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Can pressure be negative?

Um, why should I answer your personal questions among the hundreds I receive every single day, when you personally gloat over the fact that you have me on ignore? How would you even see my response?

FYI, I HAVE addressed your question many times now by pointing out that your formula is arbitrarily. It's using a *RELATIVE* number (to one side of the plates) that provides you with a relatively LOWER pressure between the plates vs. the higher pressure outside of the plates. It's like asking me why a math formula that calculates pressure relative ONLY to the top of the wing comes up with a MINUS sign in front of the formula! It's a *RELATIVE* number.

Your formula is also fundamentally flawed because there is a physical limit to the 'flatness' you can expect to achieve from real atoms, therefore the effect isn't "infinite' as your formula implies. Pretty much all your favorite math formulas are usually 'idealized' and they tend to fall apart at the actually level of atoms and subatomic particles. Zig's still convinced it's an infinite effect. How about even showing it comes CLOSE to that number in real life?

Your answer is so wrong, there is no way to even begin a real discussion. In fact, I don't believe you are capable of a real discussion, certainly not when there is any mathematics involved. Why did I try this? :( What was I thinking? :boggled: Back on ignore you go! :boxedin:
 
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?

Hmm, let's see. I'll go through the article and embolden each mention of kintetic energy of the electron. And put in red each mention of the kinetic energy of a photon

"...and the rest contributes to the electron's kinetic energy..."
"...increase in frequency of incident beam increases the maximum kinetic energy with which the photoelectrons are emitted..."
"...the maximum kinetic energy of the emitted photoelectron depends..."
"...The maximum kinetic energy Kmax of an ejected electron..."
"...The maximum kinetic energy of an ejected electron is..."
"...Because the kinetic energy of the electron must be positive..."
"... potential V0 if related to the maximum kinetic energy of the photoelectron..."
"...directly measured potentials, not electron kinetic energy..."
"...calculated maximum electron kinetic energy is..."
"...increase in the maximum kinetic energy calculated for an electron..."
"...and measuring the kinetic energy distribution of the electrons emitted..."
"...Additional kinetic energy is required to move an electron out of the conduction band..."

So that's 12 to the electrons and 0 to the photons. What point were you trying to make?
ITs quite clear also that the equations show that the electron kinetic energy is related to the frequency of the photon. Which was one of the very first things I learnt about QM when I was at school.
 
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"?
Except it has already been demonstrated.
 
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"?
Except it has already been demonstrated.
That's an important point.

That point has already been made many times in this thread, starting with the OP:


Why is the measured pressure exerted by the Casimir effect negative?

How does that happen? What about Casimir experiments that use spheres, why is there more on the one side of a sphere than on the other?

Why is the measured pressure exerted by the Casimir effect negative?

Negative pressure in a vacuum has been measured (Casimir effect)


Has lab-based experiment justification. Is also a result of QED, the most precisely tested theory in the history of physics.

I know that MM.
So now do you aknowledge that with a given configuration a negative pressure is produced by the Casimir effect (as stated in your citation :jaw-dropp)?

Sadly for you, multiple experiments say you're wrong.

I'm not sure what your point is. Are you asking how we can determine that the pressure we measure in Casimir effects is actually a pressure and not some non-pressure force? Well, that's easy: measure its area dependence. If it scales with area, then obviously it's a pressure, pretty much by definition. And it does.


The Casimir effect hasn't failed the physics test. It doesn't require faith because it can be demonstrated quantitatively and objectively (one of the essential separators between science and religion). It's not physically impossible. And it does indeed occur in the lab. Any argument that it doesn't occur, after having been repeatedly pointed to references showing that it does, would appear to be a demonstration of dishonesty or willful ignorance.

Mozina, why have you not addressed this? The equation used was developed on theoretical grounds and has been *empirically* demonstrated in a lab.
If there were no negative pressure, what is the explanation for the experimental confirmation of the equation?


Michael Mozina doesn't like the physicists' theoretical explanation(s) of the Casimir effect, and has tried to come up with alternative mechanisms that could produce the negative pressures seen in laboratory experiments.

That's fine, even laudable. Unfortunately, Michael Mozina began to deny the experimental results themselves when posters explained why his proposed alternative mechanisms could not explain those experimental results. Michael Mozina also argued against the standard explanation of those results, as though his attacks on that standard mechanism could make the empirical fact of negative pressure go away.

In short, Michael Mozina has been denying and ignoring the results of laboratory experiments. In Michael Mozina's imaginary universe, the experimental measurements showing negative pressure can be made to disappear by writing stuff like this:

You seem to have a physical disconnect somewhere between your beloved math formulas and empirical physics.

Let's take a good hard look at what you CANNOT justify or verify in the lab.

...snip...

Virtually your *ENTIRE* theory (at least 96%) is dependent upon what you CANNOT demonstrate in the lab, and only 4% is based on empirical physics that you can justify in an empirical manner. How is that not a 'religion" in terms of having "faith in the unseen" (in the lab)?

You guys and girls really do have a physical disconnect between particle physics and your math formulas. You're great at the macroscopic level, but in the microscopic realm, you're like a fish out of water. You just don't "get it".

Your stuff never fails the math test, but it always fails the physics test.

...snip...

It's really too bad your stuff always fails the physics test like all good "religions'. Unfortunately you all chose to put your faith in things that are physically impossible to demonstrate and never occur in the lab, just like any good religion. Those who dare to question your faith in the unseen (in the lab) are put to the "fire" of personal attack, so at the level of peer pressure, it works *EXACTLY* like a religion, right down to the possibility of loosing your funding and being ostracized by the rest of the community. It sure has all the "smell" of a religion since apparently all of it requires "acts of faith" in the unseen in the lab, and numerous ones too, starting with "negative pressure in a vacuum".

You don't have any empirical evidence that a vacuum can hold "negative pressure", in fact you can't even tell us what you would add or subtract from a pure vacuum (devoid of all kinetic energy) to create a 'negative pressure vacuum". Your theory fails the empirical physics test *AND* apparently it even fails the theoretical physics test.

As a stanch lover of empirical physics it's really hard to not to see the comparison. This mythical negative pressure in a vacuum god is about as useless and as impotent in the lab as most religious deities.

Unfortunately none of your claims can be demonstrated in a lab. It's therefore a complete act of faith on the part of the 'believer' and anyone that questions your faith in the unseen (in the lab) is attacked as an individual. Since you can't run around calling folks "evil" or a "spawn of satan" in your quaint little religion, they are called a "crank' or a "crackpot", or you degrade their math skills, virtually execute them, virtually silence them, or all the above in your case.

If you can't do that with your mythical magical negative pressure god, why should I bow your your impotent deity, or your peer pressure? Why is it my fault that your negative vacuum pressure sky deity is a no show in the lab?

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


Michael Mozina has written that kind of stuff before, quite often in fact, but repetition does not erase the obvious contradiction between Michael Mozina's rhetoric and the replicable laboratory confirmation of negative pressure in Casimir experiments, including experiments cited by Mozina himself.

I don't see how that contradiction can be explained by Michael Mozina's inability to bark math, although that may be a contributing factor.

I don't see how that contradiction can be explained by Kruger and Dunning, although their research is undoubtedly relevant.

Hypocrisy may be involved.
 
I'm not sure I understand the contention with the Casamir effect. Negative pressure is the result of the non zero gravitational potential in the field whose extent is infinite (ie universal) this readily explains virtual pair production at wavelengths of a particular size and range and this in turn explains the negative force. Why is this even controversial? Sure virtual is a controversial term, but these particles exist as much as a photon or an electron do, their existence is just fleeting because of the debt to energy conservation laws, which is usually met by the energy field. No mystery no voodoo just field theory.

Perhaps the something from nothing monicker causes contention with some people who don't understand the mechanics of the process. Technically although we don't know the exact source of these particles, no one is claiming that any laws of the universe are being broken here, conservation of energy or otherwise.

The only contention I can see is if indeed this mysterious repulsive force explains "dark energy" and the variable Hubble constant and hence the increasing speed of expansion in the universe.
 
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Ahem... I'm still waiting for MM to post the equation which outlines the supposed "kinetic energy" of a photon. I'd really like to see where this comes from...
 
Ahem... I'm still waiting for MM to post the equation which outlines the supposed "kinetic energy" of a photon. I'd really like to see where this comes from...

He has been suspended for 7 days so don't hold your breath.:)
 
Hmm, let's see. I'll go through the article and embolden each mention of kintetic energy of the electron. And put in red each mention of the kinetic energy of a photon

Hi Tubby, I'm a bit puzzled by your position here. You're mistaken. The photon does have ordinary kinetic energy; the relativistic formula for KE is KE = Etot - mc^2; i.e. it's defined to be all of your energy other than your rest mass. With m=0, the photon's energy is *all* kinetic.
 
The phenomenon involved here is the photoelectric effect which can be explained as follows: it takes some definite amount of energy to emit an electron from a metal surface.

This amount of energy is known as the work function (Φ) of the metal which is different for different metals. When photons or electro magnetic radiation are incident on the metal surface the electrons of the metal absorbs the energy from them just enough to get emitted.

If a photon possesses energy at least as large as the work function, the photon energy can be transferred to the electron and the electron will have enough energy to escape from the metal.

A photon with an energy less than the work function of the metal will never be able to emit electrons. Photons do not have any rest mass, but they have momentum and hence energy given by:

Energy of photon E = h ƒ

When electromagnetic radiation of frequency f is incident on a metal surface that has a work function Φ, the maximum kinetic energy of the emitted electrons is given by:

Kmax = h ƒ - Φ


Does Kmax not represent the kinetic energy of the photon?

Perhaps MM is right about something after all.;)
 
Hi Tubby, I'm a bit puzzled by your position here. You're mistaken. The photon does have ordinary kinetic energy; the relativistic formula for KE is KE = Etot - mc^2; i.e. it's defined to be all of your energy other than your rest mass. With m=0, the photon's energy is *all* kinetic.

This is largely a semantic issue, but I think that's a bad definition of kinetic energy. In particular, there's no reason to treat the energy of a photon as being any different from the energy of static electromagnetic fields. It's just the integral of the (appropriately scaled) square of the field. Would you call the energy stored in a capacitor "kinetic energy"? I wouldn't. Which is why I wouldn't call the energy of a photon "kinetic energy" either. Kinetic energy should be reserved for the energy associated with the motion of mass.
 
This is largely a semantic issue, but I think that's a bad definition of kinetic energy. In particular, there's no reason to treat the energy of a photon as being any different from the energy of static electromagnetic fields. It's just the integral of the (appropriately scaled) square of the field. Would you call the energy stored in a capacitor "kinetic energy"? I wouldn't. Which is why I wouldn't call the energy of a photon "kinetic energy" either. Kinetic energy should be reserved for the energy associated with the motion of mass.

It is a semantic issue, but I agree with ben. "Kinetic" comes from the Greek for motion, and "kinetic energy" refers to energy associated with motion. Photons move - in fact, they cannot be at rest - therefore, their energy is kinetic.

More formally, people refer to the terms in the Hamiltonian that contain time derivatives as "kinetic terms" and their contribution to the energy as "kinetic energy". Photons correspond to field configurations with non-zero time derivative; therefore by that definition as well their energy is kinetic.
 
Hi Tubby, I'm a bit puzzled by your position here. You're mistaken. The photon does have ordinary kinetic energy; the relativistic formula for KE is KE = Etot - mc^2; i.e. it's defined to be all of your energy other than your rest mass. With m=0, the photon's energy is *all* kinetic.

Doh! Now you went and gave away the answer, Ben :mad:

Or... you gave away an answer which could actually generate constructive discussion.

I was hoping to pin MM down and make him do some actual math, or - dare I say it? - make him back up his claims with some kind of solid argument. Now that you've gone and given it all away, MM is just going to keep hand-waving.

;)

ETA: Now if only we can get MM to do the math behind the potential energy of a gravitational field following the inverse square law, we might get him to understand that physical systems can contain negative energy.
 
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More formally, people refer to the terms in the Hamiltonian that contain time derivatives as "kinetic terms" and their contribution to the energy as "kinetic energy". Photons correspond to field configurations with non-zero time derivative; therefore by that definition as well their energy is kinetic.

Yes, the fields have time derivatives, but the calculation of the energy in those fields requires no time derivative. You can do it with a simple integral of the field intensity, just as you can for static fields.
 
Yes, the fields have time derivatives, but the calculation of the energy in those fields requires no time derivative. You can do it with a simple integral of the field intensity, just as you can for static fields.

The field strength involves time derivatives of the gauge potential. Not only that, for (say) a single propagating photon, the energy density is time-dependent.

Suppose the photon had a mass (or consider a W or Z boson if you prefer). You could still compute the energy by a very similar integral, with an extra contribution associated with the mass (but without time derivatives). Yet you'd certainly consider the energy in a moving lump of massive gauge field kinetic, right?
 
Suppose the photon had a mass (or consider a W or Z boson if you prefer). You could still compute the energy by a very similar integral, with an extra contribution associated with the mass (but without time derivatives). Yet you'd certainly consider the energy in a moving lump of massive gauge field kinetic, right?

Sure. But I don't see that as sufficient motive to call photon energy "kinetic" when it's massless. This is a semantic debate anyways, and one's preferences in this regard have no effect on the actual subject of this thread, so we should probably move on.
 
Sure. But I don't see that as sufficient motive to call photon energy "kinetic" when it's massless.

Except that I can take the limit where the mass goes to zero (or alternatively where the kinetic energy gets large) and get arbitrarily close to the massless case. So I think it's pretty strange to name everything as kinetic except the (smooth) massless limit, especially when we're talking about energy carried by moving lumps.

But I agree it's off topic.
 
Hi Tubby, I'm a bit puzzled by your position here. You're mistaken. The photon does have ordinary kinetic energy; the relativistic formula for KE is KE = Etot - mc^2; i.e. it's defined to be all of your energy other than your rest mass. With m=0, the photon's energy is *all* kinetic.
I was thinking about the relativistic formula earlier but in the form T = (gamma-1)mc2. For a photon we'd have m=0 but with v = c we'd have infinite gamma. In that sense is the kinetic energy not undefined? But then I thought about momentum and that is gamma*mv and yet I'm quite happy with photons having momentum. Are there expansions of these formulas that give something meaningful in the limit of m->0 and v->c? I'm a little rusty (And lazy) here.
Anyway, as I was taught it, KE is equal to the amount of work required to accelerate the body from rest to their current velocity (or from any other arbitrary velocity if you want to make things difficult for yourself). In that sense I don't see how photons can carry any kinetic energy.

It is a semantic issue, but I agree with ben. "Kinetic" comes from the Greek for motion, and "kinetic energy" refers to energy associated with motion. Photons move - in fact, they cannot be at rest - therefore, their energy is kinetic.
On that semantic issue I'd disagree completely then. Photons move I agree, but there is no energy dependence related to the speed with which they move. That is to say, all photons have speed c (in vacuum at least) but that tells us nothing about how much energy a photon carries. If the energy a photon carries does not depend on the velocity with which they move then how can that energy be said to be kinetic if kinetic energy is the energy of motion?
 
I was thinking about the relativistic formula earlier but in the form T = (gamma-1)mc2. For a photon we'd have m=0 but with v = c we'd have infinite gamma. In that sense is the kinetic energy not undefined? But then I thought about momentum and that is gamma*mv and yet I'm quite happy with photons having momentum. Are there expansions of these formulas that give something meaningful in the limit of m->0 and v->c?

Yes, the m=0 v=c limit is perfectly well behaved and finite.

Anyway, as I was taught it, KE is equal to the amount of work required to accelerate the body from rest to their current velocity ... the energy a photon carries does not depend on the velocity with which they move then how can that energy be said to be kinetic if kinetic energy is the energy of motion?

I'm not surprised you were taught it that way; that's the most straightforward way to learn what energy is, and in nonrelativistic contexts it's right. But I've never heard it invoked as part of the definition. The definition of kinetic energy, as far as I'm concerned, has always been E - M---the portion of the kinematic energy which isn't just rest mass.
 
I'm not surprised you were taught it that way; that's the most straightforward way to learn what energy is, and in nonrelativistic contexts it's right.
Is it not still right for massive relativistic particles? Sorry, my brain doesn't allow me to think for myself on Friday evenings when it comes to physics.

But I've never heard it invoked as part of the definition. The definition of kinetic energy, as far as I'm concerned, has always been E - M---the portion of the kinematic energy which isn't just rest mass.
I'm now thinking that a definition along the lines of "the energy a body has due to its momentum" would kind of satisfy me. Under such a definition then, yes, photons have kinetic energy. But like people say, it is all semantics.
 
Is it not still right for massive relativistic particles? Sorry, my brain doesn't allow me to think for myself on Friday evenings when it comes to physics.


I'm now thinking that a definition along the lines of "the energy a body has due to its momentum" would kind of satisfy me. Under such a definition then, yes, photons have kinetic energy. But like people say, it is all semantics.

It is indeed all semantics but the Devil is in the details hence interpretations.

It's all atm a matter of how exactly you explain something. It's also all a matter of how fundamentally you want to explain something. IMHO there has never been a better time to be alive in physics, even more so than the early 20th century because it's all just still up for grabs. :)

It must fascinate those who want definitive answers, just as it creates 98 page threads on the exact ToE.

It's all good though, as long as our reach doesn't exceed our grasp. Houses of cards are hard to build that way; one should ask if they need to be built at all, if the foundations aren't sound.

I'm a great believer though in KISS, the more complex and involved the foundations get without evidence the further they are likely to be from the truth. The universe probably runs on simple stuff, but we aren't advanced enough to know how yet. Simple explanations and language we barely can conceive mathematical or otherwise are all we have for the time being. As long as we can reach a consensus on what we know that is about all we can expect, even if it is the same way of describing something in different terms. My only worry is that a lot of people want to describe stuff in terms they cannot possibly prove and then build on that, as if that is sciences remit. Philosophy is fine but it is not pure science yet , when you build a structure, start with the foundations and work up, don't reach up into the second floor when they foundations aren't in place unless you call it like it is, pure philosophy. Here's to the dreamers but that's all they are. :)
 
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