Hawking radiation is one of the best-known physical processes in astronomy. Hawking radiation causes a black hole’s mass and energy to escape over time. It’s a brilliant theory, and it says that black holes have a finite lifespan. If the Hawking radiation is true. Because as famous as it is, Hawking radiation is unproven. The theory is not even theoretically proven.

Hawking proposed the idea of black hole radiation while exploring ways to integrate Einstein’s classical theory of gravity with the quantum theory of atoms and light. We don’t have a full quantum theory of gravity, so Hawking used a semiclassical approach, treating matter as quantum while gravity is treated classically. From this, Hawking showed that quantum fields can escape the event horizon of a black hole.

The Hawking process is typically represented by the production of virtual pairs. One approach in quantum physics assumes that pairs of particles can spontaneously appear and disappear in the vacuum of empty space due to Heisenberg’s uncertainty principle. The “fuzziness” of quantum theory means that space can never really be empty. In empty regions of space these particles have no real effect and are therefore virtual particles. But near a black hole’s event horizon, one member of a virtual pair could be trapped by the black hole while the other could escape as real radiation.

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The virtual particle visualization is appealing, but not without problems. Hawking’s approach can lead to things like the firewall paradox, where the region near a black hole’s event horizon should be both empty and full of realized virtual particles. Without a full quantum approach to gravity, we cannot easily resolve these paradoxes.

Hawking radiation near an event horizon. Photo credit: NAU.

However, there are semi-classical approaches to gravity other than that used by Hawking. Most of them also predict that black holes will radiate but argue with a different approach. For example, one approach is to view the matter trapped in a black hole as a quantum wave function bound by strong gravity. Since the gravitational pull of a black hole at the event horizon is never infinite, the wave function is essentially bound to a finite bin. Through a process called quantum tunneling, quantum objects can escape any finite container in time. So you get radiation from black holes without the need for virtual particles.

That’s where a new study comes in. For this work, the team investigated a different formulation of Hawking radiation that is somewhat similar to the wave function approach. They found that a black hole’s event horizon is nothing special in terms of Hawking radiation. Any lumped mass, from neutron stars to pet rocks, has a gravitational well that acts like a finite container. So quantum particles can always escape. This has long been known, but the team showed that when expressed in terms of Hawking’s virtual particles, virtual particles can become real near any mass, not just black holes. Black holes are by far the most effective producers of Hawking radiation, but if you wait long enough, even your favorite rock will radiate its mass.

This model does not change our understanding of black holes, but it could have significant implications for long-term cosmology. If given enough time everything disappears into a cloud of radiation, the universe will disappear into a cold sea of radiation.

**Reference:** Wondrak, Michael F., Walter D. van Suijlekom, and Heino Falcke. “Gravitational pair production and evaporation from black holes.” Physical Review Letters 130.22 (2023): 221502.

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