Schrödinger's Cat: A Paradox of Quantum Mechanics

Image description: Schrödinger's Cat thought experiment illustrates the idea of superposition of states in quantum mechanics, although the use of the cat's "death" is poorly suited. Image source: © Mylène Simoès, Art Director

The Paradox of Schrödinger's Cat

Schrödinger's Cat is a thought experiment proposed by the Austrian physicist Erwin Schrödinger (1887-1961) in 1935 to illustrate the paradoxes of quantum mechanics. This experiment is meant to demonstrate the strangeness of quantum superposition, where a system can exist simultaneously in multiple states until it is observed.

The Principle of Superposition

The basic idea of the experiment relies on the notion of superposition of states, a fundamental principle of quantum mechanics. According to this principle, a quantum particle, such as an electron, can be in multiple states at the same time until a measurement is made. This phenomenon is difficult to understand within the framework of classical physics, where an object is always in a well-defined state.

The Course of the Experiment

The thought experiment imagines a cat locked in a closed box, accompanied by a device that depends on the quantum state of a radioactive atom. This device contains a Geiger counter that detects the decay of a radioactive atom. If the atom decays, the counter triggers a mechanism that releases poison, thus killing the cat. If the atom does not decay, the poison is not released and the cat remains alive.

According to quantum mechanics, as long as the box is not opened and the state of the cat is not observed, the cat is in a state of superposition. It is both alive and dead, which is paradoxical within our classical understanding of the world.

A Better Analogy to Explain Superposition

Imagine two virtual twins in two different places, without knowing which one is "real" until you look.

Before looking, the twins are "superposed": they exist in both places at the same time, for example, one is in one room of your house and the other in a different room, but we cannot say which one is real.

Once you look (or make a measurement), one of the twins becomes "real," meaning you discover in which specific place they are. The superposition of the two distinct states collapses, and the state of the system reduces to a defined state (one twin in one room), eliminating the other possibilities.

What actually happens in the framework of quantum mechanics is that once the measurement is made, the quantum system ceases to be in a state of superposition and becomes in a definite state, corresponding to one of the possible states of the superposition. This is the notion of the collapse of the wave function, where probabilities become a measured state, and the other state is then "out of the picture" of this measurement.

Implications of the Experiment

The Schrödinger's Cat experiment is a way to criticize the Copenhagen interpretation of quantum mechanics, which argues that quantum reality does not exist before observation. According to this interpretation, as long as the state of the quantum system (in this case, the radioactive atom) is not measured, it exists in a superposition of multiple states.

Reflection on the Paradox

The paradox of Schrödinger's Cat raises questions about the nature of reality. If a cat can be both alive and dead, it suggests that our perception of reality at the macroscopic scale cannot be explained by the laws of quantum mechanics alone.

This paradox highlights the problem of the collapse of the wave function: according to the Copenhagen interpretation of quantum mechanics, before any measurement, a quantum system exists in a superposition of several possible states, represented by a wave function.

When a measurement is made, this superposition "collapses," and the system instantaneously adopts a well-defined state. Thus, measurement, as an act of observation, plays a fundamental role in determining the state of the system, raising questions about the nature of reality and observation in quantum mechanics. This process seems to imply that a quantum phenomenon does not have a determined reality until it has been observed.

Application and Modern Reflections

Although Schrödinger's Cat is a thought experiment, it has inspired many discussions and research on quantum measurement, the collapse of the wave function, and interpretations of quantum mechanics. Moreover, it has paved the way for demonstration technologies such as quantum computers, quantum imaging, and quantum atomic clocks, where superposition phenomena are exploited concretely.

Conclusion

Schrödinger's Cat remains a powerful metaphor for the complexity and strangeness of quantum mechanics. It strikingly illustrates the tensions between quantum physics and our classical intuition. Despite its apparent simplicity, this thought experiment continues to stimulate deep debates about the nature of reality and the role of observation in the quantum world.