Space and time - Topics

Light cones: why saying now about Alpha Centauri is a convention and not a fact The Andromeda paradox: two walkers passing each other disagree about whether a fleet has already launched LIGO measured a stretch of space 1/1000 the width of a proton The CPT theorem: reverse time and you must also flip charge and parity Michelson and Morley found nothing in 1887 and that null result became physics' most useful measurement The train and the two lightning strikes: how Einstein's 1905 thought experiment killed absolute simultaneity The twin paradox: the traveller really is younger and acceleration is what breaks the symmetry The Wheeler-DeWitt equation has no t in it: what the problem of time means for quantum gravity The speed of light is not really about light: it is the exchange rate between space and time Why length contraction is a convention about simultaneity and not a physical squash The universe is 13.8 billion years old and 93 billion light years wide: how both can be true Gravity Probe B's gyroscopes and the dragging of spacetime by a rotating Earth Recombination 380000 years after the Big Bang: how the universe went from opaque to transparent Two observers disagree about distance and duration but always agree on the spacetime interval Why free fall and weightlessness are locally indistinguishable and what that does to clocks What you see through a telescope is not what remains after correcting for light travel The cosmic microwave background is the oldest light we can ever see Interstellar's Miller planet: does one hour near a black hole really cost seven years? Inside a black hole the words before and after swap roles GPS satellites gain about 38 microseconds a day: uncorrected that is 10 kilometres of drift Minkowski in 1908: space by itself and time by itself are doomed to fade into mere shadows Scott Kelly came home 5 milliseconds younger than his twin brother Tidal locking: why the Moon's rotation period equals its orbital period Quantum entanglement cannot send signals and that is what saves causality How much younger would you be after a round trip to Proxima Centauri at 0.9c? Presentism has no observer-independent now to hang on once relativity is in play Cosmological redshift is not a Doppler shift: space stretched the wavelength on its way here In special relativity a cause can be simultaneous with its effect for some observers