B Can someone please explain this paragraph from A Brief History Of Time

[Rishabh Narula]

This argument is an instance of the pitfalls that you can encounter in
talking about infinity. In an infinite universe, every point can be regarded as
the center, because every point has an infinite number of stars on each side
of it. The correct approach, it was realized only much later, is to consider the
finite situation, in which the stars all fall in on each other, and then to ask
how things change if one adds more stars roughly uniformly distributed
outside this region. According to Newton’s law, the extra stars would make
no difference at all to the original ones on average, so the stars would fall in
just as fast. We can add as many stars as we like, but they will still always
collapse in on themselves. We now know it is impossible to have an infinite
static model of the universe in which gravity is always attractive

what does it mean when you say the stars all fall in on each other?And what does the line uniform distribution of stars outside this region mean?and what does this line mean-

According to Newton’s law, the extra stars would make
no difference at all to the original ones on average, so the stars would fall in just as fast.

again fall in?what does fall in mean?
would really appreciate some help in understanding the meaning in simpler words or some help in visualizing this.

 

[Nugatory]

If we have a finite number of stars, their mutual gravitational attraction will pull them towards one another. Wait long enough and they will be pulled into a single clump; that's what Hawking means by "fall in".

One of the interesting and somewhat unobvious consequences of Newton's law is that if you are inside a hollow shell of matter the gravitational force on you from the shell is zero. Thus, a finite-sized group of stars will fall in on itself whether it is surrounded by empty space or inside an enormous hollow shell. Hawking's point is that all the other stars in the universe will act like that hollowshell; they won't stop the infalling.

 

[bksree]

if you are inside a hollow shell of matter the gravitational force on you from the shell is zero.

Only if you are at the centre. Otherwise the mass of the shell closest will increase larger force than that away from you, isn't it?

 

[Bandersnatch]

Only if you are at the centre. Otherwise the mass of the shell closest will increase larger force than that away from you, isn't it?

No, it works everywhere inside, as long as the matter in the shell is uniformly distributed. Conceptually, it works out that the mass closest to you attracts more strongly, but at the same time there is much more overall mass pulling from the other side and the two effects cancel out exactly.
The solution is called Shell theorem. Wikipedia has a very nice article about it.

 

[Rishabh Narula]

hey thankful to all the respones...i get the hollow shell theorem as pointed out by @Bandersnatch .But if the point is that the infalling won't stop as pointed by @Nugatory ...why do we see the stars as stationary and not infalling?

 

[Nugatory]

But if the point is that the infalling won't stop as pointed by @Nugatory ...why do we see the stars as stationary and not infalling?

Hawking is saying that the argument Newton made in his letter to a Bentley (that’s the “this argument” in the first sentence of the text you quoted) doesn’t work so an infinite static universe is not possible (the last sentence).

It turns out that our universe is not static - it’s expanding.

 

[DannoXYZ]

Rate of expansion has been revised with Hubble's Constant now being much smaller than what Hubble himself calculated. But is Hubble's Law still valid? That looking back in time at most distant objects show they are expanding at faster rate than closer ones? That would still support idea that universe will eventually contract in on itself.