Can pressure be negative?

[Ziggurat]

Why would you say that? Even light has POSITIVE kinetic energy. Does your vacuum contain any photons?

Light has energy. Calling it "kinetic energy" is stupid. It's simply electromagnetic field energy, the exact same energy that static electromagnetic fields have. Which it would also be stupid to label as "kinetic".

But yes, we can let our vacuum contain photons and other fields.

 

[Michael Mozina]

Except that the formula is wrong. We know it's wrong. It's wrong for multiple reasons. So how can a wrong formula demonstrate anything?

The bottom line is that you can't get a single *REAL* vacuum down to even an absolute ZERO pressure state, let alone a negative one.

And you're dancing. Can a vacuum have energy? Yes or no. Easy question.

I already *ANSWERED* your question! The only energy it might contain is KINETIC energy (just like high temp atoms in the chamber), all of which would produce POSITIVE pressure on any surface in the vacuum chamber! The fact you don't like my answer doesn't mean I didn't give it to you.

 

[Michael Mozina]

Light has energy. Calling it "kinetic energy" is stupid. It's simply electromagnetic field energy, the exact same energy that static electromagnetic fields have. Which it would also be stupid to label as "kinetic".

It's "stupid" to resort to name calling in debate and it's STUPID to call it anything OTHER THAN kinetic energy. A photon moves at the speed of light (always) and it always possesses kinetic energy.

But yes, we can let our vacuum contain photons and other fields.

Then your photons will always exert positive pressure on all surface inside the vacuum. Where do these "fields" come from? Surely not the vacuum itself?

You're totally SOL IMO. *IF* you had some *THING* (external to Guths hot clump) to work with, you might play around with charges and such and achieve some kind of 'relative force' that is 'negative" compared to the surface of Guth's clump.

In a pure vacuum however you have nothing whatsoever to work with so you're totally out of luck. There's no way to achieve a "negative" pressure on the surface of your clump because there is NOTHING outside of Guth's clump to work with! The external pressure could be POSITIVE if photons are hitting the surface of the clump, but you don't even have that much to work with! There's nothing outside of the clump, and therefore no pressure outside of the clump. It cannot achieve any pressure because there is nothing there to work with. It's a ZERO pressure scenario, not a NEGATIVE INFINITY pressure scenario.

 

[Ziggurat]

The bottom line is that you can't get a single *REAL* vacuum down to even an absolute ZERO pressure state, let alone a negative one.

It's quite ironic that you're chiding me for dealing with an idealized system, when you're appealing to the ideal gas law. The ideal gas law is wrong.

I already *ANSWERED* your question! The only energy it might contain is KINETIC energy (just like high temp atoms in the chamber), all of which would produce POSITIVE pressure on any surface in the vacuum chamber! They fact you don't like my answer doesn't mean I didn't give it to you.

Electromagnetic energy is not kinetic energy. It is electromagnetic energy. Kinetic energy is the energy of mass motion, and photons have no mass.

But you haven't really given me a direct answer. I want a simple yes or not: can a vacuum have energy? Don't give me labels for it, don't try to tell me what that energy can or cannot do. We'll get to that later. First, let's get the most basic stuff down. Can a vacuum have energy? Yes or no.

 

[mike3]

One need not do anything so exotic.

Consider a simple piston filled with water, with cross-sectional area A. If you press on the piston with force F, you increase the pressure inside the piston by F/A. If you pull on the piston with force F, you decrease the pressure by -F/A. One might think that pulling on the piston merely cancels some of the ambient atmospheric pressure, but in fact one can pull on the piston with considerably more force than the ambient atmosphere applies. For very pure water, one can pull on the piston hard enough to produce something like 20 atmospheres of negative pressure before the water "rips" (ie, forms a void which will then expand as you pull).

Hmm. How come then, in light of that, liquid water in free space will turn to vapor? And that's just zero pressure, not negative. Is it because here we have a confining space, unlike the case of free space?

 

[mike3]

Can a vacuum have an energy?

I'm curious about this. What would a negative-pressure _vacuum_ look like? Would this have a negative mechanical effect on materials placed there (exert "pullure" instead of "pressure" )?

 

[Michael Mozina]

Hmm. How come then, in light of that, liquid water in free space will turn to vapor? And that's just zero pressure, not negative. Is it because here we have a confining space, unlike the case of free space?

Actually, space never even achieves a "zero' pressure, just a "lower" (positive) pressure.

 

[Michael Mozina]

I'm curious about this. What would a negative-pressure _vacuum_ look like? Would this have a negative mechanical effect on materials placed there (exert "pullure" instead of "pressure" )?

They cant even tell you what they would add or subtract from a "pure" (zero pressure) vacuum to make it contain "negative"" pressure. They really don't even understand the basic concept of 'pressure" in a vacuum in the first place. It's all about kinetic energy of the particles, and there is no such thing as "negative kinetic energy". The lowest limit of pressure in a vacuum is *ZERO* and it can NEVER be less than zero. If there is any kinetic energy in the system (in atoms or neutrinos or light), then there is 'positive" pressure in the system. If we could actually remove all neutrinos, all atoms, all light, every single moving thing from the chamber, it would hold ZERO kinetic energy and therefore zero pressure, not negative kinetic energy or negative pressure.

 

[Tubbythin]

. The formula PV=nRT demonstrates that the limit of pressure in a vacuum is zero, but Guth said claimed his vacuum contained a "negative" pressure, not a RELATIVELY LOWER pressure.

Nope, that is the limit of pressure for an ideal gas. A vacuum is no more an ideal gas than an elephant is. You wouldn't use the ideal gas law to calculate the pressure exerted by an elephant would you? So why use in another completely inappropriate situation.

 

[Michael Mozina]

The discussion of whether pressure can be negative has come up again in the forum.
I have started this thread for people to present the evidence either way.

My understanding is that pressure can be negative.

Your understanding of pressure in a vacuum is flawed RC and rwguinn blew away your 'understanding' even before I posted a single thing to this thread.

 

[Tubbythin]

They cant even tell you what they would add or subtract from a "pure" (zero pressure) vacuum to make it contain "negative"" pressure.

Pressure isn't defined by what you put in or take out. Your question is a non-sequitur.

They really don't even understand the basic concept of 'pressure" in a vacuum in the first place.

We're not the ones trying to use the ideal gas equation for a vacuum!

It's all about kinetic energy of the particles, and there is no such thing as "negative kinetic energy".

No it isn't. It is a vacuum. It clearly has nothing to do with kinetci energy of particles.

The lowest limit of pressure in a vacuum is *ZERO* and it can NEVER be less than zero.

No it isn't. Not even if you type it in capital letters and put an asterix around it. You are trying to use the ideal gas law. The vacuum is obviously not an ideal gas.

If there is any kinetic energy in the system (in atoms or neutrinos or light), then there is 'positive" pressure in the system. If we could actually remove all neutrinos, all atoms, all light, every single moving thing from the chamber, it would hold ZERO kinetic energy and therefore zero pressure, not negative kinetic energy or negative pressure.

It has nothing to do with kinetic energy. Just like the pressure exerted by an elephant has nothing to do with kinetic energy.

 

[Tubbythin]

Your understanding of pressure in a vacuum is flawed RC and rwguinn blew away your 'understanding' even before I posted a single thing to this thread.

Please show us the post in which rwguinn showed how the Cassimir pressure of a vacuum is positive or admit that the above statement was a lie.

 

[edd]

Why would you say that? Even light has POSITIVE kinetic energy. Does your vacuum contain any photons?

You do realise photons don't obey pV=nRT? Not that much else does.

You keep making the same mistake Michael - basically that everything is a gas.

 

[Michael Mozina]

Nope, that is the limit of pressure for an ideal gas. A vacuum is no more an ideal gas than an elephant is. You wouldn't use the ideal gas law to calculate the pressure exerted by an elephant would you? So why use in another completely inappropriate situation.

Oh boloney. If we added just ONE atom to your mystical sort of "vacuum" then the formula and the recorded pressure EXACTLY matches the ideal gas law and it is a COMPLETELY valid method of determining the pressure. It has a POSITIVE pressure.

If we now remove that SINGLE ATOM from that very same vacuum chamber, the pressure drops to zero. That *IS* the appropriate function for the pressure of a vacuum. The lowest limit of pressure in a vacuum is zero.

This whole conversation demonstrates just how little any of you understand KINETIC ENERGY. You seem to have some irrational belief that the pressure in a vacuum is somehow completely physically detached from kinetic energy. It's not. When the temperature goes up, the atoms have more kinetic energy and exert more pressure on the sides of the chamber. As you cool the atoms, they have less kinetic energy and exert less pressure on the sides. If you removed ALL the kinetic energy you achieve ZERO pressure in a vacuum. There is no such thing as "negative kinetic energy", so there is no such thing as 'negative pressure'. You folks are utterly clueless at the level of particle kinetic energy, and that's your same problem as it relates to MR theory as well.

 

[Michael Mozina]

You do realise photons don't obey pV=nRT? Not that much else does.

You keep making the same mistake Michael - basically that everything is a gas.

No, you keep making the same mistake, basically ignoring the fact that everything is related to kinetic energy, including those photons that exert *POSITIVE PRESSURE* on everything they run into.

 

[Ziggurat]

Hmm. How come then, in light of that, liquid water in free space will turn to vapor? And that's just zero pressure, not negative. Is it because here we have a confining space, unlike the case of free space?

Basically. In space, it's got a free surface that individual molecules can evaporate from, but it doesn't inside our cylinder (until a bubble forms).

 

[edd]

Michael, if you really want to describe pressure as solely due to kinetic energy from particles then I won't stop you, but you'll have to recognise that everyone else, especially in the cosmological context of negative pressure, is not talking about the same restricted circumstances as you.

You'll just have to live with that.

 

[Ziggurat]

I'm curious about this. What would a negative-pressure _vacuum_ look like? Would this have a negative mechanical effect on materials placed there (exert "pullure" instead of "pressure" )?

That would depend on how they interacted with the vacuum. But yes, the Casimir effect (an example of negative pressure in a vacuum) can pull objects together from inside. Michael likes to pretend that it's really a push from outside, but that explanation actually fails rather dramatically under serious scrutiny.

 

[Michael Mozina]

Please show us the post in which rwguinn showed how the Cassimir pressure of a vacuum is positive or admit that the above statement was a lie.

I already went through that whole Casimir debate with you folks. There is LESS pressure on the inside of the plates and MORE pressure on the outside. It's also a total red herring since you don't have a plate to work with in a VACUUM. More importantly the TYPE OF MATERIAL makes a difference, demonstrating *CONCLUSIVELY* that there is no area in the vacuum chamber with a "negative pressure" or the type of material would be irrelevant.

 

[Ziggurat]

Oh boloney. If we added just ONE atom to your mystical sort of "vacuum" then the formula and the recorded pressure EXACTLY matches the ideal gas law

No it doesn't. The ideal gas law is wrong. Even for a single atom.