Black Holes Are Completely Ridiculous

Full disclosure here: I'm not a scientist. Just a deep space enjoyer who has watch enough YouTube videos about black holes to sound like I'm an expert (I'm not).

Also, quick shout-out to one of my favorite bite-sized YouTube videos on this whole topic. If you want the "wait... are we seriously talking about living inside a black hole?" existential crisis without committing to a two-hour physics lecture, give it a watch. It's short, easy to follow, and Kurzgesagt - In a Nutshell deserves your view.

There are a lot of things in space that are hard to wrap your head around.

The scale of Jupiter. The distance between stars. A neutron star cramming more mass than our Sun into something about the size of a city. The fact that when you look at Andromeda, you are seeing light that left it before humans even existed.

But black holes are in a completely different category.

They aren't just really dense objects.

They are places where space and time themselves get so screwed up that our best descriptions of reality start arguing with each other.

And I absolutely love them for it.

The Event Horizon Isn't Even the Weird Part

There isn't a shell there. There isn't necessarily some glowing boundary that you smash into. From the perspective of general relativity, if you fell into a sufficiently massive black hole, you could cross the event horizon without noticing anything particularly special happened at that exact moment.

That's insane.

You could pass the point of no return without even knowing you passed it.

There would now be no possible path through spacetime that takes you back outside. You could turn your ship around, fire the engines as hard as physically possible, point a laser outward, scream for help, whatever.

Doesn't matter.

"Out" is no longer one of your available directions.

That's the part about black holes that gets me.

People usually describe them as having gravity so strong that light can't escape, which is technically useful, but it almost undersells what is happening.

It isn't really that light is trying to climb upward and just isn't strong enough.

Spacetime itself has been curved so severely that every future path points farther inward.

The singularity isn't merely an object sitting in front of you.

In the simplest description, it's in your future.

What the hell do you even do with that?

And Then Quantum Mechanics Shows Up and Ruins Everything

General relativity actually handles black holes pretty beautifully.

Then quantum mechanics walks into the room.

Stephen Hawking showed that black holes aren't completely black. Quantum effects around the horizon allow them to emit what we now call Hawking radiation. Given an absurdly long amount of time, a black hole can theoretically radiate away its mass and evaporate.

Cool.

Problem solved.

Except... what happens to the information about everything that fell into it?

Quantum mechanics really, really doesn't like information being permanently destroyed.

If you burn a book, the information in the book becomes hopelessly scrambled across smoke, heat, ash, photons and everything else, but in principle the underlying quantum information still exists.

A black hole appeared to be different.

Throw the book into one.

Wait for the black hole to evaporate.

Black hole gone.

Book gone.

Information gone.

Quantum mechanics starts throwing chairs.

That is the black hole information paradox, and decades of trying to fix it eventually gave us one of my favorite arguments in physics:

The firewall.

The AMPS firewall argument basically points out that several things physicists would really like to be true can't all comfortably be true at the same time.

Hawking radiation needs certain quantum entanglement relationships if the black hole is going to evaporate without destroying information. But the physics required for someone falling smoothly through the horizon appears to require another entanglement relationship.

Quantum entanglement has rules.

You can't just infinitely hand out the same entanglement to everybody like Costco samples.

Something has to give.

One proposed answer is absolutely metal:

There is a firewall at the event horizon.

Not metaphorical fire.

An unbelievably energetic region that would completely obliterate anything trying to cross the horizon.

Which solves one problem by basically telling Einstein's equivalence principle to go screw itself.

General relativity says someone freely falling across a large black hole's event horizon shouldn't encounter anything locally extraordinary at the boundary.

The firewall says:

lol.

plasma.

Do I Think Black Hole Firewalls Actually Exist?

No idea.

Obviously.

If I knew, this would be a slightly more important blog post.

But if I had to throw a completely unqualified guess into the ring, I don't think there's literally a gigantic incinerator sitting one Planck length above every old black hole's event horizon.

I think the paradox is probably trying to tell us that our idea of "inside the black hole" and "outside the black hole" being completely independent places is wrong at the quantum level.

That's already where a lot of interesting modern work is pointing.

Things like entanglement islands, holography and related ideas suggest that information we would naively describe as belonging to the black hole interior may somehow be encoded in what we consider the exterior radiation.

Which sounds ridiculous.

But so did quantum entanglement.

And black holes have a fantastic habit of turning sentences that sound ridiculous into legitimate physics papers.

The important thing is that the firewall paradox itself does not prove firewalls exist. It shows that some combination of our assumptions about quantum mechanics, locality, smooth horizons and information has to change.

My money would be on spacetime being a lot less fundamental than we think it is.

Maybe "inside" and "outside" are concepts that work wonderfully for planets, people and baseballs but start falling apart when gravity and quantum mechanics are both pushed to their limits.

And if that's true...

things get even more interesting.

Okay. What If We're Inside One?

Now we're entering the fun part.

I want to be very clear:

There is currently no good evidence proving that our universe is inside a black hole.

This is not established cosmology.

I am not saying somebody pointed JWST at the edge of the universe and found an event horizon.

But the idea that universes could exist inside black holes is a real thing physicists have played with.

There are models where gravitational collapse doesn't end in an infinitely dense singularity. Instead, quantum effects, torsion or some other new physics causes the collapse to "bounce."

From the outside?

Black hole.

From the inside?

Possibly an expanding universe.

There are even Einstein-Cartan models where a universe formed inside a black hole expands after a bounce and eventually becomes large, relatively uniform and remarkably similar in broad behavior to the universe we see around us.

And I cannot stop thinking about that.

Imagine watching a star collapse from some parent universe.

The event horizon forms.

As far as everyone outside is concerned, congratulations, you made a black hole.

But behind that horizon, the geometry of spacetime continues.

The collapsing matter reaches conditions where whatever theory eventually replaces our incomplete description of quantum gravity takes over.

Instead of reaching infinity...

it rebounds.

Space expands.

Particles form.

Stars eventually form.

Galaxies form.

Billions of years pass.

Some idiot on a tiny planet builds a website about space and starts wondering whether his entire universe is the inside of somebody else's black hole.

I love it.

Which Brings Me to Our Weird Little Corner of the Universe

Here's another thing that makes this especially fun to think about.

Our cosmic neighborhood appears to be unusually empty.

There is evidence for a gigantic local underdensity commonly called the KBC Void, Local Hole, or local supervoid.

The original Keenan, Barger and Cowie work found evidence that the nearby universe has significantly less matter than regions farther away, on scales of hundreds of megaparsecs. Exactly how large and deep this underdensity really is is still debated, and newer work continues to test whether the enormous versions of the KBC Void hold up.

But the basic picture is incredible to think about.

The Milky Way.

Andromeda.

The Local Group.

Laniakea.

All this enormous structure we think of as "our part of the universe" sitting within an even more enormous region that appears less dense than the universe at large.

And our cosmic neighborhood is roughly around the central region of the proposed underdensity.

Again:

THIS DOES NOT MEAN THE KBC VOID IS THE INSIDE OF A BLACK HOLE.

I can already hear somebody typing the angry email.

Large cosmic voids are expected to exist. Structure formation naturally creates dense filaments, walls, galaxy clusters and enormous underdense regions between them.

A void is not an event horizon.

But forget proving anything for a second.

Just play with the idea.

Suppose universes really can form inside black holes.

What would the inside eventually look like?

Would you expect the original black hole to look like a black sphere floating somewhere in our sky?

Probably not.

The horizon would be part of the global geometry of our spacetime. The "outside universe" wouldn't necessarily correspond to some direction we could point a telescope toward.

There might literally be no way for us to observe it.

Which raises an absolutely cursed possibility:

If we were inside a black hole, would we even be capable of figuring that out?

Maybe there is no experiment that lets the fish discover the aquarium.

Maybe what we call the Big Bang was connected to gravitational collapse somewhere else.

Maybe every black hole forms another universe.

Maybe some of the black holes in our universe contain universes full of galaxies, planets and people having this exact same argument.

Black hole.

Universe.

Black holes inside that universe.

More universes.

More black holes.

Cosmic Russian nesting dolls forever.

Is that what actually happens?

Probably not.

Maybe.

Who knows.

That's the fun part.

Black Holes Are Where Physics Starts Showing Its Seams

That's ultimately why black holes are probably my favorite objects in the universe.

They aren't just destructive.

They are questions.

They force general relativity, quantum mechanics, thermodynamics and information theory into the same tiny room and make them fight.

What happens to information?

Is the event horizon smooth?

Are firewalls real?

What replaces the singularity?

Is spacetime fundamental?

Does a black hole have an interior in the way we normally imagine an interior?

Can an entire universe form behind an event horizon?

Could ours have?

We have ideas.

Really good ideas.

We have equations that work disturbingly well.

We have pictures of actual black holes.

We have gravitational waves from black holes colliding.

And yet the moment you ask the seemingly simple question,

"Okay, but what actually happens in there?"

physics basically shrugs.

And I think that's awesome.

Because somewhere in the answer to that question is probably something fundamental about reality that we haven't figured out yet.

Maybe the firewall doesn't exist.

Maybe the singularity doesn't exist.

Maybe information is smeared across an event horizon.

Maybe space itself emerges from quantum entanglement.

Maybe black holes create universes.

Maybe we're sitting inside one right now.

I don't actually think the KBC Void proves that.

I don't think anything currently does.

But when the universe hands you objects that literally remove regions of spacetime from causal contact with the rest of reality...

I feel like we're allowed to have a little fun wondering what might be on the other side.