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theoretical physicist; Professor, Department of Physics, University of California, Santa Barbara

Physics has always been regarded as playing out on an underlying stage of space and time. Special relativity joined these into spacetime, and general relativity taught us that this spacetime itself bends and ripples—but it has remained part of the foundations of physics. However the need to give a quantum-mechanical description of reality challenges the very notion that space and time are fundamental.

We specifically face the problem of reconciling the principles of quantum mechanics with the physics of gravity. At first, physicists believed this meant that spacetime could violently fluctuate to the point of losing meaning—though only at extremely short distances. But attempts to reconcile quantum principles with gravitational phenomena indicate a more profound challenge to the foundational role of spacetime. This comes to the fore when studying both black holes and evolution of the Universe. Spacetime structure is seemingly problematic also at very long distances. 

Quantum mechanics appears to be an inevitable aspect of physics and is remarkably resistant to modification. If quantum principles govern nature, it seems likely that spacetime arises from more fundamentally quantum structures. This is the theme that spacetime is emergent, perhaps roughly similar to the emergence of fluid behavior from the interactions of atoms. 

The problem with fundamental spacetime is even more strongly hinted at from multiple developing perspectives. Notable among these hints is the physics of black holes, where it appears that evolution that respects quantum principles must violate the classical spacetime dictum that information does not propagate faster than the speed of light. Something is apparently very wrong with the standard spacetime picture. Additional evidence mounts when one considers the large-scale structure of the Universe, given quantum principles and the presence of dark energy. Here ultimately spacetime undergoes strong quantum fluctuations at very long scales, and seems to lose meaning. More hints have come from candidate mathematical approaches to fluctuating spacetime.

The apparent need to retire classical spacetime as a fundamental concept is profound, and confronts the reality that a clear successor is not yet in sight. Different approaches to the underlying quantum framework exist; some show promise but none yet clearly resolve our decades-old conundrums in black holes and cosmology. The emergence of such a successor is likely to be a key element in the next major revolution in physics.