I cannot skip a paper that talks about making wormholes. Even if they are fake wormholes. And even if, after reading the paper, I’m not sure that the researchers have made fake wormholes, I simply can’t not tell you about it.

The paper in question makes use of something called transformation optics to create a wormhole for sound waves. So let’s break that down.

Please transform my optics

Transformation optics is one of my favorite things (right after raindrops on roses and definitely replacing all mention of kittens). Designing optics is hard. You often know what you want an optical system to do, but figuring out how to achieve that is a matter of experience combined with trial and error.

Transformation optics makes use of the mathematical similarity between optics and general relativity. It turns an optical problem into trajectories through space. Simply bend and stretch space to get your trajectories right. Bending and stretching in space correspond to changes in refractive index in optics. It is a relatively simple matter to return to the real world and… Um, yes, at this point, you run into some problems.

In our mathematical space-time, we can bend and stretch space however we like. However, finding corresponding materials with the right refractive indices can be problematic—they simply may not exist. Or the refractive indices may have impossible values. So, yeah, it works, but transformation optics doesn’t solve all your problems.

Using waves to mimic space

The flip side of transformation optics is that the behavior of a wave going through an optical (or mechanical) system is the same as an object traveling through curved space. It is therefore possible to study impossible objects like black holes, white holes, and, as in this case, wormholes.

There, we run into the same problem: a wormhole requires the fabrication of some pretty exotic light-bending materials. But that's in part because we would normally study this on a flat surface with refractive index variations. To get around this, the researchers cheated.

Basically, they created a curved 2D surface in a 3D world. What does that mean? Well every picture you’ve ever seen of a wormhole? They 3D printed that. So, you have two plastic plates with holes in them, joined together by a short length of pipe. Now smooth out the edges so that the plates and the pipes flow together, and you have the wormhole.

How does that work? Consider a sound wave on the top plate moving toward the hole. Since everything is nice and smooth, the wave simply travels down the hole and emerges at the other side. At the other side, the wave travels off in all directions. Now, if we only consider a 2D top view, it looks like a considerable amount of the sound wave simply disappears from sight. Likewise, from the bottom side, the hole appears to be a weirdly shaped source of acoustic waves (called a caustic).

This sounds trivial… because it is. Experimentally, this is something that I would expect nearly any undergraduate to be able to do. But that doesn’t make it wrong. The sound waves do behave exactly as you would expect matter to if it approached and went through a wormhole.

Physics by analogy still isn’t physics

This experiment is what is called physics by analogy, which means it's time for you to get my regularly scheduled broadcast on the dangers of physics by analogy. The idea is that since we can’t find a wormhole to observe, and we can’t make a wormhole, we should look for an analogy and study that.

The analogy is a mathematical one: wave propagation and space-time have similar (but not identical) equations. Therefore, we can learn a lot by studying optics or acoustics experiments that mimic black holes or wormholes or anything else.

But this is, to put it simply, silly. More generously, it isn't anything more dramatic than a shortcut for hard computations. You cannot discover any new physics this way because you're not studying an actual wormhole. All you can do is explore what the existing equations can tell us about nature. That may or may not be faster than programming a computer to do the same thing. And it certainly can’t tell you if the equations are right or not.

Which leaves us with the only best reason for doing this sort of experiment: it is absolutely fun and visualizes some very complicated phenomena in a very simple way.

Physical Review Letters, 2018, DOI: 10.1103/PhysRevLett.121.234301 (About DOIs)