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).
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.
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.
The phenomenon described is not due to the Oberth effect.
What would be happening would be that the human in the spacecraft, and the spacecraft, are accelerating at exactly the same rate because they are accelerating due to gravity. Thus, the human feels no pressure accelerating him (no outside force acting upon him) from e.g. his seat. And his internal organs feel no pressure accelerating them from each other. They (the craft and the human and all his internal organs) are in free fall together and feel no forces acting upon them despite the whole system (craft-human-organs) being accelerated to tremendous velocities.
It's super un-intuitive, but you're indirectly harnessing the gravitational potential energy of lowering your propellant into a gravity well and leaving it there. The overall orbital energy gain of the spacecraft can exceed the chemical energy of the fuel.
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.
If you sit on a merry-go-round, and spin it very fast, you feel the "centrifugal force" trying to keep you in an inertial frame. That's because you're having to hold on to the ride. If you're in a spacecraft in orbit around Earth, you don't feel the force keeping you in a circular motion, because both you and the craft are experiencing the same force.
The worse thing about going past a blackhole would be tidal forces, which would exert differential stretching to your craft and you. I don't know the numbers for a blackhole that small. Also you'd need to aim very precisely - if you miss, it's a long way round to get back there and if you aim too well, it might take a chunck out of your craft and your left foot.
I was wondering that actually if the hole goes through your leg does it leave a hole or does your whole body get sucked in.
I imagine it is the whole body unless you are travelling really fast at the time. Like near speed of light.
Because the gravity outside the event horizon will still be crazy strong going out for several km (earth is a good comparison in the gravity is still fairly strong about 6000km from the centre)
Black holes warp the spacetime around them, so your idea of distance is isn't really valid. Also the idea of time is warped as well, so the black hole doesn't quite just pass through your leg as you expect.
Nor does the black hole "suck" anything in.
What would happen from your perspective if such a black hole were to pass you at high velocity would be the same as if you were to pass the black hole at high velocity. From your perspective, you would begin orbiting that object, carried along with it. So would all the matter near you. But it would be matter, not objects, as the tidal forces would spaghettify all objects very quickly.
> gravity is still fairly strong about 6000km from the centre
Sounds like the centre is what creates gravity. But there's weightlessness at the Earth center. It's the sum gravity force of all Earth particles that creates total gravity.
Depends on the size of the hole. If it's small enough, all you might get is one long bruise due to tidal/gravity effects, without losing a single atom of your body
This is incorrect. Any matter within about three Schwarzschild radii will never escape the black hole - i.e. will be carried along with it - even if the black hole passes you at the speed of light.
And if the black hole passes you at a velocity lower than the speed of light, then the radius grows. That matter will begin orbiting the black hole and will leave your body just as fast as the black hole left.
What's the Schwarzschild radius of a black hole the size of an atom? Or would something that size evaporate before it got a chance to snatch any of my leg?
The whole idea of encompassing an object that warps space into a sphere in our unwarped space is just a little invalid, especially when examining them on the scale of their constituent particles. I really don't know the answer, but I do not think that a black hole could be the size of an atom. I would expect the strong (or was it weak) nuclear force to overcome the mutual gravity and so compact an object would not be able to form.
I'm just a layman on the topic, and would greatly appreciate any insights by physicists in audience.
> 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.
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.
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:
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.
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
I can't ELI5 (don't like such things anyway) but if you know calculus, the v^2 follows directly out of trying to integrate momentum (or quantity of motion, a fun phrasing) with respect to velocity. Momentum is mv, units are kilogram * meter/second. Integrate with dv, you get 1/2 mv^2, units are kilogram * meter^2/second^2. (Double-check, take the derivative of that with respect to v, and you get mv.) This 1/2 thing times otherthing^2 relationship actually shows up all over the place in math and physics, it's quite beautiful, and incidentally another reason to prefer using tau=2pi instead of pi...
Imagine a car is moving at a speed of 10 m/s. The driver hits the brakes. How much distance does it need to stop?
The main idea is that brakes have a constant force, and the change in speed is always constant. Let's say they reduce the speed in 1 m/s each second.
The first second the car travels 10 m and the new speed is 9 m/s.
The second second the car travels 9 m and the new speed is 8 m/s.
The third second the car travels 8 m and the new speed is 7 m/s.
...
The ninth second the car travels 2 m and the new speed is 1 m/s.
The tenth second the car travels 1 m and the new speed is 0 m/s.
So the total distance until it stops is 10+9+8+7+6+5+4+3+2+1. If you make a pile of bloks and you put first 10, then 9 over them, then 8 over them, .... you get a nice triange. The base is 10 and the heigh is 10, so the total number of blocks is 10*10/2. [1] [2]
So the car needs 10*10/2 m to stop. You can repeat the calculation with other initial speeds, and the result is V*V/2.
I'm not sure if it's intuitive, but the energy is proportional to the distance to stop.
Another posibility is to throw a toy car verticaly, and calculate how hight it goes. It's the same calculation. The maximal height is V*V/2. I think it's easier to imagine that energy is proportional to maximal height.
[1] If you actually count them, the result is 55 insted of 50, more details in https://en.wikipedia.org/wiki/Gauss_sum , but 10*10/2 is a good aproximation.
[2] If you split the time in half seconds and use smaller and smaller blocks, and then use calculus, you get 10*10/2.
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.
The tidal forces would be no different than using jupiter for an assist. You would still have the probe pass by at several thousand, several tens of thousands of kilometers. A close-in gravity assist may look better on paper, but the practicalities and speeds of such a thing are risky. One small error and the mission would be over quick, launched out at a radically incorrect trajectory.
Isn't the whole advantage that you could get much closer to the center of mass of the black hole compared to a planet, thus gaining much more kinetic energy per unit thrust, but also risking higher tidal forces?
On paper yes, but doing so also reduces the time for the burn. Probes have very low-thrust engines. Even during a jupiter slingshot they barely have time to accellerate much on thier own. Often they do not bother, relying totally upon the grav assist to accellerate. The danger too of a closer approach is that something gets miscalculated. Get too close and an inevitable tiny misalignment will throw you onto a wild unwanted trajectory. There is no gps out there. Knowing exactly where and how a probe is moving isnt easy.
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).
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?
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.
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).