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What If Planet 9 Is a Primordial Black Hole? (arxiv.org)
285 points by anpat 26 days ago | hide | past | web | favorite | 345 comments

Some potentially interesting uses for it:

- definitive sink to send all of our nuclear waste (it would be the /dev/null of the Solar System)

- gravitational energy generator (limitless, until we have no more mass to throw in)

Another idea would be to use the black hole as a gravitational lens to construct a large telecscope. I wonder what the focal length and the effective lense diameter would be. Also the orbital velocity of the image sensor (in orbit around the black hole) might be too high to make this useful.

See also [1].

[1] https://physics.stackexchange.com/questions/25498/the-sun-as...

I can help on this one! In short, it won't work very well. The sun is a much better object to use.

Basically, gravitational lenses don't have a "focal length" per se. It's not an exact analogy to a classic glass lens. There is a minimum distance you need to be away from an object to use it as a g-lens. But as you get further away than that, it'll work better.

Now, the minimum distance you need to be away from the massive object decreases as the mass of the object increases. So for the sun, you'd have to place a camera about 500AU away to use it. That's too far to be practical at our current technology level. For a smaller object like a planet sized blackhole, you'd have to be orders of magnitude further away. Not very helpful!

Now, it's possible I'm mistaken as I'm thinking about some calculations I did on schwarzchild geometry and I didn't consider what would happen very close to the blackhole where curvature is very high, but my intuition says that it won't be very helpful at all.

That doesn't mean the blackhole won't be helpful though!!! I think there's an ENORMOUS number of useful experiments we could do. And, I think blackholes can be used as very powerful computers, possibly quantum ones, but I don't know the details.

Wouldn't you place it at the Sun-IX L1?

Edit: never mind, I just remembered there's a specific distance from a massive object you have to be to hit the focal length sweet spot. I wonder if having a smaller radius makes the focal length shorter?

The concept of waste is a very narrow view of the world and one trapped in our 20th century concepts.

Waste implies we have no use for it any further. This concept views the world on very limited time scales (wherein we can continue to take from the world and turn things into waste that we have no use for any longer).

Instead we should always be thinking about reuse of materials. Waste should never be a terminal state so much as the waste products of one process should be converted into a useful input to something else.

Ultimately with limited materials on planet earth, virtually everything needs to exist in a cycle (water cycle, carbon cycle, etc).

The problem with the blackhole idea is that all atoms / baryonic matter could be used for something. When you send them to a blackhole, they literally cease to exist as baryonic matter (or at a minimum are never usable again). Thus, you are literally taking that material out of use (technically they will eventually be converted into energy as hawking radiation).

Exactly - in our universe on a fundamental level all waste is just atoms and elementary particles in the end.

With enough clean & cheap energy nothing really prevents you from reassembling those particles into something useful.

Why would you want to throw away "nuclear waste" - we should be burning it! It's only viewed as waste because of politics and ignorance. Instead of stupidly burying it we should be building the various reactors that will easily burn our existing waste - dramatically reducing the overall radioactivity and producing energy at the same time!

All the fear mongering around nuclear power is beyond hysterical at this point :(

"Nuclear waste" is a term that is used to describe the byproduct, but also all the equipments that came in contact with radioactive material (cf [0]). So while we can indeed reuse some of the waste on other future nuclear facilities, there will still always be some kind of waste.

[0] https://en.wikipedia.org/wiki/Radioactive_waste#Low-level_wa...

100% this. Sending nuclear waste to a black hole? Now that's the epitome of stupid.

you must have missed the second bullet point.

Personally, I'm also concerned with magnitudes of "low-level" waste, which I believe is not really burnable. Not that it would be reasonable to launch entire reactors and ancillaries.

If it is still radioactive, it can almost certainly be reprocessed and shoved back into a reactor.

Parent is talking about stuff like tyvek suits. Also, steel components like reactor vessels and steam generators. Also, fission products are extremely radioactive and non-burnable. None of it is the least cause for "concern", of course.

I think the most obvious way to utilize a close-to-earth black hole is for space-flight. By utilizing the Oberth effect [1] it is possible to gain extra kinetic energy when firing engines very deep inside the black hole's gravity-well (while performing a sling-shot). The Penrose Process [2] may also allow for some kinetic energy boost.

Unfortunately both effects do not seem to offer the kind of multiple-orders-of-magnitude gain required to make interstellar travel practical.

[1] https://en.wikipedia.org/wiki/Oberth_effect

[2] https://en.wikipedia.org/wiki/Penrose_process

I'm pretty sure I read a short story about this in an anthology called "Black Holes" by Jerry Pournell back when I was a youngster. In short, the black hole was a "gift" from some advanced race who dropped it into orbit of our star by braking it with Pluto (hence its erratic orbit) for the purpose of interstellar travel.

You can use a small black hole for mass transmutation of lighter elements (like all that surplus hydrogen just sitting idly in your local gas giants or star) into much more useful heavier elements (metals, carbon, oxygen, etc.):


The extreme temperatures and pressures in the accretion disk are basically used to fuse the lighter elements together into heavier ones, which are then pulled out by machinery in close orbit.

Machinery made of... plasma \o/

Because anything in close orbit is by definition plasma.

It doesn't have to be that close.

However if it is a primordial blackhole it is probably not much rotating... thus no much energy to get by the Penrose process.

I thought that just by letting masses getting sucked in while pulling ropes tied to alternators we could generate electricity... is it too naive? The amount of energy given by the fall into the blackhole would be superior than that used to bring the masses there in the first place.

I don't think that this would work: whoever is holding the alternator would also be pulled into the black hole while generating the electricity.

Maybe the "right" way to convert potential energy is via conversion to heat and black-body radiation in the accretion disk? Might be difficult to capture significant percentage of that energy, though. See also [1]

[1] https://arxiv.org/pdf/astro-ph/0307333v1.pdf

I would have started by an orbiting station (or even an orbiting ring-shaped station), staying at a safe distance from the horizon, from which the masses would be droped.

Is it still too unrealistic?

For the radiation energy, it sure makes sense! Moreover isn't it any hard radiations emitted when the hadrons' quarks are torn from each other on reaching the events horizon?

If your station is in orbit, then so is the mass you want to drop into the black hole. Orbital mechanics works quite differently to what you are used to from the surface of a planet.

But the black hole is very small as far as I know, maybe we could build an orbital ring around it, attach alternators to the orbital ring and then feed "ropes" to the alternator one one end and on the other end let them fall into the black hole.

By "ropes" I mean charged particles and by alternators I mean just very powerful electromagnets that can extract the energy of the charged particles falling into the black hole.

My point was that things attached to the outer core of an orbital ring are not in 0 G, but they feel the actual gravity at the particular height the orbital ring is orbiting -- on Earth if you would be sitting on an orbital ring situated at a height of say 300km, you would feel as though you were sitting on a 300km mountain; maybe on a primordial black hole you could build an orbital ring just a few km from the black hole and have spokes going down very close to the black hole (maybe active structures to overcome our current material strength limitations) and let charged particles fall into the black hole and extract their energy as they fall into the black hole.

Or maybe the black hole is small enough that a very crude electromagnets field could just encompass all of the black hole and it could very easily extract all that sweet energy of a charged particle falling into the black hole with an electromagnet an amateur could build in his garage.

Isn't there some way to stockpile the kinetic energy in several passes, instead of only one slingshot, then finally use all of that for a final throw?

Not really. The thing is, you're not so much stockpiling kinetic energy as you are using the mass of the body to amplify the force you apply. There is an optimal time to apply that force, which is when you are closest to the body you are orbiting.

So what may happen is if you have a low-thrust engine, you will do a burn at the optimal time, then stop and wait an orbit until you reach the optimal time again. But you're not "stockpiling" anything so much as you are just thrusting at the optimal time. And once you reach escape velocity you have to keep thrusting, there is no more opportunity to do another pass.

I can't recall which probe, but NASA has used the same body for multiply slingshots. But that's stealing orbital energy from the body you pass in the same direction it's orbiting.

I think the mass of the body only matters in how much momentum it has. If you fly by an asteroid you will deflect its course. You could convert the entire mass of earth into spaceships and slingshot them past Jupiter and it would barely register.

You could make multiple passes, if needed, gaining a + delta V each time by designing the orbit such that the spacecraft is on a transfer orbit back to PBH to make another pass. (If, as guelo mentioned, you did not at some point reach escape velocity of the solar system at that orbit.) It would be a very long wait between passes though...

Seems like a Jupiter gravity assist would always be much more practical.

The limit is the escape velocity of the orbit.

Not if you have time. And three bodies. If you've achieved escape velocity from the 2nd body (eg, Earth) but not the first body (eg, the Sun) you can keep swinging around the 1st body stealing speed off of the 2nd one each time.

Takes some careful orbits and a long time, but NASA does it all the time.

It would be very cool to have a binary black hole system somewhere in the solar-system. I.e. two black holes in a tight orbit around each other at very high orbital velocities. This could allow for very powerful slingshots to bring probes to interstellar speeds.

If you have the energy to get to black holes rotating around each other, you probably can build ships that don't need to slingshot. 'Cept perhaps for braking maneuvers.

There are much easier ways to deal with nuclear waste. If you could afford to launch it into space, that's really good enough. You can put it on a collision course with the sun, or just into an out of the way orbit around the sun (or even the Earth as long as it doesn't degrade and fall back while still active.)

But really it's easier still to manage it on earth itself. Which is why, even at SpaceX prices nobody does that (that and rockets have a nasty habit of suffering rapid unscheduled dissembly.)

> You can put it on a collision course with the sun

Counterintuitively, this is far more expensive than launching it into said black hole, or just out of the Solar System all together.

If you're using a Hohmann transfer, sure. But IIRC a Bi-elliptic transfer will get you on a collision with the Sun for less delta-v than escaping the solar system. That's where you speed up to get high and slow then burn to cut your velocity when there. For ratios of oribital radii over 15 it's more always more efficient than the standard Hohmann transfer and since the ratio is very large in this case it works much better. But in this case you're just trying to get within the radius of the Sun and don't even need the third burn to slow down.


I assume you have to "slow down" ~107,000 kmph to fall into the sun, how much do you have to "speed up" to escape?

~11 km/s to escape the sun, ~30 km/s to fall into it. Escaping is easier.

Just getting into the sun isnt enough. The sun's outer layers are not very dense. A blob of uranium might survive long enough to come out the other side, at least on the initial few orbits.

I don't think we have to be very precise. It doesn't really matter if a blob of uranium orbits the sun for a few thousand years.

In that case, why get it into the sun at all? Just put it into an orbit that doesn't intersect Earth.

Interesting- why is that?

Orbital mechanics.

The earth is orbiting the sun at 30 kilometers per second. So if we launched something into space, since it started on earth, it would have that speed (similar-ish to throwing a ball from a moving car). So that object would now also be orbiting the sun at 30 km/s. We would need to slow it down that much in order to "fall" into the sun.

Once something was in earth orbit, it would only take about 12 km/s of delta v (change in velocity) to escape the solar system.

More info and math here: https://space.stackexchange.com/questions/3612/calculating-s...

Although you could do it with about 17km/s if you put it into a Holman transfer orbit to Venus and used a low flyby and aerobrake maneuver of Venus and that puts you into a flyby of Mercury which changes the orbital plane such that the ellipse intersects the corona.

Sad note that also limits your launch window to once every 113 years as I recall from the last time I did the math :-(.

From a technical perspective you push into an elliptical orbit that intersects Venus, you do a slight aerobreak (skim the surface of the atmosphere) to dogleg toward a Mercury intercept, and then as you pass Mercury it tightens your ellipse still further and you head out, and come back and fly through the outer corona of the Sun (which is its hottest point). At which point you're in a degenerate orbit that will go out and come back through the Sun's corona until you've been completely consumed/burned up.

Why do you have to slow it to 0 to hit the sun? Can't you just cruise at whatever speed you're cruising and redirect it with thrusters towards the sun?

In space (and any frictionless medium), you can't "redirect" an existing velocity vector with thrusters. You can only add a velocity vector. This means that if your desired direction is perpendicular to your current direction, your current speed is no good to you at all. If you're heading due "north" at 10mph, and you want to be heading due "east" at 10mph instead, you have to 1) fully negate your "north" velocity, and 2) come up with 10mph of "east" velocity from scratch.

Now, the trajectory of an object in solar orbit is exactly at right angles to the direction it needs to go in to hit the sun. No part of this velocity is helpful for getting to the sun - in fact it actively prevents it! The only vector that takes you directly into the sun is one with no sideways component - if you imagine yourself falling right in, any sideways nudge will cause you to miss it by a hair and go flinging off into a highly elliptical orbit. If you just ignore this and just thrust directly at the sun, hoping to overpower everything by brute force, then like a ballerina pulling her arms in, the more you try to get close to the sun with your thrusters, the faster your orbit will go; the closer you manage to get, the further out you'll be flung when you inevitably miss.

All this ignores that the sun is not a point, but quite a large ball - you can get away with some small horizontal velocity. A highly elliptical orbit will still do what you want if its lowest point is below the surface.

Once you're in orbit (say around the sun), you have to cancel the orbital velocity to fall into the object you're orbiting around. If you point at the sun and accelerate 1 km/s directly at it, you're still moving 30 km/s "sideways". All you'd end up doing is making the orbit more elliptical-shaped.

At least that's how I see it, but I am far from being an authority on this topic.

(I mean this in all seriousness)

You should play Kerbal Space Program. It will very quickly give you an excellent intuition for basic orbital mechanics.

Thanks, I actually have it installed, but never made it through all the tutorials. I'll probably give it another shot, would make it much easier to get an understanding of simple questions like this one.

I'm no rocket scientist but perhaps that would cause the object to shift from the Earth's kinda-circular orbit into a highly-elliptical orbit, its existing sideways velocity (relative to the sun) causing it to be flung past the sun at more of a straight line and hence way out into the solar system instead.

You can but then you're using a lot of propellant rather than gravity to reach your target.

No, once something was in Earth orbit, it would only take 1.7 km/s to escape solar system, per your link.

The escape velocity from the sun is 42.1 km/s, while the earth is orbiting at 29.78 km/s. The 29.78 km/s is 3 times closer to the escape velocity than to 0 km/s.

This is the first order approximation reason.

Here's a delta-V map of the solar system: http://i.imgur.com/SqdzxzF.png

That seems like a orbit/landing chart. Doesn't seem accurate for head-on collisions. Or am I reading it wrong?

As a counter example, someone mentioned nearly leaving and then cheaply coming back directly into the sun.

Interesting! I would have never guessed that starting from Earth, getting to Mars is harder (by about 5%) than escaping the Solar system.

Well, there's "I want to match Mars' orbit around the Sun, so I can orbit the planet," and then there's "I want to intersect Mars' orbit just as the planet is passing through so I smack into its surface and leave a big crater."

The latter takes a lot less delta v, but it has its drawbacks. Leaving the solar system, you don't have to budget for that rendezvous.

Ever been on a merri-go-round?

While spinning it’s hard to get to the center. Once it stops, it’s easy.

The earth is spinning around the sun. To get to the sun, you need to slow down.

The Earth is moving around the sun at ~30km/s. The escape velocity of the sun is ~42km/s. So if you start from Earth, it only takes 12km/s of change in velocity (called "delta-V" by rocket scientists / KSP players) to leave the solar system, but more than twice that (30km/s) to slow down enough to hit the sun.

Can't you use some kind of gravity assist to do the slowdown or adjust the trajectory into the sun?

The Parker Solar Probe is using seven Venus flybys to fly pretty close to the Sun (but not into it).

You could, but you could more easily use one to escape the solar system entirely.

This Is Why We Don't Shoot Earth's Garbage Into The Sun (Ethan Siegel): https://www.forbes.com/sites/startswithabang/2019/09/20/this.... Also, is loading nuclear waste onto rockets and shooting them through our atmosphere really that great of idea? Agreed better to deal here with on earth

Quote: "Quite to the contrary, the gravitational pull of the Sun far exceeds the gravitational pull of Earth! The only reason we don't notice it is because you, me, and the entire planet Earth are in free-fall with respect to the Sun, and so we're all accelerated by it at the same relative rate."

That seems off...googling suggests that the acceleration due to the sun at earth's orbit is a tiny fraction of 1G, and conversely, to have an acceleration of 1G would require going well inside Mercury's orbit.

There was an alternative plan to go the other direction.

Load the material into enclosures, bury them hundreds of feet below the sea floor near a subduction zone. Cheaper than rockets, still bloody expensive, and they may be worried about radioactive burps.

>definitive sink to send all of our nuclear waste

That seems like a lot of effort when you can just send it to nowhere and it will very, very, very likely never hit anything, ever.

I was wondering about the necessary delta-v to do that vs. throwing it into the sun and found this [1] interesting table.

Quite informative, it seems like the naive approach of going straight into the sun is much more expensive (dv of 24.0 km/s) than escaping the solar system (dv of 8.8 km/s).

But, it seems that there is a trick where you use 8.8km/s to almost escape the solar system, then turn around with very little dv cost and plunge into the sun.

[1] https://en.wikipedia.org/wiki/Delta-v_budget#Interplanetary

Yes, that's basic orbital mechanics. It takes longer that way though. There are many examples in orbital mechanics where you can trade efficiency for patience.

The general rule for circular orbits is that the escape velocity is √2 times the orbital velocity.

At no point on the esacape trajectory can the object's speed fall below √2 times that of a circular orbit at that distance (or else the object would not escape.) At whatever distance you decide to set the controls for the heart of the sun, you must kill its angular velocity with respect to the sun (because, if it has more than a slight angular momentum, it will follow an elliptical orbit that goes around the sun.) Therefore, at every point on the minimal escape trajectory, the delta-v to redirect the payload into the sun is the escape velocity at that distance. With your strategy, the cost of sending the payload into the sun asymptotically decreases towards the cost of sending it on an escape trajectory.

I was amazed the first time I heard about the Interplanetary Transport Network.


Well, agreed (or you can throw the trash into Sol). And what do you think about the other use, as an energy source?

That makes me wonder how we would get the energy from there back to earth. I mean we already have a for current purposes infinite energy source right at our doorsteps (the sun), we just need to harvest it.

It could be used on-place to fill some energy reservoir. Which could be sent anywhere, or also could be used for space travel for example.

Yes I agree that currently a Dyson sphere is the most sensible project humanity could ever think of!!

Getting nuclear waste off the surface is extremely risky & challenging. Rockets occasionally explode and scatter their payload. Even a 0.01% (1 out of 10,000) failure rate might be considered too high a risk. I think that a lot of people would not take the risk that 1 out of every 10,000 rockets with nuclear waste will explode and scatter the contents into the upper atmosphere.

Especially high level radioactive waste which is some of the only stuff you might care enough about to worry about a "permanent" solution. The existing nuclear payloads are either RTGs or occasionally fresh fuel that is only lightly radioactive compared to high level waste.

A /dev/null for the solor system would indeed be an awesome feature :) Though the most dangerous part of this would be the risk of loading the material onto a rocket. Sigh.

Or maybe very dangerous physics experiments would be arranged to occur in space in a collision trajectory with Devnull, so that it would then be automatically annihilated if it went wrong...

Probably the more interesting use is that of an energy source!

Let’s set up a petition to name the (hypothetical) primordial black hole at the nether edges of the Solar System “Devnull”!

Use as a sink has two issues:

- getting nuclear waste out of Earth's gravity well is risky, if the rocket explodes it scatters nuclear waste throughout the atmosphere. And very energetically, given the amount of rocket fuel needed to achieve this kind of flight

- getting to planet 9 (black hole 1?) will require loads of fuel or lots (>100 years) of time. Lots of time for something to go wrong, lots of time for errors in orbital calculations to accumulate, small target to hit

> small target to hit

Extremely small. The paper has a to-scale illustration of the hypothesized black hole.

I wonder what happens to a bus-sized object if you send that black hole through it...

"definitive sink to send all of our nuclear waste" ... where it would slingshot and return back to earth like a nuclear waste comet

>> definitive sink to send all of our nuclear waste

That depends on the orientation of this thing. If it is spinning in just the right way, dumping anything into it would be like activating the death star. Even a week astrophysical jet pointed at earth would be a very bad thing. The last place you want to be standing when feeding a black hole is above/below it.

So first things to know when we find it:

- how is the axis (like finding the head of a fluffy shitzu dog)

- how much is it spinning?

> - definitive sink to send all of our nuclear waste (it would be the /dev/null of the Solar System)

Now, black holes probably aren't known to be gateways to other locations, but that comment made me wonder what if they are, and dumping our hazardous waste in them has far reaching consequences somewhere else, then that in turn made me think of what other cosmic-scale consequences of alien technology might be out there.

I mean, if a civilization has planet-spanning tech, their "waste products" could be on the scale of planets too. Somewhere, a species must be burning through solar systems like we're burning the Amazon.

Might as well throw it into the sun if you have that much trash. Maybe they got around to building space elevators.

I don't mean just literal garbage, but more like side-effects of "exotic" technology that messes with the fabric of space and stuff like that.

Like was it The Three Body Problem or some other story where an alien species’ faster-than-light travel tech causes the universe to expand faster and faster, making it harder and harder for younger species to produce enough energy for FTL.

I feel that once we have a method to deliver all that nuclear waste to the black hole, we'll then have a process to utilize it instead.

You want to feed the beast?

How could you not feed such an adorable tiny creature?

That's the thing - if you get too close, you can't not feed it!

He's so irresistible!

If ever there is a space drive that needs to built for Interstellar travel, can we harvest gravitational energy from this black hole to power it?

how would this sink differ from using the sun as /dev/null - other then that you might disrupt potential stellar consciousness?

It takes substantially more delta V to send a rocket into the sun than to send it out of the solar system entirely. Getting a rocket to a particular object in the outer solar system may take more delta V than either of those options unless you're really patient.

That's obviously false. If you send a rocket outside solar system (as in giving it exactly the escape velocity) then you just wait until it's velocity with respect to sun drops close to zero and then just nudge it back directly at the sun. This takes a lot of time of course, but it does not require you more energy than what is required to leave solar system.

I am only picking at you because you mentioned a rocket. If you said "shoot it to burn in sun" then you would be (mostly) right.

I don't have an intuition for why it is harder to go to the sun than leave the solar system. How do you think about that?

Using dummy numbers. But, lets say you need to be going 100mph to escape the suns gravity. The Earth is moving around the sun at 75mph. So you only need to speed up 25mph to leave.

But to fall to the sun, you need to slow down 75mph. And speeding up and slowing down in space take the same amount of energy.

But why do we have to slow down? Can't we aim at sol, such that our velocity doesn't matter?

That's how an orbit works. If you think about a low orbit around the Earth, you are constantly falling towards Earth and missing, because you are going so fast around it. Likewise, the Earth is constantly falling towards the Sun, but because it is going sideways so fast it keeps going around instead. You say "aim towards sol" - but in order to do that you effectively need to stop going sideways. Once you have done that, it doesn't matter whether or not you are also travelling towards the Sun - you will be soon. That's what we mean when we say that in order to hit the Sun you need to slow down.

This assumes there is no planet or moon handy to whip around, to end up going in a completely different direction, with no extra energy expenditure.

It is tricky (but possible, with cleverness and a careful schedule) to gain or lose energy this way, but it doesn't matter. If your closest approach is well within the sun's photosphere, it doesn't matter how fast you're going when you get there. So, you can do it with essentially zero delta-v, starting and ending with the same total energy as an object would have co-orbiting with earth, but on an extremely eccentric orbit.

It's not terribly rare (on a geological timeline, at least) for comets to dispose of themselves this way.

Anyway, what is so great about dropping them in the sun? Jupiter swallows comets frequently. Mars is a squalid dump, and so is Venus, at least below the clouds.

Great explanation. I think many people have the wrong default intuition for what an orbit is. I don't think they realize that it means going so fast that you fall perpetually around an object rather than just eventually hitting the object.

Stephenson has a great explanation of orbital mechanics in Anathem. Also in Seveneves, but not as detailed, as I recall.

Thanks for a great explanation of this. This idea has never made sense to me until now :)

The radius of Sol's gravitational influence is much larger than the radius of its coherent mass.

The sun is always at one focus of the elliptical orbit. You just can't get the orbit close enough to plasma-brake near perihelion without also pushing your aphelion way out. So you have to aim away from Sol in order to get there at lower energy. Basically, a Voyager probe that stops at the very edge of the gravity well and then plunges straight down. Spiraling down while decelerating is faster, but costs more energy. But as you get closer, you can harvest energy from the solar wind and solar radiation, with solar sails, so the amount of delta-v you have to load onto the launch rocket does not represent your entire delta-v budget.

There are ways to trade off time for delta-v, but at that scale, the ways that really make a difference mean that the person that sets them in motion will be ancient or dead before they finish.

THANKS! I got it now :)

In order to get to the Sun rather than just speed past it, you have to decrease your velocity dramatically.

When you're in a stable orbit, you are actually spinning around the sun at a huge pace. To gain enough velocity to leave the solar system, you have to increase that pace by an amount that is less than the pace you already have.

As a terrible analogy, it takes less energy to overtake a car that is travelling in front of you at a higher speed than it does to slow yourself to a complete stop.

Not correct. An extremely eccentric orbit has the same energy as some circular orbit. All you need is for the aphelion to be well inside the sun itself, and the sun will take care of turning the energy into raw heat.

Escape the huge Sun's gravity well, or only escape Earth's gravity well (but stay in Sun's well).

Agreed. Well, Sol consciousness is already disturbed by occasional comets, isn't it? :)

The use as an energy source, however, would even outlast the lifetime of the Sun as a star.

There was a story like that called "iHole", but it appears to have disappeared from the 'net..

This is probably what you're thinking of:


I don't think so. It was about a product release by an apple-like corp that was a small personal hole that you could use as a garbage bin.

There is no such thing as nuclear waste, just unprocessed fuel. Things that can't be processed can be dumped under 50cm of sand and you build a kindergarten on top of it. It will be a lower radiation hazard than a one hour plane trip.

- That's Jupiter already.

- That's Sun.

Interesting hypothesis. Primordial black holes -- if they exist -- have a tendency to be very small, and very small black holes evaporate extremely quickly. The smallest ones couldn't have survived to the present day. An Earth-mass black hole is big enough to have survived until now, but I was under the impression PBHs that big were posited to be very rare.

Even a moon mass black hole would survive. The temperature would be about 2.7 kelvin, same as the CMB is now. So any blackhole created at or after the big bang will have stayed the same or gained mass.

The threshold I could find for surviving since the big bang is around 10^11 kg which is quite a bit smaller than the moon. 10^11 kg is quite a bit compared to human scale, more than the mass of the Three Gorges Dam, but quite small on the astronomical scale (10^11 times smaller than the moon).

So while an earth mass black hole is small (9mm), it will last quite long indeed. The evaporation time is proportional to the mass^3.

Something about your comment triggered a primal fear in me of tiny, immortal 9mm bullets of non-existence fired randomly in space at all directions until I remembered an earth-mass black hole, while about a centime wide, it's still has a full gravity field the size of Earth's. If one's coming our way, we probably can't do much, but we'll see it coming.

Seems unlikely. It emits close to nothing, it's temperature is less than 2.7 kelvin, and we'd have to be spectacularly lucky to see a micro lensing event with such a small mass.

Hell we barely notice when a several km comet enters the solar system and I've read about fairly large asteroids getting closer to the earth than the moon before being noticed.

We are going to notice a 9mm black hole? Maybe if it punctured Jupiter first.

A black hole with the mass of Earth out of sync with it surroundings will surely emit a lot on UV and X-rays. Space isn't completely empty, and the stuff on its path won't orbite it.

We would detect one entering our Solar System.

the fast moving mass of earth will distort orbits via gravity. that will be noticed

If you removed the earth, except for the moon the solar system would change very little.

Sure if the earth came between Jupiter and it's moons there might be some chaos, but the likelyhood of that is minimal. Even if it did happen we would notice, but the earth massed black hole would be very unlikely to make it into the inner solar system at that point.

The earth is a minor gravitational force in the solar system, most likely the effects would take significant time to notice... well after the nearest approach. Small gravitational tugs take many samples and significant time to notice. For example the outer solar system still doesn't add up... and we have no idea why.

This sounds like a super fun simulation to run. Do you have recommendations on simulation software?

I found these in a quick search. I have a couple 64 core machines available.




The outer solar system is pretty well accounted for. For many years we thought it wasn't, and kept hunting for another (sufficiently-massive so not Pluto) planet beyond Neptune. The discrepancy turned out to be an overestimation of Neptune's mass, which was resolved by Voyager and subsequent observations. We've now ruled out anything of Neptune's mass or greater existing out to at least 100 AU and likely several multiples of that.

Ah, thanks for the update.

Probably not. It would be something that would likely have to be observed over multiple complete orbital cycles and basically somebody basically run into it by accident. By which, of course, the earth in this hypothetical, would be long dead.

This whole topic and discussion makes me think about "The Nothing" that rips apart the world in the movie The Neverending Story. I had nightmares from the movie and The Nothing for weeks after watching the movie when I was around ten years old. 1984 might really be approaching, not the book, but The Neverending Story which was released then.

You should read Seveneves by Neal Stephenson. The plot of that book follows the events after a primordial black hole streaks through the Moon, causing it to implode. It's packed with detail, but interesting for anyone interested in space and engineering.

Hmmm, that's not what happened in the Seveneves I read... The reason for the moons destruction was never mentioned iirc. The 'Agent' was never revealed.

Good yarn though.


I've been thinking that it was a misrouted Viking.

> immortal 9mm bullets of non-existence fired randomly in space at all directions until I remembered an earth-mass black hole, while about a centime wide

Now imagine what a strong force an electron could provide :D

Total (unqualified) spitballing, but wouldn't a black hole "bullet" that hit the Earth experience effectively 0 frictional losses? By definition it won't be pushing stuff out of the way, so it seems like an inbound black hole would punch right through the Earth. Local gravity is probably going to be quite nasty during the transit, but beyond that, it seems like a fairly survivable event.

Well, do you know how we "see" black holes? The "body" of a black hole is nothing but darkness, but we see the radiation given off by the mass accelerating toward the black hole event horizon that can often reach significant fractions of c. Given the high mass density of a planet compared to space, a massive amount of hard radiation will be released.

Also the relative velocities of earth and the black hole is extremely important. Depending on the relative velocities, it's possible that the blackhole simply gets lodged in the earth's core and we're 100% dead, another 100% dead scenario is if it plunges in, out the other side, but then "falls back" for another pass and so on.

Won't it pull some of the matter it passes through under the event horizon? The extremely curved spacetime will likely turn rock to dust in near vicinity. Then, as the mass grows, so would the gravitational pull, while the speed relative to the planet being punctured would lower due to conservation of momentum.

If the tiny bullet-class black hole makes it through a planet, it could gain significant additional mass, and leave quite an exit wound.

An earth’s mass black hole has a ~1cm sized event horizon. It would suck up matter several times that, but it’s not enough to gain significant mass relative to it’s size on a single pass. This stays true as you scale things down.

Something in the 10^12 kg range is going to have an atomic scale event horizon so it’s not picking up significant mass as it shoots through the earth.

If a random PBH of 9mm traveled at near light speed through Earth, would it even pull anything into the event horizon that is not within close range of that 9mm diameter?

For example, it travels through Earth and then leaves a 9mm hole throughout the planet.

I'm not good at physics, but it seems to me if this 9mm object had the mass of earth, then at a distance of about 6400 km it would exert 1G, and at minimal distance, it would exert about a zillion Gs. So even assuming it didn't pull in matter per se, the gravity and tidal forces ought to destroy earth about as effectively as colliding with a regular earth sized planet.

No it wouldn't, but I'm thinking it would be able to transfer some kinetic energy to the earth which would be a very very bad thing.

In some exotic models, dark matter acts like bullets that are always flying through you, but with a tiny chance could interact and leave something basically like a bullet wound: https://arxiv.org/abs/1907.06674

It would go right through Earth. There might be pretty big explosions though as it feeds on earth's mass.

I read a fiction book about a tiny black hole getting caught in the planets gravity well then falling threw Earth back and forth just fucking stuff up every time. Scary stuff.

Maybe ...

Earth by Brin

Hyperion Cantos by Simmons

Ilium by Simmons

Doomsday Effect by Thomas Wren

If I thought for a few more minutes I could probably come up with a half dozen examples (or expanded the definition to include eating the moon or mars, or included ones where it would be a massive spoiler)...

A gravitationally captured/trapped black hole eating earth is relatively common plot device in science fiction.

Sounds familiar, could that be the Big Mistake in Hyperion or the Hole Man short story? https://en.wikipedia.org/wiki/The_Hole_Man https://hyperioncantos.fandom.com/wiki/Big_Mistake

Do you remember its name?

How do you suppose we would see it coming?

If it's not moving quickly, we would notice its gravitational interaction with the well-known dynamics of the various large objects already in our solar system. A 9mm black hole's gravitational influence would be fairly subtle though compared to a multiple Earth-mass black hole; the bowling ball sized black hole mentioned in the paper as an upper bound is not the kind of thing that could wander into the inner solar system unnoticed.

This comments, reminds me of the dual vector attack in the Death's End novel of Remembrance of Earth's Past trilogy from the writer Liu Cixin. Its was a small object fired at the solar system by a highly advance race, where 3d space was projected into a 2d plane so the whole solar system was destroyed.

Pretty big plot point! Don't want to give away the story

Calculator I used: https://www.omnicalculator.com/physics/black-hole-temperatur...

A black hole with 10^11 Kg seems to have a temperature of about 1.2 * 10^12 K. Even though it would be small, it would be emitting crazy amounts of all sorts of photons.

It's a black hole, that's not going to be throwing off black body radiation like that. It would be just hawking radiation plus whatever effects from matter in its accretion disk. That would still amount to pumping out 35.6 GW of hawking radiation, but being small isn't something that would inhibit it releasing energy, it's the other way around. So in other words, tiny black holes give off gobs of energy whereas larger ones do not.

Hawking radiation is indistinguishable from black body radiation.

I'm curious how an object this size with a planet's mass is not sweeping all the dust and rocks in its area as it moves through its orbit, like Jupiter cleans up the outer solar system. Even interstellar space has around 1e6 loose ions per cc. Is it radiating faster than it's eating?

Gravity doesn't care what size the object is, from far enough away. It would sweep the same area as any other density of planet with the same mass. For something at 5 earth masses, that's not a very big area.

Things can’t collide with a small object.

Almost none of the orbital sweeping effect comes from collisions. Even Jupiter is tiny, in the scope of space. The majority of orbital sweeping comes from repeated gravitational deflection.

Having a tiny radius will reduce the amount of of collisions, but it won't have a significant impact on orbital clearing.

The comment mentioned "eating" specifically, where I felt collisions would be important. You're right that for clearing the size of the object isn't very important.

They are looking at several objects between approx. 1-20 Earth-masses (presumably they were bigger during primordial creation; this is their size, now, after 13 billion years of evaporation).

The bigger the black hole, the slower it evaporates. By the time you get up into planetary sizes they basically don't change mass on the time scale of stars. (From this wonderful calculator [0], a black hole with a mass of 6.0x10^24 kg - about that of the earth - would have a lifetime of ~5.75x10^50 years)

So if there's a 1-20 Earth-mass black hole out there, it hasn't evaporated from anything appreciably larger to get to that point.

0: http://xaonon.dyndns.org/hawking/

There go my hopes humanity could use this to convert mass to energy.

How long would we have to wait for it to evaporate before we could use it for energy production?

Earth-mass black holes have surface temperatures well under that of the cosmic background radiation and will not have evaporated, they'd have been literally accreting mass just from the background. You need to get down to the billion-ish tons range to find black holes that can have appreciably evaporated over the life of the universe.

If they're in a solar system, they could keep devouring rocks and mini-planets to keep existing, no?

An Earth mass black hole has a Schwarzschild radius of about 9mm. It would devour mostly CMB photons, individual gas molecules, and very rarely small dust particles.

Thanks for teaching me how tiny it would be. I had no idea!

With the gravity of a whole planet it would still attract a lot of stray matter. I don't know how many collisions that would generate though. Mostly just deflected orbits and captured satellites, I assume.

But surely a lot more than a "regular matter" 9mm pellet.

Yes, it would whisk small stuff around, and affect the orbits of Kuiper belt bodies just like the hypothetical Planet Nine is theorized to do. It would give a big kick to any objects that happened to pass very close by, but such encounters would be extremely rare. I don’t think the density of tiny stuff is enough for an accretion disc to form around it, but could be wrong. If one did form, we would probably detect its radiation signature.

Actually even a moon mass blackhole (0.1mm) would be sustained just by the CMB background. Anything more than that would increase it's mass (and diameter).

Couldn't we somehow shield it and wait for it to produce energy eventually?

Sure, possible. But the timescales involved are daunting, as is the energy/tech required to shield a blackhole with sometime so efficient that the temperature gets down below 1 kelvin.

It's not all that much energy for the timescales involved, you'd be better off just putting say an acre of solar panels in orbit around the sun.

Tiny blackholes are amazing efficient engines for turning mass into energy, once small enough they could be quite a power plant and not picky about what you feed them.

I’m sure that would give off huge amounts of gamma radiation.

Only when they cleared their orbits, around the beginning of the solar system. Doubt they'd see much impact now.

Can't reply directly to the child.

The earth is quite larger than a black hole with an earth mass. Also the earth has a huge atmosphere to help capture things.

A black hole with the same mass as the earth would be almost impossible to hit in comparison (9mm). No atmosphere to slow things down, very hard to hit, and you have to come within 1.5 * the radius before you can't escape without thrust.

So sure, some dust would be capture, but nothing anywhere close to what the earth captures a day.

If you click on the time of the child comment it’ll open that thread separately and you can reply there. Also how you save comments.

Someone told me the other day thought I’d pay it forward!

The time of the child comment? Where is that? I don't see times with comments.

Usually "x hours ago". But isn't there a reply below each comment anyway?

In very busy threads it gets hidden for very recent comments. Or something like that, I don't know the exact algorithm.

Earth is hit by a couple dozen tons of space material a day.

Yes. They'd mostly be sustained by the solar wind.

https://www.vttoth.com/CMS/physics-notes/311-hawking-radiati... -- "Hawking radiation calculator" which will tell you how long a black hole will survive based upon e.g. mass. Very fun to play with.

Very interesting. I guess having a stable black hole like that in our solar system would be an incredible opportunity to send a probe and check our current understanding of physics.

If anything it'd be a lot more valuable then just finding another rock in space.

A planet-mass black hole in our solar system would be in the top five most revolutionary discovery in physics of all time. It is hard to overstate how insane this would be.

The odds of finding blackholes everywhere in the universe suddenly increased?

Unless there's some unknown effect of having a black hole that makes life more likely to appear when PBH orbit the star, in which case it might decrease estimates of the frequency of life in the universe.

It's theorized that the existence of Jupiter played a role in clearing out the inner solar system of kinetic kill vehicles, helping create a stable environment for life to evolve on Earth.

I apologize in advance for the dumb question. How is an earth mass black hole formed? I thought they form from collapsing large stars.

Primordial black holes were formed during the big bang. That's the only way we think such objects could form.

What mechanism could create them? By the time there was matter, I thought the only pressure forces were ambient light and baryonic acoustic waves, neither of which should be strong enough...

You're right that there is no mechanism to make primordial black holes in the standard theory. That's because they're new, separate things, so to have them, you of course need to add in new ingredients.

They can be created by the collision of topological defects in the early universe, which in turn can be created in phase transitions during the cooling of the universe, like imperfections in quickly frozen ice. Or they can be created by simple collapse following inflation, if inflation is modified to create large inhomogeneities.

You don’t need to matter to create a black hole, a Kugelblitz black hole is made entirely of light.

Fascinating! But, if that's the case, the densities of light after the big bang would have created endless numbers of these?

That's the idea.

My understanding is that things were incredibly homogeneous until the time in which baryonic acoustic waves started creating compressed and expanded areas. If there were black holes in that homogeneous soup, when things were so dense, it seems like everything would turn into a black hole or get sucked it.

Assuming light going into a black hole adds to its mass?

But assuming some light and matter didn't get sucked in and that's what remains, then I can understand why black holes are proposed as a possible answer to the mystery of dark matter.

Wolfram Alpha says that the Schwarzschild radius for the mass of the visible universe is bigger than the visible universe, so all I can say is something is missing from my understanding (but I already knew that).


What you might be missing is that the radius of a black hole is proportional to its mass, so in fact, two half-visible-universe-mass-BHs would take much less volume than a vis-universe-mass one, moreso for four quarters, etc.

Additionally, it's been proposed this universe is a 4d-spatial black hole. In this case, you can re-consider aggregating the entire 3d-spatial universe as a 4d black hole, and the Schwarzschild radius calculation works again.


That’s isn't something I missed, as it happens.

Given some of the answers on the Physics StackExchange, I think my error is using a static approximation like the Schwarzschild solution for a dynamic situation — given my grasp of the Einstein field equations is “ooh pretty symbols” this isn’t a huge surprise, though my lack of detailed understanding is a personal frustration.

I'm not sure what your question is, exactly, but a couple small addition to your observation about black holes forming from collapsing radiation.

You're quite right that formation by collapse excludes Schwarzschild.

What you're heading towards is the Vaidya metric -- I don't know of any easy overview of it, but one can think of it in terms of Schwarzschild.

Schwarzschild is a static solution: an eternal, time-independent, everywhere-vacuum, pointlike mass. There is no radiation in Schwarzschild.

Vaidya has the same spherical symmetry, but the central mass is time-dependent. The central mass can radiate away or absorb incoming radiation, but with the condition that the radiation is spherically symmetrical.

Radiation in Vaidya is technical: it is a "null dust" -- it follows null geodesics and does not self-interact, so it shares properties with light ((classical) light rays have no charge, and don't clump; breaking down light rays into particle-like elements leaves each element having no rest mass in its own Local Inertial Frame).

As long as it is sufficiently close to a null dust, Vaidya can model practically any collapse of radiation to a black hole. Unfortunately the spherical symmetry is a hard constraint for the exact solution, and is easily broken by matter self-interactions. However one can certainly play around with numerical approximations to the Vaidya solution, and if one does that enough for a particular family of perturbations of the exact solution, an intuition is likely to develop.

However, a kugelblitz from a collapsing null dust is within the gift of the exact Vaidya solution.

You can take a "swiss cheese" approach to a very early expanding universe filled with radiation (of the technical type above) peppered with Vaidya regions which evolve. Vaidya is time-dependent and can deal with a tapering off of incoming radiation as long as it's always spherically symmetrical, with the result that eventually you have a "cheese" that is an expanding radiation-filled spacetime and "holes" which are vacuoles which asymptote towards Schwarzschild, and around each "hole" a thin-shell Israel junction. The asymptotic behaviour is because the dust crossing the junction into the Vaidya vacuole is (a) cosmologically redshifted within the "cheese" and (b) diluted by the metric expansion of the "cheese". The radiation already inside the junction at early times collapses onto the central mass. The two combine to effectively shut off the incoming radiation, leaving behind a very close approximation of the Schwarzschild vacuole.

(Aside: for massive dusts, one would use a Lemaître-Tolman-Bondi metric instead of Vaidya, and one still runs into problems when breaking the conditions of spherical symmetry and no-self-interactions in the dust. LTB has a couple neat properties which are suitable for physical cosmology "swiss cheese" models if one assumes that the radiation exiting the galaxy-cluster "holes" is negligible -- starlight arriving in our galaxy cluster from distant galaxies likely adds basically nothing to the mass of our cluster as a whole, and our galaxy-cluster isn't losing much weight through its starshine out to infinity; ditto for neutrinos and heavier particles).

In summary, primordial black holes are pretty easy if the very early universe is filled with a homogeneous, isotropic dust -- radiation as a null dust, or some massive dust, or even some combination. In the initial dust one expects Jeans instability, and a power-law distribution for the total masses of the resulting vacuoles. Plenty of small primordial black holes, fewer big ones. What would cut off really small black holes originating along these lines? Unknown. If nothing is found, this type of model loses its attractiveness.

There are several other models for primordial black holes, but they look a lot less like the kugelblitzes you talked about a few comments above.

One other note, although I don't have space to develop it here: we can form black holes from gravitational radiation alone, even in a spacetime with zero matter. Gravitational radiation is not the same as matter radiation in the technical sense above: apart from the mathematical details of which tensors encode it (Riemann vs stress-energy) more physically gravitational radiation strongly self-interacts, so we generally can't treat it as a null dust.

> personal frustration

GR doesn't really come easily to anyone, even (and sometimes especially) with people who are mathematically gifted. Even Einstein and Hilbert struggled with it, and in the last century only a small number of exact solutions -- none of them better than a fair-enough-to-be-useful approximation to astrophysical observations -- have been found. There have been many many many false starts. Consequently one has to develop an understanding of where exact theory starts to diverge from exact observation (and what one can do about it with arcane tricks), and I don't think that's really possible without understanding the exact theory first.

Lastly, I don't know what you're thinking about here:

> the Schwarzschild radius for the mass of the visible universe is bigger than the visible universe

The visible universe is not even slightly approximated by the interior part of Schwarzschild metric. In particular, galaxy clusters are flying apart rather than collapsing to a point. Additionally, there are no apparent tidal stresses on galaxy clusters, even the ones at highest redshift: the Weyl tensor, which essentially encodes tidal stresses, is nothing like a black hole solution (not even under time-reversal wherein we get a "white hole", because we would then see spaghettification in reverse: galaxies evidencing ellipsoidal early galaxy-clusters with later galaxy-clusters becoming markedly rounder).

Event horizons can appear all over the place, including in perfectly flat spacetime (for Rindler observers, for example). There are lots of very-not-like-black-hole-spacetime settings in which there are global event horizons. The salience is in what trajectories radiation and other types of matter take, rather than that the matter-in-the-bulk can be partitioned by horizons that produce Lorentz-contraction observables.

Going back to our swiss-cheese model above: in the far far far future the cheese part is essentially empty of radiation because it has all diluted away with expansion, while the holes are also empty of radiation because it has all fallen onto the central mass. Both empty, highly curved spacetimes, but with very different trajectories for the null dust: flying to infinity versus flying into a point.

Thanks! That’s an absolutely fantastic description which clears up several of my misconceptions.

A PBH in our Solar System would quickly become a very popular target for science, allowing us to test both relativity theory and quantum physics at an unprecedented level. And who knows, we might even be able to use it as a slingshot for interstellar travel, or as a gravitational lens - that would be great!

Edit: so, how do we locate it exactly?

With regards to how you locate it, the paper proposes looking at existing data from FERMILAT, a gamma ray telescope which could possibly pick up anihilations from a dark matter cloud surrounding such a black hole. Since the object itself is so tiny (I’ve never seen a 1:1 scale figure in an astronomy paper before) you need to detect its presence in some other way.

What decides how good a slingshot something is isn't its density or mass (provided the mass >> than that of the object being slingshot), but rather its velocity relative to the sun. Planet 9 travels very slowly relative to the sun, and so is a poor slingshot, regardless of its density.

It took New Horizons about ten years to reach Pluto and this black hole would be on the order of ten times further away from the sun than Pluto. We would of cause still point all kinds of instruments at it from Earth but we would not be able to fly circles around it and throw stuff into it any time soon.

Actually I believe new horizons had a pretty primitive propulsion system and achieved most of it's delta V from a sling shot around jupiter.

More recent solar sails and plasma based propulsion systems have achieves significant improvements since then. If it was a priority I don't see why would couldn't go 10x as far within in the next decade.

Nuclear pulse engine can significantly cut the time.

It blows my mind to think about sub-stellar mass black holes can exist. As I was ignorant of their theoretical possibility, I thought the lower bound limit to a blackholes mass was the Chandrasekhar limit [1] (or even greater as that simply is the boundary between white dwarves and further collapse to either a neutron star or black hole).

To learn that the conditions of the early universe could have create sub-stellar mass black holes means there could be tons of small black holes out there lurking in interstellar space.

What would happen if one of them got close enough to our Sun to begin accreting gas from the Sun?

- Could it eat the entire Sun and cause our solar system to go dark?

- As it gained sufficient mass from the Sun, it would switch from orbiting the Sun to them being a binary system. Would that destabilize the orbits of planets?

- Or disturb a ton of Oort cloud bodies and potentially cause tons of comets and raise the risk of impact events?

- At what point would the Sun's fusion stop?

- Obviously this depends on the binary dance of the 2 bodies, but rapidly the sun would be pulled apart.

- Would there be X-rays and particle jets like a micro-version of a quasar?

I really hope someone is modeling this!

[1] https://en.wikipedia.org/wiki/Chandrasekhar_limit

My understanding is that mass in a black hole doesn't operate any differently at a distance. So if there was an earth-mass black hole in a tight orbit around the sun, it shouldn't have a significant impact on the orbits of objects in the solar system, even after it consumed the entirety of Sol's mass. Maybe Mercury would get messed up, depending on how tight the orbit is.

HOWEVER. Getting a black hole into a tight orbit - I don't know how that might happen. A black hole from outside the solar system would be coming in on a parabolic path past the sun. It would shoot right back out of the solar system.

If the BH managed to absorb enough mass from the sun, I would imagine that would throw off it's trajectory enough it could become captured into an orbit. On each subsequent fly by / through the sun, it would capture additional mass. This would reduce the period of the orbit.

In a situation like this, you have an increasingly massive object passing through the solar system in an irregular pattern. That can't be good.

Your premise is extremely unlikely to begin with. Let's assume somehow it comes to pass, still the time scales of these phenomenon are on the order of millions of years. If today a blackhole were to start eating the Sun, it would take a couple of million years to finish that meal. Given a max human lifespan of 120 years. You and your next 10^4 generations have nothing to worry about.

For it to eat the entire sun, you are correct.

But for it to destabilize the orbit of planets through the orbit of a binary system, it is quite plausible once the blac hole has gained sufficient mass for it to count as a binary system and not simply a planetary mass black hole orbiting a stellar mass sun[1].

[1] https://arstechnica.com/science/2013/01/binary-star-systems-...

I seem like that adds up. If somehow black hole consumed enough material from the sun so that it's orbit stayed inside the sun it would start growing exponentially.

First the outer layers of the sun are not very dense, but as a larger fraction of the matter for the black hole comes from the sun the slower (relative to the sun) the black hole would get. The slower the black hole is that closer it would come to the center of the sun. The closer to the center of the sun the denser the sun is.

The pressure of the sun is at least 10,000 times greater than the center of the earth which is 3,500 kilobar. Wouldn't the amazing gravity gradient and the 3,500 kilobar pressure result in a very well fed black hole that would double in mass within say a few days? Sure a accretion disk would form and start pushing back the matter at the north and south poles to reduce the feeding rate.

Sure black holes generally increase is size slowly, but they aren't usually inside a gas cloud of 1.4grams/cm^3 at a pressure of 3,500 kilobar and having an entire suns worth of mass to provide resistance to the accretion disk allowing for matter to fall in quicker.

I've heard numbers like you mentioned for atom sized size black holes that fell within the earth, but the main problem is that the likelyhood of swallowing an atom is so small that it grows incredibly slowly.

How can you tell for sure whether something is a black hole or a regular rock of the same mass, when you're too far away to resolve the size of the object in a telescope?

Do you have to send a probe close and throw something in it?

Black holes are of course much smaller than anything else of the same mass. So the gravitational gradient is much higher, which causes some detectable side effects.

One possibility is gravitational lensing. The abstract also mentions detecting "annihilation signals from the dark matter microhalo around the PBH". Not that I understand that completely, and I don't believe (corrections welcome) that dark matter annihilation by black holes has been confirmed experimentally.

I believe high energy particles are released by black holes as they consume matter, so that might be another way to detect a small black hole instead of a similar mass rock.

> One possibility is gravitational lensing.

From a distance there is no difference in lensing between a black hole and an regular object of the same mass.

From close there would be a difference, but if you could resolve something that small you would be able to directly image the object.

I disagree. An earth like object isn't going to bend light much, the maximum of 1G acceleration is just not that noticeable.

An earth mass blackhole is going to have 10, 1000s, or even millions of Gs depending on how close the light gets.

Seems like you'd get stars periodically blinking from places they shouldn't be, as light trajectory should have missed earth, but gets bent by the black hole. So while the black hole itself would be invisible at that distance, and occultations would be invisible, seeing stars in the wrong place would still be visible.

Yes, it will happen, but the radius of the effect is simply too small to see it.

You are only bending a minute faction of the light from the star traveling toward you. It's not enough light to see, and it's too small to resolve (unless you could resolve the black hole in the first place).

Think about it: The redirected light only has the "size" of the gravitation field in question. For an earth sized mass thats 9mm. Even if you did 9km (which wouldn't bend much) you couldn't see it, never mind 9mm.

Would evaporation give out any signal? Anti-particles could be a signal, no?

Hawking radiation is a signal, one so strong it's on the order of a large nuclear bomb when a black hole finally winks out of existence.

However if it's big enough to be messing with orbits around neptune it's likely pretty large. If it's large then the hawkings radiation isn't going to be visible.

So if it's moon mass it's going to be 0.1mm and at 2.7 kelvin or so. Earth mass is 9mm, but even colder. Generally for the orbital changes they are seeing in a wide variety of objects it seems like the mass is even larger still.

Even seeing pluto is hard, even a jupyter mass black hole is only 3 meters or so. So no I don't think we could detect it via hawking radiation.

We might however see high energy particles resulting from the somewhat messy feeding that black holes are known for. If we sent a probe it could plausibly get close enough to directly observe hawkings radiation. Pretty amazing thought, maybe we will get that lucky.

>one so strong it's on the order of a large nuclear bomb

Surely a "large nuclear bomb" isn't very strong when you're talking about things on black hole scale?

Sure, but that's when it's near zero size/mass.

Ah, looks like I was "a bit" low. Wiki claims 5×10^6 megatons of TNT when it finally evaporates. Apparently about the energy the shoemaker level struck Jupiter.

A black hole of this mass is colder than the cosmic background.

I’d say occultation observations, ie looking closely when it passes in front of a star, would allow you to prove things one way or another given the huge difference in density and radius.

Yes, but presumably it's paste Neptune. Even if it's between the mass of earth (9mm) and jupyter (3M) we aren't going to be able to notice a direct occultation.

However a lensing event seems possible, it greatly increases the effective radius of detection and is much more distinctive. Not sure what the smallest lensing event witnessed is, all the ones I'm aware of were a solar mass or more.

The first paragraph of the article is all over this lensing as a way to explain ultra-short crossing times from the OGLE experiment running in Poland. Reference [6] is titled "Constraints on Earth-mass primordial black holes from OGLE 5-year microlensing events [...]" so there's my weekend reading sorted.

How does that work, aren't distant stars point like? I guess the bigger problem is the shadow will be very thin and we're unlikely to correctly guess where it lands.

Right, stars are (effectively) point sources light. But if you take images of stars like the LSST is where you can get the entire sky every few days you'll find anomalous stars that weren't on previous exposures.

I don't know the physics enough to know if a small (earth to neptune mass) blackhole could redirect enough star lights to detectable by an earth (or earth orbit) telescope.

With a huge database containing the time series of the sky you can start searching for unknown objects that stick out because they are a change from the previous exposures for that part of the sky.

With enough data mining of those previous exposures you could find likely candidates and get telescope time to check out where you expect it to be next.

This is similar to how things like Oumuamua and 2I/Borisov were found.

From the abstract:

> This scenario could be confirmed through annihilation signals from the dark matter microhalo around the PBH

Generally by it's radiation signature. Black holes don't just swallow matter without a trace. As infalling matter is accelerated to immense speeds, the tidal forces rip it into atoms and internal friction heats it up to very high temperatures.

We'd basically see extremely energetic particles coming from a nearby source. Since no other known physical process can generate such high energy particles (not even fusion in main sequence stars), we'd have to conclude that there is probably some extremely dense object there.

Theoretically how do we detect such particles from so far away?

The same way we do so already: using telescopes and particle detectors.

A rocky, very dark thing of the same mass would be much larger in diameter, and occult stars behind it. This is a somewhat common method of discovering things, through occultation.

I suspect if you look at the smallest object detected through occultation at the distance past Neptune and it's going to be measured in km not meters (or mm).

I am reminded of a tale from Yukinobu Hoshino's science fiction anthology-series 2001 NIGHTS, which had a then-tenth planet named Lucifer, with a retrograde orbit and composed of antimatter- a former sun that collapsed not into a black hole, but a gas giant made of antimatter, left over from the Big Bang.


Were such theories about a hypothetical planet beyond Pluto already existent when this series was created, back in the mid-'80s?

Out of curiosity, would we know if a random planet or star system was made of antimatter rather than matter? Is there a spectral difference? Would gamma radiation from interaction with any local matter be the only indicator?

We are constantly interacting with rocks of various sizes and thd small amounts of hydrogen in even nearly empty space. Presumably an antimatter planet would be also only much more violently and would by now be gone.

Or be constantly flashing if it wasn’t.

Is there enough in the ISM (interstellar medium) to have made antimatter star systems impossible over the long term?

I don't enough to destroy anything that existed and was massive. But it would be giving off the gamma rays from collisions and we haven't been detecting them.

More science fiction: Larry Niven wrote about Jack Brennan, a human turned Pak Protector who set up a small environment around a micro black hole to provide gravity in the outer solar system. I don't think it was earth-mass size though, more likely a singularity

All black holes have singularities, what do you mean? (An Earth mass black hole is pretty tiny — Schwarzschild radius order of a centimetre).

A sub-Moon-mass singularity of 1e22 kg provides an acceleration of 9.8 m/s^2 at a radius of 261 km, while having a Schwarzchild radius of 1.5e-5 m.

The combined system of that singularity plus a hard spherical shell at radius 261 km (anchored by some sci-fi means that does not contribute significant mass) has a density of 134 g/mL, 6 times as dense as pure osmium, but being mostly empty space inside. And having a surface area of 856000 km^2, 0.6% the land area of Earth. Such a body could retain an atmosphere and sustain ecological cycles.

I had the question "how much mass does it take for a black hole to have 1G at the same radius as its event horizon" and the answer appears to be about 10^12 times that of the Sun.

Further science fiction tangent: Asimov wrote a novel called Nemesis that (among other things) discussed the concept of a red or brown star hidden in the outer solar system. That concept was a strange hypothesis debated back to at least the 80s.


Theories about another planet existing beyond the furthest planet we know are practically as old as our knowledge of planets themselves! I would be surprised if this didn't have an entry on TV Tropes.

I was referring specifically to the "being made of exotic material" part. Though yes, theories of Planet X being a massive gas giant have been around for a long time.

..but Lucifer is just another name of Venus.

Wouldn't we see some radiation from dust falling in these primordial black holes?

There a lot of talk about the idea of an Earth mass Primordial Black Hole punching through Earth and the effects of such an event. However, Planet nine is predicted to be about five Earth masses, not one.

I don't have a physics degree, but how close would Earth have to get to such an object before the Earth is within that object's Roche limit?

The Roche limit depends on the density difference between the two bodies in question. For a black hole (of any size), the density is basically infinite compared to regular planets, so the Roche limit is basically zero. As in, less than one meter.

The radius of the event horizon for a black hole even the size of Jupiter is only 2.5 meters.

The gravity field of a spherical object only depends on its mass and not its density/radius. So for the Roche limit only the density/radius of the object at risk of breaking apart (called minor object on wikipedia) (earth) should matter, not the one of the major object (black hole).

Wikipedia gives the formula d=R_m*(2m_M/m_m)^{1/3} which matches that intuition.

Ah, you're right.

So plugging in numbers you get 2.15 times the radius of Earth, which becomes 13 750 km.

For comparison, Earth-Moon distance is approx. 400 000 km.

Yes, but you don't have to be within the Roche limit to have substantial tidal effects, which could destroy habitability at a much greater distance. Just look at Io.

Very close, in astronomical terms. At a guess, no further than the distance from Earth to the Moon, and probably much less. I'd love to see real numbers though. :)

Edit: According to https://en.wikipedia.org/wiki/Roche_limit the Roche limit d = Rm * (2 * MM/Mm)^(1/3) for Rm = radius of the satellite, MM = mass of the primary, and Mm = mass of the secondary. So in this case d = 6380 * (2 * 5/1)^(1/3) = 13,745km. In astronomical terms, very nearly a bulls-eye.

Wouldn't we expect such a black hole to slingshot dust and other material to some non-negligible percentages of the speed of light? Wouldn't some of that eventually hit the earth's atmosphere? What would that look like? Tunguska?

Not really, it would just form a (tiny) accretion disk. Dust and other material would fall into it, emit gamma rays and be gone forever. It's a black hole.

Or both. Accretion disks can shoot a tiny bit of matter out the poles while most falls in. IIRC, the dynamics that cause this effect are not well understood.

The black hole would act identically to a regular object of the same mass.

It doesn't have any special ability to slingshot things to crazy speeds.

Remember - the slingshot doesn't add speed from the gravity of the object, it ads speed from the rotation of the object around the sun.

It does have a special ability to slingshot, because you can get much closer to its center of mass than you could with a planet and get higher accelerations.

That would help with a rocket burn gravity assist, but not a slingshot.

I thought slingshot was just another name for a gravity assist. Can you explain in more detail when it is or isn’t useful to get very close to the massive object?

There's two kinds of gravity assist.

Slingshot uses the motion of the planet around the sun, and pulls the probe along with it in the same direction that the planet is orbiting the sun.

The probe comes in perpendicular to the solar orbit, and leaves parallel to the solar orbit, and faster (relative to the sun). Relative to the planet there is no change in speed.

Kind of like bouncing a ball against a moving car. Relative to the car nothing happened, but relative to the ground the ball is faster.

Gravity burn is more complicated. (Oberth effect)

Imagine a stationary rocket (bolted to the ground). All the energy of the fuel is in the exhaust, and none in the rocket.

Now imagine the reverse - a really fast rocket, now way more of the energy is in the rocket (and less in the exhaust).

So what do you do? You fall toward a planet, and at the point where you are moving fastest, you fire your rocket. Now not only does your fuel have the energy inherent in it, it also has all the energy from falling toward the planet.

And this is the big idea here: You leave that fuel behind as exhaust! So when you climb back out of the planet you don't carry the fuel with you.

Normally falling toward a planet, and leaving the planet exactly cancel out. But you used the oberth effect to leave the fuel behind at the point where the fuel has the most kinetic energy.

What mechanism would act to slingshot material to such high speeds?

Basically just momentum transfer from a gravitational assist.

That wouldn't get anything up to relativistic speeds. The universal rule of slingshot manoeuvres is that you depart from the slingshotting body at the same speed you approached it, just in a different direction (in the slingshotting body's frame of reference). Therefore, the maximum velocity gain you can achieve with a slingshot is governed by the velocity of the slingshotting body - the black hole. Which, as far out in the extremities of the solar system that it is, is not very fast.

If the black hole is rotating isn't the reference frame in motion about the center of the black hole in addition to the orbital velocity about the sun? i.e. frame dragging?

You mean https://en.wikipedia.org/wiki/Penrose_process - it doesn't look like it is very efficient or effective, and you'd have to get so close to the black hole that you'd be shredded to get anything at all.

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