Author: Carlo Rovelli

ISBN: 9780141981727

Short and very readable book about the frontiers of todays' physics. No physics knowledge required, though welcome.

EXCERPTS

The gravitational field is not diffused through space; the gravitational field is that space itself. This is the idea of the theory of general relativity. Newton’s ‘space’, through which things move, and the ‘gravitational field’ are one and the same thing.

We are not contained within an invisible rigid infrastructure: we are immersed in a gigantic flexible snail-shell. The sun bends space around itself and the Earth does not turn around it because of a mysterious force but because it is racing directly in a space which inclines, like a marble that rolls in a funnel. There are no mysterious forces generated at the centre of the funnel; it is the curved nature of the walls which causes the marble to roll. Planets circle around the sun, and things fall, because space curves.

Riemann’s curvature, and indicate with the letter ‘R’. Einstein wrote an equation which says that R is equivalent to the energy of matter. That is to say: space curves where there is matter. [= energy]

Due to this curvature, not only do planets orbit round the star, but light stops moving in a straight line and deviates.

But it isn’t only space that curves; time does too. Einstein predicted that time passes more quickly high up than below, nearer to the Earth.

When a large star has burnt up all of its combustible substance (hydrogen) it goes out. What remains is no longer supported by the heat of the combustion and collapses under its own weight, to a point where it bends space to such a degree that it plummets into an actual hole. These are the famous ‘black holes’.

But this is still not all. The whole of space can expand and contract. Furthermore, Einstein’s equation shows that space cannot stand still; it must be expanding.

In short, the theory describes a colourful and amazing world where universes explode, space collapses into bottomless holes, time sags and slows near a planet, and the unbounded extensions of interstellar space ripple and sway like the surface of the sea … And all of this, which emerged gradually from my mice-gnawed book, was not a tale told by an idiot in a fit of lunacy, or a hallucination caused by Calabria’s burning Mediterranean sun and its dazzling sea. It was reality. Or better, a glimpse of reality, a little less veiled than our blurred and banal everyday view of it. A reality which seems to be made of the same stuff which our dreams are made of, but which is nevertheless more real than our clouded quotidian dreaming.

‘quanta’, that is, in packets or lumps of energy.

Einstein showed that light is made of packets: particles of light. Today we call these ‘photons’.

In accordance with the assumption to be considered here, the energy of a light ray spreading out from a point source is not continuously distributed over an increasing space but consists of a finite number of ‘energy quanta’ which are localized at points in space, which move without dividing, and which can only be produced and absorbed as complete units.

If Planck is the father of the theory, Einstein is the parent who nurtured it. But like all offspring, the theory then went its own way, unrecognized by Einstein himself. In the second and third decades of the twentieth century it was the Dane Niels Bohr who pioneered its development. It was Bohr who understood that the energy of electrons in atoms can only take on certain values, like the energy of light, and crucially that electrons can only ‘jump’ between one atomic orbit and another with fixed energies, emitting or absorbing a photon when they jump. These are the famous ‘quantum leaps’.

Heisenberg imagined that electrons do not always exist. They only exist when someone or something watches them, or better, when they are interacting with something else. They materialize in a place, with a calculable probability, when colliding with something else. The ‘quantum leaps’ from one orbit to another are the only means they have of being ‘real’: an electron is a set of jumps from one interaction to another. When nothing disturbs it, it is not in any precise place. It is not in a ‘place’ at all.

In quantum mechanics no object has a definite position, except when colliding headlong with something else. In order to describe it in mid-flight, between one interaction and another, we use an abstract mathematical formula which has no existence in real space, only in abstract mathematical space. But there’s worse to come: these interactive leaps with which each object passes from one place to another do not occur in a predictable way but largely at random. It is not possible to predict where an electron will reappear, but only to calculate the probability that it will pop up here or there. The question of probability goes to the heart of physics, where everything had seemed to be regulated by firm laws which were universal and irrevocable.

Electrons, quarks, photons and gluons are the components of everything that sways in the space around us. They are the ‘elementary particles’ studied in particle physics. To these particles a few others are added, such as the neutrinos which swarm throughout the universe but have little interaction with us, and the ‘Higgs bosons’ recently detected in Geneva in CERN’s Large Hadron Collider. But there are not many of these, fewer than ten types in fact. [In supersymetry only the quarks (and antiquarks) exist.]

The nature of these particles, and the way they move, is described by quantum mechanics. These particles do not have a pebble-like reality but are rather the ‘quanta’ of corresponding fields, just as photons are the ‘quanta’ of the electromagnetic field.

They disappear and reappear according to the strange laws of quantum mechanics, where everything that exists is never stable, and is nothing but a jump from one interaction to another. Even if we observe a small empty region of space, in which there are no atoms, we still detect a minute swarming of these particles. There is no such thing as a real void, one that is completely empty.

Around every galaxy astronomers observe a large cloud of material which reveals its existence via the gravitational pull that it exerts upon stars, and by the way it deflects light. But this great cloud, of which we observe the gravitational effects, cannot be seen directly and we do not know what it is made of. Numerous hypotheses have been proposed, none of which seem to work. It’s clear that there is something there, but we don’t know what. Nowadays it is called ‘dark matter’. Evidence indicates that it is something not described by the Standard Model, otherwise we would see it. Something other than atoms, neutrinos or photons.

Loop quantum gravity. The idea is simple. General relativity has taught us that space is not an inert box, but rather something dynamic: a kind of immense, mobile snail-shell in which we are contained – one which can be compressed and twisted. Quantum mechanics, on the other hand, has taught us that every field of this kind is ‘made of quanta’ and has a fine, granular structure. It immediately follows that physical space is also ‘made of quanta’. The central result of loop quantum gravity is indeed that space is not continuous, that it is not infinitely divisible but made up of grains or ‘atoms of space’. These are extremely minute: a billion billion times smaller than the smallest atomic nuclei. The theory describes these ‘atoms of space’ in mathematical form, and provides equations which determine their evolution. They are called ‘loops’, or rings, because they are linked to each other, forming a network of relations which weaves the texture of space, like the rings of a finely woven immense chain mail. Where are these quanta of space? Nowhere. They are not in a space because they are themselves the space. Space is created by the linking of these individual quanta of gravity. Once again the world seems to be less about objects than about interactive relationships.

But it’s the second consequence of the theory that is the most extreme. Just as the idea of a continuous space that contains things disappears, so the idea of an elementary and primal ‘time’ flowing regardless of things also vanishes. The equations describing grains of space and matter no longer contain the variable ‘time’. This doesn’t mean that everything is stationary and unchanging. On the contrary, it means that change is ubiquitous – but elementary processes cannot be ordered in a common succession of ‘instants’.

The passage of time is internal to the world, is born in the world itself in the relationship between quantum events that comprise the world and are themselves the source of time.

There is no longer space which ‘contains’ the world, and there is no longer time ‘in which’ events occur. There are only elementary processes wherein quanta of space and matter continually interact with each other. The illusion of space and time which continues around us is a blurred vision of this swarming of elementary processes, just as a calm, clear Alpine lake consists in reality of a rapid dance of myriads of minuscule water molecules.

This hypothetical final stage in the life of a star, 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 it would measure about one and a half kilometres in diameter. Inside this black hole 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. 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. 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.

But time does not pass at the same speed for her as for those outside the black hole, for the same reason that in the mountains time passes faster than at sea-level. Except that for her, because of the extreme conditions, the difference in the passage of time is enormous, and what for the observer on the star would seem an extremely rapid bounce would appear, seen from outside it, to take place over a very long time. This is why we observe black holes remaining the same for long periods of time: a black hole is a rebounding star seen in extreme slow motion.

Another of the consequences of the theory, and one of the most spectacular, concerns the origins of the universe. We know how to reconstruct the history of our universe back to an initial period when it was tiny in size. But what about before that? Well, the equations of loop theory allow us to go even further back in the reconstruction of that history. What we find is that when the universe is extremely compressed quantum theory generates a repulsive force, with the result that the great explosion or ‘Big Bang’ may have actually been a ‘Big Bounce’. Our world may have actually been born from a preceding universe which contracted under its own weight until it was squeezed into a tiny space before ‘bouncing’ out and beginning to re-expand, thus becoming the expanding universe which we observe around us. The moment of this bounce, when the universe was contracted into a nutshell, is the true realm of quantum gravity: time and space have disappeared altogether, and the world has dissolved into a swarming cloud of probability which the equations can, however, still describe.

Our universe may have been born from a bounce in a prior phase, passing through an intermediate phase in which there was neither space nor time.

Physics opens windows through which we see far into the distance. What we see does not cease to astonish us. We realize that we are full of prejudices and that our intuitive image of the world is partial, parochial, inadequate. The Earth is not flat, it is not stationary. The world continues to change before our eyes as we gradually see it more extensively and more clearly. If we try to put together what we have learnt in the twentieth century about the physical world, the clues point towards something profoundly different from our instinctive understanding of matter, space and time.

‘What is heat?’

What they came to understand is that a hot substance is not one which contains caloric fluid. A hot substance is a substance in which atoms move more quickly. Atoms and molecules, small clusters of atoms bound together, are always moving. They run, vibrate, bounce and so on. Cold air is air in which atoms, or rather molecules, move more slowly. Hot air is air in which molecules move more rapidly. Beautifully simple. But it doesn’t end there. Heat, as we know, always moves from hot things to cold. A cold teaspoon placed in a cup of hot tea also becomes hot. If we don’t dress appropriately on a freezing cold day we quickly lose body heat and become cold. Why does heat go from hot things to cold things, and not vice versa?

It is a crucial question, because it relates to the nature of time. In every case in which heat exchange does not occur, or when the heat exchanged is negligible, we see that the future behaves exactly like the past. For example, for the motion of the planets of the solar system heat is almost irrelevant, and in fact this same motion could equally take place in reverse without any law of physics being infringed. As soon as there is heat, however, the future is different from the past. While there is no friction, for instance, a pendulum can swing forever. If we filmed it and ran the film in reverse we would see movement that is completely possible. But if there is friction then the pendulum heats its supports slightly, loses energy and slows down. Friction produces heat.

The difference between past and future only exists when there is heat. The fundamental phenomenon that distinguishes the future from the past is the fact that heat passes from things that are hotter to things that are colder. So, again, why, as time goes by, does heat pass from hot things to cold and not the other way round? The reason was discovered by Boltzmann, and is surprisingly simple: it is sheer chance. Boltzmann’s idea is subtle, and brings into play the idea of probability. Heat does not move from hot things to cold things due to an absolute law: it only does so with a large degree of probability. The reason for this is that it is statistically more probable that a quickly moving atom of the hot substance collides with a cold one and leaves it a little of its energy, rather than vice versa. Energy is conserved in the collisions, but tends to get distributed in more or less equal parts when there are many collisions. In this way the temperature of objects in contact with each other tends to equalize. It is not impossible for a hot body to become hotter through contact with a colder one: it is just extremely improbable.

In the second lesson I related how quantum mechanics predicts that the movement of every minute thing occurs by chance. This puts probability into play as well. But the probability which Boltzmann considered, the probability at the roots of heat, has a different nature, and is independent of quantum mechanics. The probability in play in the science of heat is in a certain sense tied to our ignorance. I may not know something with certainty, but I can still assign a lesser or greater degree of probability to something. For instance, I don’t know whether it will rain tomorrow here in Marseilles, or whether it will be sunny or will snow, but the probability that it will snow here tomorrow – in Marseilles, in August – is low. Similarly with regard to most physical objects: we know something but not everything about their state, and we can only make predictions based on probability. Think of a balloon filled with air. I can measure it: measure its shape, its volume, its pressure, its temperature … But the molecules of air inside the balloon are moving rapidly within it, and I do not know the exact position of each of them. This prevents me from predicting with precision how the balloon will behave.

In this same sense, the probability that when molecules collide heat passes from the hotter bodies to those which are colder can be calculated, and turns out to be much greater than the probability of heat moving toward the hotter body.

The branch of science which clarifies these things is called statistical physics, and one of its triumphs, beginning with Boltzmann, has been to understand the probabilistic nature of heat and temperature, that is to say, thermodynamics.

The cold teaspoon heats up in hot tea because tea and spoon interact with us through a limited number of variables amongst the innumerable variables which characterize their microstate. The value of these variables is not sufficient to predict future behaviour exactly (witness the balloon), but is sufficient to predict with optimum probability that the spoon will heat up.

Now, in the course of the twentieth century thermodynamics (that is, the science of heat) and statistical mechanics (that is, the science of the probability of different motions) have been extended to electromagnetic and quantum phenomena. Extension to include the gravitational field, however, has proved problematic. How the gravitational field behaves when it heats up is still an unsolved problem.

The gravitational field, as we saw in the first lesson, is space itself, in effect space-time. Therefore when heat is diffused to the gravitational field, time and space themselves must vibrate… But we still don’t know how to describe this well.

Such issues lead us to the heart of the problem of time: what exactly is the flow of time?

Time seems to ‘flow’, whereas the quantity of butter or location in space do not ‘flow’. Where does the difference come from? Another way of posing the problem is to ask oneself: what is the ‘present’? We say that only the things of the present exist: the past no longer exists and the future doesn’t exist yet. But in physics there is nothing that corresponds to the notion of the ‘now’. Compare ‘now’ with ‘here’. ‘Here’ designates the place where a speaker is: for two different people ‘here’ points to two different places. Consequently ‘here’ is a word the meaning of which depends on where it is spoken. The technical term for this kind of utterance is ‘indexical’. ‘Now’ also points to the instant in which the word is uttered, and is also classed as ‘indexical’. But no one would dream of saying that things ‘here’ exist, whereas things which are not ‘here’ do not exist. So then why do we say that things that are ‘now’ exist and that everything else doesn’t? Is the present something which is objective in the world, that ‘flows’ and that makes things ‘exist’ one after the other, or is it only subjective, like ‘here’? This may seem like an abstruse mental problem.

Physicists and philosophers have come to the conclusion that the idea of a present that is common to the whole universe is an illusion, and that the universal ‘flow’ of time is a generalization that doesn’t work. When his great Italian friend Michele Besso died, Einstein wrote a moving letter to Michele’s sister: ‘Michele has left this strange world a little before me. This means nothing. People like us, who believe in physics, know that the distinction made between past, present and future is nothing more than a persistent, stubborn illusion.’

The passage of time is obvious to us all: our thoughts and our speech exist in time; the very structure of our language requires time – a thing ‘is’ or ‘was’ or ‘will be’. It is possible to imagine a world without colours, without matter, even without space, but it’s difficult to imagine one without time.

There is a detectable difference between the past and the future only when there is flow of heat. Heat is linked to probability; and probability in turn is linked to the fact that our interactions with the rest of the world do not register the fine details of reality. The flow of time emerges thus from physics, but not in the context of an exact description of things as they are. It emerges, rather, in the context of statistics and of thermodynamics. This may hold the key to the enigma of time. The ‘present’ does not exist in an objective sense any more than ‘here’ exists objectively, but the microscopic interactions within the world prompt the emergence of temporal phenomena within a system (for instance, ourselves) which only interacts through the medium of a myriad of variables.

Our memory and our consciousness are built on these statistical phenomena. For a hypothetically supersensible being there would be no ‘flowing’ of time: the universe would be a single block of past, present and future. But due to the limitations of our consciousness we only perceive a blurred vision of the world, and live in time. Borrowing words from my Italian editor, ‘what’s non-apparent is much vaster than what’s apparent’. From this limited, blurred focus we get our perception of the passage of time. Is that clear? No, it isn’t. There is so much still to be understood.

Time sits at the centre of the tangle of problems raised by the intersection of gravity, quantum mechanics and thermodynamics.

We do not yet have a theory capable of drawing together all three pieces of our fundamental knowledge of the world.

The heat of black holes is a quantum effect upon an object, the black hole, which is gravitational in nature. It is the individual quanta of space, the elementary grains of space, the vibrating ‘molecules’ that heat the surface of black holes and generate black hole heat. This phenomenon involves all three sides of the problem: quantum mechanics, general relativity and thermal science. The heat of black holes is like the Rosetta Stone of physics, written in a combination of three languages – Quantum, Gravitational and Thermodynamic – still awaiting decipherment in order to reveal the true nature of time.

What role do we have as human beings who perceive, make decisions, laugh and cry, in this great fresco of the world as depicted by contemporary physics? If the world is a swarm of ephemeral quanta of space and matter, a great jigsaw puzzle of space and elementary particles, then what are we? Do we also consist only of quanta and particles? If so, then from where do we get that sense of individual existence and unique selfhood to which we can all testify? And what then are our values, our dreams, our emotions, our individual knowledge? What are we, in this boundless and glowing world?

I’ve set out to show how the world looks in the light of science, and we are a part of that world too. ‘We’, human beings, are first and foremost the subjects who do the observing of this world; the collective makers of the photograph of reality which I have tried to compose. We are nodes in a network of exchanges (of which this present book is an example) through which we pass images, tools, information and knowledge. [This is the Quantum Qbistic interpretation of the "observer problem" of Quantum Mechanics (QM) that I am also a proponent myself. If you wan't to get to know more about the basics of QM and it most common interpretations I highly recommend reading Something Deeply Hidden by Sean Carroll.]

But we are also an integral part of the world which we perceive; we are not external observers. We are situated within it. Our view of it is from within its midst. We are made up of the same atoms and the same light signals as are exchanged between pine trees in the mountains and stars in the galaxies.

The images which we construct of the universe live within us, in the space of our thoughts. Between these images – between what we can reconstruct and understand with our limited means – and the reality of which we are part, there exist countless filters: our ignorance, the limitations of our senses and of our intelligence. The very same conditions that our nature as subjects, and particular subjects, imposes upon experience.

Our knowledge consequently reflects the world. It does this more or less well, but it reflects the world we inhabit. This communication between ourselves and the world is not what distinguishes us from the rest of nature. All things are continually interacting with each other, and in doing so each bears the traces of that with which it has interacted: and in this sense all things continuously exchange information about each other.

So what then is the difference between the thermostat’s and my own ‘sensing’ and ‘knowing’ that it’s warm and deciding freely to turn off the heating or not – and ‘knowing’ that I exist? How can the continuous exchange of information in nature produce us, and our thoughts? The problem is wide open, with numerous fine solutions currently under discussion. This, I believe, is one of the most interesting frontiers of science, where major progress is about to be made.

There is one issue in particular regarding ourselves which often leaves us perplexed: what does it mean, our being free to make decisions, if our behaviour does nothing but follow the predetermined laws of nature? Is there not perhaps a contradiction between our feeling of freedom and the rigour, as we now understand it, with which things operate in the world? Is there perhaps something in us which escapes the regularity of nature, and allows us to twist and deviate from it through the power of our freedom to think? Well, no, there is nothing about us that can escape the norms of nature. If something in us could infringe the laws of nature we would have discovered it by now. There is nothing in us in violation of the natural behaviour of things. The whole of modern science – from physics to chemistry, and from biology to neuroscience – does nothing but confirm this observation. The solution to the confusion lies elsewhere. When we say that we are free, and it’s true that we can be, this means that how we behave is determined by what happens within us, within the brain, and not by external factors. To be free doesn’t mean that our behaviour is not determined by the laws of nature. It means that it is determined by the laws of nature acting in our brains. Our free decisions are freely determined by the results of the rich and fleeting interactions between the billion neurons in our brain: they are free to the extent that the interaction of these neurons allows and determines. Does this mean that when I make a decision it’s ‘I’ who decides? Yes, of course, because it would be absurd to ask whether ‘I’ can do something different from what the whole complex of my neurons has decided: the two things, as the Dutch philosopher Baruch Spinoza understood with marvellous lucidity in the seventeenth century, are the same. There is not an ‘I’ and ‘the neurons in my brain’. They are the same thing. An individual is a process: complex, tightly integrated. When we say that human behaviour is unpredictable, we are right, because it is too complex to be predicted, especially by ourselves.

Life on Earth gives only a small taste of what can happen in the universe. Our very soul itself is only one such small example. We are a species which is naturally moved by curiosity.

It is not against nature to be curious: it is in our nature to be so.

Here, on the edge of what we know, in contact with the ocean of the unknown, shines the mystery and the beauty of the world. And it’s breathtaking.