Scientists may have found a substance that allows them to finally link the opposing models of quantum and classical physics. In time, this finding could allow them to understand why the classical model breaks down at an quantum level, why quantum physics doesn’t seem to work at visible scales, and how the two can be reconciled.

We know these two models of understanding the physical world — the quantum for really small bits and the classical for larger bits — don’t mix well together. Usually when one is in charge, the other is completely absent. Most of the rules of classical physics break down at the quantum level — gravity, for example, doesn’t seem to be doing much on the atomic level even as it’s literally holding the universe together overall. There’s nothing in the rules of classical physics that can explain quantum entanglement, either.

Scientists know that there must be something tying the two models together, but we’ve yet to find even a clue of what that is. Now, thanks to a newly developed material, scientists have a chance to see quantum mechanics in action on a scale visible to the naked eye — offering hope of finding a bridge between the two models.

“We found a particular material that is straddling these two regimes,” says team leader N. Peter Armitage, from Johns Hopkins University.

“Usually we think of quantum mechanics as a theory of small things, but in this system quantum mechanics is appearing on macroscopic length scales. The experiments are made possible by unique instrumentation developed in my laboratory.”

The material Armitage developed is a topological insulator, a class of material first theoretically predicted in the 1980s, and first produced in 2007. Topological insulators are conductive on their outer layer while being insulators on the internal one. This causes the electrons flowing along the material to do some pretty weird stuff. For example, Armitage and his team found that a beam of terahertz radiation (sometimes called THz or T-rays – an invisible spectrum of light) passing through their bismuth-selenium topological insulators can be made to rotate slightly — an effect only observed at the atomic scale up to now.

This rotation could be predicted with the same mathematical systems that govern quantum theory — making this** the first time researchers have witnessed quantum mechanics occurring on the macro scale**. It could form the basis on which the quantum and classical models can be linked, the ‘theory of everything’ that scientists have been trying to find for decades.

The experiment is definitely “a piece of the puzzle” but according to Armitage, there’s still a lot of work to be done before this link is fully understood. He hopes that one day we’ll have a completed picture of physics, and new materials like the team’s topological insulator might be the way we get there.

The full paper “Quantized Faraday and Kerr rotation and axion electrodynamics of a 3D topological insulator” has been published in the journal *Science*.

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