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HOW HELD A MIRROR TO FUNDAMENTAL BY NACHIKET GIRISH

THE CURRENT DEADLOCK OF SUPERSYMMETRY HAS RAISED NEW QUESTIONS ABOUT WHAT MAKES A SOUND PHYSICAL

you have seen physics in the news supersymmetry—increasingly finding no simplest case of the problem, and then adding If lately, you likely get the impression support from experimental data. The debate further terms—the quantum corrections—to that now is an exciting time to be a . over how to explain the lack of support take into account the more complex features Just this month, the for physics for supersymmetry has shaken the very of the problem which the original solution was awarded to a woman, Donna Strickland, foundations of scientific philosophy. had ignored.4 for the first time in 55 years. The observation One possible but highly controversial of gravitational waves three years ago heralded THE AND BEYOND explanation was that these corrections a whole new era of observational . fortuitously canceled each other out. Three years before that was the massive Our story begins in the 1970s, with the Alternatively, it was possible that there was triumph of with the discovery development of the Standard Model, the a hidden mechanism which balanced them of the Higgs Boson. broadest and most successful quantum theory out. Attempts to solve this problem led to the What you might not remember, however, physics has ever seen.2 This theory explains development of one of the most exciting new is what these discoveries represent. The almost every single phenomenon we can tools of theoretical physics—supersymmetry. observation of gravitational waves was the observe and has justifiably been called “the Developed in the 1970s by several verification of a prediction Einstein made a pinnacle of human achievement.”2 , supersymmetry postulates hundred years ago, while the Higgs Boson Despite the Standard Model’s great success, that every particle or fermion has a was experimental confirmation of a fifty-year- however, it is not bereft of problems. One of corresponding partner which is a force particle old hypothesis. This symbiotic relationship its most significant difficulties is known as the or boson. A photon of , for instance, is a between theory and is the “.” Theoretical calculations boson, while an electron is a fermion. These defining principle of .1 Theorists are of the of the Higgs Boson and other partner particles are thought to cancel the the pioneers who plow through unexplored related particles have revealed a troubling quantum corrections caused by the regular routes in search of new destinations, while difficulty—quantum corrections should have particles.5 Not only does supersymmetry thus experimentalists check every step to evaluate caused the to be far, far greater than neatly resolve the hierarchy problem, it has the whether the theorists are heading in the right what had actually been observed.3 Quantum added benefit of proposing an explanation for direction. corrections are terms which theorists must —the mysterious class of matter But even as we celebrate these monumental add to their equations when solving problems we know exists but have yet to observe—by achievements of science, physics itself is going using a method known as perturbation offering the supersymmetric partners of through a period of uncertainty, with one theory. In this method, a complicated problem our regular particles as possible dark matter of the hottest of particle physics— is solved by first writing the solution for the candidates.6 It almost seems too good to not

14 Berkeley Scientific Journal | FALL 2018 as “naturally” from fundamental principles as possible; that is, there should be an underlying principle which explains all the predictions of the theory. According to this concept, such a theory is preferable to one in which theorists can artificially fine-tune the values of their predicted constants so that the constants conveniently combine to give the expected result.8 In the case of supersymmetry, in order to correctly cancel the quantum corrections to the Higgs mass, theorists required the superparticles to be more or less equal in mass to their partners. But as the LHC kept reaching progressively higher and mass scales without finding any superparticles, hopeful supersymmetry proponents claimed even higher masses for the partner particles. In order to salvage the situation, supporters of supersymmetry had to fine-tune the values of a variety of other constants, so that when combined, they still yielded the correct Higgs mass.3 Paradoxically, at some point this explanation amounts to the same reasoning inherent in the non-supersymmetric Standard Model—that the serendipitous cancellation Figure 1: The standard model, the theory of almost . of the quantum corrections, or in this case, the superparticles attaining the required fine- be true. much more helpful, as it would have at least tuned values, is just a coincidence. The problem was that verifying the provided theorists with some direction. On While in principle there is nothing predictions of supersymmetry would require the contrary, the experimental data neither wrong with this kind of theory—we might particle accelerators capable of reaching supports nor disproves any of the predictions simply be lucky to live in a where no accelerator of that time could of supersymmetry. Though the LHC has been fundamental constants are custom-made reach. To remedy this, physicists built the able to confirm and reaffirm the Standard for our existence—it flies in the face of the Large Hadron Collider (LHC) in 2008. It Model, it has failed to fulfill its founding principle of naturalness, which demands an was, and remains, the largest machine ever purpose.8 The upshot is that a large number explanation for this coincidence. In following built, designed to generate sufficiently high of physicists are left with a theory they this approach, moreover, proponents of energies to explore the new physics beyond spent several decades developing to such a supersymmetry can keep raising the target the standard model.7 degree that even its critics acknowledge its particle-mass level for experimentalists to mathematical potential—without any idea of OPTIMISM TURNS TO DESPAIR its validity. Supporters of supersymmetry suggest that “Even as we celebrate It is here that the mood of this narrative physicists simply underestimated the masses becomes less upbeat. In the ten years that the of the supersymmetric particles; perhaps, the these achievements of LHC has been operational, it has not detected superparticles are actually heavier than what science, physics itself a single supersymmetric or dark matter even the LHC can currently detect.9 Not only particle. Nor has it given any clues whatsoever might some consider this a suspiciously ad is stuck in a period of for the existence of supersymmetry. The hoc claim, but increasing the predicted masses uncertainty, with one of discovery of the Higgs, though a phenomenal of the superparticles also raises questions the hottest theories of success, was but an additional confirmation of regarding the principle of naturalness. the Standard Model. particle physics— The current state of affairs, affectionately NATURALNESS IN PHYSICAL THEORIES supersymmetry— dubbed the “nightmare scenario,” leaves physicists in a unique quandary. Naturalness is a principle in scientific finding no support from Supersymmetry has not been disproved— philosophy which demands that the physical experimental data.” in fact, that outcome would have been parameters of a should arise

FALL 2018 | Berkeley Scientific Journal 15 Figure 2: Ptolemy vs. Copernicus. Since a large number of epicycles of very convenient sizes and orientations need to be added to get the required accuracy in Ptolemy’s model, we consider his geocentric theory to be fine-tuned and "unnatural." We thus prefer the much simpler heliocentric theory proposed by Copernicus. reach by simply fiddling with their constants— of nature.”8 There is, however, no quantitative the solar system or those of the expanding and thus keep justifying their research (and reason to reject a theory simply for being universe, have had to develop new their funding). These modifications make the “unsatisfying.” theories whenever the reigning ones have theory more fine-tuned and thus less natural On the other hand, historically, similar required overly contrived modifications.11 and elegant. cases have demonstrated that the simpler This dilemma leaves physics at an The trouble is that the concept of theory is usually correct. For example, Ptolemy interesting crossroad, one in which the debate naturalness is purely human, and does not clung to the geocentric idea of the solar system over the validity of a theory is, to a certain necessarily have anything to do with the by proposing that heavenly bodies moved extent, philosophical rather than scientific universe we live in. Indeed, when there is no around the in multiple nested circles in nature. Young researchers entering the experimental evidence clearly falsifying one called epicycles. By arbitrarily adjusting the now have a difficult choice to make: theory or another, the choice of which one number and size of epicycles in his model, he continue efforts at justifying the absence to use often becomes a matter of taste. For could make his theory as accurate as desired.10 of experimental evidence by fine-tuning— instance, in discussing a possible modification In contrast, Copernicus proposed a much hoping that some evidence shows up sooner to supersymmetry in his paper “The State simpler heliocentric model of the solar system, rather than later—or break away and explore of Supersymmetry after Run I of the LHC,” with Newton later providing the underlying radically different ideas. Which way should author Nathaniel Craig, a physicist at the physical explanation for this model. Despite future research go? This debate, regardless Institute for Advanced Study at Princeton the matching accuracy of both theories, of how it is resolved, will have an immense University, evaluates the modified theory Copernicus’ more elegant heliocentric model impact on the future of theoretical physics. by noting that “there is no reason it can’t be of the solar system is what ultimately proved It is indeed an exciting time to be a physicist. there, but it’s fairly unsatisfying as a theory to be the more correct one. Be it theories of

REFERENCES 5. Wolchover, B. (2016, August). What Retrieved from http://arxiv.org/ 1. Feynman, R., Leighton, R., & Sands, No New Particles Means for Physics. abs/1309.0528. M. (1963). The Feynman Lectures on Quanta Magazine. Retrieved from 9. Baer, H., Barger, V., & Mickelson, D. Physics: Volume 1 (2nd Edition ed., https://www.quantamagazine.org/ (2013). How conventional measures Vol. 1). what-no-new-particles-means-for- overestimate electroweak fine-tuning 2. Oerter, R. (2006). The Theory of Almost physics-20160809/. in supersymmetric theory. Physical Everything: The Standard Model, the 6. Tata, X. (2015). Supersymmetry: Review D, 88(9). doi:10.1088/0031- Unsung Triumph of : Aspirations and Prospects. Physica 8949/90/6/068003. Penguin. Scripta, 90(10). doi:10.1088/0031- 10. Jones, A., (1998). Ptolemaic System. In 3. Shifman, M. (2012). Reflections and 8949/90/10/108001. Encyclopaedia Britannica. Retrieved Impressionistic Portrait at the Confer- 7. Khalil, S. (2003). Search for supersym- from https://www.britannica.com/sci- ence “Frontiers Beyond the Standard metry at LHC. Contemporary Physics, ence/Ptolemaic-system. Model,” FTPI, Oct. 2012. Retrieved 44(3), 196. doi:10.1080/001075103100 11. Hawking, S., Mlodinow, L. (2010). The from http://arxiv.org/abs/1211.0004. 0077378. Grand Design: Bantam. 4. Was, Z. (1994). Radiative corrections 8. Craig, N. (2013). The State of Super- (No. CERN-TH-7154-94). P00020602. after Run I of the LHC.

16 Berkeley Scientific Journal | FALL 2018