Can pressure be negative?

[Michael Mozina]

Assume your sealed chamber in your lab to be 100m in length.

What would the pressure in the ideal sealed chamber be as the piston progresses from the sealed end?

You are specifically and intentionally changing the conditions related to my complaint because Guth's "vacuum" neither occurred in a sealed chamber, nor did he utter a peep about any external pistons in relationship to his vacuum. You're reaching there Skwinty.

It's not reasonable IMO to compare Guth's very specific claim to any random condition including additional external hardware that you might find laying around in your own imagination (not Guth's actual paper).

 

[Reality Check]

Bait and switch much Zig?
...

There is no bait and switch.


You are having problems grasping the simple physics that allows negative pressure to exist. Zig has presented an used your example to illiustrate this since you have some mental block against understanding

• the force/area definition of pressure where attractive forces acting on a surface always give negative pressures).
• the P = -dE/dV definition of pressure where a constant energy density gives negative pressure.
• the negative pressure in the theory and experiments of the Casimir effect.
• the concept of lowering the force exerted by gases until the forces exerted by quantum fluctuations dominate (Casimir effect experiments are done in vacuum chambers).

Once you grasp this example then we can go onto the pressures exerted by the cosmological constant as a step to the pressure exerted in inflation.

This is how people are educated. Stat with a simple example and build up.

No one claims that inflation has an external anything to wiork against. So do not bring up that straw man again.

ETA: Forgot that it is you who presented the scenerio and so it must be you who is doing the bait and switch !

Originally Posted by Michael Mozina
Sure. Crank up the charge between two objects in a vacuum and you'll get an increase in kinetic energy between the objects in the vacuum. It's an increase of *particle kinetic energy*.

 

[ben m]

Michael, nobody cares if you call it "negative pressure" or not.

The GR equations care about energy densities.

Guth's inflation hypothesis is a hypothesis about an energy density.

Dark energy is a hypothesis about an energy density.

Plug Guth's hypothesized energy density into GR and watch how it evolves. That's what matters. Do you think Guth did this wrong? Do you think GR itself is wrong?

Everyone other than you will look at Guth's hypothesized energy density and say, "huh, that energy has dE/dV > 0, as though it had negative pressure". You don't have to, there is nowhere in GR that you're required to plug in P. (You do, however, need E and dE/dV, but the hypothesis tells you what they are.) Everyone other than you will, having solved the Friedmann equation including the hypothesized energy, will say "huh, compared to the evolution of a universe filled with ordinary kinetic energy and gas-like pressure, this universe evolves in the opposite direction." Awfully suggestive of negative pressure for everyone other than you.

E matters. dE/dV matters. The rest of the world has a standard, non-kinetic-energy-related way of defining "P" in terms of these things. You don't like that label? Your loss. Ignoring the label doesn't change the sign of dark energy's (or the inflaton's) dE/dV, so it doesn't have the slightest effect on the cosmology.

 

[Pythonic]

Any negative force just sucks.

 

[Ziggurat]

Bait and switch much Zig? My "complaint" was specifically directed at Guth's claim

I don't care what your complaint is about. That isn't the topic of this thread. Now answer the damn question already. Is there a positive or negative pressure in the scenario that you proposed?

 

[Calrid]

As long as you treat it as a "positive" kinetic energy/pressure state, or even a ZERO kinetic energy/pressure state, you're welcome to do so.

0 in the vacuum seems as likely as 0°K.

 

[Skwinty]

You are specifically and intentionally changing the conditions related to my complaint because Guth's "vacuum" neither occurred in a sealed chamber, nor did he utter a peep about any external pistons in relationship to his vacuum. You're reaching there Skwinty.

It's not reasonable IMO to compare Guth's very specific claim to any random condition including additional external hardware that you might find laying around in your own imagination (not Guth's actual paper).

I'm reaching out to you Michael.

By stretching a vacuum, a negative pressure and negative associated energy density is achieved.

Go back to the lab experiment I proposed for you, seeing that you are a keen proponent of empiricism, and answer the question.

Also take a look at the link.
http://www.astro.ucla.edu/~wright/cosmo_constant.html

It is quite standard to utilise the piston and chamber to illustrate the physics.

 

[edd]

It's not a negative energy density.

 

[Skwinty]

It's not a negative energy density.

Oops, I meant to say energy density associated with the negative pressure.

 

[I Ratant]

AAAAAAAAAAAAAAAAAAAAAAAAHHHHHHHHHHHHHHH!

The horror, the horror, noooooooooooooooooo

.
Ask, and ye shall receive.

 

[I Ratant]

And if the universal conditions had sufficient force to remover the ideal piston from the ideal sealed chamber?

.
Don't dally, lest the cat suffocate!

 

[I Ratant]

To (likely) 99.9% of folk it's still a theoretical masturbation exercise. Most of us don't work to those lveles of precison. Top the rest of us you measure absolute gas pressure which CANNOT be less than zero at these levels of measurement. Hence my comments about definitions. Guage pressure is a different kettle of fish, obviously you can have a negative pressure differential (yet another definition...)

...

.
But, but, 99.999991% of any technical discussion here on the 'net involves endless jerking off, if only to keep the thread going.

 

[Michael Mozina]

I don't care what your complaint is about. That isn't the topic of this thread. Now answer the damn question already. Is there a positive or negative pressure in the scenario that you proposed?

Getting a little testy are you?

Ok....

*IF* you are claiming that the SECOND OBJECT, via charge/gravity/whatever, has an ATTRACTIVE effect on the FIRST OBJECT, creating a type of "negative pressure/attractive force" on some surface of the first object, I'm "ok" with that claim.

It's the OBJECT that creates the so called "negative pressure" (attraction) to the first object, it's not because the space between the objects holds any type of 'negative pressure'. Quite the contrary. The space between and around the two objects is literally *FULL* of POSITIVE photon kinetic energy that is being exchanged between and around the two objects.

At the level of QM and physics, it's the TWO objects exchanging photons that ultimately attracts the two objects, it's not because the space in and around the objects contains "negative pressure".

I don't know how much more clear I can make it.

In terms of our original disagreement however, all of this is 'moot'. Guth didn't have any pistons, no second objects, nothing of the sort. He had nothing but a "vacuum" that may or may not contain any photons, neutrinos, or other kinetic energy. It's pretty much impossible to actually achieve a ZERO kinetic energy vacuum, but assuming you achieved it somehow, you'd end up with 'zero' pressure in the vacuum. That ZERO point is the LOWEST POSSIBLE KINETIC ENERGY STATE of the vacuum. Guth's magic negative pressure *VACUUM* has nothing to do with the attraction of multiple objects INSIDE of a vacuum. You're confusing two separate issues.

At no point in human history can or has a 'vacuum' ever achieved even a ZERO kinetic energy/pressure state. It's impossible to keep the neutrinos out of the vacuum, not to mention the fact that it's impossible to get every atom out of the vacuum.

You're essentially moving the goal posts and confusing a SECOND object with a VACUUM containing *NO* second objects.

 

[Michael Mozina]

I'm reaching out to you Michael.

Yes I know my dear Skwinty. It has to be one of your most redeeming qualities actually.

By stretching a vacuum, a negative pressure and negative associated energy density is achieved.

Go back to the lab experiment I proposed for you, seeing that you are a keen proponent of empiricism, and answer the question.

Also take a look at the link.
http://www.astro.ucla.edu/~wright/cosmo_constant.html

It is quite standard to utilise the piston and chamber to illustrate the physics.

FYI, your piston scenario will never actually create a "negative pressure" inside of the chamber no matter how hard you pull on that piston. Assuming you create more 'space' inside the chamber, the atoms and photons and neutrinos might "spread out". The best you could hope to achieve is a 'ZERO' kinetic energy state. Your analogy fails to mention the fact that neutrinos will continue to blow through your chamber, and any atoms and particles present in the chamber simply spread out and become "less dense", but they still exist inside the piston chamber. When you stop the piston for a moment, those atoms will still hit the top of the piston and create "positive pressure'/kinetic force against the surface of the piston.

FYI, Guth didn't have any external chambers or pistons on the outside of his mass thingy so the analogy fails for at least two reasons that I can think of and have mentioned.

 

[Michael Mozina]

0 in the vacuum seems as likely as 0°K.

Yep. That's exactly what it would take too. You'd have to remove all heat/photons as well as all atoms inside the vacuum, all neutrinos, and all forms of kinetic energy. It isn't likely, that's for sure.

 

[Ziggurat]

It's the OBJECT that creates the so called "negative pressure" (attraction) to the first object, it's not because the space between the objects holds any type of 'negative pressure'.

Sure it is. The energy density of the space between the plates is constant and positive. And it's the increase/decrease in the total energy between the plates as the plates are pulled apart/brought together which exerts the force and hence the pressure.

The space between and around the two objects is literally *FULL* of POSITIVE photon kinetic energy that is being exchanged between and around the two objects.

I KNOW it's positive energy. That's a central part of my argument, and the fact that you're insisting on what my argument requires suggests that you still haven't figured out what I'm saying.

At the level of QM and physics, it's the TWO objects exchanging photons that ultimately attracts the two objects, it's not because the space in and around the objects contains "negative pressure".

I don't know how much more clear I can make it.

You are, in effect, arguing against potential energy because you don't like to take the derivative in order to get the force, since you can calculate the force directly without using the potential. That's clear, but it's also stupid and wrong.

The view that the plates are attracting each other and the view that the space between the plates is pulling them together are not different. They are exactly the same. And we can tell that these views are exactly the same because they make exactly the same predictions. There cannot BE a difference between the two views unless they make different predictions, but they don't. They are completely identical.

In terms of our original disagreement however, all of this is 'moot'.

We'll sort that out AFTER we sort out the scenario you proposed. Which we still haven't done, because you keep trying to derail this discussion.

So I'll try again. The energy density of the electric field (or photons, if you like) between the plates is constant and positive. The more space there is between the plates, the more energy there is between the plates. The energy is positive, and it scales with the volume. Do you actually disagree with any of this paragraph?

 

[Michael Mozina]

The view that the plates are attracting each other and the view that the space between the plates is pulling them together are not different. They are exactly the same. And we can tell that these views are exactly the same because they make exactly the same predictions. There cannot BE a difference between the two views unless they make different predictions, but they don't. They are completely identical.

No Zig they are not "identical". If they were identical, when you turned on your device, you would actually create a "negative pressure" inside the vacuum chamber and we'd see a large change in the actual pressure of the vacuum chamber. We don't. If it created "negative pressure" in between those plates, the other positive pressure items would get "sucked in" and the pressure of the chamber would drop. It doesn't work like that even if we hold the plates apart when turning on the power. The "pressure' inside the chamber stays exactly the same. Even when the plates move together, the pressure is the same because the plates simply moved, they didn't exit or enter the chamber.

You're ignoring the difference between quantum mechanical transfers of kinetic energy between TWO OBJECTS and the actual "pressure" of the vacuum chamber. The pressure of the chamber doesn't change when you turn on the power, so they are definitely *NOT* identical!

 

[Ziggurat]

No Zig they are not "identical". If they were identical, when you turned on your device, you would actually create a "negative pressure" inside the vacuum chamber and we'd see a large change in the actual pressure of the vacuum chamber. We don't.

The pressure is between the plates. It isn't outside the plates.

If it created "negative pressure" in between those plates, the other positive pressure items would get "sucked in"

What happens if you stick a dielectric partway into the gap in a capacitor and then let go?

It gets sucked in.

Oops.

Now answer my question. The energy density of the electric field (or photons, if you like) between the plates is constant and positive. The more space there is between the plates, the more energy there is between the plates. The energy is positive, and it scales with the volume. Do you actually disagree with any of this paragraph?

 

[Michael Mozina]

The pressure is between the plates. It isn't outside the plates.

That's just a "lame" answer IMO. Suppose we put a spacer between two charged plates so they can't touch, but there is 'space' between the plates so air, and other particles can get between the plates too. Now we turn on the power so the plates have a charge. If the area between the plates really experienced "negative pressure"', the particles in the rest of the vacuum chamber would get 'sucked in" and the whole pressure inside the vacuum should drop. It doesn't. I don't care one *IOTA* about a *THIRD* object, so "foregetaboutit". If the PRESSURE was actually "negative', then they whole vacuum pressure would change. It does not. Your argument is toast.

What happens if you stick a dielectric partway into the gap in a capacitor and then let go?

Who cares about a *THIRD* object Zig? What happens to the "pressure" in the vacuum chamber itself when you turn on the power? *ABSOLUTELY NOTHING HAPPENS!* They cannot then be "identical" processes as you claim, otherwise the total vacuum pressure would also change when we turn on the power It doesn't have any effect on the pressure of the vacuum, therefore your argument goes down in flames.

 

[Ziggurat]

That's just a "lame" answer IMO. Suppose we put a spacer between the plates so they can't touch, but there is 'space' between the plates so air, and other particles can get between the plates too. Now we turn on the power. If they area between the plates really experienced "negative pressure"', the particles in the rest of the chamber would get 'sucked in" and the whole pressure inside the vacuum should drop. It doesn't.

Let's say we've got a chamber filled with He at 1 atmosphere. Now we introduce a smaller container into the chamber. This smaller container is filled with nitrogen at 10 atmosphere pressure. The walls of the smaller container have microscopic pores such that helium can diffuse through it, but nitrogen cannot. What happens to the pressure in the outer container?

According to your reasoning, it should increase. It doesn't.

According to your reasoning, the high-pressure nitrogen should prevent any low-pressure helium from entering the smaller container. It doesn't.

I don't care what your opinion on the matter is. Your opinion is ignorant and wrong.

Now answer my question. The energy density of the electric field (or photons, if you like) between the plates is constant and positive. The more space there is between the plates, the more energy there is between the plates. The energy is positive, and it scales with the volume. Do you actually disagree with any of this paragraph?