Black Holes

6 of 11. Other traces. Arrow keys walk.

Light

We never see the hole.We see what it does to everything else.

“Invisible” is too crude. The horizon itself does not shine. Around it, light, gas, stars and spacetime all leave traces: different kinds, independent, mutually reinforcing.

Event Horizon Telescope reconstruction of the shadow of the black hole in Messier 87, a bright asymmetric ring around darkness.

Reconstruction · EHT Collaboration / ESO · eso1907a

M87*. A model image from millimetre-wave visibilities. The dark interior is the black-hole shadow: lensed photon paths, not a picture of the horizon surface.

Event Horizon Telescope reconstruction of Sagittarius A*, the black hole at the centre of the Milky Way.

Reconstruction · EHT Collaboration / ESO · eso2208

Sagittarius A*. A second, independent shadow, this time in our own galaxy. Different mass, different dynamical time, same kind of trace.

Hubble Space Telescope image of giant elliptical galaxy M87 with a 3,000-light-year plasma jet streaming from the central black hole.

Documentary image · NASA / ESA / Hubble / A. Lessing / E. Baltz / M. Shara / J. DePasquale (STScI) · heic2411b

Plasma, collimated for three thousand light-years, leaving a 6.5-billion-solar-mass engine. The jet is the exhaust of accretion, not a picture of the hole.

Composite observatory image of the M87 jet in X-ray, optical, infrared and radio, a knotted stream leaving the galactic core.

Documentary image · NASA / CXC / STScI / VLA · CXC 2026

The same jet in X-ray, optical, infrared and radio. Knots are shocks in the outflow, still powered from the unseen centre.

NASA Chandra illustration of Cygnus X-1: a stellar-mass black hole drawing gas from a blue companion star into a glowing accretion disk.

Illustration · NASA / CXC / M. Weiss

Cygnus X-1. A stellar-mass black hole argued from dynamics plus an X-ray companion: accretion shining long before matter reaches the horizon.

Hubble image of LRG 3-757, a nearly complete Einstein ring: a background galaxy lensed into a circle by a massive foreground galaxy.

Documentary image · ESA / Hubble / NASA · potw1145a

Gravitational lensing. Mass between us and a background galaxy bends the light into a ring. A black hole is the extreme of the same geometry.

  • Stellar motion

    Stars at the Galactic Centre orbit an unseen 4-million-solar-mass object. The orbits are the trace. Nobel Prize 2020, Genzel and Ghez.

  • Accretion and X-rays

    Gas falling inward heats and shines, in X-rays and in radio jets, long before it reaches the horizon.

  • Radio interferometry

    Many radio dishes, listening as one ear the size of Earth. The EHT is this idea pushed to the event-horizon scale.

  • Lensed light

    Photons that skim the photon sphere can orbit once or more and then escape. They draw the bright ring. Photons that do not escape define the shadow.

  • Absence

    Sometimes the evidence is a mass with no surface: bursts, pulsations or a last stable orbit that would not exist on a hard star.

Hard · this walk’s channel

Gravitational waves

A channel that does not use light at all. Binary black holes, even when they carry no accretion disk, still disturb spacetime.

None of these photograph the horizon. The living field is a computed lensing model.