Chapter 148: Schrödinger's Cat

When all time began to assume a regular rhythm, Hua Feng started to ponder the implications of parallel universes in relation to this apocalyptic catastrophe.

Parallel universes refer to other realms that have branched off from a certain universe, existing parallel to the original while being both similar and distinct.

A multiverse is a theoretical collection of infinite or finite possible universes, encompassing everything that exists and can possibly exist: all space, time, matter, energy, and the physical laws and constants that describe them. The individual universes contained within a multiverse are called parallel universes.

In the 1950s, while observing quantum phenomena, some physicists discovered that the state of a quantum varied with each observation. Since all matter in the universe is composed of quanta, these scientists hypothesized that, given the distinct states of each quantum, the universe might not be singular, but rather composed of multiple similar universes. The formulation of the parallel universe concept owes its existence to the scientific discoveries of modern quantum mechanics.

Certain scholars have used the following analogy to describe parallel universes: they might exist within the same spatial framework yet possess different temporal frameworks, much like two successive trains speeding along the same railway line; or they might exist within the same temporal framework yet possess different spatial frameworks, much like cars simultaneously traveling on the upper and lower decks of an overpass.

Parallel universes are other realms that have split from a certain universe, coexisting parallel to the original while remaining both similar and different. Within these universes, there are realms born under the exact same conditions as our own, planets identical to the one inhabited by humans, or planets with the same historical trajectory, and there may even be individuals entirely identical to human beings. Furthermore, within these diverse universes, the progression of events leads to different outcomes: species that have gone extinct in our universe may be continuously evolving and thriving in another.

Two mutually parallel universes neither overlap nor intersect, remaining as separate as well water and river water. Although on rare occasions accidental events allow the two universes to perceive each other's existence, generally speaking, they remain worlds apart—hearing the sounds of each other's dogs and roosters, yet never interacting throughout their entire existence.

Scholars have also described parallel universes using the analogy that they might share the same spatial system while possessing different temporal systems, akin to two successive trains hurtling along the same railway line; or they might share the same temporal system while possessing different spatial systems, much like automobiles simultaneously driving on the upper and lower tiers of an overpass.

The concept of parallel universes did not initially arise from the time travel paradox; it originated from quantum mechanics, specifically due to quantum uncertainty. The inception of the parallel universe concept is a direct benefit of the scientific breakthroughs of modern quantum mechanics.

The Copenhagen Interpretation

Since the 1920s, many physicists have proposed various "interpretations" of quantum mechanics, aiming to provide a reliable explanation for the measurement problem and to make the collapse of the wave function comprehensible. In quantum mechanics, the state of a microscopic particle is described by a wave function. When a microscopic particle is in a certain state, its mechanical quantities (such as coordinates, momentum, angular momentum, and energy) generally do not possess definite values, but rather a series of possible values, each appearing with a certain probability (whereas macroscopic objects in a given state possess definite values for their mechanical quantities). In other words, the motion of microscopic particles is characterized by uncertainty and probability. The wave function describes the probability of a microscopic particle's spatial distribution.

The famous "single-electron double-slit interference" experiment in physics is precisely a manifestation of the uncertainty and randomness inherent in the motion of microscopic particles. In this experiment, single electrons passing through a double slit unexpectedly produce an interference pattern.

From the perspective of classical mechanics, an electron can only pass through one slit at a given moment; it is impossible for it to pass through both slits simultaneously and create interference. However, according to quantum mechanics, the motion state of an electron exists in the form of a wave function, making it possible for the electron to pass through both this slit and that slit at the exact same moment, thereby producing interference.

Yet, whenever scientists attempt to determine which slit the electron actually passed through using instruments, the electron is invariably detected at only one of the locations. The two instruments never detect the electron simultaneously; the electron can only pass through a single slit each time.

This seems to imply that the observer's act of measurement alters the state of motion of the electron. How is this anomalous phenomenon to be explained? Physicist Niels Bohr proposed the renowned "Copenhagen interpretation": when unobserved, the electron exists with a certain probability at the locations of both slits; however, the moment it is measured—for instance, if the electron is detected at the left slit—it acquires a precise position, its probability at that point becoming 1, and its probability at all other points becoming 0. In other words, the wave function of the electron "collapses" to that point at the very instant of measurement.

Bohr introduced the observer and their consciousness into quantum mechanics, linking them to the state of motion of microscopic particles. Nevertheless, the explanations of the observer and the "collapse" were not entirely clear or convincing, and they faced skepticism from many scientists. For instance, how does the collapse occur? Does it happen instantaneously, or does it wait until the photons enter human eyes and stimulate electrical pulse signals on the retina?

The Many-Worlds Interpretation

Is there a way, then, to bypass this so-called "collapse" and "observer," stripping away the subjective element of the observer from physics, a discipline meant to study objective laws?

Hugh Everett thus put forward a bold proposition: if the wave function does not "collapse," it must necessarily continue to increase linearly. That is to say, even after observation in the aforementioned experiment, the electron remains in a superposition state of the left and right slits.

Everett further suggested that the world itself is also in a superposition; when the electron passes through the double slit, it is not merely the electron that occupies a superposition state, but the entire world. In other words, once the electron passes through the double slit, two superimposed worlds emerge—in one world, the electron has traversed the left slit, while in the other world, the electron has passed through the right slit.

In this way, the wave function has no need to "collapse" in order to randomly choose between left or right, because it manifests as a superposition of two worlds: people living in one world find that the electron passed through the left slit in their reality, whereas in the other world, the observers see the electron on the right.

Taking "Schrödinger's cat" as an example, Everett pointed out that both cats are real. There is a live cat and a dead cat, but they reside in different worlds. The issue is not whether the radioactive atom in the box has decayed or not, but rather that it has both decayed and not decayed. When the observer looks into the box, the entire world splits into two versions of itself. These two versions are identical in every other aspect. The sole difference lies in the fact that in one version, the atom has decayed and the cat is dead; while in the other version, the atom has not decayed and the cat is still alive.

The aforementioned two worlds—"the atom has decayed, the cat is dead" and "the atom has not decayed, the cat is alive"—will evolve completely independently and parallel to each other, much like two parallel realms. The quantum process gives rise to "two worlds," which is Everett's avant-garde "many-worlds interpretation."

The advantage of this interpretation is that the Schrödinger equation always holds true, the wave function never collapses, and consequently it simplifies the foundational theory. Its drawback is that the premise is exceedingly bizarre, and the price paid is that all these parallel worlds are equally real. Little wonder someone remarked: "In the history of science, the many-worlds interpretation is undoubtedly the boldest and most ambitious theory ever proposed."

Rudimentary Thoughts in Historical Development

In the 5th century BC, Democritus introduced the concept of "infinite worlds," arguing that "infinite worlds" were formed by atoms moving through their own motion. He stated: "Atoms move arbitrarily in the void, and due to their turbulent and chaotic motion, they collide with one another; upon colliding, because of their diverse shapes, they entangle with one another, thereby forming worlds and the things within them, or rather, forming infinite worlds."

In the 1st century BC, Lucretius pointed out that beyond our "visible world," there exist "other worlds" inhabited by "other races of men and beasts."

In the 4th century BC, Epicurus articulated the idea of the plurality of worlds: "There are infinitely many worlds, some like our world, some unlike our world." "In all worlds, there are animals, plants, and other things such as those seen in our world."

In 1714, Gottfried Wilhelm Leibniz put forward his concept of "possible worlds," hypothesizing that beyond the realm of the actual world (the observable universe), an infinite number of "possible worlds" exist.

He believed that the world is composed of an infinite number of monads, which possess no causal relationships among themselves, but rather exist in a sort of pre-established harmony. Although each monad is independent, they differ in degree of nobility and rank. Leibniz interpreted the emergence of a specific actual event—such as a particular individual—as the result of the combination of numerous monads, with the various outcomes of combination linked to the dominant role of the superior monads among them. This implies that the world could have been otherwise, and any event is contingent, as is the entire universe.

In 1957, physicist Hugh Everett presented his own thoughts on the quantum measurement problem. He pointed out that in quantum mechanics, multiple parallel worlds exist, and in each world, the outcome of every quantum mechanical measurement differs; hence, distinct histories occur within different parallel universes.

The many-worlds interpretation posits that observing a measurement apparatus causes that apparatus to split into two. Furthermore, along this measurement chain, such splitting continues indefinitely. Accompanying this splitting, a complete duplication of the universe must necessarily occur. In other words, whenever a quantum measurement takes place, every branch of the universe, along with the components within that branch, leads to a possible measurement outcome. Every individual situated within a specific branch of the universe will believe that their measurement result and the universe they inhabit are the sole existing realities. That is to say, a single measurement generates a brand-new universe.

These distinct new universes, unless entirely identical, can never possibly overlap. The publication of this theory marked the formal inception of the concept of parallel universes.