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.
I really want primordial black holes to turn out to be the missing antimatter from the Big Bang, but I don’t think that could ever be tested. And of course a mechanism for this would probably need new physics since antimatter interacts the same as normal matter wrt gravity.
There were suggestions that the antimatter-matter asymmetry is because antimatter was preferentially segregated into a dense phase of hadronic matter, like quark matter nuggets. This would be interesting because if such nuggets could be found and captured, they'd be a potential source of energy by annihilation with ordinary matter.
It has the nice feature of explaining why the density of baryons and the density of dark matter are not too different. Naively, there's no reason to expect the two to be anywhere close to each other. It also offers an explanation for the observed matter-antimatter asymmetry in the universe.
Don't believe any theory until it's been well tested.
When I asked similar questions physicists, turns out all of them were thought off and tested long time ago. E.g. small black holes would still cause detectable and attributable radiation.
What happens when an antimatter black hole collides with a matter black hole? We'd see gravitational waves but no photons, right? Would the grav waves reflect the tremendous energy release somehow?
The "no hair theorem" states that black holes preserve exactly three numbers: mass, charge, and angular momentum. Baryon number, lepton number (as you would see in antimatter) are not conserved, the information is lost.
And, no, antimatter does not have negative mass, in any of the three contexts (mass-energy equivalence, inertia, and gravitational).
Therefore, a black hole fed entirely by antimatter would be indistinguishable from a black hole fed entirely by the equivalent matter.
Given how much rotating black hole differs from non-rotating one I believe a black hole consisting exclusively of swirling photons could have some very unique properties that out best models don't necessarily capture.
For example some resonances that could expell enough energy through gravitational waves to make it unstable and explode into a version of a big bang.
Not necessarily. You could load up one black hole with electrons and the other black hole with anti-protons ... both would have negative charge, -Q ... but the latter would have a much greater mass, since anti-protons are much heavier than electrons, despite having equal charges.
"Antimatter" is really only "anti" in a few respects. First, the lepton or baryon number has the sign flipped. An electron has a positive lepton number; a positron would have a negative lepton number. The charges are also flipped, where applicable (anti-neutrons still have a charge of zero). Isospin is flipped; spin is not. Mass is not, in three ways.
First, if you converted an anti-proton into pure energy (ignoring charge conservation, et al), it is the same energy as you would get out of a proton.
Second, an anti-proton exerts the same gravitational pull as a proton. A world of anti-matter would cause you to fall toward it the same way the Earth would.
Third, a force exerted on an antiproton causes it to accelerate along the same vector as the force, so the inertia is the same as regular matter.
People have theorized, myself included, on what some kind of matter where mass was negative (let's dub it nega-matter) would look like, which is to say that a force pushing it away would contrarily draw it toward you. You can simulate it in particle life, likely, but what would happens is that all of the nega-electrons would feel electrostatic repulsion and therefore clump together, as would all of the nega-protons. These two lumps of enormous charge (we just don't get that in real life, electrostatics are dozens of orders of magnitude more powerful than gravity) would create an "attractive" field between them of nearly incalculable strength and, because they would move opposite to the vector of the force, promptly zoom away at many nines of the speed of light. In short, they would sort themselves out and fly away to distant corners of the universe with great haste.
> would be indistinguishable from a black hole fed entirely by the equivalent matter
> one black hole with electrons and the other black hole with anti-protons
But electrons and anti-protons are not equivalent. Positrons are.
That's what I meant -- if the same energy of electrons and positrons are used, the black holes would not be indistinguishable, their charges would be different.
Well, if you load up a black hole with, say, electrons, it will have an electrical field similar to a point charge -Q, just the way the mass M behaves as a point mass emitting a gravitational field (or distortion in spacetime, take your pick).
Additionally, when you get Hawking radiation into the mix, negatively-charged particles (electrons, muons, tauons ... but probably mostly electrons) would be preferentially emitted, since pair production near the horizon would preferentially eject the negative particle via electrostatic repulsion and just as preferentially recapture the positive member of the pair.
Not that you asked, but the angular momentum looks different than the other two, it actually changes the shape of the event horizon to something more oblate.
In a black hole, all you see from the outside is total mass. This is actually mass-energy - a release of energy inside the black hole doesn't change the total mass-energy inside.
it sounds a lot like it now that i read this, large dispersed amounts of antimatter that apparently doesn't give off light yet has massive gravitational pull
As a Brit, I've never noticed NASA spelt Nasa before, and I'm on the fence about Unesco, not because I recall seeing it title-caps, but because it doesn't look offensive to me.
That said, when I googled for the title-caps variants of both words, I saw the Guardian, Independent and Times all used it for Nasa, but only the Guardian and Times seem to use it for Unesco. There was also a page from the NYT that used Unesco, so a counter example from the US, but even the UK government pages refer to it in all caps, as do most other UK articles.
I certainly find it a weird generalisation that "UK English" (sic) prefers title-caps, as we tend to like our abbreviated names and pretty much always use all-caps when every letter comes from a separate word, and only title-caps when we use runs of letters from each different word - quango (quasi-NGO), Bakerloo (Baker St-Waterloo), Beeb (compared with BBC), Lib Dems, etc. We also have a few things that aren't consistent for government things - Defra (always used to be DEFRA, but they seem to have rebranded), MOD (always caps), Ofqual (=Office of Qualifications, always title case), Ofcom (=Office of Communications, now seems to officially be title case, but always used to be OFCOM), UCAS (University and Colleges Admission Scheme, always all-caps), etc.
Maybe the stylistic choice is based on how pronounceable it is as a word, but whatever I don't think there are any hard and fast rules!
U.S. newspapers have a tendency to convert abbreviations to title case, even when everyone else uses all-caps. For instance, I've only ever seen "Covid-19" written in newspapers: everyone else always used "COVID-19", "COVID", or just "covid", or "the coronavirus".
HN automatically makes various changes to submission titles. The lesson that should be learned is that after submitting, there is a short period of time where the user can edit the title back to what it should be, in cases such as this.
Fascinating mission. I don't understand how microlensing events resulting from primordial black holes can be differentiated even statistically, given that they "can't be formed by any known physical process", but in any case it's exciting to discover there's much more of something up there than we knew. Even if they are "just" rogue planets.
We have a variety of ways to make insanely precise measurements of light to the point of counting photons. I’m guessing that factors into being able to detect microlensing.
Mostly because time slows as things approach event horizon.
I know that in the frame of reference of infalling object everything seems like business as usual but for far away observer (us) nothing ever crossed any event horizon.
You can't do away with this problem without dismissing the notion of any well defined simultaneity altogether (which people do, but I believe it's a cop out).
As far as I'm concerned there just wasn't enough time since the beginning of the universe for any blackhole to form, from our point of view, because of the infinite time dilatation. And I think our (remote observer) point of view is what counts because most places in the universe are remote observers.
If you are interested in the rest of my cosmological beliefs I believe that black holes are primary form of matter, the rest is just a dust between them.
I believe that big bang wasn't the beginning of time and space but rather highly energetic event caused by collision of two superdense clusters of trillions of supermassive balckholes. What inflates space is the kinetic energy imparted on the dust by this collision. The mass flies away in all direction with random velocities and the spacetime is "dragged" by it.
We all came from the dust swirling between them that got heated by the event to the energies of quark gluon plasma and higher.
We are also possibly under the influence of very strong, very long wavelength gravity waves originating from outside of observable universe. Larger remaining chunks of the objects that collided to give birth to our universe might be the source.
question for the initiated: why do space agencies divide their resources and attention between different projects? Wouldn't it make sense to focus first on making the moon and it's resources accessible kind of like airplanes then systematically expand to Mars, asteroid belt, and so on instead of spreading out too thin without conquering the closest
- Betting everything on a single huge project is risky. You wait for years for completion, and if some critical part of the project costs more/takes longer than expected (much like Artemis today) you risk achieving nothing at all.
- Small, cheap projects that don't bankrupt the agency if they fail, when they succeed, generate ongoing positive PR and scientific results. NASA famously promoted this concept after one or two expensive projects had problems (of course, not famously enough that I remember more details).
- Some of those smaller projects are critical, like climate monitoring. You can't just set aside those sorts of tasks for a literal moonshot.
- Smaller projects contribute towards the big ones: NASA has given SpaceX a fair bit of money over the years that is paying off with Starship and Artemis.
What the heck is "Nasa"? Not the sword I'll prefer to fall on, but even if you're in the UK, let's honor that this organization is in the United States and they self-describe as "NASA."
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).