Is a vacuum really nothing?

The other day, someone asked me this question: “Since there is nothing in a vacuum, it should be absolute zero, right? Does a light vacuum generate high temperature?”

Before answering this question, it is important to clarify what a vacuum is.

Is a vacuum really nothing? Seriously, you may really misunderstand it!

01
Is there a devil in the vacuum?

Vacuum originally means “space without matter”, and the word vacuum comes from the Latin adjective vacuus, which looks quite graphic: the letter u looks like a container, and two u’s in a row emphasize emptiness. By the way, there are only a few words in English that contain two u’s in a row, which is special.

Aristotle in ancient Greece had suggested that a vacuum could not exist. In the Middle Ages, a thought experiment was proposed: it was considered that when two flat plates were rapidly separated, there should be a vacuum between them – even if only for a moment. in the 14th century, Jean Brittain proved that when the other end of the bellows was completely sealed, ten horses together could not pull the bellows lever.

Some philosophers have suggested that nature is so averse to vacuums that it does not allow them to appear, and that even if they were created in an instant, matter would immediately come to fill the spaces, a view called the “terror vacuum”. It has even been suggested that God could not create a vacuum even if he wanted to.

A similar idea is that a void must lead to the appearance of Satan, God’s adversary, in it, and to avoid this, God Almighty would immediately fill the void. So the void, even if it appears, cannot exist.

This is actually a worldwide consensus: the vacuum is horrible, so one should try to avoid it, and God is helping us, so the vacuum is generally short-lived. For example, people of any nationality in the world are afraid of empty houses, because when a house is left unoccupied for a long time, ghosts will come and make their home, so it is called a haunted house. We often say “power vacuum”, which means that the lack of management of a place is very dangerous.

By the way, I believe in science is not afraid of ghosts, so there is a good haunted house, do not send me, like the following I can absolutely smile.

Vacuum is really nothing? Seriously, you may have really misunderstood it!

In fact, this statement also explains another physical problem: why you can drink water with a straw? Today we all know that it’s because atmospheric pressure presses water into our mouths (don’t tell me you have suction!). . But back then, people didn’t know about atmospheric pressure and thought that God hated vacuums, so he immediately sent water to flood the tube that was sucked out of the air, thus sending water to our mouths by the way.

Although the above idea seems absurd, it does explain many problems associated with pumping water relatively well for quite a long time. Therefore, we should take a historical view of those theories that have become obsolete.

In fact, the same is true for all those hypotheses in physics, you have no idea why they hold, but as long as you accept them first, you have a theory to describe and predict physical phenomena, otherwise you will not be able to move an inch unless you have the ability to invent another new set of theories. Even if a hypothesis is really disproved in the future, that is quite normal, because physics is only subject to experiment.

02
The study and use of vacuum

In 1654, Otto von Gehrig, the mayor of the German city of Magdeburg, invented the first vacuum pump and performed his famous Magdeburg hemisphere experiment. The results showed that the Magdeburgers were unable to separate two partially emptied air hemispheres due to atmospheric pressure outside the hemispheres.

Is a vacuum really nothing? Seriously, you may have really misunderstood it!

Subsequently, Robert Boyle improved Garrick’s design and further developed the vacuum pump technology with the help of Hooker. Thereafter, research on partial vacuum continued until 1850, when Auguste Toppler invented the Toppler pump, and in 1855 Heinrich Geisler invented the mercury displacement pump, which achieved a vacuum with an air pressure of about 10 Pa.

It was at this vacuum level that many electrical properties became observable, which renewed interest in further research, so that the study of vacuum gave a great impetus to the development of electromagnetism. Subsequently, it was found that vacuum was increasingly inseparable from various studies and applications, and vacuum research was of increasing interest.

The first widespread use of vacuum was in incandescent light bulbs to protect the filament from chemical degradation. The chemical inertness produced by vacuum is also applicable to electron beam welding, cold welding, vacuum packaging and vacuum frying.

Modern ultra-high vacuum technology is widely used in the study of atomically clean substrates, because only a very good vacuum can keep atomically clean surfaces for quite a long time (about minutes to days). And ultra-high vacuum completely eliminates air barriers, allowing particle beams to deposit or remove material without contamination, the principle behind chemical vapor deposition, physical vapor deposition, and dry etching, which are critical to the fabrication of semiconductor and optical coatings, as well as surface science.

Because convection is dramatically reduced in a vacuum, this provides insulation for thermos flasks. Vacuum effectively lowers the boiling point of liquids and promotes low temperature outgassing for freeze drying, adhesive preparation, distillation, metallurgy and process purging.

The electrical properties of vacuum make electron microscopes and vacuum tubes possible, including cathode ray tubes. Vacuum interrupters are commonly used in electrical switchgear, and vacuum arcing processes are of industrial importance for the production of certain grades of steel or high-purity materials. The elimination of air friction by vacuum helps to reduce losses in flywheel energy storage and in the operation of ultracentrifuges.

03
Partial vacuum vs. perfect vacuum

Nowadays, in the majority of cases, we say that a vacuum is a space where the air pressure is much lower than the standard atmospheric pressure. By implication, we can call it a vacuum as long as the space contains only an extremely thin amount of gas. Of course, if you are a very strict person, you can call it a “partial vacuum”. But I’m afraid people will think that’s a bit of a stretch.

But the vacuum you have in mind is called “perfect vacuum” or “free space “, which means.

The space where there are no particles with energy and momentum, i.e., where there are no matter particles (e.g., atoms, electrons, etc.) or field particles (e.g., photons), and where all components of the Einstein tensor under general relativity are zero.

Obviously, this ideal state with no particles at all cannot be achieved in the laboratory, although there may happen to be no matter particles in a very small volume for some very short period of time. Even if you remove all matter particles, there would still be countless photons and neutrinos, as well as other aspects of dark energy, virtual particles and vacuum rise and fall.

So you should see why our requirements for a vacuum have shrunk considerably!

Practice shows that a truly empty space is not available and does not exist in practice. But the word “vacuum” has been widely used, and it would be too extensive if we really fought against it. That’s why it has been used until now.

In other words, the so-called “vacuum” is actually a proper fake! But people still deliberately call it “vacuum”, not because of scientific rigor, but purely for historical reasons, and not because of the “here is no silver bullet” type of lies.

In reality, all vacuums contain more or less gas molecules, but the number of gas molecules per unit volume in vacuum is much smaller than in the atmosphere, and can even be ignored. In this sense, vacuum is not a definite state, but a relative meaning.

04
Vacuum mainly depends on pumping

You may not understand, it is just to remove all the gas from the container, is it so difficult?

It is not as simple as you think. Think about it, what is the way to effectively drive away those air molecules?

For thousands of years, people have thought of many ways, for example, a closed space to enlarge; then, let a gas filled with a closed cavity and then through a chemical reaction will be consumed into a solid, etc..

But the real effective method is the seemingly simpler and more brutal pumping – of course with the combination of various high precision vacuum pumps to achieve. The reason why it looks simpler is that this method is the easiest to think of and seems simple, but it is not simple at all.

In order to obtain ultra-high vacuum, in addition to the technical specifications of the vacuum pump used, there are strict requirements for the selection of seals, chamber geometry, combination of materials and vacuum pumps and working procedures, which are collectively called vacuum technology.

Practice shows that no matter how brilliant the vacuum technology you use, there is always some trace of gas or other material molecules still left in the container. There are of course various reasons for this, besides gas leakage, for example, there is the problem of outgassing of the inner wall of the container, because any substance (even metal or not), when the gas pressure in the space it is in is low to a certain extent, will release gas, generally the molecules of the substance.

The ancient Greek philosopher Aristotle had foresight on this point. He once said that there is no hole in space, because even if there is, it will be filled automatically by the surrounding denser matter. In fact, this is the phenomenon of “diffusion” in physics – as long as the density of particles in space is not uniform, thermal movement will cause the transfer of matter from a place of high density to a place of low density.

Think about it, the container used to contain the vacuum itself is also composed of particles, so these particles naturally can not avoid the problem of diffusion. So, even if there is a region in space where there is no matter, the matter that surrounds it is always weakly deflating – diffusing matter into the space, so the preservation of the vacuum is also very difficult.

So, no matter how much effort you put into pumping a confined space into a vacuum, it’s inevitable that more and more particles of matter will slowly appear inside it, because it’s wrapped in matter itself.

05
Vacuum degree and vacuum level

The degree of vacuum is called vacuum level. In general, the vacuum level is characterized mainly by the pressure. The degree of vacuum is divided into 5 levels, from low to high, in the following order

low vacuum, with pressure above 100 Pa, which can be obtained with the help of ordinary steel and vacuum pumps

medium vacuum, with pressure between 100 and 0.1 Pa, generally obtained with the help of stainless steel and vacuum pumps

high vacuum, between 0.1 and Pa, achieved with stainless steel, elastomer seals and high vacuum pumps.

ultra-high vacuum, with pressures between Pa and Pa, achieved with low carbon stainless steel, metal seals, special surface treatment and cleaning, baking and high vacuum pumps

Very high vacuum, with pressures below Pa, achievable with vacuum sintered mild stainless steel, metal seals, special surface treatments and cleaning, baking and additional suction pumps.

Complete vacuum characterization, which requires additional parameters such as temperature and chemical composition. One of the most important parameters is the mean free distance (MFP) of the residual gas, which indicates the average distance the molecules move between two consecutive collisions. The MFP of air at atmospheric pressure is very short, 70 nm. as the gas density decreases, the MFP increases, to about 100 mm for room temperature air at 100 mPa.

Is a vacuum really nothing? Seriously, you may have really misunderstood it!

When the value of MFP is larger than the size of a chamber, pump, spacecraft or other container, it means that gas molecules moving in the container collide almost exclusively with the walls of the container, the molecules interact with each other in a completely negligible way, and the continuity assumptions of fluid dynamics do not apply. This vacuum state is the high vacuum, and the study of fluid flow in this state is called particle gas dynamics.

The highest vacuum in nature is not obtained from the laboratory, but from the vast expanse of space. For example, the atmospheric pressure on the surface of the Moon is about Pa, so there is a very high vacuum above the Moon. But even with such a high vacuum, each cubic centimeter of space still contains up to hundreds of thousands of gas molecules, although its MFP is up to tens of thousands of kilometers!

And for the kind of interstellar space far from various celestial bodies, which on average has only a dozen or even less molecules per cubic centimeter, no pressure can be generated at all, and the conventional vacuum has failed. As far as we know, the space far from any galaxy contains only an average of one molecule per cubic centimeter, and the average free range of photons there is up to 10 billion light years! It is unimaginable! This is probably the closest thing to a perfect vacuum in nature today.

In interstellar space, being far away from all matter (not counting neutrinos, photons and dark matter), there are almost no forces acting on it, so any object in it (if there is one) is almost absolutely free, so it is appropriate to call it “free space”, and the reference system based on objects moving in such space can be The reference system based on objects moving in this space can be considered as an ideal inertial system.

06
An example of high vacuum application

A Crookes radiometer, which is commonly used to measure electromagnetic fluxes, works in a high vacuum region, its main body being a glass bubble pumped into a high vacuum and containing a set of rotatable vanes. When the blades are exposed to light, the gas molecules near the blades absorb the light and collide with the blades to generate pressure, and the pressure difference caused by the different absorption rates of light on both sides of the blades causes the blades to rotate, and the faster the rotation speed, the stronger the light is, thus providing a quantitative measurement of the intensity of electromagnetic radiation.

Is a vacuum really nothing? Seriously, you may have really misunderstood it!

One of the questions here is why is the radiometer pumped into a high vacuum inside? Because higher air pressure will lead to greater air resistance, only when the air is extremely thin, the pressure difference generated by the temperature difference between the two sides of the blade can exceed the air resistance and make the blade rotate.

Another question is, is the higher the vacuum of the glass bubble, the better? No! If the air in the glass bubble is too thin, molecular collision blade pressure is too small, the blade can not rotate, so the density of air must be in a more appropriate range, that is, high vacuum this region.

07
The nature of the perfect vacuum

These are the things about the traditional vacuum. But there is much more to vacuum than that.

The following part of the story may not be very clear to you, and that’s okay, it’s normal not to understand.

Since the 1930s, with the development of quantum theory, it has been recognized that even a perfect vacuum is not always empty.

The most important theory is the “Dirac Sea” model of vacuum proposed by Dirac in 1930. He believed that the perfect vacuum is actually filled by an infinite number of electrons with negative energy. If you let high-energy gamma rays into the vacuum, it is possible to hit an electron from it, and leave a hole in the vacuum, the hole is positron, and sure enough, two years later Anderson found positron.

Vacuum is really nothing? Seriously, you may really misunderstand it!

The second one is the so-called vacuum rise and fall theory, which was confirmed with the discovery of the Lamb shift and the anomalous magnetic moment. According to quantum electrodynamics, collisions between electrons can be realized by exchanging imaginary photons, which can produce positive and negative electron pairs. Thus the vacuum can be regarded as an ocean full of virtual photons and electron pairs. According to Heisenberg’s uncertainty principle, the inside of vacuum is not calm, but can emerge with huge energy in a short time. Therefore, the vacuum can derive a large number of particles in a very short time, and then disappear instantly.

Is a vacuum really nothing? Seriously, you may really misunderstand it!

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