The two postulates of classical mechanics are taken for granted in everyday life, but quantum mechanics predicts mysterious phenomena that go beyond them. These phenomena cannot be explained by classical mechanics, and it is possible that future science will solve this problem.
Classical mechanics is the fundamental theory that physicists used to describe the world until the early 20th century, and it contains two assumptions. The first assumption is that measurements of physical properties can be made without changing other physical properties of the object being measured. This assumption is consistent with our intuition of the world as we experience it every day, and it plays a fundamental role in understanding phenomena such as the motion of objects and the transmission of forces. For example, measuring the hardness of a stone doesn’t change its color, and a rock flying toward a glass window doesn’t cause the window to know in advance that it will break. In this way, classical mechanics is based on the belief that physical phenomena are independent and predictable.
The second assumption is that physical effects propagate through space without exceeding the speed of light. This means that when information or energy travels from one place to another, its speed does not exceed the speed of light, which is the absolute limit. This assumption supports the belief that all events in the universe are continuous, and that cause and effect are strictly observed. It also fits well with our intuition, in that it must take time for one event to affect another.
These assumptions of classical mechanics are natural to our understanding of the world and are useful in explaining most of the physical phenomena we encounter in our daily lives. However, in the 20th century, physicists began to observe a variety of phenomena that could not be explained by these two assumptions. With the development of more precise measurement techniques and experimental equipment, these phenomena gradually began to reveal themselves, exposing the limitations of classical mechanics.
Quantum mechanics replaced classical mechanics as the foundation of modern physics by accurately predicting more phenomena than classical mechanics. However, many of the phenomena that quantum mechanics predicts are very mysterious and go beyond our common sense. According to quantum mechanics, a pair of particles with zero sum momentum in the same direction will remain connected no matter how far apart they are. Now imagine that one of these two particles is placed on Earth and the other is sent to Venus. If you measure the horizontal momentum of the particle on Earth and get a value of +1, then the horizontal momentum of the particle on Venus is -1. How can a particle that far away be affected by the results of a measurement that happened instantaneously on Earth?
Quantum mechanics also shows that momentum in different directions is related. For example, the horizontal momentum and the vertical momentum are related, and measuring one affects the other. As a result, we measure the horizontal momentum of the Earth particle and get ＋1. If you then measure the vertical momentum and then measure the horizontal momentum again, you now have a 50/50 chance of getting ＋1 and -1 instead of just ＋1. The second vertical measurement has made the horizontal momentum value indeterminate. Furthermore, the Earth particle is connected to the Venus particle. So if you measure the horizontal momentum of the Venus particle and get -1, and then measure the vertical momentum of the Earth particle, the value of the horizontal momentum of the Venus particle at that moment is also uncertain. So, if we measure the horizontal momentum again, we will get -1 and ＋1 with a 50/50 chance. How can a measurement made on Earth instantaneously affect the physical properties of a particle so far away? It seems impossible to give a causal explanation for this phenomenon that satisfies the assumptions of classical mechanics.
It is very difficult to experimentally verify such a mysterious quantum phenomenon. However, after a series of experiments by Alain Aspect in the 1980s, it became undeniable that these quantum phenomena do exist in the microscopic world. Quantum mechanics accurately predicts these phenomena, but it cannot explain them causally in a way that we can understand. Physicists generally react to these limitations of quantum mechanics in two ways. The first is that quantum mechanics has been very successful when applied to nature, so it doesn’t matter that these quantum phenomena seem strange to us. The second is that quantum mechanics will be replaced by a better theory in the future, and by then the mysterious quantum phenomena will have some form of explanation.
These characteristics of quantum mechanics are the subject of constant debate and research in modern physics. Concepts that were unimaginable during the reign of classical mechanics are now at the center of science, and these changes are having a profound impact on the way we understand the universe and nature. The difference between classical and quantum mechanics is more than just a difference between two theories; it is an important example of how human knowledge develops and evolves. In this process, we can see that our perception of nature is becoming increasingly complex, and we realize that science’s path to truth is never straightforward.
