What is the difference between an event horizon and a singularity? - Page 2

Argos · #31 (**permalink**) 29-June-2008, 02:39 PM

Quote:

Originally Posted by tommac

I get this ... however ... I am not sure if this answers my questions.
At the EH time relative to us stops. If time stops relative to us then relative to us no additional events will happen to anything that reaches the EH. It is stuck there forever.

Tim Thompson addresses it very well on post #14, Tommac.

If you leave your position of distant observer and go there to check it [by diving into the gravity well of the BH, for example], you´ll see that nothing is frozen at the EH, and you can cross it.

Disinfo Agent · #32 (**permalink**) 29-June-2008, 09:29 PM

Quote:

Originally Posted by tommac

Agreed! Just wanted to make sure i was looking at it correctly. Now wouldnt that mean that the EH is the singularity as viewed by us? ... let me fill in that jump of logic that i just made.

If time stopped ... then for the SOL to remain constant space must also be infinitely compressed basically to 0 thickness ... that means that a BH has 0 volume relative to us. In a previous post on this thread I mentioned that I believe a BH to be 2d basically a circle that is perpendicular to any and all lines of site it also seems to not have any time thus it is 2d ... Does this seem reasonable?

I don't follow you. What are you calling "black hole"?

A black hole is composed of several parts. The singularity is one of them, and the event horizon is another. The singularity has zero width (at least in the classical models currently available), the event horizon is a spherical surface with a nonzero radius. You seem to be speaking as though the two were the same. See the diagram here.

Does this clarify things for you?

tommac · #33 (**permalink**) 29-June-2008, 10:37 PM

Quote:

Originally Posted by Argos

Tim Thompson addresses it very well on post #14, Tommac.

If you leave your position of distant observer and go there to check it [by diving into the gravity well of the BH, for example], you´ll see that nothing is frozen at the EH, and you can cross it.

I get this point ... and since I do not plan on jumping into a black hole any time soon ( I am sure a few people on this board are disappointed ) then again I will state ...
relative to us no stars have or ever will collapse beyond the EH.

tommac · #34 (**permalink**) 29-June-2008, 10:44 PM

Quote:

Originally Posted by Disinfo Agent

I don't follow you. What are you calling "black hole"?

A star collapsing into an EH.

Quote:

Originally Posted by Disinfo Agent

A black hole is composed of several parts. The singularity is one of them, and the event horizon is another. The singularity has zero width (at least in the classical models currently available), the event horizon is a spherical surface with a nonzero radius. You seem to be speaking as though the two were the same. See the diagram here.

Does this clarify things for you?

I am proposing that they are the same. The event horizon has to be of 0 thickness and is stuck in time

now what do you mean by radius ... radius of a sphere or circle?
I agree that it has a radius but ... the radius of a circle not of a sphere , from a distant observer that is.

Disinfo Agent · #35 (**permalink**) 29-June-2008, 10:46 PM

Quote:

Originally Posted by tommac

relative to us no stars have or ever will collapse beyond the EH.

If you adopt a very strict sense of what a black hole is, then I will agree with you. In fact, the EH itself never actually forms, for a distant observer. However, the truth is that we don't really know exactly what black holes are. All we have are models of black holes, and we know those models are probably imperfect, because we haven't yet succeeded in marrying quantum mechanics to general relativity.

A more pragmatic point of view is also possible, though: if it walks like a duck, quacks like a duck, etc.... you might as well call it a duck.

Real world black holes may not be 100% exact incarnations of the Schwarzschild solution, or of the Kerr solution, but they should behave very similarly to those models (at least when you look at them from the outside).

Quote:

Originally Posted by tommac

I am proposing that they are the same.

If you mean the singularity and the event horizon (in the Kerr or the Schwarzschild black hole models), then they are definitely not the same.

Quote:

Originally Posted by tommac

now what do you mean by radius ... radius of a sphere or circle?

A sphere. The event horizon is spherical, as I noted previously.

tommac · #36 (**permalink**) 30-June-2008, 03:07 PM

Quote:

Originally Posted by Disinfo Agent

If you mean the singularity and the event horizon (in the Kerr or the Schwarzschild black hole models), then they are definitely not the same.

A sphere. The event horizon is spherical, as I noted previously.

I thought about this on the way into work ... and I realize my wording is not great and in fact I realize that they may not be the same after all. From our vantage point the EH is really the singularity because a more defined singularity can never be created.

Now a sphere ... hmmm ... I cant picture this sphere. the sphere would have 0 volume but is not a point. It needs to have 0 volume because space is fully compacted at the event horizon. But it has a radius ... I agree with that . How does that work. I can envision a circle ( kind of like a projection of the EH ) that circle would have 0 thickness but oddly enough be a circle from any vantage point. However I cant picture a sphere with 0 volume. Please elaborate.

John Mendenhall · #37 (**permalink**) 30-June-2008, 03:47 PM

Quote:

Originally Posted by Tim Thompson

Actually, no it doesn't. The whole point of the "coordinate singularity" is that it is only an illusion. Despite the title "relativity" we must remember that events are absolute. Everything that happens in one reference frame happens in all reference frames. If one observer sees something blow up, all observers will see it blow up. If two things smash into each other in one reference frame, they will smash into each other in all reference frames. And if something falls into the black hole in one reference frame, it will fall into the black hole in all reference frames. It only "looks like" things don't fall in, it only "looks like" time stops, but it is no more correct to say that time actually stops than it is to say that an optical illusion is real because we can see it. What we "see" is not reality, what really happens is reality.

Crushing point. Pun intended.

Seriously, well saind, Tim.

tommac · #38 (**permalink**) 30-June-2008, 04:49 PM

Quote:

Originally Posted by John Mendenhall

Crushing point. Pun intended.

Seriously, well saind, Tim.

The bigger question is when will it happen? I agree that this is true outside of the EH but at the EH in will happen in infinite time? and behind the EH I dont think that Tims reasoning of events holds true. Firstly because information about any event can not be communicated to the rest of the universe, secondly because we are probably dealing with imaginary space-time. An new event could have happened in our past at a microscopic level of zero consequence. It could happen after the end of time or before the big bang who knows.

Pittance · #39 (**permalink**) 30-June-2008, 04:58 PM

A few things were said above that aren't technically correct.

Matter that falls into an EH is not turned into energy. Especially not before it hits the EH. If you fell into a super massive BH, you would experience no pain or distortion as you hit the EH. If you did the same with a stellar BH, you would be torn to shreds from the tidal forces before you hit the EH. That is because the singularity is so much closer to you in a stellar, than a supermassive. Matter may indeed be turned into energy at some point after it hits the EH, but never before. The event horizon is a fairly boring thing in most regards. It is meaningless except for the fact that once something passes this boundary of blackness (from the outside), it will never return. Nothing special happens to matter and energy at this point, its just screwed out of its return-ticket.

"From a distant observer, no star has collapsed into a singularity". This is a bit weird. From a distant observer, we stopped receiving protons once the matter or energy hit the EH boundary. So we have no idea what happened. It all could have turned into a ham sandwich and we will never know. Either way, time did not stop. Our outside observation of the time has been halted because no photons can come to us. Also, time and light are quantum, not continuous. Some people have said that we would see a clock falling into a BH and see it hit the EH, and see that image forever. This is very wrong. We would see if falling, see the time slow, see it red shift, and eventually start to flicker and then nothing. This is because the photons emitted take longer and longer to come back to us because they are moving much slower the closer to the EH they are. A photon emitted exactly at the EH will remain motionless in regards to the singularity since the 2 forces are equal.

I believe the reason we think there is a singularity, instead of just an EH sized and shape sphere of mass, is because of the tidal forces observed. I "think" that the gravity given off by a 10-sun mass at a point is felt differently than a 10-sun mass given off by a 10-sun EH sphere. Which is why smaller BHs have such wild tidal forces, and super massive BHs are much more gentle indomitable forces.

There, I don't think I butched physics too much. I'm definitely not a physicist, but ive done a lot of casual research into BHs.

sabianq · #40 (**permalink**) 30-June-2008, 05:38 PM

Quote:

Originally Posted by Pittance

A few things were said above that aren't technically correct.

Matter that falls into an EH is not turned into energy.

What is a quasar?

is the matter that is falling into it not the source of the x-ray radiation?

alainprice · #41 (**permalink**) 30-June-2008, 05:58 PM

Is energy released as relatively dense matter falls into the BH? Yes

Is all matter converted to energy by some unknown process as it crosses the EH? No

Pick your question and you will have your answer.

Pittance: Why do you think there is a difference between a point and a shell or even a solid sphere from a distance?

sabianq · #42 (**permalink**) 30-June-2008, 06:05 PM

this is why i think matter is indeed changed to energy as it crosses the event horizon of a black hole

It should be noted, that in order for an object to actually
reach the speed of light, it must have no mass, since E=mass*speed of
light^2

from here
http://en.wikipedia.org/wiki/Event_horizon

Quote:

In general relativity, an event horizon is a boundary in spacetime, an area surrounding a black hole, beyond which events cannot affect an outside observer. Light emitted from inside the horizon can never reach the observer, and anything that passes through the horizon from the observer's side is never seen again.

is this a region where the gravitational attraction is equal to the speed of light?

meaning that as something falls into an event horizon of a black hole, it must be accelerated to the speed of light otherwise it would not be a black hole.

as stated above, if an object is approaching a relativistic velocity (meaning reaching the speed of light), then by if you follow special relativity dogma, that object must be turned into energy as it approaches and becomes equal to the speed of light.

Please, anybody tell me where I am lost in this reasoning.

Disinfo Agent · #43 (**permalink**) 30-June-2008, 06:34 PM

Quote:

Originally Posted by tommac

I thought about this on the way into work ... and I realize my wording is not great and in fact I realize that they may not be the same after all. From our vantage point the EH is really the singularity because a more defined singularity can never be created.

Now a sphere ... hmmm ... I cant picture this sphere. the sphere would have 0 volume but is not a point. It needs to have 0 volume because space is fully compacted at the event horizon.

Listen carefully: space is not fully compacted at the event horizon. That only happens (if at all) at the singularity, which is beyond the horizon, inside the black hole. It's not at the event horizon that all the mass in the black hole is supposed to be compacted into infinite density, or that time is supposed to stop. All of that is supposed to happen at the singularity.

The event horizon is merely the spherical boundary of the black hole. It's an imaginary boundary, like meridians on a globe, or the edge of Earth's atmosphere. And it's the point of no return (or rather the surface) for an infalling object. Once you cross it, you're inside the black hole, and you can no longer turn back. You must keep falling; in a Schwarzschild-type black hole you keep going until you hit the singularity along with the rest of the infalling matter.

Quote:

Originally Posted by tommac

However I cant picture a sphere with 0 volume. Please elaborate.

It's a hollow sphere -- or a spherical surface, more correctly. As such, it has a radius and a surface area, but zero volume. Like the surface of the Moon, or of a lake.

Quote:

Originally Posted by Pittance

"From a distant observer, no star has collapsed into a singularity". This is a bit weird.

Weird but, I believe, true. In general relativity, the reference frame of a distant observer is no less legitimate than the reference frame of an observer falling into the star that's collapsed into a black hole.

korjik · #44 (**permalink**) 30-June-2008, 07:14 PM

Quote:

Originally Posted by Disinfo Agent

Weird but, I believe, true. In general relativity, the reference frame of a distant observer is no less legitimate than the reference frame of an observer falling into the star that's collapsed into a black hole.

It should be true, but it is also pretty irrelevant. Any situation where the distribution of matter is important would put you in a frame that is not distant, and therefor the assumption that time stops at the EH is no longer good.

To put it simply, you can assume that matter crosses the EH and reaches the singularity, because any time the distinction is needed it is true.

Disinfo Agent · #45 (**permalink**) 30-June-2008, 07:21 PM

Quote:

Originally Posted by korjik

It should be true, but it is also pretty irrelevant. Any situation where the distribution of matter is important would put you in a frame that is not distant, and therefor the assumption that time stops at the EH is no longer good.

And yet it's likely that we will only ever be able to observe black holes from outside the event horizon. All the empirical data we'll ever have about them will be from afar, because an observer who got too close would never return, and any data that such an observer might gather would never reach the rest of the scientific community.

In a sense, that observer who crosses the horizon in finite time and gets to see the black hole fully formed, is what will remain forever a theoretical extrapolation.

Disinfo Agent · #46 (**permalink**) 30-June-2008, 08:19 PM

Quote:

Originally Posted by sabianq

this is why i think matter is indeed changed to energy as it crosses the event horizon of a black hole

It should be noted, that in order for an object to actually
reach the speed of light, it must have no mass, since E=mass*speed of
light^2

[...] is this a region where the gravitational attraction is equal to the speed of light?

meaning that as something falls into an event horizon of a black hole, it must be accelerated to the speed of light otherwise it would not be a black hole.

as stated above, if an object is approaching a relativistic velocity (meaning reaching the speed of light), then by if you follow special relativity dogma, that object must be turned into energy as it approaches and becomes equal to the speed of light.

Please, anybody tell me where I am lost in this reasoning.

The event horizon is not a region where the gravitational attraction equals the speed of light. If you think about it, that wouldn't even make sense: the gravitational pull is a force -- how could it equal a speed? The event horizon is also not a region where all infalling objects reach the speed of light; quite the contrary.

The EH is simply the distance of no return: if you got that close to a black hole, you would need to travel at least at the speed of light to escape from it -- which you can't do. So, if you step into the EH, you will inevitably fall towards the centre of the BH. It makes no difference how fast you were going when you arrived there.

sabianq · #47 (**permalink**) 30-June-2008, 08:35 PM

Quote:

Originally Posted by Disinfo Agent

The event horizon is not a region where the gravitational attraction equals the speed of light. If you think about it, that wouldn't even make sense: the gravitational pull is a force -- how could it equal a speed? The event horizon is also not a region where all infalling objects reach the speed of light; quite the contrary.

oh.. well, I was under the assumption that the gravitational acceleration is the acceleration of an object caused by the force of gravity from another object and is measured in meters per second

http://en.wikipedia.org/wiki/Gravitational_acceleration

and i further assumed that in order to escape the gravitational pull, one would have to accelerate faster than the gravitational acceleration to escape the gravity well
isn't this called an "escape velocity"?

http://en.wikipedia.org/wiki/Escape_velocity

Quote:

the escape velocity is the minimum speed an object without propulsion needs to attain in order to "escape" from gravity, i.e., so that gravity will never manage to pull it back

and since light travels at the speed of light, then I was assuming that if light could not escape a gravitational field then the gravitational acceleration of that field must be equal to or exceed the speed of light. since we know that nothing can exceed the speed of light including gravity, then the escape velocity of an event horizon must be equal to the speed of light which would account for the fact that light cannot escape a black holes gravity well.

am i wrong?
I am just trying to really understand this

Disinfo Agent · #48 (**permalink**) 30-June-2008, 08:39 PM

What you are missing is that the matter which falls into a black hole does not travel as fast as that. So, it does not need to be converted into energy before it enters the black hole.

tommac · #49 (**permalink**) 30-June-2008, 08:51 PM

Quote:

Originally Posted by sabianq

What is a quasar?

is the matter that is falling into it not the source of the x-ray radiation?

Isnt it from electrons being ripped off the matter that is falling in or something like that? I think the stuff that falls in doesnt get seperated it is before it falls in that the seperation occurs.

korjik · #50 (**permalink**) 30-June-2008, 08:53 PM

Quote:

Originally Posted by sabianq

this is why i think matter is indeed changed to energy as it crosses the event horizon of a black hole

It should be noted, that in order for an object to actually
reach the speed of light, it must have no mass, since E=mass*speed of
light^2

from here
http://en.wikipedia.org/wiki/Event_horizon

is this a region where the gravitational attraction is equal to the speed of light?

meaning that as something falls into an event horizon of a black hole, it must be accelerated to the speed of light otherwise it would not be a black hole.

as stated above, if an object is approaching a relativistic velocity (meaning reaching the speed of light), then by if you follow special relativity dogma, that object must be turned into energy as it approaches and becomes equal to the speed of light.

Please, anybody tell me where I am lost in this reasoning.

If you are in the frame of the falling object, it dosent evaporate into energy.

tommac · #51 (**permalink**) 30-June-2008, 09:04 PM

Quote:

Originally Posted by Disinfo Agent

Listen carefully: space is not fully compacted at the event horizon. That only happens (if at all) at the singularity, which is beyond the horizon, inside the black hole. It's not at the event horizon that all the mass in the black hole is supposed to be compacted into infinite density, or that time is supposed to stop. All of that is supposed to happen at the singularity.

From the distant observer these two are the same.
If time stops at the EH or if light can not escape then time has stopped. The light at exactly the EH would take an infinite time to reach us.

As one approaches a stop in time in order to keep the speed of light constant locally distances need to contract appropraitely. So as time slows ... the speed of light is constant so distances need to shorten. as we approach the stopage of time the space in that area needs to contract ( I am not sure if the dimensions curl up or what ) for example :

From wikipedia

Quote:

Today, it is defined by the International Bureau of Weights and Measures as the distance travelled by light in absolute vacuum in 1⁄299,792,458 of a second.

so
m=c * 1⁄299,792,458 seconds

As seconds relatively shorten ( local observation vs distant observation ) then so must m. Removing the constants ...

m is proportial to t.

So if time is stopped the meter is a point. Or if you dont believe in that nonsense ... as time approaches stopping the meter approaches a point.

Now that all happens at the EH ( from the view point of a distant observer ).
So what is the sphere of the EH that you discuss ... At the point that a star collapses to the exact size of an EH the star no longer has any volume as any mesurements ( say in meters ) would make any sense since meters are a point ... per the above argument.

I think there is still a radius however but no volume. So the way I visualize this is a circle from any vantage point. However the circle is absolutely flat.

sabianq · #52 (**permalink**) 30-June-2008, 09:07 PM

Quote:

Originally Posted by Disinfo Agent

The event horizon is not a region where the gravitational attraction equals the speed of light. If you think about it, that wouldn't even make sense: the gravitational pull is a force

So is gravity a force like magnetism then?

i had always thought it was caused by the geometric shape of spacetime and was not actually a force.

tommac · #53 (**permalink**) 30-June-2008, 09:07 PM

Quote:

Originally Posted by Disinfo Agent

It's a hollow sphere -- or a spherical surface, more correctly. As such, it has a radius and a surface area, but zero volume. Like the surface of the Moon, or of a lake.

the strange part though is that it certainly is not hollow and the distance from one pole to the other ( when seen dead on ) is 0.

tommac · #54 (**permalink**) 30-June-2008, 09:09 PM

it is funny because I am arguing that it is the ONLY relevant perspective. As it is the perspective of any observer you will ever meet.

Quote:

Originally Posted by korjik

It should be true, but it is also pretty irrelevant. Any situation where the distribution of matter is important would put you in a frame that is not distant, and therefor the assumption that time stops at the EH is no longer good.

To put it simply, you can assume that matter crosses the EH and reaches the singularity, because any time the distinction is needed it is true.

tommac · #55 (**permalink**) 30-June-2008, 09:12 PM

I look at the EH as a point in which space time is so curved that light will begin to orbit rather that travel outwards ( or runs on a treadmill ). slightly out from the EH light can escape into space ... slightly in and it gets sucked back to the singularity.

Quote:

Originally Posted by Disinfo Agent

The event horizon is not a region where the gravitational attraction equals the speed of light. If you think about it, that wouldn't even make sense: the gravitational pull is a force -- how could it equal a speed? The event horizon is also not a region where all infalling objects reach the speed of light; quite the contrary.

The EH is simply the distance of no return: if you got that close to a black hole, you would need to travel at least at the speed of light to escape from it -- which you can't do. So, if you step into the EH, you will inevitably fall towards the centre of the BH. It makes no difference how fast you were going when you arrived there.

tommac · #56 (**permalink**) 30-June-2008, 09:14 PM

Quote:

Originally Posted by sabianq

So is gravity a force like magnetism then?

i had always thought it was caused by the geometric shape of spacetime and was not actually a force.

well I think you are getting into TOE. I think both are right ...
it is a force ... but it is not your average force. But it is at minimum force-like as it acts as a force on matter.

alainprice · #57 (**permalink**) 30-June-2008, 09:14 PM

Quote:

Originally Posted by sabianq

So is gravity a force like magnetism then?

i had always thought it was caused by the geometric shape of spacetime and was not actually a force.

Bingo! Not a force, but the geometry of spacetime itself.

If I happen to be a particle stuck in the accretion disk, I will cross the EH at a speed I consider to be 0. The particles near me are not moving relative to me. So when I reach the point where 'space' is 'moving' at the speed of light, I am still stationary with respect to this 'space'. Therefore, I consider myself motionless but all observers outside the EH will measure my speed as c when I am at the so called EH boundary.

sabianq · #58 (**permalink**) 30-June-2008, 09:16 PM

Quote:

Originally Posted by Disinfo Agent

What you are missing is that the matter which falls into a black hole does not travel as fast as that. So, it does not need to be converted into energy before it enters the black hole.

I guess I dont get it.
(probably to stupid)

but from what i have read...

gravitational acceleration is the acceleration of an object caused by the force of gravity from another object. this gravitational acceleration is equal to the escape velocity.

if the escape velocity is equal to the speed of light, then objects falling into a body like a black hole will reach the escape velocity at the event horizon.

why would and what would stop a mass's velocity/speed from being equal to the gravitational acceleration? I would have to say nothing except for the fact that mass cannot reach c. so it must convert to energy in order to cross the event horizon.

I dont know

korjik · #59 (**permalink**) 30-June-2008, 09:26 PM

Quote:

Originally Posted by tommac

it is funny because I am arguing that it is the ONLY relevant perspective. As it is the perspective of any observer you will ever meet.

There is no situation where a distant observer would be able to tell wether the mass of a black hole is a surface distribution at the event horizion or a central distribution at the singularity. As a result, the distinction between the two is kinda pointless.

tommac · #60 (**permalink**) 30-June-2008, 09:37 PM

Quote:

Originally Posted by korjik

There is no situation where a distant observer would be able to tell wether the mass of a black hole is a surface distribution at the event horizion or a central distribution at the singularity. As a result, the distinction between the two is kinda pointless.

That is exactly my point. The singularity is the same as the event horizon for a distant observer.

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