# Thread: What speed of gravity is used in space probe trajectory calc. in a static field?

1. ## What speed of gravity is used in space probe trajectory calc. in a static field?

The speed of gravity used in space probe trajectory calculations is the speed of light, except that in my opinion, space/time distortion has to be allowed for. If you think about the concept used in the film 'Event Horizon', where gravity projection can enable instant travel, you may see where I'm going with this controversial post. Gravity reaches out from inside a black hole and yet light cannot, because light does not distort the fabric of space/time as gravity does. Equipment is now available to conduct an experiment to possibly show such gravity projection in action, but this is so against the mainstream would the result ever be published? What do others think of this controversial topic?

2. ## Re: What speed of gravity is used in space probe trajectory calc. in a static field?

Mainstream has it that gravitational force travels with the speed of light. Thus if the gravitational force between the Sun and Earth were suddenly to disappear by the vanishing of the Sun, it would take about 8 minutes for the effect to be experienced on the Earth. The Earth would then fly off along a tangent to its original curved path.
Against the mainstream is that gravitational force is instantaneous in a static field. Thus when the Earth is moving around the Sun, the attraction of the Earth is toward the instantaneous position of the Sun and not where the Sun had been. If the Sun’s position was retarded by the 8 minute delay of light speed the pull of gravity would be toward the optical and not the actual position of the Sun.
A gravimeter (gravitational force detector) employed during the lunar eclipse of 28th Sep 2015 might have provided the answer to the opposing views. So when the eclipse occurred visually, if the maximum gravitational force of Sun-Earth with reference from Earth had shifted 5.7x10-3 degrees east, the gravitational force would be acting instantly. Fig1.
See
Zhu, Yin (2011). “Measurement of the speed of Gravity” arXiv:1108.3761

Also regarding the film ‘Event Horizon’, where the ship uses gravity drive:
See
Zhu, Yin (3 June 2016). Can we use the Gravitational field?
Fig1SpeedOfGravity95pc186.jpg

3. ## Re: What speed of gravity is used in space probe trajectory calc. in a static field?

Originally Posted by spartan45
Thus when the Earth is moving around the Sun, the attraction of the Earth is toward the instantaneous position of the Sun and not where the Sun had been. If the Sun’s position was retarded by the 8 minute delay of light speed the pull of gravity would be toward the optical and not the actual position of the Sun.
It's true that observer effects cause some confusion on this one. Your assumption is ignoring Lorentz invariant static fields which show that the observer effect is the same for a staic field or one with objects in motion, simply by switching reference frames.
Also, Angular momentum and energy momentum must be conserved.

According to General Relativity, when a gravitating object is in motion, the vector points to the actual position of the object it is gravitating toward, not toward the light delayed image. This implies that the Earth would change vector in a straight line away from the Sol System instantly the moment the Sun vanished with no delay since no additional propagation was needed to "communicate" the sudden absence of the Sun.
However...
Where this thought experiment gets tricky is pretty straight-forward: The scenario requires ignoring the very equations required to answer it. Energy Conservation. Mass and energy simply do not and cannot violate the conservation of energy in these equations and in order to get the Sun to instantly vanish, that requires violating conservation.
Once energy conservation is accounted for and not ignored, the Earth cannot see an instant vector change since the geometry of space must conform to the complete set of equations. While one could interpret that to claim that there would be about an 8 minute delay, that is rendered into an absurdity as is the entire thought experiment because saving the conservation of energy reintroduces the necessity of the presence of the Suns gravitational effect.

In short: The thought experiment is a mere absurdity and answering it would only give an absurd answer.

4. ## Re: What speed of gravity is used in space probe trajectory calc. in a static field?

Originally Posted by Neverfly
According to General Relativity, when a gravitating object is in motion, the vector points to the actual position of the object it is gravitating toward, not toward the light delayed image.
.
Excellent reply. No need now to conduct the experiment pictured in Fig 1 of my post because with this quote you have given the answer to the question : 'What speed of gravity is used in space probe trajectory calculation in a static field?' - It is instant, not the light delayed image of the gravitating body.
At last a decent answer to what I consider a tough question. 10/10.

5. ## Re: What speed of gravity is used in space probe trajectory calc. in a static field?

Originally Posted by spartan45
Excellent reply. No need now to conduct the experiment pictured in Fig 1 of my post because with this quote you have given the answer to the question : 'What speed of gravity is used in space probe trajectory calculation in a static field?' - It is instant, not the light delayed image of the gravitating body.
At last a decent answer to what I consider a tough question. 10/10.
Experimentation is always needed to confirm mathematical deduction and every theory, no matter how sound, even as sound as Relativity, demands questioning and critical examination.
That said, you pretty much just cherry picked what you wanted to read in my response and ignored the fact that the post makes two things clear:
1.) Gravity is not shown to propagate it instantaneous speed...
In fact, if confirms that it, like everything else, is limited to 'c.'
2.) The thought experiment is not a tough question but is an absurdity since it requires the laws of physics to be both violated and maintained at the same time.

6. ## Re: What speed of gravity is used in space probe trajectory calc. in a static field?

The quote mining and the explanation:
Spartan45 is referring to a Lorentz scalar. A four vector lorentzian transform of spacetime and energy/momentum. There are a few factors that need to be understood:
-The Sun is a massive object but is still far away from our instrumentation on Earth.
-The Suns presence is a Constant Gravitational Field.
-The Earths motion in the act of falling around the Sun is very near a constant velocity.
Since the Earths motion is pretty consistent around a constant gravitational influence, it "rides the waves" so to speak, coming down in a different position then it was in when it rode the crest of the wave.
With a finite speed of gravity, this means that the effects of velocity-dependent interactions cancel out. Which I pointed out the the quoted portion, above. You can rotate the scalar and get the same result, each time. It's invariant.
Spartan45 quote mined that in order to claim support for his argument. However, the problem does not end there... even if his attention span did.

Simply put, we cannot measure it from our point of view. At this scale in local space, the effect is so small, that even rounding a figure off is more than enough to destroy the result of running the equations. The Earth experiences accelerations in other directions from the Moon and Jupiter. But even if we could get very accurate figures to handle the scale we are talking...
What we need is a changing velocity vector and changing gravitational field so that it is not invariant.
Which is what LIGO has been studying with strong supportive results.

All of this is secondary to the primary point: The four vector requires the Energy/Momentum conservation. Which renders the "tough question" Spartan45 posed into a contradiction.

7. ## Re: What speed of gravity is used in space probe trajectory calc. in a static field?

Originally Posted by Neverfly
1.) Gravity is not shown to propagate it instantaneous speed...
In fact, if confirms that it, like everything else, is limited to 'c.'
2.) The thought experiment is not a tough question but is an absurdity since it requires the laws of physics to be both violated and maintained at the same time.
The New Horizons probe launched June 19 2006 swung past Jupiter for a gravity boost in February 2007, achieving a close approach to Pluto on July 14 2015. Light from the Sun takes about 43 minutes to reach Jupiter and 5.5 hours to reach Pluto. These figures demonstrate how important it is to establish that it is the actual position of the object it is gravitating toward, not toward the light delayed image (applies to a static Gravitational field such as the Solar system).
Maybe this post question was badly worded, some might say an absurdity, but it works for me.
Incidentally, I agree gravity is not shown to propagate at instantaneous speed and, well, I may have done a little quote mining.

8. ## Re: What speed of gravity is used in space probe trajectory calc. in a static field?

Originally Posted by spartan45
The New Horizons probe launched June 19 2006 swung past Jupiter for a gravity boost in February 2007, achieving a close approach to Pluto on July 14 2015. Light from the Sun takes about 43 minutes to reach Jupiter and 5.5 hours to reach Pluto. These figures demonstrate how important it is to establish that it is the actual position of the object it is gravitating toward, not toward the light delayed image (applies to a static Gravitational field such as the Solar system).
Maybe this post question was badly worded, some might say an absurdity, but it works for me.
Incidentally, I agree gravity is not shown to propagate at instantaneous speed and, well, I may have done a little quote mining.
Ah, I see... well, tell me... Considering that you are talking about trajectory mechanics, what systems are used to calculate them? Newtons? Keplers? You mentioned the use of the Oberth effect... Are these accurate enough? What perterbations are involved?
Are orbital maneuvers employed, at all?
How do we aim for LaGrange points?

9. ## Re: What speed of gravity is used in space probe trajectory calc. in a static field?

Originally Posted by Neverfly
Ah, I see... well, tell me... Considering that you are talking about trajectory mechanics, what systems are used to calculate them? Newtons? Keplers? You mentioned the use of the Oberth effect... Are these accurate enough? What perterbations are involved?
Are orbital maneuvers employed, at all?
How do we aim for LaGrange points?
my brain just popped!!

10. ## Re: What speed of gravity is used in space probe trajectory calc. in a static field?

Originally Posted by spartan45
my brain just popped!!
Ok, well, aside for if we aim a probe toward the Sun, in which pretty precise calculations are involved and theory of Relativity is used, Newtonian mechanics are elsewhere used in the solar system. Why?
Because it's precise enough. There are perturbations and orbital maneuvers are used. Aiming for La Grange points is the same principle as above: Newtonian physics is precise enough considering the massive area of space where LaGrangian cancellation of gravity is in effect.
The short of it is, at these scales and phenomenally massive distances, sizes and speeds, using space probes as evidence of the importance of aiming for the actual position or light image is an extremely imprecise argument.

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