Black Hole Physics Question

[WhatThePhysics]

Removed by author.

[factotum]

Black holes would work exactly the same if whatever is in the middle of them isn't infinitely dense--so long as whatever *is* there is dense enough to provide a surface gravity strong enough to prevent light leaving then you're good. By definition, if light can't escape from the black hole then neither can any information about the interior, so the inside could be a particularly dense white fluffy bunny and it wouldn't affect anything outside the event horizon.

[shawnhcorey]

If light cannot escape a black hole, then neither can gravity. Einstein's Field Equations are based on the assumption that gravity travels at the speed of light. No light, no gravity.

[Chronos]

So far as we know, there is in fact an infinite-density singularity at the center of black holes. Where are you getting the idea that there's not? I mean, we may well be wrong, and quantum gravity might say there's something else there of finite size, but nobody has any working theory of quantum gravity yet, and finding one is probably so far off in the future that we can't even anticipate when it'll happen.

That said, so long as whatever it is is inside of the horizon, it doesn't matter at all.

If light cannot escape a black hole, then neither can gravity. Einstein's Field Equations are based on the assumption that gravity travels at the speed of light. No light, no gravity.

This is incorrect. Black holes certainly have gravity, and can also have an electromagnetic field (mediated by photons).

[shawnhcorey]

This is incorrect. Black holes certainly have gravity, and can also have an electromagnetic field (mediated by photons).

Something is definitely incorrect. But if black holes have gravity, then they are not black holes.

[hamishspence]

Whatever those "too small and too high in mass to be neutron stars" objects (stellar mass black holes) are, we have to call them something. "Black hole" is simply the most convenient term.

Supermassive black holes aren't too small to be neutron stars - but they are far, far too massive to be regular stars. Again "Black hole" is convenient.

If light cannot escape a black hole, then neither can gravity.

This:

How does gravity escape a black hole

explains how.

[shawnhcorey]

This:

How does gravity escape a black hole

explains how.

And what causes space-time to curve, magically space fairies?

[Lvl 2 Expert]

If light cannot escape a black hole, then neither can gravity.

Why?

Light behaves like a mix between a particle and a wave, gravity more like a mix between a field and curved space-time. I'm not the greatest physics hero here, so this is an honest question: is there a reason why gravity has to abide by this rule, or any evidence that it does?

EDIT: And as a counter question: if gravity does not follow those same rules, how does the universe prevent people from building faster than light radios based on gravity waves?

[shawnhcorey]

Light behaves like a mix between a particle and a wave, gravity more like a mix between a field and curved space-time. I'm not the greatest physics hero here, so this is an honest question: is there a reason why gravity has to abide by this rule, or any evidence that it does?

I'm not saying gravity travels at the speed of light. Einstein said that when he formulated his field equations. Since Schwarzschild used Einstein's equations to produce these magical black holes, it cannot escape their event horizons.

Here's an article that speculates event horizons do not exist. Of course, if there's no event horizon, it's not a black hole. What could form instead? A quark star.

[Tirunedeth]

Why does gravity need to escape a black hole? I would certainly agree that changes to the gravitational field (or, more precisely, the metric) must propagate at the speed of light, and cannot escape the event horizon, but that isn't relevant to the Schwarzschild metric and other static situations (and, even more generally, simply means that you can ignore any internal dynamics of the black hole).

[shawnhcorey]

If gravity cannot get out of a black hole, then it has no gravity outside of the event horizon. It would be completely undetectable since it would have no gravity to pull on nearby stars. It also would not pull in any particles, so it would not give off any radiation. Nobody would know where one is.

[Chronos]

There are a few models out there that say that black holes don't actually exist, and that what we think are black holes are actually things called "gravitars" that are ever so slightly larger than black holes (and hence don't have horizons). But it's those models which require the magic, not Einstein's.

Quark stars, meanwhile, are a much more mainstream idea, and in fact there's some reason to believe that the neutron star in the Crab Supernova Remnant is actually a quark star instead. But a quark star isn't all that fundamentally different from a neutron star: It's a little denser, but it's still made up of baryonic matter, and it still has a fundamental limit for how big it can get (just a little bit bigger a limit than standard neutron stars). And if you bring a quark star past that limit, something still needs to happen to it.

EDIT:

If gravity cannot get out of a black hole, then it has no gravity outside of the event horizon. It would be completely undetectable since it would have no gravity to pull on nearby stars. It also would not pull in any particles, so it would not give off any radiation. Nobody would know where one is.

But we do detect them, we do know where they are, and they do pull on nearby stars, therefore we can conclude that they do in fact have gravity, just like every single scientific model of them predicts.

Conventional theory says they have gravity. We observe them to have gravity. If your own personal model says they don't, then it's your own personal model which is incorrect.

[shawnhcorey]

But we do detect them, we do know where they are, and they do pull on nearby stars, therefore we can conclude that they do in fact have gravity, just like every single scientific model of them predicts.

Conventional theory says they have gravity. We observe them to have gravity. If your own personal model says they don't, then it's your own personal model which is incorrect.

It's not my personal model. Einstein and Schwarzschild say they don't have gravity.

[Lvl 2 Expert]

It's not my personal model. Einstein and Schwarzschild say they don't have gravity.

But did they actually say that black holes don't exist/don't have gravity, or did they say they do (probably) exist and are you using a single bullet point from their work to prove how much smarter you are then them?

Because in that second case I can guarantee you they thought of this and they had an explanation.

[hamishspence]

As mentioned in the above link - it's Einstein's later work on general relativity (not his early work on special relativity) which covers the curvature of space-time.

And it's safe to say that supermassive black holes cannot be "quark stars" - they're far too massive.

[shawnhcorey]

But did they actually say that black holes don't exist/don't have gravity, or did they say they do (probably) exist and are you using a single bullet point from their work to prove how much smarter you are then them?

Einstein assumed that gravity travels at the speed of light. This is necessary for his field equations to work. Of course, if you had read about his work, you would already know this.

Schwarzschild used Einstein's field equation to speculate about massive objects and concluded that light does not travel fast enough to escape them. Of course, if you had read about his work, you would already know this.

Therefore super-massive objects have no gravity, it cannot travel fast enough to escape them. Of course, if you understood logic, you would already know this.

As mentioned in the above link - it's Einstein's later work on general relativity (not his early work on special relativity) which covers the curvature of space-time.

And it's safe to say that supermassive black holes cannot be "quark stars" - they're far too massive.

Super massive objects cannot be black holes. If black holes exist, they have no gravity. The only thing for them to be are quark stars.

[Chronos]

Don't blame people smarter than you for your lack of understanding of their work. Einstein and Schwarzschild did not say that black holes have no gravity. They said something that you interpreted as meaning that they don't have gravity. Your interpretation is wrong. Their interpretation of what they said is that they do have gravity, and it is their interpretation, not yours, that matches observations.

[InvisibleBison]

Einstein assumed that gravity travels at the speed of light. This is necessary for his field equations to work. Of course, if you had read about his work, you would already know this.

Schwarzschild used Einstein's field equation to speculate about massive objects and concluded that light does not travel fast enough to escape them. Of course, if you had read about his work, you would already know this.

Therefore super-massive objects have no gravity, it cannot travel fast enough to escape them. Of course, if you understood logic, you would already know this.

You're assuming that gravitons (or whatever mediates the effect of gravity) are themselves affected by gravity. Do you have any evidence to back this up? Because if it's not true, black holes can have gravity.

[factotum]

Therefore super-massive objects have no gravity, it cannot travel fast enough to escape them. Of course, if you understood logic, you would already know this.

What you're basically saying here is that gravity is affected by gravity. Following that logic to its ultimate conclusion means that larger objects have less gravity because they have more gravity...do I really need to say how ridiculous that is?

[shawnhcorey]

What you're basically saying here is that gravity is affected by gravity. Following that logic to its ultimate conclusion means that larger objects have less gravity because they have more gravity...do I really need to say how ridiculous that is?

Gravity curves space-time. Everything that travels thru this curved space-time is effected by it.

Do I really have to say how ridiculous saying space-time is not curved is?

[Khedrac]

Gravity curves space-time. Everything that travels thru this curved space-time is effected by it.

Do I really have to say how ridiculous saying space-time is not curved is?

Yes you do. Anything not subject to gravity is not going to be affected by gravity full stop.
Also to say that gravity "curves space-time" is only true from a particular perspective - that of modelling gravity - e.g it's not true when looking at straight lines.

To clarify - light travels in straight lines when unaffected by an outside force (as per Newton's laws) - when gravity bends the path light takes it pulls it off a straight line and on to a curve. Thus to say that space-time is curved for all purposes is wrong - it's not curved when calculating the straight line path between two objects.

[Leewei]

LIGO astoundingly proved the existence of gravitational waves, which must propagate through space similar to light. These waves, however, only represent change to space-time. The effect of gravity around planets, stars, and other cosmic bodies is apparent, even when gravitational waves from these bodies are too subtle to notice.

An apparent horizon may well impede gravitational waves from leaving its interior, but it still has an enormous gravity well effective far beyond its Schwarzschild radius. Objects falling into this well will increase the strength of the gravity even after they pass the horizon -- their gravitational pull doesn't magically disappear. What does happen is that any interaction inside the horizon takes so enormously long to leave the interior that it almost certainly won't be observable until the singularity decays.

[Tirunedeth]

Einstein assumed that gravity travels at the speed of light. This is necessary for his field equations to work. Of course, if you had read about his work, you would already know this.

Schwarzschild used Einstein's field equation to speculate about massive objects and concluded that light does not travel fast enough to escape them. Of course, if you had read about his work, you would already know this.

Therefore super-massive objects have no gravity, it cannot travel fast enough to escape them. Of course, if you understood logic, you would already know this.

The assumption Einstein made as that the laws of physics (in particular, the speed of light) are (locally) the same for all observers. That changes to the metric propagate at the speed of light is a consequence of GR, not an assumption.

It isn't just a lack of speed that prevents things from escaping a black hole. If it were just an excessively high escape velocity, an object undergoing continuous acceleration could escape a black hole. Instead, within an event horizon, all future-directed paths point towards the singularity. This distinction isn't directly relevant, but does pretty clearly indicate that your understanding of GR isn't as great as you think it is.

You still haven't established that anything needs to escape black holes in order to cause gravity. During a (sufficiently slow) radially symmetric collapse, no changes to the gravitational field occur. Therefore, there isn't any need for anything to escape the black hole, and no contradiction occurs.

Further, if what you are saying is true, then the Schwarzschild metric should break down for an object with radius less than its Schwarzschild radius. Since this doesn't occur, we can conclude that the contradiction you have inferred does not occur.

[Lvl 2 Expert]

Of course, if you had read about his work, you would already know this.

Of course, if you had read about his work, you would already know this.

Hey, I actually came into this thread with the following disclaimer:

I'm not the greatest physics hero here, so this is an honest question

Of course, if you had read the posts you're replying to, you would already know this.

Of course, if you understood logic, you would already know this.

And here I'm afraid I'm going to have to borrow your point. Your telling me basically all modern physics research is wrong because obviously heavy objects can't have gravity and two of the great minds of the last century proved this without even realizing it, and all they should have done to find out of put two of their own sentences after each other. It's so simple! Now someone call NASA to have me put in charge of keeping satellites in orbit, they've been calculating gravity wrong! See, while I don't have a lot of education in physics, I do have a lot of experience with morosophies, seemingly complex systems of ideas or believes that are quite obviously unlikely to be true. And I've learned that most ideas starting with "if only Einstein had been as smart as me he would have seen what his ideas really mean" are morosophies. That is not to say Einstein must have been right about everything, and must have always known what the implications of his calculations would be. It doesn't even mean black holes are real or work as the schoolbooks think they do. There are plenty of physicists with alternate explanations for the observations we have. But I would expect the faults to be slightly more complex than the two sentences you keep repeating in this thread. Because otherwise someone would have caught the error by now. Of course I can't be certain based on just two sentences, I don't like to judge too quickly, hence I asked for clarification.

But of course I only know that because I understand logic.

[shawnhcorey]

During a (sufficiently slow) radially symmetric collapse, no changes to the gravitational field occur.

So, for some irrational reason, you think that the gravity field magically continues to exist and magically follows the mass as it moves even tho there is no communication between the mass and the field?

[hamishspence]

So, for some irrational reason, you think that the gravity field magically continues to exist and magically follows the mass as it moves even tho there is no communication between the mass and the field?

Gravity is not, in general relativity, an energy field:

From the above link:

In general relativity gravity is not an energy field. Instead, mass distorts the relations between space and time. If we go back to our earlier example, if we place mass Bob in an empty universe, the relations of space and time around it are distorted. When we place mass Alice nearby, the distortion of spacetime around it means that moves toward mass Bob. It looks as if Alice is being pulled toward Bob by a force, but it’s actually due to the fact that spacetime is distorted.

[shawnhcorey]

During a (sufficiently slow) radially symmetric collapse, no changes to the gravitational field occur. Therefore, there isn't any need for anything to escape the black hole, and no contradiction occurs.

Now I see what your problem is. You seem to think that this magical black hole somehow becomes fixed in space so there's no need for a link between it and its gravity.

Nothing is fixed in our universe; everything moves. If it has no mass, it only moves at the speed of light. If it has mass, not only does it move but it is constantly being accelerated and the acceleration is constantly changing. There are no inertia frames in our universe, so a mass cannot become fixed.

That means when a mass is accelerated, there must be some link between it and its gravity field for the field to follow it.

[Douglas]

That means when a mass is accelerated, there must be some link between it and its gravity field for the field to follow it.

Or the gravity field gets accelerated too. For all I know, a gravity field as extreme as a black hole's could be self-sustaining, and also affected by other objects' gravity.

[Tirunedeth]

So, for some irrational reason, you think that the gravity field magically continues to exist and magically follows the mass as it moves even tho there is no communication between the mass and the field?

Where, in GR, is there any requirement for continuous communication between mass and the gravitational field?

Now I see what your problem is. You seem to think that this magical black hole somehow becomes fixed in space so there's no need for a link between it and its gravity.

Nothing is fixed in our universe; everything moves. If it has no mass, it only moves at the speed of light. If it has mass, not only does it move but it is constantly being accelerated and the acceleration is constantly changing. There are no inertia frames in our universe, so a mass cannot become fixed.

That means when a mass is accelerated, there must be some link between it and its gravity field for the field to follow it.

Fixed locations are entirely possible in non-inertial reference frames. Consider the north and south rotational poles of the Earth (and, indeed, the entire rotational axis). Further, one of the central points of GR is that there is no global frame of reference, so by claiming that we can't use a frame of reference in which the black hole is fixed in position, you are assuming that GR is false.

In any case, we are not discussing the real Universe here. We are discussing the Schwarzschild metric, which is an idealized situation in which there is only a single radially symmetric mass, stationary at the origin. This situation approximates many situations in reality, even if it cannot be exactly realized. As I understand it, your claim is that even in this idealized situation GR is self-contradictory, so bringing up the effects of the entire rest of the Universe (which, for a black hole separated from other significant masses by typical interstellar distances, are merely tiny perturbations) merely obfuscates the issue.

[hamishspence]

When I google quark stars, in addition to various references to a certain Ferengi barman , I find reports that the stellar phenomenon, if it exists (a very big if), is likely to max out at around 10 solar masses.

So it is unlikely that even the first black hole candidate to be discovered (Cygnus X-1) is one of those.