In this blog post, we will examine the limitations of the Copenhagen interpretation through Schrödinger’s cat, a representative thought experiment in quantum mechanics.
Quantum theory is a physical theory that explains the microscopic world. The microscopic quantum world refers to a world that is approximately 1 nanometer in size. Until the 19th century, classical mechanics, represented by Newtonian mechanics, was considered to be the truth of physics. However, with the development of quantum theory, classical mechanics underwent a major change.
Quantum theory, which began in 1900, and the theory of relativity, which began in 1905, had a major impact not only on science but also on human culture and society. In classical mechanics, represented by Newtonian mechanics, “matter” is the center of the world. Matter has mass, position, kinetic energy, and potential energy. The role and purpose of classical mechanics is to predict the future by calculating the motion of matter and confirming it through experiments. In the materialistic worldview, matter can exist independently. Even if someone observes or experiments with it, matter is “real.” This theory is called determinism or realism. Two representative figures who believed in this determinism and conducted research were Einstein and Schrödinger. In 1935, Schrödinger conducted a famous thought experiment called “Schrödinger’s cat.”
In this thought experiment, an alpha particle and a cat appear. The cat is placed in a box that is completely isolated from the outside world. This box is connected to a container filled with poison gas. The poison gas is blocked by a valve and cannot enter the box, and the container with the poison gas is also completely isolated from the outside world, so it is impossible to see if the valve is open. The valve is connected to a machine that detects radiation. The machine is set to detect alpha particles emitted during the decay of radium, which has a 50% probability of decaying per unit time, and opens the valve when it detects such particles. If the valve is opened, the cat will die from inhaling the poisonous gas. Then, after that unit of time has elapsed, the cat will have a 50% chance of being alive or dead. Regarding this thought experiment, determinists believe that even without checking after the unit of time has elapsed, it will be determined whether the cat is dead or alive. In other words, Schrödinger argued that a cat that is both dead and alive does not exist. He believed that quantum mechanics is incomplete and unrealistic.
A representative school of thought that opposes the determinism and realism described above is the Copenhagen school, which argues that the cat that was both dead and alive exists. Representative figures of the Copenhagen school include Niels Bohr and Werner Heisenberg. Simply put, the Copenhagen interpretation states that before measurement, wave functions are expressed as a probabilistic superposition of multiple states, and when an observer makes an observation, a “collapse of the wave function” occurs, and the wave function is determined to be in a single state. In classical mechanics, physical objects were thought to exist in one of two states, either as particles or as waves, but the Copenhagen school of Bohr believed that physical objects could exist in both states simultaneously. Later, Heisenberg announced the “uncertainty principle,” which states that the position and momentum of a particle cannot be measured accurately at the same time. In the Copenhagen interpretation, Schrödinger’s cat, mentioned earlier, is interpreted as being in a state of both dead and alive while the box is closed, but the moment the box is opened to check the cat’s state, the cat must be determined to be in one of the two states. In other words, the key point of the Copenhagen interpretation is that the value of a physical quantity has meaning only after it is measured.
Here, I would like to raise the following question. In the Copenhagen interpretation, are the observer and the act of measurement still described in terms of the classical world? Under the laws of quantum mechanics, the observer, the measuring device, and the act of measurement must all follow the laws of quantum mechanics, which apply equally to all other objects in the universe. However, the rules in the Copenhagen interpretation are expressed as wave functions that follow the deterministic process mentioned earlier. It is true that the probabilistic rules of the Copenhagen interpretation fit well with reality. However, this does not mean that this interpretation ultimately explains everything in quantum theory. I believe that the physical state of a substance may be determined independently of measurement, or it may be determined by interaction with the measurement process, as claimed by the Copenhagen interpretation. However, I do not agree that the latter claim of the Copenhagen school is 100% correct. This is because the acts of observation and measurement that we perform may not be perfect enough to determine all the properties of an object. According to the Copenhagen interpretation, physical quantities do not have fixed values but are determined only through observation. Let’s take Major League Baseball as an example. A pitcher named Ohtani from the LA Dodgers threw a ball to the catcher. Suppose that the speed of the ball is measured by the team and the broadcasting station. The MLB team measures the speed at 163 km/h, while NHK measures it at 161 km/h. Does this mean that the actual speed of the ball thrown by Ohtani is 163 km/h and 161 km/h? I don’t think so. I believe that the difference in measurement results is due to the fact that measuring instruments are not 100% perfect. In order for the Copenhagen interpretation to be convincing, it must be shown that the measurements were obtained through observation within the laws of quantum mechanics that they claim to follow. Simply saying that it works when tested in an experiment is not enough. That is why the Copenhagen interpretation is still not 100% accepted in academia.
Human perception is imperfect. The act of observation itself is subjective because it occurs through our brains. For this reason, the act of observation in the Copenhagen interpretation must be redefined. Although it does not actually exist, if a third party with an omnipotent perspective were to observe Schrödinger’s cat thought experiment mentioned above, would he be able to know the state of the cat inside the box? Could it be that the results appear as a probability wave function because our observations are imperfect?
I have argued against the Copenhagen interpretation, which is currently accepted as mainstream in quantum theory. If I can prove that this interpretation is wrong, it will be a major contribution to the history of science. However, rather than accepting this interpretation at face value, I think it is not a bad idea to think about it differently at least once. The quantum world has not yet been fully explored, but I believe that one day we will understand its true nature.
