At smaller scales, our intuitive view of reality no longer applies. It’s almost as if physics is fundamentally unsettled, a truth that becomes harder to ignore as we zoom in on the particles that pixelate our Universe.
To better understand it, physicists had to create an entirely new framework to place it, based on probability over certainty. This is quantum theory, and it describes all sorts of phenomena, from entanglement to superposition.
However, despite a century of experiments showing how useful quantum theory is in explaining what we see, it is 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 that the Moon is not there when you don’t see it?”
Many physicists have wondered over the decades whether there is some way that the physics we use to describe macroscopic experiences can also be used to explain all of quantum physics.
Now a new study has also determined that the answer is a big no.
Specifically, neutrons fired in a beam in a neutron interferometer can exist in two places at the same time, something that 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 determinately in one or the other state available to it. 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 having an effect on the result.
Macro systems—those we can reliably understand using only classical physics—obey the Leggett-Garg inequality. But systems in the quantum field violate that. 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 Vienna University of Technology.
“However, Bell’s inequality is concerned with the question of how strongly the behavior of one particle is coupled to that of another quantum entangled particle. The Leggett-Garg inequality is only concerned with a single object and asks the question: how is its state at specific moments in time relative to the state of the same object at other specific moments in time?”
The neutron interferometer involves firing a beam of neutrons at a target. As the beam travels through the apparatus, it splits in two, with each of the beam teeth traveling on separate paths until they recombine later.
Leggett and Garg’s theorem states that a measurement in a simple binary system can effectively give two results. Measure again in the future, those results will correlate, but only up to a certain point.
For quantum systems, the Leggett and Garg theorem no longer holds, allowing correlations above this threshold. In effect, this would give researchers a way to tell 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, then we need an object that is macroscopic in a certain sense, that is, that has a size comparable to the size of our ordinary everyday objects.”
To achieve this, the spacing 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 every single neutron travels both ways at the same time,” says physicist Niels Geerits of the Vienna University of Technology. “However, the two partial beams are a few centimeters apart. In a sense, we are dealing with a quantum object that is large by quantum standards.”
Using several different measurement methods, the researchers probed the neutron beams at different times. And, of course, the measurements were too closely related for the classical rules of macro-reality to be at play. The neutrons, their measurements suggested, were actually traveling simultaneously on two separate paths, separated by a distance of several centimeters.
It’s just the latest in a long line of Leggett-Garg experiments showing that we really do need quantum theory to describe the Universe in which we live.
“Our experiment shows: Nature is really as strange as quantum theory claims,” says physicist Stephan Sponar of the Vienna University of Technology. “No matter what classical, macroscopic realist 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 review papers.