Condensed matter physics is so cool!

You can make materials called "spin ices" that act like universes where the usual laws of quantum electrodynamics are modified! The speed of light is much lower - and even better, the fine structure constant can be larger.

(1/n)

A "spin ice" is a crystal made of tetrahedra. In the low-energy states, in each tetrahedron 2 electron spins point out and 2 point in.

So, if you *pretend* the electron spins are an electric field, the divergence of the electric field is zero. It's just an analogy....

(2/n)

But the analogy goes deeper! As time passes, the spins move around in ways that mimic the usual laws of electricity and magnetism - but with a speed of light that's much slower: about 10 meters per second.

(3/n)

"Defects" where the 2-in 2-out rule for spins are broken act like charged particles, since the (pretend) electric field points in or out at these places.

And since these defects have extra energy, these charged particles have *mass*.

(4/n)

Unlike our universe, a spin ice also allows for magnetic monopoles, where the *magnetic* field points in or out. I don't understand how yet.

And by adjusting interactions in your spin ice (called RK and 3NN potentials), you can change the fine structure constant!

(5/n)

The fine structure constant measures the strength of interactions between charged matter and the electromagnetic field. In our universe it's about 1/137. We believe that if it were much larger the electromagnetic field would be "confined" like the strong force.

(6/n)

So, spin ices offer a playground where people can study physics in worlds with magnetic monopoles - and different choices of fine structure constant. But this work is just beginning!

The paper is here:

https://arxiv.org/abs/2009.04499 

(7/n, n = 7)