The Future of Fundamental Physics Nima Arkani-Hamed Abstract: Fundamental physics began the twentieth century with the twin revolutions of relativity and quantum mechanics, and much of the second half of the century was devoted to the con- struction of a theoretical structure unifying these radical ideas. But this foundation has also led us to a number of paradoxes in our understanding of nature. Attempts to make sense of quantum mechanics and gravity at the smallest distance scales lead inexorably to the conclusion that space- Downloaded from http://direct.mit.edu/daed/article-pdf/141/3/53/1830482/daed_a_00161.pdf by guest on 23 September 2021 time is an approximate notion that must emerge from more primitive building blocks. Further- more, violent short-distance quantum fluctuations in the vacuum seem to make the existence of a macroscopic world wildly implausible, and yet we live comfortably in a huge universe. What, if anything, tames these fluctuations? Why is there a macroscopic universe? These are two of the central theoretical challenges of fundamental physics in the twenty-½rst century. In this essay, I describe the circle of ideas surrounding these questions, as well as some of the theoretical and experimental fronts on which they are being attacked. Ever since Newton realized that the same force of gravity pulling down on an apple is also responsible for keeping the moon orbiting the Earth, funda- mental physics has been driven by the program of uni½cation: the realization that seemingly disparate phenomena are in fact different aspects of the same underlying cause. By the mid-1800s, electricity and magnetism were seen as different aspects of elec- tromagnetism, and a seemingly unrelated phenom- enon–light–was understood to be the undulation of electric and magnetic ½elds. NIMA ARKANI-HAMED, a Fellow Relativity and quantum mechanics pushed the of the American Academy since trend toward uni½cation into territory far removed 2009, is a Professor in the School from ordinary human experience. Einstein taught of Natural Sciences at the Institute us that space and time are different aspects of a sin- for Advanced Study. His interests gle entity: space-time. Energy and momentum are range from quantum ½eld theory united analogously, leading to the famous equiva- and string theory to cosmology and lence between mass and energy, E =mc2, as an im- collider physics. He has published his work in the Journal of High Ener- mediate consequence. Einstein further realized gy Physics, the Journal of Cosmology that space-time is not a static stage on which phys- and Astroparticle Physics, and Nucle- ics unfolds, but a dynamic entity that can curve and ar Physics, among other places. bend. Gravity is understood as a manifestation of © 2012 by the American Academy of Arts & Sciences 53 The Future space-time curvature. This new picture ways. The nature of the interactions is of Funda- of space-time made it possible to conceive almost completely dictated by the rules mental Physics of ideas that were impossible to articulate of quantum mechanics, together with the in the Newtonian picture of the world. requirement that the interactions take Consider the most important fact about place at points in space-time, in compli- cosmology: we live in an expanding uni- ance with the laws of special relativity. The verse. The distance between two galaxies latter requirement is known as the princi- grows with time. But the galaxies are not ple of locality. rushing apart from each other into some One of the startling general predictions preexisting space, as though blown out of of quantum ½eld theory is the existence of an explosion from some common center. anti-particles such as the positron, which Rather, more and more space is being gen- has the same properties as the electron but Downloaded from http://direct.mit.edu/daed/article-pdf/141/3/53/1830482/daed_a_00161.pdf by guest on 23 September 2021 erated between the galaxies all the time, the opposite electric charge. This predic- so from the vantage point of any one gal- tion has another striking consequence: axy, the others appear to be rushing away. namely, that even the vacuum has struc- This picture, impossible to imagine in ture and dynamics. Newton’s universe, is an inevitable con- Suppose we attempt to check that some sequence of Einstein’s theory. small region of space-time is empty. Be- Quantum mechanics represented a more cause of the uncertainty principle, we need radical departure from classical physics, higher energies to probe short distances. involving a completely new conceptual Eventually there is enough energy to make framework, both physically and mathe- an electron and a positron, without vio- matically. We learned that nature is not lating either the conservation of energy deterministic, and only probabilities can or the conservation of charge. Instead of be predicted. One consequence is the fa- seeing nothing, probing the vacuum at mous uncertainty principle, by which we small distances yields particle/anti-particle cannot simultaneously know the position pairs. It is useful to think of the vacuum and velocity of a particle to perfect accu- as ½lled with quantum fluctuations, with racy. Quantum mechanics also allowed “virtual” particles and anti-particles pop- previously irreconcilable phenomena to ping in and out of existence on faster and be understood in a uni½ed way: particles faster timescales at shorter and shorter and waves came to be seen as limiting as- distances. pects of the underlying description where These quantum fluctuations give rise to there are no waves at all, only quantum- measurable physical effects. For instance, mechanical particles. the cloud of virtual electrons and posi- trons surrounding an electron is slightly The laws of relativity and quantum perturbed by the electron’s electric ½eld. mechanics are the pillars of our current Any physical measurement of the elec- understanding of nature. However, de- tron’s charge, then, will vary just slightly scribing physics in a way that is compat- with distance, growing slowly closer in to ible with both of these principles turns the electron as more of the virtual cloud out to be extremely challenging; indeed, is pierced. These virtual effects can be cal- it is possible only with an extremely con- culated very precisely; in some circum- strained theoretical structure, known as stances, theoretical predictions and exper- quantum ½eld theory. A quantum ½eld the- imental observations can be compared to ory is characterized by a menu of particles an astonishing level of precision. The vir- that interact with each other in various tual corrections to the magnetic proper- 54 Dædalus, the Journal ofthe American Academy of Arts & Sciences ties of the electron, for example, have been same quantum-½eld-theoretic language. Nima theoretically computed to twelve decimal Electromagnetism is associated with inter- Arkani- Hamed places, and they agree with experiment to actions between electrons and photons of that level of precision. a speci½c sort. Strong interactions arise The second-half of the twentieth cen- from essentially identical interactions be- tury saw a flurry of activity, on both ex- tween quarks and gluons, while weak inter- perimental and theoretical fronts. These actions connect particles like the electron developments culminated in the 1970s and the neutrino in the same way, with with the construction of the Standard massive cousins of the photon known as Model of particle physics, a speci½c quan- the W and Z particles. tum ½eld theory that describes all known Differences appear at long distances for elementary particles and their interactions subtle reasons. The electromagnetic inter- Downloaded from http://direct.mit.edu/daed/article-pdf/141/3/53/1830482/daed_a_00161.pdf by guest on 23 September 2021 down to the smallest distances we have action was the ½rst to be detected and un- probed so far. There are four basic inter- derstood because the photon is massless actions: gravity and electromagnetism, and the interaction is long-ranged. The W which were familiar even to the ancients, and Z particles are massive, thus mediat- as well as the weak and strong interactions ing an interaction with a short range of that reveal themselves only on nuclear about 10-17 cm. The difference with quarks scales. Atomic nuclei consist of neutrons and gluons is more subtle still: the virtual and protons. An isolated neutron is un- effects of the cloud of gluons surround- stable, living for about ½fteen minutes be- ing a quark make the “strong charge” of fore disintegrating into a proton, electron, quarks slowly grow stronger at longer dis- and an anti-neutrino. (This process is also tances. At a distance of roughly 10-14 cm, responsible for radioactivity.) Fifteen min- the interaction is so strong as to perma- utes is enormously long compared to the nently con½ne quarks inside protons and typical timescales of atoms and nuclei, so neutrons. the interaction responsible for triggering But from a fundamental short-distance this decay must be very feeble–hence, perspective, these are details: the character weak interaction. The earliest incarnation of the laws is essentially identical. This of the strong interaction was noticed in the fact illustrates the central reason why we attraction keeping protons inside nuclei, probe short distances in fundamental counterbalancing their huge electrical re- physics. It is not so much because we care pulsion. about the “building blocks of matter” and Some familiar particles, such as elec- the associated set of particles we may dis- trons and photons, remain as elementary cover, but because we have learned that point-like entities in the Standard Model. the essential unity, simplicity, and beauty Others, like the proton, are understood to of the underlying laws manifest most be bound states, around 10-14 cm in diam- clearly at short distances.