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A theory of the Speed of gravity and light

Astrometry Astronomics Observing Optics
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#1 MaximusStarHunter

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Posted 23 May 2024 - 07:59 PM

So, this theory is probably wrong. People say that the speed of gravity is the same as the speed  of light. The speed of the speed of light is also affected by the refractive index of a medium. That means that in glass and water, the speed slows down. However, as far as I know, gravity does not get affected by this. Since it does not, will gravity move faster than light when light passes through glass. Or would the light set a new constant, in which the gravity can no longer travel, Since gravity propogates light. This all may be wrong, I don’t know.



#2 archer1960

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Posted 23 May 2024 - 08:30 PM

Keep in mind that different wavelengths of EM radiation have different indices of refraction in any given material. It is only in free space (i.e. a vacuum) that all wavelengths travel at the same speed. There is no reason why gravity would need to slow down to match the speed of (any given wavelength of) light in any other medium. So gravity waves may well arrive at some point before the visible light does. The same is true of other wavelengths of radiation, such as radio waves, IR, UV, x-rays, etc.


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#3 The0s

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Posted 23 May 2024 - 08:59 PM

Things can actually travel faster than light provided they are in a medium. Case in point: Cherenkov radiation, the stuff that causes some submerged nuclear reactors to glow blue, which is caused by highly energized particles traveling faster through the water than light (though the speed of the particles still can't exceed c, the speed of light in a vacuum). Thus, light does not limit the speed of other particles while in a medium; only in a vacuum is the speed of light the maximum possible speed.


Edited by The0s, 23 May 2024 - 09:03 PM.

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#4 Bubbagumps

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Posted 24 May 2024 - 06:15 PM

By speed of Gravity, I assume you mean how quickly a change in a gravitational field is transmitted through space.

 

E.G. - When you have two objects at rest that are separated by a distance then move one of the objects, how long will it take for the other body to 'feel' the effect of a change in the gravitational field ?

 

If you asked Isaac Newton, the answer would be that the change would be felt instantaneously. Gravity was seen as a force that operated via instantaneous action at a distance. Nobody was ever really comfortable with this idea but just brushed it aside.  In the classical world, Time and Space were seen as invariant. t was assumed a priori that all observers agree on distances and time intervals.  Time always passed the same in every reference frame. Signals could be transmitted instantaneously without violating any existing law or principle.  

 

This changed with Einstein and General Relativity. In Classical Mechanics, the medium in which gravity operates is Space. In Relativity, the medium is Spacetime. They are totally different animals. In relativity theory, both special and general. the speed of light in a vacuum ©,  is extremely important. It appears in just about every equation or derivation you will come across.  It is the only universal measuring stick and represents an upper bound on speeds within any reference frame. Whenever you come up with an equation for some variable in Special or General Relativity - mass, distance, speed, Gravitational Wave propagation speed, etc, it will have the variable c in it somewhere.  Since no signal or object can move faster than light, whenever you have an equation for velocity for some phenomenon, you know right away that whatever you come up with has to be <= c. In the case of gravitational waves, it just works out that the velocity is just c. It flows directly from the equations. 

 

Gravity is simply a manifestation of the geometric properties of Spacetime that arises as a result of the arrangement of matter and energy in any given frame of reference. In the case of changes in the structure of Spacetime making their way from one reference frame to another, when one object changes its position for instance, the change is propagated via waveforms that make their way through spacetime. Such waveforms are propagated with the speed of  light in a vacuum.  In the case of the scenario mentioned above, it would be akin to a ripple on a spacetime pond that is making it's way across the surface of the pond.  If you were a stationary observer in empty space watching another stationary object in the distance and the object changed it's position, the change in the gravitational field would be noticed the very instant you observed the object to move-> i.e.. The time of travel of the waveform in your reference frame would be coincident with the time it took light to travel the distance in your reference frame, which is always c for any observer.

 

As far as the speed of light in any arbitrary medium other than a vacuum, the variable speed of light in differing media is related to the electro-optical properties of matter and has nothing to do with the structure of Spacetime or how gravitational waveforms are transmitted through Spacetime. The presence of the material will not impede the propagation of a gravitational waveform in any way. The fact that matter is present will result in there being an alteration of the shape of the waveform as it passes by, but it will not 'slow it down' in that reference frame. The speed of the waveform will still be the speed of light in a vacuum. The signal will arrive a little bit earlier than the light in that medium. This does not violate any law or principle.

 

The important takeaways here are 1) there is an upper limit to the transmission of any signal through spacetime and that limit is the speed of light in a vacuum. 2) Light propagating in a vacuum will always be observed to possess the same velocity in any reference frame. 3) Gravitational waves/disturbances in spacetime propagate at c. 

 

There is a direct analog to Electromagnetism to use when thinking about the subject. Just as any accelerating charge will produce an EM wave, any accelerating mass will produce a gravitational wave. Two objects moving apart from one another with constantly velocity will not result in the propagation of a waveform. Instead, the magnitude of certain components in the local metric and a few other tensors will simply diminish uniformly with the passage of local time in a given reference frame. The same applies to Electromagnetism. Two charges moving uniformly away from each other at constant velocity will not produce an EM wave. The magnitude of the potential will just diminish uniformly with the passage of time. 

 

Quick Note - I use waveform above so as to not create any confusion with Gravitational Wave events you read about in the news.  Technically, the underlying physics related to how changes in the structure of spacetime are propagated is the same. But the term Gravitational Waves in common used today is usually a reference to gravitational waves emanating from specific events - black hole mergers, neutron star mergers etc... That is what modern GW detectors hunt down. 


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#5 archer1960

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Posted 25 May 2024 - 10:59 AM

So, here's a somewhat related question for those of you who know more about it than I do: at the theoretical level, is there any equivalent to the index of refraction that applies to gravity waves? I have never seen any reference to such a thing, but that doesn't mean it hasn't been discussed by those who study such things...



#6 Bubbagumps

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Posted 25 May 2024 - 12:12 PM

So, here's a somewhat related question for those of you who know more about it than I do: at the theoretical level, is there any equivalent to the index of refraction that applies to gravity waves? I have never seen any reference to such a thing, but that doesn't mean it hasn't been discussed by those who study such things...  

 

Is there an equivalent to the index of refraction within a material for Gravity Waves ? No. 

 

However, there a phenomenon that is similar in a roundabout way - Gravitational Lensing. In a vacuum, the path that gravity waves take is the same path that light will take. You can Google the net for images of gravitational lensing examples. 

 

HV7pp5x9eqByRRtDNhMphe-970-80.jpg

 

 

Gaia_Lensing_explained-1024x576.jpeg


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#7 Jeff B1

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Posted 25 May 2024 - 03:12 PM

I find it confusing to equate gravity with speed.  



#8 Bubbagumps

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Posted 25 May 2024 - 04:25 PM

The speed of a physical object like a car, a plane, or a molecule of a gas is simply a measure of the amount of distance the object covers for a given standard unit of time in any arbitrary reference frame. The objects are undergoing a change in position. 

 

When discussing speed in the context of a phenomenon like gravity, you are referring to propagation speed in a medium rather than the speed of an object. What is the medium ? Spacetime. What is being propagated ? Energy. When mass/energy undergoes a change in it's physical configuration, the gravitational potential at each point in space will change over a given period of local time as observed in any arbitrary reference frame. It takes time for these changes in potential to make their way through space.  It does not happen instantaneously. 

 

A good analogy is a capillary wave on the surface of water. The molecules are being physically displaced from an equilibrium position. They don't get carried along with the wave.  Energy is transmitted through the water and this causes the displacement. If you just focus on a single cross-section,  you will see the water move up and down in a periodic fashion.  The water itself is not being transported. The propagation speed refers to the frequency of the up and down cycles multiplied by the total distance between peaks on the water. The velocity of propagation of the wave is simply a measure of the period changes in energy that are occurring as a result of energy being propagated through the medium.

 

In the case of gravity, if you could somehow observe a distance object accelerate and change it's position and the object was massive enough to result in observable changes in the position of other objects between you and the source, you would see changes in the position of the objects in your line of sight due to change in potential. You would see the change in position of the objects occur uniformly as this ripple in spacetime made it's way to your position. If you know the distance to the object and recorded how long it took for this change in potential to reach your position, you would come out with a propagation speed of c. 


Edited by Bubbagumps, 25 May 2024 - 04:27 PM.


#9 Jeff B1

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Posted 25 May 2024 - 04:33 PM

Guess the speed of that apple that bonged Newton on the noggin was measured?  grin.gif



#10 archer1960

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Posted 27 May 2024 - 12:10 PM

Is there an equivalent to the index of refraction within a material for Gravity Waves ? No. 

 

However, there a phenomenon that is similar in a roundabout way - Gravitational Lensing. In a vacuum, the path that gravity waves take is the same path that light will take. You can Google the net for images of gravitational lensing examples. 

 

HV7pp5x9eqByRRtDNhMphe-970-80.jpg

 

 

Gaia_Lensing_explained-1024x576.jpeg

Yes, I'm familiar with gravitational lensing; not really similar to what an index of refraction does in a medium, but still makes for some interesting effects.



#11 Olimad

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Posted 27 May 2024 - 11:37 PM

Since there are optical man-made objects that converge ligth and others that diverge It. Do you envisage gravitational diverging lensing effets??? 



#12 R Botero

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Posted 28 May 2024 - 04:28 AM

Worth watching this one:  https://youtu.be/NBZ...1-uxmkYkd-TYgWI


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#13 archer1960

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Posted 28 May 2024 - 08:39 PM

Since there are optical man-made objects that converge ligth and others that diverge It. Do you envisage gravitational diverging lensing effets??? 

Seems like it should be theoretically possible if you had a mass distribution that was shaped like a doughnut. How that would occur is a much more difficult question...



#14 Olimad

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Posted 29 May 2024 - 01:12 AM

Seems like it should be theoretically possible if you had a mass distribution that was shaped like a doughnut. How that would occur is a much more difficult question...

IF we get to observe diverging gravitational lensing effects, It would give us information about the mass distribution.

 

It is impossible to explain a doughnut shape with GR.

The modelized "great repeller" (based on data processing) gives us an Idea of a doughnut shape of mass distribution. But It can't be explained by GR alone. GR explains part of the doughnut, but fail to explain the whole.

 

But to explain such mass distribution you have to think out of the box of GR, or put GR inside a bigger box...

 

 

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Edited by Olimad, 29 May 2024 - 09:51 AM.


#15 Bubbagumps

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Posted 29 May 2024 - 08:04 AM

Seems like it should be theoretically possible if you had a mass distribution that was shaped like a doughnut. How that would occur is a much more difficult question...

The round-appearing lensing effects are labeled Einstein Rings. 

 

A traditional lens used in optics has a principle axis and a focal point.  Gravitational lensing effects have the analogous equivalent of a focal line. Think of them as akin to a distorting funhouse mirror except they have a degree of spherical symmetry in relation to a particular axis.

 

974115b149a70655005ccb58e9fdb993.jpg

 

 

Gravitational Lensing produces a wide arrange of optical effects that depend primarily on the shape of the mass distribution causing the spacetime curvature and the relative orientation of the object(s) being lensed in relation to any distant point in space. There are different types of possible effects. An object being lensed by spacetime distortion can appear smeared out in an arc:

 

18lp68ds97nx6jpg.jpg

 

 

Or the result can be what appears as multiple copies of the object situated in what is known as an Einstein Cross:

 

GravitationalLensEinsteinCross_UT.jpg

 

 

Multiple objects can also be lensed to produce a combination of optical distortions:

 

maxresdefault-59e8d857396e5a001012e50b.j



#16 Olimad

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Posted 29 May 2024 - 09:56 AM

I think we talked about the mass distribution, having kind of doughnut shape, producing possible different gravitational lensings (converging and diverging), not about the image produced and seen from earth.


Edited by Olimad, 29 May 2024 - 09:26 PM.


#17 archer1960

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Posted 29 May 2024 - 01:04 PM

The round-appearing lensing effects are labeled Einstein Rings. 

 

A traditional lens used in optics has a principle axis and a focal point.  Gravitational lensing effects have the analogous equivalent of a focal line. Think of them as akin to a distorting funhouse mirror except they have a degree of spherical symmetry in relation to a particular axis.

 

974115b149a70655005ccb58e9fdb993.jpg

 

 

Gravitational Lensing produces a wide arrange of optical effects that depend primarily on the shape of the mass distribution causing the spacetime curvature and the relative orientation of the object(s) being lensed in relation to any distant point in space. There are different types of possible effects. An object being lensed by spacetime distortion can appear smeared out in an arc:

 

18lp68ds97nx6jpg.jpg

 

 

Or the result can be what appears as multiple copies of the object situated in what is known as an Einstein Cross:

 

GravitationalLensEinsteinCross_UT.jpg

 

 

Multiple objects can also be lensed to produce a combination of optical distortions:

 

maxresdefault-59e8d857396e5a001012e50b.j

Yes, but those are converging effects, not diverging like we were discussing.


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#18 yuzameh

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Posted 29 May 2024 - 01:21 PM

Personally, and admittedly irreverently, I find gravity more than fast enough when I trip.


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#19 Olimad

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Posted 29 May 2024 - 01:29 PM

Personally, and admittedly irreverently, I find gravity more than fast enough when I trip.

The last image shown by Bubbagumps seems to have been made after a good trip...



#20 Bubbagumps

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Posted 29 May 2024 - 02:12 PM

New discoveries are pretty routine today with the tools available.  The examples range from microlensing events all the way up to lensing produced by clusters of galaxies. The effects range from strong, to weak, to micro. 

 

Most of the interesting examples are via lensing occurring on galactic scales. 

 

The number has grown significantly since the first survey by Hubble. Given what was found in a very small patch of sky, there could possibly be 500,000 such examples available for study:

 

https://www.esa.int/...istant_universe

 

Although many lenses have spherical symmetry present, a complete and perfectly symmetric ring structure is impossible to achieve in reality. The object creating the aberration would need to be completely symmetric and uninform and the source would need to be positioned directly in the central axis on the line of sight. 


Edited by Bubbagumps, 29 May 2024 - 02:16 PM.


#21 Olimad

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Posted 29 May 2024 - 09:39 PM

Not really in the central axis. And to be really detectable the source would have to be seen directly from earth, and additionaly, the object responsible of the diverging gravitational lensings has to be off-axis, in order to see a diverging light ray (emited from the source) too.

 

The region of the "dipole repeller" (seen as a void) would be a good place to look for diverging gravitational lensings. As a result, you would see the source with different brightness images.

 

Again, GR fail to explain the "dipole repeller", because It does not take into account any répulsive effects of gravity.

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Edited by Olimad, 29 May 2024 - 10:27 PM.


#22 EJN

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Posted 29 May 2024 - 11:51 PM

I'm not following the relevance of this to gravitational lensing, the Dipole Repeller and the Shapley Attractor represent deviations from a smooth Hubble flow in the large-scale peculiar velocities of galaxies and clusters.



#23 Olimad

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Posted 30 May 2024 - 12:13 AM

I'm not following the relevance of this to gravitational lensing, the Dipole Repeller and the Shapley Attractor represent deviations from a smooth Hubble flow in the large-scale peculiar velocities of galaxies and clusters.

If all the matter get to the sharpley attractor, does that mean that the universe stopped to expand there? Since there is a converging paths to there? Or that the accélération of expansion decrease?

Does Gravity impact hubble flow?


Edited by Olimad, 30 May 2024 - 12:27 AM.



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