What is a vacuum? All most people can think of is a space without even air. But if you have a little understanding of quantum mechanics, you will know that even if there is nothing in space, there are still quantum fluctuations. As the saying goes, “vacuum is not empty.” So is there a vacuum in an absolute sense in theory? To what extent does the scientific community understand vacuum today? In this issue, we divide vacuum into seven levels and analyze them one by one to see which level you have reached in your understanding of vacuum.
Level 1: Vacuum – no particles
There are actually many scenes involving vacuum in life, such as vacuum packaging, vacuum thermos cups, etc. In industry, any environment below a standard atmospheric pressure can be called “vacuum”. However, according to different air pressures, they can be divided into different vacuum levels such as low vacuum, high vacuum, and ultra-high vacuum. For example, in the “Magdeburg Hemisphere Experiment” that I learned in middle school, there is a high vacuum environment inside the metal ball that many horses cannot pull away.
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In most people's imagination, as long as we remove all the air from the container without leaving any, it should be a true vacuum. But this kind of vacuum can only be said to have no air, but it does not mean that it is really empty. Just like we equate space outside the atmosphere with a vacuum environment, but in fact, there are also various radiations and particles such as cosmic rays in space. At least if you can see light, it means there must be photons in it. Even if you isolate all external radiation (including cosmic microwave background radiation), there will still be radiation in the container itself. After all, as long as the temperature is not absolute zero, everything theoretically radiates heat.
Therefore, if there is an ideal environment that is completely closed, does not contain a single particle, and does not produce any radiation itself, then it can indeed be regarded as an absolute vacuum in the electromagnetic sense. It's just that such a vacuum obviously does not exist in reality.
Second layer: Vacuum is not empty – virtual particles
Although there is no vacuum in the electromagnetic sense in reality, assuming there is such a vacuum, is there really nothing in it? Of course not. Although there are no real particles, there are virtual particles in it.
What are virtual particles?
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I believe that no matter how much you know about quantum mechanics, you must have heard of the “Heisenberg Uncertainty Principle”. It can be said to be one of the core and lowest principles of quantum mechanics. Simply put, it means: for some specific physical quantities (conjugate quantities with non-commutative operators), such as position and momentum, time and energy, it is impossible for us to know the precise values of both at the same time.
Taking position and momentum as an example, when we accurately measure the position of a particle, its momentum cannot be accurately measured, and vice versa. Note: This “unable to accurately measure” does not mean that the technology cannot be achieved, nor does it mean that it is locked by sophons, but that it is theoretically impossible. This is why the name of the principle was changed from the “uncertainty principle” at the beginning to the “uncertainty principle” later.
In the same way, we cannot determine the pair of physical quantities time and energy at the same time. If we limit time to a very short scale (time is precise), then energy becomes very uncertain. This phenomenon is the same even for a vacuum: in a very short time, the energy of the vacuum may not be 0, and some energy will appear out of thin air. The manifestation is that a pair of positive and negative particles will be randomly generated in the vacuum. This pair of particles generated out of thin air will We call them virtual particles.
Why is it “virtual”? Because it only stays on paper. That is to say, during the calculation process, we found that if we imagine that there are two particles here, the whole process will be very convenient to describe. And the key is that these two particles will soon annihilate each other and disappear, and the energy that appears out of thin air will be returned to the vacuum, and everything will be as if it never happened. This is the so-called “vacuum quantum fluctuation”. It can be seen from this that “conservation of energy” is not actually an iron law, it is more about the overall situation.
So, from a macroscopic point of view (the time scale is lengthened), a vacuum is a vacuum, and there are no particles (real particles) in it; but from a microscopic point of view (the time scale is shortened), the vacuum is filled with virtual particles generated by quantum fluctuations, so we It is said that “vacuum is not empty”.
The third layer: Vacuum is not empty——Quantum field
You may have questions: “Aren't virtual particles just imaginary things in calculations? Can they be considered real?”
Yes, virtual particles are just an imaginary and intuitive concept, not real particles that actually exist. But in terms of its physical meaning, the interactions involved do indeed exist. Because the influence of virtual particles on real particles (such as Lamb shift, Casimir effect, etc.) can be actually detected. Therefore, it can be considered that virtual particles really exist, but they do not exist in the form we think.
In fact, when quantum mechanics has developed into the second half, physicists have combined quantum mechanics, special relativity, and classical field theory together to form a comprehensive quantum field theory. In quantum field theory, whether they are virtual particles or real particles, they can be regarded as “ripples” appearing in the field (excitation of the quantum field). The only difference is that real particles can continue to exist and spread, and can be directly detected; virtual particles are just short-lived ghosts. They usually only come out to make soy sauce and then go back. We can only determine their existence through indirect phenomena.
Therefore, from the perspective of quantum fields, whether you are a real particle or a virtual particle, vacuum is always accompanied by one thing – the quantum field.
The fourth level: Vacuum is not empty – relative vacuum
Just as there is no absolute time in the theory of relativity, vacuum may not be absolute for space.
As quantum field theory matures, physicists are beginning to consider how to incorporate general relativity. While exploring the quantum field theory of curved space-time, physicists theoretically discovered a possible strange phenomenon: when you accelerate in a vacuum at absolute zero, you will find that the vacuum at this time actually has temperature! But this temperature is limited to your perspective. To outside observers, this space is still a vacuum without temperature (without any radiation except virtual particles). This strange phenomenon in which temperature depends on the choice of reference frame is called the “Unruh effect.”
In general relativity, acceleration and gravitational fields are equivalent (equivalence principle). Then near some super strong gravitational fields (such as black holes), it may produce thermal radiation due to the Unruh effect, which means that the black hole has a temperature. If a black hole has a temperature, it will inevitably release energy (real particles) outwards. Where does this energy come from? That's right, it's transformed from virtual particles. This is the real origin of “Hawking radiation”.
Therefore, whether the vacuum is empty or not (whether there are only virtual particles or real particles in it) depends entirely on how you choose the frame of reference.
Let’s talk a little more. When we usually discuss vacuum, why don’t we additionally consider the issue of reference system? Because although there is no “absolute reference system” in theory (Newton's absolute space-time), there is one thing that is indeed special and is often used as a “reference standard” when considering large-scale issues such as the evolution of the universe. It is the cosmic microwave background. Radiation (co-moving coordinate system). People often ask “Who is the time of the age of the universe relative to?” Here you can understand that it is relative to the background radiation. It is the time of the universe itself, also called “Cosmic time”. Since the earth's gravitational field is very small, the clock slowdown effect of relativity is very weak, so we can put it and the background radiation into the same frame of reference. The same is true for vacuum. Normally, the Unruh effect has very limited influence on vacuum, or even cannot be detected at all (Hawking radiation has not been verified so far).
Level 5: True and False Vacuum – Decay of False Vacuum
We just said that various quantum fields exist in the vacuum. For most quantum fields, they have a lowest energy state, called the “ground state”. The field energy in the ground state is the lowest and the most stable. But there is a field in our universe that is not currently in a stable ground state. It is the Higgs field that gives mass to elementary particles. Although the state of the Higgs field has been maintained for 13.8 billion years, since it is not in the most stable ground state, it means that this stability is only temporary (metastable state), and it may fall to a lower energy state at any time. .
So you can think that the current vacuum in the universe is actually a “false vacuum.” Maybe one day, the potential energy of the Higgs field somewhere suddenly drops to a lower state (a phase change occurs in space), and at this time the “false vacuum” will become a “true vacuum”. After that, this momentum will spread in all directions at nearly the speed of light. This phenomenon is called “False vacuum decay.” Vacuum decay is somewhat similar to the effect of two-dimensional foil. The entire space falls into two dimensions at the speed of light and will not stop. But unlike dichroic foils, vacuum decay will cause many elementary particles to lose their mass. As a result, everything in the universe will be wiped out, and the entire physics will be completely rewritten.
In short, when the Higgs field drops to a lower ground state, space will become a true true vacuum.
Level 6: Different Vacuums – Vacuum in String Theory
Although the current quantum field theory is sufficient to deal with most situations, when facing some extreme problems, we have to place our hope in the future ultimate theory (Theory of Everything, ToE). As a star among the candidates for the ultimate theory, string theory has always been controversial, but it provides us with a new perspective on understanding vacuum and even the entire universe from a mathematical perspective.
In the process of studying superstring theory, string theorists discovered that our world should be ten-dimensional. But from the perspective of the theory of relativity, even if time is included, it is only four dimensions (three-dimensional space plus one dimension of time constitute four-dimensional space-time). Where are the other six dimensions? Just when string theorists were at a loss, the “Calabi–Yau manifold” proposed by the Chinese mathematician Shing-tung Yau caught everyone's attention. Soon, they produced a paper called “The Vacuum Structure of Superstrings.”
String theorists believe that the dimensions of the Calabi-Yau space formed by the Calabi-Yau manifold are different from ordinary three-dimensional spaces. The dimensions of the Calabi-Yau space have sizes (compactification) and cannot be Infinitely extended. The reason why the six extra dimensions cannot be found is precisely because they are curled up in the microscopic Calabi-Yau space, which is as small as the Planck scale.
Originally everything seemed perfect, but if nothing else goes wrong, something unexpected is about to happen: there is not only one kind of Calabi-Yau manifold, and it is currently believed that the number of it is, if not infinite, an astronomical number that is quite huge. These different kinds of Calabi-Yau manifolds create unique spaces with different physical laws, and no one knows which one our universe belongs to. This weird result of string theory seems to indicate that the multiverse may actually exist. Therefore, in these different universes, they have different physical laws, including the concept of vacuum.
Level Seven: The End of Vacuum – Nothingness
In fact, before the introduction of the Calabi-Yau manifold, string theorists had already discovered that some extra dimensions can only exist at the microscopic level (compactification). Due to the existence of quantum uncertainties at the microscopic level, it seems that these extra dimensions cannot exist stably.
In 1982, in a paper discussing “Kaluza-Klein space instability”, Edward Witten, later “the father of M theory”, proposed a more terrifying decay than “false vacuum decay” . Witten discovered that in the vacuum described by the Kaluza-Klein theory, when the extra dimension shrinks beyond a certain threshold, the vacuum will collapse into a point. At this time, the dimension will no longer exist. This is the end of the vacuum. .
Different from the gravitational singularity of a four-dimensional space-time like a black hole, although this decay formation point is just a point at the beginning, it will rapidly expand and become larger at the speed of light. What’s even more amazing is that although it will get bigger, because it has lost its dimension, to us it is still a “point” no matter how big it gets.
This is the terrifying thing about Kaluza-Klein vacuum decay: Compared with the singularity of a black hole, it not only ends space and time, but also allows this end to spread around; compared with false vacuum decay, this This kind of decay does not change the state of the vacuum, but directly “annihilates” the vacuum!
If the space formed by the Big Bang is only the space we are familiar with in the narrow sense, then the “end of space” means that even any form of space in the broad sense will cease to exist. Is this the legendary “nothingness”?
Papers & Reviews
(1) P. Candelas ab, Gary T. Horowitz, Andrew Strominger, Edward Witten. Vacuum configurations for superstrings. Nuclear Physics B (1985). 258:46-74
(2) Edward Witten. Instability of the Kaluza-Klein vacuum. Nuclear Physics B (1982). 195(3):481-492
Articles & News
(1) WIKIPEDIA: False vacuum
(2) WIKIPEDIA: Calabi-Yau manifold
(3) WIKIPEDIA: Compactification_(physics)
(4) WIKIPEDIA: Kaluza–Klein_theory
(5) blog.sciencenet.cn/blog-677221-1342155.html
(6) WIRED: How the Physics of Nothing Underlies Everything