“The final stage in the life of a star,” observes physicist Carlo Rovelli in Seven Brief Lessons on Physics, “where the quantum fluctuations of space-time balance the weight of matter, is what is known as a ‘Planck star’. If the sun were to stop burning and to form a black hole, the event horizon would measure about six kilometers in diameter. The black hole’s event horizon is not a physical barrier, but simply the boundary where the black hole’s gravitational field becomes so intense that the escape velocity exceeds the speed of light. Nothing, not even light, can escape the event horizon.
“Inside the black hole’s event horizon, the sun’s matter would continue to collapse, eventually becoming such a Planck star. Its dimensions would then be similar to those of an atom,” writes Rovelli about the strange object named after Max Planck, the German theoretical physicist, the originator of quantum theory, whose discovery of the quanta won him the Nobel Prize in Physics in 1918.
Not a Singularity
According to the Heisenberg uncertainty principle of quantum mechanics, the matter within a black hole does not collapse into a true singularity of infinitesimal volume. Instead, the immense energy density of the collapsing material can provide a quantum repulsive force well before a singularity is achieved. It has been said that the singularities of black holes are where God divides by zero (attributed to Einstein but quote came decades later). As it turns out, the central regions of black holes are where God divides by an extremely small number.
Takes the Age of the Universe to Rebound
“The entire matter of the sun condensed into the space of an atom: a Planck star should be constituted by this extreme state of matter,” writes Rovelli. “A Planck star is not stable: once compressed to the maximum, it rebounds and begins to expand again. This leads to an explosion of the black hole.”
For stellar-mass black holes, it takes roughly 14 billion years — about the age of the Universe — for the Planck star to rebound. Hence, only the oldest primordial black holes, which are still theoretical, might be expected to be rebounding.
The ‘Quantum Gravity’ Signal
“A black hole is a rebounding star seen in extreme slow motion, concludes Rovelli. “It is possible that in the furnace of the first instants of the universe black holes were formed and that some of these are now exploding. If that were true, we could perhaps observe the signals that they emit when exploding, in the form of high-energy cosmic rays coming from the sky, thereby allowing us to observe and measure a direct effect of a phenomenon governed by quantum gravity.”
Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona via Rovelli, Carlo. Seven Brief Lessons on Physics (p. 46/47). Kindle Edition.