Shulou(Shulou.com)11/24 Report--

A few days ago, someone asked me such a question: "since there is nothing in the vacuum, it should be absolute zero, right? will the illumination vacuum produce high temperature?"

Before you answer this question, you have to figure out what a vacuum is.

01. Is there a devil in the vacuum? The original meaning of vacuum is "space without matter". The word vacuum, which comes from the Latin adjective vacuus, also looks pretty figurative: the letter u is like a container, followed by two u, emphasizing emptiness. By the way, there are only a few words with two u in succession in English, which shows that they are special.

Aristotle of ancient Greece once proposed that a vacuum could not exist. In the Middle Ages, a thought experiment was proposed: it was considered that when two plates were rapidly separated, there should be a vacuum between the two plates-even if only for a moment. In the 14th century, Jean Bridan proved that when the other end of the bellows was completely sealed, none of the ten horses could pull the bellows pole together.

Some philosophers have proposed that nature hates vacuum so much that it does not allow vacuum to appear. Even if it occurs in an instant, matter will immediately fill these spaces. This view is called "terror vacuum". It was even suggested that even if God wanted to, he could not create a vacuum.

A similar idea is that people think that every void will inevitably cause God's opponent, the devil Satan, to appear in it, and to avoid this situation, Almighty God will immediately fill the void. So even if there is a hole, it cannot exist.

In fact, this is a worldwide consensus: vacuum is terrible, so we should try to avoid it, God is also helping us, so vacuum is generally short-lived. For example, people of any nationality in the world are afraid of empty houses, because the house is uninhabited for a long time, ghosts will come to settle down, so it is called haunted house (Haunted house).

By the way, I believe that science is not afraid of ghosts, so there are good haunted houses, do not send me, like the following I can absolutely accept.

In fact, this statement also explains another physical problem: why can you drink water with a straw? As we all know today, this is because atmospheric pressure presses water into our mouths (don't tell me you have suction! ). But at that time, people did not know the atmospheric pressure and thought that God hated the vacuum, so he immediately sent water to drown the pipes that had been sucked away by the air, so he sent water to our mouths.

Although the above idea may seem absurd, it does explain a lot of problems related to pumping for a long time. Therefore, we should look at those outdated theories from a historical perspective.

In fact, the same is true of the assumptions in physics. You have no idea why they are true, but as long as you accept them first, you will have the theory to describe and predict physical phenomena, otherwise you will be unable to move forward. Unless you have the ability to invent a new set of theories. Even if a hypothesis is overturned in the future, it is normal, because physics is only subject to experiments.

02. Research and utilization of vacuum in 1654, Otto von Gehrik, mayor of Madeburg, Germany, invented the first vacuum pump and carried out his famous Magdeburg hemispheric experiment. The results show that due to the atmospheric pressure outside the hemisphere, the horse team is unable to separate the two hemispheres that drain the air.

Subsequently, Robert Boyle improved Gehrik's design and further developed vacuum pump technology with the help of Hook. Since then, the study of a partial vacuum has continued until 1850, when Augustoppler invented the Topler pump, and in 1855 Heinrich Geissler invented the mercury replacement pump, which achieved a vacuum with a pressure of about 10Pa.

It is at this vacuum level that many electrical properties can be observed, which re-arouses people's interest in further research, so the study of vacuum plays a great role in promoting the development of electromagnetics. Subsequently, people found that all kinds of research and applications are more and more inseparable from vacuum, and vacuum research has attracted more and more interest.

Vacuum was first widely used in incandescent bulbs to protect filaments from chemical degradation. The chemical inertia produced by vacuum is also suitable for electron beam welding, cold welding, vacuum packaging and vacuum frying.

Modern ultra-high vacuum technology is widely used in the study of atomic clean substrates, because only a very good vacuum can keep the atomic clean surface for a long time (about a few minutes to a few days). Ultra-high vacuum can completely eliminate air barriers and allow particle beams to deposit or remove materials without contamination, which is the principle behind chemical vapor deposition, physical vapor deposition and dry etching. they are essential for the manufacture of semiconductors and optical coatings and surface science.

Due to the significant reduction of convection in the vacuum, this provides thermal insulation for the thermos. Vacuum can effectively reduce the boiling point of liquid and promote low temperature outgassing, which is used 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 interrupter is often used in electrical switchgear, and vacuum arc technology is of great industrial significance for the production of some grades of steel or high purity materials. The elimination of air friction through vacuum is helpful to reduce the energy storage of flywheel and the loss of overspeed centrifuge.

Partial vacuum and perfect vacuum now, in most cases, we mean a space where the air pressure is much lower than the standard atmospheric pressure. The implication is that as long as the space contains only extremely thin gases, we can call it a vacuum. Of course, if you are a very serious person, you can call it a "partial vacuum". But I'm afraid people think it's icing on the cake.

The vacuum of "completely empty" in your mind is called "perfect vacuum" or "free space". It means:

There are no particles with energy and momentum in space, that is, space that excludes any matter particles (such as atoms, electrons, etc.) and field particles (such as photons), and 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 in a very small volume, there may happen to be no matter particles for a very short period of time. Even if you remove all the particles of matter, there will still be countless photons and neutrinos, as well as other effects such as dark energy, virtual particles and vacuum fluctuations.

So you should understand why our requirements for vacuum have been greatly reduced.

Practice shows that the real empty space can not be obtained at present, and does not exist in practice. However, the word "vacuum" has been widely used, and if the fight against counterfeiting is real, the scope involved is too wide. So I've been using it like this until now.

In other words, the so-called "vacuum" is actually a proper fake! But people still deliberately refer to it as a "vacuum", which has nothing to do with the rigor of science, purely for historical reasons, let alone a lie of "there is no silver 320 here".

In reality, vacuum contains gas molecules more or less, but compared with the atmosphere, the number of gas molecules per unit volume in vacuum is much smaller, or even can be completely ignored. In this sense, vacuum is not a definite state, but a relative meaning.

04, vacuum mainly depends on pumping, you may not quite understand, isn't it so difficult to remove all the gas from the container?

It's really not as simple as you think. If you think about it, is there any effective way to get rid of those air molecules?

For thousands of years, people have thought of many ways, for example, to enlarge an airtight space, or to fill a closed cavity with a gas and then turn it into a solid by chemical reaction.

But the really effective way is to pump air more simply and roughly-with the help of a combination of sophisticated vacuum pumps, of course. The reason for saying "looks simpler" is that although this method is the easiest to think of, it 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, there are strict requirements for the selection of seals, the geometric shape of the chamber, the combination of materials and vacuum pumps, and the working procedures. The related technologies are collectively referred to as vacuum technology.

Practice shows that no matter how clever the vacuum technology you use, there are always traces of gases or other molecules remaining in the container. Of course, there are various reasons for this, in addition to leakage, for example, because of the Outgassing of the inner wall of the container, because any substance (even metal) will give off gas, usually the molecule of the substance, when the pressure in its space is low to a certain extent.

Aristotle, an ancient Greek philosopher, had foresight on this point. He once said that there will be no holes in space, because even if they do, they are bound to be automatically filled by denser substances around them. In fact, this is the phenomenon of "diffusion" in physics-as long as the density of particles in space is uneven, thermal motion will cause matter to transfer from places with high density to places with low density.

If you think about it, the container used to hold the vacuum is also made up of particles, so these particles can't avoid the problem of diffusion. So, even if there is a non-material area in the space, the matter that surrounds it is weakly deflated all the time-diffusing the matter into the space, so it is also very difficult to preserve the vacuum.

Therefore, no matter how hard you try to vacuum a confined space, it is inevitable that more and more particles of matter will appear inside it, because it is wrapped in matter.

The degree of vacuum and vacuum-level vacuum is called vacuum. In general, the vacuum degree is mainly characterized by the pressure. The vacuum degree is divided into five levels from low to high, which in turn are:

Low vacuum, pressure below 100Pa, can be obtained with the help of ordinary steel and vacuum pump

Medium vacuum, pressure between 100and 0.1Pa, generally obtained by stainless steel and vacuum pump

High vacuum, pressure between 0.1 and Pa, can be achieved by stainless steel, elastomer seal and high vacuum pump

Ultra-high vacuum, pressure between Pa, low carbon stainless steel, metal seals, special surface treatment and cleaning, baking and high vacuum pump

Extremely high vacuum, pressure lower than Pa, can be achieved by vacuum sintering of low carbon stainless steel, metal seals, special surface treatment and cleaning, baking and additional getter pumps.

Complete vacuum characterization requires more parameters, such as temperature and chemical composition. One of the most important parameters is the mean free path (MFP) of the residual gas, which indicates the average distance over which the molecule moves between two consecutive collisions. The MFP of air at atmospheric pressure is very short, which is 70nm. With the decrease of gas density, the MFP increases, and the MFP of air at room temperature in 100mPa is about 100mm.

When the MFP value is larger than the size of the chamber, pump, spacecraft or other container, it means that when the gas molecules move in the container, they almost only collide with the container wall, the interaction between the molecules can be completely ignored, and the continuity assumption of hydrodynamics is not applicable. This vacuum state is high vacuum, and the study of fluid flow in this state is called particle gas dynamics.

The vacuum with the highest vacuum in nature does not come 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 an extremely high vacuum on the moon. But even if the vacuum is so high, there are still hundreds of thousands of gas molecules per cubic centimeter, but its MFP is as high as tens of thousands of kilometers!

For the kind of interstellar space far away from various celestial bodies, with an average of more than a dozen or less molecules per cubic centimeter, there is no pressure at all, and the conventional vacuum has become ineffective. As far as we know, there are on average only one molecule per cubic centimeter in space far away from any galaxy, and the average free path of photons there is as high as 10 billion light-years. It's so unthinkable! This is probably the closest vacuum in nature to a perfect vacuum.

In interstellar space, because it is far away from all matter (regardless of neutrinos, photons and dark matter), there is almost no force, so any object (if any) here is almost absolutely free. so it is also appropriate to call it "free space", and the frame of reference based on objects moving in this space can be regarded as an ideal inertial frame.

An example of high vacuum applications is the Crooks radiometer, which is commonly used to measure electromagnetic radiation fluxes. The main body of the radiometer is a glass bubble pumped into a high vacuum with a set of rotating blades. When the blade is exposed to light, the gas molecules near the blade absorb light and collide with the blade to produce pressure. Due to the different light absorptivity on both sides of the blade, the pressure difference causes the blade to rotate, and the faster the rotation speed is, the stronger the light is. Thus, the quantitative measurement of electromagnetic radiation intensity is provided.

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

Another question is, is the glass bubble the higher the vacuum, the better? No! If the air in the glass bubble is too thin, the pressure caused by the impact of the molecules on the blade is too small, and the blade cannot rotate, so the density of the air must be in a more appropriate range, that is, the high vacuum area.

07. The essence of a perfect vacuum is those things of the traditional vacuum. But the meaning of vacuum is much more than that.

You may not understand the following part, it doesn't matter, it's normal if you don't understand.

Since the 1930s, with the development of quantum theory, people have realized that even a perfect vacuum is not always empty.

The most important theory is the vacuum model of "Dirac Sea" put forward by Dirac in 1930. He believes 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 shoot an electron out of it, leaving a hole in the vacuum, which is the positron, and sure enough, Anderson discovered the positron two years later.

The second is the so-called vacuum fluctuation theory, which is confirmed with the discovery of Lamb shift and anomalous magnetic moment. According to quantum electrodynamics, collisions between electrons can be achieved by exchanging virtual photons, which can produce positive and negative electron pairs. So a vacuum can be thought of as an ocean full of virtual photons and electron pairs. According to Heisenberg's uncertainty principle, the vacuum is not calm, but a huge amount of energy can emerge in a short period of time. Therefore, a vacuum can derive a large number of particles in a very short time and then disappear instantly.

In addition, the spontaneous breaking of the symmetry of the vacuum under the gauge field theory gives the Higgs particle mass, which provides a solution to the fundamental problem of the origin of mass in physics. The quark confinement theory in quantum chromodynamics points out that vacuum is the condensed phase of quark matter.

Later, people also found the richer connotation of vacuum from the perspective of quantum information, which involves qubits, quantum entanglement and so on.

Therefore, the perfect vacuum is not empty. If there are no ghosts in the empty house, there is nothing but ghosts in the vacuum!

What is the temperature of the vacuum? Finally, let's look at the questions given at the beginning of this article:

Since there is nothing in the vacuum, it should be absolute zero, right? Will the vacuum of light produce high temperature?

To answer this question, we must first make it clear what does the "vacuum" mean here? Is it a perfect vacuum? Or the interstellar space in the universe? Or the vacuum of different degrees of vacuum created in the laboratory?

If it is a perfect vacuum, then there is nothing, then how can particles with irregular thermal motion be made? Then, of course, it is impossible to define temperature, so it is not that its temperature is absolute zero, but that it does not exist at all!

If it means interstellar space, where there are only a few particles per cubic centimeter, it does not meet the requirements of thermodynamics-a system of a large number of particles with irregular thermal motion, so it is still impossible to define temperature.

Wait a minute! So the background temperature of the universe is 2.725K, what's going on?

Well, I almost forgot that there are photons and neutrinos in interstellar space. They all produce background radiation, which can lead to temperature!

For the photon part, we have to start with Planck's blackbody radiation formula.

Any object with temperature will radiate electromagnetic waves outward in the form of electromagnetic waves, which is thermal radiation. The curve of the relationship between radiation intensity and wavelength (or frequency) varies with different temperature. the higher the temperature, the shorter the peak wavelength, as shown in the following figure, which is the result of Planck formula.

Modern cosmological research shows that there is a kind of isotropic radiation in the universe, which is called cosmic microwave background (CMB) radiation, which is one of the most powerful evidence of the Big Bang theory. In the past, when there was no signal, the old TV showed something like this, which was what CMB looked like.

The intensity and wavelength curve of the cosmic microwave background radiation is shown below, which is exactly equivalent to the blackbody radiation curve of 2.725K, so the corresponding temperature of the cosmic background radiation is 2.725K.

On the other hand, as long as we associate the photon energy with the temperature, we can also get this temperature value. According to Planck's formula, the temperature is inversely proportional to the wavelength at the peak of the curve, which is also known as Venn's displacement law, that is:

Where 2.821439 is Boltzmann constant and Planck constant, accordingly, we can always match the frequency of photons corresponding to the peak in radiation with the temperature, and replace it with the data in the above picture-- the peak frequency is 160.23GHz, and the value obtained is about 2.725K-I have calculated by hand, why don't you try it?

The image below is a nine-year map of the cosmic microwave background radiation drawn by a NASA probe called WMAP, which shows that the average temperature of the universe is very uniform, with only slight local temperature fluctuations, so it can be thought that the average temperature of the universe is about 2.725K.

For neutrinos, it also causes a kind of radiation called cosmic neutrino background (CNB) radiation, but the temperature corresponding to that radiation is different from that of photons, with an average of 1.95K. Because neutrinos do not interact with photons at all, there is no balance between the two temperatures.

Therefore, there are two independent temperatures in interstellar space.

If it is a vacuum in an ordinary laboratory, then there are many substances inside and outside the vacuum. The vacuum is just a cavity surrounded by matter. Of course, these substances can emit thermal radiation, that is, photons. According to the law of blackbody radiation, the space filled with these photons must also have a temperature, and the specific temperature is determined by the temperature of the vacuum container. If you shoot light into this space again, of course it will cause the temperature to rise.

In this way, it seems that as long as it is a man-made vacuum, because it can not do without the container, it can not leave the thermal radiation? Basically! Therefore, the vacuum in the container, no matter how vacuum it is, even if the matter particles reach the level of interstellar space (which is impossible), because the wall of the container will emit photons, it must have a temperature.

However, if the vacuum container is cooled to near absolute zero by cryogenic technology-lower than the temperature of the cosmic microwave background radiation-then all the photons in the cavity are absorbed by the container wall. There are no photons in the cavity, and the cavity can be regarded as a space without thermal radiation.

This article is from the official account of Wechat: University Physics (ID:wuliboke), by Xue Debao.

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