Is Absolute Vacuum a Utopia?

Vacuum and Ultrahigh Vacuum: From Magdeburg to the LHC — The Magdeburg hemispheres are an experimental device by Otto von Guericke, used in 1654 to demonstrate the existence of vacuum and the concept of air pressure. Horses pull two airtight hemispheres, which have an air extraction valve. Image source: Deutsche Fotothek (new window)

Nothingness vs. Vacuum: Radically Different Concepts

Do not confuse vacuum with nothingness.

Nothingness is the absolute absence of everything: no matter, no energy, no space, no time. It is a metaphysical concept, impossible to observe or reproduce experimentally.
Vacuum is a physical and philosophical concept referring to the absence of matter in a given volume. Unlike nothingness, vacuum can contain energy (electromagnetic fields, quantum fluctuations).

In everyday language, a "empty" container (a glass, a bottle) is actually filled with air. At atmospheric pressure, one cubic millimeter of air contains about 2 × 10¹⁹ molecules, or two quintillion. This density illustrates the ambiguity of the term: if vacuum is never total, does it deserve its name?

Vacuum in Physics: An Experimental Limit

Physicists define vacuum as the state obtained after removing all detectable matter from a given volume, according to current technological limits. It is therefore a relative vacuum, never absolute.

Ultrahigh vacuum is achieved at pressures below 10^-7 Pa (10^-9 mbar). At this level, molecular density drops to about 2 million molecules per cubic centimeter.
For comparison, interstellar space contains on average 1 atom per cubic centimeter, and intergalactic vacuum drops to 1 atom per cubic meter.

How to create a partial vacuum?

A vacuum pump is used to extract molecules from a sealed volume. The quality of the vacuum is measured by the residual pressure, expressed in:

Pascal (Pa) (SI unit): 1 Pa = 1 N/m².
Millibar (mbar): 1 mbar = 100 Pa (industrial use).
Torr: 1 Torr ≈ 133.322 Pa (historical, based on the pressure exerted by 1 mm of mercury).

On Earth, the best ultrahigh vacuums reach 10^-10 to 10^-12 Pa (e.g., particle accelerators like the LHC (large Hadron Collider)). Even under these conditions, molecules remain, and the vacuum remains relative.

Pressure in pascals (Pa): 1 Pa corresponds to a force of 1 newton (N) applied uniformly over 1 square meter.
Definition of the newton (N): SI unit of force. 1 N is the force capable of giving a mass of 1 kg an acceleration of 1 m/s².
Example: An acceleration of 1 m/s² means that an object's speed increases by 3.6 km/h every second (1 m/s = 3.6 km/h).

Vacuum and Electromagnetic Fields: Invisible Energy

Contrary to popular belief, vacuum is not "empty" of energy:

Electromagnetic waves (light, radio, X-rays) propagate through vacuum without a material medium. They correspond to variations in electric and magnetic fields, carrying energy.
In quantum mechanics, vacuum is the site of quantum fluctuations: particle-antiparticle pairs spontaneously appear and disappear (Casimir effect (quantum effect where two metal plates attract each other in a vacuum due to quantum vacuum fluctuations)).

A total vacuum would therefore imply the absence both of matter and radiation/energy. Such a state does not exist in the observable universe (where cosmic background radiation fills space at 2.7 K), nor in laboratories.

Einstein and Relativity: The End of Absolute Vacuum?

In Relativity: The Special and General Theory (1916), Albert Einstein (1879-1955) devotes an appendix to the problem of space and vacuum. He cites René Descartes (1596-1650) and Emmanuel Kant (1724-1804), and rejects the idea of a space empty of fields:
« Physical objects are not in space, but these objects have spatial extension. Thus, the concept of empty space loses its meaning. »
Albert Einstein, preface to the 9th edition (1952).
For Einstein, spacetime is a dynamic entity, shaped by matter and energy (equation E = mc²). The "vacuum" then becomes a field of potentials, where even the absence of matter leaves room for geometric properties (curvature of spacetime).

The Magdeburg Hemispheres: A Historical Demonstration of Vacuum

In 1654, Otto von Guericke (mayor of Magdeburg and scientist) conducted a landmark experiment to demonstrate the effect of atmospheric pressure:

Two metal hemispheres 30 cm in diameter were sealed together hermetically.
The air inside was pumped out (using a rudimentary vacuum pump), creating a partial vacuum.
The external atmospheric pressure (≈ 10⁵ Pa) exerted a theoretical force of ≈ 7,000 N on the hemispheres (30 cm diameter).
In 1657, before Emperor Ferdinand III in Ratisbon, 16 horses (8 on each side) failed to separate them.