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Author Topic: What is the speed of gravity?  (Read 6491 times)

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.
 

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).

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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.
 

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 »
 

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.

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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..
 

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.
 

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
 

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.
 

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?
 

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.
 

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.
 

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...)
 

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. 
 

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.
 

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/
 

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.
 

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?
 

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)
 

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?
 

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
 

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.
 

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Re: What is the speed of gravity?
« Reply #45 on: 06/03/2016 22:45:46 »

 

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