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. Itâs
currently devouring its binary companion star, which is how we see it - but thereâs no
possibility of it ever coming close enough to the Earth to cause trouble here. We know there are plenty of these âstellar massâ black holes wandering the galaxy that we donât 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 doesnât 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 weâve talked about them before. Weâve also talked about how these âPBHsâ 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 âmicroâ 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 theyâd frequently pass in front of more distant stars, magnifying
those starsâ 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 donât devour stars like Cygnus X-1, and they donât 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 letâs figure it out.
First to allay your concerns - if an asteroid-mass primordial black hole hit the Earth we wouldnât be destroyed - which is good, because if dark matter really is made of these things then
itâs 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. Letâs talk specifics. Weâll 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 itâs 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 donât happen to be there when it happens - because it may let us detect signs of past PBH impacts. Letâs 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 âseeâ black holes like Cygnus X-1 or
the supermassive black holes in quasars - from the radiation generated by the infalling,
or accreting material. But thereâs a limit to how much radiation
a black hole can produce. That same radiation causes an outward pressure that partly counters
the black holeâs 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 thereâs
a limit defined by its mass. We call this the Eddington limit - itâs
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 youâve 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 Earthâs atmosphere, and a black holeâs 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 canât 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 weâd 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 Earthâs mantle like a supersonic Mach cone. These seismic waves would reach all points on the Earthâs 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 havenât seen this happen during the few years that weâve had seismometers doesnât 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 earthâs atmosphere nor its surface would keep a good record of micro black holes hitting the earth. Earthâs
dynamic interior and its eroding atmosphere would quickly erase the tiny scar of the PBH passage. But the same canât be said of the moon. Without any real atmosphere or tectonic activity, our moonâs 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 holeâs 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 âejecta blanketâ
around it. But when a black hole hits, it doesnât stop- it goes straight through.
The pressure from the superheated plasma around it creates a shockwave that pushes outward, creating a kind of âline explosion.â 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 âupâ than âoutâ, 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 couldnât be produced by regular asteroid
impacts, and a line of that fancy quartz should point from one crater to the other, though weâd 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 donât know. We probably wouldnât 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 thereâs 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. Thereâs a link in the description. Next as always if you support us on Patreon, we canât 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 theyâre first released, you actually help spread the show to other Space Time subscribers and to new viewers. So if youâre 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 weâre 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.â Well to that Iâll let Einstein respond: âIt 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.â And that quote is ften paraphrased as âEverything should be made as simple as possible, but no simplerâ Occamâs razor is a guiding
principle, not a rule. If the current model canât be made to fit observation then itâs missing something. In the case of general relativity, it works almost everywhere - so if we find
a place it doesnât work we canât 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 weâve 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 couldnât 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 shouldnât notice
anything special about that boundary - except the fact that they canât 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. Itâs 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 youâre 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 âalmostâ. 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. Itâs
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 canât possibly be wrong. I agree that this is a compelling reason to believe in fuzzballs - which Iâm going to call Schrodingerâs hairball from now on. MrGreenAKAguci00, however points out that thereâs nothing special in this hypothesis because cats are multidimensional. And Iâll 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. Donât say you never learned anything on this show.
It already has, and it is us.