Vacuum interstellar

The vacuum is it really empty?

We must not confuse the void and nothingness that is the absence of everything. The vacuum is primarily a philosophical concept but it means the absence of matter. Concretely a vacuum is not total or at least it is very difficult to meet. In common sense, when we say that a container is empty, it is filled with air. An empty glass or an empty bottle contains approximately 2x10¹⁵ molecules per cubic millimeter, i.e. two million billion molecules.
The terminology of the vacuum is ambiguous: if a vacuum is not empty, deserve its this name?
Physicists define a vacuum as what remains when we eliminated all it is experimentally possible to remove a portion of space and emptiness is defined by an experimental limit.
There is therefore no empty but ultra-high vacuum. The UHV is obtained when the pressure is very low 10-7 Pa, unity of the international system. This is an area in which the molecules are highly scarce.
How can the void?
To vacuum, it takes a watertight enclosure and pump air with a vacuum pump, we define the quality of vacuum by residual air pressure, expressed in Pascal (Pa), or in the industrial millibar (mbar) or torr (mm Hg. We understand now that the gap can only be partial. A strong UHV on Earth corresponds to a pressure of about 10^-8 Pa.

At this pressure there are still 2 million molecules per cubic centimeter. The difference in the density of interstellar gas is huge, it is about 1 atom per cubic centimeter, is not the vacuum does not exist.
The electromagnetic waves through a vacuum, there is a vacuum in electromagnetic field variations, but these fields do not require hardware support.
The vacuum is the absence of both matter but also radiation, so it does not exist.
Albert Einstein devoted Annex 5 of his book Relativity - Special and General Theory (Relativity - The Special and the General Theory, translated by Robert Lawson, 1961) to relativity and the problem of space. He cites Descartes and Kant and gives reason to Descartes, denying the existence of the vacuum, ie, he says, a blank field.
He notes in his preface to the 9th edition of the book: "physical objects are not in space, but these objects have a spatial extent. In this way, the concept of empty space loses its meaning."

Image: A pressure of a Paschal (Pa) is a uniform, acting on a flat surface of 1 square meter, exercise perpendicular to the surface a force total of 1 newton.

Image: A newton (N) is the force capable of communicating with a mass of 1 kilogram an acceleration of 1 m / s ² (which can be read A newton is the force capable of communicating with a mass of 1 kg increased speed of 1 meter per second every second, or 3.6 kilometers per hour per second).

Magdeburg hemispheres

Magdeburg hemispheres made in 1654 were at the time, one of the most surprising of Otto von Guericke, mayor of Magdeburg, to highlight the "empty".
Two hemispheres empty a little more than thirty centimeters in diameter were assembled so perfectly that the air could be pumped them with the means at the time. So the pressure of the atmosphere surrounding that hold together firmly, since the inside was almost empty. In Regensburg, in 1657 before the Diet (assembly official) and the emperor, we tried to separate the two hemispheres and two couplings of 8 horses could not do it. This experience has demonstrated the action of atmospheric pressure on the hemispheres and thus on the entire surface of the Earth.

Image: The Magdeburg hemispheres drawn by Anton Bekannt GNU Free Documentation License.
Legend of the design:
1. Strings (horses derive left and right out of the picture)
2. Pieces of wood on which are fixed ropes,
3. Ring leather stained with oil and wax, to promote the sealing of the two hemispheres,
4. Tap extracting air,
5. Ring of money keeping ropes.
Source: German Wikipedia, original upload 2. Apr 2004 by Anton.