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Old 06-17-2018, 08:24 AM   #1
francisdrake
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Default Do we see into the future in ORBITER?

I have my problems with relativistic effects and loss of causality when travelling faster than light. Orbiter - to my knowledge - has a Newtonian setup. I assume the lightspeed here is infinitly high.

So if I look at a distant planet, e.g. Jupiter, I can see the position where he is right now, not where he has been about one hour ago? (no light speed lag)

Does this influence spacecrafts on long distance travels?
Because they are pulled towards gravity sources where they are located right now, not where they had been some minutes ago, according to their distance divided by the speed of light?
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Old 06-17-2018, 10:27 AM   #2
Thorsten
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You might want to sit down with a pen and some paper and work out how different the gravity vector to the current or to the lightspeed corrected position of a planet actually are.

Then of course you might consider the fact that gravity itself also propagates perturbations at lightspeed...
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Old 06-17-2018, 12:13 PM   #3
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Does this influence spacecrafts on long distance travels?
Because they are pulled towards gravity sources where they are located right now, not where they had been some minutes ago, according to their distance divided by the speed of light?
Yes, the speed of gravity is finite. And so you might think that the direction of the gravitational force should not act towards the instantaneous position of the gravitating body. But this isn’t the case.

In analogy with a relativistic treatment of electromagnetism, there are counter-balancing force terms that offset the aberration induced by gravity’s finite speed. The net result is that gravity always acts towards the gravitating body’s instantaneous position rather than its time-retarded position - so long as the spacecraft moves with near constant velocity.

Of course, a spacecraft will accelerate slowly (in relativistic scales) and so there is a very slight aberration effect. This effect is very small and no larger than order c^-2 (~ 10^-17). Unless you are doing very precise astrometric work, you can ignore these corrections.

see for example https://arxiv.org/pdf/gr-qc/9909087.pdf

Last edited by MontBlanc2012; 06-17-2018 at 02:46 PM.
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Old 06-17-2018, 04:15 PM   #4
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Thanks for pointing me to the paper of "Aberration and the Speed of Gravity". I will read it and try to understand it.

That there shall exist forces offsetting the aberration of gravity sounds miraculous. Also one might argue, if gravitation is really an electromagnetic force? (Even if it behaves like one.)

Obviously that topic is more complex than I thought. I am aware that the difference between instantaneous position and time-retarded position is small, while the spacecraft is far away, and can be neglected if the spacecraft is close.
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Old 06-18-2018, 01:14 AM   #5
MontBlanc2012
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Yes, it does seem miraculous that the offsetting forces do appear in the equations of motion. Iím sure there is a clear explanation for this offset and I will rummage through the internet in search of one. If I am successful, I will post.
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Old 07-10-2018, 07:39 PM   #6
Col_Klonk
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Originally Posted by MontBlanc2012 View Post
 Yes, the speed of gravity is finite.
Do we know that as absolute (lightspeed was thought to be finite), as we has very little understanding how gravity works at present, besides the obvious attraction. ?
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Old 07-10-2018, 08:51 PM   #7
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Originally Posted by Col_Klonk View Post
 lightspeed was thought to be finite

It is still finite.
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Old 07-10-2018, 09:13 PM   #8
Shifty
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Originally Posted by Col_Klonk View Post
 Do we know that as absolute (lightspeed was thought to be finite), as we has very little understanding how gravity works at present, besides the obvious attraction. ?
On the contrary, we have a great deal of understanding of how gravity works. Certainly there are still open questions, but gravity is very well defined, its mechanisms are largely explained, and its phenomena can be predicted with a high degree of accuracy in almost every situation.
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Old 07-13-2018, 07:39 PM   #9
Col_Klonk
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 On the contrary, we have a great deal of understanding of how gravity works. Certainly there are still open questions, but gravity is very well defined, its mechanisms are largely explained, and its phenomena can be predicted with a high degree of accuracy in almost every situation.
Yet, GUT is so far away
We can predict, but we still don't know why/how !

Lightspeed is not constant
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Old 07-13-2018, 07:49 PM   #10
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Originally Posted by Col_Klonk View Post
 Lightspeed is not constant

Sure, but vacuum speed of light is still a universal constant. And that is the velocity that really matters. Also propagation speed of light is maybe only a few centimeters per second, but its still finite.



Vacuum speed of light MIGHT be no constant. But thats a big MIGHT, since a lot of time as passed without finding the predicted phenomena.
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Old 07-20-2018, 10:30 PM   #11
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