Time & Gravity · A track where the open problem is the subject · Step 1 of 13 · 6 min read
Time & Gravity
A common intuition is that experience slides along a worldline and that gravity is a slope in the rate of time. Is any of that physics? Half of it is, and it is among the best-measured facts we have. The other half has no variable in any equation. These thirteen pages walk from the measured half to the open half without inventing a step.
Fixed boundary · printed on every T page · asserted by the lintNothing on this page is a theory of everything, and this site proposes none. Every claim carries one of six labels, each tied to a ledger row: MEASURED, DERIVED, BOUND, PROPOSED, OPEN, or SINGLE RESULT. Preprints never rise above PROPOSED. Each page ends with one open question that the next page takes up.
Plate T.00-V · A worldline across a clock-rate gradient, with a marker for the 'now' the intuition imagines. General relativity has the line and the shading; it has no marker. Drawn by timegravity/make_plates.py from tools/timegravity_kernel.py.About this illustrationThis plate is a deterministic drawing, not an AI-generated image and not a photograph. The curve is the kernel's own trajectory; the shading exaggerates a clock-rate difference of about one part in 10^16 per metre so that it can be seen at all.
The question, drawn once
A picture of the question, not a result. The horizontal axis is coordinate time, the vertical axis is height, and the shading is how fast a clock runs at each height, darker meaning slower. The marker is 'now' as the intuition imagines it, a point sliding along the worldline; move it with the slider or the arrow keys. General relativity has the worldline and the shading. It has no marker. Which pages carry that difference is what the map below is for.
The slider moves a picture, not a variable. No equation on the pages that follow has a term for it.
The thirteen steps, in reading order
Step 1 of 13T.00The hubEnds with: Is any of this physics?Live
Step 7 of 13T.06Does gravity collapse the wave?Ends with: If gravity neither collapses nor is shown to entangle, what is a superposition of slopes?Live
Step 9 of 13T.08A horizon has a temperatureEnds with: Does a real horizon have the hairless geometry that this thermodynamic law assumes?Live
Step 10 of 13T.09A horizon rings like a bellEnds with: If the classical horizon has no hair and only grows, what happens to quantum information when the black hole evaporates?Live
Step 11 of 13T.10Page's counting argumentEnds with: Where does the information go, in our universe?Live
Step 12 of 13T.11What selects the world a clock sees?Ends with: If the electromagnetic bath only records and decoheres what couples to photons, how could our clocks ever detect a sector of matter that never scatters light at all?Live
Step 13 of 13T.12Two clocks that only meet in gravityEnds with: Does the 84.3% of matter that leaves no electromagnetic record couple to our clocks only through the curvature of proper time, or through a portal just below today's optical-clock bounds?Live
Archival Tribute · The Hands Behind the Instruments
The Humans Who Built the Clockwork (1905–2026)
Across all thirteen steps of this track (T.00–T.12), every equation and every row in the grounding ledger began with human beings: machinists cutting platinum wedges to the micrometer, physicists threading helium bags through brick towers, clockmakers booking airline seats for atomic standards, experimentalists trapping microwave cat states and comparing optical clocks against dark-matter waves, and theorists counting states by hand. Before we step into the instruments, we honor the people whose patience and craft built this record.
1905–1916 · Bern, Prague & Berlin
Albert Einstein & Karl Schwarzschild
Thought experiments, field equations & the exact spherical metric
In 1905 Einstein wrote down the slowing of a moving clock; in 1911 in Prague he asked how a clock near the ceiling of an accelerating cabin compares with one on the floor; in November 1915 he completed the field equations in Berlin. Weeks later, while serving on the Russian front, Karl Schwarzschild derived the exact exterior metric around a spherical mass and mailed it back to Berlin.
Canal rays, a 22.5 m brick tower, commercial airliners & a Scout D rocket
Herbert Ives and G. R. Stilwell photographed hydrogen canal rays in 1938 to catch the quadratic speed shift. In 1960 Robert Pound and Glen Rebka Jr. sent 14.4 keV iron-57 gamma rays down Harvard's 22.5-meter Jefferson Tower to measure the height shift inside a building. In 1971 Joseph Hafele and Richard Keating flew four cesium clocks around the world on commercial jets, and in 1976 Robert Vessot and Martin Levine launched a hydrogen maser 10,000 km up on a Scout D rocket.
Neil Ashby documented how every GPS satellite clock balances +45.7 against −7.2 microseconds per day. Chin-wen Chou and David Wineland resolved a 33 cm height step with aluminum-ion clocks in 2010, and Tobias Bothwell and Jun Ye resolved 1 millimeter across a strontium cloud in 2022. Meanwhile Eöt-Wash torsion pendulums, Pierre Touboul's orbiting MICROSCOPE cylinders, and Jeffrey Hangst's ALPHA-g trap at CERN tested universal free fall across matter and antimatter.
1938–2026 · Columbia, Harvard, Vienna, Beersheba, London & Oxford
Spin echoes, quantum clocks in superposition & two-mass entanglement proposals
Using the resonance and spin-echo language built by Isidor Isaac Rabi, Norman Ramsey, and Erwin Hahn, Magdalena Zych, Fabio Costa, Igor Pikovski, and Časlav Brukner asked in 2011 what happens when one ticking quantum clock sits at two heights at once; Yair Margalit and Ron Folman built the magnetic-gradient analogue on an atom chip in 2015. In 2017 Sougato Bose and co-authors and Chiara Marletto and Vlatko Vedral proposed testing whether gravity entangles two superposed micron masses.
1987–2026 · Budapest, Oxford, Gran Sasso, Seattle, Wuhan & Vienna
Lajos Diósi (1987) and Roger Penrose (1996) proposed that gravity collapses superpositions at a rate set by gravitational self-energy. Deep under the Gran Sasso mountain, Sandro Donadi, Catalina Curceanu, Lajos Diósi and co-workers (2021) and the XENONnT Collaboration (2026) listened for the spontaneous X-rays that model predicts, while Jun-Ge Lee (Seattle), Wen-Hai Tan (Wuhan), Tobias Westphal and Markus Arndt (Vienna) pushed short-range gravity and matter-wave coherence to their laboratory limits.
Roy Kerr found the exact spinning horizon in 1963; Bryce DeWitt (1967) and Don Page and William Wootters (1983) showed how conditional probability recovers ticking time from a stationary state. Jacob Bekenstein, Stephen Hawking, and William Unruh linked horizon surface gravity to thermal states, Don Page (1993) derived the unitary turnover curve, and LIGO, Virgo, KAGRA, and Maximiliano Isi tested Kerr ringdowns and Hawking's area law on merging black holes. Wojciech Zurek, Erich Joos, H. Dieter Zeh, Serge Haroche, and Jean-Michel Brune showed how the photon bath selects pointer states, while Planck, Douglas Clowe, Jun Ye, Ekkehard Peik, and Nils Huntemann tested gravity and clocks across dark matter.
The thirteen steps move through four regimes without skipping an intermediate link. Steps 2 to 4 (T.02, T.01, T.03) establish the classical metric clock-rate gradient from laboratory optical clocks, GPS satellites, and torsion-balance equivalence tests. Steps 5 to 7 (T.04, T.05, T.06) bring quantum superpositions into that gradient: proper-time clock interferometry, the three-channel gravity-entanglement dispute alongside short-range inverse-square limits, and underground radiation bounds on spontaneous gravitational collapse. Steps 8 to 11 (T.07, T.08, T.09, T.10) confront the timeless Wheeler–DeWitt constraint and causal horizons: relational Page–Wootters clocks, Rindler–Unruh–Hawking thermal wedges, Kerr quasi-normal ringdown spectroscopy, and Page's unitary entropy curve. Steps 12 and 13 (T.11, T.12) ask how a macroscopic record forms at all: electromagnetic pointer selection and Quantum Darwinism for ordinary matter that scatters photons, set beside the five-sixths of non-relativistic gravitating matter that leaves no electromagnetic record and meets our clocks through spacetime curvature.
How to read a T page
Every page in this track ships six things, and the build refuses a page that lacks one: a hand-written instrument whose knockout is the argument; a standard-library verifier that recomputes the page's central number and fails on a planted defect; a grounding ledger whose every DOI resolves in the site's offline Crossref cache; a label on every claim, derived from the ledger by a lint and decided by a person; a SKILL.md so any agent can rebuild the toy; and a Mexican Spanish twin in the same commit.
At least two collaborations, peer-reviewed DOIs, older than the replication window.
Follows by mathematics or deterministic kernel from a named MEASURED parent row (computed, not directly measured).
An exclusion limit: what the data rule out, not what they show.
One collaboration, or anything inside the last twenty-four months.
A proposal or a preprint; a preprint can never be more.
A question the field has not closed.
The labels describe evidence, not confidence. MEASURED needs at least two collaborations with peer-reviewed results. BOUND is an exclusion limit. DERIVED — printed on its badge as DERIVED (NOT MEASURED) — follows by mathematics or deterministic kernel from a named MEASURED parent row without claiming a new experiment ran. SINGLE RESULT is one collaboration, or anything published inside the last twenty-four months. PROPOSED is a proposal, and a preprint can never be more. OPEN is a question the field has not closed. A lint checks the mechanics: rows, dates, collaborations, DOIs, the same label sequence in both languages. A human checklist decides the label itself.
This hub carries no claims at all, and the lint asserts that. It states the intuition once, above, as a question in the third person, and that sentence is banned on every other page of the track.
