# The Schrodinger's Cat Paradox and Quantum Weirdness Explained

Posed by Erwin Schrödinger, Schrödinger's Cat paradox is a key thought experiment that illustrates the quantum theory of superposition. This ScienceStruck post looks at one of the most famous illustrations of quantum theory.

ScienceStruck Staff

Last Updated: Feb 28, 2018

To underscore the strangeness of quantum mechanics and the probability interpretation, Erwin Schrödinger said, "I don't like it, and I'm sorry I ever had anything to do with it."

There are a few things that are as distressing to a cat (and physics) lover as the Schrödinger's Cat paradox. Yes, quantum mechanics, which explains the crazy behavior of particles at the subatomic level, is baffling to say the least, but it is also one of the most successful theories of science that has been developed in the twentieth century. We (including many classical physicists) believe that the world exists in a definite state, and we can discern the state by making careful observations.

This has been casually dumped by the quantum theory which believes that there is no unique state of a microscopic system, such as an atom. It is only with 'observation' that the state, a particle is in, can be recognized. Till then, it continues to remain in several states. This is known as 'quantum superposition.' The superposition of particles is broken down by measurement. Confusing and paradoxical? Well, that is what quantum theory is all about.

What does quantum theory have to do with a cat? Only if we could ask Erwin Schrödinger, the Nobel Prize winner, whose feline paradox proposed in 1935 gained so much prominence that recently Google decided to honor his 126th birthday with a cat-themed Doodle. Schrödinger's Cat is one of the most popular thought experiments (Gedankenexperiment) illustrating quantum indeterminacy―a standard description in quantum physics to explain the incompleteness in the description of a physical system.

This has been casually dumped by the quantum theory which believes that there is no unique state of a microscopic system, such as an atom. It is only with 'observation' that the state, a particle is in, can be recognized. Till then, it continues to remain in several states. This is known as 'quantum superposition.' The superposition of particles is broken down by measurement. Confusing and paradoxical? Well, that is what quantum theory is all about.

What does quantum theory have to do with a cat? Only if we could ask Erwin Schrödinger, the Nobel Prize winner, whose feline paradox proposed in 1935 gained so much prominence that recently Google decided to honor his 126th birthday with a cat-themed Doodle. Schrödinger's Cat is one of the most popular thought experiments (Gedankenexperiment) illustrating quantum indeterminacy―a standard description in quantum physics to explain the incompleteness in the description of a physical system.

What is the Schrödinger's Cat Paradox

The Schrödinger's Cat paradox is a thought experiment that Austrian physicist, Erwin Schrödinger intended as a discussion for the EPR (Einstein, Podolsky, and Rosen) article. The EPR article looked at quantum entanglement, where multiple particles are linked in such a way that when we measure one particle, the quantum states of other particles can be discerned.

Schrödinger, who was not in tune with the standard interpretation of quantum physics, in his communications with Albert Einstein came up with the paradoxical thought experiment to disprove the Copenhagen Interpretation. To explain his theory, Schrödinger placed his cat in a steel box (theoretically, of course) which also contains a radioactive substance, a Geiger counter, a small flask of hydrocyanic acid, and a hammer. When the atom of the radioactive substance decays, it is detected by the Geiger counter which, in turn, activates the hammer to release the acid and kill the cat. Since the radioactive atom exists in a state of superposition, where there is 50 percent chance of it having decayed or not, it extends to other events that are related to it. This also applies to the fate of the cat, which as Schrödinger put it, could be "living and dead ... in equal parts" until it is observed. So, according to the quantum theory, the cat could be living and dead simultaneously.

The superposition collapses when a person lifts the lid of the box to see whether the cat is dead or alive. This is known observer's paradox or indeterminacy, and it leads to 'decoherence', where the probabilities decohere into a single reality. In this case, the observer or the measurement system is entangled with the experiment.

Schrödinger, who was not in tune with the standard interpretation of quantum physics, in his communications with Albert Einstein came up with the paradoxical thought experiment to disprove the Copenhagen Interpretation. To explain his theory, Schrödinger placed his cat in a steel box (theoretically, of course) which also contains a radioactive substance, a Geiger counter, a small flask of hydrocyanic acid, and a hammer. When the atom of the radioactive substance decays, it is detected by the Geiger counter which, in turn, activates the hammer to release the acid and kill the cat. Since the radioactive atom exists in a state of superposition, where there is 50 percent chance of it having decayed or not, it extends to other events that are related to it. This also applies to the fate of the cat, which as Schrödinger put it, could be "living and dead ... in equal parts" until it is observed. So, according to the quantum theory, the cat could be living and dead simultaneously.

The superposition collapses when a person lifts the lid of the box to see whether the cat is dead or alive. This is known observer's paradox or indeterminacy, and it leads to 'decoherence', where the probabilities decohere into a single reality. In this case, the observer or the measurement system is entangled with the experiment.

How Does it Explain Quantum Weirdness

Measuring Microscopic and Macroscopic Objects

The Schrödinger's Cat paradox explains the apparent contradiction between things that are observable on the microscopic level and those that are visible to the naked eye. While the above-mentioned paradox may work well for microscopic elements, the same rules cannot be applied to larger, everyday objects. Microscopic particles, like electrons can only be observed by measurement. While in some experiments, they act wave-like, in others, they act particle-like, following straight line trajectories, or colliding and scattering. It is impossible to perform experiments without altering the state of the microscopic elements. As opposed to this, everyday objects are always in a single state, and we can take measurements without altering that state.

When Do Superpositions Collapse

Although the Copenhagen theory does say that the superpositions collapse when the box is opened and an observation is performed, it fails to mention how such an observer-induced collapse happens, or why a specific outcome happened as opposed to the other. This brings us to the famous tree-in-the-forest question, which states that, "When a tree falls in a lonely forest, and no one is nearby to hear it, does it make a sound?" Just because we do not observe the result, does it mean that it does not happen?

Apart from the Copenhagen Interpretation, there is Hugh Everett's many-worlds interpretation which would interpret that when the box is opened, the observer would see a dead cat in one reality and the other, an alive one, in another reality. Despite the depressing thought of the dead cat, the Schrödinger's Cat paradox has raised some important questions about quantum theory and its alleged weirdness. It illustrates the problems of observing subatomic systems and drawing conclusions, especially when it is applied to the larger macroscopic world.