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Author Topic: What is the speed of gravity?  (Read 4401 times)

Offline Ethos_

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What is the speed of gravity?
« on: 19/12/2013 18:04:54 »
In the following link, the speed of gravity is reckoned to be the same as c. This raises a question in my mind that I need help with.

http://math.ucr.edu/home/baez/physics/Relativity/GR/grav_speed.html

Thought experiment:

Let's imagine we're parked at a safe orbital distance for a supermassive black hole. The mass of the black hole is producing gravitational energies that originate within the event horizon, depending upon the aggregate mass it contains. Because the speed of gravity is the same as light, how does this energy reach beyond the event horizon?

If the speed of gravity is the same as light, wouldn't that prevent gravitational energies from being felt outside the domain of the event horizon? I realize there must be something about this that I don't understand.

I would appreciate someone with the expertise to explain what appears to me to be a paradox. If they can provide the necessary information and help me understand where I'm lacking, there will be no need for me to take this question to the New Theories section.

Thanks................................Ethos
« Last Edit: 23/12/2013 09:40:53 by chris »


 

Offline syhprum

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Re: Speed of gravity?
« Reply #1 on: 19/12/2013 20:57:39 »
It is not without good reason that they are called black holes we can only guess what lies beyond the event horizon, it may be a relatively large body of density not much greater than that of a neutron star or as oft times suggested a "singularity" of quasi infinite density.
all we can say is that it must be rotating conserving the angular momentum of the pre collapsed body from which it formed.
one would expect it to be symmetrical so there should be no outpouring of gravitational waves and the distortion of space time to which GR attributes gravity should not be affected by an event horizon.
to pose a question what known about the rotational rate of neutron stars does the equatorial velocity approach c   
 

Offline CPT ArkAngel

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Re: Speed of gravity?
« Reply #2 on: 19/12/2013 21:37:51 »
 

Offline Ethos_

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Re: Speed of gravity?
« Reply #3 on: 19/12/2013 22:59:17 »
See this, it might interest you.

https://www.simonsfoundation.org/quanta/20121221-alice-and-bob-meet-the-wall-of-fire/
Thanks for the link ArkA......

While I'm familiar with Hawking radiation and the loss of information this hypothesis calculates, it stands in opposition to the entanglement of Alice and Bob. And as this article details, one or the other must be scraped for us to move beyond the paradox. But frankly, that question is well removed from the one I've asked.

Allow me to try to be more precise in my asking. And please understand, I'm not proposing a new theory here nor is it my ultimate aim. I've just hit the proverbial brick wall of understanding and I need your help to find an answer if there is one.

My question:
If gravity propagates at the speed of light, and everything inside the event horizon is governed by that limit, how can the mass of the black hole be evident or measureable to the outside world? On the other hand, if gravity were to propagate at c and above, I could understand gravity's appearance beyond the event horizon. If the speed of gravitational propagation is less than c, how can the mass of a black hole gravitationally influence another body in space?

 

Offline CPT ArkAngel

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Re: Speed of gravity?
« Reply #4 on: 19/12/2013 23:55:49 »
The GR black hole is probably wrong because a valid theory of quantum gravity is necessary to solve the problem.

Inside the event horizon, light cannot escape because space flows toward the singularity at the speed of light or faster. This is why there is a paradox of lost information, basically. The Hawking solution did not answer the paradox. Susskind's answer was the holographic principle; an image of everything that goes inside is represented in 2D at the event horizon.

The simplest solution is that matter is spinning at the speed of light at the event horizon, but then some principles have to be changed.

There is simply no single answer to your question, it is still speculative. This is why it is so interesting...  :o)
 

Offline Ethos_

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Re: Speed of gravity?
« Reply #5 on: 20/12/2013 00:17:13 »


There is simply no single answer to your question, it is still speculative. This is why it is so interesting...  :o)
Thank you ArkA....., your brave answer is, frankly, what I had suspected. I'm not an expert about these matters, but I have gathered enough information over the years to acknowledge that present understanding of the Standard Model does not answer everything.

As a result of our conversation, I have decided to take this topic to the New Theories section because I believe there is one option that may answer these questions. And because that view is not mainstream, this is not the proper section to present it.  Thanks once again my friend.................
 

Offline JP

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Re: Speed of gravity?
« Reply #6 on: 20/12/2013 00:23:30 »
If gravity propagates at the speed of light, and everything inside the event horizon is governed by that limit, how can the mass of the black hole be evident or measureable to the outside world? On the other hand, if gravity were to propagate at c and above, I could understand gravity's appearance beyond the event horizon. If the speed of gravitational propagation is less than c, how can the mass of a black hole gravitationally influence another body in space?

The answer is that (classically) gravity doesn't propagate outward from a black hole.  The gravitational field formed as the star collapsed into a black hole and it stays in existence after the black hole forms.  In a sense, it would seem to be emanating out from the event horizon, since classically nothing inside the event horizon could communicate to the external universe. 
 

Offline Pmb

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Re: Speed of gravity?
« Reply #7 on: 20/12/2013 00:39:38 »
Quote from: Ethos_
Let's imagine we're parked at a safe orbital distance for a supermassive black hole. The mass of the black hole is producing gravitational energies that originate within the event horizon, depending upon the aggregate mass it contains. Because the speed of gravity is the same as light, how does this energy reach beyond the event horizon?
First of all you made the question more complex than it needs to be. There’s no need to talk about supermassive black holes. Any black hole will do. Basically you want to know how gravitons escape from the event horizon, correct? John Baez answered this question in the Usenet FAQ at http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/black_gravity.html
Quote
Purely in terms of general relativity, there is no problem here.  The gravity doesn't have to get out of the black hole.  General relativity is a local theory, which means that the field at a certain point in spacetime is determined entirely by things going on at places that can communicate with it at speeds less than or equal to c.  If a star collapses into a black hole, the gravitational field outside the black hole may be calculated entirely from the properties of the star and its external gravitational field before it becomes a black hole. Just as the light registering late stages in my fall takes longer and longer to get out to you at a large distance, the gravitational consequences of events late in the star's collapse take longer and longer to ripple out to the world at large.  In this sense the black hole is a kind of "frozen star": the gravitational field is a fossil field.  The same is true of the electromagnetic field that a black hole may possess.
 

Offline Pmb

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Re: Speed of gravity?
« Reply #8 on: 20/12/2013 00:41:18 »
Quote from: CPT ArkAngel
The GR black hole is probably wrong because a valid theory of quantum gravity is necessary to solve the problem.
On what basis do you justify this assertion?
 

Offline Ethos_

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Re: Speed of gravity?
« Reply #9 on: 20/12/2013 00:59:38 »
If gravity propagates at the speed of light, and everything inside the event horizon is governed by that limit, how can the mass of the black hole be evident or measureable to the outside world? On the other hand, if gravity were to propagate at c and above, I could understand gravity's appearance beyond the event horizon. If the speed of gravitational propagation is less than c, how can the mass of a black hole gravitationally influence another body in space?

The answer is that (classically) gravity doesn't propagate outward from a black hole.  The gravitational field formed as the star collapsed into a black hole and it stays in existence after the black hole forms.  In a sense, it would seem to be emanating out from the event horizon, since classically nothing inside the event horizon could communicate to the external universe.
Forgive me if I sound unreasonable because that is not my purpose here and I certainly appreciate every contribution on this subject. Nevertheless, I can't understand the gravitational influence being transmitted thru the event horizon from the mass that lies within, and ending up at the event horizon without breaking the speed of light. One way or another, the mass we register at or beyond the event horizon is making it's way thru that speed barrier we call c.

Evidently, I'm missing something here JP because I do respect your knowledgeable opinions, I'm just not seeing it yet.
 

Offline Ethos_

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Re: Speed of gravity?
« Reply #10 on: 20/12/2013 01:05:40 »

First of all you made the question more complex than it needs to be. There’s no need to talk about supermassive black holes. Any black hole will do. Basically you want to know how gravitons escape from the event horizon, correct? John Baez answered this question in the Usenet FAQ at http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/black_gravity.html

Thank you for this link, it explains a lot. Thank you once again Pmb

After looking this link over, it has become very clear to me how these processes unfold.

Looking at general relativity first:

In the same way that light takes longer and longer to escape in the final stages of collapse, so too do the gravitational consequences of events react in the same way.

Looking at this from the quantum perspective:

Gravitons don't exist in GR but like virtual photons, they do in quantum mechanics. And both gravitons and virtual photons can escape prior to final collapse just like in classical physics. And because virtual particles are not confined to light cones, they can travel faster than light and escape even after final collapse occurs.

In conclusion:

My question dealt with the quantum graviton and how it could manage to escape from the event horizon. Because this author suggests that virtual particles are not confined to light cones, they can travel faster than light. In short, virtual gravitons can escape because they do. Thereby yielding the gravitational field associated with the black hole.
« Last Edit: 20/12/2013 03:58:37 by Ethos_ »
 

Online yor_on

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Re: Speed of gravity?
« Reply #11 on: 20/12/2013 13:57:04 »
I'm not sure of this, but reading Baez there, Pete, the argument seems to become one of 'time dilations', comparing over frames of reference? I think it's one way to look at it, but to me 'gravity' also seems as a 'net', updated at 'c'? And the Black Hole, passing the event horizon becoming a place where that net becomes a 'infinite well'? That though is a question of geometry, isn't it? Do I really need gravitons to define this?
 

Offline Ethos_

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Re: Speed of gravity?
« Reply #12 on: 20/12/2013 16:55:14 »
I'm not sure of this, but reading Baez there, Pete, the argument seems to become one of 'time dilations', comparing over frames of reference? I think it's one way to look at it, but to me 'gravity' also seems as a 'net', updated at 'c'? And the Black Hole, passing the event horizon becoming a place where that net becomes a 'infinite well'? That though is a question of geometry, isn't it? Do I really need gravitons to define this?
If we ever have a hope of unifying GR and quantum mechanics, the micro and the macro, something on the order of a graviton will need to be found. To date, the graviton is one remaining force carrier to be experimentally detected. Until and if it is ever found, it remains the only explanation we have to deal with quantum gravity. The paramount goal of physics; ........unification.
 

Offline syhprum

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Re: Speed of gravity?
« Reply #13 on: 21/12/2013 14:06:00 »
There are good theoretical grounds for thinking individual gravitons are undetectable, they would be vastly less massive than neutrinos and they are hard enough to detect.
 

Offline A Davis

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Re: Speed of gravity?
« Reply #14 on: 22/12/2013 18:36:51 »
What is the temperature of a black hole, is it hot or very cold, does it lose it's heat at the point of formation. Once it has formed it cannot lose anymore heat.
 

Online yor_on

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Re: Speed of gravity?
« Reply #15 on: 22/12/2013 18:44:38 »
Sweet question AD :)

Gas that falls into a black hole is said to become hot enough to emit x-rays, propagating to the galaxy's center. By measuring that radiation you are thought to be able to find a black holes weight/mass. Assuming this stand correct when a Black Hole forms too, compressing? Would that show up for us?
==

The reason the gas is thought to heat up is it getting compressed, piling up by gravity like water in a drain, interacting with itself, creating intense 'heat/radiation' that then gets emitted. Eh, the idea also presumes supermassive black holes at a galaxy's center btw.
« Last Edit: 22/12/2013 18:50:23 by yor_on »
 

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Re: Speed of gravity?
« Reply #16 on: 22/12/2013 18:53:58 »
I don't know, maybe all is particles, maybe waves, maybe both, or, maybe 'energy'? :)
But gravity do not look like EM. And the Higgs boson describe inertia to me, not 'mass'. So how would a graviton be described?
 

Offline Pmb

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Re: Speed of gravity?
« Reply #17 on: 22/12/2013 21:22:21 »
Quote from: A Davis
What is the temperature of a black hole, is it hot or very cold, does it lose it's heat at the point of formation. Once it has formed it cannot lose anymore heat.
The answer is given here
http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/971111e.html
Quote
What is the temperature of a black hole?


The Answer
The temperature of a black hole is determined by the 'black body radiation temperature' of the radiation which comes from it. (e.g., If something is hot enough to give off bright blue light, it is hotter than something that is merely a dim red hot.)
For black holes the mass of our Sun, the radiation coming from it is so weak and so cool that the temperature is only one ten-millionth of a degree above absolute zero. This is colder than scientists could make things on Earth up until just a few years ago (and the invention of a way to get things that cold won the Nobel prize this year). Some black holes are thought to weigh a billion times as much as the Sun, and they would be a billion times colder, far colder than what scientists have achieved on Earth.

However, even though these things are very cold, they can be surrounded by extremely hot material. As they pull gas and stars down into their gravity wells, the material rubs against itself at a good fraction of the speed of light. This heats it up to hundreds of millions of degrees. The radiation from this hot, infalling material is what high-energy astronomers study.

David Palmer
for Ask an Astrophysicist

Quote from: A Davis
Once it has formed it cannot lose anymore heat.
Black Holes loose energy, and therefore heat, through Hawking radiation.
« Last Edit: 22/12/2013 21:24:31 by Pmb »
 

Online yor_on

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Re: Speed of gravity?
« Reply #18 on: 22/12/2013 22:29:15 »
That is still a hypothesis though, isn't it? http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/hawking.html

Nobody has measured any Hawking radiation, that I've heard of, although we believe us to know several locations for Black Holes. Hawking radiation is to me a way to explain a equilibrium, as it otherwise would be so that information (and energy) irrecoverably disappear from the measurable universe, behind that event horizon, all as I get it. Also it allows a graceful 'death' to all Black holes, even though they might be the last things 'radiating' then, everything else reaching some heat saturation? But that one is also a tough thing swallowing, as I don't see how interactions will stop, between matter and radiation due to a saturation.
=

There is angle to it where one discuss useful energy relative unusable. In that case one might want to define Hawking radiation as able to transform whereas all other radiation is in some 'lowest state', or possibly just 'stable state', of heat. But radiation must exist if we find heat, and so must matter. It's those two that interacts. Think of radiation in a vacuum to see why.
« Last Edit: 22/12/2013 22:42:38 by yor_on »
 

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Re: Speed of gravity?
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