# The Naked Scientists Forum

### Author Topic: What does a pressure in a vacuum mean?  (Read 3371 times)

#### yor_on

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##### What does a pressure in a vacuum mean?
« on: 04/03/2013 16:30:24 »
Been wondering how to define a pressure to a vacuum?
Have we experiments defining it?

The stress energy tensor mention it, and I saw someone giving it 'gravitational implications' too.
Would that include a vacuum (space).
How?

#### Ethos_

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##### Re: What does a pressure in a vacuum mean?
« Reply #1 on: 04/03/2013 19:06:18 »
Been wondering how to define a pressure to a vacuum?
Would that include a vacuum (space).
How?
Are we talking about zero point energy here? We know that the vacuum of space is not nothing, it is profuse with energy that gives rise to virtual particles. Maybe this pressure you're speaking of is the absolute energy pressure called the space/time fabric?

#### yor_on

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##### Re: What does a pressure in a vacuum mean?
« Reply #2 on: 05/03/2013 17:16:07 »
It's just me trying to understand what is meant by a 'pressure' in the stress energy tensor. Either a pressure is transfered by particles, or it is something more esoteric that is meant? You can see 'space' many ways it seems, as 'devoid of matter' (classical description), or as containing one or more 'states of energy', as in 'the Mexican hat' description used for the Higgs model, to 'zero point energy' and 'virtual photons', or my favorite 'indeterminism' :). To me it need to be measurable, preferably directly, not deduced by indirect mean, to be perfectly consistent with reality. Although as we go down in scale that last wish seems to become more and more elusive.

#### yor_on

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##### Re: What does a pressure in a vacuum mean?
« Reply #3 on: 07/03/2013 16:32:09 »
Let me put it otherwise.

If I have a ideal perfect vacuum inside a chamber. Then pull out a perfectly fitted piston to enlarge the room inside that chamber, as in a engine. What would the vacuum inside be now? If I did this at some place where the vacuum inside the chamber, before pulling the piston, is the same as the vacuum inside it then (assume that I fitted the piston to the chamber in deep space for example) will the piston want to go back into the chamber after me pulling it out. (Assume some bottom plug that I 'locked' after putting in the piston)

And would this be what is meant by a 'pressure' of the vacuum?
But first, explain how a perfect vacuum can become 'emptier', and of what? :)
=

Btw: would you call this a 'density' if so?
« Last Edit: 07/03/2013 16:41:20 by yor_on »

#### Pmb

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##### Re: What does a pressure in a vacuum mean?
« Reply #4 on: 07/03/2013 20:50:13 »
The vacuum actually is not completely empty but contains virtual particles.
As I understand it these particles create the vacuum pressure.

#### JP

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##### Re: What does a pressure in a vacuum mean?
« Reply #5 on: 08/03/2013 00:41:42 »
Pressure is a force/unit area.  For a pressure to exist, something has to be exerting force on the walls of your box.  If you're in a classical vacuum, nothing does this, so you have zero pressure.

Quantum mechanically, virtual particles pop into and out of existence and can exert a very small pressure (in theory).  If you have a system where more virtual particles are allowed on one side of the wall than the other, you can get a net force.  This is an explanation for the Casimir force.

#### yor_on

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##### Re: What does a pressure in a vacuum mean?
« Reply #6 on: 08/03/2013 19:39:31 »
Yes JP, and I agree on the description of it, although using indeterminism instead as a personal preference. Although I have seen it suggested as a effect of matter acting on matter too, the Casimir force, but I don't really know. It's not that I don't like the idea of a space filled with energies, and pressure, but I would like to see it validated by experiments.

#### yor_on

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##### Re: What does a pressure in a vacuum mean?
« Reply #7 on: 08/03/2013 19:43:51 »
For example, assume that I compress a region of a vacuum. We can do that easily, will it now have a higher 'energy density'? Will it be measurable, How strongly can we compress a 'space'? And can I assume it to, at some compression, present me with a resistance?
=

It's a little like a expansion, assumed to bring with it that 'energy' as it comes to be. It's kind of confusing, especially if you consider it from entropy. Even if I assume that entropy is a description of a (locally uneven, and generally defined as 'useful') equilibriums way to 'uniform equality of non useful energies' I still want to understand why, and how, a vacuum is allowed do lift in new energy into a universe.
« Last Edit: 08/03/2013 19:50:29 by yor_on »

#### yor_on

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##### Re: What does a pressure in a vacuum mean?
« Reply #8 on: 08/03/2013 23:03:38 »
Okay, I know. you can't compress a vacuum. And if 'energy' is a field? It should be everywhere, right? Ignoring matter as a, ahem, matter of fact. Can we prove that? But thinking of a pressure it imply, as you say, a imbalance. Does the vacuum show that? It seems perfectly friction less to me, nothing stops a 'uniform motion' in space as i know, well, except a collision? And thinking of matter, how big can 'matter waves' be?

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##### Re: What does a pressure in a vacuum mean?
« Reply #8 on: 08/03/2013 23:03:38 »