The systematic analysis of citations can reveal more than what journals and scientists are the most impactful. A case in point: Thomson Reuters has just released its citation-based study of the "hottest research fronts" of 2014.

The metaphorical phrase evokes, in my mind at least, a wildfire advancing through a drought-stricken forest. Insofar as hot fields consume research dollars, the metaphor is an apt one. Drama aside, because the Thomson Reuters study is both comprehensive and neutral in its evaluation of hot fields, its conclusions are worth examining.

The study identified 9700 research fronts on which scientists work the same distinct set of related problems. Those fronts were then assigned to one of 10 broad disciplines, such as clinical medicine, geoscience, and physics.

Some physics fronts, such as density functional theory, continue to generate citations decades after they were first opened; they might be termed "warm." A hot front is one whose core papers not only beget lots of citations but are also recent. You can find the precise definition of hotness that Thomson Reuters used in the study's introduction. Roughly speaking, a top-10 physics front in 2014 ended up being one whose core papers, published no earlier than 2011, had already generated about 2000 citations.

The extent to which the top 10, reproduced above, surprises you could be a matter of perspective. As someone whose job entails following research in physics and its close relatives, I'm familiar with all but one of the hottest fronts, the exception being nonlinear massive gravity. Thanks to an informative blog post by Sabine Hossenfelder, I'm up to speed. In the 2013 post she introduced the topic in the following way:

Massive gravity, a modification of general relativity in which gravitons have mass, has an interesting history. Massive gravity was long believed to be internally inconsistent, but physicists at Stockholm University now claim to have constructed a consistent theory for massive gravity. This theory is a viable alternative to general relativity and can address some of its problems.

Hossenfelder went on to describe the field's history and its newfound excitement. "This is presently a very active area of research and one that I’m sure we’ll hear more about," she concluded.

The number 1 hottest research front, the Higgs boson, shares more than you might think with nonlinear massive gravity. The two core papers in the table are doubtless the discovery papers from the ATLAS and CMS teams. When you search arXiv for Higgs papers published in 2012 and later, the vast majority turn out to be theoretical, not experimental. One path to hotness, it seems, is to inspire theorists.

The ability to inspire theorists likely accounts for some of the hotness of the remaining eight fronts in the Thomson Reuters table. Another source of hotness could be a relatively low barrier to performing some kinds of experiments. Searching arXiv for silicene papers reveals papers whose titles and low numbers of authors suggest modest tabletop apparatus.

If Thomson Reuters had evaluated hotness in the late 1990s, blue light-emitting diodes could well have emerged as a hot research front. At a recent public lecture in Washington, DC, one of the developers of blue LEDs, Shuji Nakamura, recounted attending a meeting of the Japan Society of Applied Physics in the early 1990s. At that time a team from 3M had coaxed zinc selenide to emit blue-green laser light at 77 K. Talks about zinc selenide and its II–VI relatives occupied the weeklong duration of the conference, Nakamura recalled. By contrast, just one lightly attended session was devoted to the topic, blue LEDs made from gallium nitride, that would earn him a share of the 2014 Nobel Prize in Physics.

Hotness, as defined by Thomson Reuters, measures the research community's validation of a topic. Because hotness is defined retrospectively, it cannot identify important topics before a core paper is written and later cited. Blue LEDs were not hot in 1993 when Nakamura published a paper describing a tiny heterostructure that shone as brightly as a candle.