On the smallest scales, our common-sense view of reality no longer applies. It’s as if physics is fundamentally inconclusive, a fact that becomes harder to ignore the closer we get to the particles that divide our universe into pixels.
In order to understand it better, physicists had to invent A whole new framework Quantum theory is a theory of probability, not certainty. This is quantum theory, and it describes all sorts of phenomena, from entanglement to superposition.
But despite a century of experiments showing how useful quantum theory is in explaining what we see, it is hard to shake off our “classical” view that the building blocks of the universe are reliable components of space and time. Even then, we cannot imagine that quantum theory is useful in explaining what we see. Einstein was forced To ask his fellow physicist, “Do you really think the moon isn’t there when you’re not looking at it?”
Many physicists have wondered for decades whether there is some way in which the physics we use to describe macroscopic experiments can be used to explain all of quantum physics.
A new study has also found that the answer is a big “no.”
Specifically, neutrons are fired in a beam at neutron interferometer It can exist in two places at the same time, which is impossible under classical physics.
The test is based on a mathematical assertion called Inequality in Leggett-Garg theorywhich states that a system is always in one of two possible states. Essentially, Schrödinger’s cat is either alive or dead, and we are able to determine which of these two states it is in without our measurements having any effect on the outcome.
Large systems—those that we can reliably understand using classical physics alone—obey the Leggett-Garg inequality. But systems in the quantum world violate it. A cat is both alive and dead at the same time, an analogy for quantum superposition.
The idea behind this is similar to the more famous idea Bell’s inequality“, who was awarded the Nobel Prize in Physics in 2022,” Physicist Elisabeth Kreuzgruber says, From Vienna University of Technology.
“However, the Bell inequality concerns the question of how strongly the behavior of a particle is related to another quantumly entangled particle. The Leggett-Garg inequality concerns only one 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?”
A neutron interferometer involves firing a beam of neutrons at a target. As the beam travels through the device, it splits into two parts, with each end of the beam taking separate paths until it is later recombined.
The Leggett-Garg theorem states that a measurement on a simple binary system can actually give two results. If you measure it again in the future, those results will be correlated, but only up to a certain point.
For quantum systems, Leggett and Garg’s theory no longer applies, allowing correlations above this threshold. In effect, this would give researchers a way to distinguish whether a system needs a quantum theory to understand it.
“However, it is not easy to investigate this issue experimentally,” Physicist Richard Wagner says From the Vienna University of Technology. “If we want to test macroscopic realism, we need an object that is macroscopic in a certain sense, that is, one whose size is similar to that of our everyday objects.”
In order to achieve this, the distance between the two parts of the neutron beam in the interferometer is on a larger than quantum scale.
“Quantum theory says that each neutron travels on both paths at the same time.” Physicist Niels Gerets says From the Vienna University of Technology. “However, the two partial beams are separated by several centimeters. In a sense, we are dealing with a quantum object that is massive according to quantum standards.”
Using several different measurement methods, the researchers examined the neutron beams at different times. Indeed, the measurements were so closely correlated that the classical rules of total reality could not apply. Their measurements suggested that the neutrons were in fact traveling simultaneously on two separate paths, separated by a distance of several centimeters.
It’s just the latest release in A long series of Leggett-Garg experiments This shows that we do indeed need quantum theory in order to describe the universe we live in.
“Our experiment shows that nature really is as strange as quantum theory claims.” Physicist Stefan Sponar says: From Vienna University of Technology. “No matter what classical 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 Letters.
