Black hole, hands on
Walk round it and watch the sky bend behind it. Then change its mass, from an Earth squeezed to the size of a grape up to the giant in M87. Do not try the grape at home.
- Width of the event horizon
- 25 million km
- The black hole at the centre of our galaxy. Its horizon is about 18 Suns wide.
- Width of the dark shadow it casts
- 66 million km
- About 2.6 times wider than the horizon itself, because the hole also swallows light that passes close by. This is the black disc in the picture.
- Stretch across your body at the horizon
- Under 0.001 g
- Far too gentle to notice. You would cross the horizon without feeling a thing.
- One hour for you, hovering there
- 3.3 hours far away
- Time runs slower deep in gravity. The closer you hover to the horizon, the faster the rest of the universe ages.
- Longest possible fall from horizon to centre
- 67 seconds
- By your own watch. Firing rockets to resist only makes it shorter.
Now the honest part. The five numbers come from the exact formulas for a black hole that does not spin. In the ray-traced view every pixel follows a light ray along its curved path until it falls in, hits the disc or escapes; I step the path numerically and it is good to about three per cent. What is real there: the bending of light, the size of the shadow, the way the far side of the disc is lifted into view, the speed of the orbiting gas and the brightening of the side moving towards you. What is only representative: the disc’s colours, its thinness and sharp edges, and the sky behind, which is the real Milky Way (ESO/S. Brunier) but not as seen from any real black hole. Real black holes spin. Mine does not. The picture is the same for every mass; only its scale changes. The simple sketch uses a made-up star field and a rougher rule for bending, and its sizes are not to scale. What falling in would be like