Gravity, Celestia, and the universe

[jlf]

I just spent a bit of time reading a thread about a flight mode using a simplistic gravity model, which I agree with fridger, you just cant do it.

If I remember my physics classes, and I was a history major, so dont get mad if I get something wrong, isn't the following true:

Gravity, as experienced by an object in space, and on earth for that matter, is a combination of forces from every other object in the universe, however slight it may be.

I mean, our solar system is in orbit around the galactic core due to the gravity well of the mass in the galactic core. Our galaxy is moving due to the gravitational interaction with other galaxies.

And a simplistic version of gravity for a fly by of jupiter is rather inappropriate considering that is probably one of the densest gravitational fields in our solar system.

[tony873004]

I just spent a bit of time reading a thread about a flight mode using a simplistic gravity model, which I agree with fridger, you just cant do it.

If I remember my physics classes, and I was a history major, so dont get mad if I get something wrong, isn't the following true:

Gravity, as experienced by an object in space, and on earth for that matter, is a combination of forces from every other object in the universe, however slight it may be.

I mean, our solar system is in orbit around the galactic core due to the gravity well of the mass in the galactic core. Our galaxy is moving due to the gravitational interaction with other galaxies.

And a simplistic version of gravity for a fly by of jupiter is rather inappropriate considering that is probably one of the densest gravitational fields in our solar system.

Jupiter's massive gravity field makes it perfect for slingshots to the outer planets and beyond.

Yes, every object in the universe gravitationally pulls every other object in the universe. There's a saying "pick a flower and you've moved the farthest star". The g field approaches, but never reaches 0.

[selden]

Don't forget that part of the complexity of "slingshot" trajectories that go past the giant planets is that those planets are highly oblate, and so are their gravitational fields. They don't exert exactly the same force in all directions. This is one of the reasons that Saturn's rings are so thin.

Let's just say that the calculations to describe these effects are, umm, highly complex

[t00fri]

--The /infinite/ range of the gravitational force has been alluded to above,

--The non-spherical effects.

--Things really get complicated if the various sources for the resulting gravitational field, i.e. the massive bodies, are not very far separated! In some analogy to the resulting complexity of electric fields, when their expansion in form of terms due to a sum of independent electrical point charges ceases to hold...In the electrical case on then has to consider contributions from electrical dipoles, and higher multipoles to describe adequately the actual shape of the electrical field.

Back to gravitation:

So, planetary systems with lots of moons and oblate planets are really tough;-)

Bye Fridger

[Guest]

pick a flower and you've moved the farthest star". The g field approaches, but never reaches 0.

I'm not sure thats true. At some point it must dip low enough that its effect is below one quantum of energy.

[Matt McIrvin]

pick a flower and you've moved the farthest star". The g field approaches, but never reaches 0.

I'm not sure thats true. At some point it must dip low enough that its effect is below one quantum of energy.

I'd say the biggest caveat is that the gravitational effect of your picking that flower only propagates at the speed of light, so you haven't moved that star yet, and you'll move it only if it is within your future light-cone, which, because of dark energy and the resulting accelerated expansion of the universe, may not be the case!

[Seb]

propagates at the speed of light

But isnt the speed of Gravity a lot faster than the speed of light?

[selden]

No.

[Guest]

pick a flower and you've moved the farthest star". The g field approaches, but never reaches 0.

I'm not sure thats true. At some point it must dip low enough that its effect is below one quantum of energy.

I'd say the biggest caveat is that the gravitational effect of your picking that flower only propagates at the speed of light, so you haven't moved that star yet, and you'll move it only if it is within your future light-cone, which, because of dark energy and the resulting accelerated expansion of the universe, may not be the case!

Another caveat is :- "below one quantum " assumes that we have a theory of Quantum Gravity. This is still giving some 'little' (cue Fridger) problems and until then Einstein rules relatively OK.
(Relativity is a field theory not subject to problems of minute subdivisions)

[Seb]

http://www.metaresearch.org/cosmology/speed_of_gravity.asp
- "Conclusion: The speed of gravity is 2x1010 c"

Over my head, but interesting..

Comments?

[Matt McIrvin]

http://www.metaresearch.org/cosmology/speed_of_gravity.asp
- "Conclusion: The speed of gravity is 2x1010 c"

Over my head, but interesting..

Comments?

I've gotten into arguments with that guy more than once. He's wrong. He's picked a strawman definition of "the speed of gravity" that is not useful.

This page gives links to more information.

[RND]

Dont forget also that every object in space bends gravity according to its mass. For example mars going around the Sun is caused by two aspects (if im right).

1. The centrifugal force making the planet orbit the Sun,

2. Gravity. Imagine a huge rubber sheet laid out and hovering about the ground by legs. If you put a metal ball the size of a football in the centre of the rubber sheet, it would make quite a large bulge. Similarly if you put a standard football made of plastic in the rubber sheet it would make somewhat less of a bulge. A similar thing happens with objects in space. But in this case its the Sun thats in the centre of the "rubber sheet" making the bulge and the planets each in turn make their own bulges while also going around the edge of the Suns bulge.

Hope you get what I was trying to convey...

[Ask H Larsen]

Dont forget also that every object in space bends gravity according to its mass. For example mars going around the Sun is caused by two aspects (if im right).

1. The centrifugal force making the planet orbit the Sun,

2. Gravity. (...)

The planet would be held in an elliptic orbit by gravity. Gravity IS the centripetal force which holds the planet in orbit. The centrifugal force, on the other hand, is a fictitious force which would be introduced to account for a non-inertial coordinate system in which the planet is stationary.

Other coordinates could be chosen such that there is no centrifugal force, but instead e.g. a coriolis force, which is also fictitious. It's just a compensation for non-inertial coordinate systems.

[Drake]

I just spent a bit of time reading a thread about a flight mode using a simplistic gravity model, which I agree with fridger, you just cant do it.

Not sure if this is off-topic, but the new release of Orbiter (which is free) allows you to simulate launch, orbits, and flybys and includes non-spherical gravity, though I am not sure how it is implemented. There is no better way to learn and understand orbital mechanics while having a huge amount of fun.

http://www.orbitersim.com

[Michael Kilderry]

No, it's not offtopic as this thread is about gravity, and it is appreciated when people tell the Celestia community about programs like this. So thanks.

Michael Kilderry

[Michael Kilderry]

No, it's not offtopic as this thread is about gravity, and it is appreciated when people tell the Celestia community about programs like this. So thanks.

Michael Kilderry

[Guest]

"pick a flower and you've moved the farthest star".

(or it will be moved in a few millions of years !!!)

[Michael Kilderry]

Trust me, it would take more than a few million years for something moving at the speed of light to move to the farthest star, more like billions of years.

Michael Kilderry