What's Inside a Black Hole?
The short answer
In the simplest kind of black hole, Einstein's math says the center isn't a place in front of you. It's in your future. Inside one like the black hole at our galaxy's middle, you'd reach it in about a minute at most. What's actually at the center? Nobody knows.
Two NASA scientists put it this way in 1997: once you cross the edge, "you WILL reach the center, it's as inevitable as next Monday."
Why even light can't get back out
The edge of a black hole is called the event horizon. Inside it, falling inward becomes the direction of time. The Nobel committee wrote: "Time then replaces space and all possible paths point inwards." Sean Carroll's lecture notes say "you can no more stop moving toward the singularity than you can stop getting older."
That holds for light too. A beam pointed straight back out still leaves at full speed, but it is carried inward too. Fighting doesn't change the ending. A rocket burn can add some time, up to a fixed limit, but "the rocketeer is still doomed to impact on the central singularity" (Lewis and Kwan 2007). So the popular line that struggling only gets you there sooner is false as a rule.
How long you'd have
For the simplest black hole, the most time anyone can spend inside is π × GM/c³ (G is the gravitational constant, M the mass, c the speed of light). So it grows with the mass. For 4.3 million times the Sun's mass, like the one at our galaxy's middle, that's about 66 seconds. For M87's black hole, 6.5 billion Suns, it's about 28 hours.
In NASA's 2024 simulation, a camera fell for about 3 hours to the edge, circling almost twice on the way. Once it crossed, it had 12.8 seconds before it was stretched apart. Size matters at the edge too. You'd cross a giant black hole's edge without feeling a thing. A small one tears you apart first.
What's at the center
Einstein's math says everything at the center, the singularity, is crushed to infinite density. Physicists think that's where the math breaks down. NASA says the singularity "may be either a physical structure or a purely mathematical one," and astronomers don't know which. Finding out needs a theory of quantum gravity, and nobody has one yet.
The view only works one way. From inside, you'd still see the stars, because their light keeps falling in after you. From outside, nobody ever sees anything you do in there. Your last image from before the edge slows, reddens and fades.
One caveat: all of this is about the simplest, non spinning black hole. Real ones probably spin, and their insides are even less understood.
Key numbers
| What | Number | Source |
|---|---|---|
| Most time anyone can spend inside a black hole of 4.3 million Suns, like the one at our galaxy's middle | about 66 s (66.5 s) | Our arithmetic from Lewis and Kwan, 2007 and NASA Goddard (Schnittman), 2024 |
| Most time anyone can spend inside M87's black hole | about 28 hours (27.9 h) | Our arithmetic from Lewis and Kwan, 2007 and ESO, 2019 |
| Most time anyone can spend inside a black hole of 1 Sun | about 15 millionths of a second | Our arithmetic from Lewis and Kwan, 2007 |
| The longest possible time inside the simplest black hole | π × GM/c³ | Lewis and Kwan, 2007 |
| Mass of the black hole in NASA's plunge simulation | 4.3 million times the Sun's mass | NASA Goddard (Schnittman), 2024 |
| The simulated camera's fall to the edge | about 3 hours, from nearly 640 million km (400 million miles) away | NASA Goddard (Schnittman), 2024 |
| Time from crossing the edge until the camera is stretched apart | 12.8 s | NASA Goddard (Schnittman), 2024 |
| Distance left to the center at that moment | 128,000 km (79,500 miles) | NASA Goddard (Schnittman), 2024 |
| Mass of M87's black hole | 6.5 billion times the Sun's mass | ESO, 2019 |
| Date the question behind the "as inevitable as next Monday" answer was submitted | August 8, 1997 | NASA Imagine the Universe, Ask an Astrophysicist (Palmer and Lochner), 1997 |
What the film simplifies
- The film shows the simplest, non spinning black hole. Real ones probably spin, and inside a spinning one Einstein's math lets a path miss the center, so "the center is in your future" is proven only for the simple kind.
- "Like tomorrow" means you can't avoid the center, not that it takes a day: in a black hole like our galaxy's, the most time you'd have is about a minute.
- "Thrash all you want" is true in that nothing gets you out. A well judged rocket burn can still add some time, up to a fixed limit.
- The signpost, the destination board and the SOS signals are cartoon. Nothing inside marks the center, and his SOS doesn't really stall and sink back toward him: light leaves him at full speed and is carried inward with him.
- The film stops before the worst part. Even in a giant black hole, tides stretch you apart before you reach the center: in NASA's simulation of one with 4.3 million Suns, the camera lasted 12.8 seconds after crossing.
At a glance
Runtime 1 minute 6 seconds · Topics: Black holes, General relativity, Space and time · Captions on YouTube · Full script below
Test yourself
The flashlight points his beam straight back out. Why doesn't the light escape?
Answer
Inside the edge, falling inward is the direction of time, for light too. Every path points inward, so the beam is carried in with him.
The longest time inside grows in step with the mass. M87's black hole is about 1,500 times heavier than one of 4.3 million Suns, which gives about 66 seconds. About how long does M87's give?
Answer
About 1,500 × 66 s, or roughly 100,000 seconds. That's about 28 hours.
Can a rocket save you once you're inside a black hole?
Answer
No. A rocket burn can add some time, up to a fixed limit, but every path still ends at the center.
The script
Flashlight: What's inside a black hole? Give me one second.
Host: Shine a light in, and none of it bounces back.
Flashlight: Attics. Caves. The back of your couch.
Flashlight: This is just a bigger couch.
Host: It's a giant black hole.
Flashlight: I'm in. I'm light with a handle. Nothing's faster.
Host: Even his beam, pointed straight back out, gets carried in.
Host: Because inside the simplest black hole, the center isn't a place in front of you.
Host: Einstein's math puts it in your future. Like tomorrow.
Flashlight: Tomorrow? Which way's tomorrow? Left? Up?
Host: As two NASA scientists put it: as inevitable as next Monday.
Flashlight: Monday? Not on my shift.
Host: Thrash all you want. The center's still on his schedule.
Flashlight: I can still see the stars!
Host: Nobody out there will ever see his SOS.
Flashlight: Then what's at the center? Show me.
Host: Einstein's math says everything there gets crushed to infinite density.
Host: But physicists think that's where the math... breaks.
Flashlight: So what's really there?
Host: Nobody knows.
Flashlight: Yet!
Sources
- Nothing that crosses the event horizon, not even light, can escape.NASA Science, Black Holes
- At the center, matter is crushed to infinite density; the singularity may be a physical structure or a purely mathematical one, and astronomers don't know which; nothing about what happens inside can reach an outside observer.NASA Science, Black Hole Anatomy
- Past the event horizon, "inwards" becomes "towards the future": "you WILL reach the center, it's as inevitable as next Monday" (David Palmer and Jim Lochner, submitted August 8, 1997).NASA Imagine the Universe, Ask an Astrophysicist (Palmer and Lochner), 1997
- In NASA's simulation of a black hole of 4.3 million Suns, the camera falls about 3 hours to the edge from nearly 640 million km (400 million miles), is spaghettified 12.8 seconds after crossing, with 128,000 km (79,500 miles) left to the singularity; a supermassive black hole is the one to fall into, while stellar mass ones rip objects apart before the horizon.NASA Goddard (Schnittman), 2024
- Inside, time replaces space and all possible paths point inwards; you will not feel anything crossing the horizon of a supermassive black hole; from outside no one can see you fall in; at the singularity the theory ceases to apply; a theory of quantum gravity is needed.Nobel Prize in Physics 2020, popular information
- Black holes hide a singularity in which all the known laws of nature cease.Nobel Prize in Physics 2020, press release
- Inside, the view of the outside universe does not disappear; all light is headed toward the singularity; real black holes probably spin.Hamilton, JILA, Journey into a Schwarzschild black hole
- We are doomed to hit the central singularity in a finite time however we fire our rockets; what happens there, no one knows.Hamilton, JILA, Falling to the singularity
- Inside the horizon the radius becomes timelike: you can no more stop moving toward the singularity than you can stop getting older; with charge or spin the center is not necessarily in your future.Carroll, Lecture Notes on General Relativity, 1997
- The most time anyone can spend below the horizon of a non spinning black hole is τ = πm (π × GM/c³); rockets can increase the remaining time only up to that maximum, and the rocketeer still hits the singularity.Lewis and Kwan, 2007
- For a standard, non rotating black hole, avoiding the singularity is as impossible as not moving forward in time; general relativity, in predicting a singularity, predicts its own demise.Stanford Encyclopedia of Philosophy, Singularities and Black Holes
- M87's black hole has 6.5 billion times the mass of the Sun.ESO, 2019