A few months back, I got a call from a writer at a physics magazine, asking for comments on a controversy within AMO physics. I read a bunch of papers, and really didn’t quite understand the problem; not so much the issue at stake, but why it was so heated. When I spoke to the writer (I’m going to avoid naming names as much as possible in this post, for obvious reasons; anyone I spoke to who reads this is welcome to self-identify in the comments), he didn’t really get it, either, and after kicking it around for a while, it failed to resolve into a story for either of us– in his case, because journalistic reports need to have a point, in my case because I didn’t have time to write up an inconclusive blog post.
This has nagged at me for a while, though, and last week at DAMOP, I took the opportunity to ask a bunch of people who seemed likely to have opinions on the subject about it. I’m not sure the result is any more conclusive than it would’ve been some months back, but I have some free time right now, so I’ll write it up this time, and see what happens. Again, I’m going to leave the details of responses a little vague, in hopes of not getting anybody other than me in trouble; I’m not a professional journalist, and didn’t solicit quotes as a reporter, so it would feel wrong to directly attribute words to specific individuals in writing it up on the blog. Ultimately, I’m giving my own take on the matter after discussion with a bunch of people who know the specifics better than I do, and this shouldn’t be taken as more than that.
The whole thing starts with a Nature paper from 2010 by Holger Müller, Achim Peters, and Steven Chu (because writing Nature papers is how Steve Chu unwinds) that sadly is not on the arxiv, because Nature. This paper re-interpreted some earlier results that used the interference of atom waves to measure the acceleration of gravity (from 1999 and 2001) as a measurement of the gravitational redshift instead. This was followed not long after by a comment from a French group including Chu’s co-Nobelist Claude Cohen-Tannoudji. This drew a response, then a counter-response, and a back-and-forth argument that got fairly heated for scientific literature, and can be traced in the citation history of the original paper. The Clash of the Nobel Laureates angle to the whole thing gave it a little media juice, and one or two of the papers sound annoyed enough to make me suspect that if either side had access to a kraken, it would’ve been released long ago.
So, what’s going on, here? The experiments at the heart of the whole business use an atom interferometer, shown schematically above in a figure taken from the arxiv version of a Phys. Rev. Letter by Mike Hohensee and the Chu group (RSS readers need to click through). The idea is that you start with a bunch of atoms at the lower left at time zero, split them in half, and launch half of them upward. Some time later, you stop the bunch that was launched, and launch the bunch that was stopped. When the two bunches get to the same position, at the upper right, you mix them together, and look at how many come out on the two possible exit paths. Since quantum mechanics tells us that atoms behave like waves, there will be an interference pattern here that depends on the “phase” of these waves, or more precisely on the difference in phase between the two.
In the absence of gravity, the two paths followed by the atoms should be more or less identical, and correspond to the straight dashed lines in the figure. When you turn gravity on, the atoms move on the curved paths instead, and you find that the phase difference between them depends on the strength of gravity; thus, you can use this to measure the strength of gravity. This was demonstrated by Kasevich and Chu back in 1991 when dinosaurs roamed the Earth and I was a callow undergrad. (Full disclosure: I worked for Kasevich as a post-doc at Yale in 1999-2001.)
That was a nice paper, and everybody was happy with it and the two better measurements in 1999 and 2001. The key to the 2010 paper by Müller et al. is a re-interpretation of the basic scheme. Rather than thinking of the atoms as waves undulating along these paths, they pointed out that you could think of the atoms as little clocks, oscillating at a frequency known as the “Compton frequency.” These clocks, according to relativity, “tick” at different rates depending on their state of motion and their position relative to the Earth. In this picture, the atoms on the upper path “tick” at a different rate than those along the lower path, and the result of this difference is a phase shift that shows up in the interference pattern. Thus, the measurements Peters and company made in 1999 and 2001 can be viewed as a measurement of the “gravitational redshift,” and a test of relativity.
That re-interpretation ruffled a lot of feathers, for reasons that were unclear to me. In the end, the simple calculation of the gravitational redshift ends up depending on exactly the same strength-of-gravity parameter g as the acceleration measurement, so it seems like just a matter of terminology. Somewhat more formally, you can cast the phase difference that you measure in the experiment mathematically as the sum of three terms, one having to do with the interaction between the lasers used to push the atoms around, and the other two having to do with the motion of the atoms. When you work it all out, it turns out that you can make the laser-interaction term exactly equal to one of the other two, and which one you pick determines whether you call it an effect of acceleration on the atoms, or an effect of gravity on the internal “clocks” of the atoms. Either way, the difference is the same, and tells you something about gravity.
My initial take on this was that the negative response was prompted by the fact that it seems kind of cheesey to get a second Nature paper out of re-analyzing ten-year-old experimental data. Had this been an entirely new experiment, I would’ve said “Hey, cool!” but since it was just a new interpretation of old results, it was more “That’s cool, but…” This doesn’t explain the vehemence of the responses, though, or the way it dragged on through a couple of years of dueling papers.
So, what was the result of my asking around? First and foremost, the dominant reaction I got when I brought this up was “Oh, God, not this mess…” Even people who went on to give strong opinions one way or the other started by rolling their eyes at the whole controversy; a few refused to talk about it at all.
Among those who went on to give opinions, the general responses can be broken into two classes: funding, and personalities. The funding argument is basically that the mostly-European groups that have objected to the reanalysis have a vested interest in keeping the interferometer from being seen as a redshift measurement, because there’s a European mission to make gravitational redshift measurements in space, and if you can do just as well on the ground, that puts millions of Euros of grant money at risk. The personality argument is basically that while the interferometry experiments are very clever, the clock/redshift interpretation is overselling them in a way that bothers some people; Cohen-Tannoudji in particular is seen as a very level-headed guy, not prone to overhyping matters, which inclines some people toward his side.
Outside and between these two camps is a third view, which I’ve moved toward after last week’s conversations, which is basically the “shut up and calculate” analogue: that whether you call it a test of redshift or an accelerometer, what you’re really doing is looking for a violation of the Equivalence Principle. The name you give it is just semantics, and what’s important is that a) the Equivalence Principle works, and b) testing it at high precision is Really Cool. The controversy has cooled off significantly, and there may be some movement toward, if not a reconciliation of the two views, than at least something along these lines.
Of course, lots of new stuff could happen. In particular, Müller and company earlier this year doubled down on the clock interpretation of the interferometry experiments with a Science paper on a “Compton Clock” (not on the arxiv, because Science). This is based on the observation that, if you tilt your head and squint, you can write the required laser frequencies for the the interferometer experiment as fractions of the Compton frequency (which is around 1025 Hz, ten billion times higher than the laser frequency, give or take). The Compton frequency depends only on the mass of the atoms, so if you view everything as a fraction of that, stabilizing the lasers based on the interferometer signal amounts to referencing your “clock” directly to the mass of the atoms.
This is only indirectly related to the earlier controversy, though it recapitulates the essential features: it’s a really cool experiment, based on a clever idea that strikes a lot of people as over-selling. Questions about the “Compton clock” got even more eye-rolling than questions about the original controversy, with a lot of people thinking it’s basically too clever for its own good.
For their part, Müller’s folks stick to their guns on this– a person I spoke to from that group insists that there aren’t any other frequency sources involved, and that the “clock” frequency they get is extremely repeatable and comes naturally out of the experiment, suggesting it really is something to do with their atoms. Unfortunately, they’re not really set up to do the test that would be most convincing: their interferometer uses cesium, which has only a single abundant isotope suitable for the experiment. As with so many other experiments in AMO physics, this would be much better if they used rubidium, which has two abundant isotopes, rubidium-85 and rubidium-87. In that case, they could run the “clock” for both, and show that the fraction they measure differs by 2/87ths, an amount that really wouldn’t show up anywhere else.
Of course, the unfortunate reality is that Müller’s lab is only set up to work with cesium, while the obvious lab to do this in rubidium is run by one of the European groups opposed to the whole idea. So I wouldn’t expect a completely dispositive resolution of this any time soon. It will probably remain a source of (low-level) controversy for a good while yet…
And that’s what people were arguing about at DAMOP last week.