From: mk_thisisit
Quantum mechanics presents many paradoxes that are contrary to human intuition [00:00:00]. It is a field of physics that is generally contrary to common sense but has been very practical, leading to numerous discoveries such as transistors, logic gates, microprocessors, and artificial intelligence [00:26:40].
Wave-Particle Duality
One of the central paradoxes in quantum mechanics is the wave-particle duality, often referred to as the corpuscular-wave nature [00:01:55]. This concept states that particles like photons and electrons can behave as both waves and particles. It is constantly inconsistent with human intuition [00:02:10]. For example, a photon changes its behavior if observed [00:00:46]. This dualism is something that scientists “have to get used to” because it is simply the way reality works on the nano scale [00:22:17], [00:24:44].
Photon Behavior
A single photon, which basically passes through two slits at the same time, interferes with itself [00:20:11]. However, everything changes if a detector is placed behind one of the slits to check which slit it passes through; the photon suddenly starts behaving differently [00:20:21]. This phenomenon, known as the “observer effect,” implies that the presence of the observer (or detector) changes the quantum phenomenon [00:21:00]. Some explanations, like Stephen Hawking’s, suggest that the presence of photons disturbs the quantum system [00:21:23], while others suggest the very nature of things is such [00:21:37]. It is not human consciousness, but the detector itself that influences the photon [00:21:42].
Electron Behavior
Similarly, if electrons are released as a beam at appropriately high energies, they start to behave like waves, undergoing interference and diffraction [00:22:26]. This demonstrates the dual nature of particles [00:22:18].
Schrödinger’s Cat Paradox
Schrödinger’s cat is a well-known thought experiment illustrating quantum superposition [00:27:24].
Setup
In this experiment, a cat is locked in a soundproof and isolated room with a radioactive atom (e.g., Uranium 235) and a bottle of poison [00:27:36]. The atom has a half-life (e.g., one hour), meaning there is a 50% probability it will decay within that time [00:28:31]. If the atom decays, it triggers a lever that breaks the poison bottle, killing the cat [00:28:49].
Superposition
From the viewpoint of an outside observer, until the box is opened, the atom is in a superposition of decayed and undecayed states. Consequently, the cat inside the box is considered to be simultaneously alive and dead [00:29:59]. This state can be mathematically represented by a wave function, where the state of the cat (psi) is a linear combination of two elementary base states: a live cat and a dead cat, each with a 50% probability [00:30:28].
Wave Function Collapse
According to the Copenhagen interpretation of quantum mechanics, when an observer looks inside the box, the wave function collapses, and the cat assumes one of the base states – either alive or dead [00:31:31]. This thought experiment beautifully illustrates how quantum mechanics works at the nano scale for objects like electrons and photons, where a quantum object can be in multiple states simultaneously [00:31:11], [00:32:17].
Double-Slit Experiment
The classic double-slit experiment demonstrates the wave-particle duality, especially when observing individual photons [00:33:50].
Classical Light Behavior
In a classical setup, when a laser beam (a plane wave) passes through two slits, an interference pattern (diffraction image) is observed on a screen [00:34:04]. This pattern, with bright and dark fringes, is expected and confirms the wave nature of light [00:34:47]. The waves from each slit reinforce or cancel each other out depending on their path difference [00:36:13].
Single Photon Behavior
The paradox arises when a single photon is sent through the two slits [00:34:56]. Even though only one photon is sent at a time, if the experiment runs long enough, a diffraction image still forms on the screen [00:36:02]. The question then becomes: with what does a single photon interfere? The answer is “itself with itself” [00:38:29]. This is extraordinary, as it means a photon passes simultaneously through both slits and interferes with its own probability wave [00:38:37]. This behavior is impossible in the macroscopic world [00:38:58].
Implications
This type of discovery, based on mathematical reasoning and confirmed by countless experiments, has led to significant technological progress, such as the development of the PN junction of the transistor [00:24:20], [00:33:13]. Quantum mechanics functions on the nano scale, while Newtonian mechanics is sufficient for our everyday macroscopic world [00:39:04].