Gravitational Waves From Black Holes... How?

[Sermil]

OK, with all the cool detections of gravitation waves from colliding black holes, there's something I don't understand -- do the waves come from the singularity or the event horizon of the black hole?

On the one hand, the waves can't come from the event horizon, because the event horizon doesn't have mass. It's not really a thing, it's just the line in space where the escape velocity exceeds the speed of light. The gravitational waves are carrying away energy and removing mass from the black hole, so they must be originating from the singularity, which is where all the mass is.

On the other hand, the waves can't come from the singularity, because they move at the speed of light. And anything that moves at the speed of light inside a black hole ends up at the singularity -- it can't get out. In particular, information should never be able to leave the black hole. Gravitation waves carry information about how an object is accelerating, and any information in a black hole should only be able to travel towards the singularity; it can't leave. (Thought experiment: Throw two rocks into a black hole, on a course that should lead to a collision inside the event horizon but before they reach the singularity. The acceleration due to the collision should cause gravitational waves. You should no more be able to "see" those gravitational waves than you can see the collision with electromagnetic waves.) So they must come from the event horizon, which is the only part of the black hole that can 'see' the outside universe.

So, clearly I cannot choose the glass next to me the gravitation waves cannot come from the event horizon, and clearly they cannot come from the singularity. What am I missing?

[Rakaydos]

It's not the black hole itself that's emitting the gravity waves. Its the rapidly vibrating bar of space between two black holes as they spiral around one another, tidally dragging each other closer to mutual absorption.

That's why the "droplet noise" stops as soon as the holes merge- because there isnt space between them anymore.

[Yora]

Everything that is ever observed about black holes comes from the space just outside of them. Nothing inside of them can be observed, but their extreme gravity had all kinds of interesting effects on the space around them, which can be so extreme that they can be detected from billions of lightyears away. That provides a lot of evidence from which information about black holes can be deduced.

[Anymage]

As was touched on above, neutron stars can also create gravity waves. It's not about two black holes causing them through a head-on collision. It's that they keep rotating around each other like water circling the drain, and the drag they have on the fabric of space causes ripples. That works with any appreciably massive objects in a tight mutual rotation. (Technically any two massive objects orbiting a mutual center of mass, but with sub-astronomical figures you're looking at negligible results.)

Then again, the specific point doesn't do anything about the underlying question. How can the singularity's mass affect the outside universe if the singularity is inside the event horizon. I can tell you as an intelligent layperson that event horizons are pretty active from the perspective of an outside observer; from the perspective of the outside universe, any incoming matter is stretched infinitely thin and stopped in time at the event horizon. Whether or not that apparent shell explains how they have gravitational influence, I don't have the background to say.

[Douglas]

I have no idea how accurate this is, but I've seen a black hole's gravity characterized as a Relativistic "fossil" preserved by its own time dilation - nothing, not even gravity, can come out from the event horizon itself, but there's a layer of immensely strong gravity just slightly above it that no longer has a source but has slowed its own dissipation almost to a stop by nearly freezing time.

[Lvl 2 Expert]

But if gravity can't escape a black hole, how do other things get pulled in?

#daretoask

[halfeye]

But if gravity can't escape a black hole, how do other things get pulled in?

#daretoask

Gravity is immune to gravity?

[Anymage]

But if gravity can't escape a black hole, how do other things get pulled in?

#daretoask

Just to repeat what Douglas and myself said. To an outsider, matter falling into a black hole is stopped in time (and stretched infinitely thin) at the event horizion. There's a shell of matter around it. At infinitesimally thin shells around that, you have regions where time is not quite stopped, but slowed to a crawl. So the gravity of stuff that had fallen in millions of years ago is just reaching you on its sluggish crawl out.

As mentioned before, though, that's just my takeaway as an intelligent layman. And I have no clue what things would look like from the perspective of someone falling into a black hole. Those, you'd have to dig up the information for actual qualified astrophysicists and email them.

[Grey_Wolf_c]

Just to repeat what Douglas and myself said. To an outsider, matter falling into a black hole is stopped in time (and stretched infinitely thin) at the event horizion.

That has always confused me. If it takes infinitely long time to cross the event horizon from the perspective of the rest of the universe, how can Black Holes grow? Wouldn't it take infinitely long to gain any mass at all? How is it that the supermassive black hole in the middle of a galaxy got that large, if the matter should still be frozen in time at the event horizon?

GW

[Khedrac]

That has always confused me. If it takes infinitely long time to cross the event horizon from the perspective of the rest of the universe, how can Black Holes grow? Wouldn't it take infinitely long to gain any mass at all? How is it that the supermassive black hole in the middle of a galaxy got that large, if the matter should still be frozen in time at the event horizon?

GW

I think that this has something to do with the fact that while it appears to an observer that an object falling in takes infinitely long to do so (see below) it does not for the object falling in or the black hole itself. As such, the black hole can grow while the objects falling in fade away.

Objects falling in to Black Holes: the light cone from an object falling in to a black hole is such that some of the light emitted/reflected by the object will always be visible (which is more a case of the light taking infinitely long to get away from near the black hole). However, the longer the interval between the object "falling in" and the observation, the smaller the proportion of the light that still shows it falling in is visible - which means the object fades away becoming less and less visible as it appears to take infinite time to fall in. Eventually the object bcomes effectively invisible.
One could try to state that the object actually "fell in" a long time ago, but because time is subject to gravity the definition of "a long time ago" just broke and this is not actually the case...

[halfeye]

That has always confused me. If it takes infinitely long time to cross the event horizon from the perspective of the rest of the universe, how can Black Holes grow? Wouldn't it take infinitely long to gain any mass at all? How is it that the supermassive black hole in the middle of a galaxy got that large, if the matter should still be frozen in time at the event horizon?

GW

The event horizon grows as mass is added, so as mass approaches one side, the other grows a tiny bit, and that constantly happens so mass approaching the event horizon is taken inside it by the growth of the horizon, even if it never actually reaches the position of the horizon it was originally approaching.

On the other hand, the whole time dilation to infinity thing is one of the singularity aspects of black holes, it may be that inside the event horizon gravity beyond "infinity" overcomes time dilation. Black holes do rotate, and that couldn't happen if time was infinitely dilated within them.

[Rockphed]

Gravity waves are created because objects have orbits that should go at a certain speed but do not because of local time dilation. So far we have only see neutron star and black holes give off gravity waves, but theoretically an observer outside the solar system would see gravity waves from comets. I'm pretty sure that the more elliptical an orbit the more gravity waves it gives off. What is happening is that when an object experiences time dilation in its orbit, it slows down, but the energy has to go somewhere, so it gets emitted as a gravity wave.

[gomipile]

That has always confused me. If it takes infinitely long time to cross the event horizon from the perspective of the rest of the universe, how can Black Holes grow? Wouldn't it take infinitely long to gain any mass at all? How is it that the supermassive black hole in the middle of a galaxy got that large, if the matter should still be frozen in time at the event horizon?

GW

The idea that a far away observer sees infalling matter take an infinitely long time to reach the event horizon is based on the idea of continuously emitted light. In reality, light is emitted as photons, and it doesn't take a very long time for the last photon to be observed.

The same is true of the black hole as a whole. It only takes a small amount of time for a far away observer to see the collapsing star's matter stop emitting photons.

[halfeye]

The idea that a far away observer sees infalling matter take an infinitely long time to reach the event horizon is based on the idea of continuously emitted light. In reality, light is emitted as photons, and it doesn't take a very long time for the last photon to be observed.

The same is true of the black hole as a whole. It only takes a small amount of time for a far away observer to see the collapsing star's matter stop emitting photons.

Actually, isn't time dilation the wrong way of looking at this phenomemon? So far as we can tell, from the point of view of the object itself the time proceeds as normal. The correct view would seem to thus be time expansion on the part of the observer, which is very odd when you think about it.

[Yora]

We normally define speed as distance times time, in which distances are constant and time has a fixed pace, and speed is relative to the reference frame of the observer.

But that's not actually the case. Speeds are fixed, and it is time that is relative to the frame of reference of an observer. Not sure about distance, I think that one is relative too.

But back to the question: When we say "observe" when refering to matter falling into a black hole, we really just mean "receiving emmited photons". Because of time dilation outside the event horizon the intervall between photons reaching us becomes increasingly longer until at the event horizon we can no longer be reached by any photon anymore. The image "signal" we receive in the form of photons no longer updates, but that doesn't actually mean that the object has stopped in time and no longer emits any photons. It does, but space has curved so far inside the event horizon that there are no more possible paths that lead to the outside, so those photons can never come out.

Thinking of gravity as a force makes sense in Newtonian physics, but when we're dealing with the physics of gravitstional time dilation and event horizons (general relativity) things really only make sense if gravity is treated as warped space.

[Bucky]

The idea that a far away observer sees infalling matter take an infinitely long time to reach the event horizon is based on the idea of continuously emitted light. In reality, light is emitted as photons, and it doesn't take a very long time for the last photon to be observed.

Also, the light gets redshifted, lowering its frequency and increasing the wavelength. At some point the frequency decreases so far that you practically can't tell it from an unchanging static field.

[halfeye]

We normally define speed as distance times time, in which distances are constant and time has a fixed pace, and speed is relative to the reference frame of the observer.

But that's not actually the case. Speeds are fixed, and it is time that is relative to the frame of reference of an observer. Not sure about distance, I think that one is relative too.

Speeds are also relative. That's what makes the whole system so difficult. There is no fixed point on which to base it.

When we say "observe" when refering to matter falling into a black hole, we really just mean "receiving emmited photons". Because of time dilation outside the event horizon the intervall between photons reaching us becomes increasingly longer until at the event horizon we can no longer be reached by any photon anymore. The image "signal" we receive in the form of photons no longer updates, but that doesn't actually mean that the object has stopped in time and no longer emits any photons. It does, but space has curved so far inside the event horizon that there are no more possible paths that lead to the outside, so those photons can never come out.

That's twisted enough to be relativity.

Thinking of gravity as a force makes sense in Newtonian physics, but when we're dealing with the physics of gravitstional time dilation and event horizons (general relativity) things really only make sense if gravity is treated as warped space.

Warped space, warping time. Is the transmission of gravity affected by its own warping of space-time?

[gomipile]

Warped space, warping time. Is the transmission of gravity affected by its own warping of space-time?

The gravity is the warping of spacetime. Also, yes. Spacetime can only distort as fast as the speed of light measured within itself. That's what gravitational waves are.

[FireJustice]

https://www.youtube.com/watch?v=MTY1Kje0yLg

This demonstration can be really enlightnening to start to see gravity as deformation of Spacetime.

The blue fabric is the "fabric of space time"

See how the marbles spin towards the center, that is supposed to represent a massive gravitatinal object. There's even a demonstration of binary marbles spining around each other's center of mass and translating to the center.


The gravity waves are the vibrations of the fabric, that propagate as waves, as it bends a little as the gravitational objects.

Now put ins scale as the marbles are stars, gravity wise.

[gomipile]

https://www.youtube.com/watch?v=MTY1Kje0yLg

This demonstration can be really enlightnening to start to see gravity as deformation of Spacetime.

The blue fabric is the "fabric of space time"

See how the marbles spin towards the center, that is supposed to represent a massive gravitatinal object. There's even a demonstration of binary marbles spining around each other's center of mass and translating to the center.


The gravity waves are the vibrations of the fabric, that propagate as waves, as it bends a little as the gravitational objects.

Now put ins scale as the marbles are stars, gravity wise.

I find that analogy very misleading, first because the intrinsic curvature is wrong. The rubber/fabric sheet has negative intrinsic curvature, while spacetime near a mass has positive intrinsic curvature.

Also, the sheet analogy depends on Earth's gravity to pull the marbles down so the sheet can push them in to an orbit. In General Relativity, objects in freefall follow paths that are "as straight as possible" (spacetime geodesics,) which create the impression of a force of gravity without a force in the Newtonian sense.

Geodesics on the fabric sheet are nothing like the paths the marbles take.

[Yora]

Where things get really crazy is the question how things are attracted towards masses if gravity is not a pull but a curverture of paths? If you would be motionless in relation to another mass, there would be no path that could be curved.
It's because you can't ever be motionless in spacetime even relative to another object. if your relative positions and velocities are fixed and not moving, both objects are still moving through time. The pull of gravity is the passage of time through spacetime.

Speeds are also relative. That's what makes the whole system so difficult. There is no fixed point on which to base it.

There is. The one constant in spacetime is c. The speed of causality. Which is always 299.999 km/s. If your relative perception of time changes, so does your relative perception of distance so that everyone agrees that an object traveling at c (a photon) is always traveling at exactly c.

[FreddyNoNose]

But if gravity can't escape a black hole, how do other things get pulled in?

#daretoask

exactly!...................good point.

[Tvtyrant]

Where things get really crazy is the question how things are attracted towards masses if gravity is not a pull but a curverture of paths? If you would be motionless in relation to another mass, there would be no path that could be curved.
It's because you can't ever be motionless in spacetime even relative to another object. if your relative positions and velocities are fixed and not moving, both objects are still moving through time. The pull of gravity is the passage of time through spacetime.



There is. The one constant in spacetime is c. The speed of causality. Which is always 299.999 km/s. If your relative perception of time changes, so does your relative perception of distance so that everyone agrees that an object traveling at c (a photon) is always traveling at exactly c.

So then if everything ends up in the same blackhole it really would be motionless right? Time can't escape a blackhole, and if everything was part of the hole there would be neither time nor space anymore.

[Yora]

Neither time nor gravity are objects or energy. They are not constrained by the rule that no mass or energy can come out of a black hole.

In fact, they are both spacetime.

And gravity is not even being emited out of a black hole. The effective pull of gravity is spacetime getting pulled into it. It's not leaving, it's entering.

[halfeye]

I find that analogy very misleading, first because the intrinsic curvature is wrong. The rubber/fabric sheet has negative intrinsic curvature, while spacetime near a mass has positive intrinsic curvature.

Also, the sheet analogy depends on Earth's gravity to pull the marbles down so the sheet can push them in to an orbit. In General Relativity, objects in freefall follow paths that are "as straight as possible" (spacetime geodesics,) which create the impression of a force of gravity without a force in the Newtonian sense.

Geodesics on the fabric sheet are nothing like the paths the marbles take.

I wholeheartedly agree that that analogy is misleading, you could maybe make a version in orbit where gravity wouldn't have strong effect, but even using a strong magnetic or electrostatic field as the attractive force, there would still be at least a few other things that would be wrong with it:

The tension is from the edge, things will tend to roll to the middle, there isn't an apparent edge to space.

If you put springs under the sheet to mitigate the tension from the edge objection, then the placement of the springs make nodes of resistance, which would also be unlike real space.

There is significant friction between the rolling objects and the surface.

There is tension across the surface, we don't know that is correct, it's a possible model, there is at least one other:

https://en.wikipedia.org/wiki/Pin_Art

[Sermil]

Warped space, warping time. Is the transmission of gravity affected by its own warping of space-time?

Yes, it has to to be.

Remember that when LIGO heard the kilonova back in October, the light waves and the gravitation waves were detected at the same time. This shows that not only do gravitation waves travel at the same speed as light, but they also follow the same (curved-to-our-three-dimensional-senses) path as light. If they had gone in the straight-to-our-three-dimensional-senses from the collision to our instruments, they would have arrived years before the light did.

[halfeye]

There is. The one constant in spacetime is c. The speed of causality. Which is always 299.999 km/s. If your relative perception of time changes, so does your relative perception of distance so that everyone agrees that an object traveling at c (a photon) is always traveling at exactly c.

My understanding is that that is halfway true, it could have been time or distance that was absolute and inviolable, but on what amounted to the toss of a coin, it was the speed of light that was made constant. The maths would alledgedly still work if you made time constant and the speed of light variable, it wouldn't make any more sense, but it wouldn't make significantly less. It's like electrons having a negative charge, it's a convention, for human understanding to work with, but it's not fundamentally ordained by some god.

[Douglas]

My understanding is that that is halfway true, it could have been time or distance that was absolute and inviolable, but on what amounted to the toss of a coin, it was the speed of light that was made constant. The maths would alledgedly still work if you made time constant and the speed of light variable, it wouldn't make any more sense, but it wouldn't make significantly less. It's like electrons having a negative charge, it's a convention, for human understanding to work with, but it's not fundamentally ordained by some god.

Maybe the math would have worked out for some sub portion of Relativity, but not for the whole thing. Among other things, time dilation has been experimentally measured by synchronizing two atomic clocks, flying one of them around on an airplane for a while, then bringing them back together and comparing. If time were absolute, they would have still been synchronized. They were not. If distance were absolute, the disparity between them would have been different from what it actually was.

It is quite definite physical reality, not convention at all, that it is the speed of light that is constant, not distance or time.

[jayem]

I think it's partially true that time and space could be kept constant and c changes.
Trivially we could use Earth time and co-ordinates and privilege this. However it isn't a toin coss to choose the way we did (just as it wasn't when we chose to use solar centric measurements), and obviously we lose stuff.

For example in that example, of the radium clocks, you'd have to remake them to account for the fact that the laws of physics change between the frames. So rather than saying Time Dilates, you say it's a bad clock because it doesn't measure the speed it's going wrt earth and correct for the fact that the 'rate of radioactive decay' changes because if how fast I'm going.

There is some precedent for this argument, after all a pendulum clock doesn't correct for gravity. But there there is an obvious reason why things should be different, whereas why the hell should a clock on Venus depend on what Earths doing.
However if a good 'clock' needs to integrate distance and speed measuring, local 'timekeeping' and a conversion computer, it's not exactly Occam friendly.
And then we'd also have the radius of an atom depending on where we were, as the permitivity (and permitity) changes, so a Vesuvian would change (their own) 'height' depending on the relative seasons.

[halfeye]

Yes, it has to to be.

Remember that when LIGO heard the kilonova back in October, the light waves and the gravitation waves were detected at the same time. This shows that not only do gravitation waves travel at the same speed as light, but they also follow the same (curved-to-our-three-dimensional-senses) path as light. If they had gone in the straight-to-our-three-dimensional-senses from the collision to our instruments, they would have arrived years before the light did.

If you mean the neutron star collision, then that is a good example.

<edit>

Maybe the math would have worked out for some sub portion of Relativity, but not for the whole thing. Among other things, time dilation has been experimentally measured by synchronizing two atomic clocks, flying one of them around on an airplane for a while, then bringing them back together and comparing. If time were absolute, they would have still been synchronized. They were not. If distance were absolute, the disparity between them would have been different from what it actually was.

It is quite definite physical reality, not convention at all, that it is the speed of light that is constant, not distance or time.

I think it's partially true that time and space could be kept constant and c changes.
Trivially we could use Earth time and co-ordinates and privilege this. However it isn't a toin coss to choose the way we did (just as it wasn't when we chose to use solar centric measurements), and obviously we lose stuff.

For example in that example, of the radium clocks, you'd have to remake them to account for the fact that the laws of physics change between the frames. So rather than saying Time Dilates, you say it's a bad clock because it doesn't measure the speed it's going wrt earth and correct for the fact that the 'rate of radioactive decay' changes because if how fast I'm going.

There is some precedent for this argument, after all a pendulum clock doesn't correct for gravity. But there there is an obvious reason why things should be different, whereas why the hell should a clock on Venus depend on what Earths doing.
However if a good 'clock' needs to integrate distance and speed measuring, local 'timekeeping' and a conversion computer, it's not exactly Occam friendly.
And then we'd also have the radius of an atom depending on where we were, as the permitivity (and permitity) changes, so a Vesuvian would change (their own) 'height' depending on the relative seasons.

I'm going to have to think about this a bit more.

<edit 2>

The thought occurred to me that if gravity is affected by gravity, then gravity waves can be gravitationally lensed.