You write about the explosion of physics-related papers appearing in publications other than those identified specifically as “physics journals.” What does this tell us about the growth of the discipline, and how does what you've learned about physics apply to the expanding interdisciplinary nature of other areas of science?
Until a few decades ago, it was very easy to identify a physics paper: If a paper was published in a physics journal, it was a physics paper, otherwise it was not. Our study, using network science, shows that those times are long gone. Physicists publish increasingly in multidisciplinary journals and in fields that are not traditionally seen as physics, such as biology, medicine, and the social sciences. When a paper is published in a physics journal, we call it a “core” physics paper; we identify a paper as “interdisciplinary” physics by its citation patterns—a paper published in a nonphysics journal is interdisciplinary physics if it both cites and is cited by many more physics papers than expected. A good example is the first paper on chaos theory, by meteorologist Edward Lorenz, which explored whether we can forecast the weather. It appeared in the Journal of the Atmospheric Sciences. Looking at the evolution of the citation network—that is, changes in how and which papers cite other papers over time—we find that interdisciplinary physics is now larger than core physics itself.
Overall, the growth of science is exponential—every 19 years, the whole scientific literature doubles, which means we’ve advanced from 3,000 new papers a year in 1900 to more than 200,000 papers today. This is the case for physics, and it is probably the case for most other disciplines. Although we have not done so yet, our method of citation network analysis can be applied to any discipline. In fact, we expect that many disciplines are becoming increasingly interdisciplinary. Researchers are becoming more and more specialized but at the same time rely on other specialists to solve complex problems at the intersection of different fields. With emerging fields such as biological physics or network science, boundaries are eroding, and it is common to see research teams comprising experts from multiple areas.
What has the general public, as well as the research community, gained by the broadening nature of scientific disciplines?
There are several reasons that researchers are departing from traditional topics in their fields. In the case of physics, the availability of Big Data sets in many areas of science, from online social networks to gene sequences, is incredibly attractive to physicists, who can apply their well-developed models and data analysis methods to new problems. Second, traditional subfields of physics, such as nuclear physics, are stagnating, perhaps because the big questions in these fields have already been answered. All in all, researchers from different disciplines teaming up is a huge gain for the public, as only a multidisciplinary approach can solve today’s complex problems. For example, to address climate change we need an understanding of both the technical, physical processes and solutions that bring it about and the socioeconomic dynamics between nations to ensure that everybody is pulling on the same rope.
For researchers—and science in general—the value of increased interdisciplinary collaboration means more diversity and a faster mixing of ideas, leading to more efficient solutions and greater technological progress. Our paper dispels the myth that disciplines have strict boundaries and shows how network data, such as citations, can provide novel insights by quantifying how interdisciplinary or “insular” a discipline or corpus of literature is. This can have important consequences in developing the best strategies for knowing what research questions to explore next and finding out what fields should team up or be funded to advance society most.
In your paper you note that “the wider physics literature represents around 22 percent of all scientific literature since the 1980s.” What does this tell us about role of physics in science overall and the impact of physics on our lives, including how that impact has changed over the years?
After World War II, physics became a substantial driver of science and technological progress, permitting the application of mathematics to practical problems and leading to engineering breakthroughs that have improved society in many aspects, from the laser to the microwave oven. While physics made up only around 8 percent of the scientific literature in 1910, since the 1960s its fraction has increased to 22 percent, of which 12 percent is interdisciplinary physics.