10 ms·

Primordial black holes are a fascinating topic. One of my favorite hypotheses is that "planet nine," a large possibly 1-5 Earth mass planet suggested by some or

by api 2y ago

Primordial black holes are a fascinating topic. One of my favorite hypotheses is that "planet nine," a large possibly 1-5 Earth mass planet suggested by some orbital models to exist beyond Neptune and Pluto in the far outer solar system, may be a primordial black hole. If such a thing existed it'd be a black hole about the size of a billiard ball and would be extremely hard to detect. It would not emit Hawking radiation (Hawking temperature still below the CMB), so the only thing it would emit would be when it encountered something and tore it apart. In that case you'd see X-rays, gamma rays, etc., but maybe only briefly. Only way to find it might be to model orbits accurately enough to predict its position and then look for gravitational lensing. If this companion object did exist it'd be a gigantic discovery of huge importance. It'd be within reach of probes, making it possible to study and even do experiments on to investigate things like quantum gravity. It could also be used in tight gravity assist flybys to accelerate probes to incredible velocities, maybe making interstellar probes a lot more practical. It'd be our very own way to yeet stuff to the stars, assuming these things could withstand insane g-forces (so probably not humans unfortunately).

ryandrake 2y ago
> It could also be used in tight gravity assist flybys to accelerate probes to incredible velocities, maybe making interstellar probes a lot more practical. Would 1-5 earth masses really provide enough of a yeet to appreciably affect the speed of a probe? Jupiter is about 300+ earth masses and we're not flinging probes out to stars using him.
- bigyikes 2y ago
  You can get a lot closer to the center of mass of the black hole, which should drastically increase acceleration since it falls off with distance squared.
  - gus_massa 2y ago
    The satelite acelerates when it aproach the planet, but most of it is compensated when it goes away. There is a nice graphic in https://en.wikipedia.org/wiki/Gravity_assist#/media/File:Voyager_2_velocity_vs_distance_from_sun.svg https://en.wikipedia.org/wiki/Gravity_assist#/media/File:Voy...
    - pixl97 2y ago
      You toss off some mass as you pass the black hole and it gives a powerful boost.
      - gus_massa 2y ago
        Do you have a source for that?
        vundercind 2y ago
        Star Trek fans will recognize the name as that of a versatile light starship class often seen acting as science vessels. https://en.m.wikipedia.org/wiki/Oberth_effect https://en.m.wikipedia.org/wiki/Oberth_effect FWIW I’ve read several explanations of why this works, including some confidently claiming that one or more of the others was wrong, and a couple of which kinda made sense as I was reading them, but not a one of them has made a lick of sense to me after I thought about it for a while. Despite all the attempts at understanding it, I still couldn’t tell you why it works (aside from “this math says it does” which is a shit answer) [edit] the other thing you can do, even at the same time is: https://en.m.wikipedia.org/wiki/Gravity_assist https://en.m.wikipedia.org/wiki/Gravity_assist But the specific effect in question seemed to be the Oberth Effect, given the mention of throwing off mass. Gravity assist just relies on the body in question being really heavy and in (orbital, say) motion in some fashion that’s useful to you. Kinda “pulls” you along. You steal a negligible amount of energy from a huge body, which translates into some decent speed for your very-light spacecraft.
        MeImCounting 2y ago
        OK so I was totally confused by the Oberth effect and how it could possibly be and so did some research. Now I have no idea why or how kinetic energy has a quadratic relationship with velocity, but it does. Something something work something something square of velocity, who knows. If someone could explain that to me like I'm 5 I would totally appreciate it. But if we just take that as a given then we can develop an intuitive understanding of the Oberth Effect pretty easily if we remember that velocity is only relevant to a reference frame. So when you burn at periapsis (at top speed aka when youre closest to our black hole) your energy relative to the black hole is increased a lot more because for a given unit of fuel you add the same amount of velocity, and doubling your velocity is more than doubling your energy. That energy is what carries you up and away from the black hole and towards your apoapsis (or the stars) It makes sense if we just pretend to understand why it is that somehow magically KE is proportional to the square of its velocity IDK
- dooglius 2y ago
  We did, Voyager 1 and 2 used Jupiter to get out of the solar system
  - Sharlin 2y ago
    And the Pioneers and New Horizons too.
caf 2y ago
Assuming you don't go so close that tidal effects tear the probe apart, the g-forces should only be those imposed by the probe's thrusters firing during the assist. When the thrusters aren't firing it's just in free-fall.
- kadoban 2y ago
  Hm, that can't be how that works, can it? How do you get any extra velocity then if there's never any acceleration beyond what your thrusters provide?
  - messe 2y ago
    It's called the Oberth Effect. In short, in Orbital mechanics, burning your thrusters deeper within a gravity well, results in a greater increase in kinetic energy than burning them further out. This is because momentum ~ v, while kinetic energy ~ v^2. If you're travelling faster—as you would be as you approach the black hole and fall deeper and deeper in your orbit—then you can expend to same amount of momentum to receive a disproportionally larger increase in kinetic energy. Because your potential energy falls off with distance to the black hole at the same rate regardless of the speed you're travelling at, your total energy upon escaping the black hole is much larger than it would be had you burned your thrusters outside its gravity well.
    - deleted 2y ago
      [deleted]
    - HeatrayEnjoyer 2y ago
      But you are still burning the same amount of chemical energy...
      - ibeforee 2y ago
        Wikipedia says: In terms of the energies involved, the Oberth effect is more effective at higher speeds because at high speed the propellant has significant kinetic energy in addition to its chemical potential energy.[2]: 204 At higher speed the vehicle is able to employ the greater change (reduction) in kinetic energy of the propellant (as it is exhausted backward and hence at reduced speed and hence reduced kinetic energy) to generate a greater increase in kinetic energy of the vehicle.
Loughla 2y ago
I feel like if you were close enough for a gravity assist, wouldn't the tidal forces just tear the probe apart?
- Tostino 2y ago
  Not necessarily. If you replaced the sun with an equivalent mass black hole, none of the orbits of the planets would change.
  - tsimionescu 2y ago
    Sure, because none of the planets are anywhere near close enough to the sun to experience tidal forces. But if you were to approach the sun VS a sun-mass black hole, at some distance the difference would become noticeable through tidal forces (ignoring the massive difference in emitted radiation, of course).
  - jebarker 2y ago
    I'd like to see a simulation of what that would look like. I mean, if by magic the sun was replaced with an equivalent mass black hole in an instant would anything be visible from earth before the inevitable freeze?
    - utensil4778 2y ago
      From Earth's perspective, you wouldn't see anything interesting except for the sun vanishing. Gravitationally, all that matters is the absolute mass, so all the dynamics of the solar system stay the same. A black hole of 1 solar mass has a radius of something like 3km. Totally invisible from Earth. You probably wouldn't even see any gravitational lensing. All we would see is the sun there one moment, and then nothing the next. Life on Earth would continue for a while, but the planet would eventually freeze over. So, nothing interesting. The sun vanishes and then some time later you freeze and/or starve to death.
      - jebarker 2y ago
sandworm101 2y ago
At that size, very few things would ever get close enough to be torn apart, let alone fall in. We should look for objects suddenly changing course ivo the potential black hole.
perihelions 2y ago
- "In that case you'd see X-rays, gamma rays, etc., but maybe only briefly." One speculative possibility is that we could find a continuous gamma annihilation signal from its dark matter halo, https://arxiv.org/abs/1909.11090 https://arxiv.org/abs/1909.11090 https://hn.algolia.com/?query=What%20If%20Planet%209%20Is%20a%20Primordial%20Black%20Hole%3F&type=story https://hn.algolia.com/?query=What%20If%20Planet%209%20Is%20...

by api 2y ago

ryandrake 2y ago

> It could also be used in tight gravity assist flybys to accelerate probes to incredible velocities, maybe making interstellar probes a lot more practical. Would 1-5 earth masses really provide enough of a yeet to appreciably affect the speed of a probe? Jupiter is about 300+ earth masses and we're not flinging probes out to stars using him.

bigyikes 2y ago
You can get a lot closer to the center of mass of the black hole, which should drastically increase acceleration since it falls off with distance squared.
- gus_massa 2y ago
  The satelite acelerates when it aproach the planet, but most of it is compensated when it goes away. There is a nice graphic in https://en.wikipedia.org/wiki/Gravity_assist#/media/File:Voyager_2_velocity_vs_distance_from_sun.svg https://en.wikipedia.org/wiki/Gravity_assist#/media/File:Voy...
  - pixl97 2y ago
    You toss off some mass as you pass the black hole and it gives a powerful boost.
    - gus_massa 2y ago
      Do you have a source for that?
      - vundercind 2y ago
        Star Trek fans will recognize the name as that of a versatile light starship class often seen acting as science vessels. https://en.m.wikipedia.org/wiki/Oberth_effect https://en.m.wikipedia.org/wiki/Oberth_effect FWIW I’ve read several explanations of why this works, including some confidently claiming that one or more of the others was wrong, and a couple of which kinda made sense as I was reading them, but not a one of them has made a lick of sense to me after I thought about it for a while. Despite all the attempts at understanding it, I still couldn’t tell you why it works (aside from “this math says it does” which is a shit answer) [edit] the other thing you can do, even at the same time is: https://en.m.wikipedia.org/wiki/Gravity_assist https://en.m.wikipedia.org/wiki/Gravity_assist But the specific effect in question seemed to be the Oberth Effect, given the mention of throwing off mass. Gravity assist just relies on the body in question being really heavy and in (orbital, say) motion in some fashion that’s useful to you. Kinda “pulls” you along. You steal a negligible amount of energy from a huge body, which translates into some decent speed for your very-light spacecraft.
        MeImCounting 2y ago
        OK so I was totally confused by the Oberth effect and how it could possibly be and so did some research. Now I have no idea why or how kinetic energy has a quadratic relationship with velocity, but it does. Something something work something something square of velocity, who knows. If someone could explain that to me like I'm 5 I would totally appreciate it. But if we just take that as a given then we can develop an intuitive understanding of the Oberth Effect pretty easily if we remember that velocity is only relevant to a reference frame. So when you burn at periapsis (at top speed aka when youre closest to our black hole) your energy relative to the black hole is increased a lot more because for a given unit of fuel you add the same amount of velocity, and doubling your velocity is more than doubling your energy. That energy is what carries you up and away from the black hole and towards your apoapsis (or the stars) It makes sense if we just pretend to understand why it is that somehow magically KE is proportional to the square of its velocity IDK
dooglius 2y ago
We did, Voyager 1 and 2 used Jupiter to get out of the solar system
- Sharlin 2y ago
  And the Pioneers and New Horizons too.

caf 2y ago

Assuming you don't go so close that tidal effects tear the probe apart, the g-forces should only be those imposed by the probe's thrusters firing during the assist. When the thrusters aren't firing it's just in free-fall.

kadoban 2y ago
Hm, that can't be how that works, can it? How do you get any extra velocity then if there's never any acceleration beyond what your thrusters provide?
- messe 2y ago
  It's called the Oberth Effect. In short, in Orbital mechanics, burning your thrusters deeper within a gravity well, results in a greater increase in kinetic energy than burning them further out. This is because momentum ~ v, while kinetic energy ~ v^2. If you're travelling faster—as you would be as you approach the black hole and fall deeper and deeper in your orbit—then you can expend to same amount of momentum to receive a disproportionally larger increase in kinetic energy. Because your potential energy falls off with distance to the black hole at the same rate regardless of the speed you're travelling at, your total energy upon escaping the black hole is much larger than it would be had you burned your thrusters outside its gravity well.
  - deleted 2y ago
    [deleted]
  - HeatrayEnjoyer 2y ago
    But you are still burning the same amount of chemical energy...
    - ibeforee 2y ago
      Wikipedia says: In terms of the energies involved, the Oberth effect is more effective at higher speeds because at high speed the propellant has significant kinetic energy in addition to its chemical potential energy.[2]: 204 At higher speed the vehicle is able to employ the greater change (reduction) in kinetic energy of the propellant (as it is exhausted backward and hence at reduced speed and hence reduced kinetic energy) to generate a greater increase in kinetic energy of the vehicle.

Loughla 2y ago

I feel like if you were close enough for a gravity assist, wouldn't the tidal forces just tear the probe apart?

Tostino 2y ago
Not necessarily. If you replaced the sun with an equivalent mass black hole, none of the orbits of the planets would change.
- tsimionescu 2y ago
  Sure, because none of the planets are anywhere near close enough to the sun to experience tidal forces. But if you were to approach the sun VS a sun-mass black hole, at some distance the difference would become noticeable through tidal forces (ignoring the massive difference in emitted radiation, of course).
- jebarker 2y ago
  I'd like to see a simulation of what that would look like. I mean, if by magic the sun was replaced with an equivalent mass black hole in an instant would anything be visible from earth before the inevitable freeze?
  - utensil4778 2y ago
    From Earth's perspective, you wouldn't see anything interesting except for the sun vanishing. Gravitationally, all that matters is the absolute mass, so all the dynamics of the solar system stay the same. A black hole of 1 solar mass has a radius of something like 3km. Totally invisible from Earth. You probably wouldn't even see any gravitational lensing. All we would see is the sun there one moment, and then nothing the next. Life on Earth would continue for a while, but the planet would eventually freeze over. So, nothing interesting. The sun vanishes and then some time later you freeze and/or starve to death.
    - jebarker 2y ago

sandworm101 2y ago

At that size, very few things would ever get close enough to be torn apart, let alone fall in. We should look for objects suddenly changing course ivo the potential black hole.

perihelions 2y ago

- "In that case you'd see X-rays, gamma rays, etc., but maybe only briefly." One speculative possibility is that we could find a continuous gamma annihilation signal from its dark matter halo, https://arxiv.org/abs/1909.11090 https://arxiv.org/abs/1909.11090 https://hn.algolia.com/?query=What%20If%20Planet%209%20Is%20a%20Primordial%20Black%20Hole%3F&type=story https://hn.algolia.com/?query=What%20If%20Planet%209%20Is%20...