I remember when I first learned about black holes I imagined these gigantic cosmic vacuum cleaners that must eventually suck up everything.

They're not really like that.

I imagined wormholes to other dimensions.

Also probably nonsense.

And I imagined that it was just a matter of time before a black hole finally found its way to the Earth.

So that part might actually happen.

Proper big black holes are very, very far.

The nearest known is - the Cygnus X-1 black hole - at 1000 light years distant.

It’s currently devouring its binary companion star, which is how we see it - but there’s no possibility of it ever coming close enough to the Earth to cause trouble here.

We know there are plenty of these “stellar mass” black holes wandering the galaxy that we don’t see - but the chance of one coming close enough to cause serious trouble in our solar system is tiny.

However there is one scenario which could allow for black holes to pass through our solar system - and even the planet - with startling frequency.

In fact it may have already happened.

The early universe was a wild place.

All space everywhere was a boiling particle soup.

A glob of that material in the modern universe would immediately collapse into a black hole.

But back then the universe had the same insane density everywhere - matter was smoothly spread out with only tiny density fluctuations.

And so gravity had no strongly preferred direction to collapse towards.

The expanding universe then thinned out this matter to more sensible levels, and those little fluctuations in density eventually collapsed into stars and galaxies instead of black holes.

Really it was the smoothness of the early universe that saved all of matter from collapsing into black holes.

But that doesn’t mean that no black holes were formed.

There would have been some regions that just happened to be especially dense.

Countless black holes might have formed at the earliest of times, while still leaving plenty of matter left for stars.

We call these primordial black holes, and we’ve talked about them before.

We’ve also talked about how these “PBHs” may have been created in such stupendous numbers that they account for 86% of the mass of the universe, and are therefore an explanation for dark matter.

Depending on when they formed, PBHs could have virtually any mass - from tiny ‘micro’ black holes all the way up to the supermassive black holes in the centers of galaxies.

Over the decades, astronomers and cosmologists have managed to systematically rule out most of the windows of possible masses if PBHs are to account for most of the dark matter.

For example, if there were enough of these black holes then they’d frequently pass in front of more distant stars, magnifying those stars’ light with gravitational lensing.

The rarity of this phenomenon, along with several other clever methods, has allowed us to pretty much rule out all masses greater than about 10^19 kilograms - or around 15% the mass of our Moon.

Meanwhile, if primordial black holes had masses smaller than around a trillion kilograms then they'd have all evaporated by now by leaking away their mass in Hawking radiation.

This leaves a small and very contentious window of possible masses, comparable to the masses of large asteroids.

Black holes this big don’t devour stars like Cygnus X-1, and they don’t warp the passage of light from distant stars strongly enough to easily spot them.

But fortunately for astronomers, and perhaps less fortunate for everyone else - if dark matter really is made of asteroid-mass black holes then there must be an absolutely insane number of them out there to account for most of the mass in the universe.

Which means we can wait for them to come to us.

Given how much dark matter we know that there is in the Milky Way, we calculate that there's probably about 10^18 kilograms of dark matter in the solar system at any given time.

If that dark matter is tiny black holes then we might expect dozens, maybe even thousands, of them to be in the solar system right now.

And, just like asteroids, given enough time some of them will quite literally cross paths with the earth.

So what happens if a black hole hits the Earth?

It turns out this is actually a scientifically very useful question.

So let’s figure it out.

First to allay your concerns - if an asteroid-mass primordial black hole hit the Earth we wouldn’t be destroyed - which is good, because if dark matter really is made of these things then it’s probably already happened.

This PBH would be small - its event horizon - its surface of no-return - would be around the size of an atom.

And it would be moving fast when it hit the earth - it must have fallen in from interstellar space, and so should be traveling from several 10s to 100s of km per second.

It would punch straight through the planet like a bullet through cotton candy, barely slowing down in the process.

Let’s talk specifics.

We’ll say our black hole has the mass of the Martian moon phobos - 10^16 kg like a large asteroid.

That gives it an event horizon the size of a hydrogen atom.

At interstellar speeds it spends around a minute passing through the Earth.

Given that size and speed, our Phobos-mass black hole might consume a few thousand tonnes, which is tiny for the Earth, and it’s even tiny for that black hole.

The planet barely notices the passage.

But I do not recommend standing right under a falling black hole - locally the encounter is catastrophic.

Which is actually a good thing if you don’t happen to be there when it happens - because it may let us detect signs of past PBH impacts.

Let’s talk about what happens when our black hole first enters the atmosphere.

In the immediate surroundings of any black hole, gravity accelerates matter to incredible speeds.

Near the event horizons, particles collide with each other sizable fractions of the speed of light.

This generates hilariously high temperatures - much hotter than the cores of a star.

This is how we “see” black holes like Cygnus X-1 or the supermassive black holes in quasars - from the radiation generated by the infalling, or accreting material.

But there’s a limit to how much radiation a black hole can produce.

That same radiation causes an outward pressure that partly counters the black hole’s intense gravity.

Try to feed a black hole too fast and it starts to blast away its own food.

The bigger the black hole the faster it can feed, but there’s a limit defined by its mass.

We call this the Eddington limit - it’s the maximum brightness that the material around a black hole can radiate before radiation pressure cuts off the fuel.

The tiny event horizon of a micro black hole is a very narrow choke-point for matter trying to flow in.

Its Eddington limit is very low.

For an asteroid-mass black hole at the absolute top end of our mass range, the cloud of plasma is barely microns in size, but it still shines with the power of a hundred hiroshimas every second.

If such a black hole passed into our atmosphere, it would look like the brightest shooting star you’ve ever seen, producing a destructive shockwave before hitting the ground and tunneling through the planet.

Now, this description is not too different from the Tunguska event- a massive impact that occurred in 1908 in the middle of Siberia.

Witnesses described a stripe of light in the sky as bright as the sun and a thunderous sound like cannons that leveled trees and broke windows over hundreds of kilometers, making a shockwave that traveled around the world.

The fact that there was no clear crater or meteor from an impact led some physicists in the 1970s to suggest that maybe this was a black hole punching through the earth.

But there was only one shockwave detected in the Earth’s atmosphere, and a black hole’s exit on the other side of the planet should have made another.

These days standard narrative is that the Tunguska resulted from an asteroid or comet exploding in the atmosphere.

But we can’t be 100% sure.

This was, after all, 1908 - and if the black hole exited in the middle of the ocean it could have been missed.

Tunguska-level devastation is only for black holes at the top end of our mass range.

The smaller the black hole, the more likely it is we’d miss it as it passed through the atmosphere.

But the passage of the black hole through the Earth itself might still be detected.

This black hole bullet would generate a shockwave through Earth’s mantle like a supersonic Mach cone.

These seismic waves would reach all points on the Earth’s surface.

Even at the lowest mass possible for a primordial black hole it will produce the equivalent of a magnitude 4 earthquake.

That's small, but definitely noticeable.

And it would look very different from a regular earthquake because it would be felt across the entire Earth surface.

Problem is, no such thing has even been detected.

Now part of the trouble is that these impacts would be rare.

For the smallest PBH masses, there may only be one black hole hitting the earth every million years.

For the Phobos-mass black holes or larger, you may only get one in the history of the earth.

The fact that we haven’t seen this happen during the few years that we’ve had seismometers doesn’t really tell us much.

It might also be possible to detect the tiny gravitational influence of a PBH passing close to the Earth on a near miss - but even after years of gravity monitoring none of these seems to have shown up.

OK, so spotting a live PBH seems unlikely.

But what about finding signs of past impacts?

Unfortunately, neither the earth’s atmosphere nor its surface would keep a good record of micro black holes hitting the earth.

Earth’s dynamic interior and its eroding atmosphere would quickly erase the tiny scar of the PBH passage.

But the same can’t be said of the moon.

Without any real atmosphere or tectonic activity, our moon’s surface has an almost complete history of its impacts written on its surface.

Telling apart craters made by black holes and those made by regular old rocks is hard, but not impossible thanks to some new theory work.

Recently, some scientists have calculated how the shape of a black hole’s crater would be different from that of an asteroid.

When an asteroid hits the moon, it stops very quickly, making something of a big round explosion, sending matter out in all directions.

This makes a crater basin at the point of contact, and a gentle sloping ‘ejecta blanket’ around it.

But when a black hole hits, it doesn’t stop- it goes straight through.

The pressure from the superheated plasma around it creates a shockwave that pushes outward, creating a kind of ‘line explosion.’ If the regular asteroid is like a pile of TNT being detonated at the surface, the black hole is more like a borehole filled with TNT being set off.

The result is that the crater may be much deeper, while the matter ejected around the crater goes more ‘up’ than ‘out’, and falls closer to the center, making a much steeper blanket.

One of the great things about this hypothesis is that it makes some very specific predictions.

For one, craters come in pairs- if you have an entrance wound, you have an exit wound.

Moreover, the incredible heat of the plasma around the black hole should quickly sear the rock around it into exotic high-pressure phases of quartz and pyrite that otherwise couldn’t be produced by regular asteroid impacts, and a line of that fancy quartz should point from one crater to the other, though we’d need to actually go back to the moon to test that part.

No such crater has ever been found - but a serious search has not yet been done.

OK guys, to wrap up- have any black holes ever hit the earth?

We don’t know.

We probably wouldn’t have noticed if they had.

Finding a single such case though would be huge.

For one thing, this would inspire an entire sub-genre of black-hole-impact science fiction horror, which is a positive outcome that should not be discounted.

But the discovery would also tell us that primordial black holes are a thing, and it would tell us about their masses.

That could lead to figuring out the nature of dark matter and to understanding the crazy black-hole-spawning chaos that defined the birth of this space time.