Into Sagittarius A*: A Ray-Traced Fall Into a Black Hole
Fall into Sagittarius A*, the black hole at the center of the Milky Way, in a real-time ray-traced film: bent light, the photon sphere, the event horizon, spaghettification, the singularity and what might lie beyond. Outside the event horizon, light paths are traced through the curved space of a non-spinning (Schwarzschild) black hole; the view inside the horizon and the second act are interpretations. An original ambient score plays along with the film.
Approach
Sagittarius A*, the black hole at the center of our galaxy. It holds 4.3 million Suns of mass inside a horizon 25 million km wide, about 18 times the width of the Sun. We've given it a bright disk of hot gas so you can watch what gravity does to light.
Bent light
The arc above the black hole is the far side of the disk. Its light climbs over the top of the hole to reach us; a fainter image sneaks under the bottom. One side is brighter because that gas is rushing toward us at about half the speed of light.
Last stable orbit
At 3 Schwarzschild radii, orbits stop being stable. Any gas that drifts inside this line spirals down in a few turns. That's why the disk has a sharp inner edge.
Photon sphere
At 1.5 radii, light itself can circle the black hole. Looking sideways, the bright ring cuts the sky in two. Below it, the darkness. Above it, the whole universe, squeezed and smeared.
Event horizon
There's no wall here and nothing to feel. For a black hole this big, the tides are still gentler than Earth's. But once you cross, every path into the future leads inward. A watcher far away never sees you cross: your image slows, reddens and fades.
Inside
Space and time trade roles. The singularity isn't a place in front of you anymore. It's a moment in your future, as unavoidable as tomorrow. From the horizon you have about 28 seconds on your own clock. Behind you, the outside universe shrinks to a bright window.
Spaghettification
The pull on your feet grows much faster than the pull on your head. Here that only turns deadly in the last 0.03 seconds, about 130,000 km from the center. Near a 10-solar-mass black hole, it would have torn you apart 1,700 km out, far outside its 30 km horizon.
Singularity
General relativity says density becomes infinite here, which really means the theory has stopped working. Describing what's actually here needs a theory of quantum gravity. Nobody has one yet.
Past the edge
Everything from here on is speculation. General relativity can't say what happens at the singularity, so physicists have proposed several answers. None has been tested. Here are three of the most serious.
A bounce
In loop quantum gravity, space has a smallest possible size, so matter can't be crushed forever. The collapse bounces, and the black hole turns inside out into a white hole: the time-reverse of a black hole, which nothing can enter and everything leaves. To you the bounce is instant. To the outside universe it could take billions of years.
A new universe
Some physicists go further: the bounce happens beyond our space entirely, and the crushed matter expands into a separate universe with its own big bang. Lee Smolin and Nikodem Popławski have each built versions of this idea. If it's right, our own universe could have begun inside a black hole in another one.
Through the ring
Real black holes spin, and in the exact math of a spinning black hole the singularity is a ring, not a point. Miss it and the equations carry you through, into a region where gravity pushes instead of pulls, or out into another universe. Most physicists expect infalling matter would wreck this inner structure, so read it as a map of the math, not a travel guide. We've drawn the ring glowing so you can see it.
Made by JP Madrigal.