# Can you compress a vacuum void?

• NeilWallace
PV is. So while the energy in the system does change, the specific energy (which is equal to the total energy in the system cubed) remains the same.f

#### NeilWallace

If you have a vacuum within a piston say and compress its volume what happens to it? I am trying to understand how a vacuum that is 'nothing' - no molecules / energy within it can have properties such as Volume and a sort of existence that obeys physical laws. I guess I am ultimately thinking about how the universe might have originated from nothing.

If you have a vacuum within a piston say and compress its volume what happens to it?

You've started too far into the process. Back up one step. How did you get the vacuum in the first place?

See, if the container/piston is simply sitting on the table, the air pressure on outside will ensure that the piston reduces to its minimum volume state.

There's only two ways to get a non-zero volume vacuum in your container:
1] start with the piston in its minimum volume position and pull it out until the container's volume as max
2] extract the air from the volume while holding the piston out

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If you have a vacuum within a piston say and compress its volume what happens to it? I am trying to understand how a vacuum that is 'nothing' - no molecules / energy within it can have properties such as Volume and a sort of existence that obeys physical laws. I guess I am ultimately thinking about how the universe might have originated from nothing.

If the piston was isolated so that there was no outside air pressure acting on it, then nothing would happen. The piston would move in and out and nothing would change except for the volume of the void would get smaller and larger as the piston moves in and out.

When talking about volume and distance, it doesn't matter if there is anything in that space or not, it makes no difference.

Edit: What makes you think that the universe originated from nothing? It's entirely possible that there have been an infinite number of big bangs followed by collapses in the past. (Though current evidence doesn't quite support this)

Thanks - So in the case of the piston the vacuum is maintained by a force. Is the vacuum of outerspace maintained by a force too? or is it a different kind of vacuum?

Thanks - So in the case of the piston the vacuum is maintained by a force.
Well, a force that must counterbalance air pressure.

If you took your device into space, you could move it in and out at will.

Is the vacuum of outerspace maintained by a force too? or is it a different kind of vacuum?
You're looking at this backwards. If there were such a force, what do you think would happen to space if the force went away? Would it fill with something?

On the other hand ... yes, the vacuum of space in maintained by gravity. Gravity keeps the Earth's atmsophere gathered around the planet, rather than letting it uniformly fill the solar system...

Thanks - So in the case of the piston the vacuum is maintained by a force. Is the vacuum of outerspace maintained by a force too? or is it a different kind of vacuum?

As dave said, you would only have to counteract air pressure. Imagine you have a straw. Now, close up one end with your finger and suck in on the other end. The straw collapses. Why? Air pressure from the atmosphere presses in, while the air that was inside the straw gets sucked out by you, resulting in a difference of pressure. This causes the straw to collapse. Same principle with a piston. If you were in space this would not happen, as there would be no outside air pressure to collapse the straw, and also no air to suck out as well.

If you have a vacuum within a piston say and compress its volume what happens to it? I am trying to understand how a vacuum that is 'nothing' - no molecules / energy within it can have properties such as Volume and a sort of existence that obeys physical laws. I guess I am ultimately thinking about how the universe might have originated from nothing.

I'm going to go with the assumption that the vacuum is contained within a chamber, whose volume can be changed by a piston. This makes more sense than a vacuum in a piston.

With this tightening of terminology now consider what is meant by a vacuum.

The absence of matter - definitely

The absence of energy - not true within a chamber since the chamber walls will constantly supply blackbody radiation.

Can you 'compress' the vacuum by moving the piston.

For matter the term makes no sense since there is none.

For energy the the the specific energy (=energy per unit volume) does not alter since
the volume (V) = cross section (As) x length controlled by the piston (L)
and the irradiating area Ai = perimeter(P) x L.
The irradiation is proportional to the irrating area.

$$SpecificEnergy = \frac{{k{A_i}L}}{{{A_s}L}} = const$$

where k is the irradiation constant

I think the answer is obvious if you tighten up on your terminology.

I don't think you are compressing anything. A vacuum is simply a space with the absence of matter, correct? You cannot compress nothing. All you will do is simply reduce the volume of the chamber that has the vacuum.

I'm going to go with the assumption that the vacuum is contained within a chamber, whose volume can be changed by a piston. This makes more sense than a vacuum in a piston.
Yes, I'd modified it to container/piston, but it doesn't hurt to be explicit.

Can you 'compress' the vacuum by moving the piston.

For matter the term makes no sense since there is none.
Actually, what he said was 'compress the volume'.

But point made. Not sure why the OP thinks there would be a problem compressing a volume containing vacuum.

Actually, what he said was 'compress the volume'.

Actually this makes no sense either.

You can't compress a volume. A volume is what it is, so many decilitres.
But I didn't think I'd press (:rofl:) the terminology point further.

You can compress the contents of a volume so that they occupy a smaller volume. That makes sense.

Actually this makes no sense either.

You can't compress a volume. A volume is what it is, so many decilitres.

You can compress the contents of a volume so that they occupy a smaller volume. That makes sense.

It's an iffy point. A general definition of 'compressing' is 'making something smaller'. You can make the volume of a container smaller. But that's really reaching...

Anyway it's academic, since, well, you can't really compress a vacuum.

Or is it

"applying a compressive stress"

How do you stress a volume?

Either way I think everyone understands the situation lol.

No, 'To compress' does not imply make smaller.

Take a block of steel
Jam it in between two concrete walls
Apply a blowtorch to the steel

Is the steel block now under compression?

Is the volume of the the steel block larger or smaller?

From wikipedia: "Physical compression is the result of the subjection of a material to compressive stress, resulting in reduction of volume."

From Dictionary.com: "to press together; force into less space."

So you say the steel block in my example gets smaller?

So you say the steel block in my example gets smaller?

By the defintion, I would say that you could TRY to compress a steel block, but you couldn't. You would merely apply force that would try to compress it.

Sorry about the piston .. but this was the information I was angling for from DaveC426913:

'On the other hand ... yes, the vacuum of space in maintained by gravity. Gravity keeps the Earth's atmsophere gathered around the planet, rather than letting it uniformly fill the solar system... '

so the vacuum of outerspace is caused by a force - matter and its gravitational effect on its surroundings.

If there was no matter in the universe then their would be no vacuum in outerspace. Presumably there would be nothing at all to speak of. The vacuum extends as far as the gravitational effect from the universe's matter is able to influence which is presumably into infinity.

Relatively speaking then is it possible to turn the question on its head and say rather than the matter creates the vacuum, that the vacuum creates the matter and the gravity.

As to why I think that the universe came from nothing rather than a big bang or multiple universes i just think Nothing is the only cause that doesn't need a further antecedent whereas the multiple universes and so on seem a bit messy with antecedents.

I realize that a vacuum is different from Nothing but it seems to be getting close. That is why I asked about the properties of vacuums/outerspace to see if you can get something out of nothing by messing around with a Voids Volume

Apologies I am just a beginner!

...result of the subjection of a material to compressive stress...

Are you saying that the heating does not result in a compressive stress in the steel block?

I would say that if there were no matter in the universe, it would all be a 100% perfect vacuum.

As for the beginning of the universe, I just say that it's equally likely that it started from nothing, or that there was always something there. Or even better, say that we have no idea lol.

Are you saying that the heating does not result in a compressive stress in the steel block?

To venture a guess I would say that the cube tries to expand and can't in the direction between the walls. (Assuming the walls don't give at all, which they probably would) However if nothing is stopping it in other directions it can expand in those directions, which would increase its volume. In effect you could say that the walls apply a force to the cube, but by the definition I saw they would not be compressing it since it is not being reduced in volume.

so the vacuum of outerspace is caused by a force - matter and its gravitational effect on its surroundings.
Well, no. I was being facetious.

The vacuum of outerspace is the default. It is there in a sense because the universe has expanded from a very small volume to an extremely large volume, while there is only a tiny, tiny amount of matter to fill that volume. Even if all the matter in the universe were distributed evenly, you would find large volumes of vacuum surrounding each particulate matter.

If there was no matter in the universe then their would be no vacuum in outerspace.
Well, also no. In the first few epochs of the universe, there was no matter, yet there was definitely a lot of volume to space; it was all energy.

As to why I think that the universe came from nothing rather than a big bang or multiple universes i just think Nothing is the only cause that doesn't need a further antecedent whereas the multiple universes and so on seem a bit messy with antecedents.
It tends to be futile to philosphize about cosmology. We have neither the words nor the metaphors to describe it well. For example, there are countless threads right here on PF demanding an unambiguous definition of the word 'nothing' in the context of cosmology. The only useful way to describe it well is mathematically.

Of course the steel block expands in directions where it is free to do so.
Consequently its volume increases.

With regard to the interwall axis, the walls do indeed provide reactions opposing expansion along this axis.
This, of necessity, imposes a compressive stress along that axis on the steel block.
There is no such stress imposed upon the block in the directions of free expansion.

So a material object can be subjected to a compressive stress and yet increase in volume.

What I am saying is that the WP article should read something along the lines

...subjection of a material to a compressive stress, which may result in a reduction of volume...

Of course the steel block expands in directions where it is free to do so.
Consequently its volume increases.

With regard to the interwall axis, the walls do indeed provide reactions opposing expansion along this axis.
This, of necessity, imposes a compressive stress along that axis on the steel block.
There is no such stress imposed upon the block in the directions of free expansion.

So a material object can be subjected to a compressive stress and yet increase in volume.

What I am saying is that the WP article should read something along the lines

...subjection of a material to a compressive stress, which may result in a reduction of volume...

I'm not sure that compressive stress and compression are the same thing. I believe compressive stress is simply the forces acting on the object. Compression is what can result from the stress that causes something to lose volume and be compressed.

I'm not sure that compressive stress and compression are the same thing. I believe compressive stress is simply the forces acting on the object. Compression is what can result from the stress that causes something to lose volume and be compressed.

This is a Physics forum and therefore scientific in orientation.

Science often needs to use a narrower definition of a word than is in common parlance because the common usage contains vagaries or inconsistencies.

Whilst stress and force are often interchanged in common English, iin Physics they are very different quantities with different self-consistent definitions.

Do we not all agree to use only the scientific definitions in this forum for scientific terms?

Although there are many ways of applying a compressive stress, the scientific meaning of that phenomenon is always the same.

However the resulting response (strain) of the object of that stress may be to shrink (volume decrease), to expand (volume increase) or to remain unstrained (no volume change).

Other than to correct my uses of force and stress, I'm not sure what you are trying to say. The WP article looks fine to me, as it goes hand in hand with the definitions that I found for compression. Applying compressive force CAN cause compression, but doesn't have to. If the object doesn't get reduced in volume then it isn't compressed. If you can find something else that says otherwise please tell me, as I only have a limited pool of information to draw from. It's entirely possible that the meanings will be different in a physics book somewhere or something.

If the object doesn't get reduced in volume then it isn't compressed.

I have described a natural situation, not requiring ideal components, where a body can be subject to a compressive stress and yet still expand in volume.

A body subject to a compressive stress is compressed.

I really don't see how I can make it any plainer.

There was a recent thread here about sand dilatation, which is another example.

Incidentally the whole issue becomes moot if we ask

How do you compress a container of balls?
Are we talking about compressing the voids between the balls?
Or just the balls themselves?
What if the voids are filled with fluid?
What then reacts to the compression, the balls or the fluid or both?

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I have described a natural situation, not requiring ideal components, where a body can be subject to a compressive stress and yet still expand in volume.

A body subject to a compressive stress is compressed.

I really don't see how I can make it any plainer.

There was a recent thread here about sand dilatation, which is another example.

Incidentally the whole issue becomes moot if we ask

How do you compress a container of balls?
Are we talking about compressing the voids between the balls?
Or just the balls themselves?
What if the voids are filled with fluid?
What then reacts to the compression, the balls or the fluid or both?

Did your cube's volume increase? The dimensions changed because two sides were pressed in, which caused the cube to expand in the other dimensions, but did the volume change? I have no way of knowing. I suppose we could say that a particular dimension of the cube was compressed.

As to your question above, if you compress the container and it has a reduction in volume, then it has been compressed, no matter what is inside. The rest of your question depends on the makeup of the balls and voids/fluid.

Is this a denial of the phenomenon of thermal expansion?

Is this a denial of the phenomenon of thermal expansion?

Sorry, I've been replying to this thread over the course of several days and I forgot the details of your example.

I'd question whether your block is under compression at all. The cube wants to expand slightly, but can't in one direction. It still expands in the other ones.
I don't know if that counts as being compressed or not.

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OK so you now accept thermal expansion as a physical fact.

Slide a loose fit ring onto a metal bar.

Heat the bar, but not the ring (eg by heating the bar at a reomte end).

The ring now becomes a tight fit or even a very tight fit.

Do you deny this to be a physical fact?

The bar is now under two dimensional compression from the ring.

Examples in nature abound.

This is junior high school physics.

OK so you now accept thermal expansion as a physical fact.

Slide a loose fit ring onto a metal bar.

Heat the bar, but not the ring (eg by heating the bar at a reomte end).

The ring now becomes a tight fit or even a very tight fit.

Do you deny this to be a physical fact?

The bar is now under two dimensional compression from the ring.

Examples in nature abound.

This is junior high school physics.

Again, I ask if this is actually compression. I'm not denying anything, I actually don't know. When I looked up what compression was, I posted the definitions I got. Can you show me something that says that compression doesn't always result in reduction in volume?

Since you like online and eschew libraries try googling thermal stress.

Here is the first definition on page 1 of a very long list

(mechanics) Mechanical stress induced in a body when some or all of its parts are not free to expand or contract in response to changes in temperature.