In this blog post, we’ll explore why quantum mechanics—which originated from phenomena in the microscopic world that were difficult to explain using classical physics—feels so counterintuitive, and we’ll consider different perspectives on quantum mechanics by recognizing that what is considered common knowledge can vary depending on the era and the individual.
Quantum mechanics is a flagship field of modern physics. It provides a clear explanation for phenomena occurring in the microscopic world at the atomic level—phenomena that were difficult to explain using classical physics. However, understanding this field is no easy task. Richard Feynman once remarked, “Nobody understands quantum mechanics.” Why is quantum mechanics so difficult to grasp?
The reason is that quantum mechanical phenomena defy common sense. Long ago, there was a belief in the physics community that the field had reached its limits. However, as experiments on events in the microscopic world continued, phenomena that could not be understood through classical physics were discovered. Examples include the fact that the spectrum of light emitted by atoms is observed to be discrete, or that electrons are ejected from a metal when it is exposed to a specific wavelength of light. The microscopic world is the realm of extremely small particles, such as atoms and electrons. This world is different from the macroscopic world we constantly see and experience. For example, while we can easily observe the movement of a baseball, it is very difficult to see the movement of hydrogen atoms or electrons within a water molecule. It is even difficult to know that such particles exist within a water molecule at all. It is the fundamental nature of quantum mechanics—which studies this microscopic world—that places it in the realm of the counterintuitive.
Physicists’ interpretations of quantum mechanical phenomena vary. Whether in the microscopic or macroscopic world, experimental results are indisputable facts. However, the experimental results observed in the microscopic world are difficult to accept based on common sense. Therefore, physicists do not stop at simply formulating the theory of quantum mechanics but strive to interpret these phenomena. Among the various interpretations of quantum mechanics, the mainstream interpretation is the Copenhagen interpretation, proposed by Niels Bohr and others. The Copenhagen interpretation is quite counterintuitive. According to the Copenhagen interpretation, the state of a particle exists as a superposition of multiple states described by a probability function; when an act of observation is performed on the particle, that function collapses, and the particle is determined to be in a single state. If we apply this interpretation to the macroscopic world, we can appreciate just how difficult it is to accept this interpretation based on common sense. Suppose there is a room from which no light escapes. According to the Copenhagen interpretation, the light inside this room exists simultaneously in both the “on” and “off” states. Then, the moment you open the door to check, it is determined to be in one of those two states—either on or off. How can two states be superimposed, and how can the act of opening the door determine just one of them?
Although the Copenhagen interpretation is difficult to accept, numerous experimental results since the 20th century have proven that the theory of quantum mechanics itself is an indisputable fact. Physicists have successfully developed a theory that can explain even the events occurring in the microscopic world, but they have run into a philosophical problem regarding its interpretation. Physicists are still striving to offer a more plausible interpretation of quantum mechanical phenomena. Here, we must note that quantum mechanics is not the only thing that defies common sense. The Standard Korean Dictionary defines common sense as “knowledge that people generally know or should know.” Consider Einstein’s special theory of relativity. Is this theory acceptable to common sense? The special theory of relativity, based on the fact that the speed of light is constant in all inertial frames, overturned many common-sense notions—such as the idea that time flows at a constant rate or that mass remains constant regardless of an object’s velocity. How many people could accept, based on common sense, the fact that the mass of the same object changes when its velocity changes?
Common sense varies depending on the individual or group receiving the information. Brilliant physicists would likely accept the theory of special relativity as common sense. For a simpler example, consider atmospheric pressure. If you fill a cup with water, cover it with a coaster, and then turn the cup upside down, neither the coaster nor the water falls out. Many people who have never heard of this fact would be surprised if shown this demonstration. It is difficult to intuitively grasp that 1 atmosphere—the atmospheric pressure we experience in daily life—is equivalent to the pressure exerted by a column of water approximately 10 meters high. Furthermore, common sense changes over time. Although the Earth is actually round, from an individual’s perspective, it is so vast that it is difficult to intuitively know or sense that it is spherical. Therefore, in ancient times, the fact that the Earth is round was not considered common knowledge. However, in modern times, no one denies that the Earth is round. Another example would be the geocentric and heliocentric models. As this example shows, even if something cannot be understood intuitively, common sense can change over time. Among physical phenomena, there are many that were once considered unconventional but are now accepted as common sense, and even today, the level of common sense varies from person to person. Through this variability of common sense, we must understand that quantum mechanics is not the only thing that defies common sense. Quantum mechanics is the common sense of our time, and it will become everyone’s common sense. Rather than insisting that the Copenhagen interpretation is counterintuitive and focusing on devising various interpretations—even resorting to imagination to resolve it—it would be better to accept quantum mechanical phenomena as they are and devote our efforts to uncovering or applying other useful insights based on that fact.
