At the tiniest scales, our intuitive view of reality no longer applies: It’s as if physics is fundamentally inconclusive, and that truth becomes increasingly hard to ignore as we zoom in on the particles that pixelate the universe.
To understand this better, physicists A whole new framework Put that into a theory based on probability rather than certainty: quantum theory, which explains all sorts of phenomena, from quantum entanglement to superposition.
But despite a century of experiments showing how useful quantum theory can be in explaining what we see, it’s hard to shake the “classical” view that the building blocks of the universe are reliably fixed entities of time and space. Einstein was forced He asked a fellow physicist: “Do you really believe that the moon isn’t there when you’re not looking?”
For decades, many physicists have wondered whether the physics we use to explain our macroscopic experience can also be used to explain all of quantum physics.
A new study now concludes that the answer is a resounding no.
Specifically, neutrons emitted in a beam form Neutron Interferometer It is possible to be in two places at the same time, something that is not possible in classical physics.
This test is Leggett-Garg inequalityThis specifies that the system is always in one state or another – essentially, Schrödinger’s cat is either alive or dead, and it is possible to determine which state the system is in without any measurements affecting the outcome.
Macroscopic systems (those that can be reliably understood using only classical physics) obey the Leggett-Garg inequality, but systems in the quantum realm violate this inequality: a cat is simultaneously alive and dead, an analogy for quantum superposition.
“The idea behind it is Bell’s inequalityThe Nobel Prize in Physics is scheduled to be awarded in 2022 Physicist Elisabeth Kreuzgruber says of the Vienna University of Technology.
“But the Bell inequality is about the question of how strongly the behavior of one particle is related to other entangled particles. The Leggett-Garg inequality is about only one object, and raises the question of how its state at a particular time is related to the state of that same object at other particular times.”
In a neutron interferometer, a beam of neutrons is aimed at a target. As the beam passes through the instrument it splits into two beams, each of which travels along a different path before being recombined.
Leggett and Garg’s theorem states that when you make a measurement in a simple binary system, you effectively get two results: if you measure it again in the future, the results will be correlated, but only up to a certain point.
For quantum systems, Leggett and Garg’s theorem no longer applies, and correlations above this threshold are tolerated. In effect, this allows researchers to distinguish whether or not quantum theorems are necessary to understand a system.
“But this issue is not so easy to investigate experimentally.” Physicist Richard Wagner says: “To test macroscopic realism, we need objects that are in some sense macroscopic, that is, objects that are comparable in size to the objects we use every day,” say researchers from Vienna University of Technology.
To achieve this, the space between the two parts of the neutron beam in the interferometer is on a macroscopic scale rather than a quantum.
“Quantum theory says that every neutron travels down both paths at the same time.” Physicist Niels Goeritz says: “However, the two partial beams are several centimetres apart. In a sense, we are dealing with a gigantic quantum object by quantum standards,” say the researchers from Vienna University of Technology.
The researchers looked at the neutron beam at different times, using several different measurement methods, and, as expected, the measurements were too closely correlated for the classical laws of macroscopic reality to come into play. Their measurements suggested that the neutrons were actually traveling down two separate paths simultaneously, a few centimeters apart.
this is, The long history of the Leggett-Garg experiment It shows that we really do need quantum theory to explain the universe we live in.
“Our experiments show that nature is as strange as quantum theory claims.” Physicist Stephen Sponer says: “No matter how classically and macroscopically realistic a theory you come up with, it will never be able to explain reality. It won’t work without quantum physics,” said the professor from Vienna University of Technology.
This study Physics Review Letter.