The Holometer

jonah
Figure 1. A member of the holometer team works on the device. Image due to Fermilab.
Figure 1. A member of the holometer team works on the device. Image due to Fermilab.

You may have heard the buzz about the holometer, shown in figure 1, before. It’s a giant laser interferometer, much like those used to search for gravitational waves, designed to detect quantum fluctuations in the fabric of spacetime. At least, that’s the claim. The holometer just released a preprint of their first science paper. And of course,  a Fermilab press release appears in Symmetry Magazine.

The article is good, and I recommend you read it. And the holometer experiment is good, interesting science. But I have to say, I’m extremely annoyed by how much the holometer team is overselling their experiment. The scientific paper is honest, but the press surrounding the experiment really oversells it. And I blame the science team, or at least the leader of the team.

Pixelization

The headline of the symmetry magazine article is “spacetime is not pixelized!” But a more accurate title would be: “one model of quantum gravity that predicted quantum fluctuations much larger than anybody else believes has been ruled out! There are many more models not ruled out!” In particular, the model in question is a heuristic description by the head of the experiment, Craig Hogan. And as physicist Sabine Hossenfelder describes, it’s not at all convincing. Hogan also argues that there’s a connection between his model and the holographic principle, but I’ve never understood it.

Also, let me be clear that, when Hogan says “spacetime is pixelized,” he means there’s a fuzzyness due to the Heisenberg uncertainty principle. He doesn’t mean spacetime is discrete, which is a different approach to quantum gravity.

Utility as Proof-of-Concept

The holometer team also argues that their experiment is a proof-of-concept that interferometry can be used to test quantum gravity, but I’m extremely sceptical that any model, other than Hogan’s, can be tested. Sorry, I’m about to get a little technical.

The authors claim a sensitivity of ~10^{-20} m/√Hz. But the Planck scale, where we expect quantum fluctuations of spacetime, is many orders of magnitude smaller… Like 10^{-32} m/√Hz. It takes a very special model, basically Hogan’s model, to predict quantum fluctuations of spacetime on a directly measurable scale.

Other Applications?

Although I don’t think the holometer will ever be a good test of quantum gravity, it may be a useful tool for searching for other stuff. At the 2015 Midwest Relativity Meeting, I saw a talk by a member of the holometer team describing her work. She’s using the holometer data to search for so-called exotic sources of gravitational waves: things that probably don’t exist, like cosmic strings. This is good, valuable research. We don’t think this stuff is out there, and we probably won’t find anything, but it’s worth looking anyway.

Related Reading

If you enjoyed this post, you may enjoy some of these other posts.

  • In Distance Ripples, I describe how gravitational waves work.
  • In Quantum Geometry, I describe my own research in quantum gravity, called Causal Dynamical Triangulations.

Small Corrections

Thanks to Leo Stein‘s corrections, I have changed the content a little bit. Leo points out that it’s a bit unfair to compare the holometer to LIGO, because they are built to measure different frequencies. I also claimed that cross-correlating detectors is questionable, but Leo points out that the noise sources in the holometer frequency range are less likely to cross-talk between detectors as those for LIGO.