Which is more fundamental, classical or quantum physics?

The phenomenon of Quantum Entanglement, and more importantly its support that Quantum Physics is a complete expression with no hidden variables (ala Bell’s Thm) have led a number of people to conclude that Quantum physics is the fundamental reality. If this is true, then Classical Physics is really just a “illusion” that appears true at large scales.

The problem though is that, to date, no one has been able to figure out the way that Quantum Mechanics becomes Classical Physics when you have a large system or a macroscopic scale.

There’s a new paper published which claims that it represents an important step in doing just that…

“[The] work has been published in Physical Review Letters with the title, ‘Classical World Arising out of Quantum Physics under the Restriction of Coarse-Grained Measurements.’

‘Our motivation is to understand how the classical world comes out of quantum physics,’ Kofler says. ‘The established approach in research is decoherence where one has to take into account the complexity of systems and interactions with environment.’ It is interaction with the environment that brings decoherence into play, destroying quantum coherences and making it impossible to observe quantum phenomena. ‘We believe we found a process complementary to decoherence which can explain the quantum-to-classical transition.’

Instead of referring to the environment of a system, or even to the change quantum laws, Kofler and Brukner created a theoretical framework that stresses the use of measurement apparatuses. It is their restricted accuracy which limits the observability of quantum phenomena.

‘We took a rotating spin as a model system,’ Kofler expounds via email. ‘There is a condition which all classical theories have to obey, called the Leggett-Garg inequality, but which can be violated by quantum mechanics.’

Kofler and Brukner demonstrated that the time evolution of a quantum system, no matter how macroscopic the system is, cannot be treated in a classical sense. ‘Just because something is big doesn’t mean it can be described by classical physics.’ Then referring back to the case of spin, he continues: ‘Arbitrarily large spins can still have a quantum time evolution and violate the Leggett-Garg inequality.’

Next, the two realized that coarse-grained measurements are used in realistic conditions, such as situations that we are confronted with every day, as the resolution of the apparatuses usually is limited. ‘If you are bound to restrict yourself to coarse-grained measurements of the spin,’ Kofler explains, ‘you get the classical Newtonian laws of motion.’

‘Start with a spin system of macroscopic size and the Schrödinger equation that produces the quantum time evolution. Restrict the precision of your measurements and you can see the Newtonian physics emerging.’ Kofler explains that measurements in quantum mechanics generally change the system. ‘But under our coarse measurements this change is such that a classical description is possible.’ “

Read the rest here.

Author: Nick Knisely

Episcopal bishop, dad, astronomer, erstwhile dancer...