On 10/11/2011 09:38 PM, Warp wrote:
> Invisible<voi### [at] devnull>  wrote:
>> 1. If the Earth is hollow, wouldn't that *significantly* alter its mass?
>
>    The only claim that I have ever seen to explain that is that the "inner
> sun" accounts for the missing mass.

I meant more that a hollow Earth couldn't withstand the forces that our 
Earth withstands, but if Earth was hollow the forces would be smaller.

>> 2. A hollow Earth-sized structure with a vacuum inside would surely
>> collapse pretty quickly, no matter what you made it of, but if it was
>> filled with an atmosphere of some kind, at absurd pressure, it might
>> work. (Isn't that what a bubble is, after all?)
>
>    No amount of atmosphere would be able to hold the shell under Earth's
> gravity. It would just crumble and collapse. The only reason why the
> Earth does not collapse is because it's fully made of rock and metal.
> (No double entendre intended.)

I'm sure if you had the right gas at insane enough pressure it could 
work. Whether anything could *live* in that is another matter entirely...

>> 3. It's not quite correct to say that "no force" would hold people's
>> feet onto the anti-ground. There is one: centrifugal force due to the
>> Earth's rotation. Sure, it's a force several orders of magnitude too
>> small to make the slightest shred of difference, but it's real.
>
>    Even at the equator it's over 300 times weaker than g.

Quite.

(And I note that the Earth isn't perfectly spherical, so not only is 
centrifugal force strongest at the equator, but gravity is weakest, 
since you're slightly further away from the center of Earth.)

>    Anyways, the majority of these hollow Earth theories claim that there's
> an "inner sun" at the center of the Earth (which explains how the interior
> surface of the Earth is habitable). The gravity of that sun would be strong
> enough to pull anything on the inner surface of the Earth, which would then
> fall into this 'sun'.

Uh, yeah, a "star" is where a ball of gas is so massive that it 
collapses under its own gravity. Pro-tip: There's a reason you don't 
usually find objects near to a star. They tend to fall into it.

>> 4. A *star* inside Earth? Um... do these people know how big stars
>> actually are? If you hollowed out the Sun, you could fit one million
>> copies of the Earth inside it - and the Sun isn't a particularly large star!
>
>    A body of eg. the size of the inner core of the Earth could indeed not
> have enough mass to cause a fusion reaction, and hence it would not be a
> star at all. It would be just a small moon, a lifeless rock. (It could not
> be made of gas because it would be too small to maintain its shape.)
>
>    The only "star" that small would be a neutron star, which in turn would
> be *so* massive that the Earth would collapse into it in a microsecond.
> (Not to talk that a neutron star does not have a fusion reaction going on
> either. Also the radiation that a neutron star emits would kill all life
> on earth in another microsecond.)

I'm being frustrated in my attempts to find any data for star sizes. But 
it seems that most typical types of star are *vastly* bigger than our 
tiny little planet. A neutron star is indeed the only kind I can find 
reference to which is anywhere near small enough to fit inside Earth.

There's a couple of problems with that. The first problem is the insane 
amount of gravity it generates. Any plausible configuration of matter 
having a gravitational field strong enough to crush it to the point 
where fusion, or even significant heating occurs, is going to have a 
similar problem.

Then there's the huge amount of X-ray radiation that neutron stars 
apparently produce.

Then of course, neutron stars are apparently the *final* phase of the 
development of a star, which asks the question of how the **** a star 
ends up being in the center of a planet in the first place. That is an 
entire *other* debate.

Even if we assume that by some freakishly unlikely miracle the Earth has 
a star inside it, there's another problem. The Sun emits about 4 * 10^26 
W of radiation, in all directions. Of that vast amount of energy, a 
vanishingly tiny fraction of it illuminates Earth.

(According to Wolfram Alpha, the Earth orbits the Sun at roughly 93 
million miles. That gives us a sphere with a surface area of roughly 3 * 
10^23 square miles to illuminate. The Earth has a radius of about 4,000 
miles, and so covers an area of this sphere about 50 million square 
miles. 3 * 10^23 / 50 million = 1.6 * 10^-19. So the Earth receives 
roughly 10^-19 of the Sun's radiation.)

Now consider a star enclosed by a planet. That planet would receive not 
10^-19 of the star's radiation, but 10^0 of it - i.e., 100%. Said 
interior might get... a tad warm.

(Think about it. How tiny is the flame in an oven? And yet, ovens get 
pretty damned hot!)

It's almost fun to try to make the math work, just to see how many 
different ways that this idea of a hollow planet is absurdly implausible...

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