Parallel Universes #1 – Basic Copenhagen Quantum Mechanics

Having attended European Researchers’ Night (also known as Explorathon) in the Glasgow Science Centre, the extensive work of post-graduate researchers left me intrigued. Perhaps the most memorable was the talk with a Quantum Physicist about polarization of photons. Tempted, I then asked him, “What is your favourite interpretation of Quantum Mechanics?” He replied, “This is a pretty debated topic among physicists but I have to go with Many Worlds. I’m a Many Worlds person.” The Many worlds version of Quantum physics is the second most popular interpretation after the standard Copenhagen. Many worlds, also known as parallel universes is probably deemed one of the most out of this world interpretations of Quantum Mechanics and is commonly used in science fiction. Many people are fascinated by the term parallel universes, maybe it’s the appeal that alternative possible realities would exist and their lives turn out differently – though most don’t give it a second thought and just dismiss it for a fantastical perception. 

However in order to understand how the Many Worlds Hypothesis arose, fundamental quantum properties must be considered. So let’s start from the basics of the standard Copenhagen.  

Schrodinger

His thought experiment

Schrödinger, an important figure in the first development of Quantum Mechanics, proposed a popular thought experiment: Schrödinger’s Cat. He theorized putting a cat inside a box that also contained a glass of poisonous gas, a radioactive source, a hammer and a Geiger counter. When the radioactive source decays and lets out radiation, hence it is detected through the Geiger counter. The poison gets released through the Geiger counter triggered strike of the hammer and in turn the cat dies. However the chances of radioactive decay within a certain time limit is unpredictable. The atom could decay and thus lead to the cat’s death, or it doesn’t decay and the cat survives. The thought experiment states that if nobody opens the box to check the inner state, the cat is both dead and alive simultaneously until observed. The atom is in a so-called Superposition. It exists in both states, decayed and not decayed, at the same time. 

Schrödinger disagreed with this interpretation as superposition would not occur to macro living organisms like a cat and that is true. Though the thought experiment is still a correct interpretation of what is happening to the decaying atom at a quantum level.

The double slit experiment also shows results supporting the quantum superposition of the atom in Schrödinger’s cat. Consider this: A screen, a barrier with two slits and an electron gun. Firstly one slit is covered and the other is left open. Electrons are fired one at a time from the gun at random intervals, through the slit, and on to the screen. Of course, the pattern the passing electrons produced are tightly distributed in the area adjacent to the slit, what we would expect with normal particle behaviour. If electrons continue to be fired one at a time while uncovering the second slit, an intuitive thought may be that rather than one, two bands on the screen will appear, however the reality of this experiment is quite counter-intuitive. Two bands behind the slits of similar electron distributions are not observed, instead this is seen.

An interference pattern is observed on the screen in which the electrons were fired, where the electrons were the most concentrated in the centre bands and decreasing in intensity towards the two ends. This baffled physicists since classical mechanics could not explain the phenomena. The electrons were acting like waves. When a wave passes through two slits, two waves are produced, the two waves would interfere canceling each other when two peaks met, and creating an interference pattern on the screen. But the question is, if the electrons are fired towards the screen one at a time, how could such interference occur? The electrons possessed Quantum superposition and passed through both slits simultaneously, and thereby occupy wave-like behaviour. However the intriguing part of the double slit experiment is when physicists decided to add a detector to look closely at what exactly was happening. When they turned the detector on to observe the electrons passing through two slits, something abnormal occurred. The electrons went from its previous wave-like behaviour to its ordinary particle behaviour. Possibly confused, physicists then reran the experiment with the detector turned off, and there it appeared again: the wave interference pattern. This led to the conclusion that interaction with the quantum system collapsed the wave function of the electron and forcing it to appear in a chosen position/ state. (The wave-function of the quantum system explains the probabilities of areas where the electron will be found.) 

The results of this experiment were a breakthrough in modern physics and became the foundation of the Copenhagen interpretation of Quantum Mechanics or popularly known as the “standard” version. 

Thus with Schrödinger’s thought experiment, the atom was in a superposition before outside interaction which means when we open the box we would only see one outcome. Either the atom decayed and the poison killed the cat or the atom did not decay and it is still alive. According to Copenhagen our interaction collapsed the wave-function of the atom, meaning, the observer killed the cat. 

But who wants to go with the standard norm of anything? Let’s try something crazier. Enter Many Worlds. The many world theory proposes an alternative reality where the atom does not decay and the cat survives. When the electron from the gun is fired instead of the collapse of the wave function, the theory suggests that the measurement causes “splitting” of the Universe – a new Universe for every single possible outcome of the electron and unlike Copenhagen, all of the universes are equally valid. Therefore according to the hypothesis, anything that can happen does and will happen, just in alternate Universes and timelines!

The Many worlds interpretation encompasses mathematical support but is currently lacking in experimental evidence. However we may soon be able to carry out tests for potential interactions/collisions of two Universes but until then – knowing how bizarre quantum mechanics is – anything could happen.

Author – Susan Chen

Susan is a 5th year high school student currently studying three STEM subjects at Scottish Higher level-Mathematics, Physics and Chemistry (Crash Course). She particularly loves ideas in cosmology and hopes to embark on an academic journey in the area of theoretical physics.