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More than two thousand years ago, Aristotle declared that there was no vacuum in nature because objects would travel through a vacuum at an "impossible" speed. In 1277, French Bishop Etienne Etienne Tempier retorted that God could do anything and even create a vacuum.

After that, a scientist succeeded in creating a vacuum. Otto von Guericke, a German scientist, used a self-made pump to pump air out of a copper spherical shell. This may be the first high-quality vacuum environment on earth. In one of his 1654 exhibitions, even two horses could not pull apart the two hemispheres the size of watermelons sucked together by a vacuum.

Since then, vacuum has become a basic concept of physics and the basis of other theories. In von Glick's vacuum, there is no air; in the electromagnetic sense, there is no medium that can slow down light; and a gravitational vacuum is no matter and energy that bends space. In different cases, the exact meaning of a vacuum depends on the type of thing physicists want to describe. "sometimes that's how we build a theory." Said Patrick Draper, a theoretical physicist at the University of Illinois.

As modern physicists try to find more complex hypotheses for the ultimate theory of nature, they encounter more and more kinds of vacuum. Each vacuum has its own characteristics, just like the different existing states of a substance. More and more people believe that the key to understanding the origin and future fate of the universe may be to carefully consider these growing "non-existent"-that is, types of vacuum.

The Power of vacuum: in 1672, German scientist Von Glick wrote a book about vacuum, in which he described a demonstration to Emperor Emperor Ferdinand III III in which two teams of horses tried unsuccessfully to split a copper spherical shell drawn into a vacuum in half. Image source: Royal Astronomical Society / Science

"there are still many unsolved mysteries about the vacuum." "how much is there that we don't know?" said Isabel Garcia Garcia, a particle physicist at the Caffrey Institute of theoretical Physics in California.

So far, the study of vacuum has come to a dramatic conclusion: our universe may sit on a shoddy foundation-a "metastable" vacuum, which is destined to become another kind of emptiness in the distant future. and everything will be destroyed in the process.

Quantum vacuum in the 20th century, when physicists began to look at the real world from a field point of view, everything became not so simple: physical quantities have a value at every point in space (for example, electric field is used to describe how much force electrons will be subjected to at some point in space). In classical physics, the value of a field can be zero at any point, it has no effect and does not contain energy. "vacuum is boring in the classical sense," says Daniel Harlow, a theoretical physicist at the Massachusetts Institute of Technology. "nothing is happening."

But physicists have found that the fields in the universe are quantum rather than classical, which means they are inherently uncertain. There is no quantum field with exact zero energy. Harlow likens the quantum field to a set of pendulums distributed at every point in space-the angle at which they swing represents the value of the field. Each pendulum swings slightly near its lowest point.

Without interference, the quantum field will be in its lowest energy state, that is, the so-called "ground state" or "vacuum state". Elementary particles are ripples caused by a disturbance of the field. "vacuum is actually the preferred state of a system," Garcia said.

Most of the quantum fields in our universe have one and only one ground state, in which they will remain stable forever. Of course, only most of the games, not all.

True and false vacuum in the 1970s, physicists began to realize the importance of another kind of quantum field. Their values, or even the average, are not zero. Such a "scalar field" is like many pendulums tilted at a fixed angle, such as 10 degrees. This state can be the ground state: these pendulums tend to stop at this angle, when they are stable.

In 2012, experimental physicists at the large Hadron Collider confirmed the existence of a scalar field called the Higgs field in the universe. In the scorching early universe, the pendulum of the Higgs field initially pointed straight down. But as the universe cools, the state of the Higgs field changes, just as water turns into ice, and its pendulum deviates from the bottom and rises to the same angle. This non-zero Higgs field gives mass to many elementary particles. )

If there is a Higgs field around, then the vacuum is not absolutely stable. The pendulum of a field may have many metastable angles and may change from one state to another. For example, theorists are not sure whether the Higgs field is in its most stable state-a real vacuum. Some people believe that although the current state of the Higgs field has been studied for 13.8 billion years, it is only temporarily stable, or "metastable".

If that's the case, the good times won't last forever. In the 1980s, physicists Coleman and de Lucia described how the pseudo-vacuum state of the Higgs field "decays". At some point, if enough pendulums in some positions swing to a more stable angle corresponding to a real vacuum-or nothingness-they will drag their neighbors to the same angle. the local vacuum then expands at a speed close to the speed of light. With the expansion of the vacuum, the current physics will be rewritten, and the atoms and molecules on the path will go up in smoke. (but don't panic, even if our current vacuum is really metastable, given its current stability, it will last for billions of years. )

In the potential variability of the Higgs field, physicists have found the first way for vacuum to destroy everything in almost infinite ways.

The more problems there are, the more vacuum there is. When various fields exist at the same time, they interact with each other, affect each other's pendulums, and establish a new public state in which they are more stable. Physicists think of these vacuum as sunken valleys in the undulating "energy land". Different swing angles correspond to different energy values, or the height of the "energy ground". A field tends to lower its energy, just as a stone rolls down a hill. The deepest valley is the ground state, but the stone can stop in a relatively high valley-at least for a while.

Decades ago, the number of this "energy land" broke out. Physicists Joseph Polchinski and Raphael Bousso were studying string theory, the main mathematical framework for describing quantum gravity. String theory works only if the universe is 10-dimensional, and the extra dimensions shrink to undetectable. Polzinsky and Busso calculated in 2000 that these extra dimensions could be folded in a variety of ways. Each folding method creates a unique vacuum that conforms to its own laws of physics.

The explanation that string theory allows for countless vacuum coincides with another discovery nearly 20 years ago.

In the early 1980s, cosmologists put forward a hypothesis called cosmic expansion, which has become the main theory of the birth of the universe. The theory holds that the universe begins with a rapid exponential expansion, which explains why the universe is so smooth and huge. But the success of inflation theory comes at a price.

Researchers have found that once cosmic inflation begins, it continues. Most of the vacuum will explode violently outward forever. Only a limited area of space will stop expanding and become relatively stable "bubbles", which are separated by the expansion of space between each other. Inflation cosmologists believe that the earth is in one of these bubbles.

Vacuum multiverse for some people, the idea that we live in a multiverse-a world of countless vacuum bubbles-is disturbing. It makes the nature of any vacuum (such as ours) seem random and unpredictable, suppressing our ability to understand the universe. Polzinsky told physicist and writer Sabine Hossenfelder that he found the vacuum of string theory so painful at first that he even received psychotherapy. If string theory predicts all imaginable nothingness, does it mean that it predicts everything?

For others, too much vacuum is not a problem; "in fact, it's a good thing," said Andrei Linde, a prominent cosmologist at Stanford University and one of the authors of the theory of cosmic inflation. This is because the multiverse may have solved a huge mystery: the ultra-low energy of our vacuum.

Theorists estimate that when all quantum fields in the universe vibrate collectively, the energy is huge-enough to rapidly accelerate the expansion of space and tear the universe apart in a short period of time. In contrast, the observed spatial acceleration is extremely mild, indicating that most of the collective vibrations are offset and that the energy of our vacuum has a very low positive value.

In an isolated universe, the tiny amount of energy contained in a unique vacuum may seem like an esoteric puzzle; but in the multiverse, it's just a stroke of luck. If different bubbles in space have different energies and expand at different rates, galaxies and planets will only form in the slowest bubbles. So our peaceful vacuum is no more mysterious than the habitable orbit of our planet: we find ourselves here because most other places are unfit for life.

Whether you like it or not, there is a problem with the multiverse hypothesis currently understood. Although string theory seems to allow for innumerable vacuum, so far, no one has found a tiny extra dimensional fold with a small positive energy, consistent with our vacuum. String theory seems more likely to create a negative energy vacuum.

Maybe string theory is not correct, or its flaw may be that researchers' understanding of it is not mature enough. Physicists may not have found the right way to deal with positive vacuum energy in string theory. "it's entirely possible," said Nathan Seiberg, a physicist at the Princeton Institute for Advanced Studies. "this is a hot issue."

Or our vacuum itself may be random. "the popular view is that space [with positive energy] is unstable," Seeberg said. "it may decay, which may be one of the reasons why it is difficult to understand its physical theory."

These researchers suspect that our vacuum is not one of the stable states of reality, and that one day it will vibrate into a deeper and more stable valley. In this way, our vacuum may lose or produce some kind of particles. Tightly folded dimensions can be expanded, and even a vacuum may no longer exist at all.

"this is another option," Harlow said. "A real nothingness."

The end of the vacuum physicist Edward Witten first discovered the "void bubble" in 1982. In studying the vacuum in which each point has an extra dimension curled up into a small circle, he found that quantum instability inevitably vibrates the extra dimension, sometimes shrinking the circle to a point. Witten found that as the dimension vanished, it took everything away. This instability creates a rapidly expanding vacuum bubble whose mirror-like surface marks the end of space-time itself.

The instability of this tiny dimension has long plagued string theory, and various components have been designed to make them stable. In December last year, several scientists calculated the lifespan of an extra crimped dimensional vacuum. They considered a variety of strategies for stabilization, but found that most mechanisms could not prevent the formation and expansion of bubbles. Their conclusion is consistent with Witten: when the size of the extra dimension falls below a certain threshold, the vacuum immediately collapses. Similar calculations that extend the more complex model may rule out a vacuum below this dimension in string theory.

However, if there is a large enough hidden dimension, the vacuum can last for hundreds of millions of years. This means that the virtual bubbles in the theory can match our universe very well. If so, Aristotle may be more right than he thought. Nature may not like vacuum very much. On an infinitely long scale, it may not like anything at all.

Original link: How the Physics of Nothing Underlies Everything

This article comes from the official account of Wechat: Institute of Physics, Chinese Academy of Sciences (ID:cas-iop), author: Charlie Wood, translator: Tibetan idiot, revision: zhenni

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