Tuesday, June 26, 2012

June 26, 2002: When did it start?

The second question most people ask with respect to Hendrik Schön is "When did he start faking data?"

(The first question is either "Why did he do it?" or "How could he possibly think he could get away with it?" Neither of these questions can be answered without a Vulcan mind meld.)

In our investigation committee, we did not seek to answer this question, only to pursue allegations that were brought to our attention, either directly or through Bell Labs. In fact, as the number of papers under suspicion grew from five to 25 or so, we realized we needed to close the door to new allegations, and we set an arbitrary cutoff date of June 20, 2002. We had plenty of material to work with for our report.

It turns out that a few days later we got another allegation that gave a strong hint that the faking began when Hendrik was in graduate school at the University of Konstanz, before anyone at Bell Labs knew anything about him.

To be honest, we were somewhat relieved that the new information came in after the cutoff, because investigating it would have involved us in a completely new class of materials and measurements, as well as new co-authors (obviously none from Bell Labs). We also did not have access to the digital data that was so useful for the other papers, and our authority to investigate this work would have been somewhat questionable, since we had been invited in by Bell Labs.

As we would later be told, Hendrik's thesis was rather unexceptional: lots of slow experimental work trying to accomplish a rather mundane task of introducing electrically active dopants into potential solar-cell materials. Reportedly, he was not having much success despite stead efforts, but at some point he suddenly reported dramatic success in doping the materials.

There were several different experiments on different materials with different dopants. Three figures from the papers are overlaid in this animated gif:


At least in the context of the other clear cases of copying, this looks pretty clear. Obviously not all the curves are the same, but some are, and the three figures are represented as coming from three different materials. However, the committee never examined this case in detail, so there may be important caveats.

The University of Konstanz decided in 2004 to rescind Hendrik's doctorate, deeming him "unworthy." This retroactive action struck me as rather odd. A court concluded in 2010 (amusing translation here; follow either link for a relatively recent picture) that the revocation was inappropriate, but in 2011 that decision was reversed.

In 2003, Jennifer Couzin reported in Science that "a committee at the University of Konstanz examining the work of disgraced physicist Jan Hendrik Schön found inconsistencies in several papers Schön published during his studies there, but no proof that he had deliberately manipulated data."

But I think we know he had already started down the path.

Friday, June 1, 2012

Confirming Expectations

A good experiment definitively distinguishes between alternative hypotheses. But things get tricky when a well established standard view is pitted against ill-defined alternatives.

My Focus story today describes measurements of density fluctuations in ultra-cold gases. Several experimental groups have been capturing and cooling bunches of about a thousand atoms above "atom chips" to study such things as Bose-Einstein condensates. In this case, Julien Armijo, a former member of a group at the Insitute d'Optique in Palaiseau, attributes some of the density fluctuation to quantum zero-point excitation of sound waves in the atomic cloud.

For an isolated oscillator, the signature of zero-point motion is conceptually straightforward: below a certain temperature the motions no longer decrease with temperature. What's left are the intrinsic quantum-mechanical oscillations, and the freezing temperature corresponds to the minimum quantum of energy needed to excite the oscillator. The obvious "null hypothesis" to be excluded would therefore be that the fluctuations continue to decrease toward zero with further cooling.

For the atomic cloud, however, the situation is much more subtle, because the sound waves have a continuous spectrum that extends to zero energy. This means that there are always some waves--the ones with the longest wavelength--that are excited no matter how low the temperature.

A further complication is that different wavelengths vary in their effect on the density fluctuations. A sophisticated theory says that the quantum contribution from the longest wavelengths does not add to the density fluctuations at all. In fact, this theory says that, at very long wavelengths, the fluctuations go away at zero temperature--exactly what one would expect if there were no quantum fluctuations!

As it turns out, though, the experiment measures fluctuations in individual pixels that are a few microns on a side, which corresponds to including waves with wavelengths on the same scale. The theory says that these waves will cause a measurable density fluctuation even at zero temperature.

But the same theory says that at nonzero temperatures, including shorter wavelengths will decrease the contribution of thermal excitations by exactly the same amount. The two terms get bigger as the pixels get smaller, but since they cancel anyway that doesn't affect the prediction.

This is messier than just looking for fluctuations that don't freeze out, isn't it?

It's rather difficult to choose a good null hypothesis where there are no zero-point motions. After all, everyone believes that, physically, the quantum fluctuations should be there, although different theoretical treatments may make slightly different predictions. So there is no particularly obvious way to choose a model where the quantum fluctuations are absent.

Armijo measures fluctuations that don't change with effective pixel size, just as the complete theory predicts. Of course, the measurements also agree with a theory that omits the size dependence of both the quantum and the thermal contributions. What they don't agree with, he emphasizes, is a model that includes only the thermal corrections, since these are no longer cancelled by the quantum term. It's not clear that anyone thinks this would be a credible model that needs to be excluded. (Setting Plank's constant to zero, a common way to "turn off" quantum effects, seems to make both corrections go away.)

What is clear is that the claim that this is a "direct observation of quantum phonon fluctuations" needs to be parsed quite carefully.