Schrödinger’s cat

I once had a cat called Schrödinger. At the time, I thought it was a clever name—a humorous nod to the famous thought experiment. But in hindsight, it was a terrible choice. When the cat passed away unexpectedly, the irony of his name was painfully apparent. Everyone laughed. Schrödinger, my cat, was no longer with us, and with him, the joke was no longer funny. And that’s the strange thing about Schrödinger’s cat—it was never just a simple joke, but a profound puzzle about the nature of reality itself.

The original cat, of course, wasn’t real, but the scientist who gave rise to the thought experiment certainly was. Erwin Schrödinger, the Austrian physicist, was one of the central figures in the development of quantum mechanics. In the 1920s, Schrödinger, along with his peers, formulated equations that uncovered the strange and puzzling world of the very small—equations that have since become the bedrock of modern science. These equations didn’t merely describe a niche corner of science; they reshaped our understanding of the universe at its most fundamental level.

Quantum mechanics is not just another field of science—it is the framework upon which all modern science is built. It explains the behaviour of particles at the smallest scales, such as atoms and subatomic particles, unveiling a universe far more bizarre than we could have imagined. Without quantum mechanics, we would have no understanding of lasers, nuclear energy, or even the workings of the Sun. Our understanding of chemistry, molecular biology, DNA, and genetic engineering is built on the foundation Schrödinger and his colleagues established.

Yet, despite its undeniable importance, quantum theory remains a deeply unsettling field, filled with paradoxes and counterintuitive predictions. It is perhaps the greatest achievement in science, surpassing even Einstein’s theory of relativity in its practical applications, but it challenges our everyday understanding of reality. In fact, the quantum world is so strange that even Einstein himself struggled to accept its implications. He rejected some of the conclusions of Schrödinger and his contemporaries, finding them incompatible with his own view of the universe.

Einstein, along with many other scientists, preferred to think of the equations of quantum mechanics as a sort of mathematical tool—a trick that, while useful for predicting the behaviour of particles, could not possibly reflect a deeper, more coherent reality. In their view, the equations were accurate but incomplete, masking a hidden truth that aligned more closely with our everyday experience of the world.

The most unsettling idea that quantum mechanics proposes is that reality itself is not fixed. According to the theory, we cannot know what particles are doing when we are not observing them. Schrödinger’s famous thought experiment involving a cat was intended to demonstrate how radically different the quantum world is from the one we inhabit.

In the quantum realm, the familiar laws of physics that govern our daily lives no longer apply. Instead, events occur according to probabilities. Take a radioactive atom, for example—it may decay, releasing an electron, or it may not. An experiment can be set up where there is a 50% chance that one atom in a sample of radioactive material will decay within a set period, with a detector to record the event if it occurs.

Schrödinger, disturbed by the implications of quantum mechanics, used this scenario in a thought experiment of his own. He imagined placing a radioactive atom, a vial of poison, and a cat inside a closed box. If the atom decayed, the poison would be released, killing the cat. In the everyday world, we would simply say that there’s a 50% chance the cat is alive and a 50% chance it is dead—nothing strange there. But quantum mechanics suggests that, until we open the box to observe, the cat is both alive and dead at the same time, existing in a superposition of states.

For Einstein and many others, this idea was intolerable. He famously said, “God does not play dice,” rejecting the notion that the universe could operate on random, probabilistic principles. He believed there must be a deeper, deterministic reality underlying the quantum phenomena—a “clockwork” universe, still hidden from view.

Einstein spent many years trying to uncover this hidden reality, but he died before he could conduct the necessary experiments to prove it. Ironically, some of the very ideas he introduced in an attempt to disprove quantum mechanics ended up confirming the randomness and indeterminacy he had rejected.

In the summer of 1982, a team of scientists led by Alain Aspect at the University of Paris-South conducted an experiment that would provide critical insights into the nature of quantum reality. They sought to uncover the “hidden variables”—the deeper reality that Einstein had believed was behind the quantum world. The experiment studied two photons, or particles of light, emitted from the same source in opposite directions. These photons were measured for a property called polarization.

Quantum mechanics holds that the polarization of each photon does not exist as a fixed property until it is measured. On the other hand, the hidden-variable theory proposed that each photon would have a predetermined, real polarization from the moment it was created. Since the two photons came from the same source, their polarizations should be correlated. However, the nature of the correlation measured would depend on which theory of reality was correct.

The results of this experiment were clear. The correlations predicted by hidden-variable theory did not appear, while those predicted by quantum mechanics were confirmed. More astonishingly, when the polarization of one photon was measured, it instantaneously affected the other photon, even though they were moving apart at the speed of light. This violated relativity, which tells us that nothing can travel faster than light.

These findings demonstrated that there is no hidden “reality” underlying the quantum world in the way Einstein had hoped. Instead, the behaviour of fundamental particles is not what we would call “real” in the everyday sense of the word. Yet, despite this, the particles seemed to be interconnected, each aware of the state of the others, forming an indivisible whole.

The search for Schrödinger’s cat, in its quest to understand the true nature of quantum reality, may appear to have led us to a dead end—since it reveals no reality in the conventional sense. However, this is far from the end of the story. The search may yet lead us to a new understanding of reality—one that transcends, and yet includes, the conventional interpretation of quantum mechanics.

This journey into the heart of quantum reality began long before Schrödinger. Three centuries ago, Isaac Newton was studying the nature of light, unknowingly laying the groundwork for the quantum discoveries that would follow. Had he known where his work would lead, he too would have been astonished—perhaps even more so than Einstein—by the strange truths that would eventually emerge. Newton could never have imagined that his investigations into the nature of light would set humanity on the path toward Schrödinger’s cat. And yet, the search continues, leading us ever closer to a new vision of reality.