What is the speed of gravity?

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Offline chris

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What is the speed of gravity?
« on: 15/02/2016 22:09:24 »
I have heard it stated many times that the velocity at which gravity propagates is the speed of light.

What is the rationale and evidence for this argument?
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Offline h3mp

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Re: What is the speed of gravity?
« Reply #1 on: 15/02/2016 22:38:02 »
Surely if gravity is just curved space-time, then it doesn't propagate at all and has no speed :-s

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Offline chris

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Re: What is the speed of gravity?
« Reply #2 on: 15/02/2016 23:00:19 »
The theoretical force carrier of gravity is the "graviton" particle. These are purported to propagate at the speed of light, but why?
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Offline jeffreyH

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Re: What is the speed of gravity?
« Reply #3 on: 15/02/2016 23:07:21 »
The theoretical force carrier of gravity is the "graviton" particle. These are purported to propagate at the speed of light, but why?

That is a difficult question. I hope someone can give a reason.
Fixation on the Einstein papers is a good definition of OCD.

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Offline alancalverd

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Re: What is the speed of gravity?
« Reply #4 on: 15/02/2016 23:25:38 »
IIRC there was a recent-ish experiment involving gravitational lensing which suggested that the speed of gravity was nonzero and not greater than c. I think it was at the upper end of this range.
helping to stem the tide of ignorance

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Offline CPT ArkAngel

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Re: What is the speed of gravity?
« Reply #5 on: 16/02/2016 02:59:05 »
Hi Chris,

Anything having a proper mass (or rest mass) must travel at a speed lower than the speed of light, because it would need an infinite amount of energy to attain the speed of light.

Gravitational Waves and gravitons, on the other hand, are unlikely to possess a proper mass because it would generate a possible infinite number of gravitons and gravitational waves. In the case of a quantum theory of gravity, this could be solved by a limit. In any case, you need a particle without a proper mass at the end of the line. In General Relativity, gravity has the speed of light and there were already many observational proofs of it.

The gravitational waves having been detected by LIGO have exchanged energy with it in order to be observed. This means that gravitational waves have some kind of inertial mass, but no proper mass. Thus, it can interact and it can travel at the speed of light. This is the same for photons, but they are different. (Maybe not that much...)

If you put photons in a box, the proper mass of the box is supposed to increase by the same amount of photon energy as E=MC2. Maybe it is not true at small scale and there is a direction to it.

For gravitational waves, I have no idea if it is the case. But curvature of spacetime, like a gravitational wave, in my opinion; should add proper mass to a box in a similar way as a photon.

« Last Edit: 16/02/2016 03:15:25 by CPT ArkAngel »

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Offline chris

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Re: What is the speed of gravity?
« Reply #6 on: 16/02/2016 09:29:04 »
Thank you CPT ArkAngel; i understood some of that. But I don't think you've really answered my question, which was asking for the evidence that gravity propagates at the speed of light, and why...?
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Offline Thebox

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Re: What is the speed of gravity?
« Reply #7 on: 16/02/2016 09:37:27 »
Thank you CPT ArkAngel; i understood some of that. But I don't think you've really answered my question, which was asking for the evidence that gravity propagates at the speed of light, and why...?

Hey Chris, I think the ''speed of gravity'' is more related to the size of mass and terminal velocity being the ''speed of of gravity'' relative to the attracting mass.



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Offline Space Flow

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Re: What is the speed of gravity?
« Reply #8 on: 16/02/2016 10:18:45 »
There is no evidence yet of Gravity having any speed. On the other hand there is now even more evidence to support the general relativity view of Gravity rather than the particle physics view. The Standard Model is complete. There is no room for a fictitious graviton, and GR does not need one.
Having said all that lets just address the question as a single body Universe problem.
Take a Universe that is just Spacetime like your rubber sheet analogy. Now in this matter-less totally flat Universe, add one particle of matter. Does the almost infinite edge of this otherwise empty spacetime, not imediatelly feel the presence of this particle? The equations don't say anything about information transmission time. All they say is that the mere presence of this particle is felt right to infinity at a level dictated by the inverse square law.
Ask yourself how it could possibly be different.
The speed has to be infinite or nothing makes sense.
Similarly if you then somehow made this particle disappear. Would any part of the entire Universe show a deformity for a particle that doesn't exist? If the information only travelled at the speed of light, you would have observable evidence of a gravitational effect with nothing to cause it.
Yes I know you can't create or destroy Mass/energy but this is a thought experiment.
Gravity waves may propagate, and the speed that they propagate at would be related to some property of the spacetime medium that they propagate through.
But Gravity does not propagate.
That is the wrong way to think about it.
Gravity either is and if it is, it is to the end of the Universe, or it isn't and if it isn't, it isn't everywhere at once.
If conservation laws allowed either the creation or destruction of matter/Energy, then you would find that gravity was instantaneous.
As it is, gravity just is and if a mass changes position the Universe knows about it immediately. Otherwise you have delayed gravitational effects from where matter isn't, and next thing you know someone will scream "Dark Matter".
We are made of Spacetime; with a sprinkling of Stardust.
Matter tells Spacetime how to Flow; Spacetime tells matter where to go

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Offline Thebox

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Re: What is the speed of gravity?
« Reply #9 on: 16/02/2016 10:25:07 »
There is no evidence yet of Gravity having any speed. On the other hand there is now even more evidence to support the general relativity view of Gravity rather than the particle physics view. The Standard Model is complete. There is no room for a fictitious graviton, and GR does not need one.
Having said all that lets just address the question as a single body Universe problem.
Take a Universe that is just Spacetime like your rubber sheet analogy. Now in this matter-less totally flat Universe, add one particle of matter. Does the almost infinite edge of this otherwise empty spacetime, not imediatelly feel the presence of this particle? The equations don't say anything about information transmission time. All they say is that the mere presence of this particle is felt right to infinity at a level dictated by the inverse square law.
Ask yourself how it could possibly be different.
The speed has to be infinite or nothing makes sense.
Similarly if you then somehow made this particle disappear. Would any part of the entire Universe show a deformity for a particle that doesn't exist? If the information only travelled at the speed of light, you would have observable evidence of a gravitational effect with nothing to cause it.
Yes I know you can't create or destroy Mass/energy but this is a thought experiment.
Gravity waves may propagate, and the speed that they propagate at would be related to some property of the spacetime medium that they propagate through.
But Gravity does not propagate.
That is the wrong way to think about it.
Gravity either is and if it is, it is to the end of the Universe, or it isn't and if it isn't, it isn't everywhere at once.
If conservation laws allowed either the creation or destruction of matter/Energy, then you would find that gravity was instantaneous.
As it is, gravity just is and if a mass changes position the Universe knows about it immediately. Otherwise you have delayed gravitational effects from where matter isn't, and next thing you know someone will scream "Dark Matter".

Hi space, can you confirm my understanding of something, I though gravity was the invisible force between two bodies caused by the two bodies being present, what affect would gravity have on a single particle in a void if there was nothing to attract it, would the speed remain 0 would the idea of gravity even exist?

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Offline evan_au

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Re: What is the speed of gravity?
« Reply #10 on: 16/02/2016 10:52:21 »
Quote from: chris
I have heard it stated many times that the velocity at which gravity propagates is the speed of light.
This is the common view (as assumed in Einstein's General Relativity), but you can't say that it is proven.

Quantum theory and General Relativity currently have irreconcilable differences, but researchers are hoping that  mediation may still prove successful.
 
If there is to be a reconciliation, it will probably be related to the hypothetical particle carrier of gravitation, the "Graviton".

If you look at the Wikipedia page on the graviton, you can see some of the open conflicts, eg:
  • The information box states that the rest-mass of the graviton is zero. Since we know that gravitational waves can deliver momentum into a detector, this implies that the graviton must have momentum, and the only way a zero-mass particle can have momentum is if it travels at the speed of light (as per Einstein).
  • From the recent detection of gravitational waves, researchers placed an upper bound on the rest mass of a graviton: < 10-22 eV/c2. This is miniscule, but it doesn't prove that it is exactly zero. If the mass is ever-so-slightly greater than zero, then the graviton cannot travel at c (but it could be extremely close to c).
  • "gravitational waves must propagate slower than c in a region with non-zero mass density if they are to be detectable". I assume that this is analogous to the fact that light travels at speeds < c in a volume containing matter. So the speed of light (and possibly gravitation) is only c in a true cosmic void.
  • String theory has been shown to be compatible with General Relativity, and to have a particle like a graviton. But it, too cannot be reconciled with the Standard Model of particle physics at this time.
  • More speculatively, slight differences in the properties of gravitons (like traveling slightly slower than c, or leaking into other dimensions) might explain some of the characteristics of dark matter
It's a brave pleb who would bet against Einstein, but quantum gravity is clearly something that is beyond the scope of Einstein's general relativity.

An Upcoming Experiment?: As of this month, we have demonstrated an ability to detect gravitational waves. By 2020 we should have enough operational detectors worldwide to be able to locate a source like colliding neutron stars, and point our telescopes in that direction. In this case, we should be able to compare the time of arrival of the gravitational waves with the arrival of the optical/radio signal. This will give us a direct comparison of the speed of gravitational waves and light. It would be a bonus if that mass of decaying neutrons sprayed into space gave us a simultaneous pulse in the world's neutrino detectors!

A similar analysis with Supernova SN1987A allowed us to place fairly tight limits on the speed of neutrinos compared to the speed of light (it was very close to c; now we know from neutrino oscillations that it is ever-so-slightly slightly less than c).

Quote from: CPT ArkAngel
But curvature of spacetime, like a gravitational wave, in my opinion; should add proper mass to a box in a similar way as a photon.
We know how to confine light in a reflective box. I don't know of any way to confine a gravitational wave inside a box.

Quote from: Space Flow
But Gravity does not propagate.

The fact that last September researchers detected a gravitational wave oscillations that seemed to come from merging black holes about a billion light-years away suggests that sudden changes in gravitational fields do propagate through space "to infinity", as predicted by Einstein.

Just like sudden changes in electric fields propagate through space "to infinity", as predicted by Maxwell.

I don't think it's any harder to believe in propagating gravitational fields any more than it is to believe in propagating radio waves.
« Last Edit: 16/02/2016 20:02:48 by evan_au »

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Offline chris

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Re: What is the speed of gravity?
« Reply #11 on: 16/02/2016 14:02:04 »
Thanks Evan; excellent answer; now I understand what we do and don't know much better. Chris
I never forget a face, but in your case I'll make an exception - Groucho Marx

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Offline evan_au

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Re: What is the speed of gravity?
« Reply #12 on: 16/02/2016 20:11:39 »
Also see: http://en.wikipedia.org/wiki/Speed_of_gravity#Possible_experimental_measurements

This suggests that measurements of a binary pulsar radiating gravitational waves is consistent with the speed of gravity being within 1% of the speed of light. Again, it is not proof that gravity travels at exactly the same speed as light.

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Offline CPT ArkAngel

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Re: What is the speed of gravity?
« Reply #13 on: 16/02/2016 21:10:38 »
There are two LIGO interferometers, one in Louisiana and the other in Washington state.

The gravitational waves were detected by both detectors. The delay measured between them is consistent with GW moving at the speed of light within the margin of error. The margin still includes the possibility of a lower speed.

Experimentally, General Relativity is a great success. It would be extremely surprising if gravitational waves had a different speed than C. Though GR is certainly wrong at some point, it is very unlikely in this case.

Concerning the box containing photons, just imagine a box large enough in a huge and relatively flat space...



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Offline Space Flow

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Re: What is the speed of gravity?
« Reply #14 on: 16/02/2016 22:39:05 »
Quote from: Space Flow
But Gravity does not propagate.

The fact that last September researchers detected a gravitational wave oscillations that seemed to come from merging black holes about a billion light-years away suggests that sudden changes in gravitational fields do propagate through space "to infinity", as predicted by Einstein.

Just like sudden changes in electric fields propagate through space "to infinity", as predicted by Maxwell.

I don't think it's any harder to believe in propagating gravitational fields any more than it is to believe in propagating radio waves.
Although your answer was very good and extremely comprehensive as far as the propagation of Gravitational waves are concerned, my postulations were never about the propagation of Gravitational waves.
I have no argument with anything you said. The future will tell whether your highly hypothetical gravitons have anything to do with reality or not.
Anything that I stated was to address the question posed by Chris to start with and that as I understood it was not about the speed of Gravitational waves, but about the speed of Gravity.
I see no way to connect the two. One is a compression wave traveling as a compression wave through a medium and as such would have to obey rules that have to do with transmission of compression waves through the medium. This medium in other words is a physical presence with physical characteristics that influence such propagation. Even still EM waves are as far as I understand not dependent on a medium so we would be comparing apples with oranges if we assign gravitational waves the speed of light. Entirely different methods of propagation.
Gravity being the distortion of this spacetime itself is not a compression wave. It is the influence of Mass on the shape of all of spacetime, from the coordinates of the mass to infinity. It is, rather than it propagates.
If you somehow magically removed our sun from reality, all of spacetime would know immediately, as there is no known way to describe a curvature of the medium at any distance without the mass to cause it.
Although I generally love your very thorough treatment of any question, I believe you have done this question an injustice by making it about Gravity waves and Hypothetical gravitons which it never was by the tittle.
If that was the question I would not have offered an opinion.
« Last Edit: 16/02/2016 22:53:59 by Space Flow »
We are made of Spacetime; with a sprinkling of Stardust.
Matter tells Spacetime how to Flow; Spacetime tells matter where to go

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Offline Space Flow

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Re: What is the speed of gravity?
« Reply #15 on: 16/02/2016 22:50:17 »
Hi space, can you confirm my understanding of something, I though gravity was the invisible force between two bodies caused by the two bodies being present, what affect would gravity have on a single particle in a void if there was nothing to attract it, would the speed remain 0 would the idea of gravity even exist?
Even though I promised myself that I would stop responding to you, I will answer this misconception.
Gravity as is currently understood by GR is the distortion of spacetime by the presence of mass. Matter does not in any reality attract matter. If there are two particles in the universe than they will both be influenced by the distortion that each impose on spacetime. That is not the same as attracting each other. They are both under the influence of a distorted spacetime.
So having explained that part you should be able to see that gravity in its simplest terms can be described as a single particle and its effect on the so called fabric of the universe. That is the only interaction needed.
Mass and spacetime
. Not mass and mass.
We are made of Spacetime; with a sprinkling of Stardust.
Matter tells Spacetime how to Flow; Spacetime tells matter where to go

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Offline CPT ArkAngel

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Re: What is the speed of gravity?
« Reply #16 on: 16/02/2016 23:41:03 »
The detection of gravitational waves implies a gravity travelling at the speed of light.

GWs are produced by the transfer of energy from gravity to gravitational waves. It is the decrease in potential gravitational energy of two bodies spinning together while accelerating toward each other which produces gravitational waves. This is real energy so gravitational energy is real. If energy can travel faster than light then you can go backward in time and kill you grandfather before you were born...

If you want to break this law, you must be able to explain a few unknowns in physics, at least...

Concerning the term propagating (through space), it implies that space is some kind of material. Sure that space need a description, it is not so sure that what we perceive as space is a real medium. That is why I avoid the term 'propagate'. Because I don't know and it may be a misconception, or maybe not...

In the case of entanglement, there is no energy transfer. probably...


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Offline alancalverd

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Re: What is the speed of gravity?
« Reply #17 on: 17/02/2016 00:06:49 »
Propagate is a useful term because, for instance, it distinguishes between a particle travelling through space and a selfpropagating electromagnetic wave, neither of which requires any material. 
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Offline CPT ArkAngel

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Re: What is the speed of gravity?
« Reply #18 on: 17/02/2016 00:15:17 »
Do you imply that if you could make disappear all matter in the universe, Space would still exist?

Personally, I don't think so. I think this exactly where GR cannot fit with a quantum gravity theory.

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Offline Space Flow

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Re: What is the speed of gravity?
« Reply #19 on: 17/02/2016 01:13:59 »
Do you imply that if you could make disappear all matter in the universe, Space would still exist?

Personally, I don't think so. I think this exactly where GR cannot fit with a quantum gravity theory.
No I am not implying that space can exist with no matter, just as I can not imply that matter can exist with no space. It is the interaction between these two physical qualities (Matter and Space) that allow an animated Universe. I don't know of any other kind of Universe.
Breaking it down to one particle of matter is of course an oversimplification to show the Physics of the interaction.
Magically removing the sun does not imply that I believe in magic, or that this could be done. That again is just a thought experiment to transmit an idea.   
As far as GR not fitting with Quantum Gravity Hypothesis (not yet theory), is concerned. You can not possibly doubt the validity of GR when every advance we ever make again proves its predictive ability exactly right. So if some other view does not agree with GR than the logical thing to do is to assume that the problem is with the other view. Be it Quantum Gravity or any other view that does not agree with such a 100% success rate. 
That in my view is science and everything else is conjecture.

If we are to be bringing conjecture into this discussion than I feel obliged to point you to my view of gravity that doesn't actually clash with either GR or QM, and has nothing to do with weirdo ideas like Quantum Gravity, ridiculous vibrating strings, or any Multiverses that we can never prove.
https://www.facebook.com/permalink.php?story_fbid=999378540105442&id=595088680534432

We are made of Spacetime; with a sprinkling of Stardust.
Matter tells Spacetime how to Flow; Spacetime tells matter where to go

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Offline Space Flow

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Re: What is the speed of gravity?
« Reply #20 on: 17/02/2016 01:31:44 »
Propagate is a useful term because, for instance, it distinguishes between a particle travelling through space and a selfpropagating electromagnetic wave, neither of which requires any material.
And yet Alan, both the particle and the Electromagnetic wave are from the Mass/Energy part of the duality that makes an animate Universe. Neither one of those can speak for Spacetime which represents the other side of the existence coin. 
Gravity on the other hand is the effect of Mass/Energy on Space and as such can only be described as an effect of space. There is no reason other than Human matter-centrism to make it follow the same laws as Mass/Energy.
It has to follow laws of Spacetime as a medium. 
We have to give up this ridiculous idea that everything belongs to particle physics. Spacetime has been known for two and a half thousand years at least to be separate to the states of matter, while still being a physical quality of reality.
Being physical but not Mass/Energy based it has to come under greater scrutiny than is afforded it, to discover the particulars of its physicality.
And yes I do think there is such a thing as Human matter-centrism. It follows the exact same pattern we have always displayed. The one that had Humans as something special and then put the Earth in the centre of the Universe, then our sun and our Galaxy and so on.
We are made of 99.9999..... % space and yet we think of ourselves and our Universe as Matter defined. Very biassed.
"Matter-centrism"

Added:- On the subject of Gravitational waves and Energy;
http://philsci-archive.pitt.edu/11857/1/GRavitational_Wave_Energy.pdf
« Last Edit: 17/02/2016 07:35:49 by Space Flow »
We are made of Spacetime; with a sprinkling of Stardust.
Matter tells Spacetime how to Flow; Spacetime tells matter where to go

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Offline CPT ArkAngel

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Re: What is the speed of gravity?
« Reply #21 on: 17/02/2016 02:04:10 »
It is interesting... Space flow concept is interesting but I think it would be difficult to integrated with particle physics and quantization. And Dark matter accounts for about 5 times more gravitational mass than ordinary matter. How do you explain it?

But my question was for Alan.

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Offline Space Flow

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Re: What is the speed of gravity?
« Reply #22 on: 17/02/2016 02:28:54 »
But my question was for Alan.
My apologies. I misunderstood.

It is interesting... Space flow concept is interesting but I think it would be difficult to integrated with particle physics and quantization. And Dark matter accounts for about 5 times more gravitational mass than ordinary matter. How do you explain it?
Once you accept that Space is a physical entity that is not particle based, yet still quantized, then you can examine all the observational and experimental evidence that we so far have and the integration is not only self suggestive but simple and intuitive. Or so I think.
This is not the appropriate post for expressing new theories. Which the link above points to.
We are made of Spacetime; with a sprinkling of Stardust.
Matter tells Spacetime how to Flow; Spacetime tells matter where to go

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Offline Space Flow

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Re: What is the speed of gravity?
« Reply #23 on: 17/02/2016 02:31:42 »
Propagate is a useful term because, for instance, it distinguishes between a particle travelling through space and a selfpropagating electromagnetic wave, neither of which requires any material.
How does a particle traveling through space not require the space to travel through?

« Last Edit: 17/02/2016 07:36:02 by Space Flow »
We are made of Spacetime; with a sprinkling of Stardust.
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Offline evan_au

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Re: What is the speed of gravity?
« Reply #24 on: 17/02/2016 09:33:49 »
Quote from: Space Flow
(A gravitational wave) is a compression wave traveling as a compression wave through a medium.
A compression wave travelling through a medium will move a test particle alternately away and towards the source, in the direction of travel.

However, gravitational waves move test particles in a plane perpendicular to the direction of travel, as illustrated by the animations here:
http://en.wikipedia.org/wiki/Gravitational_wave#Effects_of_passing

I conclude that gravitational waves are not a compression wave.

Quote
This medium in other words is a physical presence with physical characteristics that influence such propagation
I conclude that gravitational waves do not require spacetime to be a physical medium that supports compression waves.

Quote
(A gravitational wave) is a compression wave traveling as a compression wave through a medium .... EM waves are as far as I understand not dependent on a medium.  Entirely different methods of propagation.
I agree that they have different methods of propagation - after all, gravity seems to be attractive only, while electromagnetism attracts and repels. Their polarization angles seem to be somewhat different (see animations referenced above).

Not being dependent on a medium, both are able to propagate through a vacuum.

Sometimes when we think of waves, we think of ocean waves or sound waves, which require a medium to propagate.
However, the beauty of the quantum interpretation is that we can also imagine a quantum wave as a particle; like a bullet, it can pass through the vacuum of space, without dependence on a medium.

Quote
Gravity being the distortion of this spacetime itself is not a compression wave. It is the influence of Mass on the shape of all of spacetime, from the coordinates of the mass to infinity. It is, rather than it propagates.
I agree that if a mass were present from eternity, then the shape of all spacetime will reflect the presence of this mass, from the coordinates of the mass to infinity. It just "is".
...Just as in electromagnetism, if an electric charge were present from eternity, then the electromagnetic field of all spacetime will reflect the presence of this electric charge, from the coordinates of the charge to infinity. It just "is".

If, however, the electric charge is not present from all eternity, but is modified at some point in history (by colliding an electron and a proton, for example), then an electromagnetic influence will spread out throughout spacetime as a ripple on the electric field. This disturbance (an electromagnetic wave) travels at the speed of light.

Similarly, if the mass is not present from all eternity, but is modified at some point in history (by colliding two black holes, for example), then the gravitational influence will spread out throughout space as a ripple on the fabric of spacetime. This disturbance (gravitational wave) travels at the speed of light (according to Einstein).

Although they are ripples on different underlying fields, with different polarization angles, gravitational waves are no more difficult to imagine than radio waves.

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Offline Space Flow

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Re: What is the speed of gravity?
« Reply #25 on: 17/02/2016 10:44:44 »
If, however, the electric charge is not present from all eternity, but is modified at some point in history (by colliding an electron and a proton, for example), then an electromagnetic influence will spread out throughout spacetime as a ripple on the electric field. This disturbance (an electromagnetic wave) travels at the speed of light.
Totally agree.

Similarly, if the mass is not present from all eternity, but is modified at some point in history (by colliding two black holes, for example), then the gravitational influence will spread out throughout space as a ripple on the fabric of spacetime. This disturbance (gravitational wave) travels at the speed of light (according to Einstein).
And yet conservation laws stipulate that a mass or its representative energy have been there since eternity. It just is..
You can not compare it to EM. It is literally apples and singularities. I've said it before and I'll say it again. It is a very Matter-centric attitude from a being that is 99.99999...% space. Denying the physical presence of spacetime I can make no sense off. It even appears to me that GR demands it.
If something that can have physical coordinates that can have its physical attributes manipulated can be bent and twisted, stretched and compacted, and can still be said not to have a physical existence, then I am talking to a number of Boxes.
Even Maxwell's equations don't disprove it. Yes there is a mathematical solution that shows a method of propagation is possible without a medium, yet all observations tie EMR to the medium.
How else do we explain that the shape and other physical attributes of spacetime effect the frequency of the light?
If the medium itself was not involved in the propagation of light, how then does the stretching of spacetime redshift the light that is quite obviously propagating through it?
How does the gradient of curvature of spacetime if not physically interacting with EMR manage to redshift it on the upward journey and blueshift it on the down. Why should something like light if it propagated independent of a physical medium be affected by the physical shape of that to you non existing medium.
Is any of this really logical, or just parroting your professors/teachers.

Maths is a language. As a language it can be used to accurately describe reality.
Also as a language it can be used to accurately describe things that are not a part of this reality.
Observation is the only way to judge which maths accurately describes reality, and which doesn't.

And as far as a Gravitational wave travelling at the speed of light according to Einstein, is this a dirrect quote of the man's opinion? Because as I understand it and I admit I could be wrong, GR does not specify a speed for GWs.
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Offline evan_au

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Re: What is the speed of gravity?
« Reply #26 on: 17/02/2016 21:03:43 »
Quote
And yet conservation laws stipulate that a mass or its representative energy have been there since eternity. It just is..
That is true. There was a recent announcement of detecting a merger of two black holes of around 30 solar masses each. The amplitude ramp-up tells you the masses of the incoming objects, and the ringdown phase tells about the mass of the final object.
It lost about 3 solar masses of mass-energy; it was there at the start, and not there at the end. The "missing mass" was sprayed around the universe in the form of gravitational waves, which can carry energy just as well as light or neutrinos can carry energy.

Gravitational waves do not violate conservation of mass-energy. But violent events can turn mass into energy (and black holes can turn a large fraction of mass into energy).

Quote
If something that can have physical coordinates that can have its physical attributes manipulated can be bent and twisted, stretched and compacted...

Relativity says that the presence of mass or relative velocity do indeed distort the physical coordinates of space, as measured by various observers.
And if the gravitational field is experiencing ripples, then the there will be oscillations in measuring the physical coordinates of space.
 
Quote
as I understand it and I admit I could be wrong, GR does not specify a speed for GWs.
You can find Einstein's original 1916 paper "Näherungsweise Integration der Feldgleichungen der Gravitation" here:
http://echo.mpiwg-berlin.mpg.de/ECHOdocuView?url=/permanent/echo/einstein/sitzungsberichte/BGG54UCY/index.meta
But you will need to read German a lot better than I do to see precisely what Einstein said on the subject!

I have to settle for http://en.wikipedia.org/wiki/Gravitational_wave
Quote from: Wikipedia
The existence of gravitational waves is a consequence of the Lorentz invariance of general relativity since it brings the concept of a finite speed of propagation of the physical interactions with it. By contrast, gravitational waves cannot exist in the Newtonian theory of gravitation, since it postulates that physical interactions propagate at infinite speed.

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Offline Space Flow

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Re: What is the speed of gravity?
« Reply #27 on: 17/02/2016 22:47:20 »
I think that we are now going around in circles and as such I will have to agree to disagree on this subject.
You have failed to convince me of your point of view, and I have likewise failed to convince you of mine.
There is value in the old saying; "It is no use flogging a dead horse."

Just as a passing comment, you are still equating Gravitational waves that are generated and do propagate to Gravity that just is and always has been.
I think that is doing the original question an injustice by mudding the waters.

I would have appreciated a view on this paper but alas it was obviously not to be.
http://philsci-archive.pitt.edu/11857/1/GRavitational_Wave_Energy.pdf
We all suffer from wearing the same blinkers. We only give credible mass to a view that expresses what we want to hear. It is a common human condition.

Oh well.
Thank you for the discussion.

« Last Edit: 17/02/2016 22:56:45 by Space Flow »
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Offline evan_au

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Re: What is the speed of gravity?
« Reply #28 on: 20/02/2016 01:42:35 »
Quote from: Space Flow
I would have appreciated a view on this paper but alas it was obviously not to be.
http://philsci-archive.pitt.edu/11857/1/GRavitational_Wave_Energy.pdf
I had a read of the paper, spread over 3 days...

The first thing to say is that I have never done a course on tensors, so I can't comment on the mathematics.

The second thing to ask is "What peer-reviewed physics journal did it appear in?". While I can't comment on the maths, the idea of a peer-reviewed physics journal is that they will find some experts who do understand it, to check it for me. (The http://philsci-archive.pitt.edu/ homepage describes it as a self-publishing site for papers on the philosophy of science).

As a layman, I think the crux of the article is this:
Quote from: Patrick M. Dürr
Slicing up such an approximation of the full metric into conservative and dissipative parts allows one to translate the resulting phenomena into a familiar Newtonian framework; the (heuristic or didactic) utility of such a translation, though, comes at the price of fundamentality.
He says that dividing Einstein's gravity into a static (Newtonian) component, plus an oscillating/radiating gravitational wave component is valid, but it is arbitrary, unnecessary and he doesn't like it.

Quote
we restrict our discussion to interpreting the binary systems, modelled as point particles
The modeling of the recent gravitational wave detection was not restricted to point particles. The transition from the inspiral phase to the ringdown phase involves the touching event horizons of the black holes, as extended objects.
Black holes won't experience the non-linear dissipation effects that could affect the orbit of neutron stars, like tidal bulges, tidal dissipation, tidal locking and tidal heating.

Quote
the onset of a scalar GW mode (propagating frequency dependently, subluminally and longitudinally)
The author expects that if a gravitational wave existed, it would propagate:
  • in a frequency-dependent manner: ie space would be a dispersive medium for gravitational waves. However, the chirp observed last September spanned almost an order of magnitude in frequency (50Hz to 400Hz), and arrived within a millisecond after an estimated billion years of travel. That appears to me that space is not a dispersive medium.
  • at less than the speed of light: The 6ms delay between the two sites imply that it was traveling fairly close to c. The limits deduced for the mass of the graviton suggest that it was traveling extremely close to c.
  • Longitudinally: the researchers analysed the phase differences between the two detectors, in an attempt to localize the source. They used the conventional model of gravitational wave propagation, but I expect they would have noticed some anomalies if it had been a longitudinal wave instead.
All three of these assumptions can be tested more thoroughly once more detectors come online.

Quote
If energy is not conserved quite generally, there is no need to make up a story about where it has gone when a system loses it.
The author is willing to make some significant changes to conventional physics (eg discarding conservation of energy) in order to justify his ignoring the possibility of gravitational waves.

I'm not sure I'm ready to "throw the baby out with the bathwater".

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Offline Space Flow

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Re: What is the speed of gravity?
« Reply #29 on: 20/02/2016 10:11:48 »
Thank you for your thoughts Evan..
We are made of Spacetime; with a sprinkling of Stardust.
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Offline Ahmed Ahmed

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Re: What is the speed of gravity?
« Reply #30 on: 04/03/2016 03:13:42 »
When an object falls on earth it is limited to the speed of 9.81m squared. Isnt this taught at GCSE? Just to enlighten you guys to me gravity is not waves please dont offend the law of what you cant see. Because you only see objects acting upon gravity dont tell you what is gravity it show what gravity does with an object. I have been interested in this field for many years since my age of 6 when i kept throwing my bouncy ball and see it bouncing and decreasing in height as it continuesly bouncing till stop   It showed me the ball was ripping threw the gravity as it bounced up with the kentic force it stored as it went down using gravity. There is a part which i discovered that put me to forget this whole idea and tear the page apart because how could einstine not see this conclusion. I will discuss it someday. Maybe.

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Offline chris

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Re: What is the speed of gravity?
« Reply #31 on: 04/03/2016 09:51:04 »
Ahmed - I'm afraid that you have missed the point completely, and your interpretation of how gravity works, as our US friends would say, "needs some work". I'd recommend working through this page, and then coming back with any relevant questions:

https://en.wikipedia.org/wiki/Gravity

Chris
I never forget a face, but in your case I'll make an exception - Groucho Marx

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Offline jeffreyH

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Re: What is the speed of gravity?
« Reply #32 on: 04/03/2016 12:57:26 »
If the merger of two black hole was finalised in the blink of an eye what does this say about time dilation at an event horizon. Surely this merger should have appeared to take forever if the metrics are correct.
Fixation on the Einstein papers is a good definition of OCD.

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Offline Bill S

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Re: What is the speed of gravity?
« Reply #33 on: 04/03/2016 13:25:42 »
Quote from: Space Flow
If the information only travelled at the speed of light, you would have observable evidence of a gravitational effect with nothing to cause it.

As that stands, it doesn't make sense to me.  Can you give an example?
There never was nothing.

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Offline jeffreyH

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Re: What is the speed of gravity?
« Reply #34 on: 04/03/2016 17:33:33 »
As it stands it makes no sense since the detected gravitational wave was traveling at VERY close to the speed of light.
Fixation on the Einstein papers is a good definition of OCD.

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Offline Bill S

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Re: What is the speed of gravity?
« Reply #35 on: 04/03/2016 21:46:25 »
Quote from: Space Flow
If you somehow magically removed our sun from reality, all of spacetime would know immediately, as there is no known way to describe a curvature of the medium at any distance without the mass to cause it.

Possibly this provides the example I asked for in my previous post?

One possible flaw is that you assume that gravity is curvature of spacetime. 

I may have quoted this from David Deutsch before: “Today we understand gravity through Einstein’s theory rather than Newton’s, and we know that no such force exists”. That seems pretty clear.  There is no such force as gravity.

Isn’t that what Einstein said?  I think not.  In his theory of general relativity gravity is a force field, not much different from the electromagnetic field. It is not four-dimensional curvature. 

It was Hermann Minkowski who introduced the idea of four-dimensional space-time, which Einstein initially called “superfluous erudition”.   He did eventually accept Minkowski’s interpretation as an alternative mathematical interpretation of his equations.  All this means is that his equations for the gravitational field are mathematically equivalent to a curvature of spacetime. 

Mathematical equivalence is no guarantee of physical reality.
There never was nothing.

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Offline evan_au

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Re: What is the speed of gravity?
« Reply #36 on: 04/03/2016 21:59:28 »
Quote from: JeffreyH
the merger of two black hole was finalised in the blink of an eye
Only the last 10ms (about 8 orbits) was powerful enough to be detected from Earth. I guess that is the blink of an eye, for someone far from the gravitational well.

Quote
what does this say about time dilation at an event horizon. Surely this merger should have appeared to take forever if the metrics are correct.
Time dilation becomes extreme for observers close to a black hole - time moves 10 or 100+ times more slowly than it does for a distant observer.

The merger would have taken far less than the blink of an eye (or very fast eyelids) for an observer close to the black hole.

Bear in mind that in the final few orbits, the orbital speed increased to a significant fraction of the speed of light. That is fast by anyone's standards!  (and leads to even more time dilation...)

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Offline Space Flow

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Re: What is the speed of gravity?
« Reply #37 on: 04/03/2016 23:27:51 »
One possible flaw is that you assume that gravity is curvature of spacetime.
Bill, it is not I that assumes that Gravity is curvature of spacetime, It is the Einsteiniun interpretation of General Relativity.   
Your statement goes against everything I have ever been taught about GR.   
If you are making the claim that this is not what GR claims as the cause of Gravity I wish you would present your source of information when you make such a statement. 
As I said this is not my assumption. What I actually believe about Gravity and its underlying causes is quite different to this curvature interpretation but this does not seem an appropriate post be pushing one's own ideas. 
Everything I have said is from the classical view of GR on Gravity. 
We are made of Spacetime; with a sprinkling of Stardust.
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Offline jeffreyH

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Re: What is the speed of gravity?
« Reply #38 on: 05/03/2016 13:47:40 »
Quote from: JeffreyH
the merger of two black hole was finalised in the blink of an eye
Only the last 10ms (about 8 orbits) was powerful enough to be detected from Earth. I guess that is the blink of an eye, for someone far from the gravitational well.

Quote
what does this say about time dilation at an event horizon. Surely this merger should have appeared to take forever if the metrics are correct.
Time dilation becomes extreme for observers close to a black hole - time moves 10 or 100+ times more slowly than it does for a distant observer.

The merger would have taken far less than the blink of an eye (or very fast eyelids) for an observer close to the black hole.

Bear in mind that in the final few orbits, the orbital speed increased to a significant fraction of the speed of light. That is fast by anyone's standards!  (and leads to even more time dilation...)

That rather neatly side steps the issue. If you have a momentum mv and apply gamma to it you could think of it just as well as affecting velocity as mass. So as time slows the local speed must spear to increase. Unless this view breaks down the nearer to c an obect's speed is.
Fixation on the Einstein papers is a good definition of OCD.

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Offline Bill S

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Re: What is the speed of gravity?
« Reply #39 on: 05/03/2016 14:28:02 »
Quote from: Space Flow
If you are making the claim that this is not what GR claims as the cause of Gravity I wish you would present your source of information when you make such a statement. 

This one probably comes closest to what I said.  I'm pushed for time at the moment, but I think I could find a few more ewxamples if you need them

http://www.quantum-field-theory.net/einstein-didnt-say/
There never was nothing.

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Offline jeffreyH

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Re: What is the speed of gravity?
« Reply #40 on: 05/03/2016 15:10:54 »
Quote from: Space Flow
If you are making the claim that this is not what GR claims as the cause of Gravity I wish you would present your source of information when you make such a statement. 

This one probably comes closest to what I said.  I'm pushed for time at the moment, but I think I could find a few more ewxamples if you need them

http://www.quantum-field-theory.net/einstein-didnt-say/

Equations of motion do not describe a curved path in the absence of a force.
Fixation on the Einstein papers is a good definition of OCD.

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Offline Bill S

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Re: What is the speed of gravity?
« Reply #41 on: 06/03/2016 01:31:24 »
Quote from: Jeffrey
Equations of motion do not describe a curved path in the absence of a force.

Would that force be gravity?
There never was nothing.

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Offline Space Flow

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Re: What is the speed of gravity?
« Reply #42 on: 06/03/2016 01:44:35 »
This one probably comes closest to what I said.  I'm pushed for time at the moment, but I think I could find a few more ewxamples if you need them

http://www.quantum-field-theory.net/einstein-didnt-say/
 

 
I think such view points as is suggested by this link and yourself, are not mainstream and use a lot of quotes out of context.  This or one of many other various differing viewpoints may one day be found to be closer to the truth than current majority belief but that is certainly not the accepted by majority view of classical GR physics as I understand it today. 

By the way, this a quote from your link.   

Quote from: Albert Einstein
When working on a comprehensive paper on the special theory of relativity… there occurred to me the happiest thought of my life…. for an observer falling freely from the roof of a house there exists – at least in his immediate surroundings – no gravitational field (Einstein’s emphasis). Indeed, if the observer drops some bodies then these remain relative to him in a state of rest or of uniform motion, independent of their particular nature… This simple thought made a deep impression on me. It impelled me toward a theory of gravitation. – A. Einstein (P1982, p. 178-179)
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Offline dhjdhj

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Re: What is the speed of gravity?
« Reply #43 on: 06/03/2016 13:26:11 »
I have been following this thread with great interest and whereas I am reluctant to interrupt this discussion by obviously learned people I would like to make these observations. The speed of wave propagation through the mediums we encounter on earth is proportional to the density of the medium. The denser the medium the faster the wave. This works until we get to the very fastest wave speeds electromagnetic fields and light where they propagate through the least dense medium or a medium of no density. This brings me to the reasoning behind my 'crackpot' left field hypothesis that the cosmos may be made up incredibly small inelastic spheres fruit-packed into a matrix. At the risk of sounding 'Boxxy' I now believe the basic posit might be right. If these spheres or beads as I called them only appear real in our world if moved i.e. given a co-ordinate of time, after which they become awake and form matter as we know it, then suddenly a lot of things begin to make sense. If you park the standard model for now (I believe a sensible and suitable structure can be found) as the structure I suggested has some issues which I am working on, then the rest  of the hypothesis seems to work very well. A spinning bead provides a gravitational field which propagates to infinity through its un-awaken neighbours in accordance with the inverse square law. It satisfies ,in fact requires time dilation exactly as Einstein's equation and the mass energy equivalence equation drops out. The speed of light is C in any direction as it should be. The photon is now energy travelling as a wave function in the densest material possible only waking up a tiny piece of mass as it goes. It has duality, angular momentum, no axial momentum, spin and will happily go through two slits at a time. It also has a tiny gravitational field that causes gravitational lensing, and if you have enough of them like close to a star it will provide sufficient additional gravity to effect the nearby planet. The matrix itself will provide multiple paths for entanglement and even allow energy to flow outwards instantly negating the need for expansion. Dark matter become a requirement as kilo for kilo energy transfer would supply 2 times 10 to the 40 more gravity than normal astronomical mass, so there would be huge supplies of gravity, probably centred around galaxies which would give rise to the dark matter effect. So the answer to the question would automatically be, gravitational waves which would happily travel through this matrix, would travel at C, whilst gravitational fields would appear instantly. If the sun suddenly disappeared it would not take 8 minutes for the earth to feel it, it would be instant. Now this idea supposes that time is absolute, supports relativity, space is three dimensional and flat and that the wave speed of light and gravity is back in its right place on the graph. At the risk of being considered mad what's not to like?

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Offline jeffreyH

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Re: What is the speed of gravity?
« Reply #44 on: 06/03/2016 13:31:47 »
Quote from: Jeffrey
Equations of motion do not describe a curved path in the absence of a force.

Would that force be gravity?

Yes
Fixation on the Einstein papers is a good definition of OCD.

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Offline Bill S

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Re: What is the speed of gravity?
« Reply #45 on: 06/03/2016 22:45:46 »
Quote from: Space Flow
.....that is certainly not the accepted by majority view of classical GR physics as I understand it today.


Scientific veracity is not determined democratic vote, but the following link indicates that there are those in the scientific community who agree about what Einstein actually said.

http://www.quantum-field-theory.net/scientific-american-einstein-didnt-say-that/

This link also shows how easy it is for experts to give us "hitch-hikers" the wrong impression.
There never was nothing.