The Irresistible Pull of Gravity (Physics)

It is well known that gravity attracts and after reading the essay “The irresistible attraction of gravity” by astrophysicist Luciano Rezzolla – published by Rizzoli and selected as a finalist for the Cosmos Prize – it will also be clear why. Like it or not, we cannot escape gravity and its irresistible pull on our mind, so let us be carried away by the author on a long journey to explore the wonders of the universe.

Whether from the edge of an underground abyss or from the edge of a rock jump on the top of a mountain, from the highest rung of a ladder or from the window of a building, when we look out and look down we inevitably feel attracted. In fact, what attracts us is gravity . It attracts us not only with the body but also with the mind… in the sense that gravity is attractive, both instinctively and rationally.

Just a few seconds after birth, the Moro reflex reveals an important truth of our interaction with gravity: we know it instinctively, well before having conscious interactions with the rest of the physical universe. Then, over time, our knowledge of gravity changes as we develop the ability to observe the physical universe and understand its laws.

There is a person who knows these laws very well, even when they apply to the most extreme objects in the universe. A person who several times, in recent years, has guided us in the understanding of the most fascinating phenomena of nature , and who with patience and clarity has told us what happens in places where our imagination struggles to reach . It is Luciano Rezzolla , astrophysicist of Goethe University in Frankfurt , author of a recent essay – The irresistible attraction of gravity , published by Rizzoli and selected as a finalist for the Cosmos Prize – of which today we propose a review (with some spoilers ).

Cover of the book “The irresistible attraction of gravity: a journey to discover black holes” by Luciano Rezzolla, published by Rizzoli © INAF

From the early years of school, to explain gravity to children we start from the famous Newton’s apple and the fact that – detached from the branch – it does not float in space but falls to the ground. Already in the first chapters of this book you will discover, retracing the work of the fathers of gravity wisely narrated by the author, that this view is limited and even misleading, that gravity is not a force and that mass, alone, is not sufficient to describe it. The author recounts, in a compelling way, the astronomical observations that contributed to opening cracks in Newton’s theory of gravitation, leading the reader to discover – thanks to Einstein’s theory of general relativity – what gravity really is.

You will be joined by the author on a long journey in which, in addition to patience, the effort to imagine a reality very far from the one you are used to is required. On the way you will hardly be left behind, although the physics described are challenging. The author is able to explain in a simple but rigorous way the more complicated aspects of physics, making even the less experienced reader understand how the laws that govern it work. The text is extremely fluent, accurate, full of references to everyday life, of analogies, of comparisons that keep – like gravity – grounded. You will understand the concept of spacetimeand of curvature, you will understand how gravity is nothing more than a manifestation of the latter and that the reason why the apple falls downwards, once detached from the tree, is linked to the sign of the curvature of spacetime in which it is located the tree.

Every now and then you will be called to do mental experiments – Gedankenexperiment , in German – which do not require any instrumental apparatus but only a good dose of imagination and an excellent knowledge of physics. You will have to think about the first, the author will think about the second. These experiments, conducted using only the mind, based on logical and physical considerations, will allow you to obtain “virtual” results that are very difficult, if not impossible, to obtain in the laboratory and will allow you to investigate the limits of gravity.

Understanding the nature of gravity – and how it is inextricably linked to the curvature of spacetime – you will face the other stages of the journey, exploring the limits of gravity itself. Also in this case, the author retraces the historical steps that led – in the late 1950s, early 1960s – to the discovery of two wonders of physics: neutron stars and black holes .

The author tells how, with the advent of astronomy X – in a climate of trust and “cognitive optimism”, when stellar evolution was thought to be understood in every aspect -, astronomers discovered that in a dark corner of the sky, where there is a very ordinary small star when observed with a normal telescope, something shone with an enormously greater brightness X than might be expected. Data in hand, the astronomers soon realized that this something could have nothing to do with nuclear fusion processes. The discovery of this object, called Scorpius X-1 – along with that of another apparently similar but actually quite different object, Cygnus X-1– demolished most of the certainties that astrophysics had taken about thirty years to build. This is how, retracing the stages of the evolution of stars of great mass, the author describes how these stars are able to produce elements heavy up to iron and, having reached that point, find themselves collapsing on themselves until everything intervenes. another kind of pressure to stop the contraction.

«It has always struck me» writes Rezzolla, «that the final act of the bright and frenetic life of a massive star – as well as the catastrophic process that reveals its death – also marks the birth of one of the most fascinating objects in physics: a star of neutrons “. Perfect spheres of unimaginable density, with very high rotation frequencies and extremely high temperatures and magnetic fields, which the reader will be able to deepen in terms of observational properties, structure and internal composition, in a chapter entirely dedicated to these wonders of physics.

After the neutron stars it is the turn of black holes, samples of curvature, and you will be amazed by the fact that – although daily experience suggests that the more complex an object, organism or physical phenomenon, the greater the amount of information needed. to describe it – black holes are the simplest macroscopic physical objects of all. In addition to describing the physics of these objects, a dedicated chapter retraces the steps that led to the first image of a black hole , in the heart of M87, by the Event Horizon Telescope , explaining in detail why this image appears as we see it.

Having almost reached the end of the long journey, aware of what the curvature of spacetime is and what it entails in terms of gravity, in the last chapter the author deals with another interesting aspect: the propagation of perturbations in the curvature of spacetime. We are talking about gravitational waves , the revelation of which in 2015 marked the birth of the so-called multi-messenger astronomy, which opened a new window on the universe.

In the journey traveled with the author, you will find the answers to the many questions that each of us has asked himself at least once in his life, in addition to those that inevitably arise when reading the book. “What we experience in the course of our life on Earth is nothing more than a drop in the ocean of the physically possible”, concludes Rezzolla, and it is important not to limit the imagination: “Before advanced mathematics, complex simulations and sophisticated experiments – all however indispensable – comes the agility of our minds and their journeys of imagination. It is they who, more than anything else, allow us to extend the limits of knowledge ».

That gravity attracts is obvious to everyone, and after reading this book, it will become clear why. Like it or not, as the author states, we cannot escape gravity and its irresistible pull on our mind.

Featured image: Luciano Rezzolla is full professor of Theoretical Astrophysics and director of the Institute of Theoretical Physics at the Goethe Universität in Frankfurt. A member of the scientific committee of the Event Horizon Telescope project, his research helped capture the first photographic images of a supermassive black hole in 2019. Credits: Jürgen Lecher, Goethe-Universität Frankfurt

Provided by INAF

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