Hawking's center of mass ?

[Spyman]

Yesterday I read a book, in my language, called "COSMOS a short history" with the original title

"A brief history of time" by Stephen W. Hawking 1988.

The book is mostly a history of the progress of science, with parts of self-biography and some cosmological ideas of him for the general puplic.

 

I got stuck already on page 17 where Hawking implies there is a center of mass of the Universe.

(At least in a infinite Newtonian Steady State Universe.)

 

I don't intend to discuss if the Universe has a center of mass or not, just what Hawking implies or not.

 

What I want to know if this prof, (math ?), exists or if Hawking is just over-simplifying in a way such it favours his idea of a masscenter ?

(If it exists, how it works and if it is commonly accepted or not ?)

 

If someone has the book in English it would be nice to have this quoted so I could read that part with Hawkning's own words.

 

I will here try to translate the part back to English:

"If the numbers of stars was infinite and they where more or less evently distributed in an infinite space, wouldn't the universe collaps since the stars shouldn't have any center to be pulled against, according to Newton. This way of thinking is an example of errors of the mind when dealing with the infinite. The correct way to deal with this cind of problem - which people realized later - is to start with a finite solution and interpret how the situation changes when adding on more and more evently distributed stars outside this area. We can add how many stars we want and they will still collapse to one single body."

 

For me it somehow sounds backwards to prove the Universe has a center of mass starting with the assumption that the Universe has a center of mass.

On the other hand it's not likely for someone like Hawking to go puplic with such a huge general/basic flaw.

[swansont]

If there is a center of mass, I don't think you could measure it, since (from my understanding) we can only see the observable universe, by definition. Someone ten billion light years away would see a different observable universe.

[Spyman]

Yes, but Hawking implies that even if their view is very alike our and someone at their horizon also has a view alike ours and so on to infinity, if matter is infinite and evently distributed in an infinite space - it still would collapse to one single body.

 

I want to know why Hawking assumes this because I am stuck in the "mindtrap" he claims Newton was in.

[Ophiolite]

If someone has the book in English it would be nice to have this quoted so I could read that part with Hawkning's own words.

.

I have it somewhere. If I can put my hands on it I'll post the original of that passage for you.

[BlackHole]

According to classical gravity, gravitation in the center of the earth should be zero. This means that if we could tunnel a cave to the center of the earth, there should be no gravitation there.

 

The question is whether gravity depends only on mass & distance or also on density & pressure.

 

See gravitoelectromagnetism.

 

PS: Pressures and temperatures increase with depth inside the Earth, reaching maximum values of 364 gigapascals (GPa; 3,640,000 atmospheres) and about 6,000 kelvins at the centre. The interior temperatures are high enough to partially melt a small fraction of the crust and mantle and to completely melt the outer core. Most of the interior, however, including the inner core, is at a temperature below the melting point.

[mmalluck]

Hawking's center of mass would be pretty low right? I mean he never leaves his wheel-chair and in a seated position, his center of mass would be lower than that of a standing individual.

 

Har'dee har har.....

 

That's the first thing I thought of when I read the topic.

[Ophiolite]

BlackHole, I believe Spyman is looking for information relating to the Centre of Mass for the Universe, not the Earth.

[mmalluck]

if the system was rotating, couldn't rotational interia keep the system from collapsing in on itself?

[Spyman]

I have it somewhere. If I can put my hands on it I'll post the original of that passage for you.

I would really appreciate it !

 

According to classical gravity[/i'], gravitation in the center of the earth should be zero. This means that if we could tunnel a cave to the center of the earth, there should be no gravitation there.

I am talking about a Hawkings view of a Newtonian Universe, NOT A PLANET.

 

Hawking's center of mass would be pretty low right? I mean he never leaves his wheel-chair and in a seated position' date=' his center of mass would be lower than that of a standing individual.

 

Har'dee har har.....

 

That's the first thing I thought of when I read the topic.[/quote']LOL It's not easy to come up with a good & short description for the topic.

 

BlackHole, I believe Spyman is looking for information relating to the Centre of Mass for the Universe, not the Earth.

Correct !

 

if the system was rotating, couldn't rotational interia keep the system from collapsing in on itself?

Let's start with explaining why it would collapse in the first place, should we...

 

Actually BlackHole managed to give me some inspiration with his post:

I tried to solve this by simply growing a 1D Universe and got stuck with Newton's idea. But if I insted take a 3D spherical Universe, like a planet with a density, then the gravity at the distance X from the center will stay constant when the sphere grows, if the density remains the same, thus Hawking's are correct.