[guest49538]

As far as I know, gravity is unrelated to electromagnetism (maxwellian theory).

Einstein's special relativity is based on Lorentz transforms, the base of which (being known "a priori") allowed to postulate that nothing material can  travel faster than light.

But gravity itself is a different topic, and gravity effect propagating faster than light doesn't violate any relativity (STR or GR). IMHO, gravitational waves are perturbations in the GR gravitational field (ripples of space-time) but are not the carriers of gravitational effects. I insist: IMHO.

I've read a lot of arguments trying to prove that the speed of gravity is faster than light's speed. One of the experiments that was proposed is interesting for me: when a Jupiter's moon appear from behind Jupiter, its light takes a finite time to reach Earth. If there was a way to measure when its gravitational effect reach Earth, the question could be solved.

I have two doubts:

1) Is it that there is no shield against the action of gravity? In this case, Jupiter's moon gravity effect would be present all the time, either the moon is visible or behind Jupiter.
2) Do you think that technology to measure this effect (if it exist) can be placed into an observatory orbiting at
    any Lagrangian point, very far away from Earth? Or the effect is so tiny that it can't be detected with currently
    available technology?

I invite you to propose any other experiment, within the range of our lifetime.

[guest49538]

I modified a little bit my doubt 1), in order to represent my point with more clarity.

That gravitational effects of planets within the Solar System propagate at a certain speed is something that, occassionally, MSM publish as news. Example: when several planets become aligned into a single axis with Earth, MSM publishes that some light effects could be measured here on Earth.

If I accept GTR concept that gravity from massive objects bend the space around and that such effect vanish with distance being large enough, then newtonian physics can be applied. Is this correct?

In this case, I could imagine planets orbiting around Sun carrying with them their gravity wells. Is it correct to think that, when several planets are aligned in an axis that contains Earth, effects of such gravity wells add up (in a non linear way)? This is derived from my thought of planets orbiting with gravitational wells that vanishes asymptotically with distance, and respecting GTR postulates.

[evan_au]

Is it that there is no shield against the action of gravity? In this case, Jupiter's moon gravity effect would be present all the time, either the moon is visible or behind Jupiter.

So far as we know, there is no shield against gravity. So the proposed experiment would not prove anything.
Also see the answer below.

when several planets are aligned in an axis that contains Earth, effects of such gravity wells add up (in a non linear way)?

Yes, they do add up.

The gravitational influence decreases with the distance squared. But you cannot feel this influence, because the Earth follows a "free fall" geodesic through space, which exactly cancels out this gravitational influence for those of us on the surface of the Earth.

What matters to us is the tidal effect, which decreases with distance cubed. This means that the Moon has a much bigger effect than Jupiter. The Moon has a slightly bigger effect than the Sun. And Jupiter has a bigger effect than all the other planets put together.

So there is no need to worry about planets lining up - the effect is immeasurably small compared to the Earth, Moon and the Sun lining up.

Can the speed of gravity be faster than light?

Gravitational waves "ride" on the same spacetime as static gravitational fields, and so will travel at the same speed.

The most accurate measurement of the speed of gravitational waves was with the collision of two neutron stars. The gamma ray burst came within 2 seconds of the gravitational wave endpoint. For an event estimated at around 130 million light-years away, that is as close to identical speeds as you can get in a single physical measurement!
See: https://en.wikipedia.org/wiki/GW170817

[Halc]

The standard demonstration of gravity not having a finite speed is orbital motion.  If each of two orbiting objects was attracted to where the other 'was' instead of where it 'is', then it would have a forward component to its force vector.  It would be pulled in a different direction that towards the other object, resulting in both objects spiraling outward (a free energy gain) over time.  In fact, objects assume stable orbits, or in extreme cases, spiraling in as significant energy is radiated away in the form of gravity waves.
Earth does drop its orbit due to this radiation (energy loss at a rate of about 200 watts), and also due to friction (much greater effect).  Those effects in turn are dwarfed by the two forces that move Earth's orbit outward.

[guest49538]

Thank both for your answers. You were very kind to bother replying my question.

Probably you noticed that I used "gravitational effect" instead of newtonian forces or einstenian accelerations in the perpetual fall onto the gravitational well caused by space curvature due to a massive object to refer the cause of gravity in the motion of Earth.

Even when being far from an expert, I have my own thoughts about how gravity works and they are a mix between both theories. I don't accept blindly GTR neither I accept Newton's action at a distance. I find myself more comfortable with in-between theories like MOND, which has been rescued from oblivion by an international group of scientists and which can explain several phenomena at the Solar System (perihelion of Mercury, Flyby anomalies, perturbations of Moon's orbit, delays in Halley's comet orbit). Also, MOND provide some explanations about the rotation of galaxies without resorting to dark matter.

I believe that GTR is extremely complex in its formulation and highly non-linear, which is the cause that most of the solutions applied to cosmology are linear approximations or discrete computer-based numerical processing without any explicit analytical expression. I'm used to the second technique, as it is used with non-linear equations, which happen very often in the field of technology. I've followed in details the tricks that Schwartzchild used to obtain an exact analytical solution to the simplest theorem of Einstein about Mercury, which used only one point-like mass at the origin and analyzed the trajectory of a massless point (Mercury) in an universe where only the central point-like mass existed. I've also followed the modification of Hilbert over Schwartzchild (1917), which created a solution that Hilbert named after the late Schwartzchild. In both cases, two genial polymath had solved a problem using GTR and only one mass. I don't know of an analytical solution of GTR equations for the two body's problem, less yet GTR solutions for an universe composed by quadrillions of galaxies, even if each one is assumend as a point-like mass. I know that solutions are obtained by assuming the mass and energy isotropically distributed in the whole space, but none with two or more discrete mass.

If I'm wrong, please let me know.

I think that none of those three theories explain what gravity IS, but only what gravity DOES.

At any case, I accept the description of gravitational fields as the basis for a simplified explanation of gravitational effects (I remark the use of "simplified")

As far as I know, this concept can be traced back in time to Gauss (by 1845), when he re-wrote Newton's Theory of Gravity in terms of fields (a new concept by then) even when Gauss himself was unable to solve the problem of action at a distance (later "solved" by the use of retarded potentials). Maxwell followed Gauss with the concept of fields in electricity and magnetism (initially conceived by Faraday, and later partially certified by Gauss) and Einstein followed Maxwell by conceiving gravitational fields with intrinsic retardation of potentials (his GTR). I can "visualize" a field and understand that a ripple can be propagated along it.

So far, so good.

Besides the fact that I believe that there is not a certain explanation about what gravity is, I wonder if any of you can answer this question, accepting that gravitational waves exist:

Are gravitational waves an expression of the mediating gravitational effect between bodies or just a perturbation of the gravitational field that propagates spherically along the 3D space and that are only a transient effect due to a cosmic phenomena in the gravitational field?

I take as an example the mentioned capture of that gravitational wave by LIGO a couple of years ago.

To complete my question: Do you believe that the boson "graviton" (to be discovered) is the real carrier of the gravitational effect?

In the case of an affirmative response, then the "graviton" would propagate at the speed of light (eventually with an insignificant difference + or -).

Unless I'm deadly wrong, gravitational waves and "gravitons" are two completely different things. Is this OK?

Thanks.

[Halc]

At any case, I think that none of those three theories explain what gravity IS, but only what gravity DOES.

Science is all about what things do.  Philosophy is about what they are.  So the theories are doing what they're supposed to.

Are gravitational waves an expression of the mediating gravitational effect between bodies or just a perturbation of the gravitational field that propagates spherically along the 3D space and that are only a transient effect due to a cosmic phenomena in the gravitational field?

I cannot commit to either wording.

To complete my question: Do you believe that the boson "graviton" (to be discovered) is the real carrier of the
gravitational effect?

In the case of an affirmative response, then the "graviton" would propagate at the speed of light (eventually with an insignificant difference + or -).

They call it a graviton, yes.  That would make it discreet if such a thing exists.  Yes, the waves, be they composed of gravitons or not, propagate at light speed.  This has been measured.

Unless I'm deadly wrong, gravitational waves and "gravitons" are two completely different things. Is this OK?

Saying they're different is like saying light waves and photons are different things.  One is composed of the other, which doesn't make them 'completely different'.

[Colin2B]

Unless I'm deadly wrong, gravitational waves and "gravitons" are two completely different things. Is this OK?

Saying they're different is like saying light waves and photons are different things.  One is composed of the other, which doesn't make them 'completely different'.

I’m with @Halc here. To understand the relationship between gravitational waves and  gravitons you need to compare em waves and photons, they are aspects of the same thing.

Even when being far from an expert, I have my own thoughts about how gravity works and they are a mix between both theories.

As in the muon thread it is easy to start widening the topic far beyond the initial question. The main focus in this section of the forum is specific science questions and it might be better to start a new thread or put some of your ideas in our new theories section and keep this topic focussed on the speed of gravity.

PS if you keep posting you will soon be able to post links. The limitation is there to stop spammers abusing the forum.

[evan_au]

MOND ... which can explain several phenomena at the Solar System

MOND only makes a detectable difference at the scale of a galaxy (eg galaxy rotation curves) - although there have been proposals for space probes with sensitive instruments that may be able to test the theory within our solar system.

We know that MOND does not make a significant difference on the scale of the Solar System, because of the great care that NASA takes in tracking its space probes. This allowed them (after some years of investigation) to detect the momentum of infra-red radiation from the electronics of their space probe, causing a tiny unexpected acceleration. This does not allow for much in the way of deviations from Newton's laws in the outskirts of the solar system.

See: https://en.wikipedia.org/wiki/Pioneer_anomaly
See: https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics

Are gravitational waves an expression of the mediating gravitational effect between bodies or just a perturbation of the gravitational field that propagates spherically along the 3D space and that are only a transient effect due to a cosmic phenomena in the gravitational field?

We can paraphrase this in a more familiar field as:

Are electromagnetic waves an expression of the mediating electromagnetic effect between charges or just a perturbation of the electromagnetic field that propagates spherically along the 3D space and that are only a transient effect due to a (cosmic) phenomena in the electromagnetic field?

I would say both. In an antenna there is:
- A local disturbance in the electromagnetic field due to the RF amplifier
- A non-local disturbance in the electromagnetic field that propagates away "to infinity"
- How transient the electromagnetic field is depends on how long you have the amplifier powered up.

Translating this back into gravitational fields...
I would say both. With binary neutron stars there is:
- A local disturbance in the gravitational field due to the orbiting neutron stars
- A non-local disturbance in the gravitational field that propagates away "to infinity"
- How transient the gravitational field disturbance is depends on how long before the neutron stars collide (usually millions to billions of years - although with our current generation of detectors, we can only detect the last minute or so, when emissions are most intense).

Do you believe that the boson "graviton" (to be discovered) is the real carrier of the gravitational effect?

Relevant experts believe that the (hypothetical) graviton would have the right properties to carry the gravitational field.
-If it looks like a duck, and sounds like a duck, it probably is a duck...

But they also know of no way that we could detect an individual graviton with our current technology, because the gravitational force is around 40 orders of magnitude weaker than the electromagnetic force.
- Remember that it was only around the year 1900 that individual ultraviolet photons (wavelength about a micron) were detected, via the photoelectric effect
- And we have not yet been able to detect individual AM Radio-Frequency photons, with a wavelength of hundreds of meters, as the energy of these photons is just too low
- The gravitational waves we are currently detecting have a wavelength of around 3,000km. We would have no chance of detecting photons of this wavelength, let alone the much weaker (hypothetical) gravitons.
See: https://en.wikipedia.org/wiki/Graviton

GTR is extremely complex in its formulation and highly non-linear, ... without any explicit analytical expression.

Newton's gravity is also non-linear, and doesn't have  a general analytic expression when there are more than 2 bodies (like the Solar system).

Despite the lack of an analytic solution to Newton's gravity or Einstein's gravity, NASA still manages to plot the course of their space probes on a multi-visit tour of the Solar system. The difference in complexity between the two numerical solutions is pretty minor.

[guest49538]

Thanks to all of you for your explanations  about your understanding upon the topic of gravity.

I decided to reply about it due to some discrepances that I have regarding some assertions of yours.

When I posted the topic as a question, I was following the rules that I read for this forum but. I'm interested in knowing what concepts and beliefs forum members have over the OP topic and, if it existed, any opinion about what gravity is.

I had, previously, some concepts and beliefs myself. They are based on information that I've accessed along these years (either I agree with it or not) and my own understanding and knowledgement.

I'm writing a list of points, whitout any particular order, related to opinions expressed in the different posts:

1) I disagree with the comment: "Science is all about what things do.  Philosophy is about what they are."
    Even when physics evolved from natural philosophy, as it was known until early XIX century, they grew apart
    specially when theoretical physics exploded since 1900, giving birth to quantum physics (Planck) and
    modern cosmology (Schwartzchild, 1900 paper). In the first case, black body cavity radiation explained by
    Planck solved a 40 years problem posed by Kirchoff and it serves well even today (what is, why happens).
    The evolution of quantum physics and later, nuclear physics, provided some answers about what things do
    and some questions about what things are, and the ping-pong between answes and questions kept growing
    from simple, elementary proposals in the first quarter of the XX century. The same thing applies to cosmology,
    incredibly in the same period.
    For me (and as far as I researched in the web), science has to provide explanations about what, how and why.
    If the science behind something is solid but counter-intuitive, then philosophy can work around the dissonances.
    This could be applied to any of the relativities, if they were rock-solid theories, what I think they are not.
    As a proof, for 100 years, the debate about the rationality of STR continues even when it consumed the mind and
    work of thousand of intelligent scientist per generation.

2) The relationship proposed between electromagnetic waves and photons versus gravitational waves and gravitons
    is, for me, not valid. The reasons are many, but I'll try to sumarize:

    2.1) Electromagnetic and gravitational theories deal with behaviors of nature so different that any comparison can't
           be applied. The first one has a solid body of knowledge behind, more than 100 years of succesful applications
           and (IMHO) is undisputed in its validity since Maxwell's time. The technology that surround us is a solid proof
           of it. Gravitational theories are much less solid, with permanent adjustments (like dark matter & energy), and
           still under disputes and the seek of replacements or enhancements.
    2.2) In electromagnetism, and in the words of the founding father of quantum physics (Planck), "waves carry
           electromagnetism along the space while "quantums of action hf" are the unit to measure the emission or
           absorption of waves by "hertzian resonators"". Soon after these wise words, these entities would become
           photons and atoms (not known by then, in 1900/1901).
    2.3) There are no proofs that gravitational waves are the carriers of gravitational effects (permanent ones),
           neither that "gravitons" are the mediating entities between matter and gravity.
    2.4) In electromagnetism, the elementary dipole antenna is the mediating device between oscillators, absorbers
           and the medium. Today, more elaborated devices like fractal antennae were created for greater efficiency.
           At any case of any antenna, there are two kinds of electromagnetic fields: near fields, which surround any
           any antennae for several wavelengths and is highly non-linear, and far fields, which create the known EM
           waves, which fulfill the "wave equation", known for almost two centuries.
           In the area of gravity, as far as I know, such analogy between near and far fields are not proven and I'm not
           shure if the far field gravitational wave has a permanent existance (steady or cyclic) or a transitory existance.
           And, the "attraction effect" due to G waves is not demonstrated as permanent, in contrast with EM radiation
           from any given star.
    2.5) A three-body solution (Earth, Moon, Sun) has analytical solutions since Lagrange times, and provide five
          points (L1 to L5) which are currently used by several artificial satellites (spatial observatories) from ESA,
          NASA, Russia and China. They are not steady points, as math isn't perfect, but quite stable within a range.

To end these comments, I'd like to add that for me (as I don't believe that G waves are carriers of gravity) the nature
and speed of gravity remains open to explanations. Maybe gravity is always there and is intrinsic to matter, so there
is non-sensical to talk about the speed of gravity. Or, maybe the speed of gravity is enormous but finite, so then it is
still to be measured.

One final comment: I've read, years ago, a theory from a russian physicist which explained Mercury's perihelion phenomena without using retarded potentials. Instead, he made calculations using the concept of a Sun being a
gaseous imperfect sphere which has an unnoticed precession around it main axis, which sufficed to explain the
slow phenomena of Mercury.

Well, I hope that these comments be well received in a good spirit of an open debate.
 

[evan_au]

I'm not sure if the far field gravitational wave has a permanent existance (steady or cyclic) or a transitory existance.

We know that far-field gravitational waves have a quite permanent existence: Some of the detected events have an estimated distance of over 6 billion light years - a significant chunk of the age of the universe!

We expect that these "chirps" will spread out indefinitely through the universe at the speed of light, getting weaker with distance.

You will notice that most of the entries in the table are dated after April 1, 2019. This is when the sensitivity improvements came online - now astronomers can detect even farther and fainter events.
See: https://en.wikipedia.org/wiki/List_of_gravitational_wave_observations#List_of_gravitational_wave_events

[Colin2B]

2) The relationship proposed between electromagnetic waves and photons versus gravitational waves and gravitons
  is, for me, not valid. The reasons are many, but I'll try to sumarize:

  2.1) Electromagnetic and gravitational theories deal with behaviors of nature so different that any comparison can't
         be applied.

There are similarities between them and comparisons can be made which help us to understand them better.
Let’s start with the field surrounding an electron. A test +ve charge will be attracted to the electrons position. We know that the electric field propagates at light speed, but if the electron is moving the test charge will move towards the current position of the electron, not where it was. In this way the field behaves in a similar way to gravity.
What is less well known is that if the electron changes velocity the test charge will move towards the position the electron would have been at had it not accelerated, until such time as the change in the field propagates to the test charge’s position. How does the test charge know where the electron will be? I can offer an analogy that shows that this behaviour of fields is not so mysterious.
Imagine a boat crossing a lake, the bow wave propagates away from the boat, but even at some distance from the boat the crests point to the boat’s current position (almost,  because waves in water don’t propagate exactly as do em waves). If the boat changes direction it will take time for the change to propagate to the observer, and until it does the bow wave will point to the predicted position of the boat had it not changed course.
In the case of the accelerating electron we know the speed at which the change propagates, because the change causes a discontinuity in the field which we detect as em radiation - light, radio etc (and if we have a quantum detector we can say we detected a photon).  We can describe the electron field at any point as being dependent on the history of both the position and derivative of position (velocity) of the electron.
A similar effect occurs with gravity. As @Halc pointed out, the force felt by the moon is directed to the current position of the earth rather than it’s historical position. This led early researcher to conclude that the speed of gravity was either instantaneous or extremely fast whereas it does not need to be for similar reasons as I have described for the electron.
It’s worth pointing out that the gravity situation is slightly more complex because there is a higher order effect which allows the masses to ‘point’ towards each other’s current positions despite both being in non-linear motion around a common centre of mass. This higher order effect is also why em waves are dipole and gravitational waves are quadrupole. It also means that gravitational waves cannot be produced by a simple acceleration of a single mass. However, the gravitational waves we have detected (which are analogous to the em radiation of the electron) show that the speed of propagation, and hence the speed at which gravity changes propagate, is as close to light speed as can be measured.

[yor_on]

ok

Make it a 'net'
The 'net' is not defined to light speed

you just need to look at it from the perspective of a homogeneous and isotropic universe.

The consequences of changes is. 'c'