speed of gravity

[ox1111]

This is not technically true. All massless particles travel at c and, as I alluded to earlier, gravity does as well. We first came across c through studying light, which is why it's called the speed of light, but that's not the last place we've encountered it. They are not the same thing.

Gravity in not believed to travel at c, It is thought to be either instant or travel as fast as c2. Massless particles is used fairly common, but I believe this to be incorrect as well. we know light can and does excelerate space probes and we explain this as radiant energy, but energy with out mass has no inertia and clearly makes no sense. I don't believe in massless particles, I would venture to say that at the mass level and speed the convertion of energy is 100%. I would go as far as saying you can not have energy without mass and I know this is not conventional, but energy as we know it is motion and their is no motion without mass.

[zapatos]

Gravity in not believed to travel at c,..

 

The speed of gravitational waves in the general theory of relativity is equal to the speed of light in vacuum, c.

http://en.wikipedia.org/wiki/Speed_of_gravity

[Delta1212]

Gravity in not believed to travel at c, It is thought to be either instant or travel as fast as c2. Massless particles is used fairly common, but I believe this to be incorrect as well. we know light can and does excelerate space probes and we explain this as radiant energy, but energy with out mass has no inertia and clearly makes no sense. I don't believe in massless particles, I would venture to say that at the mass level and speed the convertion of energy is 100%. I would go as far as saying you can not have energy without mass and I know this is not conventional, but energy as we know it is motion and their is no motion without mass.

Gravity was thought to be instantaneous in Newtonian Gravity. That was overturned by General Relativity quite some time ago.

[elfmotat]

Gravity in not believed to travel at c, It is thought to be either instant or travel as fast as c2.

 

That's simply wrong. Who told you this?

 

 

Massless particles is used fairly common, but I believe this to be incorrect as well. we know light can and does excelerate space probes and we explain this as radiant energy, but energy with out mass has no inertia and clearly makes no sense.

 

Photons have momentum given by [math]p^\mu =\hbar k^\mu [/math]. When they are absorbed by a medium, they also transfer their momentum into it. Since force is just the time derivative of momentum, shining a constant stream of photons onto a surface is equivalent to applying a force to that surface.

 

 

I don't believe in massless particles,

 

That makes you a crackpot.

 

I would venture to say that at the mass level and speed the convertion of energy is 100%. I would go as far as saying you can not have energy without mass and I know this is not conventional, but energy as we know it is motion and their is no motion without mass.

 

Electric potential energy has nothing to do with mass or motion.

[ox1111]

They recently viewed stars moving at 99.9% the speed of light around the supermassive blackhole in the middle the Milky Way galaxy. The controversy with this is that the gravitational pull of the supermassive blackhole would exceed gravity squared they say the gobbling of space at the event horizon of the blackhole does not break Einstein's theory of relativity because space has no mass theory being that mass cannot be accelerated by gravity to higher than the speed of light but gravity itself may move much faster. Along the same lines space-time accelerated many times faster than the speed of light when created by the Big Bang and again they say this doesn't break the theory relativity because space-time has no mass, the matter contained within this space-time later in the expansion of the universe contains gravity. If space-time can be created at a greater speed than light and gravity is a warpage is space-time and I stands to reason that gravity can be faster than us be like without breaking Einstein's relativity.

[Delta1212]

They recently viewed stars moving at 99.9% the speed of light around the supermassive blackhole in the middle the Milky Way galaxy. The controversy with this is that the gravitational pull of the supermassive blackhole would exceed gravity squared they say the gobbling of space at the event horizon of the blackhole does not break Einstein's theory of relativity because space has no mass theory being that mass cannot be accelerated by gravity to higher than the speed of light but gravity itself may move much faster. Along the same lines space-time accelerated many times faster than the speed of light when created by the Big Bang and again they say this doesn't break the theory relativity because space-time has no mass, the matter contained within this space-time later in the expansion of the universe contains gravity. If space-time can be created at a greater speed than light and gravity is a warpage is space-time and I stands to reason that gravity can be faster than us be like without breaking Einstein's relativity.

The expansion of spacetime can create apparent speeds greater than c because nothing is actually moving. It's just having more space inserted between it and other stuff. Gravity waves cannot travel faster than c because they actually move through spacetime; they are not products of the expansion of spacetime.

[ox1111]

If ST is the fabric of our universe and warpage of ST is what causes the effect of gravity, the leading edge of the expanding ST would infact be the first gravitional wave and weather it could move mass about c is up for debate. I would guess yes and would convert matter to ST to help fuel the proccess. I would not tell you this is anything but my logic, but it is sound. Gravity waves can exceed c if ST can expand faster than c because gravity waves are just ripples in ST.

[zapatos]

If ST is the fabric of our universe and warpage of ST is what causes the effect of gravity, the leading edge of the expanding ST...

What is the 'leading edge of ST'? Do you have a citation of anything that discusses this?

...weather it could move mass about c is up for debate.

Do you have a citation for someone who is arguing that this can move mass at a rate greated that c?

Gravity waves can exceed c if ST can expand faster than c because gravity waves are just ripples in ST.

Citation?

[ox1111]

I would not tell you this is anything but my logic, but it is sound. Gravity waves can exceed c if ST can expand faster than c because gravity waves are just ripples in ST.

 

If you're referring to gravity waves the way they do in fluid dynamics, then no, it would not exceed C.

The leading edge of ST is referring to the beginning of the big bang. It is common theory now that

expansion happened at greater than c for a brief period.

Whether or not this expansion could or did create mass or accelerated that mass beyond C is Theory on my part.

I believe if you do accelerate mass beyond C it becomes ST. I have been playing around with a theory the are universe is dense and that ST, better yet I will post it sometime I am working on a few.

[Mellinia]

Scientist in China(using earth tides) have calculated the speed of gravity as c.The earth's shape is affected by the sun(gravitational force) and there will be time where different areas experience different tides(not water but land tides). They calculated the time of propagation between the changes and found that the time used for the tides to change is the same as the time for the light of the sun to reach the earth at those places.

http://www.upi.com/Science_News/2013/01/02/Findings-say-gravity-moves-at-light-speed/UPI-37801357162827/

Edited January 6, 2013 by Mellinia

[JohnStu]

gravity operate at speed of 1/1/infinity, which is precisely infinity

[John Cuthber]

gravity operate at speed of 1/1/infinity, which is precisely infinity

Like farting in a lift, that post is wrong on may levels.

Trying to do maths with infinities is always dangerous.

The assertion is unsupported and gives no evidence and, perhaps most significantly, it's clearly wrong because, as cited in the previous post, the speed of gravity has been measured and it's c.

 

John, why did you clutter up the board with that posting?

[D H]

They recently viewed stars moving at 99.9% the speed of light around the supermassive blackhole in the middle the Milky Way galaxy.

Nonsense. Citation needed, please. (Hint: You won't find one. "They" recently observed stars moving at a few percent of the speed of light around Sgr A*.)

The controversy with this is that the gravitational pull of the supermassive blackhole would exceed gravity squared they say the gobbling of space at the event horizon of the blackhole does not break Einstein's theory of relativity because space has no mass theory being that mass cannot be accelerated by gravity to higher than the speed of light but gravity itself may move much faster.

Nonsense. The only gobbling here is that this is gobbledegook.

 

Scientist in China(using earth tides) have calculated the speed of gravity as c.The earth's shape is affected by the sun(gravitational force) and there will be time where different areas experience different tides(not water but land tides). They calculated the time of propagation between the changes and found that the time used for the tides to change is the same as the time for the light of the sun to reach the earth at those places.

 

http://www.upi.com/Science_News/2013/01/02/Findings-say-gravity-moves-at-light-speed/UPI-37801357162827/

Here's the paper: http://link.springer.com/article/10.1007/s11434-012-5603-3

This is an open access journal; the PDF is freely available.

 

This paper is 100% pure excrement. It starts off on a very, very bad footing (their equation (1) is complete crap) and then progressively gets worse and worse.

Edited January 6, 2013 by D H

[John Cuthber]

Here's the paper

http://link.springer.com/content/pdf/10.1007%2Fs11434-012-5603-3

[D H]

Here's the paper

http://link.springer.com/content/pdf/10.1007/s11434-012-5603-3

Fixed that. Thanks. This new forum software bites.

 

As for why this paper is so bad, the authors start off on a very bad footing by conflating gravitation and gravity waves. Then they go one step further into the excrement by looking at gravity waves emitted by the Sun as it orbits the Earth.

 

To make matters even worse, their math starts with equation (1), which they label as the "practical Newtonian formula of solar tidal force." A better name for this would be "completely invalid formula of solar tidal force." This equation adds a lag to Newtonian gravity. You cannot just add a retarded potential to Newtonian gravity and be done with it. There's a whole lot more to general relativity than gravity propagating at the speed of light.

 

Aside #1: Adding a lag to Newtonian gravity is a favored technique amongst crackpots for "disproving" general relativity because Newtonian gravity with a lag can easily shown to be invalid. However, this Newtonian gravity + lag model is but a straw man. It isn't general relativity. All these crackpots have shown is that this straw man is invalid.

 

Aside #2: Tang KeYun, the principal author of the paper, is a crackpot of a different ilk. He's published a number of bad papers claiming to have seen the Allais effect. He apparently doesn't understand gravity or the Earth tides, which is sad for someone with a PhD in geophysics.

 

Back to the paper, the authors proceed to make matters even worse with their equation (2). This adds a false model of the Earth tides to their already-invalid equation (1). To make matters worse still is the math they don't present, the mechanism by which they calculated the "phase lag due to the delayed response of anelastic Earth to solar tidal force."

 

To make matters worse still, their equation (3) is fraught with error. A small value of δ with respect to ε is guaranteed to make ε/(ε+δ) close to one. Replace c, the speed of light, in ε=r_*/c with the mechanical engineer's definition of c (the speed of sound in air at STP) and you get even better agreement than these authors obtained! In other words, they might as well have said that gravity propagates through vacuum at the speed of sound in air at STP.

Edited January 6, 2013 by D H

[John Cuthber]

Interesting,

Is it just a coincidence that they come up with c as the answer or does it follow from their other errors?

[D H]

Interesting,

Is it just a coincidence that they come up with c as the answer or does it follow from their other errors?

Yes. It's coincidence + model errors + noisy measurements.

 

The first error is their equation (1). This alone introduces a 1.3 second error in their "δ". (The annual solar aberration is about 20" of arc. At a rotation rate of 360 degrees per sidereal day, the Earth rotates through 20 arc seconds in about 1.3 seconds.) Coincidentally, that 1.3 second error is a whole lot less than the 8 minutes it takes light to travel from the Sun to the Earth.

 

Coincidentally, the mean value of their δ, a lag of -0.9 seconds, is pretty close to what it should be (-1.3 seconds). However, this is just coincidence. Their measurements are so noisy (the δ values range from -18.2 to +36.6 for six measurements) that this -0.9 second lag is statistically indistinguishable not only from the correct -1.3 seconds but also from 0.0 seconds. (If their bad model was correct, the δ should have been 0). Suppose they had made better measurements. For example, suppose that their δ was -1.1±0.3 seconds instead of -0.9±22 seconds. The correct lag -1.3 seconds would still be within range but a lag of zero could be rejected.

 

The final problem is the coincidence that 1.3 seconds is much less than 8 minutes. Their procedure for computing α, their supposed ratio of the speed of gravity to the speed of light, hides problems with their value of δ.

[alpha2cen]

Is this the speed of gravity?

V_g; the speed of gravity

How do we know that the gravitational field becomes equilibrium?

 

 

Edited January 7, 2013 by alpha2cen

[ajb]

Imagine having some large massive body and changing it slightly. This would create a small change in the gravitational field surrounding the body. By the speed of gravity one is usually asking "at what speed would these small induced changes in the gravitational field propagate".

 

In Newtonian gravity is a theory of "gravitational statics". The gravitational field itself has no dynamics. In essence we think of the gravitational interaction as being instantaneous, or for sure very much faster than any dynamics of the bodies involved.

 

General relativity tells us that the gravitational field is dynamical. In particular, the small changes in the gravitational field I spoke of earlier are gravitational waves. So, in the context of general relativity, the speed of gravity is the propagation speed of gravitational waves. We know that gravitational waves travel at the speed of light.

[D H]

We know that gravitational waves travel at the speed of light.

We don't know that gravitational waves travel at the speed of light. We think (for very good reason) that this is the case. Physicists are, or should be, honorary Missourians: "Show me". Gravitational waves have yet to be detected. They are extremely hard to detect. Even if LIGO or some other experiment does detect them, assessing the speed is an even harder problem.

[alpha2cen]

Does space-time moving speed make the gravitational wave?

[swansont]

!

Moderator Note

alpha2cen, you have been asked — repeatedly — not to hijack discussions. If a thread prompts to ask your own question on the topic that's not directed at any post in particular, start a new thread.

[ajb]

We don't know that gravitational waves travel at the speed of light. We think (for very good reason) that this is the case. Physicists are, or should be, honorary Missourians: "Show me". Gravitational waves have yet to be detected. They are extremely hard to detect. Even if LIGO or some other experiment does detect them, assessing the speed is an even harder problem.

Okay, we calculate that they travel at the speed of light, as at the moment gravitational waves are mathematical objects in general relativity. So far, all indirect experiments suggest that physical gravitational waves travel at the speed of light, even if the direct detection of a gravitational wave has been rather elusive. For example, double pulsars allow for testing of gravitational waves [1].

 

Reference

 

[1] A.G. Lyne, M. Burgay, M. Kramer, A. Possenti, R.N. Manchester, F. Camilo, M.A. McLaughlin, D.R. Lorimer, N. D'Amico, B.C. Joshi, J. Reynolds, P.C.C. Freire, A Double-Pulsar System - A Rare Laboratory for Relativistic Gravity and Plasma Physics, Science 303:1153-1157,2004 (arXiv:astro-ph/0401086).

[D H]

Okay, we calculate that they travel at the speed of light, as at the moment gravitational waves are mathematical objects in general relativity. So far, all indirect experiments suggest that physical gravitational waves travel at the speed of light, even if the direct detection of a gravitational wave has been rather elusive.

 

Much better. It's important to distinguish (1) what we know from (2) what we think we know, from (3) what we think that we think we know, and of course from (∞) crackpot nonsense. That gravitational waves exist is pretty much fact, falling somewhere between (1) and (2), but much closer to (1) than (2). That they propagate at c is a solid (2). The distinction between (1) and (2) is perhaps hard to see from the perspective of theoreticians (such as you) who are solidly ensconced in category (3) (e.g., string theory).

 

 

For example, double pulsars allow for testing of gravitational waves [1].

 

Reference

 

[1] A.G. Lyne, M. Burgay, M. Kramer, A. Possenti, R.N. Manchester, F. Camilo, M.A. McLaughlin, D.R. Lorimer, N. D'Amico, B.C. Joshi, J. Reynolds, P.C.C. Freire, A Double-Pulsar System - A Rare Laboratory for Relativistic Gravity and Plasma Physics, Science 303:1153-1157,2004 (arXiv:astro-ph/0401086).

 

You left out what is perhaps the most important reference of all with regard to binary pulsars,

 

Hulse, R. A. & Taylor, J. H., Discovery of a pulsar in a binary system, Ap.J. 195:L51-L53 (1975)

http://adsabs.harvard.edu/abs/1975ApJ...195L..51H

 

Most important because it was the first, and also because it won the 1993 Nobel Prize in Physics.

Edited January 8, 2013 by D H

[ajb]

 

The distinction between (1) and (2) is perhaps hard to see from the perspective of theoreticians (such as you) who are solidly ensconced in category (3) (e.g., string theory).

 

Quite generally I find that theoretical physicists are not so great at distinguishing mathematical constructs and ideas from actual observed phenomena. I include myself here, but I do seem more aware of it than some.