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

Laser is the abbreviation of stimulated radiation light amplification

So in addition to visible light,

Electromagnetic waves in other bands

Can it also produce lasers?

Q1. How is the size of decibels defined? By AAA

A:

Decibels are used in many fields, and the definition is not exactly the same in different fields. Of course, the logic behind different definitions is similar, taking the logarithm of the ratio of a physical quantity to the reference value.

I guess the question is the decibel when measuring the loudness of the sound. The physical measure of loudness is the sound intensity level, which is measured in decibels. The so-called sound intensity level of a place means that the ratio of the sound intensity (or power density) to the reference intensity is taken logarithm and multiplied by 10, and its mathematical expression is that = is the reference sound intensity, which is the weakest sound intensity that can be distinguished by the human ear. If the sound intensity is = = at some point, it is 0 decibels; if =, the corresponding sound intensity level is 120 decibels. The study found that the sensitivity of the human ear to sound intensity is about logarithmic-for every ten times the sound intensity, the sound felt by the human ear is twice as high, so the sound intensity level is logarithmic. The front coefficient of 10 is for ease of use-for every ten times the sound intensity, add ten decibels.

In addition to the sound intensity level, we can also use the sound pressure level to define the sound loudness, which is also measured in decibels. It uses sound pressure (or the amplitude of sound waves) as a physical quantity to calculate the ratio, and the previous coefficient is correspondingly changed to 20. This is another definition of decibels, the two definitions are parallel, but the former seems to be more commonly used.

By is a Tibetan fanatic.

Q2. Why doesn't a hot iron pot crack when it comes to cold water? By 1000%

A:

The cracking of iron and steel devices is caused by the internal stress in the cooling process, which can be divided into thermal stress and microstructure stress. The thermal stress is caused by the different temperature caused by the different cooling rate of the inner and outer layer in the cooling process of iron and steel devices, so that the degree of thermal expansion and cold shrinkage is different. The microstructure stress is caused by the different transformation time of the internal structure in the cooling process of the device. The two kinds of stress are superimposed on each other. When the stress reaches a certain critical value, there will be deformation and cracks.

Here, take the iron pot made of low carbon iron as an example. In the life scene, the temperature in the iron pot of ordinary household gas stove is between 200and 300℃. According to the iron-carbon alloy phase diagram, the microstructure of steel does not change in this temperature range, so the microstructure stress can be ignored. In addition, the thermal conductivity of low carbon steel is larger, and the speed of heat exchange with cooling medium water is fast, so the temperature difference between the inner and outer layers is small, and the thermal stress caused by thermal expansion and cold shrinkage can not reach the critical value of cracking, so the hot iron pot will not crack in the presence of cold water.

By depth

Q3, the thing is, I used my mobile phone to record the electrostatic discharge process of the electrostatic motor teaching aids, and then I found that every arc visible to the naked eye is not always visible in the phone, but every time the arc crackles through the air. Every time it was recorded. I would like to ask why some arc can not be filmed by the mobile phone? (as a result, the arc actually seen with the naked eye is much more than that recorded by mobile phone video.) by is hidden.

A:

Hello, this situation is probably caused by the insufficient frame rate captured by the mobile phone, which can be solved by increasing the frame rate of the camera.

We know that mobile phone cameras collect optical signals through sensors such as CMOS or CCD, and convert them into digital signals for imaging. Generally speaking, the more images are captured per unit time, the smoother the video is projected, and the number of photos taken per second is called the frame rate. At present, most films are shot and shown with 24-frame rate devices, while mobile devices generally have a frame rate of about 30, that is, 30 photos per second, with an interval of about 0.03 seconds. When the duration of the arc is less than 0.03s, it may be skipped in the middle of the two photos and cannot be photographed.

The sampling frequency of the microphone recording is much higher than that of the camera, generally between 20000Hz and 50000Hz, and the interval between the two sampling is about 0.00002s to 0.00005s. Therefore, the arc sound can basically be sampled by recording. In order to capture the arc image, it is recommended to increase the frame rate of the mobile phone camera, or use more professional high frame rate photography equipment.

By single male youth

Q4. Why are white dwarfs the corpses of stars? By Anonymous

A:

White dwarfs and neutron stars can indeed be regarded as the corpses of stars with larger masses, which is the final stage of star evolution.

We have to start with the stars. As we all know, gravity is a unique protagonist in the universe, all kinds of celestial bodies voluntarily embark on the road against gravity, and that's where stars come from. A massive celestial body shrinks under the action of gravity, in which the gravitational potential energy is converted into heat, and the central temperature of the celestial body rises, igniting hydrogen nuclear fusion, releasing energy, and reaching a balance with the contraction of gravity, which is the star. However, hydrogen is limited, just like the sun, with only 5 billion years left, this black sheep is about to burn up the hydrogen. At that time, if the temperature at the center of the star is not enough to ignite helium fusion, gravity will once again gain the upper hand, forcing the star to continue to contract until the helium core is ignited, after which the energy released by helium fusion will intensify the burning of peripheral hydrogen, causing the star to further expand and cool, turning it into a red giant.

After the helium is also used up, the celestial body will leave a core of carbon or oxygen. If the star is congenitally deficient and its mass is too small, can the gravitational potential of the remaining core be enough to ignite the fusion of carbon or oxygen, then his life will come to an end and become a white dwarf. At this time, the celestial body will continue to contract under the action of gravity, and the density will increase until another force grows strong enough to resist gravity. This force is the electron degeneracy pressure, which comes from the ionized electrons at the core of the celestial body. Degenerate pressure is strong enough only at high density, so the density of white dwarfs is very high. Celestial bodies with a carbon-oxygen central sphere mass of less than 1.4 times the mass of the sun (due to the loss of a lot of mass in the evolution, its mass in the stellar stage is about 8 times the mass of the sun) will become white dwarfs after death, and the sun is obviously among them.

By Frost White

Q5. What is a neutron star? By Anonymous

A:

Take the above question, but if the star is more massive, the final destination may also be a neutron star.

The greater mass means that it can further ignite the fusion reaction of carbon, and the final central sphere is mainly iron, which is the most stable nucleus, so it is impossible to fuse again. At this time, the center shrinks rapidly under the action of gravity, and the high temperature and high pressure produced will make the photons have very high energy. These photons are enough to destroy the nucleus, electrons combine with protons to form neutrons and release neutrinos. The whole center forms a core made up of neutrons, and the gravitational collapse is restrained by stronger neutron degeneracy pressure. This is the neutron star. Its density is higher than that of white dwarfs, and its upper mass limit is uncertain, which can only be said to be about 2 times the mass of the sun, or 1.5-3 times the mass of the sun. The volume of the neutron star is generally small, and the radius of the typical neutron star is of the order of magnitude of 10km, which is not as large as that of Haidian region, but its mass is not the same. Neutron stars also have strange properties such as ultra-high magnetic field and ultra-high speed rotation, which has always been the focus of astronomy.

No matter how massive the celestial body, will eventually form a black hole, let's see the next topic decomposition, ahead, high-energy early warning.

Ps: this is actually a question from a reader, but the answer is really too long, so it is divided into four questions (originally there are only three, I added one myself), and add some workload by the way. The references of the four questions are all the following, so I won't repeat them.

Ps: bring back those who say we don't talk about physics! This time it's physics.

Reference:

[1] Liang Canbin. Theory of relativity from zero [M]. Higher Education Press, 2013.

By Frost White

Q6. Is a black hole a celestial body? By Anonymous

A:

If the star is more massive and fails to abandon enough mass to form a stable white dwarf or neutron star during collapse, it will eventually form a black hole.

A black hole is a very strange celestial body predicted by general relativity. it has a very high density and gravity is so strong that even light cannot escape, so it looks dark, as if the universe had broken a hole there, so the name black hole is apt. Of course, we already have some observational evidence that the existence of black holes has been accepted by the vast majority of people.

After Einstein put forward the general theory of relativity, Schwarzschild first obtained a static spherical symmetric solution, that is, the Schwarzschild line element of vacuum, which has singularity at ringing 0 and ringing 2M, respectively. Later, we proved that the former is true singularity and the latter is coordinate singularity, so we extend it. The picture below is the maximum extension of Krusco in Schwarzschild space-time.

This picture is very important and will be used in the next topic, and it contains a wealth of information. We can use what we can say.

In the picture on the left, An and A 'are two asymptotically flat areas with no causal relationship, B is a black hole and W is a white hole. T is time, X is space, this picture compresses two spatial dimensions, any point outside the shadow area represents a sphere, the dividing line of the shadow is two hyperbolas, and the asymptote of the hyperbola is exactly two N lines, and there is a real singularity on the dividing line, so the point in the shadow area does not exist. The two N lines are sandwiched at an angle of 45 °to the coordinate axis, and the two lines are also two radial light-like geodesic lines. On these two lines, time is meaningless and divergent (see right picture), which is the source of the singularity of the Schwarzschild line element here. It can be seen that it has no singularity in the non-shadow area.

The r of area A satisfies 2m < r < ∞, which is the time and space described by the Schwarzschild line element, and it is also the starting area of our extension. We're in this area.

After our extension, we formally prove that there is no physical singularity at ringing 2m, which means that events in region A can simply cross the N line to enter area B. On the other hand, if there is a singularity here, such traversing may have some unknown physical process.

As mentioned earlier, the N line is a radial light-like light, so all the introverted (r value getting smaller and smaller), future timelike or light-like curves in the A region will pass through the N line, enter the B region, and eventually fall into the singularity (ringing 0). On the contrary, it is impossible that the N line is an one-way "film". In fact, this is the event horizon of the black hole. Zone B is a black hole.

According to the Birkhoff theorem, the spherical symmetric solution of Einstein's equation must be a Schwarzschild element, so it is impossible for a spherically symmetrically collapsed celestial body to enter the W or A 'region. if the mass of the celestial body is large enough and continues to collapse until it is compressed to the singular point rystal 0, then there is no room for redemption.

This picture shows the collapse of stars in the Krusco coordinate system.

This picture makes a transformation of time, which can more vividly reveal to us the strange nature of the event horizon. We can see that objects outside the event horizon will not have any effect if they are extroverted, but if they are within the event horizon, they will never be able to get rid of them. In the end, they can only be attributed to singularities.

According to the extension, there is also a wormhole model. Ahead, nuclear warning!

By Frost White

Q7. What about wormholes? By Anonymous

A:

Wormhole is also a magical model based on general relativity, but like the white hole in the above question, there is no astronomical evidence to support it. Many people think that we can achieve time travel through wormholes, but the idea may be a little crude.

Let's review the Schwarzschild space-time maximum extension map in the previous question. Now when we take Thum0, we draw its embedded map, which should look like this:

This picture should be familiar to everyone, and you should have seen a lot of it in the popular science of relativity. This kind of graph is called embedded graph, which is a means for us to represent four-dimensional space-time with images in three-dimensional Euclidean space. as mentioned above, we have frozen time, but in fact we have compressed a spatial dimension. each ring on this picture is actually a sphere, and four-dimensional space-time has only two dimensions on this graph, so, in this picture, only the points on the plane are meaningful, and the points outside the plane do not exist. We just draw it in a three-dimensional Euclidean space.

This is the embedded graph of the maximum extension of Schwarzschild space-time at T 0. The upper part of the embedded graph is the positive half of the X axis and the lower part is the negative half of the X axis, so the joint ryst2M is called the throat. It is not difficult to find that if we choose a circle (actually a sphere) at the top, there must be a sphere with the same r below, and the "spool" between the two is called a wormhole, and these two spheres are the holes of the wormhole. The selection of wormhole opening is arbitrary. We know from the above question that the upper and lower half of the region are located in region An and area A 'respectively. In the distance, that is, where r is very large, time and space is gradually flat, and there is no causal relationship between the two regions.

So we can imagine, is it possible that these two non-causal spacetimes are connected at a great distance?

Exhilaratingly, relativity does not prohibit this from happening, so we have the following picture:

This is the shortcut that Schwarzschild wormhole provides for us.

But unfortunately, the Schwarzschild wormhole is impenetrable.

Still want to look at the maximum extension of Schwarzschild time and space, our extension here is not static, in which the B and W regions are dynamic, so the Schwarzschild wormhole is not always open, according to the time sequence, the wormhole does not open at all, then the opening will directly fall into the singularity, and then open to the maximum at Thum0, then gradually become smaller, and finally completely closed. What is even more frustrating is that we can never pass through the wormhole when the wormhole is open, because any type of time curve from area A can only enter area B and can never enter area A'.

But if I just want to be able to pass through the wormhole, then we have to give up the vacuum condition, and even we need strange matter to open the wormhole, and strange matter is not allowed by classical physics, but according to the quantum field theory, the distorted vacuum fluctuations in the properly curved space-time region may be this strange matter, but these ideas have not been conclusively concluded.

By Frost White

Q8. Theoretically, can all electromagnetic waves in all bands generate lasers? by Anonymous

A:

Laser is a kind of electromagnetic beam with high energy and high coherence. Coherence is the most important feature of laser, which requires a stable phase difference between two points in the light field where the distance is much larger than the wavelength. In principle, any wavelength of electromagnetic wave can form a laser, but in fact, it is difficult to generate a partial wavelength laser.

Traditionally, the generation of laser requires three elements: "excitation source", "gain medium" and "resonance structure". The excitation source drives the gain medium oscillation, which is the energy source of the laser. Microscopically, the gain medium can be regarded as some quantum or classical harmonic oscillators, whose oscillation excitates the alternating electromagnetic field, and the characteristics of the gain medium determine the wavelength of the laser. In order to improve the coherence, it is often necessary to make the generated laser reflect back and forth in the resonant structure (such as resonant cavity) to synchronize the phase, select the frequency and amplify the gain.

From the above three elements, we can analyze the difficulty of realizing a specific wavelength laser. For microwave frequency, it can be generated by antenna and resonant circuit, and it is easy to achieve better coherence because of its long wavelength and less interference by micro-thermal motion. For terahertz band, the wavelength is in the order of micron, the difficulty lies in the lack of suitable gain medium, and terahertz happens to be the characteristic frequency of collective motion in many materials, so the development of high-quality terahertz light source is an important direction at present.

In infrared, visible and near ultraviolet bands, the transition of electrons between atomic (molecular) energy levels is usually used as the oscillation principle of the gain medium, and the wavelength of the laser can be adjusted by nonlinear optical effects such as frequency doubling and frequency difference. This is also the working band of the laser we are most familiar with. However, there are still many limiting factors in this band. for example, the gain of the laser requires the population inversion of the energy level, which requires the energy level structure of the gain medium. In addition, self-focusing and other effects will cause damage to the laser itself and limit the output power of most lasers.

From the very ultraviolet band to the soft X-ray band, it corresponds to the ionization energy of the core electrons near the nucleus in the atom. Therefore, the gain medium of soft X-ray laser is usually highly ionized high temperature and high density plasma. In addition to the special properties of the gain medium, the electromagnetic wave in this band also has a strong penetration, so it is necessary to use the Bragg diffraction of multilayers to construct the mirror and the corresponding optical cavity structure. With the further decrease of the wavelength, it becomes very difficult to construct the resonance structure and find the gain medium.

However, there is such a laser source, which is very different from the traditional laser in structure and working principle, and has the ability to generate laser in all bands from microwave to X-ray, which is the free electron laser source (FEL). FEL uses synchrotron radiation emitted by charged particles moving at a variable speed. The principle is shown below. A beam of high-energy electrons, passing through a series of alternating array of polarity magnets from left to right, oscillates transversely due to the Lorentz force, and then emits synchrotron radiation in the direction of motion. If the interval between the adjacent magnets is the Lorentz factor of the electron motion, then the wavelength of the synchrotron radiation in the ground system can be obtained. Therefore, as long as the energy of the electron beam is adjusted, the high coherence radiation of all bands from microwave to hard X-ray can be obtained.

By is happy in his heart.

This article comes from the official account of Wechat: Institute of Physics, Chinese Academy of Sciences (ID:cas-iop), author: Frions

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