The article “Black holes, quantum information, and the foundations of physics,” by Steve Giddings (Physics Today, April 2013, page 30), reviews the challenges that black holes present to the foundational principles of unitarity and causality in quantum theory. The problems and paradoxes of black holes were inherent in the earliest discussions of the Hawking effect, and were brought to the attention of many physicists by Stephen Hawking himself. In reviewing the imaginative attempts, some of them quite radical, to reconcile black holes with quantum mechanics, Giddings ignores the simplest possibility of all: that because of quantum effects, a classical event horizon never forms.

The widespread belief that gravitational collapse leads inevitably to an event horizon is based on an essentially classical view of the collapsing matter and its equation of state, a view that rests, in turn, upon the assumption that quantum effects are negligible on macroscopic scales. This prejudice persists despite voluminous experimental evidence to the contrary confirming the nonlocal—but certainly not acausal—appearance of quantum phase coherence in macroscopic systems as varied as superfluids, superconductors, low-temperature atomic gases, Einstein-Podolsky-Rosen entangled photon pairs, squeezed light, and Aharonov–Bohm interference experiments. To those may be added the standard model itself, which features both quark and gluon condensates in quantum chromodynamics, and a Higgs vacuum condensate, which is uniform and coherent in space at very large distances.

The usual classical statement that local curvatures are small at the horizons of large black holes is irrelevant to the macroscopic and nonlocal phase correlations of the quantum vacuum. As a result of those nonlocal correlations, coherent vacuum stresses can grow very large in the vicinity of the apparent horizon, the smallness of the local curvature there notwithstanding. Far from violating fundamental principles, large vacuum stresses on the horizon are the result of standard one-loop calculations of quantum fluctuations in black hole spacetimes1 and are a generic feature of states that approach the ordinary Minkowski vacuum far from the black hole.2,3 Position-dependent vacuum stresses in black hole spacetimes do not violate the equivalence principle, any more than they do in the Casimir effect. Nor do such large stresses on the horizon rely on faster-than-light violations of microcausality, in contrast to the massive-remnant scenarios postulated by Giddings and illustrated in figure 4 of his article.

If quantum fluctuations and associated stress energies do become large at the locally defined apparent horizon of a forming black hole, then general arguments lead one to expect that a critical surface or phase transition should occur in the vicinity.4 At such a phase boundary layer, the energy density ρ of the squeezed vacuum can increase very rapidly. Having positive vacuum energy ρ and negative pressure p = −ρ, the interior of the “black hole” acts as a repulsive core, preventing further collapse. The resulting stable, nonsingular endpoint of complete gravitational collapse, consistent with all quantum principles, is a gravitational vacuum condensate star, or gravastar.5 It carries that name because its interior support relies on the energy of a vacuum condensate, with the same equation of state—though with a much larger magnitude—as the cosmological dark energy believed to be pervading our universe. Since ρ + 3p< 0, the only energy condition an interior vacuum condensate violates is the strong energy condition which prevents a black hole singularity from forming.

Once large quantum back-reaction effects at the horizon scale are admitted and a phase boundary layer takes the place of the event horizon, the very basis of the Hawking thermal evaporation and black hole entropy disappears. All the difficulties with black holes that arise when ≠ 0 are eliminated and there is no enormous information loss to be accounted for. Observer dependence and all contradictions with information theory or quantum cloning are removed as well.

To be sure, new physics must take place within the thin quantum boundary layer replacing the classical horizon. The physical thickness of the transition layer is of order , where RS = 2GM/c2 is the Schwarzschild radius, is the Planck length, and M is the solar mass.5 Classical general relativity receives significant corrections only in that layer. Due to the layer’s extreme thinness, distinguishing gravastars from classical black holes is astrophysically challenging, but by no means impossible, and may even be realized within this decade. Developing distinguishing predictions for astronomical observations is now an active field of research.

The important point is that an alternative exists that does not require abandoning well-established quantum principles of unitarity or microcausality. Moreover, in contrast to the rather more extreme or fanciful speculations discussed by Giddings and others over the years, this hypothesis has now surrendered itself to the normal scientific process of validation or falsification by observation and experiment.

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