[yor_on]

Let's try this. 'What is the energy density of a vacuum' by John Baez
http://math.ucr.edu/home/baez/vacuum.html

Ps: it's worth following the links if you want to see the ideas about a 'negative pressure' explained in a straightforward manner.

Furthermore, it doesn't change the question of what a 'vacuum' is, it gives you a explanation through GR of how it is thought to work (a expansion), but does not answer what a 'vacuum consist of'. So our discussion is still very much relevant.

[Bill S (OP)]

Depends on how you define a vacuum. If you think of it as a 'field' containing 'energy' aka 'virtual particles' then even a 'perfect vacuum' is 'something'.

That answers a question like: under what conditions would “an absolute vacuum” be “something”.  However, that was not the question.

But if we ignore that interpretation then what's not there, is not there. I'm not being partial here, it's more of a question of how you look at it

Any interpretation of nothing that makes it something is “playing fast and loose” with language.  Language is the medium by which we exchange information, so I think it is very important that we should not mess it about.

"If a vacuum is defined by it being 'nothing', it can have none of these properties." Well, it does have those properties, the vacuum is also called space and space 'bends' etc. It actually needs a geometry to exist, well, as I see it then.

Then, neither space, nor the vacuum can be considered as nothing.  It's as simple as that.

[jeffreyH]

The vacuum is definitely not nothing. It has energy. Space is just a name for that which we use to define distance.

[Bill S (OP)]

There is one important point though. Presuming a vacuum to be nothing but still giving us an appearance (observer dependent), what would make it 'exist'? In such a case, to me, the geometry of our universe becomes interesting, and with it my original question(s).

If a vacuum is defined as “nothing”, then any observer dependent attribute must be an illusion.  However, if we accept that our improved understanding of the nature of the vacuum is such that we now know that it is “something”, and we re-define the term “vacuum” in accordance with that; there is no problem. 

Only when we indulge in semantic contortions like insisting that nothing is something do we run into difficulties.

[Bill S (OP)]

Space is just a name for that which we use to define distance.

I think I'm OK with that (for the purposes of this discussion, anyway), but I would anticipate some eyebrow raising on the part of those who would argue that space (or spacetime) can be physically distorted.

[yor_on]

:)
Ok

You don't think observer dependencies are true then Bill?

As you write that " but I would anticipate some eyebrow raising on the part of those who would argue that space (or SpaceTime) can be physically distorted."

Or maybe it's about semantics again?

[yor_on]

 I'm sorry if you thought of it as a 'exclusive discussion' btw. It was presenting alternatives, not trying to convince anyone. As I stated before ever writing here, this is what I do to understand it myself. I test the logic and try to see if it makes sense to me, which also means that I have several views.
=

But yes, observer dependencies are correct, and true. I don't have any problems with that, it's the variant conclusions you may make from it that still holds my interest. http://www.askamathematician.com/2010/09/q-how-does-the-twin-paradox-work/

[Bill S (OP)]

I’ve obviously given the wrong impression – more than once!

You don't think observer dependencies are true then Bill?
As you write that " but I would anticipate some eyebrow raising on the part of those who would argue that space (or SpaceTime) can be physically distorted."

I have no problem with observer dependence, in general.  My comment: “If a vacuum is defined as “nothing”, then any observer dependent attribute must be an illusion” referred specifically to anything that anyone might observe, and identify as an attribute of “nothing”. 

As I see it; if it has attributes, it is not "nothing".  If it is "nothing", and attributes are observed; then either "it" has to be redefined as "something", or the observed attributes are illusions.

The comment about eyebrow raising referred specifically to Jeffrey’s statement: “Space is just a name for that which we use to define distance.”

[Bill S (OP)]

BTW. If I gave the impression I thought this was an  'exclusive discussion', I apologise.  Another wrong impression. I blame my age. 

[jeffreyH]

Space is just a name for that which we use to define distance.

I think I'm OK with that (for the purposes of this discussion, anyway), but I would anticipate some eyebrow raising on the part of those who would argue that space (or spacetime) can be physically distorted.

Space isn't a physical object. Just as time isn't. They are both constructions of mathematics. Due to Lorentz transformations they can both have a gradient that we can show on a space-time diagram.

[Bill S (OP)]

Space isn't a physical object. Just as time isn't. They are both constructions of mathematics. Due to Lorentz transformations they can both have a gradient that we can show on a space-time diagram.

You will get no argument from me, there, but then, I suspect that spacetime curvature is a concept that has led to some wrong impressions, In spite of the fact that it provides a useful way of thinking about things like gravity.

[yor_on]

Well Jeffrey :)

Both 'space' and 'time' are properties existing, long before mathematics was 'invented',. The Chinese used yin and yang for describing it, vacuum contra matter, cold contra hot, female contra male. It's been around for a really long time.
=

Although time stood by itself, they still used 'work done' as defining the distance. Going up the mountain made for a longer 'distance' than going down which, even though it makes sense, craves another way to look at it.

[Bill S (OP)]

BTW; Interesting link to askamathematician. At a cursory glance, I have less problems with that than I had with his answer to the question about time stopping for photons.