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Fogman
Econo-class Iconoclast
Joined: Jun 20, 2005
Age: 42
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Location: Twin Cities, Minnesota
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 Posted: Fri Jun 05, 2009 7:48 am Post subject: |
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| lau wrote: | | The other extreme... should you hollow out the entire of the interior of a planet, leaving just a thin (just inches or a few miles, say) shell - again, the entire interior would have zero gravity. You could not "stand on the interior surface", say. Although if would appear that you are close to lots of matter, the net effect of the rest of the mass "over your head" exactly cancels the attraction of the closer parts. |
Furthermore, if Earth was actually hollow, the Coriolis effect of the spinning planet still wouldn't be sufficient enough to provide the amount of gravity that we have due to the lack of mass. Earths' atmosphere would be less than that of Mars, as the lack of sufficient gravity would cause any gasses present on the surface to dissipate into outer space, and the surface would be just as barren and devoid of life as the moon in practically all areas of the planet aside from perhaps the equatorial regions where the Coriolis effect would be greatest. Then again, nothing would stop the osmotic flow of atmosphere from the equatorial region to the polar regions unless there were parallel mountain ranges running along both sides of the equator. -- So much for the hollow earth theory. 
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DNForrest
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Joined: Jan 19, 2008
Age: 25
Posts: 892
Location: Oregon
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| Posted: Fri Jun 05, 2009 12:16 pm Post subject: |
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| Quote: | | Water vapor also begins forming in soft tissue causing the body to swell, perhaps to as much as twice its normal volume if not constrained by a suit |
Ahh, okay, thanks for the link, that's the part I was curious about, and assumed would cause the skin to rupture in areas (we had some disturbing discussions involving this during my Thin Films class about what would happen if you were to put a squirrel/rat dog inside of one of the vacuum chambers used for plasma etching). |
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ruveyn
Phoenix
Joined: Sep 22, 2008
Age: 73
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Location: New Jersey
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| Posted: Fri Jun 05, 2009 1:05 pm Post subject: Re: How it would feel at the Earth's core. Like outer space? |
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| Fort56 wrote: | | If someone could somehow go to the Earth's core, would it feel like they were in outer space? With zero gravity. |
The Earth's care is very hot. It would not feel like outer space at all. It turns out that if the mass of the earth were confined (which it is not) to a thin outer shell it would be zero g inside. The same is true of spherical electric charge with the charge confined to a thin outer shell.
In a spherically symmetric body with symmetric distribution of the mass the gravitational force at dead center is 0.
See
http://galileo.phys.virginia.edu/classes/152.mf1i.spring02/GravField.htm
ruveyn |
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DeaconBlues
They call Alabama the Crimson Tide - call me...
Joined: Apr 22, 2007
Posts: 2263
Location: Earth, mostly
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| Posted: Fri Jun 05, 2009 1:36 pm Post subject: |
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| lau wrote: | | The other extreme... should you hollow out the entire of the interior of a planet, leaving just a thin (just inches or a few miles, say) shell - again, the entire interior would have zero gravity. You could not "stand on the interior surface", say. Although if would appear that you are close to lots of matter, the net effect of the rest of the mass "over your head" exactly cancels the attraction of the closer parts. |
One of the basic problems with the concept of the Dyson shell, unless one were to find some way of generating artificial gravity on the interior of the shell (but then, I suppose if one were able to overcome the myriad other engineering problems inherent in the construction of such a shell, artificial gravity generators would be a trivial difficulty). On the other hand, I for one would prefer to put the a/g generators on the interior of the shell, along with the solar accumulators which would be the presumptive reason for constructing such a shell in the first place, so that I could live on the shell's exterior, and still see the stars...
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pakled
"Bless his Heart"
Joined: Nov 13, 2007
Age: 52
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| Posted: Fri Jun 05, 2009 4:19 pm Post subject: |
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once again, I will demonstrate I never took physics in high school (or college... , but; gravity is a function of mass. Gravity radiates in all directions equally (as far as I know)
Shouldn't all that mass around you generate gravity as well? I think it would be 1g. I don't think you fall off Mt. Everest any more slowly than at ground level, so where you are in relation to the surface, it should be the same amount of gravity. It should be 1g 7 miles up, or thousands of miles down. There's nothing to 'cancel' gravity.
Feel free to prove me wrong. |
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CloudWalker
Deinonychus
Joined: Mar 27, 2009
Posts: 314
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| Posted: Fri Jun 05, 2009 7:42 pm Post subject: |
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I think I missed a point earlier, does hollow = vacuum?
Suppose we had the technology to construct a container to withstand the pressure at the center of Earth and put it there. Also assume that we had good enough cooling technology so that the heat there won't kill you instantly. Then a person there would be like skating in the bottom of a bowl. The spacetime distortion is a lot less than a black hole, so with enough force you could still break free but otherwise you would be in a constant free fall with changing vector.
Now, if the container is vacuum, there won't be any medium to provide the counter force for you to feel the constant gravity change. So it's quite like outer space.
If there is air then, the denser it is, the easier it is to feel the shifting of gravity. I don't know the level of gravity there, but it's definitely a lot less than that on the surface. So that will determine the likeness to the situation in outer space. Personally, I think it's at least enough to give you dizziness and constant headache. |
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DeaconBlues
They call Alabama the Crimson Tide - call me...
Joined: Apr 22, 2007
Posts: 2263
Location: Earth, mostly
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| Posted: Fri Jun 05, 2009 8:51 pm Post subject: |
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| pakled wrote: | once again, I will demonstrate I never took physics in high school (or college..., but; gravity is a function of mass. Gravity radiates in all directions equally (as far as I know)
Shouldn't all that mass around you generate gravity as well? I think it would be 1g. I don't think you fall off Mt. Everest any more slowly than at ground level, so where you are in relation to the surface, it should be the same amount of gravity. It should be 1g 7 miles up, or thousands of miles down. There's nothing to 'cancel' gravity.
Feel free to prove me wrong. |
Strictly speaking, gravity does not "radiate"; thus far, at least, it would appear to be a field effect, caused by the effect of mass on space. The attraction between two objects is directly proportional to the product of their masses, and inversely proportional to the distance between them - thus, in fact, you would fall more slowly at the top of Mt. Everest than at sea level. However, the difference is sufficiently small that you would need a laser rangefinder to determine that it exists at all. (Similarly, as per general relativity, time passes more slowly on Earth's surface than in orbit, but only atomic-decay chronometers are capable of measuring the difference.)
Our current understanding of gravity is why we don't have artificial-gravity generators; you would need to find some way to generate artificial mass first. (Of course, acceleration is identical to gravity, which is why Niven's Ringworld worked at all - its spin provided sufficient centripetal acceleration to simulate .95g, or about one Pak gravity. If you built a constant-boost spaceship capable of thrusting at 1 gravity all the way through its trip [assuming turnover at midpoint so you decelerate relative to your destination], you would have a nice 1g environment on board. You'd also be about a week away from Pluto, and a few hours from Mars.)
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I am RICHARD, Chief Warlock of the Brothers of Darkness, Lord of the Thirteen Hells, Master of the Bones, Emperor of the Black, Lord of the Undead, and Mayor of a little village up the coast. Very scenic during springtime. |
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lau
Really nice person to know. :)
Joined: Jun 18, 2006
Age: 60
Posts: 9483
Location: Somerset UK
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| Posted: Sat Jun 06, 2009 6:29 am Post subject: Re: How it would feel at the Earth's core. Like outer space? |
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| ruveyn wrote: | | Fort56 wrote: | | If someone could somehow go to the Earth's core, would it feel like they were in outer space? With zero gravity. |
The Earth's care is very hot. It would not feel like outer space at all. It turns out that if the mass of the earth were confined (which it is not) to a thin outer shell it would be zero g inside. The same is true of spherical electric charge with the charge confined to a thin outer shell.
In a spherically symmetric body with symmetric distribution of the mass the gravitational force at dead center is 0.
See
http://galileo.phys.virginia.edu/classes/152.mf1i.spring02/GravField.htm
ruveyn |
Nice link. In particular, see the bit labelled "Field Inside a Spherical Shell".
Note that the result implies that...
In a spherically symmetric body with symmetric distribution of the mass, and any size spherical hole with the same cetre point, the gravitational force at all points within that spherical hole is 0.
It matter not whether the shell is spinning, whether there is an atmosphere, or whether there is an "r" in the month. 
========
And yes, as DeaconBlues points out, Larry Niven's Ringworld does not employ gravity, but works purely because it's spinning. It's a pity it has to be built of form of matter (scrith) that doesn't seem to exist.
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kalantir
Phoenix
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Age: 21
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Location: Redmond, WA USA
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| Posted: Sat Jun 06, 2009 9:55 am Post subject: |
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If it was possible to be in the center of the earth without the heat killing you... Then gravity would be pulling you in all directions at once... which would almost certainly kill you. If you had some magical way to live... it would still be a highly unpleasant experience to be pulled in all directions at once. And then in the case that you somehow had immunity to the unpleasantness of that... Then sure... I suppose that equal gravity pulling you in every direction would create the illusion of a lack of gravity which would be like being in space.
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b9
whatever..
Joined: Aug 15, 2008
Posts: 2072
Location: australia
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| Posted: Sat Jun 06, 2009 11:37 am Post subject: |
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| kalantir wrote: | | ... Then gravity would be pulling you in all directions at once... which would almost certainly kill you. |
not in my imagination.
the earth has a mass that is the same whether you stand on it's surface or whether you "float" at it's core.
when you stand on it's surface, all the mass is "beneath" you.
that is to say that all vectors of attraction are beneath (or lower) than the horizontal plane bisecting your vertical axis. so with the canceling of the bilateral forces that are equal, the residual vector is straight down at 1g.
at the center, there is an equal amount of mass surrounding you in all directions. there is no axis. you will be pulled equally in all directions.
i would think you would have .5 g of gravity acting upon you from every angle on all three axes.
that equates to zero at your core, and an infinitesimally small (well a very tiny) deviation from zero at your extremities would be experienced.
let's take an example where your belly button is at the exact center point of all the mass in the world. your head is 3 feet away from it and your toes are also 3 feet away.
your head is only 3 feet further away from the null gravitational force exerted on the the belly button and the toes are 3 feet away on the other side.
your head is also being pulled by the gravitational force that is attracting your toes albeit 6 feet further away. your toes are also attracted to the gravitational force that attracts your head albeit 6 feet further away.
if you add the difference from zero in gravity on the head to the difference from zero in the gravity on the foot based upon their positions relative to the size of the earth, then it is apparent that the discrepancy is not significant.
| lau wrote: |
The other extreme... should you hollow out the entire of the interior of a planet, leaving just a thin (just inches or a few miles, say) shell - again, the entire interior would have zero gravity. |
if you hollowed out a planet to a shell of a few inches thick, then it's mass would
be so small as to not have any humanly appreciable gravitational effect on objects.
yes the earth would still weigh more than say a large asteroid, but the weight would be distributed over such a large volume, that the 1" thick crust on the other side of the world (8000 miles away) would have indiscernible effects on my "weight".
on a large asteroid like ceres i would weigh about 2.25 kg. that is also due to the fact that all it's mass is within 1000 km of me.
if ceres was melted and blown into a gigantic bubble the size of the earth, then it's gravitational effect on me would be reduced drastically.
i think if you could hollow out a perfect billiard ball the size of the earth to leave a shell a few inches thick, you could "tip toe" into orbit.
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as to the idea of there being zero gravity at all locii within the hollowed volume...
imagine not a hollow sphere at the center of the earth, but an imaginary cylindrical tube that is somehow bored through the entire planet, and it's other end is on the surface of the earth on the opposite side of the world.
imagine there is a short cylindrical elevator style room that fits perfectly into this tube. there is no possibility of collision with the wall of the tube because there is a magnetic rail that laterally repels the elevator.
so if the elevator is dropped into this tube, then it will accelerate downward in free fall. if the tube is not depressurized, then the elevator will reach a terminal velocity (which yields a valid result but is boringly slow given the distance), so let's say the tube is depressurized to the level of a vacuum.
the elevator accelerates down the tube and gets faster and faster. but as it gets deeper, the acceleration is reduced steadily and as the elevator passes the core, there is no pull but it is full of momentum, and it then is suddenly hurtling "skyward" as it ascends the tube away from the core to the surface on the other side of the world.
it will not make it all the way there, and it will fall back to the center and perform a diminishing cycle of oscillations until it reaches equilibrium in the locus of zero gravity. if it is a total vacuum this may take eons.
so i do not believe that there is zero gravity in a magical region that satisfies the requirement of being at the origin while being voluminous.
whatever. i thought i would talk from my surmised idea rather than parrot out research i have come across. |
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pakled
"Bless his Heart"
Joined: Nov 13, 2007
Age: 52
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| Posted: Sat Jun 06, 2009 7:05 pm Post subject: |
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all of these seem to postulate a lot of mass dissapearing from inside the earth. If you were somehow in the center of the Earth, you'd be surrounded by more mass than you can handle...
If you remove mass, you remove some of the gravity as well (not to mention the oceans, the atmosphere, and everyone else...might make it hard to get a date on Saturday night...
What I meant was that the mass is a constant. where you are is gravity as a whole of the planet, and that it would be the same at the center as on the outside. |
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lau
Really nice person to know. :)
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Age: 60
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Location: Somerset UK
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| Posted: Sat Jun 06, 2009 7:42 pm Post subject: |
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The gravitational force on any and every object within a spherically symmetric shell of matter is zero.
It makes no difference what the mass or size of the shell is.
If it were a sphere of neutronium, a light year thick, with a central hole big enough to contain the solar system, its gravitational effect on all objects anywhere within the shell would be zero.
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"Striking up conversations with strangers is an autistic person's version of extreme sports." Kamran Nazeer |
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pakled
"Bless his Heart"
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Age: 52
Posts: 6736
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| Posted: Sun Jun 07, 2009 7:01 pm Post subject: |
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| well, ok then... |
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EtotheC
Blue Jay
Joined: Jun 24, 2009
Posts: 89
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| Posted: Wed Jun 24, 2009 3:20 pm Post subject: |
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| Cloud Strife wrote: | | Hardly similar, you would probably be crushed by the pressure there. |
This guy knows, he's been to the core of a planet before... |
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Tollorin
Sea Gull
Joined: Jun 15, 2009
Age: 27
Posts: 240
Location: Sherbrooke,Québec, Canada
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| Posted: Wed Jun 24, 2009 9:54 pm Post subject: |
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| lau wrote: | The gravitational force on any and every object within a spherically symmetric shell of matter is zero.
It makes no difference what the mass or size of the shell is.
If it were a sphere of neutronium, a light year thick, with a central hole big enough to contain the solar system, its gravitational effect on all objects anywhere within the shell would be zero. |
Such a huge object will basicaly be a black hole.
Imagine that... people continuing their everyday life in the center of the biggest and most massive black hole of the universe.
Kinda cool...  |
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