How long would it take for the gravitational well created by the Sun to disappear, and the Earth and the rest of the planets fly off into space?
In the very first episode of the Guide to Space, a clean shaven version of me, hunched over in my basement explained how long it takes for light to get from the Sun to the Earth. To answer that question, it takes light about 8 minutes and 20 seconds to make the trip.
In other words, if the Sun suddenly disappeared from space itself, we’d still see it shining in the sky for over 8 minutes before the everything went dark. Martians would take about 12 minutes to notice the Sun was gone, and New Horizons which is nearly at Pluto wouldn’t see a change for over 4 hours.
Although this idea is a little mind-bending, I’m sure you’ve got your head wrapped around it. We’ve sure gone on about it here on this show. The further you look into space, the further you’re looking back in time because of the speed of light, but have you ever considered the speed of gravity?
Let’s go back to that original example and remove the Sun again. How long would it take for the gravitational well created by the Sun to disappear.
When would the Earth and the rest of the planets fly off into space without the Sun holding the whole Solar System together with its gravity? Would it happen instantly, or would it take time for the information to reach Earth?
It sounds like a simple question, but it’s actually really tough to tell. The force of gravity, compared to other forces in the Universe, is actually pretty weak. It’s practically impossible to test in the laboratory.
According to Einstein’s Theory of Relativity, distortions in spacetime caused by mass – also known as gravity – will propagate out at the speed of light. In other words, the light from the Sun and the gravity of the Sun should disappear at exactly the same time from the Earth’s perspective.
But that’s just a theory and a bunch of fancy math. Is there any way to test this out in reality? Astronomers have figured a way to deduce this indirectly by watching the interactions with massive objects in space.
In the binary system PSR 1913+16, there’s a pair of pulsars orbiting each other within just a few times bigger than the width of the Sun. As they spin around each other, the pulsars warp the spacetime themselves by releasing gravitational waves. And this release of gravitational waves causes the pulsars to slow down.
It’s amazing that astronomers can even measure this orbital decay, but the even more amazing part is that they use this process to measure the speed of gravity. When they did the calculations, astronomers determined the speed of gravity to be within 1% of the speed of light – that’s close enough.
Scientists have also used careful observations of Jupiter to get at this number. By watching how Jupiter’s gravity warps the light from a background quasar as it passes in front, they were able to determine that the speed of gravity is between 80% and 120% of the speed of light. Again, that’s close enough.
So there you go. The speed of gravity equals the speed of light. And should the Sun suddenly disappear, we’ll be glad to get all the bad news at the same time.
Gravity is a harsh mistress. Tell us a story about a time gravity was too fast for you. Put it in the comments below.
Ah, but what about entanglement? If there is any entanglement between the Sun and the planets it would be “at the same time” vs “the now that propagates at the speed of light”.
Yes? Deep question there… What about matter and antimatter.. are they entangled at the quantum scale?
L.C. Crowell where are you?
Yes I hope our long time resident expert and overall great person who shared knowledge and many an insight on UT is out there somewhere doing well…
If you did not yet know the discovered “real intelligence” by the new Intelligent Design , you will never know about gravity, entanglement, anti-matter and matter.
Study more about real science so that you could understand reality and nature.
The attractive force we call ‘Gravity’ is proportional to the differential of the electron density field between two masses.
So then is there a ‘mass equivalence’ for gravitational energy based on E=mc^2? And if so, might that not (among other energies in the universe) be akin to dark matter/dark energy?
Before we had any understanding of ‘Electromagnetic forces’. We saw the effects. Just as we now see the effects of “Dark Energy / Matter”.
It is a matter of time before we get to understand this as well.
Good article, Fraser but you omitted an answer to the obvious question of how gravity escapes from a black hole. This assumes of course that the mass of a black hole resides at the singularity. The problem is not only that gravity doesn’t have the escape velocity to escape but also that it would have to travel backwards in time to escape.
Skeptic
Excellent point. It’s my contention that all original mass that comprises a black hole does not ‘exist’ on our frame of reference so much exists ‘else when’. ‘Doesn’t have the escape velocity’. Does it not? Light most certainly does not once past the event horizon whereas gravity persists.
Granted this is only speculative but all mass/energy interring beyond the event horizon would need to accelerate beyond light speed or at least be subject to extreme time dilation by comparison to ‘normal’ space. With the exception of gravity. Ergo, speculation of gravitys’ velocity (until better understood) is just that. Is it not?
So, one must ask. Is gravity simply related to ‘mass/energy’ alone or might it have more fundamental relationship with the very fabric of space itself? Mystery.
Gravity not being able to escape a black hole would imply a particle such as the graviton. That has yet to be found. I’m sticking to Einstein’s theory of gravity being mass affecting the geometry of space. In this case, gravity being able to *escape* a black hole makes absolute sense since there is no actual particle escaping. The black hole itself is what is warping space, thus “creating” gravity.
I wonder what effect inflation both just after the big bang and that started a few billion years ago, is having on gravity of the universe at large. If inflation expands the universe, then the gravity from the receding universe has to be time lagged if gravity is not instantaneous.
Heck, I am not even sure that gravity would not be time lagged even if instantaneous.
Consider what would happen to the gravity interaction between the Sun and the Earth if space/time between them began to inflate.
Which also may go some way towards explaining of the four fundamental force carriers why gravity comes out on the weak side.. What we call gravity must have had a higher density and perhaps a different relationship to other forces closer to the start of inflation…
Another view:
“The most amazing thing I was taught as a graduate student of celestial mechanics at Yale in the 1960s was that all gravitational interactions between bodies in all dynamical systems had to be taken as instantaneous. . . . Indeed, as astronomers we were taught to calculate orbits using instantaneous forces; then extract the position of some body along its orbit at a time of interest, and calculate where that position would appear as seen from Earth by allowing for the finite propagation speed of light from there to here. . . . That was the required procedure to get the correct answers.” (“The Speed of Gravity – What the Experiments Say” , Tom Van Flandern, Physics Letters A, 250 (1-3) (1998) pp. 1-11)
Unlike light, gravity has no aberration. Its “speed” must be instantaneous.
Looks like science is still stuck back in 1905, with its “local” view of physics. Astronomers and physicists need to realize that the Universe has “non-local” behaviors too.
Thank you Brian and all of you. These thoughts give me something to work with. I also would tend to assume that gravity would behave ‘instantaneously’ as it appears quantum entanglement does (relationship?). Would you still consider gravity as a component of the electromagnetic energy spectrum as we understand it however?
No, I don’t believe gravity is part of the electromagnetic spectrum.
The speed of an electric field is instantaneous too. Professor of Physics, A. P. French, has a relevant note in his very informative book, Special Relativity (1968), p. 242-243; 267:
“Now the electric field due to a stationary source charge is radial and, of course, spherically symmetrical; that is, it is the same in all directions. It is simply the Coulomb field . . . . If the source charge is moving uniformly, the electric field is no longer spherically symmetrical. Its strength is different in different directions. But, at each instant, the direction of the electric field is still radial with respect to the position of the source charge at that same instant.
“If you think about this last result a bit—that at each instant the electric field due to a uniformly moving source charge is directed radially away from the position of the source charge at that same instant—you may begin to realize that this is a very surprising result.
“Nevertheless, the field at P points away from the present position of q1. Nature behaves in such a way that, for a uniformly moving source charge, even though the field produced at some point P originated from the location and behavior of the source charge at an earlier time, nevertheless the field points away from the position of the source charge at the present time. It is as though nature calculates where the source charge should be at the present time and acts accordingly. . . . Thus a result which at first glance may seem rather obvious is seen, upon closer examination, to be quite surprising—but nevertheless true.”
If you read that carefully, you’ll find that it is just a convoluted way of saying that electric fields act instantaneously–there is no propagation delay.
Non-local physics is only taught in the colleges as “quantum mechanics”. It applies to the microphysical realm, and is based on the quantization of space. Consider what happens when two atoms approach each other. At the one unit spatial boundary, the momentum cannot vary in space, as this space has no “inside”, and therefore no spatial positions or trajectories. The variation can only be in time, but time in our reference system is scalar (non-directional) Hence, the motions can only be mapped in as a probability distribution. (We are trying to see a “when” type of motion in a “where” type of reference system.)
The other kind of non-local physics is based on a unit SPEED limitation, and can apply to things bigger than aircraft carriers and still retain the qualities of non-locality. Unfortunately, this science is dominated by Special and General Relativity, which are both specifically and intentionally “local” theories. They work fine for coping with reference system limitations (especially at high spatial speeds), but fall apart when applied to large scale non-local phenomena. The speed of light cannot be exceeded IN SPACE, but there can still be variations in the time component. Technological use of this fact leads to some extremely weird and bizarre physical behaviors.
Imagine a guy driving down the freeway with one window slightly down. He is thinking that the air noise indicates he is going at least 80 mph, but his speedometer only indicates 60 mph. Then a cop pulls him over and gives him a ticket for going 85. Later, he discovers that the speedometer was broken. It could only indicate a maximum of 60 mph.
And so it is with non-local speeds when they are mapped into a local (“spatial”) reference system. The maximum speed that can be depicted in this system (as a change of position in SPACE) is the speed of light. Speeds that are higher than that will still be MEASURED as speeds less than that of light.
Apparently, everything has a location in three-dimensional space and three-dimensional time. If two photons are created in the same event (such as down-conversion of one violet photon into two red photons), the photons remain in the same TIME location even though they separate spatially. They are still “together” in the temporal sense, even though they may be separated by light years in the spatial sense. This is what is behind “entanglement” in general and the EPR paradox in particular.
Nice explanations – thanks!
Brian. Thank you Sir. What I wouldn’t give to sit and talk for a couple weeks. Sadly the burden would be mostly on you however. I’m diagnosed with something akin to Asperger/multiple intelligences.
Can’t speak a ‘word’ of ‘math’ I’m restricted to what I can ‘see’ in my mind. Makes things a tad sticky being how as I’m very seriously interested in these subjects. Thank you very much for your attempt to help. And it has helped. Lol. You folks can build mathematical structures and I’m stuck building ‘pictures’. I guess. Aw, but they do build. Please be well.
It seems early orbital mechanics in the 18th and 19th century dealt with resonances which appears to me to have had the underlying assumption of instantaneous gravitation using Newtonian mechanics.. Then came along Michaelson, Morley and others to try to account for refraction of light in a vacuum (think of how your spoon appears disjointed at the water/air boundry- how can a vacuum with no density change cause refraction?? Something to do with photon group velocity vs. phase amplitude but it hurts my head..) and whether Aether drift could be measured. I do feel that experiment and more deeply probing gravitatation has been brushed aside when GR/SR burst onto the world stage… In the sense that the world scientific community seems to have accepted a null result from one experiment using one instrument and moved on with a grand conclusion about the nature of the vacuum, gravity and spacetime.. Search for Cannae drive and EM drive for tantalizing hints that science may be on the verge of a healthy questioning of some assumptions in physics (if further experiments bear out Eagleworks Lab results) Einsteins theories very nearly explain many things about mass, light and relativity (and the effect of gravity as spacetime warp) but not necessarily answering what gravity is.. Afaik..
Considering we can’t exactly describe what space is, I would say it would be a little foolish to automatically assume that geometric distortions in space are limited to the speed of light. I am not saying it isn’t so but arguing for or against it, without any solid proof, is a little silly.
Interesting, well written thank you…
all I know about gravity is that my wife doesn’t care for it );->
Lol
Actually, based on the explanations from the new Intelligent Design , gravity is probably just the photon of light. If you want to know more, you can read more from this book, “PHYSICS of Intelligent Design “, Amazon, e-book section and you will see a better explanation. The author is Edgar Postrado.
“By watching how Jupiter’s gravity warps the light from a background quasar as it passes in front, they were able to determine that the speed of gravity is between 80% and 120% of the speed of light.”
This was already debunked. The people that published that study measured in fact the speed of light.
About the pulsars slowing down. How can we measure the distance between them with sufficient accuracy? Is the distance constant, increasing or decreasing? If they lose mass due to the energy put into the radio beams, the distance should increase and also their rotational speed around each other.
The distance to those pulsars (the only ones observed to behave in that way) is way to great to be certain about what happens in that system.
But we have at hand means to precisely measure the speed of gravity. Geostationary satellites! There were some Chinese scientists claiming the the variation in position of the geostationary satellites they work with, varies due to the tidal forces of the moon and the sun, and guess what? Those variations make sense for an infinite speed of gravity, not c!
In case readers are interested, I think you are referring to “Measurement of the Speed of Gravity” by Yin Zhu (arxiv.org/ftp/arxiv/papers/1108/1108.3761.pdf ). The Abstract from Appendix B says, in part : “It indicates that the gravitational force of the Sun acting on the satellite is from the present position of the Sun [i.e., no aberration] and that the speed of the gravitational force is much larger than the speed of light in a vacuum.”
Thanks for expounding on this topic!
The Speed of Gravity is mystifying.
Now it appears from replies here, that ‘C’ is actually limiting the measurements of other known & unknown forces in our universe, and some of these may be faster than speed of Light!
But I do hope Einstein is still right, and there are other explanations.
Hi Chan,
That is part of what I was alluding to in my comment above – In the Michaelson Morley experiment which sought to confirm the presence of an Aether, indeed, laser beams were used along with Nickel alloy (invar) armatures to try to decern any variation in light path travel to confirm/reject an Aether.
This link makes great reading:
http://www.pnas.org/content/11/6/306.full.pdf
Oddly enough, modern experiments seem to confirm an Aether. I am concious that the term Aether has gone our of fashion in modern cosmology – so call it “The Quantum Vacuum” if you will…
Some corrections..
1. The correct spelling is Michelson in Michelson Morley Experiment.
2. Early experiments employed Sodium light whilst later experiments employed Sodium-Neon laser.
The key to understanding gravity will likely necessitate first understanding Light and its interaction within the vacuum (since light forms the basis of many measuring apparatus)…
@weeasle, et. al
Thanks for the link and the comments.
Miller’s work notwithstanding, I think the basic concept of the Michelson Morley experiment was flawed. It assumed that there was a STATIC Aether, and that therefore the Earth’s motion “thru the Ether” could be measured. It assumed that the Earth’s motion had a magnitude and a direction relative to the Aether, and therefore used vector mathematics. Brilliant and careful as it was, the experiment produced a null result. All that it proved, however, was that THAT kind of Aether did not exist.
What now seems more likely is that there IS an Ether but it is dynamic (not static) and moves in all directions simultaneously, like an expansion. It is like the points on a TV scene when the camera zooms in, all the picture features expand and move away from each other. This is just ONE motion but which requires three-dimensions of space to describe it.
The Earth has THAT kind of motion, but in the opposite sense. If I jump out of a tree, the Earth rushes up (out) to meet me (as per Einstein). But I can do this anywhere on Earth and get the same result. The Earth’s gravitational motion is truly non-directional. And because the Ether is non-directional, none of this kind of motion will show up in a directional, vector based experiment like that of Michelson Morley.
Brian – Thank you for your reply below – I found it most enlightening.. Curious to whether u have thoughts on the Theory of Dynamic Equillibrium which was penned independently by Lemaitre in 1927 and sat unpublished in Enstein’s desk drawer in 1931. Main difference afaik was that in Einstein’s model, energy balanced entropy continually during expansion whereas Lemaitre seemed to assume a cyclical bang/crunch (I feel this may be why it lay unpublished as it maybe sounded a little unrealistic compared to the simpler BB inflation of the Standard model). Your thoughts appreciated..
I think Lemaitre was at least on the right track, but perhaps his data was insufficient.
The view that is gaining currency now is that space itself expands or is “emergent” (new spatial units are being generated by some unknown process). It is like time, in that it progresses. But it progresses in three dimensions, and we call that an expansion.
Opposing the expansion is gravitation, which is centered on an object (planet, star, galaxy). We interpret the resulting motions in terms of forces, the cosmological expansion force, which is not affected by distance, and the gravitational force, which has a 1/d^2 dependence. Because of this, there is necessarily a distance where the forces are at equilibrium, a distance I call the “gravipause” (which, in this definition, involves only ONE body, and space itself). For stars it is apparently a few light years, and for galaxies it is apparently a few million light years. Inside this distance, objects come together, and outside this distance, objects move apart (and faster the farther apart, because of the lessening influence of gravitation, and because there are an increasing number of units of space in between, like links in a chain). In the Hubble region, the math would involve 1/d^1. Beyond that, the math becomes 1/d^0 ; gravitation is presumably quantized, and at some distance, it disappears altogether.
This seems to have several implications:
1. Globular clusters are stable even though they don’t rotate.
2. No Big Bang is needed. The Expansion is due to the progression of space and time in three dimensions. These seem to be locked into a ratio called “the speed of light”. And curiously, that implies, at least in the larger picture, that the expansion of time balances out the expansion of space. From the standpoint of one or the other, the Universe expands. But from the standpoint of the ratio–which is unchanging despite its progressing parts–it is neither expanding nor contracting. From our standpoint the expansion is both centerless and edgeless (or equivalently, the center and edge are everywhere –“non-local”). …Totally mind-blowing stuff.
(Perhaps the uniform microwave background, and the uniform X-ray background, and the uniform cosmic ray background are trying to tell us something. )
3. The “Hubble constant” would be dependent on the location from which the observations are made (a large versus small galaxy).
4. The MEASURED speed of light would be somewhat dependent on the total gravitational environment. Only the speed of the (dynamic) Ether is truly constant.
5. The speed of light is a speed of a different character from ordinary speeds. Two photons moving in opposite directions separate at one unit of 3D space and one unit of 3D time in each direction. The separation speed is therefore 2/2, 4/4, 6/6, etc. The speed of light is essentially constant because that ratio is always 1/1. Alternatively, the photons are locked into the Ether, and relative to it, are not moving at all. (It is mass that is actually doing the real moving)
6. Why are stars separated by light years or light hours, but not by light weeks? (food for thought; stuff either comes together or moves apart depending on the location of the “gravipause”)
Well I hope this answers some of your questions–and without causing too many sleepless nights.
Thanks for your thoughts! Most intriguing! I havent been able to track it down yet, but if you can, I would encourage you to read Einstein’s unpublished 1931 Dynamic Equillibrium as the description you give (in particular space itself being self generating and continually replenishing) was almost exactly how I have heard others summarizing the theory…
This is now an old thread – but on the off chance you see this – I finally located a translated version of Einstein’s unpublished Theory of Dynamic Equillibrium, 1931:
http://arxiv.org/pdf/1312.2192.pdf
@weeasle
I am still getting email notifications from comments on this topic. It is not quite roadkill yet.
Thanks for the link to “Einstein’s unpublished Theory of Dynamic Equilibrium, 1931” I had been looking for one like that myself.
Another useful one is:
“Einstein’s aborted attempt at a dynamic steady-state universe”, arxiv.org/ftp/arxiv/papers/1402/1402.4099.pdf
Einstein and others had recognized that the existing “static” picture of the Universe was not stable. And then there was growing evidence that the Universe was actually expanding, which created yet more problems. He tried to solve this by postulating an influx of “new matter” to create a dynamic equilibrium. But that had problems too.
Apparently, they never got to the “gravipause” idea. Modern science hasn’t either, although by now it should be obvious.
How about this: Yes, our universe started as a singularity, but that singularity did not contain all the matter of the universe as of now? Now think how a black hole starts. Also as a singularity. Then what happens? An event horizon appears, which grows as the matter is added to the “black hole”. But the event horizon is “virtual”, it depends on the position and the speed of the outside observer. But what would be seen by an observer falling towards the singularity? For sure he will notice that matter falling towards the singularity in front and behind him will move away from him at an accelerated speed. And remember, the event horizon is a virtual thing. One static observer will see another observer falling towards the centre as a approaching the event horizon, and stopping on it. However for the falling observer there is another event horizon, or none at all. What he could observe for sure? That space expands around him at an accelerated speed, that there is a singularity from which the expansion radiates.
However, this expansion would be unidimensional not three dimensional. Three falling observers arranged in a plane perpendicular to the direction of falling towards the the singularity should observe each other as getting closer to one another. But for this, they should be at the exact same mathematical distance from the singularity, at the atom level, and at the exact same speed. Otherwise they would be acceleratingly departing from each other, but not at the same speed and acceleration as the matter in front and behind them. The question is, is all matter around us in the universe departs from us with the same acceleration in all directions? If not, my best bet is that we are actually falling towards the singularity, but the space itself around us dilates faster than us falling, and thus we are perceive ourselves as departing from the singularity. Take it as a “Fata Morgana” effect. However, we will perceive our universe as finite (observably) but new matter could be added to our universe, that is matter coming from “behind”/outside , at a greater initial speed than we started, however not faster than the speed of light.
In conclusion, what if our visible universe is a mere region around a black hole, in a greater (higher dimensional) universe?
Nevertheless, coming back to the speed of gravity, if it is infinite, than matter outside out observable universe will also exert a gravitational pull on us, but if the distance is too great, that pull is practically 0.
@ALN
There are a lot of assumptions buried in all those statements (and modern astronomy has a profusion of them). So I’ll just say this:
1. I don’t know how the Universe began, except to say that God created it. And we really seem to have no way of reliably calculating its age, except to say that it is evidently very, very old from our standpoint.
2. I favor the finite, steady state model (akin to Hoyle) except that there is no “continuous creation”. Apparently, there is some sort of cycle, possibly referred to in the Bible (Hebrews 1:10-12, Psalm 102:25-26, Isaiah 51:6 ). My guess is that matter is “recycled” between the “local” and “non-local” sectors of the Universe, and that the recycling may involve galactic explosions. Stuff coming in from a non-local source would be seen by us as a uniform background of high-energy particles, and a broad spectrum of background radiation (microwave, X-ray, etc).
3. You can work out the dimensions of gravity from the formula E= mc^2. You have to re-arrange it so it is in the non-local form: E/(1/c)^1 = m/((1/c))^3. This form of the equation says A LOT (!) and you can even derive Newton’s law of gravitation from it (except that there will be no need of G). The dimensions of c are, of course, space/time (or s/t for short). The dimensions of energy are t/s, which means it is non-directional and has only a magnitude. The dimensions of mass work out to t^3/s^3, which is again non-directional (in a spatial reference system) and has only a magnitude. It can be treated as a spherically distributed motion, and its intensity distribution will therefore be proportional to 1/r^2. Its intensity is also proportional to the amount of mass present. It is clearly a non-local motion (t/s) and therefore acts instantaneously; unlike light, gravity has no propagation delay or aberration. If the Sun suddenly disappeared, its gravity would instantly disappear too, although the light would continue for 8.3 minutes from our viewpoint.
@BrianFraser: I don’t know, I don’t believe, I suppose! (things about gravity, universe, God). I always look for proof for anything to believe. To know? I just know I exist! Nothing else.
Have a nice day! 🙂
You can live without your harsh mistress, but try living without gravity.
In the beginning – or just before that – there was singularity. What was it that kept singularity in that state? Well, gravity. Then, something went wrong with that singularity level equilibrium and, before one could say cheese, there was a rather big bang. The infinitely dense singularity – so dense that super massive black holes would look like sponges in comparison – started throwing material indiscriminately in all directions. Where was gravity then? Well, it was temporarily overcome by the force of that biggish bang. It was there all the time. It was simply getting stretched. From that day – sorry – moment, onward, the big bang has been throwing things outwards and the gravity also is getting stretched farther and farther out, while slowing down that expansion all the time. This could go on until such time that finally gravity asserts itself and starts crunching the universe back into the original singularity.
From the above, it’s amply clear that I’m no scientist. Heck, I just barely passed the science exam in the high school – aided by luck. As such, expecting any kind of supporting evidence to my assertions above would be a fruitless exercise. Since my audience in these matters is mostly me, and, since me is not too fussy about proofs and evidences, me has developed a very healthy disrespect for what are essentially bells and whistles.
Getting back to the theme, perhaps the speed of the gravity is the speed of the expansion of the universe. In the initial moments, when – as claimed by some scientists (without offering any evidence whatsoever, hah!) – the speed of expansion was greater than the speed of light. When that was the case, gravity expanded faster than light. Now that the expansion rate has tamed down, the gravity does not need to expand at the earlier break-neck velocity. It’s not as if gravity senses a distant object outside its grasp and goes after it with the diabolical intention of entrapping it. It has always been right there all the way till the edge of the universe. Simple as that. To that extent, I think in all my blissful ignorance, that the question “what’s faster, gravity or light?” is point (no pun intended) less. Cheers.
Perhaps gravity could be thought of as a non zero point vacuum expanding into a zero point vacuum – but i am going out on a limb as i am not physicist myself…
I just wanted to chime in one last time that as commenters have pointed out here there are more direct observations closer to home (than distant pulsars) that seem to contradict Frasers light hearted assertions (I did enjoy the article Fraser :).. There were some excellent comments here showing that the measurement of the pulsars may have inadvertantly overlooked some subtlety (such as relativistic gravitation effects which without knowing the exact composition of the binary system could have been overlooked). I tend to put more stock in the simple comparison of positions of satellites in proximity to earth using newtonian and GR math which lend support to a speed of gravitation greater than light..
Here is an excellent article which goes into some evidence supporting instantaneous gravitational influence in orbital mechanics (as mentioned by a number of people in this thread):
http://metaresearch.org/cosmology/speed_of_gravity.asp
Apologies for replying to myself – After re-reading the paper I linked above, I realised it is one of the best peer reviewed papers I have read – It goes into just the right detail that is analagous to the discussion sparked by this UT article – It even mention Psr1316+13 .. I hope u read it too Fraser 🙂