Seven Brief Lessons on Physics (3 page)

Particles

Within the universe described in the previous lesson, light and things move. Light is made up of photons, the particles of light intuited by Einstein. The things we see are made of atoms. Every atom consists of a nucleus surrounded by electrons. Every nucleus consists of tightly packed protons and neutrons. Both protons and neutrons are made up of even smaller particles that the American physicist Murray Gell-Mann named “quarks,” inspired by a seemingly nonsensical word in a nonsensical phrase in James Joyce’s
Finnegans Wake
: “Three quarks for Muster Mark!” Everything we touch is therefore made of electrons and of these quarks.

The force that “glues” quarks inside protons and neutrons is generated by particles that physicists, with little sense of the ridiculous, call “gluons.”

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. A handful of elementary ingredients that act like bricks in a gigantic Lego set, and with which the entire material reality surrounding us is constructed.

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 are elementary excitations of a moving substratum similar to the field of Faraday and Maxwell. Minuscule moving wavelets. 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. Just as the calmest sea looked at closely sways and trembles, however slightly, so the fields that form the world are subject to minute fluctuations, and it is possible to imagine its basic particles having brief and ephemeral existences, continually created and destroyed by these movements.

This is the world described by quantum mechanics and particle theory. We have arrived very far from the mechanical world of Newton, where minute, cold stones eternally wandered on long, precise trajectories in geometrically immutable space. Quantum mechanics and experiments with particles have taught us that the world is a continuous, restless swarming of things, a continuous coming to light and disappearance of ephemeral entities. A set of vibrations, as in the switched-on hippie world of the 1960s. A world of happenings, not of things.

The details of particle theory were built gradually in the 1950s, 1960s, and 1970s by some of the century’s greatest physicists, such as Richard Feynman and
Gell-Mann. This work of construction led to an intricate theory, based on quantum mechanics and bearing the not very romantic title of “the Standard Model of elementary particles.” The Standard Model was finalized in the 1970s, after a long series of experiments that confirmed all predictions. Its final confirmation occurred in 2013 with the discovery of the Higgs boson.

But despite the long series of successful experiments, the Standard Model has never been taken entirely seriously by physicists. It’s a theory that looks, at least at first sight, piecemeal and patched together. It’s made up of various pieces and equations assembled without clear order. A certain number of fields (but why
these
, exactly?) interacting among themselves with certain forces (but why
these
forces?) each determined by certain constants (but why precisely
these
values?) showing certain symmetries (but again, why
these
?). We’re far from the simplicity of the equations of general relativity and of quantum mechanics.

The very way in which the equations of the Standard Model make predictions about the world is also absurdly convoluted. Used directly, these equations lead to nonsensical predictions where each calculated quantity turns out to be infinitely large. To get meaningful re
sults, it is necessary to imagine that the parameters entering into them are themselves infinitely large, in order to counterbalance the absurd results and make them reasonable. This convoluted and baroque procedure is given the technical term “renormalization.” It works in practice but leaves a bitter taste in the mouth of anyone desiring simplicity of nature. In the last years of his life, the greatest scientist of the twentieth century after Einstein, Paul Dirac, the great architect of quantum mechanics and author of the first and principal equations of the Standard Model, repeatedly expressed his dissatisfaction at this state of things, concluding that “we have not yet solved the problem.”

In addition, a striking limitation of the Standard Model has appeared in recent years. Around every galaxy, astronomers observe a large cloud of material that 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 . . .

It is hardly surprising that there are more things in heaven and earth, dear reader, than have been dreamed of in our philosophy—or in our physics. We did not even suspect the existence of radio waves and neutrinos, which fill the universe, until recently. The Standard Model remains the best that we have when speaking today about the world of things; its predictions have all been confirmed, and, apart from dark matter—and gravity as described in the general theory of relativity as the curvature of space-time—it describes well every aspect of the perceived world.

Alternative theories have been proposed, only to be demolished by experiments. A fine theory proposed in the 1970s and given the technical name SU(5), for example, replaced the disordered equations of the Standard Model with a much simpler and more elegant structure. The theory predicted that a proton could disintegrate, with a certain probability, transforming into electrons and quarks. Large machines were constructed to observe protons disintegrating. Physicists dedicated their lives to the search for an observable proton disintegration. (You
do not look at one proton at a time, because it takes too long to disintegrate. You take tons of water and surround it with sensitive detectors to observe the effects of disintegration.) But, alas, no proton was ever seen disintegrating. The beautiful theory, SU(5), despite its considerable elegance, was not to the good Lord’s liking.

The story is probably repeating itself now with a group of theories known as “supersymmetric,” which predicts the existence of a new class of particles. Throughout my career I have listened to colleagues awaiting with complete confidence the imminent appearance of these particles. Days, months, years, and decades have passed—but the supersymmetric particles have not yet manifested themselves. Physics is not only a history of successes.

So, for the moment we have to stay with the Standard Model. It may not be very elegant, but it works remarkably well at describing the world around us. And who knows? Perhaps on closer inspection it is not the model that lacks elegance. Perhaps it is we who have not yet learned to look at it from just the right point of view, one that would reveal its hidden simplicity. For now, this is what we know of matter:

A handful of types of elementary particles, which
vibrate and fluctuate constantly between existence and nonexistence 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.

Grains of Space

Despite certain obscurities, infelicities, and still unanswered questions, the physics I have outlined provide a better description of the world than we have ever had in the past. So we should be quite satisfied. But we are not.

There’s a paradox at the heart of our understanding of the physical world. The twentieth century gave us the two gems of which I have spoken: general relativity and quantum mechanics. From the first cosmology developed, as well as astrophysics, the study of gravitational waves, of black holes, and much else besides. The second provided the foundation for atomic physics, nuclear
physics, the physics of elementary particles, the physics of condensed matter, and much, much more. Two theories, profligate in their gifts, which are fundamental to today’s technology and have transformed the way we live. And yet the two theories cannot both be right, at least in their current forms, because they contradict each other.

A university student attending lectures on general relativity in the morning and others on quantum mechanics in the afternoon might be forgiven for concluding that his professors are fools or have neglected to communicate with one another for at least a century. In the morning the world is curved space where everything is continuous; in the afternoon it is a flat space where quanta of energy leap.

The paradox is that both theories work remarkably well. Nature is behaving with us like that elderly rabbi to whom two men went in order to settle a dispute. Having listened to the first, the rabbi says: “You are in the right.” The second insists on being heard. The rabbi listens to him and says: “You’re also right.” Having overheard from the next room, the rabbi’s wife then calls out, “But they can’t
both
be in the right!” The rabbi reflects and nods before concluding: “And you’re right too.”

A group of theoretical physicists scattered across the five continents is laboriously trying to settle the issue. Their field of study is called “quantum gravity”: its objective is to find a theory, that is, a set of equations—but above all a coherent vision of the world—with which to resolve the current schizophrenia.

It is not the first time that physics finds itself faced with two highly successful but apparently contradictory theories. The effort to synthesize has in the past been rewarded with great strides forward in our understanding of the world. Newton discovered universal gravity by combining Galileo’s parabolas with the ellipses of Kepler. Maxwell found the equations of electromagnetism by combining the theories of electricity and of magnetism. Einstein discovered relativity by way of resolving an apparent conflict between electromagnetism and mechanics. A physicist is only too happy when he finds a conflict of this kind between successful theories: it’s an extraordinary opportunity. Can we build a conceptual framework for thinking about the world that is compatible with what we have learned about it from
both
theories?

Here, in the vanguard, beyond the borders of knowledge, science becomes even more beautiful—
incandescent in the forge of nascent ideas, of intuitions, of attempts. Of roads taken and then abandoned, of enthusiasms. In the effort to imagine what has not yet been imagined.

Twenty years ago the fog was thick. Today paths have appeared that have elicited enthusiasm and optimism. There are more than one of these, so it can’t be said that the problem has been resolved. The multiplicity generates controversy, but the debate is healthy: until the fog has lifted completely, it’s good to have criticism and opposing views. One of the principal attempts to solve the problem is a direction of research called “loop quantum gravity,” pursued by a loose band of researchers working in many countries.

Loop quantum gravity is an endeavor to combine general relativity and quantum mechanics. It is a cautious attempt because it uses only hypotheses already contained within these theories, suitably rewritten to make them compatible. But its consequences are radical: a further profound modification of the way we look at the structure of reality.

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 that 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 that determine their evolution. They are called “loops,” or rings, because they are linked to one another, forming a network of relations that 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 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.” At the minute scale of the grains of space, the dance of nature does not take place to the rhythm of the baton of a single orchestral conductor, at a single tempo: every process dances independently with its neighbors, to its own rhythm. 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.

The world described by the theory is thus further distanced from the one with which we are familiar. There is no longer space that “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 one another. The illusion of space and time that 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.

Viewed in extreme close-up through an ultrapowerful magnifying glass, the penultimate image in our third lesson should show the granular structure of space:

Is it possible to verify this theory experimentally? We are thinking, and trying, but there is as yet no experimental verification. There are, however, a number of different attempts.

One of these derives from the study of black holes. In the heavens we can now observe black holes formed by collapsed stars. Crushed by its own weight, the matter of these stars has collapsed upon itself and disappeared from our view. But where has it gone? If the theory of loop quantum gravity is correct, matter cannot really have collapsed to an infinitesimal point. Because infinitesimal points do not exist—only finite chunks of space. Collapsing under its own weight, matter must
have become increasingly dense, up to the point where quantum mechanics must have exerted a contrary, counterbalancing pressure.

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 kilometers 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. This process, as seen by a hypothetical observer sitting in the black hole on the Planck star, would be a rebound occurring at great speed. But time does not pass at the same speed for him 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 him, 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.

It is possible that in the furnace of the first instants of the universe black holes were formed and that some of these are now exploding. If that were true, we could perhaps observe the signals that they emit when exploding, in the form of high-energy cosmic rays coming from the sky, thereby allowing us to observe and measure a direct effect of a phenomenon governed by quantum gravity. It’s a bold idea—it might not work, for example, if in the primordial universe not enough black holes were formed to allow us to detect their explosions today. But the search for signals has begun. We shall see.

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 world 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 that 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 that 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 that the equations can, however, still describe. And the final image of the third lesson is transformed thus:

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. 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 learned in the twentieth century about the physical world, the clues point toward something profoundly different from our instinctive understanding of matter, space, and time. Loop quantum gravity is an attempt to decipher these clues and to look a little farther into the
distance.