What Fermilab’s Holometer Experiment teaches us about Quantum Gravity.

Tl;dr: Nothing. It teaches us nothing. It just wasted time and money.

The Holometer experiment at Fermilab just published the results of their search for holographic space-time foam. They didn’t find any evidence for noise that could be indicative of quantum gravity.

The idea of the experiment was to find correlations in quantum gravitational fluctuations of space-time by using two very sensitive interferometers and comparing their measurements. Quantum gravitational fluctuations are exceedingly tiny, and in all existing models they are far too small to be picked up by interferometers. But the head of the experiment, Craig Hogan, argued that, if the holographic principle is valid, then the fluctuations should be large enough to be detectable by the experiment.

The holographic principle is the idea that everything that happens in a volume can be encoded on the volume’s surface. Many physicists believe that the principle is realized in nature. If that was so, it would indeed imply that fluctuations have correlations. But these correlations are not of the type that the experiment could test for. They are far too subtle to be measureable in this way.

In physics, all theories have to be expressed in form of a consistent mathematical description. Mathematical consistency is an extremely strong constraint when combined with the requirement that the theory also has to agree with all observations we already have. There is very little that can be changed in the existing theories that a) leads to new effects and b) does not spoil the compatibility with existing data. It’s not an easy job.

Hogan didn’t have a theory. It’s not that I am just grumpy – he said so himself: “It's a slight cheat because I don't have a theory,” as quoted by Michael Moyer in a 2012 Scientific American article.

For what I have extracted from Hogan’s papers on the arxiv, he tried twice to construct a theory that would capture his idea of holographic noise. The first violated Lorentz-invariance and was thus already ruled out by other data. The second violated basic properties of quantum mechanics and was thus already ruled out too. In the end he seems to have given up finding a theory. Indeed, it’s not an easy job.

Searching for a prediction based on a hunch rather than a theory makes it exceedingly unlikely that something will be found. That is because there is no proof that the effect would even be consistent with already existing data, which is difficult to achieve. But Hogan isn’t a no-one; he is head of Fermilab’s Center for Particle Astrophysics. I assume he got funding for his experiment by short-circuiting peer review. A proposal for such an experiment would never have passed peer review – it simply doesn’t live up to today’s quality standards in physics.

I wasn’t the only one perplexed about this experiment becoming reality. Hogan relates the following anecdote: “Lenny [Susskind] has an idea of how the holographic principle works, and this isn’t it. He’s pretty sure that we’re not going to see anything. We were at a conference last year, and he said that he would slit his throat if we saw this effect.” This is a quote from another Scientific American article. Oh, yes, Hogan definitely got plenty of press coverage for his idea.

Ok, so maybe I am grumpy. That’s because there are hundreds of people working on developing testable models for quantum gravitational effects, each of whom could tell you about more promising experiments than this. It’s a research area by name quantum gravity phenomenology. The whole point of quantum gravity phenomenology is to make sure that new experiments test promising ranges of parameter space, rather than just wasting money.

I might have kept my grumpiness to myself, but then the Fermilab Press release informed me that “Hogan is already putting forth a new model of holographic structure that would require similar instruments of the same sensitivity, but different configurations sensitive to the rotation of space. The Holometer, he said, will serve as a template for an entirely new field of experimental science.”

An entirely new field of experimental science, based on models that either don’t exist or are ruled out already and that, when put to test, morph into new ideas that require higher sensitivity. That scared me so much I thought somebody has to spell it out: I sincerely hope that Fermilab won’t pump any more money into this unless the idea goes through rigorous peer review. It isn’t just annoying. It’s a slap into the face of many hard-working physicists whose proposals for experiments are of much higher quality but who don’t get funding.

At the very least, if you have a model for what you test, you can rule out the model. With the Holometer you can’t even rule out anything because there is no theory and no model that would be tested with it. So what we have learned is nothing. I can only hope that at least this episode draws some attention to the necessity of having at mathematically consistent model. It’s not an easy job. But it has to be done.

The only good news here is that Lenny Susskind isn’t going to slit his throat.