LIGO's detection of gravitational waves came almost exactly a century after Einstein had formulated his general theory of relativity and an ensuing paper mathematically describing the possibility of gravitational waves. Or at least that's the story as it was presented to the public (including by yours truly). And in some ways, it's even true.

But the reality of how relativity progressed to the point where people accepted that gravitational waves are likely to exist and could possibly be detected is considerably more complicated than the simple narrative described above. In this week's Nature Astronomy, a group of science historians lays out the full details of how we got from the dawn of relativity to the building of LIGO. And, in the process, the historians show that ideas about scientific revolutions bringing about a sudden, radical shift may sometimes miss the point.

Has your paradigm shifted?

The popular conception of scientific revolutions (to the extent that it exists) was shaped by Thomas Kuhn. Kuhn described a process where data gradually pushes an existing theory into crisis, allowing nearly everyone to see it doesn't work. After a period of crisis, a revolution takes place and a new theory emerges. The theory's ability to solve all the problems that precipitated the crisis quickly draws support, and a new period of theory-driven—in Kuhn's language, "paradigm-driven"—science begins.

On some level, this lines up nicely with the story of relativity. Einstein's proposals did create a new paradigm of curved space-time, and they resolved a number of problems with Newtonian gravity and rapidly picked up experimental support and acceptance. And less than a year went by before Einstein published a paper that used the new paradigm to produce gravitational waves. The paper was wrong, but he published a more correct version a few years later. The revolution that set the stage for discovery a century later was complete.

But the historians (Alexander Blum, Roberto Lalli, and Jürgen Renn) would like to rain on that particular paradigm-shifting parade. And they do so by focusing on the aftermath of Einstein's formulation of general relativity.

(It's worth noting that the lead-up doesn't support a Kuhn-like model of revolution, either. People were happily working within the Newtonian framework even after problems with it had become apparent, and there was no clear crisis period even after Einstein had come up with special relativity. In some ways, general relativity was only solving a problem that Einstein himself had created by introducing special relativity.)

Do the wave

For starters, gravitational waves didn't emerge neatly from Einstein's work on them. Einstein didn't seem to have given serious consideration to their existence until prompted to by Karl Schwarzschild (of radius fame). His first paper on them contained a math error, and his corrected version only worked if the waves propagated along a cylinder. While this paper serves as the basis for claims that Einstein predicted the existence of gravitational waves, it was clearly an approximate solution in a simplified environment.

And that's where Einstein left it. His attention shifted to trying to unify his version of gravity with electromagnetism. There wasn't really much of a community to pick up the issue at the time, as quantum mechanics was gaining traction, and a World War disrupted the scientific community and refocused it on applied physics. According to the historians, most of the work in relativity that went on during this period was focused on translating existing, well-described physical systems from Newtonian mechanics to relativity. There was far less focus on trying to determine what unique insights relativity provided about the Universe.

The lack of a strong understanding of what relativity meant also amplified problems like Einstein's math error. When math produced nonsensical things like singularities, it wasn't clear what that was telling us. Did these mathematical abstractions have no basis in reality? Would another mathematical approach produce a sensible solution? Or was relativity simply limited in what it could successfully describe? In the absence of a robust understanding of the theory, it could be hard to tell which of these was most likely.

So, while gravitational waves could pop out of the right equation, there wasn't a strong interest in determining whether they necessarily had to exist, much less how we might detect them.

Breakout

How did the field break out of these doldrums? The 1950s saw strong government support for physics in the wake of its success during the war, which expanded the community of researchers. In addition, it was becoming increasingly clear that gravitational effects would be needed to understand our expanding body of data describing the Universe and its evolution.

The relativity community also latched on to the increasing internationalization of science, and it organized annual conferences for the entire field. A consensus emerged from the growing community: the remaining physical issues with relativity had to be worked out if any individual research group was going to have confidence in the work it was doing. Working out relativity's problems was also viewed as a necessary precondition for unifying it with quantum mechanics, a problem people were interested in working on.

The existence of gravitational waves was one of these problems, and so it started to attract attention. A key breakthrough came at a conference where researchers (including Richard Feynman) recognized how the energy contained in gravitational waves could be exchanged with better-understood forms of energy in the rest of the Universe. Another researcher figured out how to take math that could describe electromagnetic waves and modify it to describe gravitational waves. The resulting mathematical structure was the genesis of the view that gravitational waves are ripples in space-time, a perspective that's survived to the present.

Revolting

These and advances in other areas of relativity put it on a firm theoretical footing. Blum, Lalli, and Renn argue that, back when general relativity was first put forward, people thought in terms of the consequences of relativity for the other theories they used to understand the Universe. By the early 1960s, the historians argue, relativity could be appreciated as having direct consequences for the behavior of the Universe—no other theory needed. This set the stage for a confidence that gravitational waves, as a necessary consequence of the theory, had to have some physical manifestation.

This understanding was also needed to build the models that told us what gravitational waves should look like, based on the events that created them. These let us pull real events out of the noise as soon as we had a detector like LIGO with sufficient sensitivity to pick them up.

This 40-year process doesn't line up well with the revolutions that Kuhn had described. There was no crisis and no period of frantic research as people scrambled to produce a new theory that could resolve apparent contradictions in the failed one. But the historians argue there's one thing here that Kuhn got right: people who thoroughly inhabit a relativistic world have a fundamentally different view of the Universe and would have trouble communicating their perspective to someone in the Newtonian world.

Kuhn viewed this as fundamentally a language problem; old terms took on new meanings under the new paradigm. But the historians seem to suggest that the change in perspective is necessary for any sort of scientific progress. Unless people can inhabit the reality of a new theory and appreciate all its consequences, it's difficult for them to resolve the implications of the theory sufficiently to make predictions—changes in language are just a byproduct.

Nature Astronomy, 2018. DOI: 10.1038/s41550-018-0472-6 (About DOIs).