In 1935, Austrian-born physicist Erwin Schrödinger described a thought experiment that magnified a glaring problem at the heart of quantum mechanics.
To this day, the problem remains, summed up by Schrödinger’s seemingly ludicrous notion of a cat existing in an undecided state of life and death.
Theoretical physicists from the Autonomous University of Barcelona in Spain think they might at last have an explanation for why Schrödinger’s cat would always appear in a single state once it’s observed.
Their proposal rests largely on the assumption that every possibility of a quantum system constitutes a universe itself, a concept known as the many-worlds theory.
From this, Philipp Strasberg, Teresa E. Reinhard, and Joseph Schindler use first principles to show how the entanglement of particles in an existing landscape drags Schrödinger’s cat out of its own equation, decidedly dead or alive but never in between.
Some of the earliest debates in quantum physics were over ways to interpret uncertainty. In the words of Albert Einstein, God “does not play dice“.
While combinations of particle states are forced into a range of possible fates on paper, they exist as physical absolutes even when nobody is looking … right?
Nope.
A century on, Einstein’s metaphorical deity is still rolling the bones in a cosmic game of craps, and physicists are still discussing what this even means beyond abstract calculations.
One attempt to make sense of this distinction in realities is to imagine all possibilities of a particle’s states as equally valid, each representing its own private universe. Of these many worlds, just one is woven into ours when it encounters our own vast network of settled possibilities, earning the right to be considered ‘real’.
In the team’s numerical demonstration, the sheer scale of interactions quickly builds in a way that suppresses possibilities until single states remain.
In other words, given the complexity of the Universe surrounding Schrödinger’s cat, one that includes the box, observers, the building they’re in, and far beyond, rapidly growing interactions between an environment and the alive and dead states over time mean the two won’t appear as a mix.
In fact, this merging of worlds happens on such a small level, so quickly, relatively few particles can quickly iron out the fuzziness of an undecided state, making the quantum haze all but vanish on the smallest of scales.
“Since objects of daily life contain a huge number of particles, this explains why the multiverse is not directly perceptible to us,” the team writes in their paper.
Problem solved, right? Yes and no. While the hypothesis helps us visualise the selection of a single state from a lottery of countless possibilities, the explanation still relies on assuming that all universes behave in this way. Those universes also don’t take into account complexities of general relativity.
It might yet be imagined that the right combination of entangled states could still cough up a mix of alive cat and dead cat, or at least, it’s not ruled out. There also remains the question of just how far quantum randomness can exert an influence in a macroscopic reality like ours.
Nonetheless, it’s not the first time theoretical physicists have suggested a need to include larger-scale pictures of existing states to make sense of why an undecided quantum blur suddenly settles on a single measurement.
Schrödinger’s cat will remain an enigma in physics for a while to come, spinning in its grave as the perfect metaphor for a field of physics that remains rich in possibility.
This research was published in Physical Review X.
