In this blog post, we’ll examine how the conventional wisdom and knowledge that humans have held have changed through the evolution of science from classical physics to quantum mechanics.
“I think, therefore I am.” This is the first proposition presented by Descartes to establish an unshakable foundation of knowledge, in opposition to the skeptics of his time who argued that, because humans are imperfect, they cannot know everything with certainty and that perfect knowledge cannot exist. Descartes began his argument for constructing complete knowledge from the most certain proposition of all: that he is currently thinking. However, there is room for serious consideration as to whether an argument based solely on thought can truly constitute complete knowledge.
Common beliefs stem from results verified through the senses and experimentation. However, no one can be certain whether these beliefs are correct or incorrect. This is because, from a higher perspective that humans have not yet reached, current knowledge may be revealed to be false. Nevertheless, humans understand the world within the limits of their perception, formulate laws based on that understanding, and accept them as truth when they judge those laws to align with reality. In other words, humans understand the world based on observable phenomena and results confirmed by experiments.
However, when technological advancements or serendipitous discoveries yield counterexamples that existing laws cannot explain, previously firmly held conventions collapse, and new conventions emerge to explain them. In physics, the transition from classical physics to quantum mechanics is a prime example of this process. Until the 18th and early 19th centuries, classical physics was regarded as a complete set of laws that explained nearly all natural phenomena observable to humans. From Descartes’ perspective, classical physics at that time was knowledge built on rational thought and seemed unlikely to crumble easily. However, as experimental equipment advanced significantly in the late 19th and early 20th centuries, phenomena that could not be explained by classical physics were discovered, leading to the emergence of quantum mechanics to account for them. Examining the process by which phenomena previously unexplained by classical physics came to be explained by quantum mechanics reveals that even knowledge built upon reason may not be absolute or complete.
As the way we understand natural phenomena shifted from classical physics to quantum mechanics, the most significant changes were the wave nature of particles and the probabilistic interpretation of position.
A prime example of a phenomenon difficult to explain using classical physics is the wave nature of particles. This can be verified through the double-slit experiment. In this experiment, electrons are fired toward two narrow slits, and the distribution of electrons on the detection screen behind them is observed.
If many electrons are fired while only the left slit or only the right slit is open, the distribution of electrons passing through a single slit appears. From the perspective of classical physics, this result is easy to predict because electrons move in a straight line. Furthermore, if both slits are open when electrons are fired, one might expect the results from each slit to simply add together, resulting in two bright bands appearing on the screen.
However, the actual experimental results were completely different from these predictions. When both slits are open, an interference pattern of alternating bright and dark stripes forms on the screen. Interpreting this from the perspective of classical physics leads to the conclusion that electrons—which are very small particles—altered their paths due to the presence of the other slit through which they did not pass. This is a result that is difficult to understand using common sense.
For example, it is similar to a situation where a person who has long entered the same classroom through the school’s main gate ends up in a different classroom one day—even though they still entered through the main gate—simply because a new entrance has been created in a different part of the school. This result cannot be explained by classical physics alone. Ultimately, the double-slit experiment demonstrated the limitations of classical physics and revealed that conventional wisdom can no longer explain all natural phenomena.
So how can these interference patterns be explained? In fact, this result is a phenomenon very familiar to physicists. This is because interference patterns are the typical form that appears when waves meet and cause constructive and destructive interference. Building on this insight, quantum mechanics introduced a new concept that interprets particles as probability waves. In fact, all subatomic particles, including electrons, can be described as probability waves.
In other words, while classical physics understood the motion of a particle as that of a rigid object, such as a stone, quantum mechanics represents particles as waves. It is interpreted such that the probability of finding a particle is high where the wave’s amplitude is large, and low where the amplitude is small. Therefore, the double-slit experiment can be understood as follows: the probability wave of a single electron passes through both slits, causing them to interfere with each other; as a result, each electron is detected according to the probability of forming an interference pattern. In this way, quantum mechanics naturally explains phenomena that could not be explained by classical physics.
