Plate 38 · Space
Where Not Even Light Escapes
Squeeze a heavy enough star into a small enough space and its gravity grows so strong that not even light can escape: a black hole. You can’t see the hole itself, but the light around it is bent into a bright ring. In 2019, people saw a picture of such a ring for the first time.
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The Edge of No Return
Throw a stone upward and the faster you throw it, the higher it goes. To leave Earth for good, it would need 11.2 kilometres a second. The heavier and smaller a world is, the faster you need to go.
Squeeze the Sun into a ball just 3 kilometres in radius, or the Earth into a marble 9 millimetres in radius, and the speed needed to escape its surface would be more than the speed of light. Inside that boundary not even light gets out. It is called the event horizon. Einstein’s theory gives the same size, and something stranger: inside it, even light aimed straight outward is carried inward.
A simulation: a non-spinning black hole against the stars
Bending Light
Light passing near a black hole is pulled into a curve by its gravity. Stars behind the hole seem squeezed around its edge, and some appear twice.
The closer the light passes, the more it bends. At about 1.5 times the horizon’s radius, light can even go round in a circle, though the slightest nudge sends it falling in or flying off. Light from far away that comes any closer never gets back out.
Where the Ring Comes From
Many black holes are surrounded by a swirling disc of hot gas, called an accretion disc, so hot that it glows.
The far half of the disc should be hidden behind the hole, but its light is bent over the top and under the bottom and reaches us anyway, so the disc seems to arch over the hole as well. The dark patch in the middle is the hole’s shadow: every line of sight that lands on it ends at the horizon. Because the hole bends light towards itself, its shadow looks about 2.6 times wider than the horizon really is.
Bright on One Side
The disc spins very fast: near its inner edge the gas moves at about half the speed of light. On the side turning towards us, the light is boosted brighter and bluer; on the side turning away, it is dimmer and redder.
That is why a black hole’s ring is usually brighter on one side.
The First Picture
In 2019 the Event Horizon Telescope, which links radio telescopes all over the Earth, showed the world its picture of the ring around the black hole at the centre of the galaxy M87, taken two years earlier. That hole is 6.5 billion times the mass of the Sun, 55 million light years away.
Its ring is only 42 micro-arcseconds across in the sky, about the size of an orange on the Moon seen from Earth. In 2022 the same team showed a picture of the black hole at the centre of our own galaxy.
The simulation blurred and tinted to hint at the telescope’s picture; the real image is radio light in made-up colours, and shows no stars
Your Turn
Turn the view to see the black hole from the side and from above. Switch off the disc to see only how starlight is bent.
Try thisSwitch the view from “Above” to “Side”: what does the disc do over the hole?
What the model leaves out
Every pixel’s light ray is traced along its curved path around a non-spinning black hole, following general relativity; most real black holes spin, which changes the ring’s shape a little. The disc is drawn as a thin glowing sheet from 3 to 12 horizon radii; its brightness and colour are illustrative, keeping only the effect that the side turning towards us is brighter. The sky behind it is a stand-in: a map of our own Milky Way, sprinkled with made-up stars.
A common misreading
A black hole is not a cosmic vacuum cleaner sucking in everything around it. Far enough away, its gravity is the same as a star’s of the same mass: if the Sun suddenly became a black hole of the same mass, the Earth would keep orbiting just as before (though it would grow cold and dark). And the dark patch in the photos is not the hole “itself” but its shadow: the horizon’s silhouette, magnified by bent light.
Sources
- Event Horizon Telescope: First image of a black hole (M87*, 2019)
- Event Horizon Telescope: Sagittarius A*, the black hole at the heart of our galaxy (2022)
- NASA: Black holes
- NASA Goddard: Black hole accretion disc visualisation
Data
Milky Way: NASA/Goddard Space Flight Center Scientific Visualization Studio, Deep Star Maps 2020; Gaia DR2: ESA/Gaia/DPAC.
Follow the question
Plate 18The Space Station Is FallingAstronauts float not because there is no gravity in space, but because they and their station are always falling.
Plate 29A Curved Piece of GlassLight bends a little as it passes from air into glass. Shape the glass into a curve and light passing through every part of it can meet at one point — which is how magnifying glasses, cameras and your eyes work.
