Can the existence of the Graviton be discounted ?

[MigL]

You go with observations where they can be made.
Where they cannot, you go with the predictive aspects of existing theory.
Unfortunately classical GR fails at very small separations and hi energies, making infinite predictions.
These same infinities appear again when trying to quantize the gravitational field, mostly as a result of gravity itself gravitating, and they cannot be eliminated by the traditional method of renormalization which has worked for QED and QCD previously.
Other approaches have been tried, and while SuperString theory seems to be a dead end, Loop Quantum Gravity still shows some promise.

As to  when we'll have a workable quantum Gravity theory, it's anyone's guess, but unless AI becomes self thinking/learning instead of simply offering the 'best fiit' answer from a multitude of stored responses, it certainly will not be provided by AI.

[iNow]

1 minute ago, MigL said:

unless AI becomes self thinking/learning instead of simply offering the 'best fiit' answer from a multitude of stored responses, it certainly will not be provided by AI.

I’d wager an annual salary this is wrong, especially once AI is running on quantum computers, but that’s off-topic here. +1 for an otherwise good post 

[MigL]

Quantum computers perform very complex calculations on large amounts of data.
The theoretical number of computations that can be performed simultaneously is 2ⁿ, where n is the number of qubits available.

That does not mean that they can think, learn or imagine.

My apologies for straying off topic.
 

[MJ kihara]

1 hour ago, MigL said:

These same infinities appear again when trying to quantize the gravitational field, mostly as a result of gravity itself gravitating,

The issue of gravity itself gravitating what is it?

[MigL]

The gravitational field itself is a source of gravity.
IE Gravity gravitates or is self coupling.

Edited November 1, 2023 by MigL

[MJ kihara]

4 hours ago, MigL said:

The gravitational field itself is a source of gravity.
IE Gravity gravitates or is self coupling.

I think self coupling and generating itself are two different issues, anyway,am struggling to understand how gravitational field is a source  of gravity...Are there examples supported by observation?

[Killtech]

9 hours ago, MigL said:

The gravitational field itself is a source of gravity.
IE Gravity gravitates or is self coupling.

How well is the self coupling experimentally verified and understood? Though if we go away from a linear field like thinking, it makes intuitively sense, i would still like to see what evidence we have for that.

Not looking for a general test of GR though. i admit that singling out an aspect of a theory without a proper competing model that differs only there may be hard to do. Maybe it is just enough to have a theory with a limited propagation speed and therefore replacing Newtons gravity with Maxwell-like equations for gravity would do the trick? What would that predict for the rotation of Mercury's perihelion?

[MigL]

This article provides a simple explanation ...

"An important property of gravity in Einstein’s theory is that it can create more gravity. The result is “non-linearity” – the gravitational influence of two bodies isn’t just the sum of their separate influences!"

From     The gravity of gravity « Einstein-Online

[Markus Hanke]

20 hours ago, Killtech said:

How well is the self coupling experimentally verified and understood?

The non-linearity of GR only really shows up in the strong field regime, so there’s no real way for us to experimentally test it. We can, however, test it observationally - in particular, gravitational wave forms observed from collisions of black holes and other heavy objects are consistent with full strong-field non-linear GR, but not with linearised GR. Generally speaking, most (not all) weak-field regimes can be quite accurately modelled with linearised GR, but strong-field scenarios generally require the full non-linear theory.

The non-linearity of the GR equations is very well understood mathematically - ref any text on systems of differential equations.

As for perihelion precession, the non-linearity would only show up for very elliptical orbits (not the case for Mercury), or for orbits in highly curved spacetimes.

[MJ kihara]

9 hours ago, MigL said:

This article provides a simple explanation ...

"An important property of gravity in Einstein’s theory is that it can create more gravity. The result is “non-linearity” – the gravitational influence of two bodies isn’t just the sum of their separate influences!"

From     The gravity of gravity « Einstein-Online

Thanks for the article,however,am still not getting the way gravitational field is source of gravity....I don't know how to put it....what am trying to say is gravitational field is gravity,i think talking of gravity and gravitational field is the same thing.

[swansont]

45 minutes ago, MJ kihara said:

Thanks for the article,however,am still not getting the way gravitational field is source of gravity

A gravitational field contains energy. Energy is a source of gravity.

[Killtech]

14 hours ago, Markus Hanke said:

As for perihelion precession, the non-linearity would only show up for very elliptical orbits (not the case for Mercury), or for orbits in highly curved spacetimes.

 That's what i have intuitively assumed. That scenario however allows for very accurate tests though, since the proximity allows to minimize the influence of unknown factors.

14 hours ago, Markus Hanke said:

The non-linearity of GR only really shows up in the strong field regime, so there’s no real way for us to experimentally test it. We can, however, test it observationally - in particular, gravitational wave forms observed from collisions of black holes and other heavy objects are consistent with full strong-field non-linear GR, but not with linearised GR. Generally speaking, most (not all) weak-field regimes can be quite accurately modelled with linearised GR, but strong-field scenarios generally require the full non-linear theory.

The non-linearity of the GR equations is very well understood mathematically - ref any text on systems of differential equations.

Of course by "experimentally", i meant to include observational tests as is the case of Mercury's perihelion precession. our ability to detect gravitational waves directly is kind of new though, so i am a bit surprised to hear that they already detailed enough to allow such analysis. Or is that more based on indirect observational data for which we have more history?

The issue with non-linearity in a purely empiric model context is that it requires far more detailed and precise data taken from different regimes to determine its exact form because unlike in the linear case, there are way more degrees of freedom/parameters the non-linearities can express themselves. Einsteins postulates are made based on experiences in a weak field regime and produce a very particular kind of non-linearity, but there is no immediate guarantee they still hold everywhere through the strong field regimes. Hence, it is important to test their exact predictions with observation data and look for deviations. Given how much dark matter GR needs to be somewhat consistent with observation and the singularities it produces, i am struggling to understand how well verified the theory is throughout all the regimes.

It is of course already important to know that gravity cannot be fully linear, but such an observation/distinction alone does not reveal that much about the detailed nature of the non-linearities in reality.

 

Anyhow i didn't know there is a linearized model of GR. Guess, it makes sense to have that as a computationally much easier proxy. Have to look how that works, thanks.

[MJ kihara]

On 10/4/2023 at 3:07 AM, swansont said:

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

 

On 10/4/2023 at 5:34 AM, Genady said:

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.

Why can't the impasse be solved by a having a chimeric particle that transition either way...hhhh kinda of gatekeeper....anyway I know it sounds unconventional and belongs to speculation section,apologies for that.

[Markus Hanke]

7 hours ago, Killtech said:

It is of course already important to know that gravity cannot be fully linear, but such an observation/distinction alone does not reveal that much about the detailed nature of the non-linearities in reality.

The non-linearity of the model is encoded in the structure of the field equations themselves, and means simply that you can’t just add two valid solutions (metrics) together, and expect the result to also be a valid solution to the equations for the particular physical scenario you are interested in. For example, in the aforementioned case of a binary BH merger, the metric of the spacetime containing the in-spiralling black holes is thus not just the sum of two “ordinary” black hole metrics, but a new and different solution in its own right, which has to be obtained from scratch by solving the field equations (which in this case can only be done numerically). The degree by which solutions fail to be linear will increase the more you move into the strong field regime - e.g. when your binary BH are still very far from each other, the overall spacetime almost (depending on your required levels of accuracy) looks like two ordinary BH spacetimes joined together; but as they continue their in-spiral and get closer together, the error of the linearised approximation becomes very large very quickly.

Since, in this scenario, the form of the gravitational wave field far away depends explicitly and directly on the geometry of the spacetime close to the in-spiralling black holes, the difference (relative to a linearised approximation) is directly observable here. In general though it is difficult to separate out the effects purely due to non-linearity, since this self-interaction is encoded in the structure of the equations themselves, and thus does not appear as a computational term that can be isolated and separately measured.

7 hours ago, Killtech said:

Anyhow i didn't know there is a linearized model of GR. Guess, it makes sense to have that as a computationally much easier proxy. Have to look how that works, thanks.

The basic idea is this - you treat the gravitational metric as a small enough deviation from flat Minkowski spacetime, so you make an ansatz of the form

\[g_{\mu \nu}=\eta_{\mu \nu}+h_{\mu \nu}\]

and demand that \(|h_{\mu \nu}|\ll 1\). Also introduce the convention that an upper bar means trace removal, ie

\[\overline{h}_{\mu \nu } \equiv h_{\mu \nu } -\frac{1}{2} \eta _{\mu \nu } h\]

Without loss of generality (this can be formally proven, but I’ll obmit that here), one can then impose a gauge condition to simplify the maths, such as

\[\overline{h}{^{\mu \alpha }}{_{,\alpha}}=0\]

Setting all of this into the original Einstein equations and working through the considerably cumbersome expressions, everything decouples and simplifies into

\[-\overline{h}{_{\mu \nu ,\alpha}}^{\alpha}=16\pi T_{\mu \nu}\]

Unlike the full original Einstein equations, this equation is fully linear, and obviously far easier to solve. 

Edited November 3, 2023 by Markus Hanke

[Killtech]

10 hours ago, Markus Hanke said:

The non-linearity of the model is encoded in the structure of the field equations themselves, and means simply that you can’t just add two valid solutions (metrics) together, and expect the result to also be a valid solution to the equations for the particular physical scenario you are interested in. For example, in the aforementioned case of a binary BH merger, the metric of the spacetime containing the in-spiralling black holes is thus not just the sum of two “ordinary” black hole metrics, but a new and different solution in its own right, which has to be obtained from scratch by solving the field equations (which in this case can only be done numerically). The degree by which solutions fail to be linear will increase the more you move into the strong field regime - e.g. when your binary BH are still very far from each other, the overall spacetime almost (depending on your required levels of accuracy) looks like two ordinary BH spacetimes joined together; but as they continue their in-spiral and get closer together, the error of the linearised approximation becomes very large very quickly.

Since, in this scenario, the form of the gravitational wave field far away depends explicitly and directly on the geometry of the spacetime close to the in-spiralling black holes, the difference (relative to a linearised approximation) is directly observable here. In general though it is difficult to separate out the effects purely due to non-linearity, since this self-interaction is encoded in the structure of the equations themselves, and thus does not appear as a computational term that can be isolated and separately measured.

I am quite familiar with non-linear equations. i work in finance and we have quite a bit of non-linear models to tackle as well. In terms of physics i have studied a bit soliton solutions which only appear in non-linear equations. But i have to admit that i am quite new to GR. I mean i knew that Maxwell equations are non-linear in GR through their contribution to energy and therefore to curvature, but for gravity i haven't somehow realized it by looking at the equations. The terms hidden in the curvature tensor easily slip ones attention. 

But again, the issue with non-linearity is that it is in fact way more complex not just in terms of solutions, but how many different kinds of non-linearities there may be. In comparison linear models are structurally almost uniquely determined. So having a lot more possibilities allows to fit any data at the cost of a significantly more complex parameterizations to calibrate - success of AI tells a story about that. But if we were to train an AI model to predict the time evolution of physical gravity systems based on our observation data, i bet it will come up with a quite different model which will deviate from Einstein's GR mostly in the extrapolating regime where we lack sufficient observations. Non-linear models are usually quite terrible at extrapolating. 

So there is a good reason to not blindly trust such models and thoroughly tests them through out all regimes. This is why i want to understand how well we have tested the particular shape of the non-linear contributions. 

10 hours ago, Markus Hanke said:

The basic idea is this - you treat the gravitational metric as a small enough deviation from flat Minkowski spacetime, so you make an ansatz of the form
[...]
Unlike the full original Einstein equations, this equation is fully linear, and obviously far easier to solve. 

Hmm, sounds familiar, a bit like perturbation theory up to the first order. Thanks for the explanation.

The linear field equation is admittedly much easier to visualize and understand the core of the theory, so i suppose I should have started with that. Ah well.

[Boltzmannbrain]

This question gets explained starting at 6:00 

 

[MJ kihara]

5 hours ago, Boltzmannbrain said:

This question gets explained starting at 6:00 

 

".....just as photon/vitual photon pop out of magnetic fields virtual graviton pops out of gravitational fields....." let me take my pill in the speculation side...red pill is certainly bitter .

Observation reign supreme, but in this case it's virtual ...due to uncertainty principle,no further questions.

It's also good to emphasis that as 'currently/conventionally' known nothing escape from a blackhole to avoid finality and leave room for further research.

[mistermack]

6 hours ago, Boltzmannbrain said:

This question gets explained starting at 6:00

That's an excellent find. I prefer the river picture that he went through first, but the whole video is to the point and well done. 

[Markus Hanke]

17 hours ago, Killtech said:

This is why i want to understand how well we have tested the particular shape of the non-linear contributions. 

I don’t see how it would be possible to separate out just the non-linear contributions, so it is difficult to test this directly, other than to compare the exact solutions against the linear approximation.

[martillo]

20 hours ago, Boltzmannbrain said:

This question gets explained starting at 6:00 

 

So now, it could be needed to begin to be considered "virtual particles" with any velocity, even "impossible velocities" (middle of the video), for the things to work? Wouldn't this be the same as the "instantaneous action at a distance" concept of of Classical Physics? Seems it is time to doubt about everything and rethink everything...

Edited November 5, 2023 by martillo

[Boltzmannbrain]

2 hours ago, martillo said:

So now, it could be needed to begin to be considered "virtual particles" with any velocity, even "impossible velocities" (middle of the video), for the things to work? Wouldn't this be the same as the "instantaneous action at a distance" concept of of Classical Physics? Seems it is time to doubt about everything and rethink everything...

I found that quite surprising too, but I don't think I would start doubting everything. 

[martillo]

1 hour ago, Boltzmannbrain said:

I found that quite surprising too, but I don't think I would start doubting everything. 

Just curious, what would you start reviewing in Physics?

Edited November 5, 2023 by martillo

[Boltzmannbrain]

11 hours ago, martillo said:

Just curious, what would you start reviewing in Physics?

Depends on how much physics the person understands I guess.  If you understand enough, you can try going to the Google Scholar search and putting in key words to find any papers written on the topic.   

[martillo]

19 minutes ago, Boltzmannbrain said:

Depends on how much physics the person understands I guess.  If you understand enough, you can try going to the Google Scholar search and putting in key words to find any papers written on the topic.   

You misunderstood me. I asked what you would start reviewing, not how to do it. I mean, is there any special topic in Physics you think could be questioned right now? But I was just curious if any. Don't worry too much about.

Edited November 5, 2023 by martillo

[Boltzmannbrain]

8 minutes ago, martillo said:

You misunderstood me. I asked what you would start reviewing, not how to do it. I mean, is there any special topic in Physics you think could be questioned right now?

I think the how will lead you to the what.  Current scientific papers are a good place to look for special topics because they will discuss the latest research and even give educated opinions in the introduction.  I am sure if you type in graviton in the Google Scholar search, you will find a lot of good information.