This blog post explores how quantum mechanics transcended the laws of causality in classical mechanics and the resulting shift in worldview.
Classical mechanics served as the fundamental theory by which physicists described the world until the early 20th century. It included two key assumptions. The first assumption is that measuring one physical property of an object can be done without altering any of its other physical properties. This assumption aligns with our everyday intuitive experience of the world and plays a fundamental role in understanding phenomena like the motion of objects or 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 shatter the moment a rock flies toward it. Thus, classical mechanics is based on the belief that physical phenomena are independent and predictable.
The second assumption is that physical influences propagate through space without exceeding the speed of light. This means that when information or energy moves from one place to another, its speed cannot exceed the absolute limit of the speed of light. This assumption supports the belief that all events in the universe occur continuously, with causality strictly maintained. Therefore, it aligns well with our intuition that it necessarily takes time for one event to influence another.
These assumptions of classical mechanics are naturally accepted in our understanding of the world and are useful for explaining most physical phenomena encountered in daily life. However, as the 20th century progressed, physicists began observing various phenomena that could not be explained by these two assumptions. These phenomena gradually revealed their 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, becoming the foundation of modern physics. However, many of the phenomena predicted by quantum mechanics are highly mysterious, defying our common sense. According to quantum mechanics, a pair of particles with a net momentum of zero in the same direction remain correlated no matter how far apart they are. Now, suppose 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 a particle so far away be instantly affected by the result of a measurement made on Earth?
Furthermore, according to quantum mechanics, momentum in different directions is also correlated. For example, if you measure the horizontal momentum, the vertical momentum is affected. As a result, if you measure the horizontal momentum of the Earth particle and get +1, then subsequently measure the vertical momentum and then measure the horizontal momentum again, you won’t just get +1; instead, you get +1 and -1 with equal probability. The second vertical measurement has made the horizontal momentum value indeterminate. Moreover, the Earth particle is linked to the Venus particle. Therefore, after measuring the Venus particle’s horizontal momentum and obtaining -1, if you then measure the Earth particle’s vertical momentum, the Venus particle’s horizontal momentum value also becomes uncertain at that instant. Consequently, if the horizontal momentum is measured again, it will yield -1 and +1 with equal probability. How can a measurement made on Earth instantaneously affect the physical properties of a particle located an enormous distance away? It seems impossible to provide a causal explanation for this phenomenon that satisfies the assumptions of classical mechanics.
Experimentally verifying such mysterious quantum phenomena is extremely difficult. However, following a series of experiments conducted by Alain Aspect in the 1980s, the fact that these quantum phenomena actually exist in the microscopic world became undeniable. Quantum mechanics accurately predicts this phenomenon, but it does not provide a causal explanation that we can understand. Physicists generally show two reactions to this limitation of quantum mechanics. The first is that, since quantum mechanics has been highly successful in describing nature, the fact that these quantum phenomena seem strange to us is not really a problem. The second is that quantum mechanics will be superseded by a better theory in the future, and when that happens, the mysterious quantum phenomena will be explained in some form.
These characteristics of quantum mechanics remain a subject of constant debate and research in modern physics. Concepts unimaginable during the era dominated by classical mechanics now stand at the heart of science, profoundly influencing how we understand the universe and nature. The difference between classical and quantum mechanics transcends a mere contrast between two theories; it serves as a crucial example illustrating how human knowledge develops and evolves. Through this process, we recognize that our understanding of nature is becoming increasingly complex, and we come to realize that science’s journey toward truth is never linear.
