Schrodinger’s cat

Schrödinger’s cat is considered one of the most famous thought experiments in quantum mechanics. It was proposed by Austrian physicist Erwin Schrödinger in 1935 in a discussion with Albert Einstein. This paradox was designed to illustrate the problem of the Copenhagen interpretation of quantum mechanics when applied to everyday objects. This thought experiment has fascinated and confused scientists and philosophers, which helped them understand the strange nature of quantum theory.

In this Answer, we’ll explore what is Schrödinger’s cat paradox with the following outline.

Copenhagen interpretation

In quantum physics, a particle can simultaneously be in every possible state until we observe or measure it. This is called the superposition state. Mathematically, we can say each state has a probability of where the particle could be. In a superposition state, the particle exists in many possible states simultaneously. When we observe it, the superposition collapses, and the particle settles itself into one of those possible states. It can only be in one state while being observed.

The Copenhagen interpretation, developed by Niels Bohr and Werner Heisenberg, emphasizes this idea and highlights the crucial role of observation in determining outcomes. When we stop observing the particle, it returns to the superposition state.

To understand this, consider that there is a food market with nine shops. You are waiting for your friend in the dining area. Your friend has gone to buy food from the food market. You don’t know which shop your friend is at to buy the food, so each shop has an equal probability. In quantum physics, your friend is in a superposition state, where he exists in all possible states, such as a food shop. Once you observe your friend, you identify the shop, and now you know the shop where your friend is. As soon as you stop observing, your friend will be again in a superposition state.

What is Schrödinger’s cat?

Erwin Schrödinger disagreed with the Copenhagen interpretation, particularly its application to everyday objects. To express his point of view, he planned a thought experiment. He proposed placing a cat inside a sealed box along with the following setup:

• Radioactive source: It has a 50% probability of decaying in the next hour.

• Hydrocyanic acid: A small flask of hydrocyanic acid is closed with a cork.

• Radiation monitor unit: It senses the decaying of the radioactive source.

• Hammer: It is attached to the radiation monitor unit. If the radioactive source emits radiation, the unit will trigger the hammer to break the flask.

Now, the cat’s fate is attached to the state of the radioactive source. If it decays, the radioactive monitor unit will detect it, break the flask of hydrocyanic acid, and the cat dies. If the radioactive source is not decayed, the cat remains alive.

According to the Copenhagen interpretation, the radioactive source is in a superposition state of decayed and undecayed. Consequently, the entire system must also be in a superposition state. So until we observe the cat, the cat is simultaneously alive and dead. Opening the box and observing the cat forces the system to collapse into one of the possible states: alive or dead.

Schrödinger criticized the Copenhagen interpretation of superposition, which states that if the cat were found dead on opening the box, it would be alive and dead again, which is impossible. He also claimed that the quantum superposition does not apply to humans and living things. This scenario challenged classical intuition and highlighted the counterintuitive nature of quantum mechanics.

Interpretations and implications

Schrödinger’s cat led to various interpretations and philosophical debates about the nature of reality and measurement in quantum mechanics. Let’s look into some of them below.

Many-worlds interpretation

In 1957, Hugh Everett proposed all possible states of a quantum event occur, each in a separate and parallel universe. In this case, the universe splits into one universe where the cat is alive and the other where the cat is dead. Since these states are decoherence, it is ensured that there is no interaction with each other between different states. Whenever the outcome is observed, the observer’s state is entangled with the cat’s, so both states correspond.

The relational interpretation

The relational interpretation, proposed by Carlo Rovelli, suggests that the state of a quantum system is not absolute but relative to the observer. In this view, different observers may have different accounts of the state of a system, and there is no single objective reality. For example, one observer might see the cat as alive, while another might see it as dead, depending on their interactions with the system. This interpretation challenges the notion of an objective reality and emphasizes the role of the observer in defining the state of a quantum system.

Objective collapse theories

Objective collapse theories propose that all superposition states collapse spontaneously and independently of any observation. This happens when some physical threshold is reached, such as temperature, mass, time, irreversibility, etc. Hence, the cat will be in a definite state before any external observation. This can be loosely described as “the cat observes itself” or “the environment observes the cat.” For example, if we put a cat into the box and do not observe it for 10 days, it will die due to hunger and thirst. There would be no longer a superposition state in that case.

Another notable example is the Ghirardi-Rimini-Weber (GRW) theory, which introduces spontaneous collapses that occur randomly but with a defined probability. These collapses prevent macroscopic objects from being in superposition states for extended periods. This ensures the objects we see in our everyday world appear in definite states.

Objective collapse theories aim to provide a solution to the measurement problem by making the collapse of the superpositions.

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Conclusion

Schrödinger’s cat illustrates how strange quantum mechanics can be for larger objects. This thought experiment imagines a cat that is both alive and dead until someone observes it. This highlights the role of observation in quantum theory. The experiment challenges our understanding and has opened debates, leading to ideas like the many-worlds interpretation and objective collapse theories. Ultimately, Schrödinger’s cat prompts us to rethink our views on reality and the impact of observation in the quantum world.