At the smallest scales, our intuitive view of reality no longer applies. It’s almost as if physics is fundamentally indecisive, a truth that becomes harder and harder to ignore as we zoom in on the particles that pixelate our universe.
In order to better understand it, physicists had to devise an entirely new framework to place it in, one based on probability over certainty. This is quantum theory and it describes all kinds of phenomena, from entanglement to superposition.
Despite a century of experiments showing how useful quantum theory is in explaining what we see, it’s hard to shake our “classical” view of the building blocks of the universe as reliable devices in time and space. Even Einstein was forced to ask his fellow physicist, “Do you really believe the moon isn’t there if you’re not looking at it?”
Many physicists have asked over the decades whether there is some way that the physics we use to describe macroscopic experience can also be used to explain all of quantum physics.
Now, a new study also finds that the answer is a big fat no.
Specifically, neutrons fired in a beam in a neutron interferometer can exist in two places at the same time, which is impossible in classical physics.
The test is based on a mathematical statement called the Leggett-Garg inequality, which states that a system is always uniquely in one or the other of its available states. Basically, Schrödinger’s cat is either alive or dead, and we are able to determine which of these states it is in without our measurements affecting the outcome.
Macrosystems – those that we can only reliably understand using classical physics – obey the Leggett-Garg inequality. But systems in the quantum realm violate this. The cat is alive and dead at the same time, an analogy for quantum superposition.
“The idea behind it is similar to the more famous Bell’s inequality, for which the Nobel Prize in Physics was awarded in 2022,” says physicist Elisabeth Kreuzgruber of the Technical University of Vienna.
“However, Bell’s inequality is about how strongly a particle’s behavior is related to another quantum entangled particle. The Leggett-Garg inequality is only about a single object and asks: what is its state at specific points in time? related to the state of the same object at other specific moments?”
A neutron interferometer involves firing a beam of neutrons at a target. As the beam passes through the device, it splits in two, with each of the beam tips taking separate paths until they are later reunited.
Leggett and Garg’s theorem states that a measurement on a simple binary system can effectively give two results. Measure it again in the future, those results will correlate, but only up to a point.
For quantum systems, the Leggett and Garg theorem, which allows correlations above this threshold, no longer holds. In effect, it would give researchers a way to distinguish whether a system needs a quantum theorem to understand.
“However, it is not so easy to investigate this question experimentally,” says physicist Richard Wagner of the Technical University of Vienna. “If we want to test macroscopic realism, then we need an object that is in some sense macroscopic, that is, that has a size comparable to the size of our common everyday objects.”
To achieve this, the space between the two parts of the neutron beam in the interferometer is on a scale that is more macro than quantum.
“Quantum theory says that each individual neutron moves along both paths at the same time,” says physicist Niels Geerits of the Vienna University of Technology. “However, the two sub-beams are a few centimeters apart. In a sense, we are dealing with a quantum object that is huge by quantum standards.”
Using several different measurement methods, scientists examined the neutron beams at different times. And indeed, the measurements were too closely correlated for the classical rules of macro reality to play. The neutrons, their measurements indicated, were actually moving simultaneously along two separate paths, separated by a distance of several centimeters.
It’s just the latest in a long line of Leggett-Garg experiments to show that we really do need a quantum theory to describe the universe we live in.
“Our experiment shows: Nature is really as strange as quantum theory claims,” ​​says physicist Stephan Sponar from the Vienna University of Technology. “It can’t be done without quantum physics.”
The research was published in Physical inspection letters.