I. Introduction:
Physics is essentially an experimental science. Physics experiment plays an important role in the development of physics and physics education. It can be said that physics cannot be understood without physical experiments. Because of this, in the research and teaching of physics, we have always attached great importance to physical experiments, from the design of experiments, the manufacture and debugging of instruments, to the control of experimental processes and the analysis of experimental results. In contrast, there are not many related thought experiments. Therefore, it is necessary to make a longitudinal historical investigation and a horizontal comparative study of thinking experiments in physics. This is helpful to the research and teaching of physics.
Second, the general investigation of thinking experiment
Galileo was a pioneer of modern physics. He made many contributions to physics. Among them, the law of falling body and the law of inertia he discovered provided two fast and solid cornerstones for modern physics. Galileo's success is attributed to his pioneering use of scientific physical experiments, and more importantly to his original scientific method of combining physical experiments with ideological experiments. Galileo's outstanding work shows that he is not only a master of physics, but also a pioneer of thought experiments.
As we all know, people's understanding of physical phenomena and laws such as force and motion has been bound by Aristotle's theory for a long time. Aristotle believes that the speed of an object is directly determined by whether it is stressed and the size of the force; Different objects fall at different speeds on the ground; Heavy objects fall fast, while light objects fall slowly. Some people have raised objections to his false statement, but because there is no exact experimental and theoretical proof, it has not been taken seriously. It was Galileo who succeeded in breaking Aristotle's wrong authority. Galileo skillfully used thought experiments to deny this erroneous theory that unified Europe for nearly two thousand years.
The speed at which an object falls is proportional to its weight. Galileo wrote in his book Dialogue between Two New Sciences and Mathematical Proof: "I very much doubt that Aristotle has verified it with experiments. When two stones, one of which is 65,438+00 times the weight of the other, fall from the same height, such as 65,438+000 cubits, the speed difference will reach such an extent that when the former falls to the ground, the latter does not exceed 65,438+00 cubits. " Galileo firmly grasped this doubt and designed a thought experiment to analyze and demonstrate. He pointed out: If Aristotle's assertion holds, that is, heavy objects fall faster than light objects, then when heavy objects and light objects fall together, the speed slows down because of the slow speed of obstacles. The slow is accelerated quickly, and as a result, the falling speed of the tied object must be between the original two objects, that is, less than the falling speed of the original heavy object. However, when two objects are tied together, they become a complex, which is heavier than the original weight. According to Aristotle's judgment, the falling speed of the complex is faster than that of the original weight, which contradicts the above conclusion. So heavy objects will not fall faster than light objects, and they should fall at the same speed. It was this thought experiment that strengthened the confidence and determination of Galileo's falling body experiment.
After denying Aristotle's law of falling body, Galileo made a further quantitative study on the motion of free falling body. Based on the qualitative observation that the speed of free fall is getting faster and faster, he assumed that the free fall is a uniformly accelerated motion, and conducted a lot of falling experiments during 1590- 1592. However, under the test conditions at that time, this hypothesis could not be immediately confirmed by experiments, so Galileo combined the thought experiment with the real experiment to solve this problem. With the help of mathematics, he worked out the relationship between the distance s and the time t of uniform acceleration from rest, that is, s/t? = constant. At this time, it does not contain any rate, so just measure S and T directly.
However, the free-falling speed of the object is still too fast to be accurately measured at that time. Galileo wanted to measure it in slow motion, that is, to use an inclined plane instead of a falling object experiment. After repeated experiments, the following results were obtained:
(1) When the inclination of the inclined plane is fixed, the ratio of the distance S that the ball rolls over to the square of the time t used is a constant, that is, s/t? =c。
(2) change the inclination of the inclined plane, s/t? The value of is changing, but the distance traveled by the ball is proportional to the square of time, and only the proportionality constant changes.
Galileo pushed this result to the extreme with his thought experiment-when the inclination angle is 90? Time. That is to say, when an object falls freely, this assertion also holds. He concluded from this that the motion of free falling body is uniformly accelerated.
Galileo was sincerely pleased with his original scientific method of combining physical experiments with ideological experiments.
After Galileo, with the development of science and technology and the deepening of cognitive activities, thought experiments have been widely and consciously used in physics, and physicists such as Mach and Einstein have appeared who are good at using thought experiments. In various branches of physics, some famous thought experiments have been produced. For example, in mechanics, Mach experimented with the idea of "Newton's rotating barrel". Deny Newton's absolute space-time and absolute motion. In quantum mechanics, there are thought experiments such as single electron diffraction experiment to prove particle fluctuation, energy potential well, ideal microscope experiment to prove uncertainty relationship, and electron beam single slit diffraction experiment. In the theory of relativity, there are thought experiments of "Einstein train" and "photon rocket" to prove the relativity of simultaneity, and so on. It can be said that without thought experiment, the development of physics is difficult and its theoretical system cannot be established.
In this regard, Einstein once commented that "Galileo's discovery and the scientific reasoning method he applied are one of the greatest achievements in the history of human thought, marking the real beginning of physics".
Third, the basic characteristics of thinking experiment
From the historical investigation of thinking experiment in physics, we can see that thinking experiment is a complex thinking and reasoning activity in the form of real experiment. Its ideological operation includes the following aspects: (1) the expectation of experiments that have never been carried out or are potentially realizable; (2) the idealized abstraction or extrapolation of real experiments, that is, ideal experiments; (3) An abstraction that does not exist in reality and contradicts experience, but is logically meaningful and physically meaningful. This kind of thought experiment operation can get definite conclusions without materialization. Thus, the thinking experiment in physics has the following basic characteristics:
1, which has the operability of the experiment.
Thinking experiment is not an actual experiment, so it is called experiment because this kind of thinking activity is carried out in the form of experiment and has maneuverability. R Harry pointed out: "The so-called thinking experiment is formed when operating the model in imagination."
According to their own understanding and thinking, the operator of the thinking experiment uses the empirical representation and language materials stored in his mind to perform imaginative operation through analysis, imagination, analogy, guess and logical reasoning.
Imagination tools used in thinking experiments are the basic means by which the imaginary subject acts on the object. When determining the program definition of simultaneity, Einstein believed that light travels in a straight line at a uniform speed. The procedure for checking the synchronization of two identical clocks is to measure the distance between the two clocks and find their midpoint. When their clocks point to seven o'clock, two observers who stand by and watch the two clocks will signal. If the two signals meet at the midpoint, the two clocks will be synchronized.
Imagining the change of object image is the most important content of thinking experiment and the inevitable process of thinking experiment operation. Imagination subject and tools exist for this purpose. In the process of observing and acting on the imaginary object, the essence and relationship of the studied things are exposed, so as to achieve the intended purpose of thinking experiment.
2, with strict logic.
In the process of thought experiment, logical reasoning is his basic direction. When a physicist conceives a thought experiment, his imagination is completely free: he can fully associate the object of study according to his own experience and scientific level at that time. Where Lenovo comes from and in which direction it extends is also random. When many image materials and empirical representations emerge in the minds of physicists, their development and connection often turn to related concepts and relationships. These concepts and relationships are not only related to the following logical thinking, but also to the predetermined goals before conception. As long as logical reasoning allows, it even accepts representations that violate "empirical facts" or are contrary to established "physical laws" to enter thought experiments.
Therefore, the operation process of thinking experiment is not only the process of imagination free development, but also the process of logical movement. In this process, logic plays a leading role, guiding and controlling imagination, keeping imagination rich, but not thinking. Imagination provides a reliable material basis for logical movement, which runs in the image activities provided by imagination. The two complement each other and blend into one.
3. Very creative.
The purpose of physicist's thought experiment is to reveal the inherent regularity of things. Therefore, its exploration is unprecedented and creative. Einstein and infeld pointed out in the Evolution of Physics: "The purpose of any theory is to guide us to know new situations, inspire us to do new experiments and discover new phenomena and laws." The same is true of thought experiments. French physicist and engineer Carnot, when studying thermal efficiency and heat engine efficiency, was inspired by the waterfall drop and carried out the thought experiment of "Carnot cycle". He envisioned two ideal isothermal processes and two ideal adiabatic processes to complete a thermodynamic cycle. Through such an ideal thermodynamic cycle thought experiment. Carnot concluded that the highest thermal efficiency of the heat engine is equal to T2-T 1/T 1, which creatively solved the problem of heat engine efficiency.
Fourth, the positive role of thinking experiment.
Thought experiment has a positive effect on the development of physics, mainly as follows:
1, which is strongly questioned and criticized.
Using thinking experiment, we often start with questioning the traditional theory, thus vividly and accurately exposing the defects of the old theory and refuting its mistakes.
Aristotle, who enjoyed supreme prestige in Europe at that time, summed up his mechanical theory: when the force pushing an object no longer pushes it, the original moving object will be at rest. This theory has dominated people's minds for more than two thousand years. Galileo questioned this, and conducted a thought experiment on the basis of the inclined plane experiment. He thought: "If a moving object has a certain speed, as long as there is no external reason to increase or decrease its speed, it will always maintain this speed-this condition is only possible on the horizontal plane, because if it moves along the inclined plane, the downward movement will be accelerated, while the upward movement will be slowed down. It can be seen that only the motion on the horizontal plane is constant, because if the speed is constant, the motion will neither be reduced nor eliminated. " Here, Galileo correctly distinguished Aristotle's defects, put forward the viewpoint of "inertia", and obtained a brand-new mechanical theory of the relationship between force and velocity, thus denying the old theory of the relationship between force and velocity itself.
2. Deeply understand the role of innovation.
The application of thinking experiment can deepen people's understanding, open up new research directions and promote the development of physics.
V the relationship between thinking experiment and real experiment
Thinking experiment and real experiment are different in principle: real experiment is a kind of scientific experiment activity. It is to make something (or process) repeat itself under the condition that many external influences are excluded as much as possible, the main factors are highlighted, and the relationship between them can be observed in detail. At first, experiment originated from practice and was higher than practice, which was the only criterion for discovering and testing physical theory. Thinking experiment is a rational thinking activity. It is not a subjective imagination divorced from reality, but based on reality, operating according to the format of the experiment, and making a deeper abstract analysis of the practice process, and its reasoning process is based on certain logical laws. It is a relatively independent scientific method.
Thinking experiment and real experiment are closely related and complement each other. The theories and laws in physics are summarized from a large number of experimental facts, and the assumptions and arguments in physics also depend on the verification of real experiments. In real experiments, in order to get a regular understanding of nature, physics strictly controls objects through instruments and equipment and makes purposeful changes. In this way, the expectation and design of the experimental process before the real experiment begins, and the analysis and summary of the empirical materials after observation must rely on a rational thinking. Sometimes the two are inevitably intertwined. Real experiments provide empirical materials for thought experiments, which process the empirical materials reasonably and provide theoretical guidance for real experiments. This can be seen clearly from Galileo's discovery of the law of falling body and the law of inertia.
In the research of physics, thinking experiment can be an irreplaceable scientific method, and it also supplements the deficiency of real experiment with its rigor and accuracy of scientific thinking.
It should be pointed out that modern physics has developed to the stage of theoretical science, and cognitive activities are getting farther and farther away from daily experience and people's intuition, and go deep into the micro and macro fields that cannot be directly perceived. The relationship between phenomena and essence of things is becoming more and more complicated and hidden. The instruments and equipment needed for physical research activities are becoming more and more sophisticated, and the position of rational thinking is more prominent. As a scientific method, thinking experiment will be applied in a wider field.
Conclusion of intransitive verbs
The investigation and analysis of thinking experiment tells us that thinking experiment is one of the important methods to deepen physics research and teaching. Einstein pointed out: "Asking a question is often more important than solving a problem, because solving a problem may be just a mathematical or experimental skill. Putting forward new problems, new possibilities, and looking at old problems from a new perspective require creative imagination, which marks the real progress of science. " Therefore, on the basis of real experiments, we should strengthen the research of thinking experiments, enlighten thinking, lead the research of physics to a deeper level, and make teaching fascinating and more effective.
Take the exam and contribute.
[1] A. Einstein and L. infeld, Evolution of Physics, Shanghai Science and Technology Press, 1962.
[2] T.S. Cooter: The Role of Thought Experiment, Necessary Tension, Fujian People's Publishing House, 198 1 year.
[3] WI.B. Bei Feric: Art Science Press for Scientific Research, 1979.
[4] Gao Wenwu: "On Thought Experiment", "Dialectics of Nature Newsletter" No 2, 1982.
[5] Lectures on the History of Physics, Beijing Physics Society, 1983.