A few days ago, someone asked me this question: "Since there is nothing in the vacuum, it should be absolute zero, right?" Does the light vacuum produce high temperatures? ”
Before answering this question, we must first figure out what a vacuum is.
01
Is there a devil in the vacuum?
Vacuum originally meant "space without matter". The word vacuum comes from the Latin adjective vacuus, which also looks quite graphic: the letter u is like a container, two u in succession, emphasizing emptiness. By the way, there are only a few words in English with two consecutive u' in a row, which shows that it is special.
Aristotle of ancient Greece once proposed that a vacuum could not exist. In the Middle Ages, an experiment of thought was proposed: considering that when two plates were rapidly separated, there should be a vacuum between the two plates—even if it was only for a moment. In the 14th century, Jean Bridan confirmed that when the other end of the bellows was completely sealed, ten horses working together could not pull the bellows pole.
Some philosophers have proposed that nature is extremely averse to vacuums, so it does not allow vacuums to appear, and even if it is generated in an instant, matter will immediately fill these spaces, which is called "terror vacuum". It has even been suggested that God cannot create a vacuum even if He will.
A similar idea is that any void is believed to inevitably lead to the appearance of God's opponent, the demon Satan, and to avoid this, Almighty God will immediately fill the void. So even if the void appears, it cannot exist.
This is actually a worldwide consensus: the vacuum is terrible, so we should try to avoid it, and God is helping us, so the vacuum is generally short-lived. For example, no matter what ethnic group in the world, people are afraid of empty houses, because the house has not been inhabited for a long time, and ghosts will come to settle down, so it is called Haunted house. We often say "the vacuum of power", which means that there is a lack of management in a certain place, which is very dangerous.
By the way, I believe in science and are not afraid of ghosts, so there are good haunted houses, don't send me, like the following I can absolutely laugh.
In fact, this statement also explains another physical problem: Why is it possible to drink water with a straw? Today we all know that this is because atmospheric pressure has pressed water into our mouths (don't tell me you have suction!). ）。 But at that time, people did not know the atmospheric pressure, thinking that God hated the vacuum, so he immediately sent water to flood the pipes that were sucked out of the air, so he brought the water to our mouths.
Although the above idea seems ridiculous, it does explain many of the problems related to pumping for a long time. Therefore, we should look at the theories that have become obsolete in the eyes of history.
In fact, the assumptions in physics are also the same, you have no idea why they hold, but as long as you accept them first, you have a theory that describes and predicts physical phenomena, otherwise you will not be able to do anything unless you have the ability to invent another new theory. Even if an assumption were to be overturned in the future, it would be normal, because physics is subordinated only to experimentation.
02
Vacuum research and utilization
In 1654, Otto von Geélik, mayor of the German city of Magdeburg, invented the first vacuum pump and conducted his famous Magdeburg hemispheric experiment. The results showed that due to atmospheric pressure outside the hemisphere, the horse team was unable to separate the two partially drained hemispheres of air.
Subsequently, Robert Boyle improved Gehrwick's design and further developed the vacuum pump technology with Hooke's help. Thereafter, the study of partial vacuum continued until 1850, when Auguste Topler invented the Toppler pump, and in 1855 Heinrich Geisler invented the mercury displacement pump, achieving a vacuum with an air pressure of about 10 Pa.
It is at this vacuum level that many electrical properties become observable, which rekindles interest in further research, so the study of vacuum has given a great impetus to the development of electromagnetism. Subsequently, people found that various research and applications are more and more inseparable from the vacuum, and vacuum research has aroused more and more interest.
Vacuum was first widely used in incandescent bulbs to protect filaments from chemical degradation. The chemical inertness generated 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-level cleaning substrates, because only a very good vacuum can keep the atomic-level clean surface for a considerable amount of time (about a few minutes to a few days). Ultra-high vacuum completely removes air barriers, allowing particle beams to deposit or remove materials without contamination, which is the principle behind chemical vapor deposition, physical vapor deposition and dry etching, which are essential for the manufacture of semiconductor and optical coatings and surface science.
This provides thermal insulation for the thermoses due to the significantly reduced convection in the vacuum. Vacuum can effectively reduce the boiling point of liquids, promote low temperature degassing, and is used for freeze drying, binder preparation, distillation, metallurgy and process purging.
The electrical properties of vacuum enable electron microscopy and vacuum tubes, including cathode ray tubes. Vacuum arc extinguishing chambers are often used in electrical switchgear, and the vacuum arc process has important industrial significance for the production of certain grades of steel or high-purity materials. Eliminating air friction through vacuum helps reduce flywheel energy storage and losses during ultracentrifuge operation.
03
Partial vacuum versus perfect vacuum
Now, in most cases, we say that vacuum is 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're a very serious person, you can call it a "partial vacuum." But I'm afraid that others will think that this is a snake painting.
And the vacuum of "utter emptiness" in your mind is called "perfect vacuum" or "free space," which means:
There are no particles with energy and momentum in space, that is, spaces that exclude any material particles (such as atoms, electrons, etc.) and field particles (such as photons), and all components of Einstein's tensor under general relativity are zero.
Obviously, this ideal state of being completely devoid of particles is impossible to achieve in the laboratory, although in a very small volume, it may happen that there are no material particles for a very short time. Even if you remove all the particles of matter, there will still be an infinite number of 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 shrunk so much!
Practice has shown that a truly empty space is currently unavailable and does not exist in practice. But the word "vacuum" has been widely used, and if it is true and false, it is too broad. So that's how it's been used until now.
In other words, the so-called "vacuum" is actually a proper fake! But people still deliberately describe it as a "vacuum", regardless of scientific rigor, purely for historical reasons, and not a lie of "there is no silver here and three hundred and two".
In reality, the vacuum contains more or less gas molecules, but relative to the atmosphere, the number of gas molecules contained in the unit volume of the vacuum is much smaller, and even completely ignored. In this sense, vacuum is not a definite state, but a relative meaning.
04
Vacuum is mainly pumped
You may not understand, isn't it so difficult to remove all the gases in the container?
It's really not as simple as you think. If you think about it, is there any way to effectively drive away those air molecules?
For thousands of years, people have thought of many ways, such as enlarging a confined space; then, filling a closed cavity with a certain gas and then chemically consuming it into a solid.
But the really effective method is the seemingly simpler and crudeer pumping – of course, with the help of a combination of a variety of high-precision vacuum pumps. The reason why I say "it looks simpler" is because 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 used, there are strict requirements for the selection of seals, chamber geometry, combination of materials and vacuum pumps and working procedures, etc., and the related technologies are collectively referred to as vacuum technology.
Practice has shown that no matter how clever the vacuum technology you use, there are always trace amounts of gas or other substance molecules that remain in the container. There are of course various reasons for this, in addition to air leakage, for example, because of the problem of Outgassing on the inner wall of the container, because any substance (even metal) will emit gas when the air pressure of the space in which it is located is low to a certain extent, which is generally the molecule of the substance.
The ancient Greek philosopher Aristotle had a foresight on this point. He once said that there will be no voids in space, because even if they do, they will inevitably be automatically filled by the denser material around them. In fact, this is the phenomenon of "diffusion" in physics - as long as the density of particles in space is not uniform, thermal motion will cause matter to move from a place of high density to a place of low density.
You think, the container used to hold the vacuum itself is also composed of particles, so these particles naturally cannot avoid the problem of diffusion. So, even if there is a real area of space without matter, the matter that surrounds it is always faintly deflated - diffusing matter into this space, so the preservation of the vacuum is also very difficult.
So, no matter how hard you try to pump a confined space into a vacuum, it is inevitable that more and more particles of matter will slowly appear inside it, because it is inherently wrapped in matter.
05
Vacuum degree and vacuum level
The degree of vacuum is called the degree of vacuum. In general, the degree of vacuum is mainly characterized by pressure. The vacuum level is divided into 5 levels from low to high, which are:
Low vacuum, pressure of more than 100Pa, can be obtained with the help of ordinary steel and vacuum pump;
Medium vacuum, the pressure is between 100 and 0.1Pa, generally obtained with the help of stainless steel and vacuum pump;
High vacuum, pressure between 0.1 and Pa, can be achieved by stainless steel, elastomer seals and high vacuum pumps;
Ultra-high vacuum, pressure between and Pa, achievable by low carbon stainless steel, metal seals, special surface treatments and cleaning, baking and high vacuum pumps;
Extremely high vacuum, with pressures lower than Pa, can be achieved by vacuum sintering of low carbon stainless steel, metal seals, special surface treatments and cleaning, baking and additional suction 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 a molecule moves between two consecutive collisions. The MFP of air at atmospheric pressure is short, at 70 nm. As the gas density decreases, the MFP increases, and the MFP of air at a room temperature of 100 mPa is about 100 mm.
When the value of MFP is larger than the size of a 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 is completely negligible, and the continuity assumption of fluid mechanics does not apply. This vacuum state is high vacuum, and studying 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 the moon is extremely high vacuum. But even with such a high vacuum, each cubic centimeter of space still contains up to hundreds of thousands of gas molecules, but its MFP is as high as tens of thousands of kilometers!
For the kind of interstellar space that is far away from various celestial bodies, the average cubic centimeter has only more than ten or even fewer molecules, which cannot produce pressure at all, and the conventional vacuum has failed. As far as is known, in space far away from any galaxy, each cubic centimeter contains only one molecule on average, and the average free path of photons there is as high as 10 billion light years! It's unimaginable! This is probably the closest vacuum in nature today.
In interstellar space, because of the distance from all matter (not considering neutrinos, photons, and dark matter), there is almost no force, so any object (if any) here is almost absolutely free, so it is appropriate to call it "free space", and the reference frame established based on the moving objects in this space can be regarded as the ideal inertial frame.
06
An example of a high vacuum application
A Croux radiometer commonly used to measure the flux of electromagnetic radiation works in a high vacuum area, and its main body is a glass bubble that is pumped into a high vacuum, with a set of rotatable blades. When the blade is exposed to light, the gas molecules near the blade absorb the light and collide with the blade to generate pressure, due to the different light absorption rates on both sides of the blade, the pressure difference caused by the blade rotates, the faster the rotation speed, the stronger the light, thus providing a quantitative measurement of the intensity of electromagnetic radiation.
One of the questions here is, why is the radiometer pumped into a high vacuum? Because higher air pressure leads to greater air resistance, only when the air is extremely thin, the pressure difference caused 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? Non also! If the air in the glass bubble is too thin, the pressure generated by the molecular collision blade is too small, and the blade cannot be rotated, so the density of the air must be in a more suitable range, that is, the high vacuum area.
07
The essence of a perfect vacuum
These are the things of the traditional vacuum. But the vacuum is much more than that.
The following part may not be well understood by you, it doesn't matter, it is normal not to understand.
Since the 1930s, with the development of quantum theory, it has been recognized that even a perfect vacuum is not actually empty all the time.
The most important theory of this is the vacuum model of the "Dirac Sea" proposed by Dirac in 1930. He argues that the perfect vacuum is actually filled with an infinite number of electrons with negative energies. If high-energy gamma rays were allowed to enter the vacuum, it would be possible to punch an electron out of it, leaving a hole in the vacuum, which is the positron, and sure enough, two years later Anderson discovered the positron.
The second is the so-called vacuum fluctuation theory, which was confirmed with the discovery of Lamb displacement and anomalous magnetic moments. According to quantum electrodynamics, collisions between electrons can be achieved by exchanging virtual photons, which can produce positive and negative electron pairs. Thus the vacuum can be seen as an ocean filled with pairs of virtual photons and electrons. According to Heisenberg's uncertainty principle, the inside of the vacuum is not calm, but a huge amount of energy can emerge in a short period of time. Therefore, a vacuum can generate a large number of particles in a very short period of time and then disappear in an instant.
In addition, the symmetry of the vacuum under gauge field theory spontaneously breaks down, giving Higgs particles mass, providing a solution to the fundamental problem of physics, the origin of mass. The theory of quark confinement in quantum chromodynamics states that a vacuum is the condensed phase of quark matter.
Later, people also found a richer connotation of the vacuum from the perspective of quantum information, which involves qubits, quantum entanglement and other issues, and will no longer be pulled here.
Therefore, the perfect vacuum is not empty. If there are no ghosts in the empty house, then there is nothing but ghosts in the vacuum!
08
What is the temperature of the vacuum?
Finally, let's look at the question given at the beginning of this article:
Since there is nothing in the vacuum, it should be absolute zero, right? Does the light vacuum produce high temperatures?
To answer this question, we must first understand, what is the "vacuum" here? Is it the perfect vacuum? Or interstellar space in the universe? Or is it a vacuum of different vacuum levels created in the laboratory?
If it is a perfect vacuum, then there is nothing, so how can it be a particle with irregular thermal motion? Naturally, it is impossible to define the temperature, so it is not that its temperature is absolute zero, but that the temperature does not exist at all!
If we refer to interstellar space, there are only so few particles per cubic centimeter, and it will 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 the temperature?
wait! The background temperature of the universe is 2.725K, so what is going on?
Uh, almost forgot, there are photons in the interstellar space of the universe, and neutrinos. They all produce background radiation, which all causes temperature!
For the photon part, we have to start with Planck's blackbody radiation formula.
Any object with a 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 temperature, and the higher the temperature, the shorter its peak wavelength, as shown in the figure below, which is the result of Planck's formula.
Modern cosmological studies have shown that there is an isotropic radiation in the universe called cosmic microwave background (CMB) radiation, which is one of the strongest evidence for the Big Bang theory. The old-fashioned television set in the past, when there is no signal, shows the following look, which is what the CMB looks like.
The curve of the intensity and wavelength of the cosmic microwave background radiation is shown in the following figure, which is exactly equivalent to the blackbody radiation curve of 2.725K, so the temperature corresponding to the cosmic background radiation is 2.725K.
At another angle, as long as the photon energy is associated with the temperature, we can also get this temperature value. According to Planck's formula, temperature is inversely proportional to the wavelength at the peak of the curve, which is also known as the Venn displacement law, namely:
Where 2.821439, for the Boltzmann constant, for the Planck constant, according to this, we can always correspond to the frequency of the photon at the peak of the radiation and the temperature, substituted into the data in the above figure - the peak frequency is 160.23GHz, the resulting value is almost 2.725K - I have calculated Oh, you also try?
The following figure is a NASA probe called WMAP that has been drawing a map of the cosmic microwave background radiation all day over 9 years, showing that the average temperature of the universe is very uniform, and there is only a slight temperature fluctuation in the local area, so it can be considered that the average temperature of the universe is almost 2.725K.
For neutrinos, it also causes a type of radiation called cosmic neutrino background (CNB) radiation, but the temperature corresponding to that radiation is different from that of photons, and its average is 1.95K. Since neutrinos do not interact with photons at all, there is no equilibrium between these two temperatures.
Therefore, there are two independent temperatures in interstellar space.
If it is a vacuum in the general laboratory, then there are many substances inside and outside this vacuum, and the vacuum is only a cavity surrounded by matter, which of course 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, of course, it will cause its temperature to rise.
In this way, as long as it is a manually created vacuum, because it is inseparable from the container, it is impossible to leave the heat radiation? Basically! Therefore, the vacuum in the container, no matter how high the vacuum, even if the material particles reach the level of interstellar space (which is impossible), because the container wall emits photons, so there must be a temperature.
However, if the vacuum container is cooled to near absolute zero through cryogenic technology, which is 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 inside the cavity, and the cavity can be seen as a space without thermal radiation.
END
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Source: University Physics
EDIT: Hidden Idiot