Scientists at MIT have discovered something new in the world of quantum physics: neutrinos travelling across hundreds of miles in the real world still follow quantum laws.
The effects of quantum physics have never been seen over such a great distance, and it gives researchers new clues as to how the infinitesimally small physics involved could have repercussions on the larger objects we interact with every day.
Neutrinos are electrically neutral particles that speed through the Universe (and everything in it) with only extremely weak interactions along the way. They also oscillate, changing between 'flavours' as they go.
Now the team from MIT has measured neutrinos leaving Chicago, Illinois, and arriving at Soudan, Minnesota – a distance of some 735 kilometres (456 miles) – and found that the 'flavour' changes along the way can most readily be explained by quantum theory.
Why is this important? Because it shows that quantum mechanics can operate on a large (735-km) scale, as well as a miniature one.
"We can't escape quantum mechanics, even when we describe processes that happen over large distances," explained one of the researchers, physicist David Kaiser. "We can’t stop our quantum mechanical description even when these things leave one state and enter another, travelling hundreds of miles. I think that's breathtaking."
One of the underlying principles of quantum physics is the idea of superposition, that objects can exist in two (often opposing) states at the same time. The most well-known example is Schrödinger's cat, which posits that a feline trapped inside a box could be alive or dead, until the box is opened up.
This new research shows the idea of superposition can be applied to neutrinos trekking across large distances, not just tiny, subatomic events. In other words, the neutrinos essentially have no fixed identity, just like Schrödinger's cat, as they travel.
To analyse the neutrino data, the scientists used a tweaked version of a maths expression called the Leggett-Garg inequality. The expression is used to test systems that have two or more distinct states and see if quantum or classical physics provides the best explanation.
"The final result is that, like all other tests performed to date under very different circumstances, quantum mechanics appears to be the correct description of the world at all distance scales, weirdness notwithstanding," said physicist André de Gouvêa from Northwestern University in Illinois, who was not involved in the research.