Can pressure be negative?

[Ziggurat]

I'm getting the distinct feeling that this is a loaded question.

It's a question whose answer has physical consequences. If that's loaded, well, tough. You can't avoid reality because answers are "loaded".

Let's assume the vacuum contains "kinetic energy' (sorry you'll have to learn to deal with physics) in particle form (including photons), and somehow you managed to completely seal your special vacuum from any additional inputs of kinetic energy from outside of the vacuum. It's a physical impossibility of course, but we'll assume you did it somehow. If you then simply increased the volume of the container/vacuum, the particles would "spread out' and become "less dense" and the pressure (quantum and otherwise) would decrease inside of the vacuum.

I didn't ask what happens to the pressure. Tell me what happens to the energy as you expand the volume. Does the energy increase or decrease as the volume expands?

Sorry but get over it. You'll have to accept the concept of particle kinetic energy sooner or later. Why else did you figure that temperature affects pressure in an ordinary vacuum?

Photon energy isn't kinetic. You'll have to accept that your definition of "kinetic" has no connection to the standard definition.

 

[Tubbythin]

Name one vacuum experiment that was ever devoid of all neutrinos? How about one vacuum devoid of all atoms? Baloney it doesn't.

Name one gas that experiment where a gas that perfectly obeyed the ideal gas law used.

Of course they do. The fact you would even say something like this is simply sad IMO.

No they don't. Kinetic energy is a measure of the amount of work that as to be done to accelerate the body from 0 speed to its current speed. Please Michael, tell us all how much energy it takes to accelerate a phtoon from at rest to a spped of c.

I need to get some coffee and take a break for a bit. This place does take it out of me after awhile, especially when I see intelligent folks like yourself make silly statements like that.

Its not silly, as I've just shown. But thank you for calling me intelligent.

 

[R.A.F.]

You know.....

Blah, blah, blah. You haven't changed a bit since your hat was handed to you by the posters over on BAUT for your "iron sun" nonsense...still thinking you can dictate how a conversation should be conducted.

I for one don't need that crap...outta here...

 

[ben m]

Are you claiming that gravity is a 'pressure' rather than a "curvature of spacetime"?

Is that what you *actually* think I said, or did you misread me as badly as possible so you'd have something to argue against?

 

[temporalillusion]

Are you claiming that gravity is a 'pressure' rather than a "curvature of spacetime"?

Take a bag of chips, squeeze it so the air inside is compressed, does it weight more or less than when you weren't squeezing it?

Take the same bag of chips, pull on the bag outwards so the volume inside is larger, does it weight more or less than when you weren't squeezing it?

 

[Michael Mozina]

Photon energy isn't kinetic. You'll have to accept that your definition of "kinetic" has no connection to the standard definition.

I think we better start with this claim since it seems to be a common problem for you folks.

http://en.wikipedia.org/wiki/Photoelectric_effect

http://en.wikipedia.org/wiki/Photoelectric_effect#Mathematical_description

Do a page search on kinetic energy and specifically look at the functions related to kinetic energy. What the hell are you talking about? What do you think moves the electrons around in their various shells?

 

[Michael Mozina]

Is that what you *actually* think I said, or did you misread me as badly as possible so you'd have something to argue against?

No Ben, I honestly have no clue what you think I'm ignoring about gravity as it relates to 'pressure in a vacuum".

 

[MattusMaximus]

But is this really "negative pressure" like what would happen if you could get a negative reading on a barometer? Below vacuum pressure, so that objects have a sort of uniform outward force applied to them, like how that when an object is submerged in an atmosphere it has a uniform inward force applied? You say it "pulls them together" -- but that isn't what I expect from negative pressure: I'd think the term would mean in outward, apart effect, not an inward together effect, regardless of whether or not it's from an external inward push or internal inward pull. It's the "inwardness" that's the problem.

You're mixing apples and oranges here; actually, more like apples and hammers. The type of pressure read by a barometer is a hydrostatic/dynamic effect of fluid mechanics based off of classical physics. The Casimir effect (and related pressure) is a quantum mechanical effect exclusively which has no analogy in classical physics.

These things are fundamentally different, so trying to compare the two in the manner in which you are doing makes no sense.

 

[Michael Mozina]

No Michael. It's been explained before. Pressure contributes to gravity. It is not the force from P=F/A that contributes to the cosmological importance of negative pressure - it's the gravitational effect that the pressure has independently of that.

Which "effect' and can you demonstrate that effect empirically here and now on Earth?

Thinking otherwise is like thinking the universe expanded because of it was hot and the matter in it has a positive pressure, which is a common misunderstanding.

Well, I don't really care if you want to have 'positive pressure" inside your "clumpy thingy". I simply see no evidence that any 'vacuum' is even capable of experiencing "zero' pressure/kinetic energy, let alone a SUB ZERO pressure. It's a nice myth and all, but it's A) physically impossible to get such a thing from a "vacuum", a B) impossible to demonstrate in any lab on Earth in my lifetime. How is that not "faith in the unseen"?

Not just mass sources gravity - energy does, and the pressure the energy or matter has is a contributing factor.

What energy? Inside or outside of the "near singularity" thing?

I'll probably leave the previous stuff about the big bang, it's too much of a derail here. Fork the thread if you wish.

I think that would be best. I didn't really mean to put you personally on the spot like that anyway, I just trust you would do your best to explain it well. I'm not so sure about the rest of these guys. I'll start and new thread and I'll open it up to everyone. It sounds like a fun thread.

 

[MattusMaximus]

Er, no. It has a some positive pressure and a LOT of KINETIC ENERGY. There is no such thing as "negative energy" other than the "vibe" I get from astronomers.

Ummm... you've never heard of a gravity well? The potential energy associated with gravitation is, by definition, negative (assuming the inverse square law, of course).

 

[MattusMaximus]

I'm talking about particle kinetic energy, specifically photons.

Please specify how you propose to calculate the kinetic energy of a photon. What is the equation?

 

[Michael Mozina]

Ummm... you've never heard of a gravity well? The potential energy associated with gravitation is, by definition, negative.

That depends on your point of view. If I put two objects in space at specific distance apart, I simply have POSITIVE potential energy that eventually turns into positive kinetic energy, which is then released again as the two objects slam into one another. Where in that process was either the total kinetic or potential energy 'negative'?

 

[temporalillusion]

Which "effect' and can you demonstrate that effect empirically here and now on Earth?

Squeeze a bag of potato chips, does it weigh more or less than it weighed before you squeezed it?

 

[MattusMaximus]

I always though that the definition of energy was that it is always positive?
E=|F*x|, KE=1/2 MV², PE=1/2kx², E=mC²...
You guys are turning my world downside-up!

No. As I told MM before, the potential energy associated with the inverse square law of gravity is, by definition, negative. Read more here.

There is no reason why the total energy (i.e. sum of kinetic & potential energies) of a system has to always be positive. If the amount of negative PE in a system is greater than the positive KE, the total energy is negative. We see examples of this all the time: for example, the total energy associated with the Earth orbiting the sun is negative - which is a good thing, because if it were not, our planet wouldn't be in a bound orbit around our star

It works the same way for the energy levels of electrons bound to an atomic nucleus. And there are plenty more examples in physics.

 

[Michael Mozina]

Squeeze a bag of potato chips, does it weigh more or less than it weighed before you squeezed it?

Relativistically speaking, more. The atoms in the bag will move faster, so it holds more total energy/mass. Its *internal* pressure increases, but does nothing to the outside pressure. There is a larger pressure difference between the inside and outside of the bag, but neither the inside or the outside of the bag experience a "negative' pressure.

 

[MattusMaximus]

That depends on your point of view. If I put two objects in space at specific distance apart, I simply have POSITIVE potential energy that eventually turns into positive kinetic energy, which is then released again as the two objects slam into one another. Where in that process was either the total kinetic or potential energy 'negative'?

Wrong. This only works if you are using the localized, small-scale formula for GPE = weight x height = mgh --> but this assumes the gravitational force (weight) is constant.

The more general case & expression of GPE shows that you are ignoring the fact that the gravitational force is not constant over long distances. The mathematics of the inverse square law clearly shows that the zero level reference for gravitational potential energy (note that I am not assuming localized effects, we're talking about large scale effects) is negative. In fact, the most that GPE can ever be for such a gravitational system is zero (when the objects are infinitely far apart), and as the objects get closer together the GPE becomes more negative.

Read up on the math. I suggest doing the calculations yourself while you're at it.

 

[Ziggurat]

I think we better start with this claim since it seems to be a common problem for you folks.

http://en.wikipedia.org/wiki/Photoelectric_effect

Read your own link, Otto. "Kinetic energy" only appears in reference to the electron, never to the photon.

Do a page search on kinetic energy and specifically look at the functions related to kinetic energy.

I did, Otto. And what it refers to is the kinetic energy of the electron, not the photon.

What do you think moves the electrons around in their various shells?

Energy. Which the photon has. But it's stupid to call it "kinetic energy", Otto.

But all of that is beside the point anyways. You still haven't answered my question: what happens to the vacuum energy as volume is increased? Let's keep this simple and make it an adiabatic process.

 

[MattusMaximus]

And MM, since you're harping so much on this photon "kinetic energy", please show us the equation that you use to calculate such a thing. A derivation of this equation from first principles would be even better.

ETA: Based upon my earlier posts, do you also now see why it is possible to have a negative energy within a system?

 

[temporalillusion]

Relativistically speaking, more. The atoms in the bag will move faster, so it holds more total energy/mass. Its *internal* pressure increases, but does nothing to the outside pressure. There is a larger pressure difference between the inside and outside of the bag, but neither the inside or the outside of the bag experience a "negative' pressure.

So in GR positive pressure contributes positively to gravity, resulting in increased weight.

This is what edd said: "Pressure contributes to gravity. It is not the force from P=F/A that contributes to the cosmological importance of negative pressure - it's the gravitational effect that the pressure has independently of that."

And you've just agreed with him that pressure contributes to gravity.

Positive pressure contributes positively to gravity, negative pressure contributes negatively to gravity (you can test this by stretching the bag, the decrease in pressure reduces the weight of the bag).

As to what that negative pressure comes from, any uniform field has negative pressure.

If you have a cylinder with a stopper at the top filled with something with a positive pressure, if you pull on the stopper you'll feel the stopper being pushed by the pressure, and as the stopper is pulled the pressure decreases, the energy on the inside of the cylinder decreases.

Now make the stuff inside the cylinder a field with a constant value everywhere in the cylinder. If you pull on the stopper there's more volume for the field to permeate. Since a uniform field has the same energy at every point, the increased volume means the energy in the cylinder has increased.

In the case of positive pressure, pulling on the stopper decreases the energy. In the case of the constant field, pulling on the stopper increases the energy.

In the case of the positive pressure, the stuff in the cylinder is pushing the stopper. In the case of the constant field, because the stopper has to be pulled and an effort expended to move the stopper, the contents of the cylinder are pulling on the stopper.

A uniform field sucks inward, which is negative pressure, which contributes negatively to gravity (repulsion rather than attraction).

 

[ben m]

No Ben, I honestly have no clue what you think I'm ignoring about gravity as it relates to 'pressure in a vacuum".

Do you want to dig through your old threads and find one of the previous 5 times I've explained it?