In this blog post, we will examine how quantum mechanics transcended the laws of causality in classical mechanics and how this changed our worldview.
Classical mechanics was the basic theory used by physicists to describe the world until the early 20th century, and it included the following 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 everyday experience of the world and plays a fundamental role in understanding phenomena such as the motion of objects and the transfer of force.
For example, measuring the hardness of a rock does not change its color, and a glass window cannot know in advance that it will break when a rock is thrown at it. Classical mechanics is based on the belief that physical phenomena are independent and predictable. The second assumption is that physical influences propagate through space at a speed that does not exceed the speed of light.
This means that when information or energy moves from one place to another, its speed does not exceed the absolute limit of the speed of light. This assumption supports the belief that all events in the universe occur continuously and that causality is strictly observed. Therefore, it fits well with our intuition that it takes time for one event to affect another.
These assumptions of classical mechanics are naturally accepted in our understanding of the world and are useful for explaining most of the physical phenomena we encounter in our daily lives. However, in the 20th century, physicists began to observe various phenomena that could not be explained by these two assumptions. These phenomena gradually began to reveal their true nature with the development of more precise measurement techniques and experimental equipment, exposing the limitations of classical mechanics.
Quantum mechanics replaced classical mechanics by accurately predicting more phenomena than classical mechanics, becoming the foundation of modern physics. However, many of the phenomena predicted by quantum mechanics are very mysterious and beyond our common sense. According to quantum mechanics, a pair of particles with the same momentum in the same direction will remain connected no matter how far apart they are. Now, suppose that one of these two particles is placed on Earth and the other is sent to Venus. If the horizontal momentum of the particle on Earth is measured and found to be +1, then the horizontal momentum of the particle on Venus will be -1. How can particles so far apart be instantly affected by measurements taken on Earth?
Furthermore, according to quantum mechanics, momenta in different directions are also related. For example, if you measure the horizontal momentum, the vertical momentum is affected. As a result, measuring the horizontal momentum of the particle on Earth gives a value of +1. If you then measure the vertical momentum and then measure the horizontal momentum again, you will not only get +1, but +1 and -1 with equal probability. The second vertical measurement has made the horizontal momentum value uncertain. Furthermore, Earth particles are connected to Venus particles. Therefore, if we measure the horizontal momentum of Venus particles and obtain -1, and then measure the vertical momentum of Earth particles, the horizontal momentum value of Venus particles also becomes uncertain at that moment. Therefore, when the horizontal momentum is measured again, -1 and +1 appear with equal probability. How can a measurement made on Earth instantaneously affect the physical properties of particles that are so far away? It seems impossible to explain this phenomenon causally while satisfying the assumptions of classical mechanics.
It is extremely difficult to experimentally verify such mysterious quantum phenomena. However, since Alain Aspect’s series of experiments in the 1980s, it has become undeniable that such quantum phenomena actually exist in the microscopic world. Quantum mechanics accurately predicts this phenomenon, but it cannot provide a causal explanation that we can understand. Physicists generally have two responses to the limitations of quantum mechanics. The first is that quantum mechanics has been very successful when applied to nature, so it is not a problem 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 when that happens, the mysterious quantum phenomena will be explained in some way.
These characteristics of quantum mechanics are the subject of ongoing debate and research in modern physics. Concepts that were unimaginable during the era of classical mechanics are now at the center of science, and these changes are profoundly influencing the way we understand the universe and nature. The difference between classical mechanics and quantum mechanics goes beyond the differences between the two theories and serves as an important example of how human knowledge develops and evolves. In this process, we can see that our understanding of nature is becoming increasingly complex, and we realize that the journey of science toward truth is by no means linear.
