BenSeidel wrote:Here is a simple article that I found in about 10 seconds of googling that sums it up reasonably well. The key point here is that pressure is always a vector, hence it can never be negative, but instead it refers to the relative direction to which pressure (force) vector is acting, ie is it an inward force or an outward force. It's a complex subject, but it is possible to build systems that really do pull and in no way can be interpreted as pushing fluids from one region to another (as classical pressure modeling dictates). These systems are generally referred to as having negative pressure, to distinguish it from systems where liquids move due to a pressure differential.
http://discovermagazine.com/2003/mar/featscienceof
Also, Just because a term may be used to describe something else, like a negative pressure room, does not mean that it has another meaning in another context.
Anyone know the blackest object in our solar system? Answer: it's the sun.
Sorry for the out of topic, but by no means pressure is a vector.
Pressure is a scalar, as confirmed by the wikipedia article, and by no mean the article linked explicitely questions this definition, although at some point it makes an infortunate mistake about pressure being a force. Also, the article does not say pressure can't be negative, pretty much the opposite, and gives the signification of negative pressure.
Now, what matters is the effect of pressure, namely the pressure force, which is a vector, but is obtained by taking the
gradient of the pressure and summing it over the surface the pressure applies (or pressure differential, differential being easier to understand as a vulgarisation term, but the mathematical object behind is broader and more abstract than its particular case, the gradient).
In my understanding, pressure is not a physical thing, only a
mathematical tool to describe forces, pretty much like acceleration field can describe the movement of an object without any physical consideration of what caused the forces accelerating the object. Pressure is obtained by choosing a zero (eg atmosphere) and integrating the forces you want to describe with it (reverse operation of the gradient). And pulling force may require negative pressure. As a matter of fact, you could replace all readings of pressure by themself minus a gazillion and still describe the same effect, as the gradient it generates stays the same (and all the pressure would be negative). If you pull a charriot, by no way you are limited by the atmospheric pressure and the surface of the rear of the charriot to pull it (and, as convoluted as it may be, it can be described by pressure).
Now, if you start specifically talk about gas pressure, or pressure caused by X, you can start essentialize pressure and say it is a physicall thing, but I don't expect pressure caused by liquid has anything to do with, say, magnetic pressures, except for their effect. And some essentialized pressures can be negative, some can't. For some essentialized pressures, the sign of the pressure has an actual meaning (eg taking vaccum gas pressure as 0), sometimes, pressure can only be defined up to an arbitrary constant, and negative pressure are not an issue, and not even meaningfull.
The article goes in great length with a lot of essentialized pressures that are bound to have different meanings, save for their effect, which is all the word pressure entails : "each man's negative pressure was a stranger to the other" (contradicting your last paragraph). I think Turner final words pretty much tells that the pressure refers to the force it generates, reguardless of the cause. Now, I really don't like this article for spending so little time on this final point and doing it in a much confusing way, stating that "it[negative pressure]'s direction—an inward, imploding, or contracting force, rather than the outward-pushing force typically defined as pressure", which is technically outright incorrect. The technically correct and non confusing statement should have been "negative pressure
implies the direction of the
related force—an inward, imploding, or contracting force, rather than the outward-pushing force typically
described by pressure", because mathematically,
pressure is not a force, it's (implied) gradient is (or kind of, it's a force/area).
As for the blackest visible object of the solar system, I know of no scientific definition of black, which is even less definite that regular colors. Black is precisely a term used to describe different things and that has widely different depending on context. If we define black as emitting or reflecting non visible light, the sun is the blackest object of the solar system, but also, black holes are anything but black. If we define black as non emitting nor reflecting visible light, which is the closest definition to the usual meaning of black, and is the one used in "black holes", then the sun becomes the less black object of the solar system.