At the tiniest scales, our intuitive view of reality breaks down. It’s almost as if physics is fundamentally indecisive, a truth that becomes increasingly difficult to ignore as we zoom in on the particles that pixelate our universe.
To understand it better, physicists had to come up with an entirely new framework to fit it into, one based on probability over certainty. This is quantum theory, and it describes all sorts of phenomena, from entanglement to superposition.
But despite a century of experiments demonstrating how useful quantum theory is for explaining what we see, it’s hard to shake our “classical” view of the building blocks of the universe as reliable fixtures in time and space. Even Einstein was forced to ask his fellow physicist, “Do you really believe that the moon isn’t there when you’re not looking at it?”
Over the decades, numerous physicists have wondered whether the physics we use to describe macroscopic experiences can also be used to explain all of quantum physics.
Now, new research has also shown that the answer is a big no.
More specifically, neutrons fired in a beam in a neutron interferometer can be present in two places at the same time. This is impossible according to classical physics.
The test is based on a mathematical statement called the Leggett-Garg inequality, which states that a system is always determined to be in one of two available states. In principle, Schrödinger’s cat is either alive or dead, and we can determine which of those states it is in without our measurements affecting the outcome.
Macrosystems – which we can reliably understand using only classical physics – obey the Leggett-Garg inequality. But systems in the quantum realm violate it. The cat is both alive and dead at the same time, an analogy for quantum superposition.
“The idea behind it is similar to the better-known Bell inequality, for which the Nobel Prize in Physics was awarded in 2022,” says physicist Elisabeth Kreuzgruber of the Vienna University of Technology.
“Bell’s inequality, however, is about how strongly the behavior of one particle is related to another quantum-entangled particle. The Leggett-Garg inequality is about only a single object and asks the question: how is its state at specific points in time related to the state of the same object at other specific points in time?”
The neutron interferometer involves firing a beam of neutrons at a target. As the beam travels through the device, it splits into two, with each end of the beam following a separate path until they are recombined later.
Leggett and Garg’s theorem states that a measurement on a simple binary system can effectively produce two results. Measure it again in the future, those results will be correlated, but only up to a point.
For quantum systems, Leggett–Garg’s theorem no longer holds, allowing for correlations above this threshold. This would essentially give researchers a way to distinguish whether a system needs a quantum theorem to be understood.
“However, it is not so easy to investigate this question experimentally,” says physicist Richard Wagner of the Vienna University of Technology. “If we want to test macroscopic realism, we need an object that is in a sense macroscopic, that is, that has a size comparable to the size of our usual everyday objects.”
To achieve this, the space between the two parts of the neutron beam in the interferometer is more macro- than quantum-sized.
“Quantum theory says that each neutron travels on both paths at the same time,” says physicist Niels Geerits of the Vienna University of Technology. “However, the two sub-beams are separated by several centimeters. In a sense, we are dealing with a quantum object that is enormous by quantum standards.”
Using different measurement methods, the researchers examined the neutron beams at different times. And indeed, the measurements were too closely correlated for the classical rules of macroreality to be at play. The neutrons, their measurements suggested, were in fact traveling simultaneously on two separate paths, separated by a distance of a few centimeters.
It is the latest in a long line of Leggett-Garg experiments showing that we really do need quantum theory to describe the universe we live in.
“Our experiment shows: Nature really is as strange as quantum theory claims,” says physicist Stephan Sponar of the Vienna University of Technology. “No matter what classical, macroscopically realistic theory you come up with: it will never be able to explain reality. It doesn’t work without quantum physics.”
The research was published in Physical assessment letters.