- The famous Schrödinger’s Cat thought experiment is a simple example of how quantum systems give rise to classical “reality.” But is such an outcome inevitable?
- A new study, relying on the “many worlds interpretation” of quantum mechanics—along with a concept known as “decoherent histories”—shows that increased energy levels suppress possibilities until only single states remain.
- This suggests that classical reality can evolve from a purely quantum foundation.
Discoveries over the past century have undeniably confirmed that we live in a quantum world. But, strangely, what we discern as “real” is undeniably classical. This conundrum underpins nearly every facet of quantum mechanics: how do quantum interactions give rise to the classical reality of our everyday experience?
This question is most famously illustrated by the thought experiment known as Schrödinger’s Cat, which essentially breaks down a quantum effect as if it operated on a macro scale. As a brief refresher, this experiment explains how the “state” of a cat—dead or alive—in an enclosed box is uncertain until that box is opened and an outside observation creates reality. However, physics can’t yet explain how cats—or, in the quantum sense, atoms—can go from two states to one. So, an idea known as the “many worlds interpretation” has been put forth, which suggests that both states occur and branch into ever-different multiverses.
A new study from scientists at the Autonomous University of Barcelona in Spain, drawing on this “many worlds interpretation,” seemingly explains why our classical reality emerges. The results of the study were published in the journal Physical Review X.
“We live in a quantum world, as far as the experiments we do can tell,” Joseph Schindler, a co-author of the study from Autonomous University of Barcelona, said to New Scientist. “So then why do I end up having all these [non-quantum] classical experiences?”
To answer this question, the researchers worked with a concept known as “decoherent histories.” This idea asserts that every physical process is the result of a a series of steps in a definite sequence that provides a record of past behavior. The researchers, physicist Michael Hall wrote for Physics, used powerful computers to simulate quantum evolution up to 50,000 energy levels. They found that large-scale branching still occurred, confirming the idea that classical reality is able to form from a purely quantum foundation.
“We ask: how does decoherence emerge as system size increases?,” Philipp Strasber, a co-author of the study from Autonomous University of Barcelona, said on X (formerly known as Twitter). “Answer: exponentially fast for slow and coarse observables (observables we humans care about) in chaotic many-body systems.”
Essentially, as objects were made bigger by several thousand particles, the research team found that they eventually ended up in a world that obeyed classical physics.
“It’s very generic, almost inevitable,” Strasberg told New Scientist. “This is good news for people who want to live in a classical world.”
Of course, questions remain. For example, this study assumes that all universes behave in the same way, leaving room for the idea of a system coughing up a reality where the cat is both dead and alive, according to ScienceAlert.
If such a quantum feline nightmare is possible, it might be worth sticking with a more classical reality.
Darren lives in Portland, has a cat, and writes/edits about sci-fi and how our world works. You can find his previous stuff at Gizmodo and Paste if you look hard enough.
