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Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: intrafinesse on 12/06/2017 00:54:08

Title: What emits gravitons and how are they emitted?
Post by: intrafinesse on 12/06/2017 00:54:08


1) What emits gravitons?  Just matter or does energy (i.e. photons) also emit gravity?  I assume spacetime does not emit gravitons.

2) Does matter warp space time by emitting gravitons?

3) Kip Thorn said that there is no matter remaining  in a Black Hole, thats it's only warped space time.    Once space time is warped by a black hole and there is no more matter left, how does spacetime stay warped?  Does it freeze in place once warped?   But wouldn't that imply a moving body would leave a trail or permanently warped space?

4) Since the black hole does have a gravitational field, similar to it's ancestor star beyond the event horizon, what is emiting gravitons? 

5)  Can gravitons escape the event horizon?

I watched some interviews with Kip Thorn and am reading his book on the science behind Interstellar and it go me thinking, hence these questions.  Is this format ok?
Title: Re: What emits gravitons and how are they emitted?
Post by: PmbPhy on 12/06/2017 01:41:13
1) What emits gravitons?  Just matter or does energy (i.e. photons) also emit gravity?
Particles don't emit gravitons. Gravitons are virtual particles which are exchanged between two interacting particles.

BTW - Energy is not photons, i.e. they are two very different things. Photons have energy. They are not energy. Energy is not a physical thing which can generate a gravitational field. Matter generates gravitational fields and all matter has mass and for every bit of matter which has mass there is energy associated with it.

I assume spacetime does not emit gravitons.
Correct. That's because spacetime is not a thing and as such does not have matter.

2) Does matter warp space time by emitting gravitons?
That seems to be the idea. BTW, the correct term is "curve" spacetime.

3) Kip Thorn said that there is no matter remaining  in a Black Hole, thats it's only warped space time.
Huh? Where did he say that?

5)  Can gravitons escape the event horizon?
First of all black holes are described by general relativity (GR) which is a classical theory. Gravitons are part of quantum theory. As yet there is no quantum theory of gravity. That said, since gravitons are virtual particles they can move faster than the speed of light and thus can escape the event horizon. For more on this see:

http://curious.astro.cornell.edu/physics/89-the-universe/black-holes-and-quasars/theoretical-questions/451-how-do-gravitons-escape-black-holes-to-tell-the-universe-about-their-gravity-advanced

http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/black_gravity.html
Title: Re: What emits gravitons and how are they emitted?
Post by: intrafinesse on 12/06/2017 02:11:12
www space.com/17086-bizarre-black-holes-kip-thorne-interview.html

This is an article where Kip Thorn says a black hole has no more matter

>>When an object like a star becomes a black hole, what happens to its mass?

Kip Thorne: The matter of which a star is made, the atoms of which a star is made, are destroyed at the center of a black hole, when the black hole is created. The matter is gone, but the mass, in the sense of mass and energy being equivalent, has gone into the warped space-time of the black hole.
Title: Re: What emits gravitons and how are they emitted?
Post by: PmbPhy on 12/06/2017 02:29:59
www space.com/17086-bizarre-black-holes-kip-thorne-interview.html

This is an article where Kip Thorn says a black hole has no more matter

>>When an object like a star becomes a black hole, what happens to its mass?

Kip Thorne: The matter of which a star is made, the atoms of which a star is made, are destroyed at the center of a black hole, when the black hole is created. The matter is gone, but the mass, in the sense of mass and energy being equivalent, has gone into the warped space-time of the black hole.
He means that there are no more atoms. He appears to think of matter as what atoms constitute. That's all. If one uses a different definition of matter than Thorne then his statement becomes incorrect. E.g.

www.dictionary.com/browse/matter
Quote
Matter definition, the substance or substances of which any physical object consists or is composed:
Clearly a black hole is a physical object and therefore consists of matter.
Title: Re: What emits gravitons and how are they emitted?
Post by: evan_au on 13/06/2017 12:05:52
Quote from: intrafinesse
1) What emits gravitons?
Assuming gravitons are real (and most physicists think that one day there will be a quantum theory of gravity which has some form of graviton):

In addition to the virtual gravitons mentioned by PmbPhy (which can for a short time travel at any speed),
... there would be real gravitons, which travel at the speed of light, and have an infinite range.
- Massive objects like closely orbiting neutron stars and black holes have been observed to radiate energy into space
- This radiation is in the form of gravitational waves, which have an infinite range
- In a theory of quantum gravity, gravitational waves would consist of coherent waves of  gravitons, produced by the acceleration of the orbiting masses
- Einstein's relativity would probably describe the coherent gravitons as produced by the mutual distortion of spacetime as the masses spiral towards each other
- This is similar to the way that accelerating electric charges in an antenna produce coherent waves of photons, which also travel at the speed of light, and have an infinite range

See: https://en.wikipedia.org/wiki/Graviton
Title: Re: What emits gravitons and how are they emitted?
Post by: jeffreyH on 13/06/2017 12:25:51
Studying features of both the electromagnetic and gravitational fields with respect to coherence should be interesting.
https://en.m.wikipedia.org/wiki/Degree_of_coherence
Title: Re: What emits gravitons and how are they emitted?
Post by: Kryptid on 13/06/2017 15:41:37
First of all black holes are described by general relativity (GR) which is a classical theory. Gravitons are part of quantum theory. As yet there is no quantum theory of gravity. That said, since gravitons are virtual particles they can move faster than the speed of light and thus can escape the event horizon. For more on this see:

http://curious.astro.cornell.edu/physics/89-the-universe/black-holes-and-quasars/theoretical-questions/451-how-do-gravitons-escape-black-holes-to-tell-the-universe-about-their-gravity-advanced

http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/black_gravity.html

I've come across that question before and it is a bit of a strange one when you really think about it. Asking how gravitons can get out of a black hole is basically like asking how gravity can escape from gravity. Does it even make sense to say that gravity can hold itself back or prevent its own escape? That sounds self-contradictory to me.
Title: Re: What emits gravitons and how are they emitted?
Post by: evan_au on 13/06/2017 22:22:29
Quote from: intrafinesse (OP)
2) Does matter warp space time by emitting gravitons?
According to Einstein's general relativity, an isolated star, neutron star or black hole will warp spacetime due to its mass.

However, an isolated mass will not generate gravitational waves, and so will not produce real gravitons. Producing gravitational waves and real gravitons requires the interaction of at least 2 masses.

So matter can warp spacetime without emitting any (real) gravitons.

Because gravity is such a weak force, it requires quite large masses in close orbit to generate significant power as gravitons. The Earth orbiting the Sun generates about 200W of gravitational waves, which is infinitesimal in terms of the Earth's kinetic energy as it orbits the Sun.

Quote
5)  Can gravitons escape the event horizon?
Newton's shell theorem states that to a distant observer, the gravitational effect of a certain mass concentrated at the center of a black hole (the singularity) is the same as that same mass distributed in a thin, spherical shell around the same point.

However, to a distant observer, the mass that formed a black hole still exists as a thin shell around the event horizon. So, in a sense, the gravitational effects of the mass do not need to escape the event horizon - they never entered it.

Newton's shell theorem might need some adjustment for a rotating black hole; for one thing, the event horizon is an ellipsoid rather than a sphere. And there's the effects of time dilation and frame dragging, which Newton never had to deal with.
Title: Re: What emits gravitons and how are they emitted?
Post by: jeffreyH on 14/06/2017 12:49:19
We cannot have matter plastered at the horizon in a physical sense since proper time continues past this point. In the case of virtual particles this is not an issue. However these virtual particles do not have an infinite range. This leaves us with a dilemma.
Title: Re: What emits gravitons and how are they emitted?
Post by: PmbPhy on 14/06/2017 23:13:53
However, an isolated mass will not generate gravitational waves, and so will not produce real gravitons. Producing gravitational waves and real gravitons requires the interaction of at least 2 masses.
Why would you say that? There's no requirement for gravitons (which are virtual particles) to require the presence of a gravitational wave just like there's no requirement for a virtual photon to require EM radiation.
Title: Re: What emits gravitons and how are they emitted?
Post by: evan_au on 15/06/2017 00:19:55
Quote from: PmbPhy
gravitons (which are virtual particles)
As I understand it, virtual particles (https://en.wikipedia.org/wiki/Virtual_particle) are important for calculating interactions in quantum theory, but their range and lifetime are limited by Heisenberg's Uncertainty Principle. They can't be directly detected by experiment.

In a quantum theory of gravity, an exchange of virtual gravitons would be responsible for the gravitational attraction of an object sitting on the Earth's surface. There is a force of attraction, but there is no acceleration of the masses (no change in distance), so there is no energy radiated via real gravitons.

Real particles can be detected by experiment; particles like the photon, electron and (hypothetical) graviton have an indefinite lifetime - we have detected photons and gravitons across distances exceeding a billion light years.

In a theory of quantum gravity, gravitational waves are coherent emissions of real gravitons which carry real energy, with an infinite range; a gravitational wave demands the presence of real gravitons, just like the quantum version of an electromagnetic wave demands the presence of real photons.

The Earth orbiting the Sun radiates real gravitons - but at a very low level (around 200W).
To get into the TeraWatt range and above you need orbiting neutron stars; to put out more energy than a galaxy (for a few milliseconds), you need colliding black holes.
Title: Re: What emits gravitons and how are they emitted?
Post by: PmbPhy on 15/06/2017 01:57:44
As I understand it, virtual particles (https://en.wikipedia.org/wiki/Virtual_particle) are important for calculating interactions in quantum theory, but their range and lifetime are limited by Heisenberg's Uncertainty Principle. They can't be directly detected by experiment.
Correct.

In a quantum theory of gravity, an exchange of virtual gravitons would be responsible for the gravitational attraction of an object sitting on the Earth's surface.
Correct. Preaching to the choir.

but there is no acceleration of the masses (no change in distance), so there is no energy radiated via real gravitons.
I don't know why you're interpreting the OP's question to refer only to real gravitons and not virtual ones. By definition a graviton is the particle that mediates the force of gravitation. That'd be like saying that photons don't mediate the EM force but only virtual ones do. It'd be unwise to make that distinction when answering a direct question regarding what mediates the EM force and the same holds for gravitons.

See: What Are Gravitions? at http://www.pbs.org/wgbh/nova/blogs/physics/2014/05/what-are-gravitons/
Quote
Individual gravitons interact very feebly, and we are only held to the planet because the Earth emits so many of them. Because a single graviton is so weak, it is impossible for us to directly detect individual classical gravitons.

Even in non GR quantum theory the difference between a real photon and a virtual one isn't simple as that. Think of two electrons separated by a light year. Since the range of the EM force is infinite they must be exchanging photons. Somewhere along the way the virtual photon became real even though its still mediating the EM Force.

The precise statement of this was explained to be by the author of the text I learned it from. I can't find it so if you'd like I can ask him again exactly how he explained it to me.
Title: Re: What emits gravitons and how are they emitted?
Post by: jeffreyH on 15/06/2017 12:17:31
A while ago in one of my threads in new theories I had proposed that virtual gravitons could become 'real' gravitons. That would be buried in my notes somewhere.