The speed of propagation of gravity

[MigL]

Thanks for clearing away some of the 'fog', guys.

[SergUpstart]

13 hours ago, Mordred said:

What is defined as the four forces involve whether or not that field has a gauge boson. Example the gauge boson for the EM field is the photon.

Gravity if it is a force would need the graviton as it's gauge boson. We haven't discounted the possibility yet.

Keep in mind were talking force fields.

I heard that the discovery of the Higgs boson means that all the fundamental particles of the Standard model have been detected. Graviton has not yet been detected. So does the Standard model provide for the existence of a graviton?

[Markus Hanke]

15 hours ago, SergUpstart said:

Maybe the question of whether gravity is a force can be solved like this. Is centrifugal force a force? Working in Cartesian coordinates, we work in a non-inertial frame of reference in which gravity is a force.

To see whether gravity is a force, simply attach an accelerometer to a freely falling test particle. You will find that the instrument reads exactly zero at all times, even though the trajectory of the test particle makes it obvious that it is affected by gravity. And then of course you have other effects, such as gravitational time dilation, that can’t be explained by forces at all.
Thus, gravity isn’t adequately described as a mechanical force.
 

43 minutes ago, SergUpstart said:

I heard that the discovery of the Higgs boson means that all the fundamental particles of the Standard model have been detected.

It completes the “zoo” of those particles which the Standard Model predicts within the energy ranges that we can probe with current technology.

44 minutes ago, SergUpstart said:

Graviton has not yet been detected.

The hypothetical graviton interacts so weakly that it would be extremely difficult to detect it directly.

46 minutes ago, SergUpstart said:

So does the Standard model provide for the existence of a graviton?

The entire idea of a “graviton” is based on the notion that gravity can be quantised using the usual framework of quantum field theory. It is in fact easy to write down a QFT for gravity - but the problem is that such a QFT is not renormalisable, and exhibits infinities that cannot be removed via any known method. Essentially, the resulting QFT is useless, in that one cannot extract many meaningful physical predictions from it. So evidently, QFT is not the right method to quantise gravity.
Based on current knowledge, it would seem that gravity is conceptually different from the other fundamental interactions, and is hence not amenable to the usual quantisation schemes. This puts a huge question mark behind the notion of a “graviton” - treating gravity as the interchange of vector bosons may not be a meaningful concept. But if it is, then it would not be difficult to incorporate it into the Standard Model (you’d just add an appropriate term to the Lagrangian).
This is an area of ongoing research.

[studiot]

4 hours ago, Markus Hanke said:

To see whether gravity is a force, simply attach an accelerometer to a freely falling test particle. You will find that the instrument reads exactly zero at all times, even though the trajectory of the test particle makes it obvious that it is affected by gravity. And then of course you have other effects, such as gravitational time dilation, that can’t be explained by forces at all.
Thus, gravity isn’t adequately described as a mechanical force.

+1

 

What more is there to be said ?

[SergUpstart]

9 hours ago, Markus Hanke said:

 

The hypothetical graviton interacts so weakly that it would be extremely difficult to detect it directly.

The entire idea of a “graviton” is based on the notion that gravity can be quantised using the usual framework of quantum field theory. It is in fact easy to write down a QFT for gravity - but the problem is that such a QFT is not renormalisable, and exhibits infinities that cannot be removed via any known method. Essentially, the resulting QFT is useless, in that one cannot extract many meaningful physical predictions from it. So evidently, QFT is not the right method to quantise gravity.
Based on current knowledge, it would seem that gravity is conceptually different from the other fundamental interactions, and is hence not amenable to the usual quantisation schemes. This puts a huge question mark behind the notion of a “graviton” - treating gravity as the interchange of vector bosons may not be a meaningful concept. But if it is, then it would not be difficult to incorporate it into the Standard Model (you’d just add an appropriate term to the Lagrangian).
This is an area of ongoing research.

I see another problem with graviton. The virtual graviton must move faster than the speed of light so that the gravity of black holes can go beyond the event horizon.

[Strange]

15 minutes ago, SergUpstart said:

I see another problem with graviton. The virtual graviton must move faster than the speed of light so that the gravity of black holes can go beyond the event horizon.

That is not a problem because virtual particles (which are not really particles) are not constrained in that way.

(Also, a quantum theory of gravity might completely change our understanding of the event horizon.)

[Markus Hanke]

2 hours ago, SergUpstart said:

I see another problem with graviton. The virtual graviton must move faster than the speed of light so that the gravity of black holes can go beyond the event horizon.

Static gravity does not propagate, so no gravitons need to escape an event horizon.
Gravitons would need to be massless spin-2 bosons, and as such move at exactly the speed of light, just like photons and gluons.

As a side note - whether superluminal motion would guarantee the ability to escape an event horizon is a question (albeit a purely academic one) that isn’t straightforward to answer.  

[studiot]

It is disappointing to say the least to put thought and effort into answering someone's questions at a sutiable level and find they can't be bothered to respond.

Especially when they are attempting to meddle with super advanced theory, whilst not understanding the basics.

Edited January 29, 2020 by studiot

[Mordred]

Sergupstart it's best to start with gravity being spacetime curvature described by the energy momentum tensor. You really are not ready for particle production via field strength for particle number density. In essence any location of a field is capable of particle production. The number density is in direct ratio with the field energy density at a given locale.

 So no gravitons do not need to escape a BH.

 Secondly gravity as spacetime itself as Marcus mentioned is static. 

[SergUpstart]

My assumption that gravity slows down the speed of propagation of electromagnetic waves, but does not slow down the propagation of gravitational waves can be easily refuted experimentally, if the effect of gravitational lensing for gravitational waves is detected, i.e. the gravity of massive bodies such as the Sun or Jupiter will equally bend the trajectories of both photons and gravitational waves.

[Strange]

52 minutes ago, SergUpstart said:

My assumption that gravity slows down the speed of propagation of electromagnetic waves, but does not slow down the propagation of gravitational waves can be easily refuted experimentally, if the effect of gravitational lensing for gravitational waves is detected, i.e. the gravity of massive bodies such as the Sun or Jupiter will equally bend the trajectories of both photons and gravitational waves.

Unfortunately, we can't pinpoint the source of gravitational waves accurately enough to test the lensing (and the chances of spotting a suitable candidate, even if we could, are pretty low).

But there is no reason why anyone would particular want to test a baseless assumption that is already contradicted by evidence and theory. Designing experiments to test some random person's random guess about how things work is a waste of time.

[Mordred]

One can mathematically formulate the effect of lensing on a GW waves however as you mentioned testing would be problematic as the detector would have to be in location and a suitable size to detect the correct range of frequencies.

Might be cheaper to build a colony on the Moon lol.

Anyways the curvature can lead to GW tails. 

Edited January 30, 2020 by Mordred

[Markus Hanke]

14 hours ago, SergUpstart said:

My assumption that gravity slows down the speed of propagation of electromagnetic waves, but does not slow down the propagation of gravitational waves can be easily refuted experimentally, if the effect of gravitational lensing for gravitational waves is detected, i.e. the gravity of massive bodies such as the Sun or Jupiter will equally bend the trajectories of both photons and gravitational waves.

The interaction of gravitational waves with background curvature due to gravitational sources, as well as with other gravitational waves, exhibits nonlinear dynamics. This becomes more pronounced in the strong field regime, und you will get effects such as backscatter, frequency shifts, tails etc that don’t exist for EM radiation (which is governed by linear dynamics). It would be very difficult to draw conclusions as to propagation speeds from gravitational lensing alone.

A better way to test the propagation speed is to have not one, but several gravitational wave detectors, preferrably in orbit. You can then just look at the delay between detectors picking up the signal, which immediately tells you the propagation velocity. Unfortunately at this point in time we have only a very small data set involving only two earth-bound detectors, so the error margin is too big for this to be very meaningful; nonetheless, every additional detection event will statistically improve the bounds, so it is just a matter of time really. Here is the current data:

https://arxiv.org/abs/1707.06101

 

[SergUpstart]

1 hour ago, Markus Hanke said:

Here is the current data:

I didn't find any waveforms there. Waveforms of gravitational waves similar to sine waves?

After all, if gravitational waves are not harmonic oscillations, then the requirement for the presence of mass in gravity can be removed.

In short, the question is formulated as follows: is the nature of gravitational waves related to self-oscillations? It is an elastic wave?

[MigL]

No, not similar to sine waves at all.
You could picture them as alternating stretching and compressing of space-time in the vertical and horizontal, about the axis which is the direction of travel. Google quadrupole wave

Interestingly, A Einstein predicted their existence, based on GR, in 1916.
But They were first proposed by H Poincare in 1905.

Edited January 31, 2020 by MigL

[Strange]

7 hours ago, MigL said:

No, not similar to sine waves at all.
You could picture them as alternating stretching and compressing of space-time in the vertical and horizontal, about the axis which is the direction of travel. Google quadrupole wave

An animation of this:

(Depending on your browser, you might have to click the image to see the animation)

From: https://cplberry.com/tag/polarizations/

For the sources of gravitational waves we have seen so far, the amplitude of the oscillation will be approximately sinusoidal (because it is created by two objects in approximately circular orbits around one another). But both the amplitude and frequency will be continuously changing.

Nice breakdown of the waveform and what it tells us in the image and text here: https://cplberry.com/2016/02/23/gw150914-the-papers/#parameter-estimation

[MigL]

Interestingly if you reduce space-time to two dimensions, as in the 'stretched trampoline with a bowling ball in the center' analogy, you do get sinusoidal waves spiraling outwards. As in this video simulation :

https://www.bing.com/videos/search?q=quadrupole+waves&&view=detail&mid=38596A8CBDA754CE970238596A8CBDA754CE9702&&FORM=VRDGAR&ru=%2Fvideos%2Fsearch%3Fq%3Dquadrupole%2Bwaves%26FORM%3DHDRSC3

 

[Markus Hanke]

21 hours ago, SergUpstart said:

requirement for the presence of mass in gravity

The source of gravity is any form of energy-momentum, not just mass. Also, because gravity is non-linear, in some sense it also forms its own source - so “gravity gravitates”. 

21 hours ago, SergUpstart said:

In short, the question is formulated as follows: is the nature of gravitational waves related to self-oscillations? It is an elastic wave?

I don’t know what you mean this, as spacetime is not a medium. Gravitational waves are periodic changes in the curvature of spacetime, and their source is a quadrupole or higher multipole moment (unlike EM radiation, which is dipole in nature).

[SergUpstart]

7 hours ago, Markus Hanke said:

Gravitational waves are periodic changes in the curvature of spacetime, and their source is a quadrupole or higher multipole moment (unlike EM radiation, which is dipole in nature).

Can this be stated in other words, "Electromagnetic waves are only transverse and gravitational waves are both longitudinal and transverse" ?

[Mordred]

No both EM and GW are transverse waves. The main difference is the polarizations of a GW wave are 45 degrees apart but with EM they are 90 degrees apart. However they are still transverse polarizations.

By the way that was a good question one seldom asked. The longitudinal component of a GW wave are transverse traceless hence the polarity tensors [math]h_+,h_×[/math] (more accurately were using GR'S permutations  tensors ). The  traceless components are the longitudinal components.

Edited February 1, 2020 by Mordred