by Ruba Alabed

In the field of astrophysics, there exist a multitude of phenomena that perplex curious scientists and the general public alike. An obvious example are black holes; perhaps less obviously so are their time-reversed version: white holes. Formed by the quantum tunnelling of a Planckian scale near-evaporated black hole, white holes pose a possible solution to the black hole information paradox, meaning the very concept of them could revolutionise physics and hence our understanding of the vast universe we call home.

To understand how the concept of white holes was conceived, one must refer to loop quantum gravity (LQG) in order to explain what happens when Hawking radiation reduces a black hole to the Planckian scale, where standard classical and relativistic theories would have time halt and hence must be abandoned in favor of LQG. As proposed by Ref. [1], a quantum tunnelling process from a black hole to a white hole at this scale could resolve the black hole information paradox.

This transition is predicted by LQG which states that spacetime is granular, and therefore there is a lower limit to the compression of a black hole: an individual granule of spacetime. Imagining the interior of a black hole as a funnel, its tube elongating and narrowing with the passage of time (see Figure 1) and hence increasingly distorting spacetime, the tube eventually reaches the Planck scale, in which both space and time are massively altered by quantum phenomena, and so Einstein’s theory of general relativity must be replaced with LQG; regions in which this occurs are called singularities.

Disclaimer: singularities are in fact not at the ‘bottom’ of a black hole as may have been presumed, that is just the poor falling star! [2]

“The quantum properties of space and time allow the inside of the black hole to ‘leap’ beyond the singularity, when classical equations would have time stop.” [2]

Within these volumetrically miniscule regions is precisely where we start to deal with white holes; particularly, it is an issue of infinite density that would force this quantum leap to occur. As any particle that undergoes the tunnel effect, there is a specific probability, given by the equations of LQG, that the spacetime fabric of the interior of a black hole can instantaneously transition to that of a white hole. Then, each of the arrows in the image below will be flipped, indicating the (partially) opposing nature of white holes to that of black holes, as they shorten and widen with time, analogous to how throwing a ball downwards causes it to then travel upwards after the bounce, albeit with less energy, as will be discussed shortly [2]. Similarly, as nothing can escape from inside a black hole, nothing can enter a white hole, and everything within it escapes, implying that all information that previously entered the black hole is not lost, rather it’s eventually set free following that said quantum transition, solving the infamous black hole information paradox and curing many physicists of their decade-long headaches.

However, don’t be fooled – gravity still works the same! It is not as though one would be repelled by a white hole; after all it is still a mass, and the force of gravity has not been altered. Additionally, it may be tempting to believe that the exterior of the black hole turns white upon the quantum transition, but this is not true, admittedly unsatisfyingly so, as this would simplify matters astronomically; pun intended.

Being the bright, insightful scientists we are, thinking about this fact proves it to be of sound reasoning, as the exterior of a black hole is not experiencing the spacetime distortions that the interior is, and so it continues to obey the principles of general relativity, meaning no such quantum leap occurs externally. Thus, one cannot distinguish a black hole from a white one by merely looking at its exterior, hence the difficulty in verifying this theory [2].

Would this process of quantum leaping from one spacetime configuration to the other occur reversibly?

This is a valid question, but the answer is no. Referring to the ball analogy previously mentioned, one can think of Hawking radiation, the heat emitted from a black hole that causes it to evaporate and hence its horizon to shrink, in the same way as the energy dissipated when the ball hits the ground: “Heat is the mark of irreversibility. Heat distinguishes past from future. There is therefore at least one clearly irreversible aspect to the life of a black hole: the gradual shrinking of its horizon” [2] (pp. 91-92). This dissipated heat cannot be gained back by the black hole, and so its exterior cannot grow once it has already evaporated, leading us to the death of a black hole.

“Thus, black holes die by tunneling into white holes.” [2]

Finally, from this aspect of irreversibility one can infer that the death of a black hole occurs as its horizon evaporates over time via Hawking radiation, and upon transitioning, its remnants slowly diminish as information exits the white hole’s horizon, causing the once imposing ‘structure’ to ‘fade away into nothingness’, without the paradoxical loss of information [1].

With all that said, countless other theories float around the scientific community, and we are yet to unanimously agree on one; this particular theory is evidently especially intriguing to me and other much more qualified minds, as the proposed properties of white holes agree with the most reputable principles of modern physics, bringing together Einstein’s theory of General Relativity, Loop Quantum Gravity, Newton’s laws of Gravitation, and the laws of Thermodynamics. White holes also allow for one of the most haunting questions in science to be answered: ‘Can information truly be destroyed?’ Hopefully, the knowledge that such a phenomenon may exist allows for a temporary peace of mind, until the day we can definitively fill that hole in our understanding by unveiling the truth behind those mysterious dark holes in our universe.

References

[1] Bianchi E, Christodoulou M, D’Ambrosio F, Haggard HM, Rovelli C. White holes as remnants: a surprising scenario for the end of a black hole. Classical and Quantum Gravity. 2018 Oct 19;35(22):225003.

[2] Rovelli C. White Holes. Penguin; 2023.