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View Full Version : Does a black hole propogate gravity?

tommac
2008-Apr-30, 01:52 PM
If nothing can escape the warping of space-time of a black hole AND gravity travels lazily at the speed of light. Then how could a black hole have a gravitational field?

phunk
2008-Apr-30, 03:49 PM
Gravity IS the warping of space time. It is not something that has to be emitted by the black hole.

phunk
2008-Apr-30, 03:53 PM
It's not gravity that travels at the speed of light, it's changes to the gravitational field. Say you have a large mass and move it suddenly. Someone feeling the gravity from that mass won't notice the change in its pull until the same time that they see the mass move, because the change takes the same amount of time to propogate as the photons take to reach him.

alainprice
2008-Apr-30, 03:57 PM

Charge is transmitted by virtual photons. How can charge escape a black hole? At least we're still talking about photons.

tommac
2008-Apr-30, 04:14 PM
It's not gravity that travels at the speed of light, it's changes to the gravitational field. Say you have a large mass and move it suddenly. Someone feeling the gravity from that mass won't notice the change in its pull until the same time that they see the mass move, because the change takes the same amount of time to propogate as the photons take to reach him.

So ... near a black hole do you have that effect? Since they will never see the singularity ( which is the black hole )???

alainprice
2008-Apr-30, 04:30 PM
The singularity + the event horizon = the black hole.

Tim Thompson
2008-Apr-30, 04:56 PM
If nothing can escape the warping of space-time of a black hole AND gravity travels lazily at the speed of light, then how could a black hole have a gravitational field?
Because gravity is not something that is emitted. Gravity is the curvature of spacetime induced by mass. The black hole has mass, and therefore it has gravity. The gravity is outside the black hole, and does not propagate from inside the black hole.

It's not gravity that travels at the speed of light, it's changes to the gravitational field. ...
Indeed so, and the changes propagate in the form of gravitational waves. Gravitational waves, predicted by general relativity, are a necessary consequence of the fact that changes in gravitational fields do not propagate instantaneously, but rather at a finite speed. Although we have not yet unambiguously observed gravitational waves, evidence from the orbital decay of binary pulsars is consistent with the predictions of general relativity (i.e., Bhat, Bailes & Verbiest, 2008 (http://adsabs.harvard.edu/abs/2008arXiv0804.0956B); Bisnovatyi-Kogan, 2006 (http://adsabs.harvard.edu/abs/2006astro.ph.11398B); Weisberg & Taylor, 2005 (http://adsabs.harvard.edu/abs/2005ASPC..328...25W); van Straten, et al., 2001 (http://adsabs.harvard.edu/abs/2001Natur.412..158V); Taylor & Weisberg, 1982 (http://adsabs.harvard.edu/abs/1982ApJ...253..908T)). So there is plenty of observational evidence in favor of the proposition that general relativity, and the finite propagation speed of gravitational waves, are valid.

alainprice
2008-Apr-30, 05:09 PM
Ask yourself if gravity attracts gravity? If yes, then a black hole wouldn't allow the outside to know its mass.

If no, we have the real world that we appear to be living in.

tommac
2008-Apr-30, 05:49 PM
Ask yourself if gravity attracts gravity? If yes, then a black hole wouldn't allow the outside to know its mass.

If no, we have the real world that we appear to be living in.Obviously I know the answer to the question ... but what I am hinting at is how does a gravitational wave escape a curvature of space so severe that space-time is warped back in on itself.

What you seem to be saying is that the warping itself of space time in the form of a gravitational wave is immune to space itself. Thus not effected by the curvature of space-time

alainprice
2008-Apr-30, 06:02 PM
A gravitational wave IS the curvature of space.

Keep working on making the proper link between gravity, spacetime and matter-energy.

tommac
2008-Apr-30, 06:14 PM
A gravitational wave IS the curvature of space.

Ok so ... relative to any point in the curved space, gravity can travel faster than the speed of light.

alainprice
2008-Apr-30, 06:51 PM
No, that is the basis of GR. Gravity always travels at the speed of light, relative to the starting point.

Relative to a starting point, nothing can move faster than 'c' locally.

tommac
2008-Apr-30, 07:00 PM
No, that is the basis of GR. Gravity always travels at the speed of light, relative to the starting point.

Relative to a starting point, nothing can move faster than 'c' locally.

If you have a place inside of the black hole's event horizon. If you shine a light in your duracell powered flashlight ourwards from inside the EH. Space-Time is curved in such a way that the light travels at the speed of light but will never get to the EH. However if you were standing the surface of a superdense star that just got eaten the gravity from that star would be felt outside of the EH. Therefor it can not travel along the same path as the light coming off of that star OR if it did it would need to travel faster than than the light from your flashlight to get to any point outside of the EH.

seanhogge
2008-Apr-30, 07:04 PM
That's akin to how it would work if gravity were subject to itself.

Gravity isn't composed of photons. Why would you expect it to behave as light would or be subject to the same influences and rules?

tommac
2008-Apr-30, 07:10 PM
That's akin to how it would work if gravity were subject to itself.

Gravity isn't composed of photons. Why would you expect it to behave as light would or be subject to the same influences and rules?

Ignoring how it works ... I am trying to make a statement here.

Gravitational waves move faster than the speed of light at least from the singularity of a black hole? ( true or false )

alainprice
2008-Apr-30, 07:11 PM
False.

My attempts to assist you have been fruitless. I will cease to participate.

Best of luck in your endeavors.

seanhogge
2008-Apr-30, 07:15 PM
If you're going to ignore how it works, how can you make statements about how it works?

tommac
2008-Apr-30, 07:17 PM
False.

My attempts to assist you have been fruitless. I will cease to participate.

Best of luck in your endeavors.

I am really sorry that you see it like that. How do you define fruitless? Meaning that I didnt totally and fully understand completely what you were saying?

tommac
2008-Apr-30, 07:21 PM
If you're going to ignore how it works, how can you make statements about how it works?

uggg ...

look ... space time is curved. Light travels based on that curature right? But since gravity causes the curvature it is does not follow the curve right?

Then how can one measure how fast gravity travels if not to compare it with the speed of light?

Is it safe to say that gravity travels at the speed of light in a perfect vacuum void of any gravitational forces?

Now if there was another BH in the way curing the space in front of the gravitational waves from our BH ... would something on the other side of our BH also experience the gravity from our BH ?

I mean i am trying to agree with all of you but there seems to be one missing piece.

NEOWatcher
2008-Apr-30, 07:41 PM
... space time is curved. Light travels based on that curature right? But since gravity causes the curvature it is does not follow the curve right?
I have always pictured that gravity IS the curvature. In other words, the detection of curvature is called gravity.

I mean i am trying to agree with all of you but there seems to be one missing piece.
It seems like you keep equating gravity with an actual object. It's not.

You either need to take a leap to the abstract way of thinking, or need to understand the mathematical definition of it. It just doesn't match anything on the scale of everyday human experience.

tommac
2008-Apr-30, 08:06 PM
I have always pictured that gravity IS the curvature. In other words, the detection of curvature is called gravity.

It seems like you keep equating gravity with an actual object. It's not.

You either need to take a leap to the abstract way of thinking, or need to understand the mathematical definition of it. It just doesn't match anything on the scale of everyday human experience.

OK I dont need an actual object for gravity.
I am saying that gravity ( I dont like this term because I think of blue shift when I think of gravity ) ... lets call it anti-gravity ... although I am sure that someone will complain here ...

I am saying that anti-gravities ( again ... I am just using that word because I am forced to ) curvature of space time over long distances ( trillions of light years ) would cause a signifigant red shift ... much stronger than the blue shift ( or red shift if the observer was inside of it ) that occurns near a gravity well

seanhogge
2008-Apr-30, 09:34 PM
look ... space time is curved. Light travels based on that curature right? But since gravity causes the curvature it is does not follow the curve right?

I'd be willing to go with that, yes.

Then how can one measure how fast gravity travels if not to compare it with the speed of light?

We're not measuring the travel of gravity. Gravity as we know it can only be observed by its effect on matter. So we're measuring the speed of reaction by matter of distance to a gravitational source. For example; an object closer to a gravitational source "feels" that source more quickly than one farther away.

Is it safe to say that gravity travels at the speed of light in a perfect vacuum void of any gravitational forces?

Gravity doesn't react to matter. It only acts upon it. So perfect vacuums are irrelevant.

Now if there was another BH in the way curing the space in front of the gravitational waves from our BH ... would something on the other side of our BH also experience the gravity from our BH ?

Yes. It would feel the net force of both black holes. Gravity doesn't interact with itself. It concatenates such that the first black hole's gravitational attraction is added to the second from the perspective of an outside observer.

I mean i am trying to agree with all of you but there seems to be one missing piece.

It rather seems that no matter how many times it's said by those who are far more qualified than we, your response is perpetually "but that can't be," instead of "I don't understand how that can be."

You and I both need to face facts that our limited expertise will require a certain amount of authoritative dicta. Only when we fully understand the underpinnings of the dicta may we begin to question it with any hope of progress.

It seems worthy of noting that most of the anti-mainstream discoveries have almost entirely been made by those intimately enmeshed with mainstream work.

alainprice
2008-Apr-30, 09:39 PM
It seems worthy of noting that most of the anti-mainstream discoveries have almost entirely been made by those intimately enmeshed with mainstream work.

Deep!

tommac
2008-Apr-30, 10:42 PM
I'd be willing to go with that, yes.

We're not measuring the travel of gravity. Gravity as we know it can only be observed by its effect on matter. So we're measuring the speed of reaction by matter of distance to a gravitational source. For example; an object closer to a gravitational source "feels" that source more quickly than one farther away.

Gravity doesn't react to matter. It only acts upon it. So perfect vacuums are irrelevant.

Yes. It would feel the net force of both black holes. Gravity doesn't interact with itself. It concatenates such that the first black hole's gravitational attraction is added to the second from the perspective of an outside observer.

It rather seems that no matter how many times it's said by those who are far more qualified than we, your response is perpetually "but that can't be," instead of "I don't understand how that can be."

You and I both need to face facts that our limited expertise will require a certain amount of authoritative dicta. Only when we fully understand the underpinnings of the dicta may we begin to question it with any hope of progress.

It seems worthy of noting that most of the anti-mainstream discoveries have almost entirely been made by those intimately enmeshed with mainstream work.

Ummm ... I am not sure that I am saying that they are wrong ... but more that I dont understand what they are saying ... when I say I dont agree ... it must be prefixed with the fact that I have only been thinking about this stuff for a month or so. I am sure even neverfly knows more than I do. I just dont agree with stuff until I understand it ... sorry if I sounded too confident in my statements.

tommac
2008-Apr-30, 10:43 PM
OK all ... instead of starting a new thread ... which I think is needed for this ... per neverflys advice I will keep it here.

Is the speed of propogation of gravity constant? That is regardless of where you observe the effects of gravity does its speed of propogation appear constant?

speedfreek
2008-Apr-30, 10:57 PM
Ummm ... I am not sure that I am saying that they are wrong ... but more that I dont understand what they are saying ... when I say I dont agree ... it must be prefixed with the fact that I have only been thinking about this stuff for a month or so. I am sure even neverfly knows more than I do. I just dont agree with stuff until I understand it ... sorry if I sounded too confident in my statements.

Tommac, I was where you are now, a few years ago, but I learned that I have to walk before I can run, so I went back to basic principles and then decided to concentrate on one single aspect of the big picture until I understood it. The aspect I chose was the expansion of the universe. In looking into the expansion I found myself having to learn a whole lot of new concepts along the way, and I am only really starting to get to grips with the big picture of expansion now, although certain aspects still elude me.

You seem to have chosen a starting point where you want to understand cosmic expansion, the underlying nature of gravity, black holes and a whole gamut of cosmological subjects, all at the same time. You will find that as you truly understand one concept, others start to click into place (until you come across quantum physics, that is!) but if you risk thinking you understand something when you really don't, you end up heaping misconceptions upon yourself all the way down the line.

You have your whole life to come to a true understanding. :)

P.S. I know virtually nothing about black holes.

tommac
2008-May-01, 12:13 AM
thanks ... but was there a need to post this in one of my threads rather than PMing me this? I was trying to keep to one thread but again it is filled with this stuff. So I will try to repost it again. Please if you have any of this heart to heart stuff you are sending please PM it to me or add it in your own thread. Please.

But thanks for the post. I see what you are saying.

Tommac, I was where you are now, a few years ago, but I learned that I have to walk before I can run, so I went back to basic principles and then decided to concentrate on one single aspect of the big picture until I understood it. The aspect I chose was the expansion of the universe. In looking into the expansion I found myself having to learn a whole lot of new concepts along the way, and I am only really starting to get to grips with the big picture of expansion now, although certain aspects still elude me.

You seem to have chosen a starting point where you want to understand cosmic expansion, the underlying nature of gravity, black holes and a whole gamut of cosmological subjects, all at the same time. You will find that as you truly understand one concept, others start to click into place (until you come across quantum physics, that is!) but if you risk thinking you understand something when you really don't, you end up heaping misconceptions upon yourself all the way down the line.

You have your whole life to come to a true understanding. :)

P.S. I know virtually nothing about black holes.

tommac
2008-May-01, 12:15 AM
Reposted as to not get lost in the Bull shorts above.

OK all ... instead of starting a new thread ... which I think is needed for this ... per neverflys advice I will keep it here.

Is the speed of propogation of gravity constant? That is regardless of where you observe the effects of gravity does its speed of propogation appear constant?

Neverfly
2008-May-01, 12:30 AM
Reposted as to not get lost in the Bull shorts above.

That Does It!

Tommac, I appreciate your desire to learn.
But I'm Publicly announcing (and you are just going to have to learn to live with that) that you are the first BAUTer to ever make it onto my ignore list.

Goodbye Tommac...

tommac
2008-May-01, 12:53 AM
OK all ... instead of starting a new thread ... which I think is needed for this ... per neverflys advice I will keep it here.

Is the speed of propogation of gravity constant? That is regardless of where you observe the effects of gravity does its speed of propogation appear constant?

alainprice
2008-May-01, 01:24 AM
Two words: General Relativity.

Good luck! Took me 3 years after I learned Special Relativity to be mentally prepared to accept the General.

publius
2008-May-01, 01:41 AM
I was about to compose a long post about this as this is right up my little alley, but seeing the "bull shorts" comment plus the flights home from CA and a little west->east jet lag which has me pretty much wiped out (3 hours later than my internal clock thinks), I think I'll pass. :)

Yes, one is going have to learn and appreciate what is being dismissed as "Bull shorts" to have any hope of understanding all this.

Short answer: gravity propagates *locally* at c (ie a local observer would say gravitaitonal field changes are propagating at 'c' right by him using his own local coordinates). Put more generally, changes in the gravitational field, the "shape" of space-time, propagate along the same null geodesics of that space-time as does light.

But, in the general case, it's much more complex because the null geodesics are determined by the shape of space-time. So as gravity propagates, it changes the background that determines how it continues to propagate. IOW, we have some very complex non-linear wave behavior here. Under appropriate limits, the EFE can be linearized and you can deal with much simpler solutions which deal with propagating gravitional waves and changes in the field in general, reminiscent, but still different, from how EM propagates.

Note that even a "short answer" took me two paragraphs. :)

Now, short answer on the black hole thing. A mass distribution collapsing into a black hole "leaves behind" the external as it collapses. Nothing has to continuously propagate out.

And indeed, any gravitational changes made inside the horizon would indeed never make it back out to the external space-time. And indeed, the events of such changes *WOULD NEVER OCCUR* at any time on the clocks of external observers anyway. So, you never see what doesn't happen. Big deal. :lol:

That may seem bizarre, but the same thing can easily been seen in nearly flat space-time with an accelerating, Rindler observer. Just as light past a certain point can never catch the Rindler observer, so changes in a gravitational field would never catch him either. He would continue through the existing space-time before the change that was chasing after him.

It's the same thing with a black hole. The changes (say some mass locally "wiggles" in it's own proper time after it crosses the horizon and causes some tiny gravitational wave) the changes never get out to "catch" any external observers.

-Richard

tommac
2008-May-01, 01:59 AM
Thanks pubilus, this post is certainly not bull shorts. Heh ... was going to write a one liner : "Holey shorts" great post pubilus. but decided against it.

However this post was awesome!!!

I was about to compose a long post about this as this is right up my little alley, but seeing the "bull shorts" comment plus the flights home from CA and a little west->east jet lag which has me pretty much wiped out (3 hours later than my internal clock thinks), I think I'll pass. :)

Yes, one is going have to learn and appreciate what is being dismissed as "Bull shorts" to have any hope of understanding all this.

Short answer: gravity propagates *locally* at c (ie a local observer would say gravitaitonal field changes are propagating at 'c' right by him using his own local coordinates). Put more generally, changes in the gravitational field, the "shape" of space-time, propagate along the same null geodesics of that space-time as does light.

But, in the general case, it's much more complex because the null geodesics are determined by the shape of space-time. So as gravity propagates, it changes the background that determines how it continues to propagate. IOW, we have some very complex non-linear wave behavior here. Under appropriate limits, the EFE can be linearized and you can deal with much simpler solutions which deal with propagating gravitional waves and changes in the field in general, reminiscent, but still different, from how EM propagates.

Note that even a "short answer" took me two paragraphs. :)

Now, short answer on the black hole thing. A mass distribution collapsing into a black hole "leaves behind" the external as it collapses. Nothing has to continuously propagate out.

And indeed, any gravitational changes made inside the horizon would indeed never make it back out to the external space-time. And indeed, the events of such changes *WOULD NEVER OCCUR* at any time on the clocks of external observers anyway. So, you never see what doesn't happen. Big deal. :lol:

That may seem bizarre, but the same thing can easily been seen in nearly flat space-time with an accelerating, Rindler observer. Just as light past a certain point can never catch the Rindler observer, so changes in a gravitational field would never catch him either. He would continue through the existing space-time before the change that was chasing after him.

It's the same thing with a black hole. The changes (say some mass locally "wiggles" in it's own proper time after it crosses the horizon and causes some tiny gravitational wave) the changes never get out to "catch" any external observers.

-Richard

NEOWatcher
2008-May-01, 12:04 PM
thanks ... but was there a need to post this in one of my threads rather than PMing me this?
Yes; this is a public board, and many people may be in the same boat you are. I took that post as good advice for anyone trying to learn new concepts, therefore good for a public statement.

What is the bull shorts comment for anyway? Are you dismissing and insulting a comment that you already acknowledged as understanding?

tommac
2008-May-01, 01:42 PM
Yes; this is a public board, and many people may be in the same boat you are. I took that post as good advice for anyone trying to learn new concepts, therefore good for a public statement.

What is the bull shorts comment for anyway? Are you dismissing and insulting a comment that you already acknowledged as understanding?

No it was targetted for all of the off-topic posts that distract from the OP.