Wormholes, alternate universes, time warps — we’ve all heard crazy theories about what happens inside a black hole. So what’s the real story?

The short answer is, physicists don’t know.

## What's inside a black hole?

We understand what happens outside the black hole as you approach its event horizon, that infamous point of no return. The event horizon is where the escape speed exceeds the speed of light: you’d have to be going faster than light (which is impossible for any bit of matter) to escape the black hole’s gravity.

Inside the event horizon is where physics goes crazy. Calculations suggest that what the fabric of spacetime looks like inside a black hole depends on that particular black hole’s history. It might be turbulent, twisted, or any other number of things. One thing’s for sure, though: the tidal forces would kill you (see below).

According to theory, within a black hole there’s something called a singularity. A singularity is what all the matter in a black hole gets crushed into. Some people talk about it as a point of infinite density at the center of the black hole, but that’s probably wrong — true, it’s what classical physics tells us is there, but the singularity is also where classical physics breaks down, so we shouldn’t trust what it says here.

In a very specific mathematical case, the singularity in a spinning black hole becomes a ring, not a point. But that mathematical situation won’t exist in reality. Others say that the singularity is actually a whole surface inside the event horizon. We just don’t know. It could be that, in real black holes, singularities don’t even exist.

Wormholes are theoretically possible, given the right conditions. But those conditions almost certainly would never exist in the real universe.

## What would happen if you fell into a black hole?

If it were a stellar-mass black hole, you’d be dead before you passed the event horizon. That’s because, if you think of a black hole as a pit, a stellar-mass black hole has steeper sides than a supermassive black hole. The tidal forces become too strong too fast for you to survive to the event horizon, resulting in your spaghettification (yes, that’s the technical term).

So let’s travel into a supermassive black hole. Passing the event horizon, you wouldn’t notice much (except some fun light effects and several extra g’s of gravity). But as you drew closer to the singularity, gravity should stretch and squeeze you as if you were dough in a bread machine. At this point you’d die.

## What would someone watching see as you fell in?

As you approached the event horizon, a second person far, far away would watch your image slow down and redden. Theoretically, at the event horizon your image would freeze. But in practice you would disappear: the photons lose energy as it becomes harder for them to climb out of the black hole’s gravitational well nearer the event horizon, and their wavelength would increase until it grew past the observer’s detection capabilities — making the image invisible. So your image would redden and dim with time, until it faded entirely.

This Q&A is adapted from the February 2017 infographic “Anatomy of a Black Hole.”

MECHAthrust

March 18, 2017 at 6:15 am

If black hole have strong enough gravity then why it is act only one side.

grego99

April 20, 2022 at 11:52 am

the label "black hole" is a bit of a misnomer, misleading name. it is not a 2 dimensional disc like hole. it is a sphere, just like a star. Imagine a star getting bigger and bigger, pulling in more mass, eventually it would be so massive, it would start pulling back into itself emitted solar flares, then eventually all of its emissions would be pulled back into itself, even light! at that stage, an outside observer would see the "star" disappear except for external matter being pulled into it, orbiting it, etc. better name would be a "dark star". so if you had a crush proof ship, and allowed your ship to get pulled in, past the "event horizon", you would most likely see the bright star and at the center. the density of this central "star" is hard for us to imagine, so scientist describe it as a singularity, a "dot" ? we just do not know. perhaps we may one day see stages of stars turning into "dark stars", perhaps large stars getting fainter quickly is this process?

@mangalkishan

June 25, 2017 at 2:29 am

if even light can't escape from the black hole and can't cross the event horizon how anouther man standing out of the event horizon can watch anouther man falling inside the black hole???As you approached the event horizon, a second person far, far away would watch your image slow down and redden. Theoretically, at the event horizon your image would freeze. But in practice you would disappear: the photons lose energy as it becomes harder for them to climb out of the black hole’s gravitational well nearer the event horizon, and their wavelength would increase until it grew past the observer’s detection capabilities — making the image invisible. So your image would redden and dim with time, until it faded entirely.How this point is possible?????????

Camille M. Carlisle

June 26, 2017 at 3:12 pm

The light that's reddening is coming from the person *before* they pass the event horizon. Think of the black hole like a hole at the bottom of a steeply sloping valley: the steep ground outside the hole is the space just outside the event horizon. It's still steep enough that light has to struggle to climb up, but it's outside the hole itself. We can't see the light from after the person crosses the event horizon, but we can see the light from just before.

NASA Fan

January 17, 2018 at 11:39 pm

Would anybody survive a trip in a worm(black)hole?

GerryH

September 26, 2019 at 5:22 am

If you could survive crossing the event horizon of a supermassive black hole, wouldn't you be travelling at the speed of light? As such, you'd be within a moment of time & therefore wouldn't be able to experience anything? A bit like waking up dead!

PaulDent

March 31, 2018 at 2:03 pm

The question " what's inside a black hole" can potentially be answered by asking what the laws of physics are inside, which are all determined by " The Metric of Spacetime"
However, due to a sign exchange between the radial and time dimension between the outside and outside, you have to consider the inexorable chute from the event horizon to the central singularity to represent the passage of time, and what was time on the outside becomes space inside, possibly being infinite in extent. So you have to formulate the internal laws in terms of this time/space exchange; and guess what? The laws could then turn out to be the same as ours, allowing everything from the Big Bang to the evolution of life to occur inside, given that the "time" from event horizon to central singularity lasts long enough. Doing the math puts the required size of a black hole to allow all that at about 10 times the visible mass of our universe. Moreover, the universe that could evolve in such a "Mother Hole" would appear to be expanding raidly at first, slow down to a plateau rate, then accelerste again at an ever increading rate (an outside being would think it must be contracting but space/time becomes time/space so an inside being thinks it's expanding); also, stars on the outskirts of galaxies would orbit way faster than they should according to Newton/Kepler). In addition, at a point 13.5B years from the event horizon, the apparent rate of expansion in the plateau phase would be about 50km/s per megaparsec.

PaulFF

November 23, 2018 at 9:58 am

Are you saying that you may not even notice it?

Clive Hodgson

July 21, 2021 at 10:54 am

If a neutron star with 99% of the mass required to become a black hole gains another 1% of mass, does the resulting black hole still contain a neutron star at its core or some other more extreme form of matter?

Note that the September 2021 issue of S&T has an interesting article about some of the extreme forms of matter in the first 10 seconds after the Big Bang.