A metamaterial is stuff that has been engineered to manipulate and steer electromagnetic waves in ways that cannot be reproduced in naturally occurring materials.
These materials are periodic structures built out of tiny electronic components such as split-ring capacitors and wires. Individually, these components have a mild interaction with passing em waves. But assembled into a repeating structure, they have a powerful influence on light.
There is no shortage of exotic things metamaterials can do: everything from invisibility cloaks to power transmission lines. But one of their most exciting applications is in cosmology because, believe or not, they can mimic the structure of spacetime.
It turns out that there is a close similarity between the way light is effected by the curvature of spacetime and the way it is influenced by the electromagnetic “space” inside a metamaterial. In fact, there is a formal mathematical analogy between these things. So the behaviour of photons inside a metamaterial is identical to their behaviour in space-time.
That’s handy because it allows engineers to recreate all kinds of exotic astrophysical objects in the lab. We’ve already talked about the first black hole made using a metamaterial and seen how it ought to be possible to recreate the Big Bang and even entire multiverses.
Now we have another exotic idea. One of the leading thinkers in this area is Igor Smolyaninov at the University of Maryland in College Park. Today, he shows how to create quantum foam inside a metamaterial.
First, a quick backgrounder about quantum foam. Nobody is quite sure what laws of physics govern spacetime on the smallest scale, that’s over the Planck length of about 10^-35 metres. However, our best guess is that quantum mechanics must somehow prevail. And if that’s the case then Heisenberg’s uncertainty principle must play an important role.
This principle implies that to discover anything about a region of space on that scale, we would have to use energies so high that they would create a black hole. (That’s why it doesn’t make sense to think of anything smaller.)
Now, because these black holes can exist, quantum mechanics suggests that they do exist, constantly leaping in and out of existence at the Planck scale.
These “virtual black holes” give spacetime a certain strange structure at the Planck scale. For want of a better word, physicists call it quantum foam.
So what’s this got to do with metamaterials? Smolyaninov points out that metamaterials are only transparent for photons of a specific wavelength when their dielectric permittivity is engineered to be below some critical value.
Should it rise above this value, the material would suddenly become opaque.
So his idea is to create a metamaterial in which the dielectric permittivity is just blow this critical value. Then any thermal fluctuations inside the material ought to raise the permittivity, making the material opaque in that region.
So any photons caught in that region will be trapped. “They experience total internal reflection at any incidence angle,” says Smolyaninov.
That region is therefore an analogue of a black hole. And the fact that these black holes will spring in and out of existence as the temperature naturally fluctuates means that the metamaterial behaves like quantum foam.
But the best thing is that this quantum foam effect ought to be straightforward to see. Smolyaninov says there are well known systems that sit at this critical juncture between transparency and opacity. He points in particular to a mixture of aniline and cyclohexane which is immiscible below 35 degrees C. Above this temperature, however, the liquids happily mix, creating regions with differing permittivity.
The interesting effect occurs in the layer between them as they mix, which becomes entirely opaque at the critical temperature. But because of thermal fluctuations, small regions are constantly flickering in and out of opacity, trapping and releasing light in the process. “This behaviour is rather similar to the behaviour of actual physical spacetime on the Planck scale,” says Smolyaninov.
In other words, at the critical temperature this stuff is analogous to quantum foam.
Smolyninov hasn’t actually done this experiment but there’s nothing about it that seems particularly tricky. You could do it in an ordinary flask or test tube. In fact, he ends his paper saying: “This effect appears to be large and easy to observe.”
Which means that sometime soon, physicists will have their own version of quantum foam to play with in the lab.
Ref: arxiv.org/abs/1101.4625 Virtual Black Holes in Hyperbolic Metamaterials