1. Vacuum - no particles

There are many scenes involving vacuum in life, such as vacuum packaging, vacuum thermos cup and so on. In industry, as long as the environment below a standard atmospheric pressure can be called "vacuum", but according to the different pressure they can be divided into low vacuum, high vacuum, ultra-high vacuum and other different degrees of vacuum. For example, I learned in high school about the "Madburg hemisphere test," that metal ball that a lot of horses can't pull open and it's a high-vacuum environment.

In most people's imagination, as long as we remove all the air in the container, leaving no air left, it should be a true vacuum. But this kind of vacuum can only mean that there is no air, it does not mean that there is really nothing. Just like we think of space outside the atmosphere as a vacuum environment, but in fact, there are all kinds of radiation and particles in space, such as cosmic rays, at least if you can see light, there must be photons in it. Even if you were to block out all external radiation (including the cosmic microwave background radiation), the container itself would still contain radiation. After all, thermal radiation theoretically exists in everything except absolute zero.

So, if there is an ideal environment that is completely enclosed, has no particles in it, and does not produce any radiation of its own, then it really does count as an absolute vacuum in the electromagnetic sense. It's just such a vacuum, which obviously doesn't exist in reality.

2. Vacuum is not empty

Although there is no electromagnetic vacuum in reality, if there is such a vacuum, is there really nothing in it? Of course not, a real particle doesn't have one, but it has a virtual particle in it.

What is a virtual particle?

No matter how much you know about quantum mechanics, you've probably heard of the Heisenberg Uncertainty principle. It can be said to be one of the core and bottom principles of quantum mechanics. In simple terms, it means that for certain physical quantities (conjugate quantities with commutative operators), such as position and momentum, time and energy, we cannot know the exact values of both.

In the case of position and momentum, when we accurately measure the position of a particle, its momentum cannot be accurately measured, and vice versa. Note: This "cannot be measured accurately" does not mean that the technology is not able to reach, nor is it locked by Tomoko, but it is theoretically impossible. This is why the name of the principle changed from the original "uncertainty principle" to the later "uncertainty principle".

Similarly, for a pair of physical quantities, time and energy, we cannot simultaneously determine. If we restrict time to a very short scale (time precision), then the energy becomes very uncertain. This phenomenon is the same even for the vacuum: in a very short time, the energy of the vacuum may not be zero, and some energy will appear out of thin air, which is shown by the random generation of a pair of positive and negative particles in the vacuum, which we call virtual particles.

Why "virtual"? Because it only stays on paper. So it turns out that if you assume that there are two particles here, then the whole process is very convenient to describe. And the point 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, as if nothing had happened. This is called the "quantum fluctuation of vacuum". From this, we can see that "energy conservation" is not an iron law, it is more from the whole.

So, in the macroscopic (time-scale stretching) view, a vacuum is a vacuum, and there are no particles (real particles) in it; But from the microscopic (shortened time scale) point of view, vacuum is full of virtual particles produced by quantum fluctuations, which is why we say "vacuum is not empty".