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.
Before we get to comments there’s a few things. First, I wanted to let you know about
a fun new episode of Above the Noise that dives into the research and debates the merits
of Space Tourism and Billionaires in Space. I know from previous episodes that many of
you have strong opinions on our space faring future, so check out the episode, drop your
thoughts into the comments and let them know, politely, that Space Time sent you. There’s
a link in the description.
Next as always if you support us on Patreon, we can’t thank you enough for your support.
But I also wanted to let you know that another great way to support the show for free is
by hitting the bell icon next to the subscribe button and getting notified of new releases.
By getting notifications and watching new episodes when they’re first released, you
actually help spread the show to other Space Time subscribers and to new viewers. So if you’re
already part of the early gang, we see you and appreciate you! And if not, hit the bell
icon and join the early gang to help the Space Time community.
Today we’re covering comments from the last two episodes, both on different ways to break
general relativity. There was the one on modified newtonian dynamics as an explanation for dark
matter, and the one on fuzzballs - the stringy theory version of the black hole.
Kidz Bop 38 is Straight FIRE says that it feels like we're really chasing our tail adding
all this complexity. "Let's just add a few more fields to GR so that our theory fits
the data.” Well to that I’ll let Einstein respond: “It can scarcely be denied that the supreme
goal of all theory is to make the irreducible basic elements as simple and as few as possible
without having to surrender the adequate representation of a single datum of experience.” And that quote is ften
paraphrased as “Everything should be made as simple as possible, but no simpler”
Occam’s razor is a guiding principle, not a rule. If the current model
can’t be made to fit observation then it’s missing something. In the case of general relativity,
it works almost everywhere - so if we find a place it doesn’t work we can’t just
throw out the theory because there must be a reason it works everywhere else. Any new
theory has to reproduce general relativity in the regimes that GR actually does work.
Gareth Dean made a point that I missed in that MOND episode. And that's that we’ve observed galaxies
that seem to be >99% dark matter as well as a few that seem to lack it entirely. He asks
whether the new MOND explains these, or only spiral galaxies? Given that the new MOND has
trouble explaining the Bullet cluster, my guess is that it also fails to explain other
anomalous galaxies. If there are any MOND researchers watching, please feel free to chime in!
The first fuzzball question is from Louis Marti, who asks what it would look like to
fall into a fuzzball?
And it was nicely answered elsewhere in the comments by Rob. To summarize: It would look
just like falling into a regular black hole for the person falling, but it would look
like you got smeared over the surface for someone watching from a distance. One of the
important things about fuzzball that we couldn’t fit into the episode is the idea of complementarity.
To be taken seriously, fuzzballs need to satisfy the equivalence principle - the founding idea
of general relativity that says that freefall in a gravitational field is fundamentally
the same as inertial motion in free space. That means that an observer falling through
a black hole event horizon shouldn’t notice anything special about that boundary - except
the fact that they can’t reverse their motion. How does this work for a fuzzball if it really
has a surface that is? The proposition is that when an object hits the fuzzball surface it sets
up vibrations in that surface - and those vibrations are somehow equivalent to the object
itself. As in, from its perspective the faller, it continues to exist - coded in the complexity
of the string vibrations. It’s holographically projected into 2 spatial dimensions, and in
its experience it keeps falling. Meanwhile from the perspective of a distant observer
it becomes a stringy pancake.
This is related to AdS/CFT correspondence and the holographic principle, which we covered
previously. It, says that vibrations in a quantum field on the surface of a 4-D hyperbolic
space are equivalent to objects inside that space. And if this sounds completely bonkers
to you, then you’re in good company. Thinking about it turns by brain into a fuzzball, by that I mean empty
on the inside.
A few if you asked how this fuzzball idea could be tested. Actually, this might be the
most testable prediction of string theory. There are simulations that suggest
that the gravitational waves created when fuzzball merge should look almost exactly
the same as those produced when classical black holes merge. Emphasis on the “almost”.
It may be that the so-called ring-down - the waves produced as the merged object settles
back into a spheroid - lasts longer for fuzzballs than for regular black holes due to the event
horizon being less cleanly defined. It’s also been suggested that the Event Horizon
Telescope could potentially detect deviations from general relativity in the form of quantum
fluctuations near the event horizon that might be amplified by gravitational lensing. None
of this has been seen yet, but upgrades to the current generation of instruments might
actually have the sensitivity to spot something.
We used a representations of the 4-D hypersphere in the fuzzball episode, made by the talented Wildstar2002,
and Wildstar actually jumped onto the comments to say hi. If you want your mind blown by
the best math-inspired computer graphics, including various many-dimensional geometries,
I recommend you check out their channel - link in the description.
Tubluer points out that any well-developed intuition for physics points to the correctness
of the fuzzball model on account of the fact that the connection between multi-dimensional
cats, hairballs and black holes can’t possibly be wrong. I agree that this is a compelling
reason to believe in fuzzballs - which I’m going to call Schrodinger’s hairball from
now on. MrGreenAKAguci00, however points out that there’s nothing special in this
hypothesis because cats are multidimensional. And I’ll add that all animals are technically
multidimensional - at least 3+1 in my experience. And for that matter all hairballs are technically
quantum. This teaches us one important lesson: next time you clean up after your cat try
not to get holographically projected down into a fuzzy smear. Don’t say you never
learned anything on this show.