More than 2,000 years ago, Aristotle declared that there is no vacuum in nature because objects travel at "impossible" speeds in a vacuum. In 1277, French bishop Etienne Tempier retorted that God is omnipotent, even creating a vacuum.
Afterward, a scientist succeeded in creating a vacuum. German scientist Otto von Guericke used a homemade pump to extract air from a copper spherical shell. This may be the first high-quality vacuum environment on Earth. In one of his 1654 exhibitions, not even two colts could pull apart two watermelon-sized hemispheres that had been vacuumed together.
Since then, the vacuum has become a fundamental concept in physics and the basis of other theories. Von Gerich's vacuum is the absence of air; the electromagnetic vacuum is the absence of a medium that can slow down light; the gravitational vacuum is the absence of matter and energy that can bend space. In different cases, the exact meaning of a vacuum depends on the type of things physicists want to describe. "Sometimes, that's how we construct a theory," says Patrick Draper, a theoretical physicist at the University of Illinois.
As modern physicists struggle to find more complex hypotheses for the ultimate theories of nature, they encounter more and more kinds of vacuums. Each vacuum has its own characteristics, just like the different states of existence of a substance. There is a growing belief that the key to understanding the origin and future fate of the universe may be to carefully consider these ever-increasing "non-existences" -- the types of vacuums.
The Power of the Vacuum: In 1672, the German scientist von Glick wrote a book on the vacuum, describing a demonstration he gave Emperor Ferdinand III in which two teams of horses attempted to put a The evacuated copper spherical shell split in half, without success. Image credit: Royal Astronomical Society/Science
"There are so many unsolved mysteries about the vacuum," said Isabel Garcia Garcia, a particle physicist at the Caffrey Institute for Theoretical Physics in California. "How much is there that we don't know?"
So far, research on the vacuum has led to a dramatic conclusion: our universe may be sitting on an inferior foundation -- a "metastable" vacuum that is destined to become another in the distant future Nothingness, and everything will be destroyed in the process.
In the twentieth century, when physicists began to see the real world in terms of fields, everything became less simple: physical quantities have a value at every point in space (for example, electric fields are used to describe certain how much force the electron will experience at a point). In classical physics, the value of a field can be zero at any point, it has no effect, and it does not contain energy. "A vacuum is boring in the classic sense, and nothing happens," said Daniel Harlow, a theoretical physicist at the Massachusetts Institute of Technology.
But physicists have discovered that the fields in the universe are quantum rather than classical, meaning they are inherently indeterminate. There is no quantum field with exactly zero energy. Harlow likened a quantum field to a set of pendulums distributed at every point in space -- the angles at which they swung represent the value of the field. Each pendulum swings slightly near its lowest point.
Without interference, the quantum field would be in its lowest-energy state, the so-called " ground state " or " vacuum state ." (Elementary particles are ripples created by a perturbed field.) "A vacuum is actually the preferred state of a system," Garcia said.
Most quantum fields in our universe have one and only one ground state, in which they remain stable forever. Of course, only the vast majority of fields, not all.
True and false vacuum
In the 1970s, physicists began to realize the importance of another class of quantum fields. Their values, even the average, are not zero. Such a "scalar field" is like a number of pendulums tilted at a fixed, specific angle, say 10 degrees. This state can be the ground state: these pendulums are more inclined to rest at this angle, at which point 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 hot early universe, the pendulum of the Higgs field initially pointed straight down. But as the universe cooled, the state of the Higgs field changed, just as water turns to ice, its pendulums all veered off straight down and rose to the same angle. (This nonzero 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 angles of metastable states and may transition from one state to another. For example, theorists are not yet sure whether the Higgs field is in its most stable state a true vacuum. Some 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 so, the good times won't last forever. In the 1980s, physicists Coleman and DeLucia described how the pseudo-vacuum state of the Higgs field "decays." At some point, if enough pendulums in some positions swing to an angle that would be more conducive to stability in terms of true vacuum - or nothingness - they will drag their neighbors to the same angle, and then this local vacuum expands at nearly the speed of light. As the vacuum expands, current physics will be rewritten, and the atoms and molecules in their path will be wiped out. (Don't panic though, even if our current vacuum is really just a metastable state, given its current stability, it will persist for billions of years more.)
Amid the potential variability of the Higgs field, physicists have discovered the first of nearly infinite ways that a vacuum can destroy everything.
The more questions, the more vacuum
When various fields exist simultaneously, they interact, affecting each other's pendulums and establishing new common states in which they are more stable. Physicists imagine these vacuums as sunken valleys in an undulating "energy land." Different swing angles correspond to different energy values, or "energy ground" heights. A field will tend to lower its energy, just like a stone will roll down a hill. The deepest valley is the ground state, but stones can rest in relatively high valleys -- at least for a while.
Decades ago, this "energy land" exploded in numbers. Physicists Joseph Polchinski and Raphael Bousso were working on string theory, the main mathematical framework for describing quantum gravity. String theory only works if the universe is 10-dimensional, and the extra dimensions shrink too far to detect. Porzinski and Busso calculated in 2000 that these extra dimensions can be folded in a number of ways. Each way of folding creates a unique vacuum that conforms to its own laws of physics.
The explanation that string theory allows for countless vacuums matches another discovery nearly 20 years ago.
In the early 1980s, cosmologists proposed a hypothesis called the expansion of the universe , which has become the leading theory for the birth of the universe. The theory suggests that the universe began with a rapid exponential expansion, which nicely explains why the universe is so smooth and huge. But the success of inflation theory has come at a price.
The researchers found that once cosmic inflation started, it continued. Most vacuums explode outward forever and violently. Only a limited region of space will stop expanding and become relatively stable "bubbles" that separate due to the expansion of space between each other. Inflationary cosmologists believe that Earth is in one of those bubbles.
For some, the notion that we live in a multiverse a world of countless vacuum bubbles is unsettling. It makes the nature of any vacuum (like ours) seem random and unpredictable, inhibiting our ability to understand the universe. Porzinski told physicist and author Sabine Hossenfelder that discovering the vacuum of string theory initially distressed him so much that he even underwent psychotherapy. If string theory predicts everything imaginable, does it also predict everything?
For others, too much vacuum isn't a problem; "in fact, it's a good thing," said Andrei Linde, a prominent Stanford University cosmologist and co-founder of the inflation theory. one. That's because the multiverse may have solved a huge mystery: the ultra-low energies of our vacuum.
Theorists estimate that when all the quantum fields in the universe vibrate collectively, the energy is enormous enough to rapidly accelerate the expansion of space and tear the universe apart in a short amount of time. But by comparison, the actually observed acceleration in space is extremely mild, suggesting that most of the collective vibrations are canceled out and that the energy of our vacuum has a very low positive value.
In an isolated universe, the tiny energies contained in a unique vacuum may seem like an esoteric puzzle; but in a multiverse, it's just hit and miss. If different bubbles in space have different energies and expand at different rates, galaxies and planets will only form in the slowest-expanding bubble. So our placid vacuum is no more mysterious than our planet's habitable orbit: we find ourselves here because most other places are inhospitable to life .
Like it or not, there is a problem with the multiverse hypothesis as currently understood . Although string theory seems to allow for an infinite number of vacuums, so far no one has found a tiny extra-dimensional fold with small positive energies consistent with our vacuum. String theory seems to be more prone to negative energy vacuums.
Maybe string theory isn't right, or maybe its flaws lie in researchers' immature understanding of it. 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 Institute for Advanced Study in Princeton. "It's a hot issue."
Or our vacuum may be arbitrary itself. "The prevailing view is that space [with positive energy] is not stable," Seberg said. "It can decay, which is probably one of the reasons why it's hard to understand the physical theory of it."
These researchers suspect that our vacuum is not one of the stable states of reality, and one day it will vibrate into a deeper, more stable valley. As a result, our vacuum may lose or generate some kind of particle. Tightly folded dimensions can unfold, and even a vacuum may cease to exist at all.
"It's 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 where every 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. This instability creates a rapidly expanding vacuum bubble whose mirror-like surface marks the end of spacetime itself.
Instabilities in this tiny dimension have long plagued string theory, and various ingredients have been devised to make them stable. Last December, several scientists worked together to calculate the lifetime of a vacuum in an extra curled dimension. They considered a variety of manipulations for stabilization but found that most mechanisms failed to prevent the creation and expansion of bubbles. Their conclusion is consistent with Witten: When the size of the extra dimension falls below a certain threshold, the vacuum collapses immediately. Similar calculations extending more complex models might be able to rule out vacuums below this dimension in string theory.
But with a large enough hidden dimension, the vacuum can last for billions of years . This means that the bubble of nothingness in the theory could be a good match for our universe. If so, Aristotle may have been more correct than he himself thought. Nature may not like the vacuum very much, and at infinite scales, it may not like anything at all.