positive vs. negative pressure

[AntonioLao]

In physics, pressure is defined as force per unit area. However, if this area is a closed surface, for example a spherical surface, then two types of pressure can be defined: positive and negative pressure. If one is called p-plus the other is called p-minus. Both are vector quantities with magnitude and direction. For a basketball to stay inflated, its p-plus must equal its p-minus. To keep the tires of a car properly inflated, the p-pluses (outward) must not be less than the p-minuses (inward). For all practical purposes, the 3D configurations of matter imply that some sort of pressure equilibrium does exist which gives matter its various shapes and sizes. For fluids, a new distinct type of pressure emerged. It cannot be classified as static equilibrium but dynamic equilibrium pressure. Although each individual fluid particle has equilibrium pressure, the neighboring particles do not giving the phenomenon of fluidity and made possible the construction of hydraulic presses.

Now, if these p-pluses and p-minuses are applied to the whole universe, the formers are equivalent to the forces of anti-gravity while the latter is the force of gravity. These universal pressures can be applied only to the spacetime continuum not to the embedding matter and energy. Dominance of p-plus gives expansion while dominance of p-minus gives contraction. Nonetheless, there should be certain homogeneity and isotropy although outward decreasing matter density is most likely probable as the distance between galaxies increases.

[vacuum-mechanics]

In physics, pressure is defined as force per unit area. However, if this area is a closed surface, for example a spherical surface, then two types of pressure can be defined: positive and negative pressure. If one is called p-plus the other is called p-minus. Both are vector quantities with magnitude and direction. For a basketball to stay inflated, its p-plus must equal its p-minus. To keep the tires of a car properly inflated, the p-pluses (outward) must not be less than the p-minuses (inward). For all practical purposes, the 3D configurations of matter imply that some sort of pressure equilibrium does exist which gives matter its various shapes and sizes. For fluids, a new distinct type of pressure emerged. It cannot be classified as static equilibrium but dynamic equilibrium pressure. Although each individual fluid particle has equilibrium pressure, the neighboring particles do not giving the phenomenon of fluidity and made possible the construction of hydraulic presses.

Dear AntonioLao,

Indeed it is a simple and clear explanation about positive and negative pressure!

Now, if these p-pluses and p-minuses are applied to the whole universe, the formers are equivalent to the forces of anti-gravity while the latter is the force of gravity. These universal pressures can be applied only to the spacetime continuum not to the embedding matter and energy. Dominance of p-plus gives expansion while dominance of p-minus gives contraction. Nonetheless, there should be certain homogeneity and isotropy although outward decreasing matter density is most likely probable as the distance between galaxies increases.

This seems be a logical explanation, but negative pressure was frequently misunderstood by some people (when talking about the universe) as being anti-gravity forces (of vacuum energy) which making our universe expand!

Now, it is interesting to note that how an attractive force (negative pressure) of vacuum energy could expand the universe; this is a counter intuitive! One good explanation which answers this strange problem was done by Professor Richard A. Mould in his text book “Basic Relativity” as below;

[According to the Friedmann solution of Einstein field equation, the cosmological constant which behaves as positive ambient mass (vacuum energy) will act as a long-range repulsive force that prevents the gravitation collapse of the existing ordinary matter.]

“It is not apparent why a combination of positive ambient mass and negative pressure should cause a long-range repulsion, it is clear that ordinary matter will not produce an effect of this kind. Therefore, the cosmological constant (which behaves as positive ambient mass) can not be thought as an ordinary mass density, its influence on the universe can be learned only through solution of the field equation!”

Sincerely,
Nimit
-------------------
www.vacuum-mechanics.com .

[AntonioLao]

By convention, if a physical vector quantity is first assignd with a positive sign then its exact opposite must be a negative sign. However, the negative pressure as the cosmological constant in Einstein's field equation is something being subtracted in order to prevent expansion. On the other hand if the universe is contracting then the cosmological constant becomes an additionto the field equation to prevent contraction. Both of these cases were anticipated by Einstein in order to create a static universe without expansion or contraction.

[Graybeard]

As well .... just the presence of energy and pressure contribute to gravity. The total gravity of an object is not just its mass.

cool bananas ... greg