time aren't separate stages. They're one geometry, and each observer slices it into 'space' and 'time' in their own way.Read →Time dilationHow much slower? At walking pace, immeasurably; on an airliner, billionths of a second a day; near light speed, years.Read →EntropyEntropy isn't mess. It's a count — how many ways the parts could be arranged without the whole looking any different.Read →Radiation: the beginning and the endRadiation, then matter, then radiation again — with the universe's free energy spent along the way, never made anew.Read →Expansion and redshiftThe universe isn't exploding into empty space. The space between galaxies is growing — everywhere at once.Read →HorizonsHorizons aren't walls. They're limits on what can reach whom — set by light's speed and the shape of spacetime.Read →Quantum timeQuantum mechanics treats time as the stage, not an actor. That works brilliantly — and it's exactly what breaks when quantum rules meet gravity.Read →
Big questions, bold ideas
Is time travel possible?Forward, yes — astronauts already do. Backward, the equations leave a door ajar, and physics hasn't found a way through it.Read →HolographySpace may be built from entanglement — like a hologram, where a 3D image is encoded on a flat surface. For model universes the evidence is strong; for ours it's an open bet.Read →Two-time physicsA second time dimension breaks things we rely on. A handful of physicists think the breakage can be tamed — and that the tamed version might explain puzzles in ours.Read →
How this app works
The scales of timeAn astronaut's watch, a light clock and the edge of the universe are the same story, told at different sizes.Read →How sure? The tagsScience isn't one pile of facts. Each claim here says how far it has got — and one paper can hold claims at several levels.Read →The five hurdlesFive things any idea about time has to get past. They're a checklist, not a verdict — and the scores are made to be argued with.Read →