Unfortunately, the GEO600 noise vanished after a new readout method was employed by the collaboration. Hogan corrected his prediction for the noise by finding a factor in his calculation, so that the noise was no longer in the GEO600 range.
But that wasn't the end of the story. Craig Hogan is a man with a mission - a holographic mission. Last year, I mentioned that Hogan got a grant to follow his dreams and is now building his own experiment at Fermilab, the "Holometer," especially designed to detect the holographic noise. (Which might or might not have something to do with holographic foam. More likely not.)
In the February issue, Scientific American's cover story asks the questionn "Is Space Digital?" The article by Michael Moyer reports on Hogan's Holometer. I really like Scientific American. Look, we have a subscription to the PRINT version, do I need to say more? But this is the worst article I've ever read in this magazine.
To begin with, it is entirely uncritical, and one doesn't actually learn anything about the, in principle quite interesting, question whether space may be digital. One doesn't even learn why the question is interesting. But even worse is that the article is also factually wrong in several places. You can read there for example:
"[Hogan] begins by explaining how the two most successful theories of the 20th century - quantum mechanics and general relativity - cannot possibly be reconciled. At the smallest scales, both break down into gibberish."
Where to start? Moyer probably meant "quantum field theory" rather than "quantum mechanics." One might forgive that, since this simplification is often made in science journalism.
I am more puzzled that Hogan allegedly explained quantum field theory and general relativity cannot possibly be reconciled. Last time I looked there were literally thousands of people working on such a reconciliation; they would surely be interested to learn of Hogan's insight. Therefore, I doubt that this is what Hogan was saying, especially since his experiment is supposed to test for such a reconciliation. More likely, he was laying out the main difficulties in quantizing gravity. Which brings me to the next misleading statement: one might say indeed that gravity breaks down at small distances, which could mean all from the formation of singularities at high densities to the breakdown of the perturbative expansion. But it's news to me that quantum mechanics "breaks down to gibberish" at short scales.
"The Planckscale is not just small - it is the smallest."
Depends on whether you are talking about the Planck length or the Planck mass!
"The laws of quantum mechanics say that any black hole smaller than a Planck length must have less than a single quantum of energy."
The laws of quantum mechanics don't say anything about black holes. And probably neither Hogan nor Moyer have ever heard of monsters.
And then I came to this sentence:
"[P]hysicists mostly agree that the holographic principle is true"
Micheal Moyer's evidence comes from talking to Craig Hogan and Leonard Susskind. He also quotes Jacob Bekenstein, Raphael Bousso and Herman Verlinde.
I am always stunned how easily science writers lose perspective. The vast majority of physicists work in condensed matter physics, nuclear and atomic physics, solid state physics, plasma physics, optics or quantum optics, and astrophysics, half of them in experiment. The idea that space may be digital is a fringe idea of a fringe idea of a speculative subfield of a subfield. I'm not saying it's not interesting. I'm just saying if you'd actually go and ask a representative sample of physicists, I guess you'd find that most don't care about the holographic principle and wouldn't agree on any statement about it.
Anyway, the best part of Moyer's article is a quotation by Hogan about the motivations for his experiment:
"It's a slight cheat because I don't have a theory."
Indeed, if you look at the Holometer website, you find an extensive list of two articles, both unpublished, one of which scores with 25 revisions in 2 years.
Hogan is also quoted with saying
"Things have been stuck for a long time. How do you unstick things? Sometimes they get unstuck with an experiment."
That is true and exactly the reason why I am working on the phenomenology of quantum gravity! But normally, before investing money into an experiment, it is worthwhile to check if not the hypothesis that would lead to a signal in the experiment would also lead to other effects that we should already have seen. Unfortunately, this is difficult to tell without a theory! The criticism in my post from three years ago was essentially that Hogan's scenario breaks Lorentz invariance, and we know that Lorentz-invariance violation is very tightly constrained already. Maybe there is a way to avoid the already existing constraints, but I'd really like to know how.
I admit that I admire Hogan for his passion, perseverance, and also his honesty to admit that he doesn't exactly know what he's doing or why, just that he feels like it has to be done. He is the archetypal American with a hands-on, high-risk, high-gain attitude. He also looks good on the photo in the Scientific American article, is director of the Fermilab Center for Particle Astrophysics, and probably doesn't care a lot about peer review.
Of course I hope he succeeds, because I really want to see some positive evidence for quantum gravity phenomenology in my lifetime!
And hey, you know, I too have an idea for an experiment that can revolutionize our understanding of the world. And mine did even get published.