Few literary works rival Carlo Rovelli’s captivating discourse on white holes, both enlightening and profoundly philosophical. Through his narrative, readers embark on a journey akin to an adventure story, guided by the boundless wonder of the researcher facing fundamental questions about the structure of the universe. Through skillful allusions to Dante’s Divine Comedy, readers traverse alongside the researcher, following Virgil’s guidance while delving into the depths of cosmic exploration, perhaps catching a fleeting glimpse of the divine along the way…

**Introduction **

Regardless of the designation it may ultimately adopt, the quest remains unwavering: to comprehend the intricate fabric of the universe and continually push the boundaries of human understanding. It entails mapping the uncharted territories of a world with nebulous boundaries, perpetually extending our frontiers to perceive the cosmos with unprecedented clarity. Ultimately, it is about laying the foundational groundwork of an enigmatic world, of which we are an integral part.

It’s about being pioneers, propelled by an untamed zeal for nascent intuitions awaiting empirical validation, while also remaining humble in the face of the magnitude of discovery and the apprehension of losing our way. This discovery evokes Alice in Wonderland confronting the mirror, traversing it, and emerging on the other side.

So, what transpired on that sunny day in Marseille, where Carlo was working ? What is the fate of a black hole upon its demise? And if – through a quantum leap – the black hole transforms into a white hole?

Initially deemed fanciful, here we are in 2024, a decade later, with a growing number of researchers championing this captivating concept: indeed, it holds merit. Although no white hole has been observed (and for good reason: it would be much slimmer than a hair!), it is this extraordinary odyssey, the pinnacle of a lifetime of research, to which Carlo invites us to accompany him.

**Part One of the Book **

**The First Observed Black Hole **

Decades ago, significant skepticism shrouded the concept of black holes. Yet, through rigorous calculations, a black hole was observed – or more precisely, heard – by a stroke of luck when Karl Jansky investigated disturbances in telecommunications. Constructing an antenna to intercept an elusive signal coinciding with the stellar rotation period, Jansky inadvertently detected “the radiation emitted by incandescent matter caught in a furious whirl around a giant black hole at the center of the galaxy.” An incredible discovery. Link to image: https://news.mit.edu/2022/first-supermassive-black-hole-sagitarrius-0512

Today, we witness images of these cosmic giants, emblematic of galactic centers, nearly each harboring a supermassive black hole.

**Black Holes: Earth Compressed into the Volume of a Ping-Pong Ball **

Einstein showed what happens to time and space around a mass like Earth. He demonstrated what no one had considered before: they are curved. It is this curvature that attracts bodies toward greater masses and constitutes the force of gravity. In this regard, Einstein paves a new path in physics, not in its differentiation but in its extension beyond Newtonian physics. This force creates an initially peculiar effect: time slows, space stretches until it tears apart.

But for such a surface to form, one must entertain the possibility of compressing a mass like that of Earth into the volume of a ping-pong ball. We’ll revisit this later.

Einstein himself doubted his own calculations because they seemed so ludicrous. Yet he was mistaken: it is now proven that compressed masses like this exist by billions upon billions, and these are black holes. They can be immense or very tiny.

**Time Appears to Halt at the Horizon of a Black Hole, Yet Time Doesn’t Exist **

Ultimately, what defines a black hole? A black hole is, among other things, the remnant of a star that has finished burning. Hence why Karl Jansky could detect its signature. When a star depletes its hydrogen reserves, it cools, contracts, and its gravitational force plunges it into collapse, giving rise to a black hole that engulfs everything in its path.

I’ll leave the joy of discovering the intricacies of black hole formation to readers. However, if there’s one aspect Carlo eloquently expounds upon to elucidate the subject of his book – white holes – it’s the phenomena occurring at the so-called “horizon”. This boundary marks the outer limit of black holes…

Due to the distortions of time induced by the powerful gravitational attraction of black holes, time near the horizon is infinitely slower than ours. We age less, or not at all, near the horizon, underscoring the relativity of temporal experience – a fundamental concept for understanding the universe’s structure. Each observer experiences time differently, a poignant reminder of the inherent subjectivity in our perception of reality.

**A Constant Play of Perspectives **

Carlo warns us: our unwavering conviction in the accuracy of our perceptions impedes our ability to learn. I like this quote from Stephen Hawking who once said, “the greatest enemy of knowledge is not ignorance, it is the illusion of knowledge.” Reality surpasses the bounds of immediate perception, urging us to transcend conventional paradigms and envision the universe through a wider lens…

Quantum physics, once a source of consternation for Einstein due to its seemingly paradoxical nature, now stands as a testament to the inexorable march of scientific progress. By reconciling classical and quantum frameworks, science advances, unfurling the tapestry of reality…

Gravity, as elucidated by Einstein’s general theory of relativity, curves the fabric of spacetime, necessitating a departure from the Euclidean geometry as we know it.

**Crossing the Horizon of a Black Hole **

Returning to black holes, we endeavour to glean insights from the enigmatic realm of white holes. As we approach the horizon, the boundary delineating the point of no return, we traverse a cosmic funnel that can stretch for millions of light-years…

At the bottom of the black hole, the deformation of space time is extremely potent. And as always under such conditions, quantum effects are expected… However, here’s where the rub lies: Einstein’s equations do not account for these phenomena. Beyond a certain point, his equations cease to function: everything is going haywire. The variables of the equations become infinite!

This point, where Einstein’s equations no longer work, is called a singularity. So, we find ourselves at the bottom of the black hole and we no longer have a map to help us. No guide. How, then, to take account of the singularity: that moment when everything goes haywire?

**Understanding the Singularity **

One might assume it occurs at the bottom of the black hole. it is actually a simple failure of general relativity to tell what going on there. So is this truly the case? For a very old black hole, whose gravitational force is immense and drags everything along its path, the star is still falling because in its own time, it’s just a fraction of a second. Einstein’s equations still hold. For now, nothing goes haywire.

Stephen Hawking suggested that old black holes can exhaust themselves through a process called evaporation, which gradually diminishes their size. The more the black hole shrinks and coils in on itself, the stronger the time deformation, and the closer we get to the fateful “Planck scale.”

Einstein’s equations are now invalidated by quantum phenomena. The singularity, the place where everything goes haywire, is therefore not at the center of the black hole, where the collapsing star is, because in reality, that would mean Einstein’s equations would continue to function. This transition challenges the conventional notion of a singular point at the heart of a black hole, suggesting instead an extended singularity with no clear endpoint.

So, how do we know what’s happening there? Once again, Carlo suggests here to change perspective, broaden one’s overall vision. He suggests going to see, as the greatest have done before him – among them Anaximander, Kepler, and Einstein himself: to go and see with “the eyes of the mind”. And try, “by demonstrations and refutations” again and again, until finding the right equation. Thinking about what we must leave behind and which maps to take… before the grand journey.

**Carole Stora-Calte**