The quantum world is unpredictable. The behavior of a quantum particle is perplexing. Where Einstein’s photoelectric effect pointed out the particle nature of a light wave, the Davisson-Germer experiment, on the other hand, demonstrated the wave nature of an electron. It’s just like the quantum particles are suffering from a split personality disorder!

Wave-particle duality
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Wave-particle duality
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Quantum mechanics cannot easily be reconciled with everyday language and observations, and hence, it has often seemed absurd to physicists. Ever since the notion of “quantum physics” came into existence, people worldwide have come up with several interpretations to explain how things actually work in the eerie quantum world. However, for the better part of the last century, the most accepted explanation for why the same quantum particle may behave differently was given by the Copenhagen interpretation. So, first of all, let’s try to understand what Copenhagen Interpretation actually says.

Copenhagen interpretation

Werner Heisenberg (left) with Niels Bohr at a Conference in Copenhagen in 1934. Photo: Fermilab, U.S. Department of Energy
Werner Heisenberg (left) with Niels Bohr at a Conference in Copenhagen in 1934. (Photo: Fermilab, U.S. Department of Energy)

The Copenhagen Interpretation of quantum mechanics was devised from 1925 to 1927 by Niels Bohr and Werner Heisenberg. It is one of the oldest proposed interpretations of quantum mechanics and remains one of the most commonly followed. According to this interpretation, physical systems do not possess definite properties before being measured. And, quantum mechanics can only predict the probability distribution of a given measurement’s possible results.

In other words, measurement affects the system, causing it to reduce to only one of the possible values immediately after the measurement. Confused? It’s normal to baffle at this point because this is not what we observe in our daily life scenarios. This is where Schrödinger came up with his famous cat experiment to point out a flaw in the Copenhagen interpretation of superposition.

Erwin Schrodinger
Image Courtesy :Wikipedia
Erwin Schrodinger

Aim of Schrödinger’s cat experiment

Before we begin with this, let me tell you one important thing. The Schrödinger’s cat experiment was not a real activity carried out in any lab. Rather it was a thought experiment devised by Austrian physicist Erwin Schrödinger in 1935, though the idea originated from Albert Einstein. Schrödinger designed it to show what the Copenhagen interpretation would look like if the mathematical terminology used to explain superposition in the microscopic world were replaced by macroscopic terms that an average person could visualize and understand. 

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The cat is both dead and alive

Pictorial description of Schrodinger's cat experiment
Image courtesy: wikipedia
A pictorial description of Schrodinger’s cat experiment

In this famous thought experiment, a cat is placed in a steel box along with a Geiger counter, a vial of poison, a hammer, and a radioactive substance, as shown above. When the radioactive substance decays, the Geiger counter detects it and then triggers the hammer to release the poison, which subsequently kills the cat. But, radioactive decay is a random process, and there is no way to predict when it will happen. So, the atom exists in a state of superposition — both decayed and not decayed at the same time.

Until the box is opened and an observation is made, an observer doesn’t know whether the cat is alive or dead. This is because the cat’s fate is tied to whether or not the atom has decayed. So, as Schrödinger put it, the cat is “living and dead … in equal parts” until observed. This is Schrödinger’s cat experiment. But what does it signify?

Inferences from Schrödinger’s cat experiment

As we have read earlier, the cat ends up being both dead and alive simultaneously. But, the existence of a cat that is both dead and alive at the same time is quite absurd and does not happen in the real world. Right? So, this thought experiment showed that the collapse of all possible outcomes to merely one outcome is not just driven by conscious observers or measurements. There is something more to the collapses. Every interaction that a quantum particle makes can collapse its state.

This means that as soon as the radioactive atom interacts with the Geiger counter, it will collapse from its non-decayed/decayed state into one definite state. The Geiger counter will either get definitely triggered, and the cat would die, or the Geiger counter wouldn’t get triggered at all, and the cat won’t die. But both won’t happen simultaneously. So, quantum state collapse is not driven just by conscious observers/measurements, and “Schrodinger’s cat” was just a teaching tool invented by Schrödinger to make this fact more obvious and simpler to grasp.

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Undoubtedly, the role of the observer and measurements remains an important question in the study of quantum physics and is an endless source of speculations, especially in the field of quantum computation. Still, throughout the years, Schrödinger’s cat analogy has been one of the best analogies to illustrate the emerging theories of how quantum mechanics works.

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