What is empty space?

[Sisyphus]

By "empty space," I mean space empty of all matter. I realize this a huge question. Maybe I should ask, if you were take a one meter cube of space from way out between the galaxies, clean out any stray ions or whatnot, what could you say about it? Presumably it has EM waves passing through it in all directions, constantly. It also has a gravitational "shape," right? And it can expand and contract, theoretically? But does that expansion/contraction mean anything without reference to objects?

 

That was a tangled mess of a question. I'll leave it open if anyone wants to touch it.

[NMajik]

It also has a gravitational "shape," right?

 

<Stupid question> Can something without matter have a gravitational shape? Or would it only be modified by the gravity of matter not contained within it? </Stupid question>

[psynapse]

How could nothing expand or contract? It's not a gas, it is nothing.

[Sisyphus]

<Stupid question> Can something without matter have a gravitational shape? Or would it only be modified by the gravity of matter not contained within it? </Stupid question>

 

By gravitational shape I was just viewing gravity as a "geometrical" property of space itself. Like the "heavy objects on rubber sheet" analogy.

 

How could nothing expand or contract? It's not a gas, it is nothing.

 

Good question. The short answer is that it's not nothing, it's space. I don't think I can explain the long answer very well (hence the original question), but I think generally space is viewed as having some inherent metric which can change relative to material objects. That's why the universe can expand, and everything can get farther apart, but nothing actually has to "move." The analogy I had in mind was the expanding balloon, where points on its surface don't move across its surface, but still grow farther apart since the surface itself is increasing in size. Except the "surface" here is three-dimensional empty space.

[Martin]

quantum gravitists like Lee Smolin work on this question and related questions

 

a partial answer was given in 1915 with Gen Rel

space is identified with its geometry (which equals the gravitational field, which equals a dynamically varying metric describing geometry)

 

in a sense, there is nothing there except the geometry!

 

for many people this is not a completely satisfying answer----indeed you may say that it is very bad and unsatisfying---it boils down to saying something like "space is nothing but the spatial relations between events"----the distances, angles, areas, volumes that the spatial metric describes between whatever residual fields are still in the space.

 

so ever since 1915 there has been a tendency to try to dig deeper and get to something more substantial and satisfying-----in particular a spacetime dynamics that obeys quantum theory (which 1915 Gen Rel does not)

 

 

probably the best most uptodate source describing the struggle to get a more fundamental theory of space time and matter is Smolin's new book The Trouble with Physics

 

Section I describes the problems physics faces. Section II does string theory. Section III is the approaches that go beyond string

find out about book here

http://www.amazon.com/gp/bestsellers/books/14560/ref=pd_ts_b_nav/102-4540543-7840144

and here

http://www.thetroublewithphysics.com/

(click on "contents" to get TOC)

===================================

HOWEVER EVEN I CAN RESPOND A LITTLE BIT

notice that when you posed the problem you DESIGNATED A PARTICULAR CUBIC METER

vacuum means space from which all the energy has removed that you can possibly remove-----simply to talk concretely about some PARTICULAR cubic meter volume of vacuum necessarily means to have a BOUNDARY AND A LOCATION

 

location is meaningless unless it is marked by material objects as references, like some stars, so YOU ALREADY HAVE THE GRAVITATIONAL FIELD OF THE landmark stars INSIDE THE BOX, the field of the nearby stars pervades everwhere including inside the box, even though it is weak it is still there

 

 

to me this is philosophically significant:, you cannot talk about a particular cubic meter of vacuum UNLESS you are willing to have outside objects defining location and this means there is a gravitational field in there inside the box

 

(geometry, curvature, referring to external objects)

 

and if you want it to be a real good vacuum, as good as you can make, then you need a surrounding boundary wall so you can pump all the gas OUT, and sweep all the dust OUT from out of the empty vacuum to the outside of the wall

 

heh heh

 

already also the wall being material will be contributing to the gravitational field most likely and or outgassing and vaporizing its own distinctive drek into the vacuum

 

=================

there is always stuff in space, always fields, including the gravitational field which describes the spatial geometric relations between events determined by the other fields

 

it is a hellishly difficult philosophical problem to say what is empty space, maybe if there were no matter there would BE no space.

 

quantum gravitists spend time trying to make models, like network models and simplex building block models, to give a QUANTUM description of the most basic degrees of freedom of space.

 

Next year 2007 will be published by Cambridge U. press a book edited by Daniele Oriti (at Utrecht----same place as Gerard 't Hooft)

 

this book will be called Approaches to Quantum Gravity: towards a new understanding of space, time, and matter

 

it is likely to be an important book. I exchanged some messages with Oriti yesterday. the book is made of a bunch of contributions by over a dozen people. I am looking forward to it. In the meantime I think Smolin's book is the best.