In this blog post, we’ll explore everything from the emergence of quantum mechanics—which challenged the predictability of classical physics—to the debates surrounding the Copenhagen interpretation and the many-worlds interpretation.
Quantum mechanics, which emerged in the early 20th century, completely upended the existing framework of physics. Until the advent of quantum mechanics, classical physics had established a stable system. All phenomena in the natural world were believed to occur according to fixed rules, and future events could be predicted based on those laws. It was believed that the future was determined by the laws of nature, and that if the exact state of motion of all particles were known, the future could also be known with certainty. Many scientists believed that these absolute “rules” could never be broken under any circumstances.
However, with the advent of quantum mechanics, this belief was shattered. This was because results that were difficult to explain using existing theories were observed in the microscopic world—a realm far smaller than atoms. A prime example illustrating this is the “double-slit experiment” conducted by American physicists Clinton Davisson and Lester Jummer at Bell Laboratories.
In this experiment, electrons are fired at a plate with two narrow, elongated holes—that is, slits. A detection screen is then set up behind the plate to detect the electrons that pass through the slits. At first glance, one might expect that since electrons pass only through the openings, two narrow bands—matching the projection of the two slits—would appear on the detection screen. However, the actual results were completely different from what was expected. Bright and dark bands alternated on the detection screen. The results appeared as if waves passing through the two slits had interfered with one another. This result was virtually impossible to explain using existing theories, which treated electrons as single particles, such as a ball.
To understand this, quantum mechanics explains that all particles possess both particle-like and wave-like properties. Not only electrons, but all particles in the microscopic world exhibit characteristics of both particles and waves. In quantum mechanics, the wave-like nature of particles is described as a “probability wave.” In regions where the wave amplitude is high, the probability of finding an electron is high, and in regions where it is low, the probability is low. In the double-slit experiment, this can be interpreted as the electron’s probability wave passing through both slits simultaneously, resulting in interference that appears on the detection screen. This interpretation directly challenges the perspective of classical physics, which held that the state of any object could be precisely predicted. However, the experimental results could not be fully explained by classical physics alone, and this led to quantum mechanics taking center stage in the scientific community.
Yet one question remains: What exactly is the nature of this “probability wave”? Can we directly observe the nature of this probability wave? No, we cannot. Niels Bohr explained that if we attempt to observe it, the probability wave “collapses.” When a widely dispersed wave is observed, it collapses into a single point, becoming 100% at that point and 0% at all other points, effectively disappearing. This phenomenon is called the “collapse” of the probability wave, and the interpretation of quantum mechanics that views observation as the collapse of the probability wave is known as the “Copenhagen interpretation.”
The Copenhagen interpretation currently enjoys the support of the majority of physicists because it provides the most accurate explanation of experimental results. However, it has several clear limitations. First, it is difficult to provide a precise explanation for the “collapse” of the probability wave function. The probability wave in quantum mechanics can be mathematically described using Schrödinger’s “wave equation.” However, the wave equation cannot explain the process by which the wave function collapses as a result of observation. It is simply treated, for the sake of convenience, as being compressed to 100% at the point of observation. Another problem is that it is difficult to precisely define the act of “observation.” Niels Bohr explained observation as the contact between macroscopic and microscopic objects, but he did not clearly explain where that boundary lies. A thought experiment that clearly illustrates these issues is “Schrödinger’s Cat.”
Consider a box containing a radioactive substance and a cat. The setup is designed so that if the radioactive substance inside the box decays, poison is released, killing the cat. According to the Copenhagen interpretation, until the box is opened, the states of the radioactive substance having decayed and not having decayed coexist; consequently, the states of the cat being dead and alive also coexist. In other words, it becomes ambiguous at which point the wave function must collapse.
The Many-Worlds Interpretation, proposed in the past by Everett III, offers a solution to this problem. Everett explained that the particle’s probability distribution does not collapse; rather, all possible locations are observed across multiple worlds. In other words, the particle’s position is not determined as a single outcome, but all possibilities are realized simultaneously across multiple worlds. This approach eliminates the problem of the collapse of the probability wave function, making the theory mathematically sound. Although it may appear as if the probability wave collapses in each individual universe, in reality, all possible outcomes are realized in other universes as well. There is no longer any need to assign special significance to the act of “observation.” In Schrödinger’s cat thought experiment, worlds where the cat is alive and worlds where it is dead would all coexist.
Although the Many-Worlds Interpretation is logically flawless, one drawback is that it is difficult to verify the existence of these multiple worlds. Once they have branched off, these parallel universes exist in a “decoherent” state, making them difficult to observe physically. Consequently, some people dismiss this interpretation as religion rather than science. However, in science, mathematical integrity is just as important as verifying the existence of physical entities. No matter how well a theory aligns with reality, it cannot be called perfect if there are gaps in its explanation. Furthermore, it is not advisable to dismiss the possibility of existence simply because we cannot observe it. In fact, there have been numerous instances in the past where the existence of phenomena was proven only later. If we persevere without giving up, the day may come when we can prove their existence.
Thus, while there is no consensus regarding the interpretation of quantum mechanics, it has explained numerous microscopic phenomena in the laboratory that classical physics could not account for. Therefore, it is undoubtedly one of the most successful theories in modern physics. Although some aspects remain unresolved, the truth will eventually come to light as time goes on. In fact, the existence of a many-worlds interpretation might be confirmed; the collapse of the wave function upon observation in the Copenhagen interpretation might be mathematically explained; a new interpretation merging these two might emerge; or an entirely different interpretation, on a completely different level, might come to light. It will be fascinating to see what answers the brilliant physicists of our time will uncover.
