Not really. A black hole isn't any more dangerous than any other object with the same mass. A black hole with the mass of Earth, for instance, would behave exactly as Earth does, except when an object collides with it, it continues on down into the center where it would hit Earth's crust if it were a planet.

If a black hole has the mass of the earth it's Schwarzschild radius would be about 9mm. If it was disturbed from an orbit well past pluto it would have quite a velocity when it hit earth.

The good news is it would pass effortlessly through the earth and not go to the earth's core and stay there. After all where is the resistance going to come from?

Now if the blackhole was brought to rest at the earth surface it wouldn't reach escape velocity (11.2 km/sec), but anything coming from way past pluto would likely have a much larger velocity.

The bad news is that having an earth skewer the earth at high speed is going to cause some pretty crazy gravitational interactions.

Also some of the matter falling into the black hole from earth would be consumed, but some of it would radiated out, no idea if that would be worse than the physical disruptions or not.

>crazy gravitational interactions

Thats putting it very mildly. Its actually quite an interesting thought exercise - As this black hole approached earth its gravity would counteract earth's gravity, meaning everything near where it "touched down" would experience near zero-g as it approached. The opposite side of the earth would experience greatly increased gravitational pull...

Then while the black hole was inside the earth, everything would be experiencing close to 2G, then zero G for a bit for the poor souls on the side of the earth where it exited.

Because of the fantastic amount of momentum it would have, and it would certainly be travelling at well over earth escape velocity I would expect it would travel straight through earth and keep going, not even coming close to stopping inside earth.

The affects on earth would be very... bad.

I'd imagine it would trigger devastating global earthquakes, as well as epic rock slides from the changing gravitational forces. Not to mention the tsunamis from the oceans sloshing around like water in a giant bath, as well as all the rockslides both above and below water. It would be a mess.

Assuming this object would be travelling at 100km/sec, and the gravity would maybe begin to badly affect us when it was 50,000 km out from the surface of the earth (?) then it would take about 16 hours to travel from this distance, impact the earth and then reach this distance out the other side.

I'm not sure how much energy would be released by the matter being consumed by the black hole - It might not be very much, after all its total gravitational pull wouldn't be very high, but would be travelling at probably at least 50+ km/second - certainly more than 11km/sec (earth escape velocity) The matter in front of the black hole might simply be swallowed up without much fuss, or if there is any "resistance" to this matter falling down past the event horizon I'd imagine it would be like a continuous series of nuclear bombs going off as matter was flash heated to the point of spontaneous nuclear fusion before it passed the event horizon.

The one good point to note in all of this is it would be extraordinarily difficult to disturb this object from its object so it fell into the inner solar system. After all, it has the same mass as earth, and we don't have to be worried about random stuff disturbing earth's orbit.

> everything near where it "touched down" would experience near zero-g as it approached[...] while the black hole was inside the earth, everything would be experiencing close to 2G, then zero G for a bit for the poor souls on the side of the earth where it exited.

All of these numbers are off by many orders of magnitude. You're fundamentally misunderstanding how gravity works.

If you were standing on the surface of the Earth and an Earth-mass black hole were magically stationary 1 km underneath your feet you wouldn't experience something close to 2G, you'd experience something closer to 40 million G.

Check out [1] for more details, and [2] for a calculator where you can play with the gravitational formula.

1. http://www.physbot.co.uk/gravity-fields-and-potentials.html

2. https://www.wolframalpha.com/input/?i=surface+gravity+calcul...

I suspect by "while the black hole was inside the earth" he meant at the centre of the Earth

Then they wouldn't be saying that the people on the other side of the earth would experience zero-G as the black hole exited, they'd be "experiencing" some combination of being ripped apart by tidal forces and being buried under rapidly shifting rubble, and all of this would be happening at some G value in the range of millions.

This would be an extraordinary sci-fi story and I would pay good money to read it.

Same here, maybe even write a simulation or two to help pin down some of the side effects.

I did do a super simple simulation trying to explore if you could escape from beyond the event horizon where time dilation becomes so extreme that even very rare events become common. One of those extremely rare events could be when a galaxy collides with a second galaxy and the black holes at the center end up merging.

Turns out you could escape from falling into a particular black hole if you had a second black hole to counteract the gravity. Unfortunately even while you are at zero acceleration, you end up inside an even larger event horizon formed by the two black holes. Your choices would be to fall into the first black hole, fall into the second, or wait for them to collide. Even the waiting isn't a particularly good option. While being outside the event horizon of a very large black hole minimizes the spagetification problem, there's no such protection inside the event horizon.

In any case if interested in reading sci-fi based on black holes check out https://en.wikipedia.org/wiki/Black_holes_in_fiction

Not quite the same thing, but you might be interested in _The Forge of God_ by Greg Bear.

I second that. Great book. The sequel, anvil of stars is pretty good too, but its a different setting

David Brin's Earth deals with a black hole stuck orbiting inside the planet. Also predicted Google glass backlash.

That somehow reminded me of the "what if" article about stopping earth's rotation from one moment to another, except affecting the poles equally. It also has a taste of turning the landscape upside down, which sounds terrible.

An interesting part about it would likely be what would be left of earth, since I guess earth's core material would be rather eager to be swallowed by the PBH, and we rely on a couple of properties of the core for the electromagnetic field. I'd imagine this would never be the same planet afterwards, at least not with the same gravitational attraction... or even comparable properties within the next geologic timescale.

Should inertia mitigate some of this? It would all happen quite quickly and be over just as quickly right? I imagine enough to release tensions in earthquakes already bound to happen, and some serious Tsunamis, but maybe not so large that they'd wipe out all life? Would it eat a lot of our mass? Would that slow it down?

Not really. If it's orbiting the sun the maximum speed is less than the sun's escape velocity. But likely much less.

One number I saw was for Comet Kyakutake which now has a 70,000 year period, was 58 km/sec. At that speed it would take about 4 minutes to skewer earth. But things would be pretty crazy for something like 3 times that.

Imagine the object is 10,000km away, the nearest edge of the earth is starting to notice less gravity, around 1/2 a G. Walking becomes difficult, homes, bridges, trees, etc would equalize to 0.5G loads, likely making all kinds of interesting noises. Driving would feel like it's on snow/ice (with half the traction, but the same mass).

About a minute later there would be zero gravity over a decent fraction of the near side of the planet. Rivers overflow their banks, cars float into the air with the slighted bump, and any step taken is your last.

Likely volcanos and fault lines would experience significant pressure changes and might well go off. The atmospheric pressure of 15 pounds per square inch (22,500 pounds per square meter) disappears.

Any cars on suspended structures like bridges would be flung into the air as the weight of the entire bridge is suddenly removed. Every car would launch as soon as it hit the smallest bump. Even just the 3000 pounds of car compressing the tires would be enough to launch the car with gravity is removed.

Most humans are still alive. People within a few 1000km in pain, getting bends, and exposed to very low pressures. The atmosphere has going from 15 psi to close to zero.

One minute later the object is 3000 km away, and now acceleration towards it is on the order of 47 M/sec^2, earth is only pulling people back with 9.7M/sec^2 so everything within 1000km is accelerating away from the earth at 4G or so.

First it's all water, most buildings, and all cars that leave the earth's surface and accelerate towards the black hole. Then it's plants, soil, and everything but the bedrock. Then the bedrock, mountains, and the entire earths crust that leaves.

Vertical (or towards the black hole if you prefer) acceleration keeps increasing. Enough to throw significant debris all over the planet, into orbit, and even above escape velocity. The significantly impacted part of the planet has gone from a few % to 25%. Air on the closest half of the planet is rushing towards the impact point at 1000s of miles per hour (accelerated at 15 psi).

Distant locals half way between where the black hole enters and exists are going to start noticing crazy winds and air pressure changes as a decent chunk of the earth atmosphere is leaving the planet.

Seconds later the acceleration is up to 40G or so, it's still 1000 km away, but nothing can resist the acceleration. The deep internal pressures of the earth make things worse as a nearest 1000km of the earth gets pulled on by 40G forces. Think of super volcano the side of a continent, but worse. On the near side of the planet there's really no surface to speak of. Just a giant cloud tormented by the ever increasing gravitation gradients. Spagatification reduces the cloud to small hot particles.

A minute later the object hits where... the surface was. The acceleration of 25% of the earth or so has just reversed direction, with even higher because it's now the earth + black hole. A minimal amount of matter was consumed by the black hole, adding a nice light show at as a few % of the mass consumed is turned into energy.

Now a fraction of the planet has gone from zero PSI sucking the rest of the earths atmosphere in (15 PSI of negative pressure), to a huge over pressure (1000s of PSI of positive pressure) as a everything that left is slammed back into earth. Everything within a 1000km radius of the impact zone saw a negative acceleration of at least 40G followed by a positive acceleration of 41G. The circulate shock wave will be easily heard round the world and will cause a world wide maelstrom just from the energy involved, even ignoring the world wide rain of white hot debris.

I don't think the earth would quite be at the level of a planetary object called synestia, but it's on it's way (look it up).

After another minute the black hole is near the center of the earth, and the entire earth sees 2G of gravity. The energy already imparted into the earth is substantially more than if you ran the moon into the earth at 130,000 mph.

The next minute isn't too bad, oceans, earthquakes, winds, and storms are all off charts in intensive compared to anything besides maybe the center of a nuclear bomb. But nothing worse than what has happened... yet.

Then there's a huge gravitation spike on the opposite side of the planet, north of 40G over a 2000 km diameter circle, but more damaging. As bad as the approach was, the exit wound is worse. It ends up taking a 1000 km deep (earth crust is only 30km or so) chunk of the earth with it. This makes the previous super volcano eruptions look like childs play. The hole left behind could easily drain all the worlds oceans, or if filled with magma would cause the earth to deflate and cause elevation changes world wide. Air pressure would drop world wide, and what was left would be heavily polluted with embers, smoke, and be way too hot to breath. In fact I'm not sure the earth would have a crust at this point.

Depending on the exit I could see all land ending up below the ocean (the average land elevation being much more even than today) or ocean levels being dramatically lower (if the black hole left through the center of the pacific ocean).

Any water within 6000 km or so of exit would be gone, land would be less.

Seems unlikely at this point that the earth rotation would be the same speed, or even the same direction. The orbit around the sun might change some, but not a big deal compared to everything else.

Seems plausible there might still be single cell bacteria alive, but not much more.

From the description it sounds like a faster black hole would do less damage since the involved masses would accelerate for a shorter time span.

What would be the optimal speed for a collision?

Hrm, I suspect something like 0.5c where relativistic effects to not increase the mass of the black hole much, but does significantly decrease the transit time.

Here, have an upvote. This is the reason HN is the best board on the interwebs.

I love this comment

The force of gravity is proportional to distance. You'd be feeling hell of a lot more gravity being close to an earth-sized black hole than you do on the surface of the earth.

I think the surface would just become liquid again.

To paraphrase Neal Stephenson, "The earth blew up without warning and for no apparent reason."

The point stands, it's not any more dangerous than an earth sized object of the same mass, in fact it's less. It isn't any easier to divert from it's orbit, it is not any more likely to collide with the earth if diverted (in fact less so on account of its small size), and it would be no more destructive on collision.

If you drop the earth into the sun, the sun won't care.

If you drop a black hole into the sun, the sun dies.

Seems a wee bit more dangerous to me.

The collision case is screaming for a "What If"-style exposition, as in https://what-if.xkcd.com/

and wildy change Earth orbit

I read this book once.

A layman here. Wouldn't a Black Hole with a mass equal to Earth's be significantly smaller than Earth? Won't the Gravitational range (Schwarzschild radius?) be significantly less than Earth's?

Yes, a black hole with the mass of the Earth would be much smaller. There's an actual size illustration of one in the article.

The gravitational attraction from a given distance from the center would be the same as for Earth. For example, if you're 4,000 miles from the center, you'll experience a 1-g acceleration. The difference is that, because of the black hole's smaller size, you can get a lot closer to it.

No at a distance it would be exactly the same. You would only start to notice the difference gravitationally once you get to be around 1 earth radius away.

The Schwarzschild radius is determined by the mass of the object, not the volume.

Radius = (2 * Gravitational Constant * Mass)/(Speed of Light^2)

In other words, 9mm

The entire mass of Earth, compressed down to the size of a bullet. That's actually a very vivid illustration, unlike most of the other large numbers that get thrown around in astronomy.

Its actually compressed down to a point. A singularity. The 9mm is just the radius from the point from which nothing can return.

For all intents and purposes of astronomic calculations, you can pretend it's 9mm in size, because A) the difference between a 9mm sized object that behaves like a black hole and a singularity with a 9mm event horizon is minimal and B) it's a 9mm black hole going 50km/s+, most math will result in the extinction of human kind anyway.

To be fair, we don't know what is behind those 9mm. Einstein's equations predict a singularity but afaik there is consensus that those don't hold there.

A rather lethal 9mm bullet. Would it just punch right through the earth if it hit or would it stop and then eat the earth from the inside?

it would just spagettify the earth long before it reaches the center.

Would it? If it were one earth radius away from the surface it would exert as much pull on someone on the surface as the earth would, right? Things couldn't start becoming spaghetti before that point, and that's pretty close to earth; surely it'd reach the earth's center very shortly after that.

Not my area of expertise, but at the distances we're talking, can we just treat them as point masses? Somewhat different accelerations of earths colliding won't matter much until the density is comedically low.

To the parent comment’s point though, would that really be any worse than an earth-sized planet colliding with the earth? If anything it sounds like it might be less destructive.

>continues on down into the center

You'll get a Nobel prize or two if you could prove that..

Not really, it’s pretty simple that an object that dense would just pass through the ground.

The ground or the entire planet cannot remain in its current form anywhere near a black hole. There would be no ground for the black hole to sink to.

Besides wouldn't the 'ground' gravitate towards the black hole than the other way around?

For the black hole the ground is merely more dense vacuum, the ground has barely enough time to move if it comes in at interplanetary velocities (50km/s+), so by the time the ground will begin to react to the intense gravity, the black hole will have passed through with the force of a few hundred nuclear explosions.

At 6000 km away the gravity at the nearest surface of earth is zero.

1 minute (at 50km/sec) later it would be 3000 km the acceleration 4.5G or so.

1.5 minutes later it would 1500 km away the acceleration would be 17G or higher.

2 minutes later it would be on the surface, and everything ripped up (including buildings, soil, bedrock, crust, and the magma below would go from being accelerated up at incredible velocities to an even more intense acceleration down.

Over that 2 minutes the black hole is going to chew the hell out of the earth for 1000s of km in all directions and that's just the start of the fun.

As it hits the surface everything within 1000km will be experiencing 40G. The pressure waves will go from negative (as the black hole pulls the atmosphere away from the earth) to a HUGE over pressure as magma, bedrock, and water get slammed back down into the earth at 40G. Said over pressure wave of intense heat and pressure will expand outward from the point of impact and create problems world wide.

Worse on the opposite side it will just take a large chunk with it (approximately the volume of which the black hole has significantly more than 1G of gravity) which could easily take a continent with it, or say most of the pacific ocean.

I could see ocean levels and air pressure lowering significantly. After all why would the air stick around a 1G planet instead of a more than 1G black hole?

The area from which the air is removed is likely to be somewhere around 12,000 km wide (the area of which the black hole applies more than 1G).

So I'm not sure I believe the "barely enough time to move" thing when for minutes the accelerations will be crazy.

Not to mention that anything with 9mm would accelerate at the speed of light to the singularity. So while moving through the earth it wouldn't meet any resistance at all.

What do you think the density of a black hole is? :)

Almost certainly not actually infinite in reality :)

We treat it that way because, despite it showing that our model of physics is breaking down at that point, it works really well for most real purposes. But look at string theory, loop quantum gravity, etc., and you will find plenty of the variants within those umbrellas do not have a singularity at the heart of black holes.

I think a black hole with earth mass would be much more dangerous than earth itself.

Humans might be plenty dangerous to the rest of the galaxy. It remains to be seen.

Nope. Replace the Earth's Moon with a black hole of the same mass. Nothing bad would happen except our nights would be darker. Our tides wouldn't even change.

Sure, fine for you and me. Might, have been a problem for Buzz Aldrin though.

Michael Collins, it's your time to shine.

One of my favorite things is to have made a joke, where it turns out the real value of the joke is to set the context for someone really funny to take it in an unexpected direction, and if the direction is heartbreaking in seven words or less, all the better.

What if a moon mass or earth mass black hole collided with the sun? Would it suck in matter and eventually consume the sun?

Probably but it would take a while. Any relative angular momentum between the Sun and the black hole would result in the hole orbiting the Sun since an earth mass is tiny compared to the Sun. It might well orbit beneath the surface of the Sun, sucking up plasma as it orbits. How long would it take to "eat" the Sun? The calculation could be done but it's tricky; the hole grows as the Sun shrinks, and that causes the mass transfer to accelerate. And the Sun is denser in the middle and the hole will spiral inward as it grows to keep the center of mass of the system in the same place, etc etc.

It's possible there are one or more black holes orbiting inside the Sun right now. They might be the reason sunspots exist.

> It's possible there are one or more black holes orbiting inside the Sun right now. They might be the reason sunspots exist.

Practicing your impression of Calvin’s dad, I see…

Depends. Is the velocity of the black hole relative to the sun higher or lower than the escape velocity of the sun?

If higher than it's going to keep going, if lower, then yes eventually it's going to consume the sun.

Wouldn't momentum transfer from the consumed mass slow it down?

Some, but the sun isn't very dense. So sure if you were ever so slightly above escape velocity the consumed matter could slow you down.

A black hole the mass of the earth is 9mm. The weight of a cubic cm of sun is around 1.41 grams. The black hole isn't really going to notice.

The pass through the sun is around 1.39102e+11 cm * 1.41 grams per cm^3 = 139 million KG. So the earth weighs around 4.2* 10^16 more than what a black hole sized hole in the sun weighs.

I thought its gravitational pull and the pressure around the black hole should cause it to absorb much more mass that that estimate.

The sun emits earth sized mass all the time, I imagine the blackhole could get sway to different directions.

The question “What would happen if the Moon were replaced with an equivalently-massed black hole? If it's possible, what would a lunar ("holar"?) eclipse look like?” is answered at https://what-if.xkcd.com/129/. It starts with “ ‘Not much’ and ‘not much.’ ”

I have that book and it's wonderful. I was probably plagiarizing unconsciously.

Are there any sources on this? Would love to read more on this theoretical scenario.

Not really the same scenario, but Neal Stephenson's Seveneves is a fictional account of what might happen if the moon suddenly exploded.

Of more concern is the fact that they suppose this primordial black hole was captured from a population of micro black holes that just wanders around out there. One of those hitting closer in in the solar system could be pretty upsetting.

Only because black holes are hard to see. A black hole has to obey Newton's laws as much as an asteroid does. If either is on a collision course with Earth, we're toast. Any black hole not on a collision course with Earth is merely an interesting curiosity.

If the black hole is moving very rapidly, and it passes through the center of mass of the Earth, and it is moving parallel to the Earth's motion around the Sun, well it's 'less bad', and the path being parallel becomes less important the faster the black hole is going. The gravitational disruption will be equal in intensity to a slow moving black hole, and huge near the path of the black hole (something about 100m from the path would experience an acceleration of about 4 billion times Earth's surface gravity, but only for a very short time, something 1000km from the path would max out at about 40x), but things far from the black hole's path will only experience a few G of net additional acceleration for an extremely short time. The change in the Earth's orbit would be minimized. The amount of energy transferred between the black hole and the earth from the actual collision would be negligible. In fact by far the greatest harm would come from an off-center collision, since that is what would determine how much momentum would be transferred, and that is what would cause our orbit to change.

However, that 4 billion g acceleration is a problem even if it is only for a very short time. If the black hole is going at .999999999c, which is far more than we can hope for, something withing about 100m it's path would be accelerated due to the gravity of the black hole so much that it ends up moving at hundreds of thousands of miles per hour, and stuff within a few dozen meters becomes relativistic. Even ignoring the stuff very close to the path of the black hole, the stuff at hundreds of meters from the path would release an epic amount of kinetic energy. My envelope isn't large enough to see just how large, but certainly enough to leave a melted/vaporized tube at least a few hundred meters across all the way through the earth, and this is a gross underestimate.

> The gravitational disruption will be equal in intensity to a slow moving black hole[...] the amount of energy transferred between the black hole and the earth from the actual collision would be negligible.

Yeah, but you're not accounting for all the kinetic energy the black hole is going to impart on the Earth, which is the main factor in this hypothetical disaster.

Let's assume for simplicity that the radius of the Earth is 5000 km, and the black hole is moving at 1000 km/s.

That means that as soon as it's approximately 5000 km out we'll have around 5 seconds where around half of the Earth isn't the Earth's surface in any meaningful sense anymore, rather it's a bunch of rocks that have just crossed the zero G barrier between the Earth and the black hole, and are now in freefall "upwards" from the Earth.

I'm too lazy now to calculate how far a rock on the surface will travel "upwards" given the approaching black hole, but let's just assume a constant 9.8 m/s^2 for the whole 5 seconds to understate this disaster (it's going to be a lot more than that). You'll fall ~120 meters in 5 seconds in freefall in Earth gravity.

So by the time the black hole passes the "crust" that crust is already hovering some hundreds of meters in the air, and will rapidly fall down and re-compress as the black hole passes.

I wouldn't be surprised if the energy transfer is enough to turn the entire surface into liquid magma, but I haven't done that back-of-the-envelope calculation.

> Let's assume for simplicity that the radius of the Earth is 5000 km, and the black hole is moving at 1000 km/s.

You are assuming the stuff near the surface is being pulled up, but the stuff under that isn't being pulled up, so rocks are 'flying away from the earth and towards the black hole'. In reality the only net force you can 'feel' from gravity is due to tidal forces, so this would have an effect more like 'squeezing' the earth and the energy would be spread throughout the entire interior. For an earth-mass black hole, however, the difference in acceleration between something 1000km away and something 2000km away is still stupidly high (about 30g), so your assumption is correct!

But generally, this is my point. A slow moving black hole is going to rip the earth apart, a fast moving black hole is 'less dangerous' because it just punches a hole and keeps going, with almost no interaction with the earth, even though that is counter-intuitive. Yes, things would get pulled away from the surface of the earth, but they would only feel that pull upward for a very, very short amount of time, not long enough to be substantial. A small black hole, say the mass of a meteor or something, might go through the earth and not even be noticed, it's far less dangerous than a meteor of the same mass would have been.

A fast moving black hole isn't 'nearby' very long, it would go from far away to far away in microseconds.

However, as the mass gets large enough, the dynamical friction on the black hole is so large that it leaves behind significant kinetic energy by melting a hole through the earth, no matter how fast it is going. https://en.wikipedia.org/wiki/Dynamical_friction

If you neglect dynamical friction, as far as I know an earth mass black hole could potentially go right through the earth and we wouldn't have to worry so much, but it depends on the specific path it takes since it could still disrupt the Earth's orbit.

Careful, as you accelerate the black hole to ever larger fractions of C the mass becomes bigger and will cause even more problems. You might well disturb all the planetary orbits, not just earth.

Would make for a pretty impressive anti-podal volcano if nothing else.

I did hear one theory that the Tunguska event was an evaporating black hole, I don't think there's much scientific evidence to back that up though.

I forget the story, but some scary advanced weapon technology gets abandoned, sold on the black market, and ends up in India. Random folks there figured out you could chip the containment vessel, feed it sewage, and bounce a laser off it to capture energy and power the village. Complications ensue.

An 5x Earth mass object doesn't need to impact Earth to create a bad time on Earth. It just has to get moderately close.

Very true. And it doesn't matter if it's black or reflective, except we're sure to see it coming if it's reflective.

For some time when that was happening it wouldn't be a solar system anymore it would be a black hole system, or a binary system with a star and a black hole

Just whatever the predominant gravitational influence is

Maybe this semantical exercise doesnt matter here if the mass of the black hole is so small