Cats in Physics

By Phil Brown on September 15, 2017

This week the Ig Nobel Prizes were awarded; for achievements that first make you laugh, then make you think. The Ig Nobel prize for physics was awarded to a study On the Rheology of Cats, but these feline house guests have a more entangled history with physics than that, as JPhys+ would like to show.

Schrodinger’s Cat

Famously, Erwin Schrodinger’s thought experiment entangled the life of a cat with a spin 1/2 system. This highlighted the layered nature of reality that quantum mechanics can sometimes suggest between the microscopic world: spin 1/2 particle and the mesoscopic world: cat.

In her very recent Letter with Journal of Physics A: Mathematical and Theoretical, Margaret Reid from Swinburne University of Technology discusses the current state of that famous thought experiment and reports on the study of ‘cat-states’.

We examine Einstein–Podolsky–Rosen’s (EPR) steering nonlocality for two realisable Schrödinger cat-type states where a meso/macroscopic system (called the ‘cat’-system) is entangled with a microscopic spin-1/2 system. We follow EPR’s argument and derive the predictions for ‘elements of reality’ that would exist to describe the cat-system, under the assumption of EPR’s local realism. By showing that those predictions cannot be replicated by any local quantum state description of the cat-system, we demonstrate the EPR-steering of the cat-system. For large cat-systems, we find that a local hidden state model is near-satisfied, meaning that a local quantum state description exists (for the cat) whose predictions differ from those of the elements of reality by a vanishingly small amount. For such a local hidden state model, the EPR-steering of the cat vanishes, and the cat-system can be regarded as being in a mixture of ‘dead’ and ‘alive’ states despite it being entangled with the spin system.

Quantum-classical transition: predictions of the element of reality state (1, 1) for the cat-system of (2) with (from left) α = 2, 10, 100. From M D Reid 2017 J. Phys. A: Math. Theor. 50 41LT01. Copyright IOP Publishing.

Falling Cats

Even before Schrodinger’s cat, another great of physics was pondering the pets. H Hernández-Coronado from CINVESTAV and C Chryssomalakos and E Serrano-Ensástiga from Universidad Nacional Autónoma de México mention in their 2015 Journal of Physics A: Mathematical and Theoretical paper, that James Clerk Maxwell had considered the problem of falling cats and how they always seem to land on their feet. The authors of this paper go on to present a quantum description for how cats fall and how they always manage to right themselves.

We present a quantum description of the mechanism by which a free-falling cat manages to reorient itself and land on its feet, having all along zero angular momentum. Our approach is geometrical, making use of the fiber bundle structure of the cat configuration space. We show how the classical picture can be recovered, but also point out a purely quantum scenario, that ends up with a Schroedinger cat. Finally, we sketch possible applications to molecular, nuclear, and nano-systems.

Sequence of the orientation change produced by a cyclic deformation in the x1–x2 plane in shape space for the three body problem, in the quantum case. From C Chryssomalakos et al 2015 J. Phys. A: Math. Theor. 48 295301. Copyright IOP Publishing.

JPhys+ would like to thank the authors who contributed the mentioned research to Journal of Physics A: Mathematical and Theoretical. No cats were harmed.

This work is licensed under a Creative Commons Attribution 3.0 Unported License. Images and quotes taken from C Chryssomalakos et al 2015 J. Phys. A: Math. Theor. 48 295301 and M D Reid 2017 J. Phys. A: Math. Theor. 50 41LT01. © IOP Publishing, All Rights Reserved.