I am occasionally asked that question by friends and acquaintances. I am referring here both to professional scientists, typically working in fields other than mine (I happen to have a lot of friends in this category, through my work), as well as to individuals who are not themselves scientists, but have some interest in science and try to keep reasonably abreast with the latest discoveries and developments.
“Has it been confirmed ?”… “Has the supersolid phase of matter really been seen in solid helium experiments ?”… “Has the controversy been finally settled ?”…
My answer, these days, is along the lines “Well, things are not so clear, yet… I think it would be fair to say that we do not really know, at this time”.
Of course, this answer often bewilders non-scientists. How is it possible that, almost eight years after E. Kim and M. H. W. Chan reported the observation of what appeared to be almost incontrovertible evidence of that highly intriguing, elusive phase of matter (see this old post of mine for a very simple introduction to this subject), the community would still be divided on the interpretation of the experiment ?
Does it not feature all the expected hallmarks of the phenomenon that one is trying to observe ? Has it not been independently verified by other groups ? Have theorists not come up with quantitative descriptions, accurately matching the laboratory observations ?
Well… yes and no. The fact is, things are a bit stalled, right now.
I belong to the (growing) skeptics party (see also, for instance, here), in that it is my opinion that what has been observed experimentally does not prove beyond reasonable doubt that solid helium has a supersolid phase ; naturally, there are differing views as well, but I also happen to think that this controversy may remain unresolved, and the experimental findings unexplained possibly for many years, for reasons that I describe below.
Because this is an interesting example of how science actually works, beyond the idealized, stereotypical image that the news and entertainment industry routinely perpetuate, I think it is worth writing about it .
The smoking gun
The bulk, arguably the totality of the evidence produced thus far in support of the supersolid phase in helium, is obtained through a single experiment, making use of a device known as torsional oscillator (TO).
The technique has been extensively utilized for decades, to study quantitatively the superfluid response of liquid helium-four.
But, while liquid helium-four becomes essentially entirely superfluid at low temperature (of the order of two degrees above absolute zero) for the solid the “smoking gun”, namely the amount of superfluid decoupling, is relatively small (~ 1% or less). The feebleness of the signal constitutes a major problem, in that it becomes rather difficult to extract it from the background, and it is also possible for any “spurious” effect, unrelated to what one is trying to observe, to produce a deceptively similar signal.
Now, whenever carrying out a laboratory experiment, one can be guaranteed of one thing: a lot of “weird” stuff will happen, even if one is repeating a tried-and-true experiment. Most of this weird stuff can be safely dismissed as unimportant, a “fluke”, for the simple reason that it will not show up consistently in the same way each time the experiment is performed — in fact, it may not show up at all. This has generally little or nothing to do with the ability of experimenter, and a lot more to do with fact that the world has its own funny way of behaving, regardless of how much humans may wish otherwise. Bottom lime, the only experimental evidence that is regarded as “legitimate”, is that which can be reproduced independently by others, upon specifying the proper physical thermodynamic conditions (pressure, temperature, sample purity, etc.).
Has the original observation of Kim and Chan been reproduced independently ? Yes, but… there are quantitative differences between various experiments, e.g., the effect does not always occur at the same temperature and does not always have the same magnitude. Moreover, it appears to be affected by experimental details of the apparatus (i.e., the torsional oscillator). So, the desired reproducibility of the phenomenon has still not been completely achieved, which of course raises the suspicion that it may not be a genuine, universal physical effect, but rather something spurious, entirely unrelated to supersolid behaviour.
If to that one adds the occurrence of other puzzling, essentially unexplained phenomena concomitantly with the putative superfluid transition of the helium crystal (such as its stiffening), one ends up with a complicated scenario, which in the absence of further experiments, leaves room for different interpretations.
And indeed, the different, mostly conflicting interpretations of the existing, incomplete, puzzling and ambiguous phenomenology, are what one hears these days at any conference or session on the subject of supersolid helium. Most of these interpretations actually aim at offering a description of what is observed in the laboratory without invoking a superfluid transition of solid helium; in fact, it is my distinct impression that at this time, most of the speakers and attendees do not really believe anymore that helium is supersolid, and that alternative, less exotic and more mundane explanations for what is observed in the torsional oscillator experiments are likely to be correct.
What next ?
Physics is an experimental science.
This means that progress can only be driven by experiment, and the present state of supersolid helium research offers a reminder of that. Only new experiments, either of the same type as the one already performed (perhaps with superior technology), or of different types, can shed further light on the enigma, rule out some present theoretical scenarios in favor of others, or even lead to the formulation of entirely new theories.
But in my opinion there are reasons to believe that these experiments will not be carried out any time soon, and we may not get to the bottom of may this, possibly for decades .
This is because physics, research in general, is also a human activity, and as such subjected to the same dynamics of any other human activity. It is costly, and it is subjected to fashions and fads.
Funding for basic research is hard to come by, these days, and the general area of superfluidity, within which supersolid research falls, has long been regarded as “mature” . That means that the number of junior researchers who are likely to become involved in it is small, and that in turn means that, as the current generation of players retire without being replaced by younger colleagues, not much may be happening on the problem of supersolid helium for many, many years.
And it is possible that, say, in twenty years, when and if someone else decides to perform the same experiment using a more sophisticated apparatus or novel experimental methodology, something completely different (or nothing at all), is observed, and the anomaly observed by Kim and Chan (and others) ends up being filed in the archives of science under “unsolved mysteries”.
It’s all good anyway…
I would not want for this to end on a sour note though, because, albeit the enigma may not have been unraveled, a lot of interesting science has been produced in the process, as it always happens. Significant theoretical and experimental progress has been made toward the understanding of the supersolid phase, some myths and originally held beliefs have been debunked, and novel insight in the solid state of matter has been achieved. This is not unlike what happened, for example, with high-temperature superconductivity (HTS), which generated a huge research industry in the late 80s and early 90s — though in that case too, the mystery of HTS was never solved, it would be absurd to maintain that all of that effort was for naught (of course, some say that, but one thing one quickly learns in life is, no matter how absurd a statement, someone is bound to make it — it is part of what makes life interesting).
And that is what makes scientific research a worthwhile endeavour regardless of whether a given problem is “solved” or not.
 I do not in fact believe that any such phase exists anywhere in the phase diagram of helium, nor in that of any other known, naturally occurring substance. Here is a more
boring technical analysis of the present experimental and theoretical understanding. Of course, it is biased, because, for lack of what the community would regard as conclusive experimental evidence, each of the investigators tend to focus on a subset of experimental facts and/or theoretical ideas which seem to make the most sense.
 Let me make immediately one thing clear: the “controversy”, “disagreement” here is a genuine difference of opinion among scientists, over what appears to be puzzling, conflicting experimental evidence. There is no doubt in my mind that every single scientist who has contributed in recent times to either the experimental or the theoretical research on the supersolid phase of helium, is a competent professional of the utmost integrity.
There is no question that every experimental datum that has been published, corresponds to what the authors observed under controlled laboratory conditions, and recorded as accurately and as faithfully as they could. The fact that the outcome of the same experiment carried out by different groups is sometimes different, only points to delicate, subtle, poorly understood aspects of the experiment, that render its full, independent reproduction under controlled conditions a tricky proposition. However, data have been communicated and shared openly, scientists have spent time visiting the labs of colleagues working on the same subjects, collaborative work has ensued — exactly as it should be all the time. Never has there been any suspicion, or impression, that any of the leading investigators might be less than forthcoming in disclosing in full every relevant details of any experiment carried out. This has made it for me both a pleasure and an honor to be involved in this effort.
 Indeed, when Kim and Chan carried out their experiment in 2003, supersolid helium research was stalled for over a decade. And it is not something that happens in physics only, subjects in science do go through periods of hibernation, due to lack of funding but most often simply for falling out of fashion.
 Now, of course statements to the effect that anything is “mature” must be taken with a grain of salt. When there is the widespread belief in the community that a subject is “mature”, it does not necessarily mean that everything that was there to be understood has indeed been understood, and that there is nothing more to discover. It may simply mean that sufficiently influential members or groups, within the community, are trying to push it toward other subjects, typically their own favorite ones.