On October 3, the 2017 Nobel Prize in Physics was announced in Sweden, and Rainer Weiss, Barry C. Barish and Kip S. Thorne were awarded for gravitational wave detection research. The gravitational wave detector LIGO is worth $600 million.
In fact, physics experiments sometimes can change the world without a penny. In physics, there is a special kind of experiment: they don't need to buy expensive instruments, they don't need a lot of manpower and resources, they just need logic. The brain; but this kind of experiment can challenge the conclusions of predecessors, establish new theories, and even lead people to rethink the world. This kind of experiment is a legendary thought experiment. Many great physicists in history have designed thought-provoking thought experiments. Galileo, Newton, and Einstein are the representatives. These thought experiments not only have an indelible effect on the development of physics, but also Subverting people's understanding of the world about the universe. This article will introduce some famous thought experiments in the history of physics from easy to difficult.
1 Principle of Inertia Since the Aristotle era, people have always thought that force is the cause of motion, and the movement of objects without force will be static. It was not until Galileo proposed the following household experiments that people knew the principle of inertia—an object that is not subject to any external force (or a combined external force of 0) will remain stationary or move at a constant speed: 
Imagine a V-shaped smooth guide placed vertically, a small ball that can roll without friction on it. Let the ball roll down from the left end and the ball will roll to the same height on the right. If you lower the slope of the right rail, the ball will still roll to the same height, at which point the ball will roll further in the horizontal direction. The smaller the slope, the farther the ball must roll in order to roll to the same height. At this point, it is assumed that the slope of the right side rail is continuously lowered to be leveled. According to the previous experience, if there is no friction, the ball will continue to roll and maintain a uniform linear motion. In any actual experiment, because the friction is always negligible, any real experiment can not strictly prove the principle of inertia, which is why the ancients did not derive the principle of inertia. However, thought experiments can be done. Only through the extension of daily experience can any rational person believe in the correctness of the principle of inertia. This simplest thought experiment is enough to reflect the edge of thought experiment!
2 The two small balls are still affected by Aristotle. People before Galileo thought that the heavier objects fall faster, and the lighter objects fall more slowly. Galileo is well known in the famous experimental people on the Leaning Tower of Pisa, but what many people don't know is that Galileo has already proved through a thought experiment that two small balls must land at the same time: If Aristotle's argument If it is correct, then imagine a heavy ball and a light ball tied together. Because the heavy falls quickly and the light falls slowly, the light ball will drag the heavy ball to give it a resistance to slow it down, so the falling speed of the two balls should be between the heavy ball and the falling speed of the light ball. However, if the two balls are viewed as a whole, the total weight is greater than the heavy ball, and it should fall faster than when the ball is dropped alone. So the two inferences are contradictory. Aristotle’s argument is wrong, and the two balls must be at the same time. With the above thought experiment, in fact, the simultaneous landing of two small balls is not only a law of physical establishment, but also logically. In this example, the thought experiment played a role that the real experiment could not achieve: even if the Newtonian theory of gravity that we learned in our high school did not apply, the two small balls were still established at the same time! Later I will talk about the equivalent principle in general relativity. The logical inevitability of this thought experiment is the key factor for Einstein to sum up the equivalent principle.
3 Newton's cannon 
As shown in the figure, a cannon that is mounted on a mountain raises the projectile horizontally at a high speed. The faster the projectile is, the farther it will fall. Once the speed is fast enough, the projectile will never land, but will periodically move around the earth. Newton’s simple thought experiment, for the first time, made people realize that the reason why the moon does not fall to the ground (and does not fly away) is the gravitation that leads to the landing of the apple! Newton's theory of gravity has contributed to a leap in people's understanding: the things in heaven are not "sacred", and the laws they follow are exactly the same as the ordinary objects on the ground.
4 bucket experiment
Hang a bucket with a long rope, let it rotate until the rope is tight, then inject the water, and the water and the bucket are temporarily in a static state, at which point the liquid level is apparent. Then suddenly the bucket is rotated in the opposite direction. At the beginning, the water surface does not follow the movement, and the water surface is still level. But then, the bucket gradually transferred the motion to the water, causing the water to start spinning, and it can be seen that the water gradually leaves its center and rises along the wall of the barrel to form a concave surface. The faster the movement, the higher the water rise. If the barrel is suddenly stopped at this time, the water will still rotate due to inertia, and the liquid level at this time is still concave. Newton believes that the concave surface of the water is not caused by the relative movement of water around the water, but by the absolute, true circular motion of the water. Therefore, the existence of absolute motion can be judged by the depression of the water surface. This thought experiment was designed by Newton to demonstrate the existence of absolute space. However, as we all know, Newton's absolute time and space view is actually wrong, which means that this thought experiment is actually a failure example. This fallacy was pointed out by philosopher and physicist Mach more than 100 years later. Mach believes that the depression on the surface of the water is not due to the movement of water relative to the "absolute space", but to the movement of all other objects in the universe, which all exert an effect on the water by gravity. Among them, the decisive object is a distant celestial body. It is the "reference system dragging" effect of distant celestial bodies that causes the liquid surface relative to their rotation to sag. Mach believes that there is no absolute space and all reference systems are equivalent. If the water surface can be kept still and all distant objects are rotated together, according to Mach's point of view, the still water surface will produce a concave surface. We obviously can't do such an experiment, but if we use the buckets of several kilometers thick to do the above bucket experiment, then people can't be sure of Newton's judgment on the flatness of the liquid surface. . Later, Mach's point of view had a decisive influence on Einstein's invention of general relativity, and the Mach principle itself was widely recognized with the gradual confirmation of general relativity.
5 Before Obersberg ’s theory of the Big Bang in the 20th century, people’s understanding of the universe was simple: the universe is infinite, the time of existence is infinite, the universe is in a steady state, and the stars in the universe are distributed at large scales. Evenly on. What people did not know at the time was that from these four basic assumptions, it was possible to logically come to a conclusion that was clearly contrary to the facts - Oberscher: If the universe is stable, infinite, and time-spaced, evenly The same illuminant is distributed. Since the illuminance of the illuminant is inversely proportional to the square of the distance, the number of illuminants in the spherical shell is proportional to the square of the distance at a certain distance, so that the integral of the illuminance of all illuminants does not converge. The sky in the night should be infinitely bright. However, the dark nights of the day are always on schedule, and the sky is not always bright. This shows that there was a problem with our understanding of the universe. Obers himself gave an explanation. He believed that the dust and non-illuminating stars in the universe absorbed a part of the light. However, this explanation is wrong, because according to the first law of thermodynamics, the energy must be conserved, so the intermediate barrier will heat up and begin to emit radiation, resulting in uniform radiation in the sky, the temperature should be equal to the temperature of the surface of the illuminator, ie The sky is as bright as the stars, but in fact no such phenomena are observed. It was not until the advent of the Big Bang theory that Oberss was able to solve it. According to the Big Bang theory, the universe was born in a big explosion 15 billion years ago. Until now, the universe is still in the process of expansion. Therefore, the existence time of the universe is limited and not in a stable state. The two basic assumptions are no longer valid, so Oberthurch is naturally disintegrated.
6 In the era after Laplace's demon Newton, classical mechanics produced great success in describing the world, and people gradually believed that the world could be mechanically described by the laws of physics. More extremely, Laplace believes in mechanical determinism, arguing that everything in the world (including humans and society) cannot escape the control of certain physical laws. "We can think of the present state of the universe as its past fruit and the cause of the future. If an intelligence knows the force of all natural movements at a certain moment and the position of all naturally constructed objects, if he can also analyze the data, then the universe The movement from the largest object to the smallest particle is contained in a simple formula. For this wise man, nothing will be vague, and the future will only appear in front of him as before." - La Plasla The "intelligence" mentioned by Plath is what the later generations call "Laplace demon." If the Laplace demon is there, then the world is too terrible: all of your actions can be calculated, and our fate is all determined by the laws of physics + initial conditions. Nothing will happen. It’s beyond calculation, so there’s so much fun in life! Fortunately, the development of chaos theory and quantum mechanics makes the Laplace demon never exist. Quantum mechanics tells us that physical quantities are uncertain and cannot be accurately measured without errors. The theory of chaos shows that as long as three or more objects are involved, the extremely small difference in initial conditions will result in a vastly different final result. From another perspective, the Laplace demon is based on the reversible process of classical mechanics, but the real system does satisfy the irreversible process of the second law of thermodynamics (the entropy-increasing principle). Therefore, the world is still full of uncertainty and full of surprises. People can also use their own subjective efforts to change their destiny.
7 Maxwell demon 
In middle school, we all studied the second law of superheat mechanics (the principle of entropy increase): the irreversible process entropy of isolated systems is always increasing. "The leaves are never separated, the water is difficult to receive; the resurgence of the ash is hard, and it is hard to beat; the circle is broken and the ambiguity is unreasonable; life is easy to be old, rejuvenation is just fantasy; raw rice cooked mature rice, irreparable..." It is the reaction of the entropy increase principle in real life, and it has now become one of the most unbreakable principles in physics. However, Maxwell once proposed a martyrdom on the principle of entropy increase, which is very confusing: an insulated container is divided into two equal squares, in the middle is a small "door" controlled by "Maxwell", the air in the container When a molecule moves irregularly, it will hit the door. The "door" can selectively put the faster molecules into one cell, while the slower molecules are placed in another cell, so that one of them will The temperature of the system is higher than the other one, and the entropy of the system is reduced. This temperature difference can be used to drive the heat engine to work, which is in contradiction with the second law of thermodynamics. It is not an easy task to refute this martyrdom. Some people may think that Maxwell demon needs to consume energy when opening and closing the door. The entropy increase generated here will offset the decrease of system entropy. However, the energy consumed by the switch door is not essential, it can be arbitrarily reduced to be small enough. The true interpretation of Maxwell's demon was not revealed until the 20th century. The question about entropy has always been difficult to understand, so I directly quote Mr. Zhao Kaihua's words in "New Concept Mechanics·Hotology": "Maxwell demon has the ability to acquire and store molecular motion information. It relies on information to intervene in the system and make it against it. In the direction of nature, in modern terms, information is negative entropy, and Maxwell demon inputs negative entropy to the system, reducing its entropy. So, how can Maxwell's demon get the information he needs? It must have a temperature and environment. Different micro-light sources illuminate the molecules, which requires a certain amount of energy and generates additional entropy. Maxwell's demon is at this cost to obtain the required information (ie, negative entropy), which compensates for the additional entropy. The reduction of entropy in the system. In general, even if there is a Maxwell demon, its way of working does not violate the second law of thermodynamics."
8 Twins Einstein's special theory of relativity established a new concept of time and space, which was difficult for people at the time. Therefore, since the introduction, the special theory of relativity has been subjected to various martyrdoms, the most famous of which is the twins. However, no matter how difficult it is, the special theory of relativity can be perfectly explained. All the defects are resolved one by one. Studying these flaws can more deeply understand the space-time view of special relativity. In the special theory of relativity, the reference time of the movement will be slowed down, the so-called slowing effect of the moving clock. Now imagine a scenario where there are a pair of twins A and B, A stays on the earth, and B flies deep into the universe with a spacecraft approaching the speed of light. After flying a certain distance, the spacecraft turned back and flew back, eventually landed back to Earth, and the two brothers met. Now the problem is coming: A thinks that B is slower when exercising, B should be younger than A; and similarly, in B's opinion, A is always in motion, A is slower, and A should be younger than B. It is. So who are the younger brothers? Is the special theory of relativity contradictory? In fact, the key to understanding twins is to know that the status of A and B is not equal: only B of the two experienced an acceleration process, and B inevitably experienced an acceleration when the spacecraft turned around. Therefore, only A is in the inertial system established by the special theory of relativity. Only A's view is correct: when the two brothers meet, B is younger than A. Similar effects have been confirmed by precision experiments. In fact, as long as you use the special theory of relativity to do detailed calculations, you can understand from the perspective of B why B is younger than A, but this has to do cumbersome calculations, which is not given here. At this point we can safely say that the special theory of relativity does not contain contradictions on this issue. But the twin brothers who went out for a trip actually came back to be younger. This has subverted the world view of most people... but this is a fact, and you don’t believe it!
9 Equivalence Principles Everyone in the middle school learned the concept of quality, but in fact there are two different qualities: inertial mass and gravitational mass. The inertial mass is m in F = ma, which is a measure of the magnitude of inertia; the gravitational mass is m in F = GMm / r^2, which is a measure of the magnitude of gravity. The reason why the two do not distinguish between the two is because the two are exactly equal. This fact is not taken for granted, and it is through this magical fact that Einstein summed up a basic assumption of general relativity: the principle of equivalence. 
Imagine a spacecraft in free space (without gravitation) that accelerates linear motion with an acceleration of a = 9.8 m/s^2. If the person inside throws a small ball, the ball will land at an acceleration of 9.8m/s^2 due to inertia; this is just like the inertial system in a gravitational field. The inertial force in the non-inertial system is proportional to the inertial mass, while the gravitational force is proportional to the gravitational mass. The fact that the inertial mass and the gravitational mass are equal causes the two effects of inertial force and gravitation to be indistinguishable. This is the weak equivalent principle. Einstein further promoted, for all physical processes (not just mechanical processes), the accelerating motion reference system in free space, and the inertial system under gravitation, which are completely indistinguishable in principle, which is the strong equivalent. principle. "All objects in the gravitational field have the same acceleration. This law can also be expressed as the mass of inertia is equal to the mass of gravity. It made me realize the full importance of it. I was extremely surprised and guessed about its existence. There must be a key to gain a deeper understanding of inertia and gravity." - Einstein.
10 Schrödinger's cat
Schrödinger’s cat is probably the most famous fictional animal in physics. It is one of the founders of quantum mechanics. Schrödinger proposed to explain that quantum mechanics is not complete: put a cat into a closed box. Then, the box is connected to an experimental device containing a radioactive nucleus and a container containing toxic gases. Imagine that this radioactive nucleus has a 50% chance of decay within an hour. If it decays, it will emit a particle, and the emitted particle will trigger the experimental device, open the container containing the poison gas, and kill the cat. According to quantum mechanics, when not observed, the nucleus is in a superimposed state of decaying and undecaying, and the cat is in a superimposed state of death and living, that is, "both dead and alive" (rather than "half dead and alive" that many people misunderstand, "Either die or live"). However, if the box is opened after an hour, the experimenter can only see the "decaying nucleus and dead cat" or "unreduced nucleus and live cat". The question now is: When did the system start to be in a superposition of two different states and become one of them? Was the cat dead or alive or dead and alive before opening the box? The original intent of this experiment was to show that quantum mechanics itself is incomplete if it can't collapse the wave function and give a reasonable explanation of the state of the cat. Schrödinger's cat is a nightmare for physicists, which magnifies the microscopic quantum mechanical effects into macroscopic daily life, making everything very strange. The interpretation of Schrödinger's cat involves a variety of profound philosophical understandings of quantum mechanics. This article does not elaborate. If you want to understand the fate of this magical cat, then go to the School of Physics to learn! Postscript: The thought experiment is a great wisdom in the history of physics. It is enough to look back at the shining light of wisdom. I hope that these thought experiments can give readers an enlightenment to the world.
In addition to CWDM, DWDM, FWDM, LAN WDM, FBT WDM, AAWG WDM, there are other WDM realted equipment that commonly used together to form up passive optical network. Like Mechanical Fiber Optic Switch, WDM PD, Pigtail PD, TAP WDM Devices, Optical Isolator, Optical Circulator, etc. In other words, Mechanical Fiber Optic Switch, WDM PD, Pigtail PD, TAP WDM Devices, Optical Isolator, Optical Circulator, etc., are the most commonly used WDM related equipment in construction of optical network. They all play important roles when setting up a integrated fiber optical network. You can always trust GL-COM in this area.
Mechanical Optical Switch is based on Advanced Optoelectronic Integrated Technology. it connects optical channels by redirecting an incoming optical signal into a selected output fiber. This is achieved using a Opto-Mechanical proprietary configuration and activated via an electrical control signal. Mechanical Fiber Optic Switch connects optical channels by redirecting an incoming optical signal into a selected output fiber. The mechanical operation offers ultra-high reliability and fast switching speed as well as bi-directional performance.
WDM Related Equipment, Mechanical Fiber Optic Switch, WDM PD, Pigtail PD, TAP WDM Devices, Optical Isolator, Optical Circulator
Shenzhen GL-COM Technology CO.,LTD. , https://www.szglcom.com