B How Can We Observe Black Holes Growing?

[mef]

TL;DR

A very simple question

IT IS WIDELY KNOWN that from the point of view of a distant, BUT FROM a FINITE distance, the time of falling into a black hole is INFINITE.

Then how does the BH absorb the surrounding matter?

It turns out that for any observer located at a finite distance R (the coordinate of the Schwarzschild metric) from the BH, no matter will ever reach the BH and it will never be able to absorb anything and increase in size.

 

[Dale]

Summary:: A very simple question

IT IS WIDELY KNOWN that from the point of view of a distant, BUT FROM a FINITE distance, the time of falling into a black hole is INFINITE.

Please don’t RANDOMLY SHOUT at us. It is rude and distracting

Then how does the BH absorb the surrounding matter?

In order to fall in, matter must lose its angular momentum. Typically this happens in an accretion disk, where there are lots of collisions that can redistribute angular momentum and energy so as to allow particles to fall in.

It turns out that for any observer located at a finite distance R (the coordinate of the Schwarzschild metric) from the BH, no matter will ever reach the BH and it will never be able to absorb anything and increase in size.

It turns out that physics is about more than the Schwarzschild coordinates.

 

[mef]

You are answering a completely different question.
What happens to particles with zero angular momentum (radial)?

 

[PeterDonis]

What happens to particles with zero angular momentum (radial)?

They fall in. They reach the horizon in a finite time by their own clock (and hit the singularity a finite time by their own clock after that).

Your misconception is a common one. I suggest reading this series of Insights articles:

https://www.physicsforums.com/insights/schwarzschild-geometry-part-1/

 

[mef]

Do you not understand what I wrote about the view, from the point of view of an outsider?

 

[Nugatory]

Do you not understand what I wrote about the view, from the point of view of an outsider?

We understand what you are asking, truly we do!

The “point of view of an outsider” is based on light from the infalling object reaching their eyes. As the object approaches the horizon it takes longer and longer for that light to reach the outsider and light from the horizon crossing never gets out at all; that’s what it means to say that the object gets closer to the horizon but never crosses it.

However, if we start with an object of mass $m$ and drop it into a black hole of mass $M$, we find that very quickly we have a black hole of mass $M+m$ - it’s just that we don’t get to see the object crossing the horizon.

Be aware that this answer contains some serious oversimplifications; the links posted by @PeterDonis would be a good start if you want a better understanding. There are also many older threads here as your question is a common one; the list of “Related Threads” below may be helpful, as well as https://www.physicsforums.com/threa...ching-a-bh-and-bh-growth.921644/#post-5814635

 

[PeroK]

Do you not understand what I wrote about the view, from the point of view of an outsider?

There are a lot of misconceptions online about an object falling into a black hole. These sources generally over-emphasise the role of a distant observer and what they see in terms of the light signals that reach them.

The argument they present, which you are repeating, is essentially that the object takes "infinite time" to reach the event horizon. This is false. There are other misconceptions, such as that the infalling object sees the whole history of the outside universe. That is also false.

One key point is that in the curved spacetime of a black hole, not all events may be observed by all observers. That the distant observer never observes an event does not mean that the event does not take place.

 

[Orodruin]

Do you not understand what I wrote about the view, from the point of view of an outsider?

Please understand that the gap of knowledge here is a big one. You are dealing with people who are very well versed in relativity and also very used to seeing precisely the type of misconceptions that you present. Instead of being dismissive and ungrateful to their replies because they do not fit with your own preconceptions, you should try to understand them.

 

[mef]

Please understand that the gap of knowledge here is a big one. You are dealing with people who are very well versed in relativity and also very used to seeing precisely the type of misconceptions that you present. Instead of being dismissive and ungrateful to their replies because they do not fit with your own preconceptions, you should try to understand them.

In addition to general words, no calculations.
OK.
Let's do it differently.
How much will the remote observer's clock show when, according to the local clock, matter reaches the horizon?

 

[mef]

There are a lot of misconceptions online about an object falling into a black hole. These sources generally over-emphasise the role of a distant observer and what they see in terms of the light signals that reach them.

The argument they present, which you are repeating, is essentially that the object takes "infinite time" to reach the event horizon. This is false. There are other misconceptions, such as that the infalling object sees the whole history of the outside universe. That is also false.

One key point is that in the curved spacetime of a black hole, not all events may be observed by all observers. That the distant observer never observes an event does not mean that the event does not take place.

I'm setting a very simple task... so that it becomes completely clear to you. How long does it take for a Newtonian apple to move from Earth to a black hole in the center of our galaxy.
From Newton's point of view

 

[Orodruin]

In addition to general words, no calculations.
OK.
Let's do it differently.
How much will the remote observer's clock show when, according to the local clock, matter reaches the horizon?

You have another misconception here: The idea that a global ”when”. The idea that there is a unique time of crossing the horizon as seen by a distant observer is a false one.

 

[PeroK]

I'm setting a very simple task... so that it becomes completely clear to you. How long does it take for a Newtonian apple to move from Earth to a black hole in the center of our galaxy.
From Newton's point of view

Newtonian gravity does not apply to black hole event horizons. As far as the apple is concerned, it reaches the event horizon in a finite time.

 

[Nugatory]

Let's write down the SPECIFIC time of the apple's fall from the Earth to the horizon of the black hole in the center of our galaxy. Take some mass of this BH. The attraction of the Earth and other gravity in the galaxy should be ignored.

By the “specific time” do you mean the proper time along the the infalling object’s worldline - that is, the time measured on the infaller’s clock between when it is released and it reaches the horizon?

If you mean something else, you will have to be more precise about exactly what the end points of the interval we’re measuring are. That’s what @Orodruin is getting at when he reminds you that there is no global “when” in curved spacetime.

 

[PeroK]

Newtonian gravity does not apply to black hole event horizons. As far as the apple is concerned, it reaches the event horizon in a finite time.

PS in terms of GR, there is a thread here about the time to fall into a black hole:

https://www.physicsforums.com/threads/how-long-does-it-take-to-fall-into-a-black-hole.1000235/

 

[Ibix]

How much will the remote observer's clock show when, according to the local clock, matter reaches the horizon?

Depends on how the remote observer defines "when". You are thinking three dimensionally, where you have little choice about what "when" means. In four dimensions, you have a choice. You can pick "when" so that the matter never crosses the horizon, or "when" so that it crosses almost as soon as it is released. It's a choice - not of physics, but of what you mean by the word "when". There's no universal meaning to it.

 

[mef]

Newtonian gravity does not apply to black hole event horizons. As far as the apple is concerned, it reaches the event horizon in a finite time.

I didn't talk about Newtonian gravity, only about his apple))

 

[mef]

You have another misconception here: The idea that a global ”when”. The idea that there is a unique time of crossing the horizon as seen by a distant observer is a false one.

there are strict initial conditions , but there is no definite answer? And then why do we need such a theory?)) How will you test students on the exam? He will write ANY answer... and he will ...this can also be))

maybe you have in terms of quantum mechanics? with such a probability, such an answer, with another probability, another answer))

 

[Ibix]

there are strict initial conditions

No there aren't. You keep failing to define "when". If you do that, there is an answer.

 

[mef]

No there aren't. You keep failing to define "when". If you do that, there is an answer.

t=0 start))

 

[PeroK]

I didn't talk about Newtonian gravity, only about his apple))

The GR calculations are in the thread I gave a link to.

 

[Ibix]

t=0 start))

You mean the Schwarzschild t coordinate? That isn't defined on the horizon so your question has no answer in those terms.

 

[PeroK]

t=0 start))

There is no "universal time in GR". Again, what you are syaing is: IF we assume universal time, then a black hole makes no sense. Which is true. Universal time (the same $t$ for everything) is not part of the theory of relativity.

 

[Orodruin]

there are strict initial conditions , but there is no definite answer? And then why do we need such a theory?)) How will you test students on the exam? He will write ANY answer... and he will ...this can also be))

No. You persist in applying your own misconceptions without any regards to being told that your question as posed is not well defined according to the theory. Obviously such a question would not be asked in an exam precisely because it is not a good question to ask.

 

[weirdoguy]

And then why do we need such a theory?

Let me guess - you didn't come here to learn, did you? You came here to force upon us all of your misconceptions just to tell us that GR is not correct.

 

[Dale]

You are answering a completely different question.
What happens to particles with zero angular momentum (radial)?

They fall in, just like particles with zero angular momentum about any spherical mass.

Do you not understand what I wrote about the view, from the point of view of an outsider?

Why should the particle care about the point of view of an outsider?

How much will the remote observer's clock show when, according to the local clock, matter reaches the horizon?

This is not a physical question, it is a mathematical question. And even as a mathematical question it is not fully specified since you have not indicated which coordinates you want to use to determine “when” things happen. If you use mathematically bad coordinates you will get a bad mathematical answer.

I'm setting a very simple task... so that it becomes completely clear to you. How long does it take for a Newtonian apple to move from Earth to a black hole in the center of our galaxy.

It is not a simple task because it is incompletely specified. You need to specify the coordinates. And even once you do so, this remains a mathematical exercise rather than a physical one

but there is no definite answer?

That is correct. You cannot have a definite answer to an indefinite question

 

[mef]

You mean the Schwarzschild t coordinate? That isn't defined on the horizon so your question has no answer in those terms.

This is defined at a finite point R (observer coordinate)
On the horizon, time is defined at least as a limit, just like in mathematical analysis. At the same time, some limits may diverge to infinity, but they exist!

but there are limits that do not exist, for example, sin(1/x) for x -> 0

 

[mef]

You have another misconception here: The idea that a global ”when”. The idea that there is a unique time of crossing the horizon as seen by a distant observer is a false one.

What makes you think I'm asking about the global when?
I'm asking about when only with the observer system

 

[PeroK]

What makes you think I'm asking about the global when?

Because he's taught relativity to hundreds of students and can see the mistakes you're making.

The only question for you is whether you have the desire to learn GR, which is hard work. Or, whether you prefer not to learn it, which is very easy to do.

You quoted some Real Analysis. 99.9% of the world's population do not understand real analysis and why $\sin \frac 1 x$ does not converge, as $x \rightarrow 0$. Real analysis is not easy to learn and many people think it's just some rubbish that mathematicians invented. It's easy for them to trash mathematics rather than learn it.

Ultimately, we've all put in the months of hard study to learn GR - and overcome the difficulties in understanding thinks like coordinate-dependence and invariance.

We can't force you to learn GR. But, you can't persuade us it's all wrong any more than you could persuade us that mathematics is all wrong. Because we've learned and understood it.

 

[Orodruin]

What makes you think I'm asking about the global when?
I'm asking about when only with the observer system

Because you are trying to construct a ”when” including both a stationary observer and an object falling through the event horizon by asking ”when” for a distant observer does an object pass the horizon. There is no such construction. The time coordinate t is just a coordinate. It does not define a universal sense of simultaneity. That no such concept exists should be clear already from special relativity. You yourself is the one standing in your own way by asserting that your ill defined questions are well defined.

On the horizon, time is defined at least as a limit, just like in mathematical analysis.

No, it is not and repeating the statement is not going to make it so. Time is undefined on the horizon because regular Schwarzschild coordinates are singular there. This also underlines that coordinate time is an exceedingly bad choice to define global simultaneity. It is somewhat useful for that outside the horizon due to its role as parametrising a timelike Killing flow. This however ends on the horizon.

 

[Orodruin]

and can see the mistakes you're making.

The hardest part of learning relativity is arguably letting go of hard ingrained preconceptions that are no longer valid.