Carlo Rovelli

"Don't trust your teachers."

"Listening to a cultivated person of today who jokes and almost boasts of his scientific ignorance is as sad as listening to a scientist who boasts of not having read any poem."

"Hawking was one of the good physicists of his generation, not the universal genius the popular press has depicted. It is a pity, because this confusion fogs the memory of a remarkable man."

"If we want to understand the past we must do so on its own terms, and disregard the future of that past, but if we want to understand the present we better not disregard the past steps that were essential for getting to the present. This is of importance especially for those of us engaged in trying to push ahead the scientific path of discovery today. We are not much interested in what scientists did wrong, there is too much of that. We are interested in what they did right, because we are trying to copy them in this, not in that."

"... theoretical physics is less clean than the way it's usually solved. You can always change parameters and save yourself. ... it's very rare that theories are ruled out by just an experiment or a group of experiments. Theories usually come with flexibility. Theoreticians can add flexibility. And so new experiments — you can just patch up your theory."

"The landscape is magic, the trip is far from being over."

"I think that physics is about escaping the prison of the received thoughts and searching for novel ways of thinking the world, about trying to clear a bit the misty lake of insubstantial dreams, which reflect reality like the lake reflects the mountains."

"The new coherent picture is not yet available. With all their immense empirical success, G(eneral)R(elativity) and Q(uantum)M(echanics) have left us with an understanding of the physical world which is unclear and badly fragmented. At the foundations of physics there is today confusion and incoherence."

"An overly pragmatic attitude is not productive in the long run."

"I argue that the best strategy for understanding quantum gravity is to build a picture of the physical world where the notion of time plays no role at all."

"[I]n a fundamental description of nature we must "forget time"... this can be done in the classical and in the quantum theory."

"I... interpret mechanics as a theory of relations between variables, rather than the theory of the evolution of variables in time."

"General relativity has changed our understanding of space and time. ...The spacetime of general relativity ...is likely to be just a classical approximation that loses its meaning in the quantum theory, for the same reason the trajectory of a particle does."

"The notion of time familiar to us may... be reconstructed in special physical situations, or within an approximation, as... the "surface of a liquid" disappears ...[at] the atomic level, or "temperature" ...makes sense only in certain physical situations and when there are enough degrees of freedom."

"I am aware that ...[this] answer ...is only one among many possibilities. Other authors have argued that the notion of time is irreducible... Until our theoretical and experimental investigations tell us otherwise... what is important is to put the alternatives clearly on the table..."

"[I]n pre-relativistic mechanics, time is a special physical quantity, whose value is measured by physical clocks, that plays the role of the independent variable of physical evolution."

"While non-relativistic time is the observable quantity measured (or approximated) by physical clocks, in general relativity clocks measure s along their worldline, not t. The relativistic... t is a freely chosen label with no direct physical interpretation. ...The physical content of a solution of Einstein's equations is not in its dependence on t, but ...in what remains once the dependence on t (and x) has been factored away."

"What is then the physical content of a solution of Einstein's equations..? ...[C]onsider what is actually measured in general relativistic experiments. ...Consider a clock at rest on ...Earth and a clock on a satellite in orbit around the Earth. ...Call T1 and T2 the readings... Each measures the along its own worldline, in the Earth gravitational field. ...the theory predicts the value of T2 that will be associated to each value of T1. Or vice versa."

"[I]n general relativistic observations there is no preferred independent time variable. What we measure are a number of variables, all on equal footing, and their relative evolution. ...[I]t can be equally read it as the evolution of the variable T1 as a function of the variable T2, or viceversa. Which of the two is the independent variable here?"

"The way evolution is treated in general relativity, is... subtle... Change is not described as evolution of physical variables as a function of a preferred independent observable time variable. Instead, it is described in terms of a functional relation among equal footing variables... as... (T1,T2)... In general relativity, there isn’t a preferred and observable quantity that plays the role of independent parameter of the evolution... General relativity describes the relative evolution of observable quantities, not the evolution of quantities as functions of a preferred one. ...[w]ith general relativity we have understood that the Newtonian "big clock" ticking away the "true universal time" is not there."

"If we formulate the fundamental theory of nature in a timeless language, we have then the problem of recovering the familiar notion of time."

"Intuitively (and imprecisely) speaking, time "flows", we can never "go back in time", we remember the past but not the future, and so on. Where do all these very peculiar features of the time variable come from? ...[T]hese features... emerge at the thermodynamical level. ...[T]hese are all features that emerge when we give an approximate statistical description of a system with a large number of ."

"Whatever the statistical state ρ is, there exists always a variable tρ, measured by the thermal clock, with respect to which the system is in equilibrium and physics is the same as in the conventional nonrelativistic statistical case! This observation leads us to the following hypothesis."

"The thermal time hypothesis. In nature, there is no preferred physical time variable t. There are no equilibrium states ρ0 preferred a priori. Rather, all variables are equivalent; we can find the system in an arbitrary state ρ; if the system is in a state ρ, then a preferred variable is singled out by the state of the system. This variable is what we call time."

"[I]t is the statistical state that determines which variable is physical time, and not any a priori hypothetical "flow" that drives the system to a preferred statistical state. When we say that a certain variable is “the time”, we are not making a statement concerning the fundamental mechanical structure of reality. Rather, we are making a statement about the statistical distribution we use to describe the... properties of the system... The "thermal time hypothesis" is the idea that what we call "time" is the thermal time of the statistical state in which the world happens to be, when described in terms of the macroscopic parameters we have chosen."

"Time is... the expression of our ignorance of the full microstate."

"In his youth Albert Einstein spent a year loafing aim- lessly. You don’t get anywhere by not ‘wasting’ time – something, unfortunately, which the parents of teenagers tend frequently to forget."

"A handful of types of elementary particles, which vibrate and fluctuate constantly between existence and non-existence and swarm in space even when it seems that there is nothing there, combine together to infinity like the letters of a cosmic alphabet to tell the immense history of galaxies, of the innumerable stars, of sunlight, of mountains, woods and fields of grain, of the smiling faces of the young at parties, and of the night sky studded with stars."

"Children grow up and discover that the world is not as it seemed within the four walls of their homes. Humankind as a whole does the same."

"The idea that a well-defined now exists throughout the universe is an illusion, an illegitimate extrapolation of our own experience"

"When two friends meet... after one has lived in the mountains and the other at sea level, the watches... will show different times. ...Neither is truer than the other. ...Times are legion: a different one for every point in space. ...there is a vast multitude of them. ...Every clock has its proper time. ...Einstein has shown how to calculate the difference... The world is not like a platoon advancing at the pace of a single commander. It is a network of events affecting each other. ...Physics does not describe how things evolve "in time" but how things evolve in their own times, and how "times" evolve relative to each other. Time has lost... its unity."

"The difference between past and future... cause and effect... memory and hope... regret and intention... in the elementary laws that describe the mechanisms of the world, there is no such difference."

"In the elementary equations of the world, the arrow of time appears only where there is heat. The link between time and heat is... fundamental: every time a difference is manifested between the past and the future, heat is involved."

"The World is Made of Events, not Things."

"But it isn’t absence that causes sorrow. It is affection and love. Without affection, without love, such absences would cause us no pain. For this reason, even the pain caused by absence is, in the end, something good and even beautiful, because it feeds on that which gives meaning to life."

"If I move one watch, put it higher, and wait a little bit... with very good clocks, and today we can... you bring it [back] down, and this [the clock that was higher] is ahead, this [the clock that remained lower] is behind. ...The clock up, has measured more time than the clock down. And it's not just a matter of clocks. Everything, ...if you are high ...a flower would blossom more, a machine would do more cycles ...more things happening up than down. Of course, the difference is small, otherwise we would have noticed it ...earlier."

"Quite remarkably, this was understood before having good clocks... It is one of great intuitions by Einstein. In 1915 he completed the theory of relativity... At the time clocks were not good enough... it was a theory, a good speculation... Today it is not a theory... It's a fact that we measure in the labaratory. ...It's so clear and definite that technology needs this to be taken into account."

"GPS... gets messages from satellites. On those satellites are clocks... The satellites broadcast the times and your little device use the time to tringulate wherever you are... [W]hen this was built the physicists told the [engineers] that were putting up the system, "Careful, up there... your clocks are going to run faster..." ...The entire project was controlled by the American army ...supervised by the generals ...[who] didn't believe it. So the first satellites were put up there with a switch... and of course it does not work if you do not take this into account. ...You would crash the car against the wall, driving with the navigator if we didn't [take] this into account ..."

"This means that the time between [points on a line] is 24 hours here but... more if you're... higher, and less if you're less [high]. ...Now it's measurable to 30-40 cm [difference in] altitude in the lab."

"What makes the difference is the mass. ...[T]he earth is a big mass and it slows down time. If you go to a bigger mass, like Jupiter, it's stronger. If you go near a big star it's stronger. If you're near a black hole... it's even more strong, so strong that if you go very near... time essentially stops down. It goes very, very, very slowly."

"If we had a starship, we could move near a black hole, stay there for a half hour and then come out. Outside hundreds of years could have gone. So we could... come away from the black hole into the far future easily."

"[T]ime is much more elastic than our intuition of this rigid passage... The time interval between one point and another... depends on where you are."

"[I]n "Helgoland" Rovelli explains his "relational" interpretation, in which an electron, say, has properties only when it interacts... When it is not interacting, the electron is devoid of physical properties: no position... velocity... trajectory. ...[T]he electron's properties are real only for the object it's interacting with and not for other objects. ...Rovelli's lyricism may depend on how many other ...accounts of quantum physics one may have read: The more that number, the more "Helgoland" will seem a poem."

"'... is a magnificent example of the adage "less is more". Tackling everything from quantum physics and cosmology to particle physics, space-time and black holes in so few words... Rovelli... managed... with aplomb. ...The Order of Time ...is a little more conventional ...the first part covering ..., the arrow of time, relativity, synchronization and the notion of Planck time... The second part imagines a world without time, while [in] the third Rovelli wonders how we perceive a flow of time in a timeless world. ...[H]e has a deep technical knowledge ...he knows what the hell he’s talking about ...Rovelli can condense complex ideas into beautifully written prose, gently guiding the reader through mind-bending ideas without... cliché or stale metaphors. Anecdotes, history, art, philosophy and culture pepper the text."

"[W]hile we scientists know how to use [quantum physics]... [w]e don't know what it's telling us about the fundamental nature of reality. It is into that chasm that... Rovelli leaps with his new book Helgoland. ...Rovelli is one of the world's leading theoretical physicists. He is also a lyrical writer whose previous bestsellers have offered beautiful meditations on "the adventure of science." ...What quantum mechanics is teaching us, Rovelli says, is that reality is a vast net of interactions where there are no things, only relationships. ...Helgoland is not... a book to learn quantum mechanics from. ...Rovelli is offering a new way to understand not just the world but our place in it ..."

"With the fate of Schrödinger's cat in the balance and Heisenberg's idea that quantum theory only describes observations, Rovelli inevitably asks the tricky questions: what is observation? What is an observer? ...He is the champion of the "relational" interpretation... [that] quantum theory does not describe the way in which quantum objects manifest themselves to "observers", but... how every physical object manifests itself to every other physical object. The world we observe is... better understood as a web of interactions and relations rather than objects."

"Rovelli talks about time as a complex collection of layers and then strips away those layers... I find an elegant grandeur in how he relates the worlds of science, philosophy and art. What’s more, scientists and philosophers, going back to ancient times, have weaved a connection between poetry and physics, or, as it was once called, "." ...I can think of no one other than Rovelli who would begin all but one chapter of his nonfiction book with excerpts from the "Odes" of Horace. That single aberrant chapter... quotes Shakespeare’s "Henry IV, Part I." ...Rovelli’s new story of time is elegant and lucidly told, whether he is revealing facts or indulging in romantic-philosophic speculation about the nature of time."

"The beauty of physics lies in its precise statements, and that is what is essential to convey. Many readers won’t have the background required to distinguish fact from speculation. Words can turn equations into poetry, but elegant language shouldn’t come at the expense of understanding. Rovelli isn’t the first author guilty of such romanticizing... But when deceptively fluid science writing permits misleading interpretations to seep in, I fear that the floodgates open to more dangerous misinformation."

"Wedging old ideas into new thinking is analogous to equating thousand-dollar couture adorned with beads and feathers and then marketed as "tribal fashion" to homespun clothing with true cultural and historical relevance. Ideas about relativity or gravity in ancient times weren’t the same as Einstein’s theory. Art (and science) are in the details. Either elementary matter is extended or it is not. The universe existed forever, or it had a beginning. Atoms of old aren’t the atoms of today. Egg and flour are not a soufflé. Without the appropriate care, it all just collapses."

"Carlo Rovelli... is one of the great scientific explicators of our time. His... '... remains the world's fastest selling science book. ...The Order of Time and ' ...illuminated ...Einsteinian relativity, gravitational waves and other ...physics. ...Rovelli's gift is to bring difficult ideas down a level. His books continue a tradition of jargon-free popular scientific writing from Galileo to Darwin that disappeared in the academic specializations of the last century."