Chapter 3-2 Wave-Particle Duality

We live in a world of light, where photons are ubiquitous. Since Newton's time, the nature of light has sparked intense debate in the scientific community. At that time, there were two completely different viewpoints: one, proposed by Newton, suggested that light is composed of particles, while the other, supported by Hooke, Huygens, and others, argued that light is a wave. Due to Newton's dominant position and prestige in the field of physics, his particle theory of light overshadowed the wave theory, becoming the mainstream scientific viewpoint for the next century.

It wasn't until 1801 that the British scientist Thomas Young conducted an experiment that reversed this trend, making the wave theory of light the mainstream. This experiment is one of the greatest in the history of human physics and one that has driven physicists to madness: the "Double-slit Interference Experiment". The process and results of this experiment are so bizarre that they led physicists to question the very reality of the world. This experiment quietly explained why many pioneers of modern physics turned to philosophy, and even theology, in their later studies.

The earliest double-slit interference experiment began with the "Single-slit Diffraction Experiment". As early as the 17th century, scientists had discovered the phenomenon of light diffraction: when light encounters an obstacle whose size is comparable to its wavelength (such as the edge of an opaque object, a small hole, or a narrow slit), it no longer follows a straight path but bends around the edges of the obstacle, continuing to propagate. A representative experiment is the "Single-slit Experiment", when light passes through a vertical slit, it diffracts, and what is seen on the screen is not a vertical bright line but a horizontal bright band (see the upper part of Figure 3.2).

Figure 3.2: Single-slit Diffraction Experiment and Double-slit Interference Experiment

The "Double-slit Interference Experiment" involves passing a beam of light through two closely spaced slits and projecting it onto a screen behind the slits. From the results of "Single-slit Diffraction Experiment", one would expect that when the bright bands from the two slits overlap on the screen, a brighter light band would be observed. However, something magical happens: the screen displays a pattern of alternating bright and dark fringes (see the lower part of Figure 3.2).

This phenomenon is quite perplexing because where the dark fringes appear, there should logically be light. Yet, when the light from the two slits overlaps, those specific positions on the screen lack light. This phenomenon seemed inexplicable by Newton's Particle Theory. Inspired by the interference of water waves, Thomas Young proposed that light behaves as a wave, and the dark fringes are a result of interference between light waves.

The revolution in human understanding of the universe has always been driven by a handful of geniuses. A century after the "Double-slit Interference Experiment" proved the wave theory of light, another genius reshaped our understanding of light and opened the door to quantum mechanics. This genius was Albert Einstein, the greatest physicist of the twentieth century.

In 1905, just after earning his PhD, Einstein published a paper titled "On a Heuristic Viewpoint of View Concerning the Production and Transformation of Light", in which he explained the photoelectric effect by proposing that light consists of energy quanta, or photons.

In simple terms, the Photoelectric Effect refers to the phenomenon where light shining on a metal surface causes the emission of electrons. This phenomenon is peculiar: normally, electrons are tightly bound to the atoms on the metal surface. Strangely, when exposed to a certain kind of light, these electrons start to break free from the atoms and scatter. Since the main characters in this phenomenon are light and electrons, it was named the photoelectric effect.

At that time, it was understood that light was a wave, and the intensity of the wave represented its energy. Logically, since electrons are bound to their orbits by atoms, the higher the intensity of the light (meaning more and denser photons), the easier it should be to eject electrons. However, experimental results showed that whether light could eject electrons from a given metal surface depended on the frequency of the light (with red light having the lowest frequency and violet light having the highest) rather than its intensity.

In a flash of inspiration, Einstein thought, what if light wasn't continuous but consisted of discrete packets called photons? This instantly resolved the issue. By increasing the frequency, individual photons could more easily eject electrons. In other words, if the energy of a single photon is greater than the binding energy of the metal atom to the electron, it can eject the electron. Increasing the light's intensity merely increases the number of photons, but only the energy related to frequency determines whether electrons can be ejected.

Einstein's revolutionary idea, which differed significantly from the classical physics framework, revived the Particle Theory of light. However, this led to a new problem: is light a particle or a wave? Both forms had experimental evidence, and both seemed correct. The dilemma was that waves and particles are fundamentally different properties. Under what circumstances does light exhibit wave characteristics, and when does it show particle characteristics?

This problem was resolved more than a decade later by the French scientist Louis Victor de Broglie. In his PhD thesis, he proposed the Wave-particle Duality of quantum mechanics, which states that all particles or quanta can be described not only in terms of particles but also in terms of waves (see Figure 3.3).

Figure 3.3: Wave-particle duality

Recalling back to high school physics textbooks, the subjects under study were always distinctly categorized as either "pure" particles or "pure" waves. The former constituted what we commonly refer to as "matter", while the latter's typical example was light waves. However, in the quantum world, wave-particle duality resolved the dilemma of these "pure" particles and "pure" waves. It provided a theoretical framework where any substance can sometimes exhibit particle-like properties and at other times exhibit wave-like properties.

Even more revolutionary is the understanding that all microscopic particles, including photons, can exhibit both particle and wave characteristics. Which characteristics are displayed depends on whether we observe them; observation leads to particle-like behavior, while non-observation reveals wave-like behavior — this is known as the "Observer Effect" (see Figure 3.4).

Figure 3.4: Observer Effect

When do light waves become particles? A fact proven by countless scientific experiments is that: light propagates as a wave and appears as a particle when observed. In other words, observation turns the wave into a particle. In other words, whether we observe it or not affects the state presented by the photon: when not observed, the photon exhibits a fuzzy wave-like state, and when observed, the photon exhibits a definite particle state.

In 1927, Werner Heisenberg, based on the wave-particle duality of light, proposed the "Uncertainty Principle". This principle suggests that: the seemingly contradictory wave-like and particle-like properties are actually complementary; both exist simultaneously and complement each other. We cannot measure one property without interfering with or destroying the other. According to this principle, in terms of nature, quanta are neither particles nor waves. Experiments or measurements that highlight one property necessarily sacrifice the other. For example, we cannot simultaneously measure the position and velocity of a microscopic particle; the more precisely the position is determined, the less precisely the velocity is known, and vice versa.

Later, scientists including Niels Bohr supported this conclusion and explained: photons, electrons, and other microscopic particles are essentially waves that exist everywhere, even randomly appearing in any corner of the universe. When observed, these microscopic particles just happen to appear where we observe them, giving us the impression that they are right there, allowing us to directly perceive their particle-like characteristics rather than their wave-like nature — this is known as "Wave Function Collapse" (see Figure 3.5). This explanation, a collaborative result of Niels Bohr and Werner Heisenberg's research in Copenhagen, is known as the "Copenhagen Interpretation".

Figure 3.5: "Wave Function Collapse" after observation

The physics community widely acknowledges that the Copenhagen Interpretation has played a significant role in the development of quantum mechanics. It not only provides a reasonable explanation for the probabilistic nature of quantum theory but also significantly influences the development and research of natural sciences and philosophy. Furthermore, Bohr proposed the "Spirit of Copenhagen", which is "a virtue of completely free judgment and discussion, advocating a strong academic atmosphere of equality, free discussion, and close cooperation".

What kind of world is ultimately presented depends on whether we observe it, and is it still uncertain? This contradicts the inherent human understanding that the world exists objectively (independent of subjective will). Einstein clearly could not accept this notion: "Does the moon cease to exist when we aren't looking at it?" He viewed the explanation of "wave function collapse" as an afterthought, much like the "wave-particle duality" and the "uncertainty principle". In fact, physicists do not know why "wave function collapse" occurs; they merely deduced this conclusion from observational results to explain the peculiar behavior of microscopic particles.

What troubled Einstein even more was that, according to his theory of special relativity, the fastest speed in the universe is the speed of light, and no object with mass can reach or exceed it. However, in the quantum world, the monument of relativity collapses, and superluminal phenomena are everywhere (see the Section "Quantum Entanglement"). This genius devoted almost all his energy in the latter half of his life to searching for a "grand unified theory", believing that there must be some hidden variable yet to be discovered, which made quantum mechanics appear so strange.

Our understanding of the entire universe is very limited. The timescale of our understanding of the macroscopic world is insufficient; things happening a hundred light-years away have nothing to do with us, and events that occur when you are born may not have concluded by the time you die. Our resolution of time in the microscopic world is inadequate, revealing only electron clouds and "uncertainty." Back then, the explanation of "wave-particle duality" was more of a reluctant compromise by physicists, or a middle-of-the-road approach, which frankly reflects "human ignorance". A century later, scientists are still repeatedly conducting countless "improved versions" of the "double-slit experiment", making it one of the most famous classic experiments in the past two hundred years of physics. The uniformly bizarre experimental results need not be elaborated here.