Exists empty space

The power from nowhere

Imagine that all material objects were removed from the world. What was left? An empty room, the total nothing? This question has been a concern of philosophers and natural scientists since ancient times. The first to think systematically about the relationship between being and emptiness were the pre-Socratics. Thales von Milet, for example, assumed that space is continuously filled with matter, namely with water, the primary substance of all things. Pure nothingness had just as little space in Thales ’world as empty space. Derived from the Latin word "plenus" for full, such a view is also known as plenism. Famous representatives of this doctrine of natural philosophy were the philosophers René Descartes and Gottfried Wilhelm Leibniz in modern times.

Democritus von Abdera developed a completely different view. Following on from his teacher Leukipp, he postulated the existence of a myriad of no longer divisible units of matter (atoms), which combine in empty space to form ever new combinations and thus form the variety of things. "In reality there are only atoms and empty space," was Democrit's credo. From today's perspective, the scope of this idea cannot be overestimated. When the Nobel laureate in physics, Richard Feynman, was once asked what most important human knowledge he would impart to an extraterrestrial civilization, he replied: "Everything is made of atoms."

But Democrit's model of atoms moving in empty space aroused the disapproval of a far more influential philosopher. We are talking about Aristotle, who was also a staunch plenist and for whom there was no movement without a driving medium. Therefore he thought the entire space was filled with an ether and explained that nature was afraid of emptiness. Latins later spoke of the "horror vacui". Like other views of Aristotle, his plenism was elevated to dogma by the church in the Middle Ages. One of the first to try to rehabilitate the empty space was Giordano Bruno. But the use of the philosopher, who was later burned as a heretic, had no consequences, because there was still no empirical evidence for the existence of a vacuum.

That changed in 1644. At Galileo's suggestion, the Italian physicist Evangelista Torricelli succeeded in proving that at least a vacuum can be created. The classic experiment for this was designed as follows: First, Torricelli filled a long glass tube with mercury and closed it with a finger. Then he dipped the tube, with the closed opening facing down, into a bowl full of mercury. When he opened the tube, the liquid metal inside flowed into the bowl. But not completely. The mercury column stopped at a height of about 76 centimeters and a vacuum or a vacuum was created in the glass above, from which the column appeared to be hanging. In reality, the ambient air was pressing on the mercury in the bowl, preventing the metal from completely draining out of the tube.

Although other naturalists, including the Frenchman Blaise Pascal, repeated and refined the experiment, the plenists did not surrender. "This apparently empty space is certainly filled with ether, which gets into it (into the glass) without difficulty," said the mathematician Leonard Euler. Others claimed that very fine mercury vapors had formed above the mercury. And they weren't wrong about that. "Today we know that there is actually a saturated vapor of mercury above the mercury," explains the Potsdam physics teacher Klaus Liebers. "At that time, however, the objection resembled pure speculation about the rescue of Aristotle."

The Magdeburg physicist Otto von Guericke, the inventor of the air pump, carried out a sensational experiment at the Reichstag in Regensburg in 1654. He put two metal hemispheres on top of each other, sealed them with a leather ring and pumped them dry. Then he had 15 horses harnessed to each hemisphere, which tried to tear the ball apart. But the air pressure that acted on the vacuum inside was so strong that the horses struggled in vain. It was only when Guericke ventilated the sphere that the two hemispheres could be separated without great effort.

It is of course practically impossible to remove all gas molecules from a closed container and thus create a completely evacuated space. With the help of special pumps, however, vacuums of the highest quality can now be produced, which are used in basic physical research or microelectronics, for example. One speaks of a high vacuum when the pressure of the molecules remaining in it is between 10-3 and 10-7 hectopascals. In the so-called ultra-high vacuum, even lower pressures are reached, namely 10-7 to 10-12 hectopascals. The mean free path of the molecules still present is correspondingly large, i.e. the distance they cover in space without collision. It can be up to 100,000 kilometers in an ultra-high vacuum. There is an almost perfect vacuum in cosmic space, which contains an average of one particle per square centimeter. Mind you in the cut. In some areas of space between the stars of a galaxy there are no atoms at all. Nevertheless, billions upon billions of photons, neutrinos and other particles flow through these regions every second.

Strictly speaking, it was not the atomists, but the plenists who, in retrospect, were right, according to the Karlsruhe physicist Henning Genz. »Spaces that are as empty as is possible in harmony with the laws of nature form physical nothing. But that does not mean that such rooms are literally empty. ”They contain, for example, the thermal radiation that occurs at any temperature and that only disappears at the inaccessible absolute zero point of the temperature. But even then, the room would not remain in physical inactivity.

Quantum mechanics is to blame for this. According to Genz, the empty space that she describes corresponds "net" to the empty space of our imagination, but not "gross". This means that from random fluctuations in the energy of the empty space, particle-antiparticle pairs (for example electrons and positrons) continuously emerge, which initially fly apart, then reunite and, as it were, disappear into nothingness through annihilation. These particles are called "virtual" because they cannot become real particles without external energy input. That empty space is capable of such a dynamic at all follows from Heisenberg's uncertainty relation for energy and time. After that, the vacuum must continuously provide energy, a lot of energy for a short time and little energy for a long time. As a result, heavy particle-antiparticle pairs such as protons and antiprotons only appear virtually for a short time in a vacuum.

Sometimes, however, virtual particles also have a real effect. As early as 1948, the Dutch physicist Hendrik Casimir predicted that an attractive force would develop between two parallel, conductive plates in a vacuum. Eight years later, Soviet and Dutch researchers were able to demonstrate the Casimir effect experimentally. This strange phenomenon is explained as follows: The influence of the virtual particles is less between the plates than in their surroundings. As a result, a so-called photon pressure acts on the plates from the outside and pushes them together a little.

In a sophisticated experiment, Swedish researchers recently even succeeded in "materializing" virtual photons. For this purpose, they pumped kinetic energy into the vacuum and let the resulting photons bounce off a kind of mirror that moved extremely quickly. As a result, the photons were converted into a real and measurable state. In principle, electrons and other particles could also be "liberated" from the vacuum by supplying appropriate energy.

According to today's ideas in physics, it was ultimately the vacuum or zero point fluctuations of fields in the early universe that led to the formation of structures such as galaxies and galaxy clusters, writes the Cologne physicist Claus Kiefer in the latest issue of the journal »Physik in our time". "We owe our existence to the vacuum, which nature is not afraid of."

The general theory of relativity founded by Albert Einstein in 1915 contains further astonishing insights into empty space. According to this, in contrast to Newton's mechanics, space is not a passive container for the physical processes, but rather participates in them. The basic dialectical idea of ​​this theory can be formulated briefly and succinctly as follows: Matter commands space how to bend, and space commands matter how it must move. If one were to remove all objects from the room, there would be no structureless nothing left here either. Because even an object-free space has geometrical properties. It can be flat or curved - depending on what is known as dark energy, which is often also referred to as vacuum energy. It interweaves the entire space and is used in modern cosmology to explain the accelerated expansion of the universe.

To this day, however, nobody knows what this energy, introduced ad hoc, is made of, which is believed to make up 72 percent of the total energy density of the universe. There is no experimental evidence of their existence. Likewise a viable approach that would be suitable for integrating dark energy into the formalism of quantum mechanics. Because despite intensive efforts, physicists have so far not been able to combine general relativity and quantum mechanics. Only when this has happened, says Henning Genz, will it be possible to say more precisely how much emptiness in space the laws of nature allow in principle.

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