Various theoreticians have pointed out that there is a formal mathematical analogy between the way certain metamaterials bend light and the way spacetime does the same thing in general relativity. In fact, it ought to be possible to make metamaterials that mimic the behaviour of not only our own spacetime but also many others that cosmologist merely dream about.
Researchers have created a metamaterial containing many “universes” that are mathematically analogous to our own, albeit in the three dimensions rather than four. They exploited the self-assembling nature of cobalt nanoparticles suspended in kerosene.
“These regions behave as transient 2+1 dimensional Minkowski spacetimes which temporarily appear and disappear inside a larger metamaterial “multiverse”,” they say.
Cobalt is ferromagnetic so the nanoparticles tend to become aligned in a magnetic field. In fact, if the density of nanoparticles is high enough, the field causes them to line up in columns. When this happens, the nanocolumns form a metamaterial which is mathematically equivalent to a 2+1 Minkowski spacetime.
So light passing through behaves as if this region has one dimension of time, aligned with the nanocolumns, and two dimensions of space, perpendicular to the nanocolumns.
That creates a single Minkowski universe. The trick that Smolyaninov and pals have pulled off is to create a multiverse containing many Minkowski spacetimes .
The secret here is to keep the density of nanoparticles just below the threshold required to form nanocolums. That’s just over 8 per cent of the fluid by volume in this case. When that happens, natural variations in the density cause nanocolumns to form in small regions of the liquid. In effect, tiny universes are leaping in and out of existence. Smolyaninov and co can even “see” these universes by their effect on polarised light passing through the fluid.
Extraordinary light rays propagating inside a hyperbolic metamaterial look similar to particle world lines in a 2+1 dimensional Minkowski spacetime. Magnetic nanoparticles in a ferrofluid are known to form nanocolumns aligned along the magnetic field, so that a hyperbolic metamaterial may be formed at large enough nanoparticle concentration nH. Here we investigate optical properties of such a metamaterial just below nH. While on average such a metamaterial is elliptical, thermal fluctuations of nanoparticle concentration lead to transient formation of hyperbolic regions (3D Minkowski spacetimes) inside this metamaterial. Thus, thermal fluctuations in a ferrofluid look similar to creation and disappearance of individual Minkowski spacetimes (universes) in the cosmological multiverse. This theoretical picture is supported by experimental measurements of polarization-dependent optical transmission of a cobalt based ferrofluid at 1500 nm.