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-   -   A scenario in which a planet's gravity pulls away from center of mass (http://www.physicsforums.com/showthread.php?t=581566)

modus Feb26-12 12:55 PM

A scenario in which a planet's gravity pulls away from center of mass
 
I'll start off by saying I have ZERO scientific background. I'm very interested in having a general working knowledge of physics and such, but I'm more concerned with creative applications for writing at the moment. Ever seen a movie with way too many scientific impossibilities? Yeah, I'd like to avoid more of those.

Anyway, is it at all possible for a human-habitable planet to have a pull upward, so that things would fall into the sky if left untethered, while other things such as land masses have just enough mass to be at equilibrium and float in the atmosphere? Say this planet was formerly like Earth, but something happened to cause this scenario and humans were advanced enough to adapt. Could perhaps a high-density "bubble" surround the planet, dense enough to offset the gravitational pull the Earth's mass provides?

So basically, what scenarios would make this possible? And what environmental consequences might make life impossible? If life is possible, what are other consequences within the environment?

Thanks for any help.

Gregoriorosso Feb26-12 04:48 PM

Re: A scenario in which a planet's gravity pulls away from center of mass
 
forget the mass in the sky: problems with atmosphere, which will disperse-unless the sphere outside can stop it. Unfortunatly the gravitational effect of a cave sphere on the objects inside the cavity were zero, although the gravitational effect of the earth were not changed.
but for creative writing you can relay on a chunk of some form of unknown matter like dark matter, captured by the gravitational field of our planet and falling inside, oscillating around the center, with a strange effect on the gravitational field only in the crust (to limit the geodynamics - earthquakes and volcanoes) and also with a sort of halo-field at 10 km over the sea level, to contain the atmosphere.
all happens in few hours, the atmosferic pressure becomes lower, like on a high mountain. the halo is not rigid and impenetrable, is like a liquid wall, you can pass through with your hand with little effort. depth and lights at your choice. don't know how to send away he dark matter
the moon (and all artificial satellites - no more communications ) will escape forever. the earth will experience a very little change in its orbit around the sun.
problems with the oceans huge tides, slow rivers- extreme meteorological changes
may be you survive on flying mountains - after millions of years a new ecology like in Pandora

modus Feb26-12 07:23 PM

Re: A scenario in which a planet's gravity pulls away from center of mass
 
So some form of foreign element en masse finds its way into our atmosphere and becomes trapped there, creating an encompassing layer. It changes gravity from the crust upward but still allows the atmosphere to be contained. That's what I'm getting from your answer, but I don't think I understand how the atmosphere and our precious oxygen could be contained. I want the gravitational pull to be upward, not just have a less-pressurized atmosphere. I guess this dark matter could have some sort of magic effect keeping the atmosphere in but still letting sunlight and heat through?

You also mentioned that objects within the cavity of a bubble would be unaffected by the pull of the bubble's mass. I don't understand that either, but I guess that's why I'm not a scientist.

kjamha Feb26-12 07:50 PM

Re: A scenario in which a planet's gravity pulls away from center of mass
 
Offsetting Earth's gravitational pull would create a gravity free environment, which would create many problems For one, convection patterns would probably cease. Heavier, denser air would not sink. The planet would roast at the equator and the North and South poles would be much colder. Oxygen from trees/plants would have a tough time circulating the globe.

How dense is your "bubble". Infrared light has a tough time making it out of a car window (which is one of the reasons the inside of a car gets so hot in the summer). Would infrared light from the planet be able to escape Earth? If not, forget about the frozen north and south poles - the planet would eventually roast.

Any movements on the surface (waves, volcanic activity, earthquakes) would send debris up to the bubble, which would then gain more mass, increasing its gravitational strength - pulling up more debris. Eventually, or over a long period of time, this debris might block out the sun.

the list goes on...

modus Feb26-12 08:38 PM

Re: A scenario in which a planet's gravity pulls away from center of mass
 
Ick, yeah, this is exactly what I thought would happen. Too many problems to explain, too distracting in terms of trying to tell a story. However, I still don't want a gravity-free environment. I'm looking to achieve a reverse-gravity environment while maintaining a life-sustaining atmosphere. Seems a little over my head though.

DaveC426913 Feb26-12 08:45 PM

Re: A scenario in which a planet's gravity pulls away from center of mass
 
Quote:

Quote by modus (Post 3786159)
Ick, yeah, this is exactly what I thought would happen. Too many problems to explain, too distracting in terms of trying to tell a story. However, I still don't want a gravity-free environment. I'm looking to achieve a reverse-gravity environment while maintaining a life-sustaining atmosphere. Seems a little over my head though.

The trouble is not finding a solution; it's that there's a conceptual problem. If everything not nailed down will have a tendency to fall up, that means humans will too. Humans sensorily define "up" as the opposite direction of pull. Which means, humans will not feel like they're standing on a planet, they'll feel like they're hanging from the bottom of a planet suspended over their heads.

modus Feb26-12 09:24 PM

Re: A scenario in which a planet's gravity pulls away from center of mass
 
That... is a good point. Up wouldn't be up anymore. I was applying my idea to objects but not people. Not sure what I was thinking.

DaveC426913 Feb26-12 10:06 PM

Re: A scenario in which a planet's gravity pulls away from center of mass
 
Quote:

Quote by modus (Post 3786239)
Not sure what I was thinking.

You were thinking of a story idea. It's OK to try things and see what happens. You tweak one variable in the world, then imagine the cascade of consequences. Sometimes they're mundane, but occasionally they're myriad.

modus Feb26-12 10:22 PM

Re: A scenario in which a planet's gravity pulls away from center of mass
 
Right. Well I've always had a small obsession with sky themes. There's some books and games that depict floating continents, and I wanted to try and demystify that sort of environment. Not easy. Thanks for your help, everyone.

DaveC426913 Feb26-12 10:26 PM

Re: A scenario in which a planet's gravity pulls away from center of mass
 
Well, they did sort of address it in Avatar.

The antigravity element was a naturally-occurring in the world. Masses that contained high concentrations would float, masses that did not would stay on the ground. Obviously, Unobtanium was not ingested or retained by their bodies in large quantities.


My sky fantasies (illustrations in my case) involve vast blimplike floating constructs or creatures, upon which people, flora and fauna cling and live out their lives.

modus Feb26-12 10:51 PM

Re: A scenario in which a planet's gravity pulls away from center of mass
 
I don't remember that from Avatar. Is that even plausible, though? An element with anti-gravity properties? That's a great way to do the floating continent thing. Seems like if I want to defy anything, I can just fabricate an element for it, haha. Very convenient, but I'm sure certain extremes don't sit well with science-oriented people.

Gregoriorosso Feb27-12 03:27 AM

Re: A scenario in which a planet's gravity pulls away from center of mass
 
I understood that you were looking for a planetary landscape with reverse gravitation everywhere on the surface, saving the planet integrity and the atmosphere, and you supposed that were effect of a surprising change of our actual terrestrial situation.
That's why I didn't propose an Unobtainium Pandora-like model.

" some form of foreign element en masse finds its way into our atmosphere and becomes trapped there"
not exactly, it sounds as an "ad hoc" solution.
My model:
the earth encounters a cloud of SM (strange matter). Interacting with condensed ordinary matter (OM), SM shields the inertial and the gravitational mass. This means that condensed SM-OM mixture has the g-effects unchanged but the g-field is not propagating outside. There is no disassembly of the planet, and that the orbit around the sun does't change, but the moon is lost. The interaction SM-OM is saturated: one SM particle for each one OM particle, inside the earth. Then you must introduce an halo effect near the surface of the planet
so the atmosphere is captured but the condensed matter when detached from the surface is OM free from gravity.

"I want the gravitational pull to be upward, not just have a less-pressurized atmosphere."
ok, a stone detached from the surface will go upward, due to the centrifugal force - you can calculate the acceleration, it is slow - the depressurization depend on my arbitrary limit at 10 km, what is outside is lost, just a secondary effect.
" I guess this dark matter could have some sort of magic effect keeping the atmosphere in but still letting sunlight and heat through "
Yes. But don't ask me what will happen. Extreme climate change, I suppose. Magnetic effects are unchanged, so no weakening of the shielding against solar wind etc
"You also mentioned that objects within the cavity of a bubble would be unaffected by the pull of the bubble's mass" it is a consequence of some Gauss integral: no Pellucidar is allowed by Newtonian physics. But Pellucidar is my dream world.

Gregoriorosso Feb27-12 03:39 AM

Re: A scenario in which a planet's gravity pulls away from center of mass
 
sorry, I forgot: if the inertia of the planet is shielded but electromagnetic forces are still acting, the motion of the planet will be altered by the interaction with magnetic fields of the sun. Slowing down the rotation, longer and longer days. Don't know how fast. Also the sun light pressure will act slowly on the planetary orbit, repulsing the planet to the outer space. This is simpler to calculate.


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