Black Hole Time Dilation And Observation Of Accretion

[c-english]

Hello.

If time is infinitely dilated at the edge of the EH, how do we observe accretion?

 

[phinds]

Hello.

If time is infinitely dilated at the edge of the EH, how do we observe accretion?

the accretion disk is outside the EH so we see most it just fine. Observations of events very close to the EH are observations in our frame of reference of events that actually happened a long time ago in our frame of reference. We know, however, that this is basically an optical illusion. That is, we don't believe that the infall of matter isn't happening just because our eyes tell us it hasn't happened yet.

 

[kuro-hi]

My guess from a quick read here
http://www.physicsoftheuniverse.com/topics_blackholes_event.html
is that the accretion disk is not the event horizon it is surounding the event horizon and therefore still able to emit light. it is the layer just before the point of no return. The site does mention another oddity tho and that is that space is being sucked into the black hole faster than the speed of light. How is that possible?

 

[Nugatory]

The site does mention another oddity tho and that is that space is being sucked into the black hole faster than the speed of light. How is that possible?

The site is wrong about that, or at least worded so carelessly that it might as well be wrong. Stuff like this is the reason for the Physics Forums rule requiring that references be to accepted sources as described in the rules.

 

[kuro-hi]

Oh my apologies i will have to refresh my memory as to accepted sites.

 

[mfb]

Accretion disks end at the lowest stable orbit - there are possible orbits even below that point (and escape is possible even below the lowest orbit), but those are unstable, and nothing orbits in exactly the right direction - material that is below the lowest stable orbit quickly falls into the black hole.
The result: the inner edge of the accretion disk is well separated from the event horizon.

 

[AshUchiha]

Hello.

If time is infinitely dilated at the edge of the EH, how do we observe accretion?

There is no such thing as "time" by the way

 

[Cruz Martinez]

There is no such thing as "time" by the way

Time is what a clock measures? How can you say there is no such thing?

 

[Chalnoth]

There is no such thing as "time" by the way

Hello.

If time is infinitely dilated at the edge of the EH, how do we observe accretion?

We never do observe objects passing the event horizon. The light from objects falling into a black hole gets redshifted until it can no longer be observed. The image of the object falling into the black hole gets redder and falls more slowly the more we observe it.

But there's quite a bit of activity before the infalling objects get that far! As those above mentioned, the accretion disk itself is a fair distance from the black hole, and the matter that makes up the disk tends to be exceedingly hot, often so hot that it glows in x-ray wavelengths. There's also the fact that real black holes rotate, and rotating black holes are quite a bit more complex than non-rotating black holes. Specifically, the rotation of the black hole causes a sort of "bulge" to form just outside the event horizon and wider at the equator. This bulge is known as the ergosphere, and much of the matter that falls into the ergosphere tends to be ejected from the poles of the black hole at extraordinarily high energies. These jets can be tremendous for larger black holes. The supermassive black holes at the centers of galaxies can spawn jets that are larger in size than the galaxy they originate from. For example:
http://www.nasa.gov/mission_pages/hubble/science/hercules-a.html

 

[virgil1612]

Hello

My first post here. I am very much interested in understanding this.

Because as of yet, I don't understand it. I have read the thread, but I found no explanation or justification to 'how a black hole grows'. Let's look at any active galaxy, maybe a quasar. So there is something huge happening in its center, around the black hole. I'm measuring the mass of the BH looking at the speed of the stars rotating around it. Say I am a magical human being with a lifespan of 1 billion year. So after 1 billion years I'm observing again this galaxy (quasar, whatever), I'm measuring its mass again. Will I find the same value? For me, from my point of reference, this BH has not grown 1 gram in one billion years? Because it seams that I cannot see anything 'actually passing the BH horizon', so for me this BH cannot grow, so I should find it has the same mass. For me, this conclusion is unacceptable (of course, as unacceptable as it is for me, it could be correct).

I guess you can feel the frustration in my words. I would be grateful to anyone showing the weak point in this logic. Or a link to something I could read on the net, that would be great.

Cheers, Virgil

 

[rootone]

Over a billion years there is a good chance that you would see the the black hole capturing and consuming objects ranging from dust clouds to entire solar systems.
When this happens the Black hole mass increases.
It's thought to be probable that black holes can merge as well, so creating a more massive black hole.

Although you won't actually see an object crossing the event horizon, it certainly can happen.

 

[mfb]

So after 1 billion years I'm observing again this galaxy (quasar, whatever), I'm measuring its mass again.

You'll measure a larger value.

Because it seams that I cannot see anything 'actually passing the BH horizon'

You cannot see the light, but the process still happens and you can still see that the mass of the black hole increased. It is not even relevant how it looks very close to the event horizon - for the mass measurement as you described it, it is sufficient if more mass falls into the accretion disk.

 

[virgil1612]

Over a billion years there is a good chance that you would see the the black hole capturing and consuming objects ranging from dust clouds to entire solar systems.
When this happens the Black hole mass increases.
It's thought to be probable that black holes can merge as well, so creating a more massive black hole.

Although you won't actually see an object crossing the event horizon, it certainly can happen.

You mean I can see/observe/measure matter passing the event horizon?

You'll measure a larger value.
You cannot see the light, but the process still happens and you can still see that the mass of the black hole increased. It is not even relevant how it looks very close to the event horizon - for the mass measurement as you described it, it is sufficient if more mass falls into the accretion disk.

But again, I read than me, as an external observer, I cannot see 'anything' passing the EH. So you mean, I cannot see the light but I can see heavy matter, baryons, passing the EH?

 

[mfb]

It depends on what you call see/observe/measure. Can you see an electron? You can use tools that allow to be quite sure you have an electron at some place, but does that mean you "see" it? In the same way, you can be quite sure stuff fell into the black hole. Everything beyond that is philosophy and I won't discuss it.

 

[rootone]

What you will observe is the light from the infalling object becoming increasingly redshifted until it is no longer detectable.
That is a consequence of gravitational time dilation, an effect which is described by relativity.
From your point of view the object just fades away, but for anyone unfortunate enough to be standing on the infalling object they will actually cross the horizon, and the matter of which they were composed is now (in some form), part of the black hole.

 

[virgil1612]

Although you won't actually see an object crossing the event horizon, it certainly can happen.

But not for me, cause I can't see it happen!

What you will observe is the light from the infalling object becoming increasingly redshifted until it is no longer detectable.
That is a consequence of gravitational time dilation, an effect which is described by relativity.
From your point of view the object just fades away, but for anyone unfortunate enough to be standing on the infalling object they will actually cross the horizon, and the matter of which they were composed is now (in some form), part of the black hole.

I know about light redshifted and gravitational time dilation, I wasn't talking about this.
>>...but for anyone unfortunate enough to be standing on the infalling object... >> Sure, but I'm not talking about him, I'm talking about a person on Earth.

It depends on what you call see/observe/measure. Can you see an electron? You can use tools that allow to be quite sure you have an electron at some place, but does that mean you "see" it? In the same way, you can be quite sure stuff fell into the black hole. Everything beyond that is philosophy and I won't discuss it.

I'm talking about measuring the mass of a black hole. It can be done looking at the velocities of objects rotating around it. This is not philosophy, this is physics. How can I 'measure' an increased mass of the BH since for me nothing passes its event horizon?

 

[mfb]

Please don't make multiple posts in a row, you can edit your posts to add something.

But not for me, cause I can't see it happen!

You also can't see my typing this post because the light will never reach you (it is blocked by the walls of the room I am in). Is there any doubt of it happening?

How can I 'measure' an increased mass of the BH since for me nothing passes its event horizon?

The matter does pass the event horizon. You just never see light from this exact event, but you see every other effect (like the increased mass).

 

[PeterDonis]

not for me, cause I can't see it happen!

Do you only believe things happen when you see them happen? If the Sun sets and goes below your horizon, do you believe it's not there any more?

If we see objects falling into a particular compact region, and the light from them redshifts until it can't be detected, and they never come out again, where do you think they went, if not inside the black hole?

How can I 'measure' an increased mass of the BH since for me nothing passes its event horizon?

Just the way you said: look at orbital velocities of objects around it. The orbital velocity of an object at a given radius will be larger after something has fallen into the hole. You don't have to see the object fall below the event horizon to measure that.

 

[rootone]

In your 'frame of reference' you won't see anything because once the object crosses the horizon no light or any other form of information is able to reach you.
You will measure less and less photons as it approaches the horizon, and eventually so few that it may as well be zero.
Maybe you will see one highly redshifted photon every few million years, that valiantly struggled to make it to your detector having started it's journey a little outside of the horizon.
In the objects fame of reference though, it crossed the event horizon and any light or information of any kind it emits will never be seen by you.

 

[virgil1612]

Please don't make multiple posts in a row, you can edit your posts to add something.
You also can't see my typing this post because the light will never reach you (it is blocked by the walls of the room I am in). Is there any doubt of it happening?The matter does pass the event horizon. You just never see light from this exact event, but you see every other effect (like the increased mass).

Sorry for the multiple posts. I'm just new and excited of being here.
There is no law of physics forbidding you, from my system of reference, to type this. When I say 'see' please understand that I mean 'measuring' or 'understanding that it happens'. It is not 'see' as in using my eyes.
It is physics forbidding matter, particles, anything, to pass the EH from my reference system. This is relevant.

 

[virgil1612]

Do you only believe things happen when you see them happen? If the Sun sets and goes below your horizon, do you believe it's not there any more?

If we see objects falling into a particular compact region, and the light from them redshifts until it can't be detected, and they never come out again, where do you think they went, if not inside the black hole?

Just the way you said: look at orbital velocities of objects around it. The orbital velocity of an object at a given radius will be larger after something has fallen into the hole. You don't have to see the object fall below the event horizon to measure that.

Please see my comment about 'seeing' from my answer to mfb.
<<where do you think they went>> From my reference frame, they are not allowed to get there. From my reference frame, physics does not allow them to get there.

 

[PeterDonis]

It is physics forbidding matter, particles, anything, to pass the EH from my reference system.

No, that's not what physics says. Physics says that what you are calling "my reference system" is incomplete; it doesn't cover all of spacetime. You can't prevent things from happening by adopting a particular reference system.

 

[virgil1612]

No, that's not what physics says. Physics says that what you are calling "my reference system" is incomplete; it doesn't cover all of spacetime. You can't prevent things from happening by adopting a particular reference system.

Thanks Peter. Could you, whenever you have time, to go into greater detail about this? Or maybe provide some reference where I could read further? Even something mathematical... While I never took a course in General Relativity, I think I could follow the equations involved.
Virgil.

 

[Nugatory]

It is physics forbidding matter, particles, anything, to pass the EH from my reference system. This is relevant.

The only thing the physics prohibits is the light from the object crossing the horizon reaching you.

However, there is a time in your reference frame such that if you send a radio message to the poor guy falling into the black hole before that time he will be able to to reply to it; but after that time although your message may reach him, he will be unable to reply to it because he will be inside the horizon by the time it reaches him. There is also a later time such that if you don't send your radio message before that later time it will never get to the infaller because he'll have reached the central singularity before the message gets to him. That sure sounds like "he passes the event horizon and reaches the singularity even though the light from it never reaches me" to me.

 

[phinds]

Please see my comment about 'seeing' from my answer to mfb.
<<where do you think they went>> From my reference frame, they are not allowed to get there. From my reference frame, physics does not allow them to get there.

No, from your reference frame you cannot SEE them get there. This is basically akin to an optical illusion. You certainly can KNOW that they get there. Do you think they magically overcome gravity and hover at the event horizon?

 

[Nugatory]

Thanks Peter. Could you, whenever you have time, to go into greater detail about this? Or maybe provide some reference where I could read further? Even something mathematical... While I never took a course in General Relativity, I think I could follow the equations involved.
Virgil.

Someone very recently posted a pretty decent paper on misconceptions about black holes that worked through most of what we're talking about here using Kruskal coordinates. I don't have time to find it now, but someone will remember it and post the link.

And in the meantime, you could try the wiki article at http://en.wikipedia.org/wiki/Kruskal–Szekeres_coordinate . You don't have to grovel through all the coordinate transforms to see what the worldline of the infalling observer looks like (any line that starts in region I and crosses into region II, and must always be steeper than 45 degrees), the worldline of you hovering outside the black hole (one of the hyperbolas of constant r in region I), and the worldlines of light signals exchanged between them (45-degree straight lines moving upwards).

You can ignore regions III and IV for now; they're interesting but not relevant to this problem.

 

[virgil1612]

Someone very recently posted a pretty decent paper on misconceptions about black holes that worked through most of what we're talking about here using Kruskal coordinates. I don't have time to find it now, but someone will remember it and post the link.

And in the meantime, you could try the wiki article at http://en.wikipedia.org/wiki/Kruskal–Szekeres_coordinate . You don't have to grovel through all the coordinate transforms to see what the worldline of the infalling observer looks like (any line that starts in region I and crosses into region II, and must always be steeper than 45 degrees), the worldline of you hovering outside the black hole (one of the hyperbolas of constant r in region I), and the worldlines of light signals exchanged between them (45-degree straight lines moving upwards).

You can ignore regions III and IV for now; they're interesting but not relevant to this problem.

Thank you, this is excellent.
Virgil.

 

[virgil1612]

Hello,

I think I finally understand.
I will quote some answers that were spot-on, except that I wasn't prepared to really understand their meaning.

phinds: <<events very close to the EH are observations in our frame of reference of events that actually happened a long time ago>>
mfb: <<You cannot see the light, but the process still happens and you can still see that the mass of the black hole increased.>>
rootone <<From your point of view the object just fades away, but for anyone unfortunate enough to be standing on the infalling object they will actually cross
the horizon>>
mfb <<The matter does pass the event horizon. You just never see light from this exact event, but you see every other effect (like the increased mass).>>
PeterDonis <<You can't prevent things from happening by adopting a particular reference system.>>
Nugatory <<The only thing the physics prohibits is the light from the object crossing the horizon reaching you.>>
phinds <<No, from your reference frame you cannot SEE them get there. This is basically akin to an optical illusion.>>

So what was my problem: I was talking about what we could see (like in seeing with our eyes), the whole stuff about light becoming redshifted and everything slowly freezing because of the gravitational time dilation. But in my head, the ghost hanging at the edge of the black hole was the real object, its particles, that somehow were not allowed to pass over the event horizon! It's kind of embarrassing, especially since not seeing the actual object is such a common occurrence in Astrophysics (you don't see Andromeda Galaxy as it is right now, but as it was 2.5 mill years ago). I really feel the need to apologize for my sometimes idiotic answers.

Thanks for your help,
Virgil.

 

[phinds]

tI really feel the need to apologize for my sometimes idiotic answers.

Absolutely unnecessary. We all hit conceptual roadblocks from time to time in cosmology and quantum mechanics where things are not always as they seem or are not as our "common sense / logic / intuition" tells us they should be.

 

[rootone]

Those AHA! moments are one of most fun parts in science.

 

[Lord Crc]

However, there is a time in your reference frame such that if you send a radio message to the poor guy falling into the black hole before that time he will be able to to reply to it; but after that time although your message may reach him, he will be unable to reply to it because he will be inside the horizon by the time it reaches him. There is also a later time such that if you don't send your radio message before that later time it will never get to the infaller because he'll have reached the central singularity before the message gets to him.

Is the minimum difference between the times you mention effectively zero? Does the black hole mass matter to any significant degree? Just curious if it's an extended event for the observer or if it's also effectively instantaneous.