Can the existence of the Graviton be discounted ?

[mistermack]

Maybe someone can quickly clear up a problem I have with the possibility that gravity is mediated by an elementary particle, the Graviton, that is massless and propagates at the speed of light. As I understand it, it's considered hypothetical at the moment, meaning it's considered as one possibility. 

I can't see how it's possible. My reasoning is pretty simplistic. Take a black hole at the centre of a galaxy. It's gravity clearly affects the stars and materials that are  orbiting it. We can calculate the mass of the black hole by how it's gravity affects everything else. But how can a boson escape a black hole, and affect the surrounding stars? As I understand it, nothing can escape a black hole. Photons are massless bosons, that travel at the speed of light, but they can't exit a black hole, so how could a graviton? Is it immune to the action of other gravitons? Is that theoretically possible? 

Please don't respond by saying that the existence of the graviton is not accepted theory, I already know that. 

The question of this OP is, can it's existence be positively discounted, for the reason I've given above? And if not, why not? 

[Genady]

21 minutes ago, mistermack said:

Maybe someone can quickly clear up a problem I have with the possibility that gravity is mediated by an elementary particle, the Graviton, that is massless and propagates at the speed of light. As I understand it, it's considered hypothetical at the moment, meaning it's considered as one possibility. 

I can't see how it's possible. My reasoning is pretty simplistic. Take a black hole at the centre of a galaxy. It's gravity clearly affects the stars and materials that are  orbiting it. We can calculate the mass of the black hole by how it's gravity affects everything else. But how can a boson escape a black hole, and affect the surrounding stars? As I understand it, nothing can escape a black hole. Photons are massless bosons, that travel at the speed of light, but they can't exit a black hole, so how could a graviton? Is it immune to the action of other gravitons? Is that theoretically possible? 

Please don't respond by saying that the existence of the graviton is not accepted theory, I already know that. 

The question of this OP is, can it's existence be positively discounted, for the reason I've given above? And if not, why not? 

Graviton would not need to escape from inside the event horizon. Gravitational field is present outside of it, and graviton is a quantum of that field.

Edited October 3, 2023 by Genady

[mistermack]

6 minutes ago, Genady said:

Graviton would not need to escape from inside the event horizon. Gravitational field is present outside it, and graviton is a quantum of that field.

I thought that it was the constant flux of gravitons that was hypothesised as creating the field ? 

[Genady]

2 minutes ago, mistermack said:

I thought that it was the constant flux of gravitons that was hypothesised as creating the field ? 

I don't think so.

[mistermack]

Photons are massless bosons that propagate at the speed of light. Like the hypothetical Gravitons. But there's no suggestion that they can have any effect across an event horizon. Can a graviton even be a graviton, inside a black hole? 

[Genady]

Black hole can be, in principle, charged. A charged nonrotating black hole creates a static electric field outside its event horizon.

Edited October 3, 2023 by Genady

[swansont]

Gravitons would be virtual particles, not limited by the same restrictions as real particles under classical rules of physics

[Genady]

AFAIK, gravitons would be real particles in gravitational radiation, i.e., gravitational waves. But gravitational waves originate outside of the black hole's horizons, so the OP question does not apply.

[Janus]

19 hours ago, mistermack said:

Maybe someone can quickly clear up a problem I have with the possibility that gravity is mediated by an elementary particle, the Graviton, that is massless and propagates at the speed of light. As I understand it, it's considered hypothetical at the moment, meaning it's considered as one possibility. 

I can't see how it's possible. My reasoning is pretty simplistic. Take a black hole at the centre of a galaxy. It's gravity clearly affects the stars and materials that are  orbiting it. We can calculate the mass of the black hole by how it's gravity affects everything else. But how can a boson escape a black hole, and affect the surrounding stars? As I understand it, nothing can escape a black hole. Photons are massless bosons, that travel at the speed of light, but they can't exit a black hole, so how could a graviton? Is it immune to the action of other gravitons? Is that theoretically possible? 

Please don't respond by saying that the existence of the graviton is not accepted theory, I already know that. 

The question of this OP is, can it's existence be positively discounted, for the reason I've given above? And if not, why not? 

The graviton would be a quantum of gravitational radiation ( gravitational waves), playing the same role that photons do for electromagnetic radiation.

Virtual photons act as the mediator for electromagnetic forces.( Which they also do with a black hole. Thus a black hole can have a electric charge and field, even though photons cannot escape the event horizon of a black hole.) 

So, in the same way, Virtual gravitons would act as the mediator for the gravitational force giving the Black hole a gravitational field even though no actual gravitons leave the EH.

 

*

[MJ kihara]

👀 gravitational waves are spacetime curvature disturbances.

[Killtech]

Maybe let's first settle much simpler question about just GR:

is a gravitational wave subject to gravitational lensing? The question is two-fold: what does the theory say and what the experiment. the latter is more relevant but probably still unclear, since it is not that long ago that we were able to even make a first detection.

a gravitinon would have to follow the same propagational behavior as the wave.

Edited October 25, 2023 by Killtech

[MJ kihara]

5 hours ago, Killtech said:

is a gravitational wave subject to gravitational lensing?

I think it will depend on the frequency of gravitational wave and the magnitude of gravitational lensing...since gravitational waves are a result of propagation of spacetime curvature disturbances,it's expected that any warping of spacetime caused by mass inducing gravitational lensing will affect  curvature of spacetime that inturn will affect the way disturbances are propagated, assuming gravitational waves propagate radially from mass source and light moves in a straight line propagated by a photon, gravitational lensing in this case will be of different kind...the detector for gravitational lensing will have to work following different principles for light and for gravitational waves.

[swansont]

18 hours ago, Killtech said:

is a gravitational wave subject to gravitational lensing?

Yes

https://www.universetoday.com/148230/gravitational-wave-lensing-is-possible-but-its-going-to-be-incredibly-difficult-to-detect/#:~:text=Gravitational waves and light both,to travel a certain distance.

[mistermack]

When I posted the OP, I thought I was probably asking a silly question, but going by the answers, I can see why I was in the dark. 

If you look at the wikipedia page on the graviton, which I had already done, the word "virtual" doesn't even appear, except in a link, in the "see also" section at the bottom of the page.  See  Graviton - Wikipedia   

The first sentence reads " In theories of quantum gravity, the graviton is the hypothetical quantum of gravity, an elementary particle that mediates the force of gravitational interaction."    

This appears to be totally at odds with the post from Janus, above, which makes the clear distinction between virtual gravitons and gravitons. If you replace "graviton" with "virtual graviton" and replace "elementary particle" with "virtual elementary particle" in the wikipedia page, then it begins to make more sense. 

So I'm of the opinion at the moment, that the wikipedia page is wrong and needs re-writing. 

 

One other question that occurred to me is how virtual gravitons comply with the description of virtual particles being transient and short-lived. 

Presumably virtual gravitons are acting on bodies that are millions or even billions of light-years distant, so they must exist for millions and billions of years to achieve that. 

[Carrock]

15 hours ago, mistermack said:

One other question that occurred to me is how virtual gravitons comply with the description of virtual particles being transient and short-lived.

What description? Photons, for example, are stable and therefor virtual photons have infinite range.

[mistermack]

2 hours ago, Carrock said:

What description?

Well I'm making no pretence of knowing about virtual particles. Reading wikipedia is generally a reliable source, but not infallible. On the wiki page on virtual particles, the first sentence reads " A virtual particle is a theoretical transient particle that exhibits some of the characteristics of an ordinary particle, while having its existence limited by the uncertainty principle."           

And later this : "The closer its characteristics come to those of ordinary particles, the longer the virtual particle exists."

If a virtual graviton can exist for a billion years and affect an object a billion light years away, then going by the second sentence, it would appear that there must be very little difference between virtual and real. 

[Carrock]

Wikipedia is certainly oversimplifying. Like, say, a photon, a virtual photon is calculated or observed to exist from when it is created to when it is destroyed, which for massless virtual particles has no upper time limit.

The second sentence is is even more misleading. Oppositely electrically charged particles experience a net attraction by exchanging virtual photons; this attraction does not happen with real photons but the arguably very different characteristics of virtual and real photons in this instance does not shorten the virtual photons' lifetime.

[Killtech]

But does gravity actually slow down gravity? Let's consider the very old gedankenexperiment: what would happen is a black hole suddenly disappeared, how long would it take for different observers around it to notice it? In particular, if we say gravity travels at \(c\) and for the outside observer clocks within a gravity field are massively slowed, then the speed measured in term of the outside observers clock should appear much slower.

Or let's rephrase it: given a massive object with a time dependent mass/energy of \( m(t)=1+\sin (\omega t) \), i.e. producing a field resembling a longitudinal wave - how fast would the curvature changes it produces propagate? what wavelength would that curvature field have locally?

going back to the graviton, or in fact any virtual particle. They appear in Feynmen diagrams as part of the integral kernel. But that kernel still contains these paths by physics, which can be interpreted as allowing only somewhat possible paths - that is paths which require more then infinite energy have a contribution/probability density of 0. So if virtual particles are constrains by the same geometry, how are they able to contribute to the amplitude starting at a point within \( r_s \) to a point outside?

Well. of course we do not have a theory of quantum gravity or in fact any quantum theory that can deal with curvature, so i guess the question probably does not have a good answer as of now?

Edited October 28, 2023 by Killtech

[MJ kihara]

On 10/27/2023 at 9:41 PM, Carrock said:

Oppositely electrically charged particles experience a net attraction by exchanging virtual photons

There is a need for a better model/ theory that clarify how and why there is the exchange of 'virtual photon' I think there might be two issues of 'virtual photons' and attraction of oppositely charged particles that are displayed in electromagnetism that makes the definition of virtual particles from Wikipedia,which is somehow valid,seem ambiguous.

[MigL]

Gravitons are a necessity of a Quantum Field Theory of Gravity.

GR is NOT a quantum field theory; why mix the two ?

[MJ kihara]

On 10/28/2023 at 1:13 PM, Killtech said:

But does gravity actually slow down gravity? Let's consider the very old gedankenexperiment: what would happen is a black hole suddenly disappeared, how long would it take for different observers around it to notice it? In particular, if we say gravity travels at c and for the outside observer clocks within a gravity field are massively slowed, then the speed measured in term of the outside observers clock should appear much slower.

You cannot create nor destroy energy...I think blackhole is more of energy,that is, enormously packed energy, whose impact induces the gravity that  we detect from blackhole. Therefore,blackhole cannot suddenly disappear maybe transition to other energy forms i.e evaporate through Hawking radiation...the distance as experienced by the photon or observer near a black is highly warped.. therefore, photons moves at speed of light near the blackhole,for outside observer the clock appear slower to compensate for  invariance of speed of light.

[Killtech]

8 hours ago, MigL said:

Gravitons are a necessity of a Quantum Field Theory of Gravity.

GR is NOT a quantum field theory; why mix the two ?

ideally the latter is the quantized version of the former and in particular in the classical limit we should yield one from the other. But since the latter does not exist yet, we have to study the specialties of the former to understand where the trouble comes from. 

7 hours ago, MJ kihara said:

You cannot create nor destroy energy...I think blackhole is more of energy,that is, enormously packed energy, whose impact induces the gravity that  we detect from blackhole. Therefore,blackhole cannot suddenly disappear maybe transition to other energy forms i.e evaporate through Hawking radiation...the distance as experienced by the photon or observer near a black is highly warped.. therefore, photons moves at speed of light near the blackhole,for outside observer the clock appear slower to compensate for  invariance of speed of light.

In a closed physical system this cannot happen and it doesn't actually matter if the black hole is made of mass or something else - the curvature is caused by the energy stress tensor after all. Therefore the back hole could be made up entirely of light of sufficient intensity in principle. Clearly the gedankenexperiment violates energy conservation in Newtons and Einsteins physics all the same, so let's just assume it's an open system and treat it analoge to a driven oscillator. 

The physicists that formulated that old gedankenexperiment didn't intend for it to be a realistic situation but wanted to illustrate a particular question about propagation of gravity. Just as it was valid to show how Newton's gravity is instantaneous compared to Einstein's, it is still an interesting case to drill down the properties of gravity to some of its essential aspects. 

[MigL]

Classically, GR tells us that a distant observer will experience events happening differently from the proper time of an infalling observer.
To the distant observer, infalling objects slow down temporally, and stop at the event horizon, while the infalling observer sees nothing amiss in his time frame and continues through the event horizon.

The gravitational field we 'see' as distant observers is nothing more than the relic gravitational field of the collapsing mass-energy that formed the Black Hole, and is now 'frozen' at the event horizon for eternity ( yes, in classic GR there is no Hawking radiation, Black Holes do not evaporate but are eternal ). Nothing, not even the gravitational field 'escapes' a Black Hole, but because we treat gravity as a point source, geodesics necessarily extend to the interior of the event horizon.

We don't have a quantized field theory of gravity; what we have is a classical geometric field theory, and Black Holes are far from the classical limit where we expect one theory to 'reduce' to the other, as evidenced by infinities that arise.

[MJ kihara]

19 hours ago, MigL said:

We don't have a quantized field theory of gravity

Where and when should we ever expect to have one or we just wait for the AI to generate one?

19 hours ago, MigL said:

The gravitational field we 'see' as distant observers is nothing more than the relic gravitational field of the collapsing mass-energy that formed the Black Hole, and is now 'frozen' at the event horizon for eternity ( yes, in classic GR there is no Hawking radiation, Black Holes do not evaporate but are eternal ).

Frozen...As in time or as stored memory?

Should we go with what classic or quantum theories say or with observation made?

Recent observations..JWST..is that there might be older galaxies than our current models predict... supposing they had a blackhole then going by your argument that it's eternal we should be expecting observation of super super supermassive blackhole that could have sucked all of us.....I doubt blackhole don't transition back to normal spacetime (average curvature).

[iNow]

2 hours ago, MJ kihara said:

Frozen...As in time or as stored memory?

As in... once an object crosses the event horizon, it's light can no longer escape gravity and thus can no longer reach us as the distant observer. For that reason, the object appears "frozen" and stuck still to us at the location immediately prior to event horizon crossing.