The incredible story of the vacuum from 'horror vacui' to quantum physics

Madrid, Apr 16 (The Conversation) Is the vacuum empty? The answer depends on the level of sophistication of physics we use.
    If we limit ourselves to the everyday physics of touching, seeing, or smelling, we might say that there is nothing around us. Air is invisible and, in that sense, seems "empty."
    However, our own breathing disproves this intuition: the exchange of oxygen and carbon dioxide in the lungs depends on a well-known physical phenomenon, diffusion, by which molecules move from regions where they are more concentrated to regions where they are less so. Our physiology exploits the fact that there is something where at first glance we see nothing.

    Something similar happened on the long road to understanding emptiness.

    Nature abhors a vacuum
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     For centuries, Western thought was influenced by Aristotle's idea that nature abhors a vacuum (the so-called horror vacui ). According to this view, if a void appeared anywhere, matter would rush to fill it immediately. The idea seemed reasonable: in everyday life, we don't find spaces completely devoid of matter.
    But physics began to move away from purely philosophical speculation when it began to rely on quantitative experiments.
    In the 17th century, Galileo Galilei became interested in a practical problem: raising water from deep wells using suction pumps. This problem was crucial for mine drainage and agricultural irrigation. However, Galileo observed an intriguing limit: water could not be raised beyond about 10 meters by suction. Why did this limit exist?
    His disciple Evangelista Torricelli , with the collaboration of Vincenzo Viviani, devised an experiment in 1643 that provided a decisive clue.

    The weight of the atmosphere
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     Torricelli filled a glass tube approximately one meter long with mercury, stopped it, and inverted it over a container of the same metal. When the stopper was removed, the mercury partially descended, but did not empty the tube. It stabilized, forming a column about 760 millimeters high at sea level.
    Above the mercury was a transparent, apparently empty region: the so-called "Torricelli void" .
    Torricelli also verified that the height of the column did not depend on the shape of the tube or the volume of the space above it. This indicated that the phenomenon was not due to "suction" from the inside, but to pressure exerted from the outside.
    The explanation was revolutionary: the mercury was suspended because the air around us has weight. The atmosphere exerts pressure on the surface of the mercury in the container, pushing it down into the tube.

    The first barometer had been born.

    In search of emptiness
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     The result was confirmed a few years later by Blaise Pascal. In 1648, his brother-in-law Florin Périer climbed the Puy de Dôme , in central France, with a barometer. He observed that the height of the mercury column decreased as the altitude increased.
    The interpretation was clear: the higher the altitude, the less air there is above us, and therefore the lower the atmospheric pressure.
    The column of mercury was supported by the weight of the atmosphere. The experiment confirmed the existence of atmospheric pressure and a surprising idea: the space at the top of the tube could actually be empty of ordinary matter.

    Vacuum pumps
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     But the systematic study of vacuum required more sophisticated instruments: vacuum pumps.
    In 1650, the German engineer Otto von Guericke built one of the first pumps capable of extracting air from a container. His most famous experiment took place in 1654 in Magdeburg: he joined two hollow metal hemispheres, extracted the air from inside, and asked two teams of horses to pull in opposite directions. The animals were unable to separate them. This spectacularly demonstrated the enormous force exerted by atmospheric pressure.
    A few years passed, and scientists Robert Boyle and Robert Hooke perfected the design of vacuum pumps, allowing for more controlled experiments.
    Boyle observed several revealing phenomena. Inside an airless cavity, he rang a bell and saw that it didn't ring. He placed a burning candle inside and saw that it went out. And whether out of stubbornness or curiosity, he put different animals inside, observing that a winged insect was unable to fly. He also noticed that a mouse or a bird was unable to breathe. The vacuum was gaining traction, and the idea jumped from the scientific realm into popular culture.
    The fascination with these experiments transcended the scientific realm. The painting * Experiment with a Bird in a Vacuum Pump *, by the British artist Joseph Wright of Derby , depicts a public demonstration of the effects of a vacuum on a bird, a symbol of the cultural impact of these discoveries.

    The basis of X-rays
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     A vacuum also played a crucial role in Wilhelm Conrad Röntgen 's discovery of X-rays in 1895. The cathode ray tubes used in these experiments required a very high vacuum. If too much gas remained inside, the electrons would lose energy by colliding with air molecules before reaching their metallic target.
    The development of improved vacuum techniques enabled decisive advances in atomic and electronic physics.
    However, the biggest surprise would come with 20th-century quantum physics.

    The quantum vacuum is not empty
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     In classical physics, a vacuum is understood as the absence of matter. But quantum field theory describes it as the lowest possible energy state of the fundamental fields that fill the universe.
    Even in the absence of real particles, these fields experience inevitable fluctuations due to the uncertainty principle . These fluctuations can be interpreted as the ephemeral appearance of particle-antiparticle pairs called virtual particles.
    They cannot be detected directly – if we could do so they would cease to be virtual – but their effects are measurable.
    A notable example is the Casimir effect , predicted in 1948 by Hendrik Casimir and accurately measured in 1997 by Steve K. Lamoreaux's team.
    If we place two extremely close metal plates in a vacuum (separated by distances on the order of micrometers or nanometers), the allowed quantum fluctuations between them are fewer than in the surrounding environment. This difference generates a small net pressure that pushes the plates together.
    A useful analogy is the vibration of a violin string: the conditions at the ends determine which notes are possible. Similarly, the plates restrict the vibrational modes of the quantum field.
    The quantum vacuum possesses measurable physical properties.

    A void filled with physics
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     Today we know that the vacuum is linked to some of the deepest concepts in modern physics, such as the Higgs field , responsible for the mass of many elementary particles; the cosmological constant , associated with vacuum energy and the accelerated expansion of the universe; and quantum electrodynamics , one of the most precise theories ever experimentally verified.
    The historical journey reveals an interesting irony: Aristotle was wrong about the details, but right about the spirit. The void never turned out to be simply nothing. (The Conversation) AMS
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(This story has not been edited by THE WEEK and is auto-generated from PTI)