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Comment by qubex | Hacker News Reader

This is both a very obvious and a very interesting proposal: locate an apparent source of gravitation by observing the gravitational effects it has upon a fleet of small (almost point-like) craft.

However, there’s something that niggles me.

If it’s a primordial black hole that has been captured by the sun, why is it (as far as I understand) apparently in the same plane (ecliptic) as all the other major bodies in the solar system? It makes sense for all planets & cetera to be in the same plane because they all originated in the same swirling (flat) planetary nebula... but a captured object could potentially orbit at any inclination (and, considering how massive it is, I doubt it would be coerced into being coplanar by comparatively minor intersections with light bodies such as those present in the Oort Cloud, which are arranged roughly spherically anyway).

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Disks minimize gravitational potential energy when angular momentum is held constant. Forming solar systems have ways of radiating energy, but they can't really radiate angular momentum. That's why galaxies and solar systems tend to be disk-shaped. The disk shape is very general and comes from fundamental principles, so I don't see why it couldn't continue to apply when one bit of mass in the system just so happens to be a black hole.

Yes but the mechanism that makes the disk form from the general cloud is physical collisions, two "particles" that have opposite inclinations compared to the average hit each other and end up closer to that average plane. But basically nothing can hit a black hole that size so where would it get its momentum from?

>But basically nothing can hit a black hole that size

Why would that be true? Individual gas molecules manage to disk-ify and turn in to planets, I don't see why a black hole couldn't. This would presumably happen in the early stages of system formation when there was a lot of stuff flying around separately.

Obviously, nothing can ‘hit’ a black hole: anything that does just gets absorbed by it as it falls in.

However, more broadly speaking, the angular momentum is conserved, so the result would be more or less the same. However, given the enormous, ridiculous difference of relative masses (and therefore angular momentum) in the case of black holes and some gas particles, I think my initial point stands.

The black hole would interact with many gas particles, not just one. How many would be reasonable? Well, Jupiter is a gas giant and it had to form somehow.

Yes, conservation of momentum is the right way to think about that.

> conservation of momentum is the right way to think about that

So you’re saying you’d expect a body that were captured at an arbitrary angle to end up orbiting in the same ecliptic as the planets roughly around the sun’s equator?

If the body was there from the start it would obey the same rules as all of the dust that was there from the start.

>anything that does just gets absorbed by it as it falls in.

I don't think this is true. Plenty of (almost everything) things would interact with the black hole's gravitation but not be close enough fall in.

You have an object the mass of a gas giant. Planets by definition sweep their local neighbourhood clear of objects. So the black hole is going to carve out an area that is largely devoid of other bodies, and particularly in the context of a gaseous planetary nebula, it’s going to swallow stuff. This will lead to a roughly spherical void around it, and a gravitational bias towards the centre of mass of the planetary nebula (i.e. radially inwards towards the sun).

"Planets by definition sweep their local neighbourhood clear of objects"

What makes you think that a black hole orbiting the sun is a "planet" by the definition you're using?

More generally, you can't prove things about the physical world just by citing somebody's arbitrary definition of a word.

Molecules are much lower density than black-holes. If an object were infinitely small, it would never be hit by physical collisions. If it were infinitely massive it would never be affected by them.

I would have thought that any model of a black hole in this scenario essentially approximates one of those conditions.

The black hole in this scenario is neither infinitely massive nor infinitely small... 10 times more massive than Earth and five cm across.

A close enough gravitational interaction, even if not a collision, could do sufficient momentum exchange.

Do not forget that a collision is just an interaction of two objects through a force. Gravitational slingshot maneuvers are a way to bounce a spacecraft off of a planet using gravity as the force. Kind of like bouncing a ball off a fast moving car and seeing it fly off at high speed.

A theoretical black hole in our outer solar system would do something similar to low mass objects such as comets and asteroids as well as interstellar gasses.

>where would it get its momentum from?

The Sun itself, surely?

If SpaceX can afford to put a car into orbit around the sun as a publicity stunt during rocket testing, I wonder how much this would cost.

Edit: Probably around $100MM: https://www.spacex.com/about/capabilities for the launch, several $100MMs for the (space based?) accelerator and spacecraft, and then a few more for operations and research. Yeah, $1 billion as described in the article seems like a good estimate: https://arxiv.org/pdf/1805.01306.pdf

Since knowledge arising from this experiment will probably only be useful as a common good, there aren't really incentives for this to happen, unless governments decide to go through with it.

NASA's 2020 budget is only $22.6 billion: https://en.wikipedia.org/wiki/Budget_of_NASA

The Apollo program cost $153 billion: https://en.wikipedia.org/wiki/Apollo_program#Costs

Okay, this thing is ten times further away from the sun (400 AU) than Pluto (40 AU), 80 times the distance of Jupiter from the sun (5 AU), very far. Still, it's much closer than our next closest star, Proxima Centauri, which is 265612 AU away, or over 660 times the distance of Planet 9.

If we get actual evidence supporting that there could indeed be a black hole within our solar system, that seems like a true opportunity to advance our knowledge of physics. The potential to study a real black hole up close seems worth massive investments given what kind of breakthroughs could he possible from it.

Yes and no. I mean, how would you study a black hole of radius ~5cm without getting really close? Consider that:

1. Time dilation dictates that it would take ages for any instrument to get near the black hole, conduct measurements and send its results back to Earth.

2. Time dilation also means you would never see anything fall into the black hole. So studying the inside of the black hole is out of question, too. But arguably this is what would be required to make any breakthroughs in our understanding of black holes.

3. Tidal forces near the event horizon of such a tiny black hole would have catastrophic consequences[1] for anything (human or object) that gets too close to it.

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

Wouldn't this mean that these tidal forces may (have) cause(d) highly accelerated matter to shoot through our solar system?

I assume you're referring to astrophysical jets[1] that are common with SMBHs. I'm afraid, though, I'm not familiar enough with jet physics to answer your question. A priori, the tidal forces only cause infalling matter to get shredded (and heated up). Whether a proper accretion disk together with a (significant) jet can form, given the size of the black hole, I don't know.

On a related note: It'd be interesting to see in how far such a small BH would actually be capable of "sweeping [its] local neighbourhood clear of objects" like planets are (as was claimed in some comment above). Only then would I expect a significant accretion disk and, possibly, a jet. The reason the answer to this question is not obvious (at least to me) is once again the tiny size of the BH: How would anything manage to fall into the black hole, given that it's so tiny and, thus, very easy to miss?

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

Also a lovely, future power-source.

How would this work?

If it's rotating, you can extract its rotational energy using the https://en.wikipedia.org/wiki/Penrose_process

Additionally, all black holes constantly emit Hawking radiation https://en.wikipedia.org/wiki/Hawking_radiation Unfortunately for the planet 9 black hole, it only loses mass on balance it's below a certain size, about 0.007 earth mass.

Primordial black holes of that size, if they exist, would be quite useful. However, they would be monumentally hard to find: the black hole above would be 62 micrometres in radius, and have an event horizon temperature of 2.93K. Assuming no changes to the cosmic microwave background, this black hole is unstable: it will eventually radiate away all its mass and disappear. You'll be waiting a while for that to happen, at 20 picowatts, it'll take 1.9e35 gigayears to die.

Amplified superradiant scattering is one option.

https://en.wikipedia.org/wiki/Black_hole_bomb

// If SpaceX can afford to put a car into orbit around the sun as a publicity stunt during rocket testing...

SpaceX had to test the rocket anyways and have some mass in the nosecone. Elon Musk, on a lark I guess, put his personal car in the nosecone as that mass. You make it sound as if they expensively launched a rocket for publicity. In really, it probably cost a negligible amount in addition to what they already had to spend for testing.

It is a scientific crime that SpaceX didn't put a gravitational-reference satellite on that launch instead.

A Sputnik with a radio repeater/reflector aboard or one of these https://en.wikipedia.org/wiki/LAGEOS would have been a gift to future generations in the form of precision gravity tests. No moving parts, can't fail, will orbit the Sun for the remainder of human-time. Only key requirement is a lot of mass and high density.

Yes, launching a car into solar orbit was cool. It was also a waste of a rare opportunity.

It was a test launch with a high chance of failure.

Why the hell would they put an actual payload into it. Are you also angry that the Apollo missions didn't put "useful" things into space when they were launching hardware with sensors

Putting an actual payload into the maiden flight of the Ariane 5 made into a very expensive test failure.

The important fact to consider here is: How much does a LAGEOS-like satellite cost? I couldn't find the answer.

Thanks for the interesting comment, but I have to say: Marketing helps science too. If SpaceX achieves all its goals, then perhaps the marketing stunt was worth it?

Still, I wonder if Elon knew such a satellite was a possibility. I would assume yes, but I'm not sure.

How much work would it take to create a LAGEOS-like satellite? Presumably they're not just lying around. I couldn't find how much the original LAGEOS satellites cost.

Tbf, the only extra "cost" there was the car itself. If they didn't use the car, they would've launched a concrete block anyway to simulate a payload.

You're going to have deltaV problems with that approach. It's kind of like asking why can't we use a 737 to fly some rovers on the Moon.

Check out the rocket equation and see what kind of mass fraction you'll need to get hundreds of km/s: https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation

I wonder if the influence of the rest of the solar system wouldn't impact it.

Consider: the reason we think there is anything out there is because we see a lot of bodies that aren't following the expected pattern. For every action, there is an equal and opposite reaction, so perhaps those out-of-order bodies also pulled Planet 9 closer to the standard plane. Repeat a few billion times over a few billion years, each little tug moving it more in-line (and the rest of the solar system out-of-line).

This still depends on it starting relatively close to the same plane, but there may be selection bias involved. Those bodies that met with the solar system and weren't aligned well with the plane may have been more likely to escape again. The one (or ones) remaining are the ones that were well-aligned.

Could such a black hole have influenced the current orbiting plane or do we believe it was captured too late?

Above he mentions that “if it were there from the start it would obey the same rules”... so I guess he does... though where evidence for this comes from isn’t clear from me (unless it’s some kind of circular logic along the lines of “if it is in the same plane then it must have been there from the start, and if the perturbations in the Oort Cloud objects is consistent with an object in the ecliptic plane, then it must be that if t’s a black hole it was captured early on”, or something like that.

I dunno, to be honest. I don’t want to appear like I’m a polemicist with an axe to grind and sour grapes, but as I said in my original post... there’s something about this whole thing that just confuses me a bit.

It may have just happened to line up correctly. But gravity is going to pull it toward the center of mass for the whole solar system. If it were too massive to be pulled in like it wouldn’t be the hard to spot phenomenon that it has been to this point.

It’s the shotgun approach to detecting gravity, literally

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