A team of MIT physicists analyzed the oscillations of neutrinos, the subatomic particles that weakly interact with matter and can pass through our bodies by the billions per second without any effect. The study showed the particles can be in a superposition state, without individual entities, when traveling hundreds of kilometers.
The superposition is a quantum mechanic idea in which objects are capable of existing simultaneously in two or more seemingly counterintuitive states, for example, electrons can spin both clockwise and anti-clockwise, or be at rest and excited, all at the same time.
About 80 years, ago, the physicist Erwin Schrödinger performed a thought experiment that explained the laws of quantum mechanics through an example of a cat trapped in a box with a radioactive source, which could be in a superposition state, both alive or dead. The scientists have since proven the particles can be in superposition at quantum, subatomic scales. However, there was still a question if such a phenomenon could be observed in the everyday world.
Neutrino particles can oscillate, or change between several distinct "flavors" while traveling through the space close to the light speed.
In the Fermilab's Main Injector Neutrino Oscillation Search (MINOS) experiment, neutrinos are produced from the scattering of other high energy, accelerated particles near Chicago, beamed to a detector in Soudan, Minnesota, which is 735 km (456 miles) away. The neutrinos leave their original location as one flavor, but they can oscillate during the journey, reaching their destination as another flavor.
A team of physicists explored and compared the distribution of neutrino flavors in Illinois and Minnesota, and concluded the quantum phenomena is adequate to explain them. When the particles sped between the reactor and detector, statistically, they were most likely in a superposition state, with no definite flavor.
The researchers also discovered that the data was "in high tension" with classical descriptions of the matter behavior. It seemed statistically unlikely that it could be explained by the models of Einstein type, in which the objects always embody definite properties.
The data from MINOS were analyzed by using a Fromaggio's reworking version of the Legget-Garg inequality. According to it, the distribution of neutrino flavors at the detector should depend on the energies at which the neutrinos were created. The flavor distributions should look very different if the neutrinos assumed a definite identity throughout their journey, versus if they were in superposition, with no distinct flavor.
"What's fascinating is, many of us tend to think of quantum mechanics applying on small scales. But it turns out that we can't escape quantum mechanics, even when we describe processes that happen over large distances. We can't stop our quantum mechanical description even when these things leave one state and enter another, traveling hundreds of miles. I think that's breathtaking," said David Kaiser, the Germeshausen Professor of the History of Science and professor of physics at MIT.
The scientists predicted the distribution of neutrino flavors arriving at the detector, both in case the neutrinos were exhibiting classical, Einstein-like theory behavior, and quantum state-like, superposition behavior. When the distributions were compared, virtually no overlap was found. The data from the MINOS experiment was found to fit squarely within the predicted distribution for the quantum case, which means it is highly likely the neutrinos did not have individual identities crossing hundreds of miles between detectors.
"What gives people pause is, quantum mechanics is quantitatively precise and yet it comes with all this conceptual baggage. That's why I like tests like this: Let's let these things travel further than most people will drive on a family road trip, and watch them zoom through the big world we live in, not just the strange world of quantum mechanics, for hundreds of miles. And even then, we can't stop using quantum mechanics. We really see quantum effects persist across macroscopic distances," Kaiser concluded.
The results of the study, to be published later this month in Physical Review Letters, represent the longest distance over which quantum mechanics has been tested to date.
Featured image: MIT physicists have found that subatomic particles called neutrinos can be in superposition, without individual identities, when traveling hundreds of miles, and not just at quantum, subatomic scales. Image credit: Christine Daniloff/MIT