Astronomers have observed the effects of a rapidly spinning black hole twisting the motion of matter around it, a result widely interpreted by cosmologists as consistent with a century-old prediction of Einstein’s general theory of relativity. The finding is a striking achievement for modern physics. Yet philosopher Michael Kuznets argues that it also exposes a deeper tension at the heart of science itself: our most successful theories may excel at prediction while remaining silent about what the world is fundamentally made of.

Recent observations suggesting that a rotating black hole drags spacetime around with it have been widely hailed as another decisive confirmation of Einstein’s general theory of relativity. The phenomenon, known as frame dragging and more formally as the Lense–Thirring effect, was first predicted in 1918 as a consequence of Einstein’s equations for rotating masses. That it now appears to have been detected in one of the most extreme environments in the universe is, by any reasonable standard, a remarkable scientific achievement.

Astronomers using NASA’s Swift observatory together with the Very Large Array radio telescope have observed the disrupted orbit of a star as it interacted with a rapidly spinning black hole. The star’s motion exhibited a subtle but persistent wobble, consistent with the prediction that the black hole’s rotation twists the surrounding spacetime and alters the trajectories of nearby matter. This behaviour is precisely what general relativity predicts should occur if spacetime itself responds dynamically to mass and motion, and the observations provide some of the clearest evidence yet that this effect is real.

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We can point out that despite the theory’s success, we still lack clarity about what spacetime is supposed to be, or whether it should be regarded as a fundamental constituent of reality at all.

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It is, in this sense, another genuine success for the physics community. The result reflects decades of theoretical development, observational ingenuity, and collective effort, and it further reinforces the reliability of some of our most sophisticated mathematical models of the universe. It is therefore understandable that the discovery has been widely celebrated as yet another vindication of Einstein’s theory.

The philosophical significance of the result, however, is less straightforward than the headlines suggest. The central question it raises is not whether general relativity works, but what its success actually tells us. Put simply, the question is whether getting predictions right also tells us what the world is really made of. This distinction between ontological insight into what things actually exist and predictive utility, the predictive usefulness, of a theory is not a peripheral issue, it lies at the heart of how explanation operates in modern physics.

Some scientific realists take predictive success to be enough. On this view, the fact that general relativity continues to deliver correct predictions, even in extreme regimes, is strong evidence that its core claims about spacetime are at least approximately true. But some find this kind of conclusion premature. We can point out that despite the theory’s success, we still lack clarity about what spacetime is supposed to be, or whether it should be regarded as a fundamental constituent of reality at all. Just because we can explain how it will behave might not be enough to be sure we know what it is.

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The theory tells us how to model the behaviour of systems, not how to read its mathematical structures as a literal inventory of what exists.

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