Can pressure be negative?

[Michael Mozina]

That I can understand, but


Wouldn't that require a negative volume?
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!

Gee, I sure hope not. They're the one's that are trying to turn reality on it's head. Due to physical limitations, no vacuum chamber or experiment on Earth is physically capable of reaching even a "zero' pressure state. The quantum pressure/kinetic force effect is related to photon kinetic energy that has NOTHING at all to do with the "pressure" in the vacuum. They just don't get the concept of particle kinetic energy. In this case the quantum photon kinetic energy is simply GREATER on the outside and LESS on the inside. We know that he photon is the carrier particle of the kinetic energy because the type of materials matters. Only materials that are attracted to magnetic fields show this effect and the photon is the carrier particle of the EM field.

That particle kinetic energy is represented by the green wavy lines and the blue arrows in the this WIKI drawing about the Casimir effect. None of the blue arrows point away from the plates. They all push into the plates, just more on one side than the other.

 

[Ziggurat]

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, energy doesn't always need to be positive. In fact, in Newtonian physics, the absolute energy never even matters, only relative energy does, so the difference between positive and negative energy (as opposed to positive and negative energy differences) is arbitrary to begin with. It then becomes largely a matter of convenience how one wants to define zero energy. An object at rest is a convenient reference for zero kinetic energy, and thus kinetic energy is always positive using this reference. But it's also convenient to define zero gravitational potential for two masses as the potential when they're infinitely far away, so that gravitational potential energy (-GMm/r) is always negative with this reference.

In general relativity, these choices are no longer arbitrary, but even so, negative energies are still possible (though certain quantities like kinetic energy will always be positive).

 

[edd]

Due to physical limitations, no vacuum chamber or experiment on Earth is physically capable of reaching even a "zero' pressure state.

Using this as an argument against negative pressure is like saying you can't make a house out of wood, because you know a builder who has bricks and if you take all his bricks away he won't have any wood.

 

[Perpetual Student]

Ha! I knew it would come to this. It's the pretty picture, not the mathematics that determines the truth.
The artist's rendering is to be believed; not the researchers who did the analysis and the mathematics, who included the negative sign below.
[latex] \dfrac{F_c}{A} = - \dfrac{(hbar)c\pi^2}{240a^4}[/latex]

 

[ben m]

Using this as an argument against negative pressure is like saying you can't make a house out of wood, because you know a builder who has bricks and if you take all his bricks away he won't have any wood.

Well said!

 

[Michael Mozina]

Using this as an argument against negative pressure is like saying you can't make a house out of wood, because you know a builder who has bricks and if you take all his bricks away he won't have any wood.

I'm sorry, but I really don't follow your analogy edd.

No 'vacuum' in the world, no 'vacuum' in any 'experiment' on Earth has, or ever could achieve, a "zero' pressure state. No such state exists in any vacuum in nature today. Even a zero kinetic energy/pressure state is a mathematical abstraction because it simply does not exist in nature today. Neutrinos blow through every experiment on Earth and even Earth. What evidence do you have that any such a state is even physically possible, EVER?

The best you might hope for is a "low" pressure environment. That's a physical fact. The pressure of the vacuum isn't even related to the Casmir effect because it occurs at *VARIOUS* pressures and the pressure has very little effect on your math formula! This whole Casimir argument is in fact a red herring because Guth never had two "clumps", let alone two perfectly flat plates to his name, just one clumpy thingy. Please don't blame me for your need for a "singular" creation event. That was you choice, not mine.

 

[edd]

Imagine for the sake of argument bricks have positive pressure and planks of wood cause negative pressure. You have a stack of bricks. Taking away bricks will never make a plank of wood appear.

Negative pressure doesn't occur by passing through zero and substances with positive and negative pressure can coexist.

 

[edd]

Oh yes, and the Big Bang is about as non-clumpy as the universe ever gets.

 

[ben m]

I'm sorry, but I really don't follow your analogy edd.

No 'vacuum' in the world, no 'vacuum' in any 'experiment' on Earth has, or ever could achieve, a "zero' pressure state. No such state exists in any vacuum in nature today. Even a zero kinetic energy/pressure state is a mathematical abstraction because it simply does not exist in nature today. Neutrinos blow through every experiment on Earth and even Earth. What evidence do you have that any such a state is even physically possible, EVER?

You keep coming back to particle-densities and kinetic energies.

That is a subset of the types of pressure that are possible according to the laws of physics.

We know that gas kinetic pressure is always positive. You don't have to keep repeating this. Vacuum energy/vacuum pressure is not due to a gas, it's due to quantum mechanics and spacetime geometry. Your gas-pressure intuition has nothing to do with the topic.

 

[DeiRenDopa]

Er, no. It has a some positive pressure and a LOT of KINETIC ENERGY.

Give me E(V) then. Find a positive E(V) which will satisfy our observations.

Oh, but that's "barking math on command". Except it isn't: it's just getting you to support your claims.

The truth is, there is no positive E(V) which will give us a positive, volume-independent pressure. It's not possible. So either we have a negative energy density and a positive pressure outside the plates, or a positive energy and a negative pressure inside the plates. One or the other. You cannot reconcile these results with a simultaneously positive pressure and a positive energy.

What he hell is negative vacuum energy? I'm talking about particle kinetic energy, specifically photons.

This isn't radiation pressure, Michael. Radiation pressure cannot explain the Casimir effect.

This exchange rather neatly illustrates, I think, the deep, unbridgeable (?) gulf that separates MM from, well, almost everyone else who's ever engaged in an exchange of posts/comments with him.

To MM, pressure (or "pressure", or perhaps *pressure*, or *PRESSURE*) is a subjective, intuitive concept. Explaining this concept to others MM invokes other - subjective and intuitive - concepts such as radiation and kinetic energy.

To most others, pressure - in physics - is a concept which has a clear, unambiguous definition. Being part of physics, that definition is - at its heart - mathematical (since at least Newton and Galileo physics has been inextricably tied to mathematics). Of course, there may be several different definitions of pressure, depending on the domain or physical environment for example; however, they are all inherently mathematical.

Unfortunately, MM does not 'do' math, and his approach to - and understanding of - physics is entirely subjective and intuitive.

That leads to endless exchanges of boring posts, not only on negative pressure, but also on magnetic reconnection, cosmology, empirical science, plasma physics, and much more besides.

FWIW, quite some time ago I concluded that unless and until MM is willing to honestly and seriously address the question of why, fundamentally, his worldview of physics is so different from that of physicists (and many others) - and how a meaningful dialog may be developed - there is little point in even reading what he posts (so he's on Ignore). Of course, what others write is often worth reading, and some of the insights and approaches presented are pure gold.

 

[Perpetual Student]

Let's say Mozina is correct and the best vacuum pressure that can be reached in a lab is some value, say p_lv.

Now, looking at the equation

[latex] \dfrac{F_c}{A} = - \dfrac{(hbar)c\pi^2}{240a^4}[/latex]

we can see that as the variable a (the distance between the two plates) gets smaller the pressure becomes arbitrarily large. When a is very large we can say the pressure is the same on both the inside and outside of the plates as the value of [latex]\dfrac{F_c}{A}[/latex] approaches zero. So, no matter what the magnitude of p_lv is (we can assume the equation above treats it as zero), it has no bearing on the outcome of the experiment since it is not a variable. There is no other explanation that the pressure inside the plates becomes an arbitrarily large negative value since p_lv is a fixed positive value. Hence, negative pressure exists inside those plates.
Paying attention Mozina?

 

[Tubbythin]

I'm sorry, but I really don't follow your analogy edd.

No 'vacuum' in the world, no 'vacuum' in any 'experiment' on Earth has, or ever could achieve, a "zero' pressure state. No such state exists in any vacuum in nature today. Even a zero kinetic energy/pressure state is a mathematical abstraction because it simply does not exist in nature today. Neutrinos blow through every experiment on Earth and even Earth. What evidence do you have that any such a state is even physically possible, EVER?

Michael, if you have a gas pressure of +1 and a vacuum pressure of -2 (in whatever system you like and whatever system of units you choose) then you have a net pressure of -1 because, last time I looked, 1 + (-2) = -1 which is a negative number.

The best you might hope for is a "low" pressure environment. That's a physical fact. The pressure of the vacuum isn't even related to the Casmir effect because it occurs at *VARIOUS* pressures and the pressure has very little effect on your math formula!

Sight the vacuum energy is the energy that is present in a volume of space even when that volume of space is empty. It is not the energy that is present only when a volume of space is a vacuum. Do you understand this?

This whole Casimir argument is in fact a red herring because Guth never had two "clumps", let alone two perfectly flat plates to his name, just one clumpy thingy. Please don't blame me for your need for a "singular" creation event. That was you choice, not mine.

What are you talking about?

 

[Michael Mozina]

Imagine for the sake of argument bricks have positive pressure and planks of wood cause negative pressure. You have a stack of bricks. Taking away bricks will never make a plank of wood appear.

Negative pressure doesn't occur by passing through zero and substances with positive and negative pressure can coexist.

IMO your analogy fails because you can't demonstrate your "magic wood" even exists. You can demonstrate real bricks and real wood exists, but your type of 'wood' never actually shows up in the lab, and no the Casimir effect is *NOT* an example of any such process, nor does it relate to Guth's plateless vacuum.

*IF* you had say mass or charge *outside* of your clumpy thingy, then sure, I might agree that you might create an internal "attraction" toward something external, that from the internal perspective points outward. Guth only only had a "vacuum" which apparently contained no bricks, but it contained some kind of magic invisible wood that never grows on Earth and that no longer grows anywhere in the universe because it no longer exists in nature. That's a better analogy.

Your (collectively) whole emotional need for a 'negative pressure in a vacuum' is really all about that need you have for a singular creation event. Alfven's "bang" required no need for one giant event which "creates all matter". That's an arbitrary choice on your part, but you have to at least stick to the consequences of your own theory. You can't make up magic density defying wood and then claim you theory is as strong and stable as an empirical house of bricks.

 

[edd]

*IF* you had say mass or charge *outside* of your clumpy thingy

I don't know how many times more I need to tell you there is no clumpy thing. The rest has been done even more to death.

Plus, I've no particular desire for dark energy over certain alternatives, but I do accept it as a possibility.

 

[Ziggurat]

Once again, Michael, can a vacuum have an energy? Yes or no.

 

[Michael Mozina]

Oh yes, and the Big Bang is about as non-clumpy as the universe ever gets.

Alright edd, please start at the beginning and give us the mathematical genesis rundown and a couple of key figures and conditions. Lets' run time backwards 13.7 billion years before the bang. How big is the physical mass/energy of the universe prior the bang? Is it bigger than a breadbox? What is "outside' of this 'thing'? Is anything 'flat" that might be used to create a Casimir effect? What made it grow from it's formerly compact size to it's present size? How big is it? When did the first Higgs form? What was it's size when the first Higgs formed?

I have to ask these questions, because I get a million and one different answers depending on whom I ask. It's not at all unlike a "religion' that starts "in the beginning' but has a billion and one "interpretations" and versions of the same story.

 

[ben m]

IMO your analogy fails because you can't demonstrate your "magic wood" even exists.

We have demonstrated it to the satisfaction of thousands of educated physicists, astrophysicists, and mathematicians. Everyone with a physics background who has looked at the issue has decided it was OK.

The underlying laws of physics (GR, QM) say it's possible for negative-pressure vacuum energy to exist, without violating any sort of logic or consistency check. The actual telescope data say that in fact it does exist.

We haven't demonstrated it to your satisfaction? Sorry, that's your fault, not ours, and certainly not Mother Nature's. Too bad. Likewise, the evidence that the Earth is round has not yet satisfied Time Cube Guy. Too bad.

 

[edd]

Lets' run time backwards 13.7 billion years before the bang.

Kind of busy right now so I'll let you edit this before I answer....

 

[Michael Mozina]

Once again, Michael, can a vacuum have an energy? Yes or no.

A vacuum *ALWAYS* has kinetic energy. It never possesses "negative energy" or "negative pressure" (whatever the hell that is).

 

[Michael Mozina]

Kind of busy right now so I'll let you edit this before I answer....

Would you prefer "To the instant of the bang"?

Edit: I would.