
I have a headache and blame Nature magazine for it. 
For about 15 years, I have worked on quantum gravity phenomenology, which means I study ways to experimentally test the quantum properties of space and time. Since 2007, my research area has its own conference series,
“Experimental Search for Quantum Gravity,” which took place most recently September 2016 in Frankfurt, Germany.
Extrapolating from whom I personally know, I estimate that about 150200 people currently work in this field. But I have never seen nor heard anything of Chiara Marletto and Vlatko Vedral, who just wrote a comment for
Nature magazine complaining that the research area doesn’t exist.
In their comment, titled “
Witness gravity’s quantum side in the lab,” Marletto and Vedral call for “a focused meeting bringing together the quantum and gravityphysics communities, as well as theorists and experimentalists.” Nice.
If they think such meetings are a good idea, I recommend they attend them. There’s no shortage. The above mentioned conference series is only the most regular meeting on quantum gravity phenomenology. Also the Marcel Grossmann Meeting has sessions on the topic. Indeed, I am writing this from a conference here in Trieste, which is about “
Probing the spacetime fabric: from concepts to phenomenology.”
Marletto and Vedral point out that it would be great if one could measure gravitational fields in quantum superpositions to demonstrate that gravity is quantized. They go on to lay out their own idea for such experiments, but their interest in the topic apparently didn’t go far enough to either look up the literature or actually put in the numbers.
Yes, it would be great if we could measure the gravitational field of an object in a superposition of, say, two different locations. Problem is, heavy objects – whose gravitational fields are easy to measure – decohere quickly and don’t have quantum properties. On the other hand, objects which are easy to bring into quantum superpositions are too light to measure their gravitational field.
To be clear, the challenge here is to measure the gravitational field created by the objects themselves. It is comparably easy to measure the behavior of quantum objects in the gravitational field of the Earth. That has something to do with quantum and something to do with gravity, but nothing to do with quantum gravity because the gravitational field isn’t quantized.
In their comment, Marletto and Vedral go on to propose an experiment:
“Likewise, one could envisage an experiment that uses two quantum masses. These would need to be massive enough to be detectable, perhaps nanomechanical oscillators or Bose–Einstein condensates (ultracold matter that behaves as a single superatom with quantum properties). The first mass is set in a superposition of two locations and, through gravitational interaction, generates Schrödingercat states on the gravitational field. The second mass (the quantum probe) then witnesses the ‘gravitational cat states’ brought about by the first.”
This is truly remarkable, but not because it’s such a great idea. It’s because Marletto and Vedral believe they’re the first to think about this. Of course they are not.
The idea of using Schrödingercat states, has most recently been discussed
here. I didn’t write about the paper on this blog because the experimental realization faces giant challenges and I think it won’t work. There is also Anastopolous and Hu’s
CQG paper about “Probing a Gravitational Cat State” and
a followup paper by Derakhshani, which likewise go unmentioned. I’d really like to know how Marletto and Vedral think they can improve on the previous proposals. Letting a graphic designer make a nice illustration to accompany their comment doesn’t really count much in my book.
The currently most promising attempt to probe quantum gravity indeed uses nanomechanical oscillators and
comes from the group of Markus Aspelmeyer in Vienna. I previously discussed their work
here. This group is about six orders of magnitude away from being able to measure such superpositions. The Nature comment doesn’t mention it either.
The prospects of using BoseEinstein condensates to probe quantum gravity has been discussed back and forth for two decades, but clear is that this isn’t presently the best option. The reason is simple: Even if you take the largest condensate that has been created to date – something like 10 million atoms – and you calculate the total mass, you are still way below the mass of the nanomechanical oscillators. And that’s leaving aside the difficulty of creating and sustaining the condensate.
There are some other possible gravitational effects for BoseEinstein condensates which have been investigated, but these come from violations of the equivalence principle, or rather the ambiguity of what the equivalence principle in quantum mechanics means to begin with. That’s a different story though because it’s not about measuring quantum superpositions of the gravitational field.
Besides this, there are other research directions. Paternostro and collaborators, for example, have suggested that
a quantized gravitational field can exchange entanglement between objects in a way that a classical field can’t. That too, however, is a measurement which is not presently technologically feasible. A proposal closer to experimental test is that by Belenchia
et al, laid out their
PRL about “Tests of Quantum Gravity induced nonlocality via optomechanical quantum oscillators” (which I wrote about
here).
Others look for evidence of quantum gravity in the CMB, in gravitational waves, or search for violations of the symmetries that underlie General Relativity. You can find a little summary in my blogpost “
How Can we test Quantum Gravity” or in my Nautilus essay
“What Quantum Gravity Needs Is More Experiments.”
Do Marletto and Vedral mention any of this research on quantum gravity phenomenology? No.
So, let’s take stock. Here, we have two scientists who don’t know anything about the topic they write about and who ignore the existing literature. They faintly reinvent an old idea without being aware of the wellknown difficulties, without quantifying the prospects of ever measuring it, and without giving proper credits to those who previously wrote about it. And they get published in one of the most prominent scientific journals in existence.
Wow. This takes us to a whole new level of editorial incompetence.
The worst part isn’t even that Nature magazine claims my research area doesn’t exist. No, it’s that I’m a regular reader of the magazine – or at least have been so far – and rely on their editors to keep me informed about what happens in other disciplines. For example with the comments pieces. And let us be clear that these are, for all I know, invited comments and not selected from among unsolicited submissions. So, some editor deliberately chose these authors.
Now, in this rare case when I can judge their content’s quality, I find the
Nature editors picked two people who have no idea what’s going on, who chew up 30 years old ideas, and omit relevant citations of timely contributions.
Thus, for me the worst part is that I will henceforth have to suspect
Nature’s coverage of other research areas is equally miserable as this.
Really, doing as much as
Googling “Quantum Gravity Phenomenology” is more informative than this Nature comment.