Why Doesn't the Moon Fall When Orbiting Earth?

[russ_watters]

Your logic is working well, but your understanding of the Big Bang is leading you astray.

The Big Bang was not an explosion and since it created the universe, it didn't happen at a single point in the universe, it happened at every point. Thus, the universe does not have a center. And, since the expansion is uniform, the universe looks extremely consistent no matter which direction we look.

 

[russ_watters]

I'm not aware of any proof of black holes at the center of the Milky Way. The center of the Milky Way would just be the center of rotation.

Not really relevant to the conversation, but... http://en.wikipedia.org/wiki/Milky_Way_Galaxy

The galactic center harbours a compact object of very large mass, strongly suspected to be a supermassive black hole. Most galaxies are believed to have a supermassive black hole at their center.

http://www.space.com/scienceastronomy/051102_black_hole.html

Sgr A* is located at the center of the Milky Way Galaxy, about 26,000 light-years away. It is estimated to have a mass equal to about 4 million Suns. Such a high concentration of matter in such a small space places tight constraints on what the object could be if not a black hole.

An alternative possibility, though one not likely in the view of most theorists, is that the object might be a cluster of millions of collapsed dead stars, called neutron stars. If that were the case, the stars would only survive for about 20,000 years. At the end of that time, they would either collapse into black holes themselves or evaporate away into space.

The more likely alternative that Sgr A* is a supermassive black hole like those found at the centers of some other galaxies. Some of those black holes are more conspicuous, declaring their presence with highly visible streams of superheated matter, called particle jets.

While particle jets have been detected near Sgr A*, they have tended to be fainter and much shorter than those found around other supermassive black holes.

In any case, it doesn't matter what is there, but to reiterate what you said, the objects in the galaxy orbit around the center of mass of the galaxy - it is not really correct to say that they orbit that object and it leads to confusion when talking about systems where there isn't any object occupying the center of mass.

 

[russ_watters]

Btw, does that black hole or the whole galaxy(since everything in the galaxy is orbiting it) orbit something?

Though not universal, as Jeff pointed out, there are galaxies that orbit each other - both the Milky Way and Andromeda have quite a number (dozens?) of satellite galaxies.

 

[rcgldr]

Which raises another question, does universe even have a center?

Assuming the universe is finite, it has a center of mass, but we can't observe enough of the universe to determine where this center is.

 

[rcgldr]

not aware of any proof of black holes at the center of the Milky Way

The more likely alternative that Sgr A* is a supermassive black hole like those found at the centers of some other galaxies.

I was too conservative in my post, as it's believed that there is a black hole at the center, but no real way to prove this. As posted, black hole or not, the Milky Way has a center of mass, but I don't know how closely this corrresponds to the center of rotation, since what you have is spiraling arms projecting out from the center of the Milky Way.

 

[russ_watters]

Assuming the universe is finite, it has a center of mass, but we can't observe enough of the universe to determine where this center is.

That isn't necessarily true (the typical analogy is that the Earth's surface has no center) and isn't what cosmologist theorize the universe looks like.

 

[rcgldr]

Assuming the universe is finite, it has a center of mass.

That isn't necessarily true (the typical analogy is that the Earth's surface has no center) and isn't what cosmologist theorize the universe looks like.

The Earth's surface has a "center", just need to go up one dimension (from 2d to 3d) in order to find it. So if the universe is a 3d surface on a 4d bubble, there's probably a center, but we can't observe it since we're stuck in 3d mode.

 

[russ_watters]

The Earth's surface has a "center", just need to go up one dimension (from 2d to 3d) in order to find it. So if the universe is a 3d surface on a 4d bubble, there's probably a center, but we can't observe it since we're stuck in 3d mode.

If you need to go up a dimension to find the center, then the center isn't on the surface, is it? The surface has no center on it. I see what you are saying, but you are changing the question: the question is does the universe have a spatial (3d) center (or does the Earth have a center on its surface). And the answer is no. Whether there are extra dimensions in which a center can be found is a separate question.

Here is a very slick explanation/demonstration: http://www.exploratorium.edu/hubble/tools/center.html

 

[superweirdo]

no no no, the center still might be in our 3d but we just need a bigger look to find it. It is we are trapped in a glassic ball where we look, we find stars so there is no way to tell if there is a center b/c we are trapped, but a person outside that ball can easily tell.

 

[ZapperZ]

no no no, the center still might be in our 3d but we just need a bigger look to find it. It is we are trapped in a glassic ball where we look, we find stars so there is no way to tell if there is a center b/c we are trapped, but a person outside that ball can easily tell.

Then what you are guessing (it is not even a hypothesis) is NOT the Big Bang, but something else that is not defined. The BB is not something you can simply use whichever way you wish.

I will again ask you to re-read the PF guideline regarding speculative posting. If you wish to work out your own theory without basing it on existing physics, please do so in the IR forum.

Now, how this thing went from a simple classical mechanics problem to a center of the universe and the BB is a mindboggler.

Zz.

 

[superweirdo]

thats what my physics teacher told me so I was citing him, but I guess since he only took masters into it and he tells us that he his more interested in philosophy than physics, I guess I crossed the line, I apolozige.