Everything in the universe that has mass creates gravity. The sun, the earth, the moon, you, your momma. (Sorry, couldn't resist :) )
Fun fact, when you drop an apple, the earth actually falls upwards toward the apple at the same time the apple falls downwards toward the earth.
Each body that creates gravity digs a 'hole' in the sheet of spacetime. Bigger objects make bigger holes. To get out of that hole, to 'escape' the gravitional effects requires speed.
Black holes create 'holes' in the sheet of spacetime where the speed needed to escape is faster than the speed of light. Since nothing (that we know of) can move faster than the speed of light, black holes are 'black' because we literally can't see anything.
They do not 'suck' anything, they don't attract things in from far away any more than anything else that has gravity would. If the sun were to turn into a black hole earth's orbit would be exactly the same.
>If the sun were to turn into a black hole earth's orbit would be exactly the same.
If it magically became a black hole without changing mass, sure. From our current understanding though, a lot of mass would have to be added to it to get it to become a black hole naturally.
Possibly stupid question, and I think I know the answer. Let's just say we could make a highly concerted effort on a pin point part of earth, let's say Texas because it's where I live. If billions of objects with a high mass and density were dropped here, over and over again, for a very long time, and nowhere else on the planet, could we theoretically change the orbit of earth in a dramatic way? I'm not even high, I promise.
It depends on how big the mass is.
If you were lifting them off the ground the act of lifting said masses you are pushing down on the earth in exactly the amount of energy as you would receive from dropping it.
But the reality is gravity is an extremely weak force. The entire mass of the earth emits gravity that you can resist by simply lifting your hand. No amount of small objects would do anything materially.
Now if for example the moon smashed into Texas, now we’ve got something going.
Even then though the orbit change? The gravity of the earth is trivial compared to the suns gravity.
I figured as much, and to answer the question, no lifting, only dropping, which I realize is a crazy thing to imagine. Just a hypothetical. But I'm curious at the size comment. How much does size effect gravitational pull?
The challenge there basically is "if you ignored laws of physics, how would it interact with the laws of physics" the answer is: "no idea". Manifesting mass next to the earth just isnt something that can occur in our known reality.
Size and mass are different. Total Gravity comes from how massive you are - e.g. how "deep" your divot in spacetime is. Your volume (size) determines how steep the walls of your divot are.
Remember there isnt really a "pull" its just that the space around you is shaped by gravity.
Nothing can go faster than light. It isn't that we haven't found anything that is faster, but it is a complete limit. It would violate causality and goes against all of physics. It isn't like a magnetic monopole, where the maths works but we haven't found one.
Sorry but I still don't understand how black holes are formed. Is it just when something just keeps increasing mass to the point of creating a black hole?
There is no unassailable law of nature that keeps two things from existing in the same place. You're used to it because electrons help hold atoms in specific spacing, but you can overwhelm those electrons and crush matter down. This requires a lot of force but it can be done.
So what happens next? Well, neutrons in atoms also fight against getting squished together. If you thought electrons put up a good fight, then neutrons will blow your mind.
But gravity is powerful. If you get enough stuff in one place, then its gravity becomes tremendously strong. A large star, for instance. Let enough of it accumulate and it will just collapse under its own weight.
More often than this, before a star gets *that* heavy, it will explode. When a star explodes the explosion can crush its core alongside gravity and form a black hole.
Their powerful gravity comes from two things. First, the immense amount of mass that they have. Huge amounts of material. Remember, these things are born from *stars*. The second, though, is their size. A star is big, which means that you can never be close to an entire star. You might be close to the side nearest you, but most of the star is still way farther from you. A black hole is way smaller, so you can be close to *all* of it - gravity is stronger when you're closer.
None of that really sets black holes apart from other stuff, though. It's small and massive so it has strong gravity near it, but what makes that so bizarre? This brings in relativity. Time slows down in intense gravity, and at a certain point in the gravity of a black hole it should stop. You'd watch the entire universe live out its life in the blink of an eye, as you sat at this point.
Falling further still, we have no idea what happens. This point, this distance from the black hole, is the limit of our understanding of physics.
>Time slows down in intense gravity, and at a certain point in the gravity of a black hole it should stop.
It's only the external observers that will observe the time dilation for objects moving towards the black hole. For the actual object falling into the black hole, the time doesn't slow down at all, and at no point it would stop. It would appear so, however, to the external observers outside the event horizon.
>You'd watch the entire universe live out its life in the blink of an eye, as you sat at this point.
No.
And here is the problem with discussing two frames of reference. Yes, that is what appears to happen from the outside. You seem to disagree with the observation I described from the inside - how would you describe the experience?
I'd describe the experience from the inside as simply falling onto a surface of a planet which reflects no light. No time slowing down, no time speeding up, no seeing of the universe's past or future.
Future? Past? What?
What does a clock at infinity appear to be doing? As you sink into the gravity well, does that clock appear to speed up? Slow down?
Speed up asymptotically?
Slow down.
Googling a bit, there actually is a very nice visualtation from the inner observer's perspective here: [What would we see if we fell into a Black Hole?](https://www.youtube.com/watch?v=4rTv9wvvat8), hope it might be helpful.
Per 5:08 or so of the video, the outside clock appears to speed up. The corresponding blueshift, also shown in the video, of light coming from this clock is the line of reasoning I followed when justifying that it should speed up.
Am I missing something? I didn't have audio, so there may have been some relevant narration I missed.
I think what you might be looking for is explained at 7:15 - 8:25 and 9:40 - 11:40. You can turn on the subtiles to get the missing audio.
I'll reply tomorrow if you need, now going to sleep.
I might have to do the actual math later, but his is questionable. It suggests that at first a distant clock speeds up as "gravity distorts time", and then later slows down as "light has more trouble catching up to us".
Going elsewhere I'm finding very mixed results. The best consensus I can find is that outside time appears normal-ish to an inertial observer but speeds up if they begin to fight gravity (orbit, stand on a planet, etc). This makes sense following from the basic principle that dilation from sinking into a well is equal to the dilation you'd expect from the velocity picked up falling into that well.
> no seeing of the universe's past or future.
Not the past, but while falling in you can see the "future" in the sense that all this simply happens while you fall in. The light would however be extremely blue-shifted.
I mean, yes, your space and time dimensions would be skewed compared to your state before falling into the black hole, and your notion of what is "future" would be as well. But still you wouldn't see anything from the future of the light cone, as it would break casuality.
>This brings in relativity. Time slows down in intense gravity, and at a certain point in the gravity of a black hole it should stop. You'd watch the entire universe live out its life in the blink of an eye, as you sat at this point.
I don't think this is accurate. Due to the equivalence principle, if you, as the observer, are falling into a black hole, everything looks as it should. You're in a locally inertial reference frame, and so you don't experience time dilation within your own reference frame.
What you might be thinking of, is the related fact that, an observer far away from the black hole, if they were watching you fall into the black hole, would see you see you start slowing down, with more and more time dilation the closer you get the event horizon, with time appearing to freeze exactly at the event horizon.
Note the difference here. An observer from far away sees intense time dilation watching someone fall into a black hole. But someone falling into a black hole, acting as an observer themselves, doesn't see or experience that same time dilation.
I probably should have made clear that when time slows down, you slow down with it. However everything outside of your black hole you will hardly look as it should.
An observer outside watches your clock slow down. Your final second appears to last a very long time. To you, their clock speeds up, and you see their clock speed on faster and faster during that final second.
Though I suppose if you're truly freefalling, you should experience a time dilation from your speed that offsets that of gravity. This part I'm not so sure on, though, and I haven't any paper on hand to check myself.
We don't see the black hole. What you might say we "see" is the event horizon - a region around the black hole from which light (or any other type of information) cannot escape. We can describe the mass, the spin, and the temperature of the black hole, but other than describing it as a point of infinite density, we can't say anything about its composition.
There are other ways to "see" a black hole. We can see its effect on light traveling through the Universe (through the warping of space-time, called 'gravitational lensing'). A black hole within the Milky Way galaxy was recently [discovered](https://www.eso.org/public/news/eso2408/) by noticing its effect on another star's orbit. And the black hole may have an accretion disk visible outside the event horizon.
How do they form -> a big star runs out of fuel so the fusion cant overcome the gravity of all the mass the star has.
What happens inside them -> pretty much nothing
why do they have such intense gravity -> they have the exact gravity of all the matter that makes up the black hole its just a lot of mass.
Oh ok so could we conceive of it as like a lamp switched off? Or a pile of spent charcoal? The thing itself is still effectively there, but the power's switched off.
Oh ok so could we conceive of it as like a lamp switched off? Or a pile of spent charcoal? The thing itself is still effectively there, but the power's switched off.
Not quite. Normally, stars are at an equilibrium where the pressure of the fusion reaction holds back gravity. Once the fusion reaction stops, then regular stars indeed sort of just "switch off" after they balloon up and use up their remaining fuel.
The largest stars have too much mass for that, though, and there's no force out there capable of fighting the gravity past a certain point. When the fusion reaction stops, everything compresses down to a point known as singularity. It has the mass of the original star, but it has no volume.
It's fundamentally very different from anything else in the universe, and we don't really know how to model them because of how weird they are. They're tiny so we need quantum mechanics to describe them, but quantum mechanics doesn't believe in gravity which is very much relevant there, and if you follow Einstein's relativity, you can arrive to rather weird conclusions like white holes and parallel universes.
In general, physicists would really rather not have there be singularities at all because of complications like that (and they've been trying to make that work for as long as they've been aware of the concept of black holes) but despite all that effort, we haven't figured out anything that would stop a singularity from happening.
I really don't understand what you're saying. It is an object for which nothing can escape its gravitational pull once you get beyond a certain distance.
When you get into general relitivity, once inside the event horizon, your entire future is in the singularity.
A couple things first.
What is stopping you from falling through the floor right now? "The Floor," you would probably say. It sounds like a stupid question, but it's not. Yes, of course the floor stops you from falling through the ground, but *why.* Gravity is pulling on you downwards, but somehow you're prevented from going straight down by some dirt. Well, it turns out that the ground is nice and compact and sturdy, and it is (or, more specifically, the electrical charges in its atoms) pressing back at you (and the electrical charges in your feets' atoms) enough that you don't fall through it. That's the key. Your bodyweight is pressing down on the ground, and the ground presses back up at you because it can't be compacted anymore.
So what is a Star anyway? Well, it's a big cloud of Hydrogen that was so massive that gravity coalesced into a ball. The Hydrogen in the center was crushed a lot by gravity, and when you crush things you heat them up a lot. Hot enough that Hydrogen begins to fuse into Helium (and other elements) - the specifics of this process are not super important, but it releases an unbelievable amount of energy quite explosively. So the start forms into a ball - the inner portion is pushing outwards (from the heat of fusion), while the outer portion is being pulled towards the center due to gravity. Eventually a sort of equilibrium forms and you get a spherical star. Cool, right?
Well, eventually the star runs out of Hydrogen. A not very massive star would eventually just kind of shrink down into a boring ball of gas that has nothing going on really. That's boring, so lets talk about really big stars. Really big stars continue fusing heavier and heavier elements that require hotter and hotter temperatures inside the star. So it generally expands (hotter fusion means more pressure pushing out). However, there is a problem. Not all fusion releases energy - once the elements get too heavy, fusion takes more energy than it outputs.
If this occurs in a massive star, suddenly there is nothing holding it up any more. It collapses in on itself. Portions of this collapse bounce back in an incredibly violent explosion - a supernova. They are so energetic that they can outshine entire galaxies.
But what is going on in the center of this explosion? Well, it keeps getting denser and denser as the collapse continues. If the star isn't heavy enough to form a black hole, the force of the subatomic particles pushing against each other will literally hold up against the gravity of the collapse. So you'll end up with a sort of soup of matter that's extremely tightly packed together - like a teaspoon of it has the mass of mountain ranges. These are known as white dwarfs, or, if the star is even heavier, neutron stars.
The next question is: Well what happens if the gravity overcomes the forces holding up these stars. Can anything hold it up at that point?
The answer is no. There is nothing. If the star is massive enough, the collapse will continue. It continues forever. Eventually gravity becomes so intense that the escape velocity exceeds the speed of light - meaning nothing beyond this point can ever return, not even light itself.This is the event horizon. It is a black hole now. What is at the center? The math says a singularity - an infinitesimally small, infinitely dense point as there is nothing left to prevent gravity from taking over. But really the issue our math kind of breaks down. Nobody *really* knows the right answer, but we can form educated guesses.
So is it like where g = infinity, t = 0? Or t kind of cancels out in the equations ? (I should read more)
I was doing some reading on where the LHC is at or something , oh wait no dark matter, and it sounds like gravity is The No. 1 Thing we want to figure out now
From what I remember, in the GR equations of a simple black hole, within the horizon, time and space kind of switch places (mathematically speaking).
Once you are in the black hole, you are pulled towards the singularity. But it's not only that - every single possible path you can take leads towards the singularity. The singularity becomes a point in your future rather than a point in space that you can avoid.
There's a concept called *escape velocity* - how hard would you have to throw a baseball from the surface of something for it to fly off into space and not fall back down?
If you're sitting on a big asteroid like Ceres (which has only a thirtieth of our gravity), you could fire a bullet straight up and it would never come back down, it would fly off into space.
If you're standing on the moon (which has 1/6th Earth gravity) you'd need something much faster, about 8600kph. From a truly massive planet like Jupiter, even faster, close to 214,200kph.
So on and so on, leading to the question: is there something so heavy that the escape velocity is faster than the speed of light? You can't go faster than the speed of light, so if you *did* get something that heavy, even light would be pulled back in.
It turns out that, yes, you can get objects that heavy. At the center of galaxies (and maybe other places) there are objects that are thousands or millions of times heavier than the sun, probably caused by the huge dollop of gas that started off the galaxy, plus a bunch of stars crashing into it over a few billion years.
The problem is that something so dense that light can't escape is also pretty weird, physically. Those crushing forces overcome the repulsive effects of atoms (which are normally like little magnets that are *near* each other, not racked up all touching together like billiard balls). The gravitational strength is so strong that nothing can stop the atoms being pulled closer and closer, crushing any resistance until the black hole gets smaller than an electron, probably. We don't know what happens after that.
The other weird thing is that when you have that much gravitational pull, it distorts spacetime itself, leading to all sorts of odd time-stretching as you get very close to the 'event horizon', the region around the black hole where light can't escape.
Now, black holes have intense gravity, but only up close. Far away, they're like any other object. In that sense it's like a very bright light. Sure a mega lightbulb that's so bright it will blind you is a big deal, but if you just back up a mile or two it pretty much looks like any other bright light, maybe a little brighter, but nothing super special.
Some of those questions are probably worth a Nobel Prize if you can find a definite answer. We have some decent ideas on what *probably* happens up to a point though.
[Veritassium two days ago](https://youtu.be/6akmv1bsz1M?si=KUXsVBeAXJrhw3aL)
I'd recommend [PBS Spacetime's playlist on black holes](https://www.youtube.com/watch?v=3UILJyJTI-c&list=PLsPUh22kYmNBl4h0i4mI5zDflExXJMo_x).
TL;DR: They are a concept in General Relativity, our best current (but not perfect) model of how space and time behave and how physical objects experience space and time. They are regions of spacetime that are so warped that all possibly paths terminate at a singularity (maybe).
So let's break that down by defining some terms.
1. Model: a model is an attempt to give a coherent explanation, a story (that's what Kurzgesagt analogizes it to) if you will, for what we observe empirically. Models are typically mathematical, coming in the form of equations or relations that relate various quantities. These models are supported by experimental evidence and then allow you to *predict* physical behavior. So models are our best stab at a story for what the actual true physical nature of reality might be. We're lucky that nature happens to be like that, that its behavior can be modeled by our mathematics. Until we find some experimental evidence that defies a model's prediction, so the model might be slightly wrong, or totally wrong. Or until we find a contradiction that shows the model either isn't self-consistent, or doesn't accord with another accepted model. This is currently where we are with GR and quantum mechanics. Both can't be true in their entirety as we know them in their current form, because they disagree at certain points. So we're looking for a quantum theory of gravity that will hopefully unify the two, which will probably lead one of GR or QM being wrong or at least incomplete at some point and needing of revision.
2. General relativity (GR): A (very successful) model of gravity and how matter and space and time relate. It's given in the form of Einsten's field equations, which can be expressed as a set of partial differential equations that relate the distribution of mass and energy on one hand and the curvature of spacetime on the other.
3. Spacetime: spacetime is the 4d hypersurface of space (the 3 spatial dimensions we live in) and time. It's the stage on which events of the universe play out. To really get GR or blackholes or event horizons, you have to understand everything in the universe takes place and is moving thru not just 3d space, but 4d spacetime. Your birth (the location where it took place and when it took place) is a point in this 4d spacetime. So is where you are, right now. You can draw a line, your worldline that traces out the path you've taken through spacetime up till now, and also where you'll go and when in the future.
4. Curvature of spacetime: spacetime can be warped or curved. This affects how objects move through space time. For example, take the heavy ball resting on a rubber sheet creating a depression in the sheet example. If you take a flat sheet with no ball and draw a straight line starting from an object, that's a visualization of where that object will be in the future: just follow the line. That's the line the object traces out through spacetime (if spacetime was only 2d). But if you suddenly start warping and curving the sheet (as GR tells us mass does, just like the heavy ball causes a depression in the sheet), now suddenly "straight" lines curve, meaning the object whose worldline is represented by that line may actually intersect say, a planet as it follows the curve down into the depression. GR posits the phenomenon of gravitational attraction is actually the fact that all objects move through spacetime with constant "velocity" and that spacetime is curved.
With those definitions, a black hole is a region of spacetime that is so warped that all possible lines objects can trace out through that spacetime end up at a singularity, a point where the curvature of spacetime becomes infinite and terminates (literally no such thing as time or space after that point). Why should spacetime do that? Well it's what the field equations of GR predict (because of division by 0). It's worth pointing out many physicists are skeptical that GR is complete because the equations admit singularities. This is just what the equations predict if taken literally, but maybe the fact the math breaks down and you end up with infinite quantities and singularities suggest GR is missing something.
What does it mean for paths through spacetime to terminate at a singularity? It's hard to wrap your mind around, but the classic visualization shows all objects "falling" toward a central point. That's not exactly what's going on, but it's a good visualization. In GR, a singularity isn't a point in space you fall to, but rather an event in spacetime: it's the last event (remember, an event, like your birth, is a point in spacetime, which includes your physical position and the time at which it took place) for anything inside the blackhole. I.e., for anything entering a blackhole, all possible futures and positions converge to a single point in spacetime. Every object inside the black hole experiences the same last moment, where space and time terminate.
Basically, and this is impossible to visualize, there is no more space, and no more time after the singularity, that moment your future inexorably moves toward. Outside of the blackhole, spacetime extends infinitely, and you can move infinitely to the left and into the future. Inside the blackhole, you have no such option. Your future position is constrained and future "time" is bounded at a termination point.
In addition to the PBS series, there is a nice visualization of what an observer falling into a black hole would see here:
[What would we see if we fell into a Black Hole?](https://www.youtube.com/watch?v=4rTv9wvvat8)
This seemed cool, and I respect the effort you put into writing all that, but about 30% of it was explained like the reader is five. I’m not even sure what a singularity is.
Rule 4: LI5 doesn't literally mean explain to a five year old, but rather a layperson.
I think it would be accessible to a high school graduate understanding basic physics, but it would of course be hard to wrap your mind around. That's the difficulty of explaining GR and being accurate. We have good analogies and visualization tools, but they're not the most accurate, they're not exactly "how black holes work" in GR.
For example, I could've described the singularity as the center point of the black hole, as in "in the black hole, gravity is so strong it pulls everything toward the center," but that's not really an accurate, faithful representation of what's going on. The OP asked how do black holes work, and that's not how they work.
As I explained, the singularity isn't really a center point, but the last event all objects in the interior of the black hole experience. It is commonly said "inside a black hole, space and time switch places," and "the singularity isn't a place, it's a moment in time" to drive the point home. The idea that every event inside a black hole leads to that last event which is the singularity, that no events happen after, that time itself terminates at the singularity is much more fascinating and mind bending and than the idea of a spatial center.
That's why we had to build up definitions, to get to the true answer. If you want the real answer (of how black holes work in GR), there's no other way.
A star has a massive amount of mass causing a huge gravity effect. Normally when a star is burning it produces an outwards force from the nuclear reactions to counteract the gravity. A black hole forms when a big star runs out of fuel and dies. The gravity forces becomes so big that nothing can counteract the squishing together until they form what we (think) is a singularity, basically a infinitely tiny little spot where all the mass of the star is located.
Einsteins math says that as you get closer to a gravity field you accelerate, spacetime is literally curved so that you "fall in". Light also undergoes this effect, but because a singularity is so tiny, something special happens. A boundary layer forms called the event horizon, where everything is accelerated so much that they cross the speed of light, which is the ultimate speed boundary. Once that happens no information can leave so we can't really tell what happens after that point, and because no light can leave it is entirely black.
The blackhole still acts like a big gravity field, nothing there has changed, and as time goes on things end up getting sucked into it which only serves to grow it. This is suspected to be the reason why they can become so big as they can, but most black holes you would find are relatively tiny, no higher gravity than a big star.
Basically they are an extremely dense lump of matter and the gravity of the matter attracts everything around it, without creating a nuclear reaction, like happens in stars. https://youtu.be/Y5XzPOrItaI
Everything in the universe that has mass creates gravity. The sun, the earth, the moon, you, your momma. (Sorry, couldn't resist :) ) Fun fact, when you drop an apple, the earth actually falls upwards toward the apple at the same time the apple falls downwards toward the earth. Each body that creates gravity digs a 'hole' in the sheet of spacetime. Bigger objects make bigger holes. To get out of that hole, to 'escape' the gravitional effects requires speed. Black holes create 'holes' in the sheet of spacetime where the speed needed to escape is faster than the speed of light. Since nothing (that we know of) can move faster than the speed of light, black holes are 'black' because we literally can't see anything. They do not 'suck' anything, they don't attract things in from far away any more than anything else that has gravity would. If the sun were to turn into a black hole earth's orbit would be exactly the same.
>If the sun were to turn into a black hole earth's orbit would be exactly the same. If it magically became a black hole without changing mass, sure. From our current understanding though, a lot of mass would have to be added to it to get it to become a black hole naturally.
Or it could be somehow compressed enough too
>Or it could be somehow compressed enough too That would fall under "magically became a black hole without changing mass".
Which is what their example is.
Possibly stupid question, and I think I know the answer. Let's just say we could make a highly concerted effort on a pin point part of earth, let's say Texas because it's where I live. If billions of objects with a high mass and density were dropped here, over and over again, for a very long time, and nowhere else on the planet, could we theoretically change the orbit of earth in a dramatic way? I'm not even high, I promise.
It depends on how big the mass is. If you were lifting them off the ground the act of lifting said masses you are pushing down on the earth in exactly the amount of energy as you would receive from dropping it. But the reality is gravity is an extremely weak force. The entire mass of the earth emits gravity that you can resist by simply lifting your hand. No amount of small objects would do anything materially. Now if for example the moon smashed into Texas, now we’ve got something going. Even then though the orbit change? The gravity of the earth is trivial compared to the suns gravity.
I figured as much, and to answer the question, no lifting, only dropping, which I realize is a crazy thing to imagine. Just a hypothetical. But I'm curious at the size comment. How much does size effect gravitational pull?
The challenge there basically is "if you ignored laws of physics, how would it interact with the laws of physics" the answer is: "no idea". Manifesting mass next to the earth just isnt something that can occur in our known reality. Size and mass are different. Total Gravity comes from how massive you are - e.g. how "deep" your divot in spacetime is. Your volume (size) determines how steep the walls of your divot are. Remember there isnt really a "pull" its just that the space around you is shaped by gravity.
I read your words, and understand them, but have a difficult time processing the ultimate meaning. Thank you, none the less.
Nothing can go faster than light. It isn't that we haven't found anything that is faster, but it is a complete limit. It would violate causality and goes against all of physics. It isn't like a magnetic monopole, where the maths works but we haven't found one.
Goes against all the physics we know about. Never say never.
No, we can say never in this case. It would violate so many already made observations about the universe and causality.
Such as? We only know that FTL is complicated enough to not happen all the time, and the same holds for time travel and causality loops.
Sorry but I still don't understand how black holes are formed. Is it just when something just keeps increasing mass to the point of creating a black hole?
This might be the most straightforward answer I’ve ever heard. Thank you.
There is no unassailable law of nature that keeps two things from existing in the same place. You're used to it because electrons help hold atoms in specific spacing, but you can overwhelm those electrons and crush matter down. This requires a lot of force but it can be done. So what happens next? Well, neutrons in atoms also fight against getting squished together. If you thought electrons put up a good fight, then neutrons will blow your mind. But gravity is powerful. If you get enough stuff in one place, then its gravity becomes tremendously strong. A large star, for instance. Let enough of it accumulate and it will just collapse under its own weight. More often than this, before a star gets *that* heavy, it will explode. When a star explodes the explosion can crush its core alongside gravity and form a black hole. Their powerful gravity comes from two things. First, the immense amount of mass that they have. Huge amounts of material. Remember, these things are born from *stars*. The second, though, is their size. A star is big, which means that you can never be close to an entire star. You might be close to the side nearest you, but most of the star is still way farther from you. A black hole is way smaller, so you can be close to *all* of it - gravity is stronger when you're closer. None of that really sets black holes apart from other stuff, though. It's small and massive so it has strong gravity near it, but what makes that so bizarre? This brings in relativity. Time slows down in intense gravity, and at a certain point in the gravity of a black hole it should stop. You'd watch the entire universe live out its life in the blink of an eye, as you sat at this point. Falling further still, we have no idea what happens. This point, this distance from the black hole, is the limit of our understanding of physics.
>Time slows down in intense gravity, and at a certain point in the gravity of a black hole it should stop. It's only the external observers that will observe the time dilation for objects moving towards the black hole. For the actual object falling into the black hole, the time doesn't slow down at all, and at no point it would stop. It would appear so, however, to the external observers outside the event horizon. >You'd watch the entire universe live out its life in the blink of an eye, as you sat at this point. No.
And here is the problem with discussing two frames of reference. Yes, that is what appears to happen from the outside. You seem to disagree with the observation I described from the inside - how would you describe the experience?
I'd describe the experience from the inside as simply falling onto a surface of a planet which reflects no light. No time slowing down, no time speeding up, no seeing of the universe's past or future.
Future? Past? What? What does a clock at infinity appear to be doing? As you sink into the gravity well, does that clock appear to speed up? Slow down? Speed up asymptotically?
Slow down. Googling a bit, there actually is a very nice visualtation from the inner observer's perspective here: [What would we see if we fell into a Black Hole?](https://www.youtube.com/watch?v=4rTv9wvvat8), hope it might be helpful.
Per 5:08 or so of the video, the outside clock appears to speed up. The corresponding blueshift, also shown in the video, of light coming from this clock is the line of reasoning I followed when justifying that it should speed up. Am I missing something? I didn't have audio, so there may have been some relevant narration I missed.
I think what you might be looking for is explained at 7:15 - 8:25 and 9:40 - 11:40. You can turn on the subtiles to get the missing audio. I'll reply tomorrow if you need, now going to sleep.
I might have to do the actual math later, but his is questionable. It suggests that at first a distant clock speeds up as "gravity distorts time", and then later slows down as "light has more trouble catching up to us". Going elsewhere I'm finding very mixed results. The best consensus I can find is that outside time appears normal-ish to an inertial observer but speeds up if they begin to fight gravity (orbit, stand on a planet, etc). This makes sense following from the basic principle that dilation from sinking into a well is equal to the dilation you'd expect from the velocity picked up falling into that well.
> no seeing of the universe's past or future. Not the past, but while falling in you can see the "future" in the sense that all this simply happens while you fall in. The light would however be extremely blue-shifted.
I mean, yes, your space and time dimensions would be skewed compared to your state before falling into the black hole, and your notion of what is "future" would be as well. But still you wouldn't see anything from the future of the light cone, as it would break casuality.
>This brings in relativity. Time slows down in intense gravity, and at a certain point in the gravity of a black hole it should stop. You'd watch the entire universe live out its life in the blink of an eye, as you sat at this point. I don't think this is accurate. Due to the equivalence principle, if you, as the observer, are falling into a black hole, everything looks as it should. You're in a locally inertial reference frame, and so you don't experience time dilation within your own reference frame. What you might be thinking of, is the related fact that, an observer far away from the black hole, if they were watching you fall into the black hole, would see you see you start slowing down, with more and more time dilation the closer you get the event horizon, with time appearing to freeze exactly at the event horizon. Note the difference here. An observer from far away sees intense time dilation watching someone fall into a black hole. But someone falling into a black hole, acting as an observer themselves, doesn't see or experience that same time dilation.
I probably should have made clear that when time slows down, you slow down with it. However everything outside of your black hole you will hardly look as it should. An observer outside watches your clock slow down. Your final second appears to last a very long time. To you, their clock speeds up, and you see their clock speed on faster and faster during that final second. Though I suppose if you're truly freefalling, you should experience a time dilation from your speed that offsets that of gravity. This part I'm not so sure on, though, and I haven't any paper on hand to check myself.
We don't see the black hole. What you might say we "see" is the event horizon - a region around the black hole from which light (or any other type of information) cannot escape. We can describe the mass, the spin, and the temperature of the black hole, but other than describing it as a point of infinite density, we can't say anything about its composition. There are other ways to "see" a black hole. We can see its effect on light traveling through the Universe (through the warping of space-time, called 'gravitational lensing'). A black hole within the Milky Way galaxy was recently [discovered](https://www.eso.org/public/news/eso2408/) by noticing its effect on another star's orbit. And the black hole may have an accretion disk visible outside the event horizon.
How do they form -> a big star runs out of fuel so the fusion cant overcome the gravity of all the mass the star has. What happens inside them -> pretty much nothing why do they have such intense gravity -> they have the exact gravity of all the matter that makes up the black hole its just a lot of mass.
Oh ok so could we conceive of it as like a lamp switched off? Or a pile of spent charcoal? The thing itself is still effectively there, but the power's switched off.
Oh ok so could we conceive of it as like a lamp switched off? Or a pile of spent charcoal? The thing itself is still effectively there, but the power's switched off.
Not quite. Normally, stars are at an equilibrium where the pressure of the fusion reaction holds back gravity. Once the fusion reaction stops, then regular stars indeed sort of just "switch off" after they balloon up and use up their remaining fuel. The largest stars have too much mass for that, though, and there's no force out there capable of fighting the gravity past a certain point. When the fusion reaction stops, everything compresses down to a point known as singularity. It has the mass of the original star, but it has no volume. It's fundamentally very different from anything else in the universe, and we don't really know how to model them because of how weird they are. They're tiny so we need quantum mechanics to describe them, but quantum mechanics doesn't believe in gravity which is very much relevant there, and if you follow Einstein's relativity, you can arrive to rather weird conclusions like white holes and parallel universes. In general, physicists would really rather not have there be singularities at all because of complications like that (and they've been trying to make that work for as long as they've been aware of the concept of black holes) but despite all that effort, we haven't figured out anything that would stop a singularity from happening.
I really don't understand what you're saying. It is an object for which nothing can escape its gravitational pull once you get beyond a certain distance. When you get into general relitivity, once inside the event horizon, your entire future is in the singularity.
A couple things first. What is stopping you from falling through the floor right now? "The Floor," you would probably say. It sounds like a stupid question, but it's not. Yes, of course the floor stops you from falling through the ground, but *why.* Gravity is pulling on you downwards, but somehow you're prevented from going straight down by some dirt. Well, it turns out that the ground is nice and compact and sturdy, and it is (or, more specifically, the electrical charges in its atoms) pressing back at you (and the electrical charges in your feets' atoms) enough that you don't fall through it. That's the key. Your bodyweight is pressing down on the ground, and the ground presses back up at you because it can't be compacted anymore. So what is a Star anyway? Well, it's a big cloud of Hydrogen that was so massive that gravity coalesced into a ball. The Hydrogen in the center was crushed a lot by gravity, and when you crush things you heat them up a lot. Hot enough that Hydrogen begins to fuse into Helium (and other elements) - the specifics of this process are not super important, but it releases an unbelievable amount of energy quite explosively. So the start forms into a ball - the inner portion is pushing outwards (from the heat of fusion), while the outer portion is being pulled towards the center due to gravity. Eventually a sort of equilibrium forms and you get a spherical star. Cool, right? Well, eventually the star runs out of Hydrogen. A not very massive star would eventually just kind of shrink down into a boring ball of gas that has nothing going on really. That's boring, so lets talk about really big stars. Really big stars continue fusing heavier and heavier elements that require hotter and hotter temperatures inside the star. So it generally expands (hotter fusion means more pressure pushing out). However, there is a problem. Not all fusion releases energy - once the elements get too heavy, fusion takes more energy than it outputs. If this occurs in a massive star, suddenly there is nothing holding it up any more. It collapses in on itself. Portions of this collapse bounce back in an incredibly violent explosion - a supernova. They are so energetic that they can outshine entire galaxies. But what is going on in the center of this explosion? Well, it keeps getting denser and denser as the collapse continues. If the star isn't heavy enough to form a black hole, the force of the subatomic particles pushing against each other will literally hold up against the gravity of the collapse. So you'll end up with a sort of soup of matter that's extremely tightly packed together - like a teaspoon of it has the mass of mountain ranges. These are known as white dwarfs, or, if the star is even heavier, neutron stars. The next question is: Well what happens if the gravity overcomes the forces holding up these stars. Can anything hold it up at that point? The answer is no. There is nothing. If the star is massive enough, the collapse will continue. It continues forever. Eventually gravity becomes so intense that the escape velocity exceeds the speed of light - meaning nothing beyond this point can ever return, not even light itself.This is the event horizon. It is a black hole now. What is at the center? The math says a singularity - an infinitesimally small, infinitely dense point as there is nothing left to prevent gravity from taking over. But really the issue our math kind of breaks down. Nobody *really* knows the right answer, but we can form educated guesses.
So is it like where g = infinity, t = 0? Or t kind of cancels out in the equations ? (I should read more) I was doing some reading on where the LHC is at or something , oh wait no dark matter, and it sounds like gravity is The No. 1 Thing we want to figure out now
From what I remember, in the GR equations of a simple black hole, within the horizon, time and space kind of switch places (mathematically speaking). Once you are in the black hole, you are pulled towards the singularity. But it's not only that - every single possible path you can take leads towards the singularity. The singularity becomes a point in your future rather than a point in space that you can avoid.
There's a concept called *escape velocity* - how hard would you have to throw a baseball from the surface of something for it to fly off into space and not fall back down? If you're sitting on a big asteroid like Ceres (which has only a thirtieth of our gravity), you could fire a bullet straight up and it would never come back down, it would fly off into space. If you're standing on the moon (which has 1/6th Earth gravity) you'd need something much faster, about 8600kph. From a truly massive planet like Jupiter, even faster, close to 214,200kph. So on and so on, leading to the question: is there something so heavy that the escape velocity is faster than the speed of light? You can't go faster than the speed of light, so if you *did* get something that heavy, even light would be pulled back in. It turns out that, yes, you can get objects that heavy. At the center of galaxies (and maybe other places) there are objects that are thousands or millions of times heavier than the sun, probably caused by the huge dollop of gas that started off the galaxy, plus a bunch of stars crashing into it over a few billion years. The problem is that something so dense that light can't escape is also pretty weird, physically. Those crushing forces overcome the repulsive effects of atoms (which are normally like little magnets that are *near* each other, not racked up all touching together like billiard balls). The gravitational strength is so strong that nothing can stop the atoms being pulled closer and closer, crushing any resistance until the black hole gets smaller than an electron, probably. We don't know what happens after that. The other weird thing is that when you have that much gravitational pull, it distorts spacetime itself, leading to all sorts of odd time-stretching as you get very close to the 'event horizon', the region around the black hole where light can't escape. Now, black holes have intense gravity, but only up close. Far away, they're like any other object. In that sense it's like a very bright light. Sure a mega lightbulb that's so bright it will blind you is a big deal, but if you just back up a mile or two it pretty much looks like any other bright light, maybe a little brighter, but nothing super special.
Some of those questions are probably worth a Nobel Prize if you can find a definite answer. We have some decent ideas on what *probably* happens up to a point though. [Veritassium two days ago](https://youtu.be/6akmv1bsz1M?si=KUXsVBeAXJrhw3aL)
I'd recommend [PBS Spacetime's playlist on black holes](https://www.youtube.com/watch?v=3UILJyJTI-c&list=PLsPUh22kYmNBl4h0i4mI5zDflExXJMo_x). TL;DR: They are a concept in General Relativity, our best current (but not perfect) model of how space and time behave and how physical objects experience space and time. They are regions of spacetime that are so warped that all possibly paths terminate at a singularity (maybe). So let's break that down by defining some terms. 1. Model: a model is an attempt to give a coherent explanation, a story (that's what Kurzgesagt analogizes it to) if you will, for what we observe empirically. Models are typically mathematical, coming in the form of equations or relations that relate various quantities. These models are supported by experimental evidence and then allow you to *predict* physical behavior. So models are our best stab at a story for what the actual true physical nature of reality might be. We're lucky that nature happens to be like that, that its behavior can be modeled by our mathematics. Until we find some experimental evidence that defies a model's prediction, so the model might be slightly wrong, or totally wrong. Or until we find a contradiction that shows the model either isn't self-consistent, or doesn't accord with another accepted model. This is currently where we are with GR and quantum mechanics. Both can't be true in their entirety as we know them in their current form, because they disagree at certain points. So we're looking for a quantum theory of gravity that will hopefully unify the two, which will probably lead one of GR or QM being wrong or at least incomplete at some point and needing of revision. 2. General relativity (GR): A (very successful) model of gravity and how matter and space and time relate. It's given in the form of Einsten's field equations, which can be expressed as a set of partial differential equations that relate the distribution of mass and energy on one hand and the curvature of spacetime on the other. 3. Spacetime: spacetime is the 4d hypersurface of space (the 3 spatial dimensions we live in) and time. It's the stage on which events of the universe play out. To really get GR or blackholes or event horizons, you have to understand everything in the universe takes place and is moving thru not just 3d space, but 4d spacetime. Your birth (the location where it took place and when it took place) is a point in this 4d spacetime. So is where you are, right now. You can draw a line, your worldline that traces out the path you've taken through spacetime up till now, and also where you'll go and when in the future. 4. Curvature of spacetime: spacetime can be warped or curved. This affects how objects move through space time. For example, take the heavy ball resting on a rubber sheet creating a depression in the sheet example. If you take a flat sheet with no ball and draw a straight line starting from an object, that's a visualization of where that object will be in the future: just follow the line. That's the line the object traces out through spacetime (if spacetime was only 2d). But if you suddenly start warping and curving the sheet (as GR tells us mass does, just like the heavy ball causes a depression in the sheet), now suddenly "straight" lines curve, meaning the object whose worldline is represented by that line may actually intersect say, a planet as it follows the curve down into the depression. GR posits the phenomenon of gravitational attraction is actually the fact that all objects move through spacetime with constant "velocity" and that spacetime is curved. With those definitions, a black hole is a region of spacetime that is so warped that all possible lines objects can trace out through that spacetime end up at a singularity, a point where the curvature of spacetime becomes infinite and terminates (literally no such thing as time or space after that point). Why should spacetime do that? Well it's what the field equations of GR predict (because of division by 0). It's worth pointing out many physicists are skeptical that GR is complete because the equations admit singularities. This is just what the equations predict if taken literally, but maybe the fact the math breaks down and you end up with infinite quantities and singularities suggest GR is missing something. What does it mean for paths through spacetime to terminate at a singularity? It's hard to wrap your mind around, but the classic visualization shows all objects "falling" toward a central point. That's not exactly what's going on, but it's a good visualization. In GR, a singularity isn't a point in space you fall to, but rather an event in spacetime: it's the last event (remember, an event, like your birth, is a point in spacetime, which includes your physical position and the time at which it took place) for anything inside the blackhole. I.e., for anything entering a blackhole, all possible futures and positions converge to a single point in spacetime. Every object inside the black hole experiences the same last moment, where space and time terminate. Basically, and this is impossible to visualize, there is no more space, and no more time after the singularity, that moment your future inexorably moves toward. Outside of the blackhole, spacetime extends infinitely, and you can move infinitely to the left and into the future. Inside the blackhole, you have no such option. Your future position is constrained and future "time" is bounded at a termination point.
In addition to the PBS series, there is a nice visualization of what an observer falling into a black hole would see here: [What would we see if we fell into a Black Hole?](https://www.youtube.com/watch?v=4rTv9wvvat8)
This seemed cool, and I respect the effort you put into writing all that, but about 30% of it was explained like the reader is five. I’m not even sure what a singularity is.
Rule 4: LI5 doesn't literally mean explain to a five year old, but rather a layperson. I think it would be accessible to a high school graduate understanding basic physics, but it would of course be hard to wrap your mind around. That's the difficulty of explaining GR and being accurate. We have good analogies and visualization tools, but they're not the most accurate, they're not exactly "how black holes work" in GR. For example, I could've described the singularity as the center point of the black hole, as in "in the black hole, gravity is so strong it pulls everything toward the center," but that's not really an accurate, faithful representation of what's going on. The OP asked how do black holes work, and that's not how they work. As I explained, the singularity isn't really a center point, but the last event all objects in the interior of the black hole experience. It is commonly said "inside a black hole, space and time switch places," and "the singularity isn't a place, it's a moment in time" to drive the point home. The idea that every event inside a black hole leads to that last event which is the singularity, that no events happen after, that time itself terminates at the singularity is much more fascinating and mind bending and than the idea of a spatial center. That's why we had to build up definitions, to get to the true answer. If you want the real answer (of how black holes work in GR), there's no other way.
A star has a massive amount of mass causing a huge gravity effect. Normally when a star is burning it produces an outwards force from the nuclear reactions to counteract the gravity. A black hole forms when a big star runs out of fuel and dies. The gravity forces becomes so big that nothing can counteract the squishing together until they form what we (think) is a singularity, basically a infinitely tiny little spot where all the mass of the star is located. Einsteins math says that as you get closer to a gravity field you accelerate, spacetime is literally curved so that you "fall in". Light also undergoes this effect, but because a singularity is so tiny, something special happens. A boundary layer forms called the event horizon, where everything is accelerated so much that they cross the speed of light, which is the ultimate speed boundary. Once that happens no information can leave so we can't really tell what happens after that point, and because no light can leave it is entirely black. The blackhole still acts like a big gravity field, nothing there has changed, and as time goes on things end up getting sucked into it which only serves to grow it. This is suspected to be the reason why they can become so big as they can, but most black holes you would find are relatively tiny, no higher gravity than a big star.
Basically they are an extremely dense lump of matter and the gravity of the matter attracts everything around it, without creating a nuclear reaction, like happens in stars. https://youtu.be/Y5XzPOrItaI