Glossary
Key ideas, in plain words.
Every idea we name in a session, gathered here and explained without equations. Each one links back to the session it belongs to.
- 38 microseconds → 11 kilometres
- A microsecond of clock error becomes a position error at the speed of light. Left uncorrected, GPS would drift about 11 km off within a day, which is useless for navigation.
Relativity You Can Hold → - A new sense
- Gravitational waves pass straight through gas, dust, and galaxies that block light. They give us a way to register events light can never show. It's like gaining hearing after a life of only sight.
Ripples in Spacetime → - An Earth-sized eye
- Resolution isn't set by how big a telescope you can build, but by how far apart you can spread detectors and how precisely you sync them. The EHT linked observatories across the planet into one.
The Shadow of a Black Hole → - Asymmetric aging
- The travelling twin really does return younger. At 80% of light-speed, 5 years for the traveller is about 8 years at home. That's a genuine difference in elapsed time, not a viewing trick.
The Twins → - Causality is safe
- The disagreement never touches cause and effect. Any two events that could actually influence one another keep their order for everyone. Only causally disconnected events flip.
No Universal Now → - Curved spacetime
- Space and time form one fabric. Mass and energy bend it, and that bend is what we experience as gravity. Wheeler put it best. Matter tells spacetime how to curve, and spacetime tells matter how to move.
Gravity Is Not a Force → - Doppler boosting
- Gas orbiting the hole at near-light speed looks brighter on the side moving toward us. That asymmetry lets us read the direction of spin from a single photograph.
The Shadow of a Black Hole → - Event horizon
- The boundary around a black hole past which nothing escapes, not even light. It isn't a surface you could touch. It's a one-way edge in spacetime.
The Shadow of a Black Hole → - Expansion of space, not in space
- Galaxies aren't moving outward from a centre. The space between them stretches everywhere at once. The Big Bang was the beginning of that stretching, and of geometry and time themselves.
The Universe Pulls Apart → - Geodesic
- The straightest possible path through curved geometry. A falling apple and an orbiting moon are both just following geodesics, coasting rather than being pulled.
Gravity Is Not a Force → - Gravitational lensing
- The same bending, scaled up. Clusters of galaxies bend and magnify the light of more distant ones, forming arcs and rings. It's a natural telescope, and we now use it to see the early universe.
Mass Bends Light → - Gravitational wave
- A ripple in the curvature of spacetime, set off by massive objects accelerating, that travels outward at the speed of light. It physically stretches and squeezes space as it passes.
Ripples in Spacetime → - Knowing what we don't know
- That the cosmos is mostly unknown is a precise result, read straight off how gravity behaves. Galaxies spin too fast. The expansion accelerates. The honest stance is to be certain about the measurements and agnostic about the cause.
The Universe Pulls Apart → - Light follows the curve
- Light has no mass, but it still follows the straightest path through curved spacetime. Near a heavy object that path bends, so the light arrives deflected. No pull required.
Mass Bends Light → - Looking out is looking back
- Light takes time to cross space, so every observation is of the past. The sky is a layered archive. Nearby stars from years ago, distant galaxies from eons ago, all arriving together. Distance is time.
The Oldest Light → - No preferred 'now'
- There's no universal present sheet sweeping across the cosmos. 'Now' is a local notion that tilts with your motion, not a single moment the whole universe shares.
No Universal Now → - Proper time
- The time a clock actually logs along its own path. Different paths between the same two events can rack up different proper times, and the traveller's path simply logs less.
The Twins → - Recombination
- About 380,000 years after the beginning, the universe cooled enough for electrons to join nuclei into neutral atoms. The fog of charged particles cleared, and light could finally travel freely.
The Oldest Light → - Relativity of simultaneity
- Whether two distant events happen 'at the same time' depends on how you're moving. Two observers in different motion can honestly disagree about the order, and neither one is wrong.
No Universal Now → - Seeds of everything
- The CMB is smooth to one part in 100,000. Those tiny denser specks were the gravitational seeds that grew, over billions of years, into galaxies, stars, planets, and us.
The Oldest Light → - Strain
- The tiny fractional change in length a wave causes. GW150914 stretched LIGO's 4-km arms by about one part in 10²¹. That's a length change near 1/1000 the diameter of a proton.
Ripples in Spacetime → - Tested with real clocks
- In 1971, atomic clocks flown around the world on airliners came back behind the clocks left on the ground, by precisely the predicted amount. The paradox is a measured fact.
The Twins → - The 1919 eclipse
- Totality is the only moment starlight shows beside the Sun. Eddington photographed it, measured the predicted shift, and the match made Einstein world-famous.
Mass Bends Light → - The accelerating universe
- In 1998, distant Type Ia supernovae came out dimmer than expected, showing the expansion is speeding up rather than slowing. The cause, which we call dark energy, earned the 2011 Nobel Prize and remains unexplained.
The Universe Pulls Apart → - The atomic clock onboard
- Each satellite carries a clock as precise as the one that defines the second itself, because the whole system depends on knowing the time to billionths of a second.
Relativity You Can Hold → - The chirp
- As the two black holes spiralled in, the wave's pitch and volume rose together over a fifth of a second, then cut off at the merger. Played as audio, the signal sounds like a chirp.
Ripples in Spacetime → - The cosmic microwave background
- That first free light, still arriving from every direction, stretched by expansion down to 2.7 degrees above absolute zero. It is the oldest light we can see, a baby photo of the cosmos.
The Oldest Light → - The dark sector (95%)
- Ordinary matter, everything we can see, is about 5% of the universe. Dark matter (~27%) is known only by its gravity. Dark energy (~68%) drives the acceleration. We can measure both, and explain neither.
The Universe Pulls Apart → - The equivalence principle
- Inside a closed box you cannot tell, by any local experiment, whether you're sitting still in gravity or accelerating through empty space. Gravity and acceleration are locally the same thing.
Gravity Is Not a Force → - The light cone
- Every event has a cone of places its light can reach. Events inside that cone can be connected by cause and effect. Events outside it, in what physicists call the elsewhere, can't influence each other at all, and that's exactly where observers disagree about order.
No Universal Now → - The shadow
- The dark central region in the EHT images. It is larger than the horizon itself, because the black hole's gravity bends the paths of nearby light into the bright ring we see around it.
The Shadow of a Black Hole → - Twice the Newtonian value
- Einstein predicted that starlight grazing the Sun bends by 1.75 arcseconds, exactly double a naive Newtonian guess. The factor of two is what made the 1919 measurement a real test.
Mass Bends Light → - Two corrections, opposite signs
- Motion slows the satellite clocks, which is special relativity. Height in gravity speeds them up, which is general relativity. The gravity effect is bigger, so the net is a gain of about 38 microseconds a day.
Relativity You Can Hold → - Why GPS is the proof you can hold
- You don't need a particle accelerator to test relativity. A working phone map is a continuous, real-time confirmation that time runs at different rates at different speeds and heights.
Relativity You Can Hold → - Why it doesn't cancel
- Motion is relative, but the trip isn't symmetric. Only the traveller accelerates and turns around, and that break in symmetry is what decides who ends up younger.
The Twins → - Why it's not just philosophy
- GPS satellites tick at a measurably different rate than ground clocks because of this curvature. Ignore the correction and your map drifts about 11 km off within a day.
Gravity Is Not a Force →