Why vacuum has negative pressure?

[scope]

hi,

does anyone know why vacuum energy has negative pressure?
is it only experimental or it is theoretical too?

i also wonder if the negative pressure of the Casimir effect is absolutely negative or if it is relatively negative relatively to the rest of the laboratory?

i would be grateful for any reply!

 

[Rajini]

There is nothing like negative pressure.
Furthermore, it is impossible to achieve zero pressure.

 

[scope]

and what about the casimir effect?

 

[Rajini]

What you want to know about Casimir effect ?
details can be seen here:
http://en.wikipedia.org/wiki/Casimir_effect

 

[scope]

i mean you said pressure can not be negative, but pressure seems to be negative for the casimir effect, so what is to be believed? is it absolute negative pressure?

 

[Rajini]

Hi,
I think if you take the magnitude of the force, then the calculated pressure is negative. If you take the value of force together with its sign (which is negative), then pressure is positive.
But i am not not 100% sure about Casimir effect. So maybe some expert will advice you.
cheers

 

[turbo]

Thinking of the Casimir effect in terms of pressure, you can posit that the virtual particle-antiparticle pairs of the quantum vacuum have an effect on macroscopic objects. If you put two flat parallel plates of a Casimir device (other shapes have been used) very close to one another, you can suppress the emergence of virtual pairs of any wavelength larger than that separation. The Casimir effect (attraction of the plates) can be thought of as the reduction of "pressure" between the plates. There other interpretations, but you should be able to dig those up if you are interested. Casimir wanted to calculate the magnitude of the Van der Waals force between molecules of the materials comprising the plates. I don't know if he had any other motivation - you should be able to dig into that though.

There is an interesting macroscopic parallel in the behavior of ships. If you position a ship closely enough to another to suppress larger waves in the space between them, the wave action of the water on their out-sides will tend to force them together.

 

[cesiumfrog]

There is an interesting macroscopic parallel in the behavior of ships. If you position a ship closely enough to another to suppress larger waves in the space between them, the wave action of the water on their out-sides will tend to force them together.

How strong is this effect?

 

[turbo]

How strong is this effect?

You can look it up. It is significant.

 

[strangerep]

[...] but pressure seems to be negative for the casimir effect, so what is to be believed? is it absolute negative pressure?

Don't "believe" anything -- work through the math... :-)

The vacuum state is just the lowest-energy state for a particular Hamiltonian
and boundary conditions. It's misleading to think that the "vacuum" of infinite
spatial extent is somehow the same as the "vacuum" between two parallel
conducting plates. In QED those cases are significantly different.

With the Casimir effect, the boundary conditions at the plates mean there's
fewer possible EM modes when the plates are closer together, hence less energy.
That manifests as a force pushing the plates together since the system will try to
move towards a lower energy state.

 

[cesiumfrog]

You can look it up.

Do you recommend any particular place to attempt so? The most I found so far was the wikipedia "Casimir effect" talk page, debating whether or not the effect between ships is a myth.

 

[sisphysicist]

As we know that vacuum is a place having nothing,therefore, the negative presssure may be due to the Newton third law !