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Don't Know Much About Gravity

...or at least as much as we think we do. Does the gravity model need to be adjusted?

In the one probe the researchers did not confirm a noticeable anomaly with, MESSENGER, the spacecraft approached the Earth at about latitude 31 degrees north and receded from the Earth at about latitude 32 degrees south. "This near-perfect symmetry about the equator seemed to result in a very small velocity change, in contrast to the five other flybys," Anderson explained -- so small no anomaly could be confirmed.

The five other flybys involved flights whose incoming and outgoing trajectories were asymmetrical with each other in terms of their orientation with Earth's equator.

For instance, the NEAR mission approached Earth at about latitude 20 south and receded from the planet at about latitude 72 south. The spacecraft then seemed to fly 13 millimeters per second faster than expected. While this is just one-millionth of that probe's total velocity, the precision of the velocity measurements was 0.1 millimeters per second, carried out as they were using radio waves bounced off the craft. This suggests the anomaly seen is real -- and one needing an explanation.

Well, gravity just like evolution, is (in the words of anti-evolutionists) only a theory. It's not reality--it's simply an attempt to model it. And for most purposes, it does a pretty good job. But one of the reasons to do space, I think, is that it gives us new laboratories to make new discoveries about basic physics, the potential of which is unforeseeable.


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Jardinero1 wrote:

If; the speed of light is a constant and the absolute upper limit of velocity in our universe; and the force of gravity is instantaneous; how do you reconcile the contradiction.

How can a photon of light be limited to C but the force of gravity is instantaneous, ergo greater than C. I have trouble with this.

Somebody throw me a lifeline.

Brock wrote:

It's been know for a while that the Voyager probes are "off" by a bit too. Isn't this just another example of the same thing?

Mac wrote:

Jardinero1 wrote: How can a photon of light be limited to C but the force of gravity is instantaneous, ergo greater than C. I have trouble with this.

A photon is limited to a finite speed. Gravity exists everywhere, so it doesn't travel, therefor it is not greater than C, just everywhere. Perhaps the reason a photon cannot exceed C is because gravity acts on it.

Monte Davis wrote:

Isn't this just another example..?

The story covers six Earth flybys: Galileo (twice), NEAR, Rosetta, Cassini, and Messenger. All but the last showed an anomaly like those seen in the two Pioneers. So first, it's five more examples... and second, at this very early stage of something that could be huge*, every example is important in building a consensus that there is indeed something that needs explaining.

*or could be systematic error, although IMHO Anderson is as meticulous as they come

Kevin B wrote:

It's either dark matter or dark energy. Everything that doesn't fit the current model is usually down to one of those.

Personally, I reckon the strings get a bit stretched on the inward run, then twang back as the craft heads out again.

Habitat Hermit wrote:

The piece portrays this as recent news which it is not. Wikipedia has a better writeup about the flyby anomaly (and also the Pioneer anomaly).

Paul Breed wrote:

My guess is this might be an error in the Earth Gravity field models? The earth is not a uniform solid, it has heavy and light areas as well as variation in altitude (mountains) How does this rotating non uniformity
interact with the space craft? I can imagine calculating it, I can also imagine a warehouse full of supercomputers....


Jardinero1 wrote:


Gravity doesn't exist everywhere. It diminishes at a rate inversely proportional to its distance from it's source just like every other wave form.

Kent G. Budge wrote:

Trained physicist chiming in here. Gravity is instantaneous only in the Newtonian model; in general relativity, it propagates at the speed of light.

I suspect a general relativistic effect. His Hehness (Glenn Reynolds) made a surprisingly reasonable suggestion for a nonphysicist, that the folks here neglected frame dragging. I actually kind of doubt it, but it could be something along that line.

The Pioneer anomaly is another matter. I find it quite intriguing, particularly since it might be evidence of conformal gravity (a competing theory with Einstein's) that explains dark matter and the cosmological constant very neatly.

Raoul Ortega wrote:

It's either dark matter or dark energy.

Actually, it's from the way phlogiston interacts with all the N-rays trapped by the aether in the Earth's epicycle.

Jardinero1 wrote:


So, is the sun's pull on the earth in the same straight line as the earth's pull on the sun or is there a delay between the earth and the sun while the two bodies act upon one another. The only way the two gravities could be in a straight line is if gravity is instantaneous, faster than C. If they are not in a straight line then the earth would eventually spin out of the sun's orbit.

This really bothers me. The more I think about it and read about the Pioneer anomaly, the more I think we don't know anything at all about gravity.

McGehee wrote:

Mac: Gravity exists everywhere, so it doesn't travel, therefor it is not greater than C, just everywhere.

Jardinero1: Gravity doesn't exist everywhere. It diminishes at a rate inversely proportional to its distance from it's source just like every other wave form.

I don't think that's what Mac was talking about; it's something I've wondered about though: how does gravity propagate? If it were possible to instantaneously alter the mass present in a particular place, would the effect of the change to the gravitational terrain propagate through the neighborhood at the speed of light? If so, then it would certainly add weight (heh) to the idea of the speed of light being an upper limit.

But what if not? What if the change were to propagate much faster than the speed of light?

Has anyone ever managed to find a way to test this, other than with equations?

McGehee wrote:

Kent G. Budge: in general relativity, it propagates at the speed of light.

That'll teach me to comment before rading all the comments. Still, an actual practical proof would be nice, if it could be done...

Mac wrote:

Jardinero1 said: Gravity doesn't exist everywhere.

Yes it does, even in a most miniscule amount. Now, I see what your question is now. If the sun ceased to exist, how long would it be for us (earth) to zip out of here because the gravity exerted on us by the sun was suddenly gone. It just makes sense that it would move at C. As for the orbit of the earth, the gravity well exerted by the sun extends in all directions, so that as we orbit, we maintain our position as we move through the field.

Jay Manifold wrote:

An experiment at MU that I blogged here indicated that the speed of gravity is 1.06c, plus or minus 20 percent. Follow-up here.

Phil Wilson wrote:

I was surprised to learn from a recent Science News article on atomic interferometers that we only know G in Newton's gravitational force equation to worse than one part in 10 exp -5. If that's so imprecise, how can we be looking at differences many orders of magnitude less in the velocity vectors of these probes?

Peter wrote:

Phil, while we may not know G to all that great of precision, I expect we can measure the product of G and Earths mass pretty precisely by tracking satellites. Last I looked, if you know this product you don't need to know G or M individually for gravity calculations.

Mike G in Corvallis wrote:

Shadow matter (AKA mirror matter, maybe AKA cold dark matter), which interacts with ordinary matter only through weak and gravitational forces, ought to exist according to particle physics symmetry theory. If it actually exists, and if there's some in our neighborhood, the Earth might have collected an appreciable amount of the stuff in its gravitational well. (Several orders of magnitude less than an Earth mass, otherwise we'd already know about it, I'd think.) If the shadow matter object is rotating, it probably isn't rotating once every 24 hours, but it might be rotating in the plane of the ecliptic. Depending on how it interacts with itself, perhaps the shadow matter has settled into a rotating disk inside the Earth rather than a sphere. Gravitational interaction with this object might explain a gain or loss in energy of the spacecraft depending on the encounter parameters.

There's a nice, nontechnical writeup on shadow matter under "Mirror Matter" on Wikipedia, with lots of interesting references.

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This page contains a single entry by Rand Simberg published on March 4, 2008 11:39 AM.

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