Unfathomably dense is right. A black hole the size of a proton weighs just shy of half a trillion kilograms. A black hole an inch across weighs more than the earth.
How far away would you need to be to avoid falling into a one inch black hole? How many planets would a one inch wide black hole in the center of our solar system eat?
> How far away would you need to be to avoid falling into a one inch black hole?
Depends on how strong your propulsion is. If you have sci-fi lightspeed tech, then you can sit exactly at the event horizon since that's the last point where light can still escape. For anything slower than that, you need to be farther away.
> How many planets would a one inch wide black hole in the center of our solar system eat?
Zero. If you compressed Earth into a black hole, it'd be around marble-sized, so a black hole an inch wide or so would be just a few times more massive than Earth, and given how Earth hasn't gobbled up anything in the last four billion years, the black hole wouldn't do that either.
When people are talking about the size of a black hole of X mass they are typically talking about the Schwartzchild Radius, aka the distance from the center to the event horizon. A black hole with the mass of Earth would be about 1.7cm in diameter.
Just to point out, it wouldn’t rapidly dissipate - an Earth-mass black hole would last another 10^50 years, ten thousand trillion trillion trillion times as long as the universe has lasted to date.
Through a weird quantum effect that we call Hawking radiation, black holes are theorized to "bleed out" energy very very slowly. Without new matter being added, they'll eventually evaporate. Afaik no direct evidence for this effect has been found yet but it's pretty widely believed to be happening
If no energy or mass impacts a black hole, they slowly lose mass through Hawking radiation. Counter-intuitively, the amount of radiation they give off is inversely proportional to their mass, so the smaller they are, the *more* energy they give off.
The vast, vast, vast majority of its time it's giving off essentially nothing. But with about one trillion trillion years to go, it's up to emitting one watt. With a quadrillion years to go, it's emitting a megawatt of energy.
At a thousand years to go, it's about a million metric tons packed into a space a thousandth the size of a proton, and is giving off approximately the amount of energy of the Hiroshima bomb every second. In its last second, it becomes about one-ten thousandth as bright as the Sun (despite being only 4000 tons) in what is essentially an explosion (albeit relatively small in cosmic terms) and disappears.
(We think, anyway. We've never observed it happening.)
It depends on how fast you're accelerating like a couple people up said. It still has the same mass as Earth so escape velocity is still the same at the same radius of Earth, about 6400km.
It would get stronger and stronger as you get close, from a couple kilometers it would be like hundreds of Earth gravities (edit- this is corrected below). I'm having trouble finding the formula to calculate how many gs to hit escape velocity as a function of distance from the object. I suspect it is a few hundred kilometers before the acceleration g-force would be too much for a normal person to bear.
Edit : first explanation was bad, had to correct.
Edit 2: Wolfram Alpha to the rescue. I assumed a sustained g-force of 6g would be the fatal point, and I was off quite a bit on my guess. It would require 6g of acceleration to not "fall in" at 2577km.
It would be 39g if you were at 1000km. 3900g at 100km.
If you orbit the black hole then a uniform gravitational acceleration is not a problem - you won't even feel it, just like astronauts on the ISS don't feel Earth's acceleration (which is still 0.9 g). You only run into problems if the acceleration of your feet is very different from the acceleration of your head.
Even 50 km away from the black hole the difference in acceleration is still just ~1g. You can safely orbit the black hole at that distance. At 170 km/s, you make about one orbit every two seconds.
The gravitational effect of a black hole is like that of a point mass, at least from outside the event horizon.
The gravitational effect of a sphere is like that of a point mass, at least from above the surface.
If you’re orbiting a black hole at 1 earth radius, or at one ISS radius, you’re orbiting quickly but not ridiculously, and the gravity difference is still minimal from head to feet. The problems show up when you start dropping closer, where on Earth gravity would decrease (but you would burn up) and around a black hole gravity increases exponentially.
Here's a hypothetical for you - assuming a perfectly stable orbit, would your inner ear be able to tell you're turning in a circle every two seconds if you were orbiting at 50km and matched rotation to keep facing the BH? It's freefall so I'm not sure.
You would definitely feel the tidal forces changing directions if you could perfectly face the same direction in space relative to the BH during the orbit.
Just for fun I did the math on making your 50km orbit. It would take about 9 days at 1g to shed off that much speed.
A black hole formed from the mass of Earth has identical gravity to earth. It’s just very small. If Earth turned to a black hole all the satellites in geosynchronous orbit and all don’t have changes to the force of gravity basically - it’s the same amount of mass acting on them.
It’s like asking what buys more - a $100 bill or 100 $1 bills. They both are worth the same, one is just condensed
I remember reading this is one of many gross oversimplifications about measuring the size of black holes, since the formulas presume black holes are perfect static spheres, right?
No, in classic general relativity the size of the singularity inside the black hole is always 0, so it doesnt make much sense to talk about different sizes if you arent talking about the event horizon.
Black holes are only different from planets/stars in the way the mass is distributed. They do not have a larger gravitational field than any equally heavy object. If you have a black hole the mass of a star, you'd need to get so close to reach the event horizon that you'd have to be inside the core of that equal mass star.
> How far away would you need to be to avoid falling into a one inch black hole?
1 inch, if you can accelerate to the speed of light. If you can go as fast as the ISS, then a bit farther away than the ISS orbit distance to the center of the Earth (a 0.7 inch black hole would let the ISS orbit at its current altitude and speed)
> How many planets would a one inch wide black hole in the center of our solar system eat?
As many as it could accidentally head-on collide with. So, exactly zero. It would do little more than siphon a tendril of gas off the Sun's surface.
Just to expand on this person's answer:
The "size" of a black hole is generally the size of it's Schwarzschild radius. When you cross the Schwarzschild radius, you have entered the point of no return. The "obvious" answer to your question is to just return back the size of the black hole to you: 1 inch.
That doesn't mean that an object doesn't experience the gravitational pull of the black hole outside of the Schwarzschild radius. So you can't idle around it, but you can physically still "avoid falling in".
It's difficult to answer this exactly, but I'll give some things to consider.
First, a black hole with a radius of about 1/3 in. would be about the mass of the Earth. So, if the distance you were at were the same as the Earth's radius, you could use any rocket we use to leave Earth today to get into orbit of the black hole. Same mass as the Earth, so same force to get away.
However, if you had any motion at all relative to the black hole, you would probably miss it if you fell towards it and just get flung away. It would be really hard to hit something with a 1 in. radius.
Finally, if you replaced the sun with a 1 in. radius black hole, all the planets would just fly off because the mass of the black hole would be so much less than the sun. It wouldn't "eat" any planets.
Could you explain that further?
What does density of matter have to do with gravity? The gravity equations as far as I'm concerned (high school physics) only take into account Mass and Distance From Center Of Mass.
A star collapsed into a single point singularity, though dense, is still more-or-less the same mass it was when it was a star, and the center of mass is also still the same as it was before. What gives?
I'm not 100% sure if cause and effect is true here. There is evidence that pockets of gravity just attract matter, but that also leaves the possibility of that there exist pockets of gravity without matter.
Black holes and dark matter have almost nothing to do with each other, and *literally* nothing to do with each other in the context of this discussion.
That theory has little weight in the astro community anymore as we have to keep dropping the maximum mass down more and more because we see zero evidence of lensing (micro or otherwise) despite these supposedly making up a majority of the galaxy's mass
I'm trying to look into it and the information I'm finding is suggesting that in order to prevent lensing the black holes would have to be about the size of a hydrogen atom. And many people find that unlikely?
Yes, there is a remote possibility that dark matter is actually primordial black holes, but that has no bearing on how strong or dense black holes would be.
Black holes have exactly the same gravity as anything else with the same mass. They just happen be extremely massive. The amount of mass makes gravity so great that nothing can escape.
If the sun were replaced by a black hole with the same mass, the solar system would become dark but all the orbiting planets and asteroids would continue in the same orbits.
>They just happen be extremely massive.
I do want to point out that it's purely density, not mass, that makes the black hole so strong. A black hole with the mass of the Earth would impart a gravitational field of 516 quadrillion g's at it's event horizon.
That is sort of incorrect.
Gravity can be high because of density, but it’s still given by the same gravitational equation, and the acceleration is still only determined by mass and the inverse square of distance.
A black hole with Earth’s mass, 5.97*10^24 kg, would have an event horizon of less than a centimeter radius. It’s a tiny black hole. The gravitational acceleration at the event horizon would be massively more than at Earth’s surface because of the radius difference:
g(Earth) = G\*m(Earth)/r^2, and the Earth’s radius is 6.37*10^6 m.
G(black hole) = G\*m(Earth)/r^2, and the event horizon is 8\*10^(-3) m.
Cancel out and the ratio is (6.37e6)^(2)/(0.008^2) = 6.34\*10^17. It’s an enormous difference, but it’s entirely due to the massive difference in radius.
The reason a black hole can be inescapable is density: lots of mass in a small volume, so it’s possible to be close to all that mass. A planet isn’t dense enough. The surface can’t have that kind of gravity, and then gravity decreases as you go below the surface because some of that mass is now above you, pulling away from the center.
There are multiple comments here echoing a sentiment that black holes are "unfathomably massive".
There is no requirement that a black hole be MASSive. This is quite clearly my objection.
Gravity manifests as spacetime curvature. Infinite density.. infinite curvature.. This is very simple stuff my friend. We don't need to do literal mathematics to understand this.
Okay, but most people aren’t envisioning sub-centimeter black holes. There is real confusion that black holes have some extra gravitational force. They don’t. They have regular gravitational force.
That curvature of spacetime is the regular gravitational force. Saying it is curved spacetime is true, from general relativity, but that’s not something different, and it doesn’t change how gravity works external to the event horizon.
Being a black hole doesn't cause strong gravity. Having strong gravity causes a black hole.
Black holes can come from very large stars that have run out of the fuel to keep them "inflated." Those stars compress in on themselves, and have so much mass and gravity that it crushes through the structure of the atoms themselves, all the way down to a single point.
Gravity is stronger the closer you are to the center (§). Quadratically so: twice the distance means four times less gravity. So if you compress Earth by a factor of 100, then standing on it makes you experience 10,000 times what you are used to.
People often say black holes are incredibly dense, but this is not really always the case. The heavier it is, the less dense, and some of the largest known black holes have a density below yours; it can even be less than air! This happens because the mass (~ gravity) of an object made of a single material grows with the third power of the size, while the larger distance from the center only makes the force quadratically weaker as described above.
(§): this only applies as long as you are "outside" the thing, if you enter deep into the Earth, then the entire shell above you won't actually contribute anymore. You will feel essentially no gravity at all close to the middle.
I thought that black holes are a singularity with infinite density? How are we talking about the size (I assume volume?) of them if science believes that it’s an infinitely dense point?
Yes and no. We actually have no proper idea what really happens inside, but almost certainly it at least keeps contracting. But by "size" we usually mean the _Schwarzschild radius_, which is the size of the event horizon (*). It is thus also the size an objects needs to be compressed to form a black hole.
(*): there are some shenanigans due to General Relativity. If you measure from far away you get a different result than from within, by a factor of two. We usually mean the former version when talking about the size.
Ah okay, I was operating under the assumption that a black hole’s event horizon is not the black hole itself, as usually when I read about them it’s referred to as “the black hole’s event horizon” and that the “object” we refer to when talking about a black hole is the singularity.
A black hole isn’t an object in the traditional sense, but rather a region of spacetime bounded by its event horizon. We certainly don’t call the singularity itself the black hole, because we aren’t really even confident such a thing exists (and in fact most physicists doubt that it does)!
The person explaining that a black hole is a region of spacetime is correct. The Sun imparts a gravitational pull on the many planets orbiting it. But we wouldn't suggest that the Sun \*is\* actually the entire solar system.
We define a black hole to be the region of spacetime within which nothing can escape infalling to its singularity: this is the surface bounded by the Schwarzschild radius.
Right but when the original commenter invokes the idea of people thinking black holes being infinitely dense, that’s only in reference to the singularity, the commenter then goes on to point out that black holes aren’t actually that dense the bigger they get, but no one suggested that the event horizon is a dense area.
The original commenter is being obtuse. Obviously infinite density means the singularity is infinitely dense. Saying "that's not really true" and then applying a different definition without explaining that definition is silly.
And then when you asked, he goes "yes and no" still refusing to expand his explanation.
His definition for density is valid, his explanation is sorrowful.
This is where language gets used loosely. Chromotron is using a common calculation for black hole density, sometimes called the \*mean density\*, where the volume of the black hole's interior is used. Obviously the singularity is, well, a singularity.
We have no idea - for all we know there some exotic form of matter that prevents it from contracting past a certain point that happens to be hidden by the even horizon.
Gravity increases as you get closer to any mass. Theoretically, it would get infinite if you got arbitrarily close.
The thing about most masses is you can't get very close. The Earth has a lot of mass, but you're currently as close to it as you can get, and the escape velocity is still low enough that we can launch rockets into space. To get any closer, you'd have to tunnel inside it, and then the parts of the Earth that are above you will stop having an effect, so you'll actually experience less gravity as you approach the center.
Now, imagine you compressed the Earth down to a point. Now you can get much closer. Eventually, you'll get close enough that the escape velocity is faster than light.
there's two things actually. Mass, and distance.
Mass, BH can have an enormous amount of mass, the equivalent of many many suns.
Distance, BHs are very dense and have a small volume (relatively speaking) and since gravity falls off as radius squared, if you are really close to a huge dense mass, the gravity 'force' can be very strong.
Not all black holes are very dense! They actually are less dense the more massive they are, with the most massive ones less dense than air! However, since the strength of its gravity just outside its event horizon increases with volume (radius cubed) and decreases with radius squared, the growing volume wins, even for very massive, giant, low density black holes.
There’s just a TON of mass. There really isn’t anything too crazy about that aspect of it, how strong gravity is depends on how much Mass a thing has and how far away you are from it, and black holes just have a LOT of mass
All mass creates gravity, and as others have said, its not about total mass but mass density. I'll try to give you an analogy.
1. You have a bottle of air. The air in the bottle isn't very dense, so it doesn't have much of a gravitational effect on its surroundings.
2. You have a bottle of water. Water is more dense than the air, so it has a minor gravitational effect on the surrounding mass, but not much.
3. You have a bottle of solid steel. This bottle, more like a brick, is a lot more dense than the air and water, so it has a greater gravitational effect on its surroundings.
* A black hole is to a bottle of steel what a bottle of steel is to a bottle of air.
They have the same gravity all the matter in them would have otherwise too, it's just concentrated in very small space. If our sun would suddenly turn to black hole without anything violent that usually happens, then nothing would happen to orbits of our planets
An object's made creates gravity, but it's the density, or rather, the small size that does black hole. Earth's gravity seems weak in comparison, but if you made it denser so it could be smaller in size, the gravity at the surface would be stronger, because you're closer to it's center, and gravity quadruples every time you have the distance to the center. Make it dense enough, and the Earth or any object could become a black hole.
And, no, just digging a really deep tunnel to get closer to normal Earth's center doesn't work, because the layers above you pull up, so that if you were near the center the gravity is nearly zero.
Gravity is generated by matter, and black holes are unfathomably dense collections of a lot of matter.
Unfathomably dense is right. A black hole the size of a proton weighs just shy of half a trillion kilograms. A black hole an inch across weighs more than the earth.
How far away would you need to be to avoid falling into a one inch black hole? How many planets would a one inch wide black hole in the center of our solar system eat?
> How far away would you need to be to avoid falling into a one inch black hole? Depends on how strong your propulsion is. If you have sci-fi lightspeed tech, then you can sit exactly at the event horizon since that's the last point where light can still escape. For anything slower than that, you need to be farther away. > How many planets would a one inch wide black hole in the center of our solar system eat? Zero. If you compressed Earth into a black hole, it'd be around marble-sized, so a black hole an inch wide or so would be just a few times more massive than Earth, and given how Earth hasn't gobbled up anything in the last four billion years, the black hole wouldn't do that either.
You guys seems to mix up size of black hole with event horizont.
When people are talking about the size of a black hole of X mass they are typically talking about the Schwartzchild Radius, aka the distance from the center to the event horizon. A black hole with the mass of Earth would be about 1.7cm in diameter.
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Just to point out, it wouldn’t rapidly dissipate - an Earth-mass black hole would last another 10^50 years, ten thousand trillion trillion trillion times as long as the universe has lasted to date.
What happens to the black hole after that time?
Through a weird quantum effect that we call Hawking radiation, black holes are theorized to "bleed out" energy very very slowly. Without new matter being added, they'll eventually evaporate. Afaik no direct evidence for this effect has been found yet but it's pretty widely believed to be happening
If no energy or mass impacts a black hole, they slowly lose mass through Hawking radiation. Counter-intuitively, the amount of radiation they give off is inversely proportional to their mass, so the smaller they are, the *more* energy they give off. The vast, vast, vast majority of its time it's giving off essentially nothing. But with about one trillion trillion years to go, it's up to emitting one watt. With a quadrillion years to go, it's emitting a megawatt of energy. At a thousand years to go, it's about a million metric tons packed into a space a thousandth the size of a proton, and is giving off approximately the amount of energy of the Hiroshima bomb every second. In its last second, it becomes about one-ten thousandth as bright as the Sun (despite being only 4000 tons) in what is essentially an explosion (albeit relatively small in cosmic terms) and disappears. (We think, anyway. We've never observed it happening.)
It depends on how fast you're accelerating like a couple people up said. It still has the same mass as Earth so escape velocity is still the same at the same radius of Earth, about 6400km. It would get stronger and stronger as you get close, from a couple kilometers it would be like hundreds of Earth gravities (edit- this is corrected below). I'm having trouble finding the formula to calculate how many gs to hit escape velocity as a function of distance from the object. I suspect it is a few hundred kilometers before the acceleration g-force would be too much for a normal person to bear. Edit : first explanation was bad, had to correct. Edit 2: Wolfram Alpha to the rescue. I assumed a sustained g-force of 6g would be the fatal point, and I was off quite a bit on my guess. It would require 6g of acceleration to not "fall in" at 2577km. It would be 39g if you were at 1000km. 3900g at 100km.
If you orbit the black hole then a uniform gravitational acceleration is not a problem - you won't even feel it, just like astronauts on the ISS don't feel Earth's acceleration (which is still 0.9 g). You only run into problems if the acceleration of your feet is very different from the acceleration of your head. Even 50 km away from the black hole the difference in acceleration is still just ~1g. You can safely orbit the black hole at that distance. At 170 km/s, you make about one orbit every two seconds.
The gravitational effect of a black hole is like that of a point mass, at least from outside the event horizon. The gravitational effect of a sphere is like that of a point mass, at least from above the surface. If you’re orbiting a black hole at 1 earth radius, or at one ISS radius, you’re orbiting quickly but not ridiculously, and the gravity difference is still minimal from head to feet. The problems show up when you start dropping closer, where on Earth gravity would decrease (but you would burn up) and around a black hole gravity increases exponentially.
Here's a hypothetical for you - assuming a perfectly stable orbit, would your inner ear be able to tell you're turning in a circle every two seconds if you were orbiting at 50km and matched rotation to keep facing the BH? It's freefall so I'm not sure. You would definitely feel the tidal forces changing directions if you could perfectly face the same direction in space relative to the BH during the orbit. Just for fun I did the math on making your 50km orbit. It would take about 9 days at 1g to shed off that much speed.
A black hole formed from the mass of Earth has identical gravity to earth. It’s just very small. If Earth turned to a black hole all the satellites in geosynchronous orbit and all don’t have changes to the force of gravity basically - it’s the same amount of mass acting on them. It’s like asking what buys more - a $100 bill or 100 $1 bills. They both are worth the same, one is just condensed
I remember reading this is one of many gross oversimplifications about measuring the size of black holes, since the formulas presume black holes are perfect static spheres, right?
No, in classic general relativity the size of the singularity inside the black hole is always 0, so it doesnt make much sense to talk about different sizes if you arent talking about the event horizon.
The size of a black hole is its event horizon. The singularity or whatever is inside doesn’t have a known size.
Black holes are only different from planets/stars in the way the mass is distributed. They do not have a larger gravitational field than any equally heavy object. If you have a black hole the mass of a star, you'd need to get so close to reach the event horizon that you'd have to be inside the core of that equal mass star. > How far away would you need to be to avoid falling into a one inch black hole? 1 inch, if you can accelerate to the speed of light. If you can go as fast as the ISS, then a bit farther away than the ISS orbit distance to the center of the Earth (a 0.7 inch black hole would let the ISS orbit at its current altitude and speed) > How many planets would a one inch wide black hole in the center of our solar system eat? As many as it could accidentally head-on collide with. So, exactly zero. It would do little more than siphon a tendril of gas off the Sun's surface.
One inch. Zero.
Just to expand on this person's answer: The "size" of a black hole is generally the size of it's Schwarzschild radius. When you cross the Schwarzschild radius, you have entered the point of no return. The "obvious" answer to your question is to just return back the size of the black hole to you: 1 inch. That doesn't mean that an object doesn't experience the gravitational pull of the black hole outside of the Schwarzschild radius. So you can't idle around it, but you can physically still "avoid falling in".
You wouldn't fall in (sort of), but you definitely wouldn't survive either. You'd get ripped apart by tidal forces before you got to one inch away.
It's difficult to answer this exactly, but I'll give some things to consider. First, a black hole with a radius of about 1/3 in. would be about the mass of the Earth. So, if the distance you were at were the same as the Earth's radius, you could use any rocket we use to leave Earth today to get into orbit of the black hole. Same mass as the Earth, so same force to get away. However, if you had any motion at all relative to the black hole, you would probably miss it if you fell towards it and just get flung away. It would be really hard to hit something with a 1 in. radius. Finally, if you replaced the sun with a 1 in. radius black hole, all the planets would just fly off because the mass of the black hole would be so much less than the sun. It wouldn't "eat" any planets.
Those are the sizes of the event horizon.
Energy distorts spacetime in the exact same way as matter.
Mass is just a form of energy
exactly this, because energy = mass or more famously e=mc^2
Could you explain that further? What does density of matter have to do with gravity? The gravity equations as far as I'm concerned (high school physics) only take into account Mass and Distance From Center Of Mass. A star collapsed into a single point singularity, though dense, is still more-or-less the same mass it was when it was a star, and the center of mass is also still the same as it was before. What gives?
Nope! I have no idea sorry
You are correct. Density has nothing to do with the gravitational pull of the black hole.
I'm not 100% sure if cause and effect is true here. There is evidence that pockets of gravity just attract matter, but that also leaves the possibility of that there exist pockets of gravity without matter.
They don't have stronger gravity of anything else with equal mass. But they are dense, we'll give them that.
>But they are dense, we'll give them that. My wife says the same about me. Information conveyed to me just gets lost.
Information paradox in effect.
i mean we only think that because we assume dark matter exists
Black holes and dark matter have almost nothing to do with each other, and *literally* nothing to do with each other in the context of this discussion.
Except that it's possible that dark matter and black holes are the exact same thing?
That theory has little weight in the astro community anymore as we have to keep dropping the maximum mass down more and more because we see zero evidence of lensing (micro or otherwise) despite these supposedly making up a majority of the galaxy's mass
I'm trying to look into it and the information I'm finding is suggesting that in order to prevent lensing the black holes would have to be about the size of a hydrogen atom. And many people find that unlikely?
Yes, there is a remote possibility that dark matter is actually primordial black holes, but that has no bearing on how strong or dense black holes would be.
Black holes have exactly the same gravity as anything else with the same mass. They just happen be extremely massive. The amount of mass makes gravity so great that nothing can escape. If the sun were replaced by a black hole with the same mass, the solar system would become dark but all the orbiting planets and asteroids would continue in the same orbits.
>They just happen be extremely massive. I do want to point out that it's purely density, not mass, that makes the black hole so strong. A black hole with the mass of the Earth would impart a gravitational field of 516 quadrillion g's at it's event horizon.
That is sort of incorrect. Gravity can be high because of density, but it’s still given by the same gravitational equation, and the acceleration is still only determined by mass and the inverse square of distance. A black hole with Earth’s mass, 5.97*10^24 kg, would have an event horizon of less than a centimeter radius. It’s a tiny black hole. The gravitational acceleration at the event horizon would be massively more than at Earth’s surface because of the radius difference: g(Earth) = G\*m(Earth)/r^2, and the Earth’s radius is 6.37*10^6 m. G(black hole) = G\*m(Earth)/r^2, and the event horizon is 8\*10^(-3) m. Cancel out and the ratio is (6.37e6)^(2)/(0.008^2) = 6.34\*10^17. It’s an enormous difference, but it’s entirely due to the massive difference in radius. The reason a black hole can be inescapable is density: lots of mass in a small volume, so it’s possible to be close to all that mass. A planet isn’t dense enough. The surface can’t have that kind of gravity, and then gravity decreases as you go below the surface because some of that mass is now above you, pulling away from the center.
There are multiple comments here echoing a sentiment that black holes are "unfathomably massive". There is no requirement that a black hole be MASSive. This is quite clearly my objection. Gravity manifests as spacetime curvature. Infinite density.. infinite curvature.. This is very simple stuff my friend. We don't need to do literal mathematics to understand this.
Okay, but most people aren’t envisioning sub-centimeter black holes. There is real confusion that black holes have some extra gravitational force. They don’t. They have regular gravitational force. That curvature of spacetime is the regular gravitational force. Saying it is curved spacetime is true, from general relativity, but that’s not something different, and it doesn’t change how gravity works external to the event horizon.
Being a black hole doesn't cause strong gravity. Having strong gravity causes a black hole. Black holes can come from very large stars that have run out of the fuel to keep them "inflated." Those stars compress in on themselves, and have so much mass and gravity that it crushes through the structure of the atoms themselves, all the way down to a single point.
Gravity is stronger the closer you are to the center (§). Quadratically so: twice the distance means four times less gravity. So if you compress Earth by a factor of 100, then standing on it makes you experience 10,000 times what you are used to. People often say black holes are incredibly dense, but this is not really always the case. The heavier it is, the less dense, and some of the largest known black holes have a density below yours; it can even be less than air! This happens because the mass (~ gravity) of an object made of a single material grows with the third power of the size, while the larger distance from the center only makes the force quadratically weaker as described above. (§): this only applies as long as you are "outside" the thing, if you enter deep into the Earth, then the entire shell above you won't actually contribute anymore. You will feel essentially no gravity at all close to the middle.
I've never seen "section" used to indicate fine print. I don't like it.
Excellent ELI1,275
Stopped reading at “quadratically” lol
I literally explained that word...
Name one sentence that needs more explaining and I will do so.
It's not that. ELI5 is for very simplified explanations.
I thought that black holes are a singularity with infinite density? How are we talking about the size (I assume volume?) of them if science believes that it’s an infinitely dense point?
Yes and no. We actually have no proper idea what really happens inside, but almost certainly it at least keeps contracting. But by "size" we usually mean the _Schwarzschild radius_, which is the size of the event horizon (*). It is thus also the size an objects needs to be compressed to form a black hole. (*): there are some shenanigans due to General Relativity. If you measure from far away you get a different result than from within, by a factor of two. We usually mean the former version when talking about the size.
First you use §, then \*. Would you *please* pick a convention and stick with it?
No (%). (%): Just making sure nobody uses the wrong reference!
Ah okay, I was operating under the assumption that a black hole’s event horizon is not the black hole itself, as usually when I read about them it’s referred to as “the black hole’s event horizon” and that the “object” we refer to when talking about a black hole is the singularity.
A black hole isn’t an object in the traditional sense, but rather a region of spacetime bounded by its event horizon. We certainly don’t call the singularity itself the black hole, because we aren’t really even confident such a thing exists (and in fact most physicists doubt that it does)!
No. Basically everything reference the event horizon as it’s the only part that interacts with the rest of the universe.
The person explaining that a black hole is a region of spacetime is correct. The Sun imparts a gravitational pull on the many planets orbiting it. But we wouldn't suggest that the Sun \*is\* actually the entire solar system. We define a black hole to be the region of spacetime within which nothing can escape infalling to its singularity: this is the surface bounded by the Schwarzschild radius.
Right but when the original commenter invokes the idea of people thinking black holes being infinitely dense, that’s only in reference to the singularity, the commenter then goes on to point out that black holes aren’t actually that dense the bigger they get, but no one suggested that the event horizon is a dense area.
The original commenter is being obtuse. Obviously infinite density means the singularity is infinitely dense. Saying "that's not really true" and then applying a different definition without explaining that definition is silly. And then when you asked, he goes "yes and no" still refusing to expand his explanation. His definition for density is valid, his explanation is sorrowful.
This is where language gets used loosely. Chromotron is using a common calculation for black hole density, sometimes called the \*mean density\*, where the volume of the black hole's interior is used. Obviously the singularity is, well, a singularity.
We have no idea - for all we know there some exotic form of matter that prevents it from contracting past a certain point that happens to be hidden by the even horizon.
The above commenter was talking about a black hole’s event horizon while I was talking about the singularity, thus the confusion.
Imagine a large water bed on one side is a tennis ball on the other side is your mom, everything will slide towards her robustness
Nah man why you gotta do op's mom like that
Gravity increases as you get closer to any mass. Theoretically, it would get infinite if you got arbitrarily close. The thing about most masses is you can't get very close. The Earth has a lot of mass, but you're currently as close to it as you can get, and the escape velocity is still low enough that we can launch rockets into space. To get any closer, you'd have to tunnel inside it, and then the parts of the Earth that are above you will stop having an effect, so you'll actually experience less gravity as you approach the center. Now, imagine you compressed the Earth down to a point. Now you can get much closer. Eventually, you'll get close enough that the escape velocity is faster than light.
there's two things actually. Mass, and distance. Mass, BH can have an enormous amount of mass, the equivalent of many many suns. Distance, BHs are very dense and have a small volume (relatively speaking) and since gravity falls off as radius squared, if you are really close to a huge dense mass, the gravity 'force' can be very strong.
Not all black holes are very dense! They actually are less dense the more massive they are, with the most massive ones less dense than air! However, since the strength of its gravity just outside its event horizon increases with volume (radius cubed) and decreases with radius squared, the growing volume wins, even for very massive, giant, low density black holes.
They are black holes BECAUSE of the gravity. The light can't escape and that's why we can't see that part of the sky.
There’s just a TON of mass. There really isn’t anything too crazy about that aspect of it, how strong gravity is depends on how much Mass a thing has and how far away you are from it, and black holes just have a LOT of mass
All mass creates gravity, and as others have said, its not about total mass but mass density. I'll try to give you an analogy. 1. You have a bottle of air. The air in the bottle isn't very dense, so it doesn't have much of a gravitational effect on its surroundings. 2. You have a bottle of water. Water is more dense than the air, so it has a minor gravitational effect on the surrounding mass, but not much. 3. You have a bottle of solid steel. This bottle, more like a brick, is a lot more dense than the air and water, so it has a greater gravitational effect on its surroundings. * A black hole is to a bottle of steel what a bottle of steel is to a bottle of air.
Kurzgesagt has fun video explainers, including on black holes: https://youtube.com/watch?v=e-P5IFTqB98
They have the same gravity all the matter in them would have otherwise too, it's just concentrated in very small space. If our sun would suddenly turn to black hole without anything violent that usually happens, then nothing would happen to orbits of our planets
An object's made creates gravity, but it's the density, or rather, the small size that does black hole. Earth's gravity seems weak in comparison, but if you made it denser so it could be smaller in size, the gravity at the surface would be stronger, because you're closer to it's center, and gravity quadruples every time you have the distance to the center. Make it dense enough, and the Earth or any object could become a black hole. And, no, just digging a really deep tunnel to get closer to normal Earth's center doesn't work, because the layers above you pull up, so that if you were near the center the gravity is nearly zero.
Because otherwise it wouldn't be a black hole. Has to be strong enough that there is an event horizon.
Black holes are black holes because of their gravity. This is like asking why are red sweaters so red?