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◐ Horizons

Established Contested

Horizons aren't walls. They're limits on what can reach whom — set by light's speed and the shape of spacetime.

Black-hole horizons

Squeeze any mass inside its Schwarzschild radius, 2GM/c², and not even light can climb back out. For the Sun that radius would be 3 km; for Earth, 9 mm; for the black hole at the centre of our galaxy, about 13 million km. Established

Seen from far away, a clock lowered towards the horizon slows towards a stop. Yet someone falling through a large black hole's horizon feels nothing special there — the horizon isn't a surface, it's a point of no return. Inside, moving towards the centre becomes as unavoidable as moving into tomorrow. Established

Do black holes last forever?

Hawking (1974) showed that, with quantum effects, black holes should glow faintly and slowly evaporate — a firm prediction of theory, not yet observed from a real black hole (lab analogues show the effect). Whether the information that fell in comes back out is the information paradox. Contested Hole H7.

Cosmic horizons

The particle horizon: the farthest matter whose light has had time to reach us — about 46 billion light-years away today. That's the edge of the observable universe. The event horizon: because expansion is accelerating, light we send now can never reach galaxies beyond about 16–17 billion light-years, and their light emitted now will never reach us. Established within the standard model of cosmology.

The Hubble distance (about 14.5 billion light-years), where recession reaches light speed, is not a horizon: we see plenty of galaxies beyond it. Established

How we know

  • Stars orbiting the galaxy's centre (Nobel Prize 2020) and images of black holes' shadows (2019, 2022).
  • Gravitational waves from merging black holes (2015 onwards).
  • Cosmic horizons follow from the measured expansion history (see Expansion and redshift).

See it in the labs

RiverCosmic horizonsSpacetime diagram

Further exploring

Other free sites that explore this well — not part of Chronoscope.

Sources: Schwarzschild (1916); Hawking, Nature 248 (1974); Genzel & Ghez (Nobel 2020); Event Horizon Telescope (2019, 2022); LIGO (2016); Davis & Lineweaver (2004); Steinhauer (2016, 2019).

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