# Gravity and the vacuum

#### Eh

In general relatvity, as in Newtonian gravity, every massive body produces a gravitational field that extends throughout the universe. The strength of that field falls off the further away you go from the center of the body. But in the vast voids between galaxies, can the overall strength cancel out to zero, leaving perfectly flat space? Or is there likely to be curvature at every point in spacetime?

Also, about the curvature associated with gravitational fields. Since curvature is a measure of the amount of energy in any given point, does the curvature caused by this gravitational field (extending throughout the universe) also contain energy at each point?

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#### drag

Greetings EH !
Originally posted by Eh
In general relatvity, as in Newtonian gravity,
every massive body produces a gravitational field
that extends throughout the universe. The strength
of that field falls off the further away you go
from the center of the body. But in the vast voids
between galaxies, can the overall strength cancel
out to zero, leaving perfectly flat space? Or is
there likely to be curvature at every point in
spacetime?
Com'mon ! Perfection is unattainable...
Originally posted by Eh
Also, about the curvature associated with
gravitational fields. Since curvature is a
measure of the amount of energy in any
given point, does the curvature caused by
this gravitational field (extending
throughout the universe) also contain
energy at each point?
Well, particles have potential energy.
As for the Zero Point Field itself, there's
not supposed to be a prefered frame of reference
for different vacuum energies, I think.

Live long and prosper.

#### mathman

Answer to question 1 - points in space where gravitational forces happen to cancel out are all over. The easiest way to look at it is Newtonian. As you approach a mass, the gravitational force is toward it. Going away toward another body, it will be toward the other body. At some point there is a cross over.

Question 2 - I don't know.

#### Hurkyl

Staff Emeritus
Gold Member
mathman
Answer to question 1 - points in space where gravitational forces happen to cancel out are all over. The easiest way to look at it is Newtonian. As you approach a mass, the gravitational force is toward it. Going away toward another body, it will be toward the other body. At some point there is a cross over.
There are more than 2 bodies in the universe...

#### Tyger

Here is how to imagine Gravity

First, Gravity doesn't act between two or more bodies, the way other forces (interactions) do. It acts on the space around a massive body. Think of it this way, the Sun is causing the very space around itself to shrink, and as it does it falls into the Sun. Imagine a section of space located far from the Sun, as it is pulled in it becomes smaller and smaller and eventually it falls into the Sun at Solar escape velocity. Of course anything in that section of space will also fall into the Sun, whether it is one atom or a comet.

Out in the far reaches between the galactic sheets the opposite is happening, space is expanding and moving outward. So you don't need two kinds of Gravity, one to explain why your feet stick to the floor and another to explain why the universe expands. The same kind of Gravity does both, only working differently on different scales. However I suspect that Gravity has more "parts" than just the R&minus;2 that we are familiar with and take so much for granted.

So don't think of Gravity as some mysterious "field", think of it directly as what happens in the space around a massive body, and you will have a clear, highly intuitive, idea of what Gravity really is and how it works.

#### John MacNeil

Gravity is inherent to the smallest particles of matter, not to massive bodies. The reason that larger bodies have greater gravity is because there are copious amounts of smaller particles clumped together and it is their cumulative gravities which interact to make the larger bodies display stronger gravity.

There is no such thing as curvature of space. Space is an empty area in which physical bodies and fields reside. When a gravity field attracts light and makes it bend toward it, then the gravity is attracting something physical, which are the photons. The gravity itself must be made of even smaller particles than photons in order for it to interact physically with the photons. As yet there doesn't seem to be any theory describing the particle nature of gravity so you'll just have to envision gravity particles as a lot tinier, and more numerous, than photons.

The idea of spacetime is a misnomer. There is no such thing as spacetime. All that spacetime was ever meant to be was a mental measurement tool for describing phenomena. What there is, is the state of existence which is relative to position, which is never static. Spacetime is like mathematics. The mathematics can be precise and repeatable and be real to us in our minds, but it doesn't occupy space as a physical entity.

#### Hurkyl

Staff Emeritus
Gold Member
If an animal looks like a duck, sounds like a duck, and all of its other qualities are also duck-like, shouldn't we call it a duck?

#### Eh

Originally posted by Hurkyl
There are more than 2 bodies in the universe...
So what does this mean for flat space in a universe with countless bodies?

#### drag

Originally posted by Eh
So what does this mean for flat space in a universe
with countless bodies?
What do you mean by "what does this mean" ?

(Maybe, a bad golf field ?
Just kidding, I couldn't resist.)

Live long and prosper.

#### Eh

What does it mean, in terms of how likely and frequently would you expect to find regions of zero net curvature, given that the universe has countless sources?

#### drag

Greetings Eh !

Highly theoreticly :
Well, if the Universe is infinite then I believe,
but I'm not certain, that there'll be no absolute
reference frame for "flat" space. If it is finite
then I guess it should have one, but the "flatness"
(if we may call it that) would probably be determined
by internal dynamics - laws.

Live long and prosper.

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