The Construction of a High-Energy Collider: Interviews with Witten, Glashow and Weinberg by Hong-Jian He* Interview: Edward Witten Discusses bert Einstein World Award of Science (2016), and Future High Energy Colliders APS Medal for Exceptional Achievement in Research (American Physics Society, 2016), to just name a few. Edward Witten is currently the Charles Simonyi The physics community has highly admired his ex- Professor at Institute for Advanced Study, Prince- ceptional creativities and contributions, just as the ton. He recently visited Beijing, where he attended citation of Isaac Newton Medal stated, “for his many the international conference of String-2016 hosted profound contributions that have transformed areas by Tsinghua University, and right before this meet- of particle theory, quantum field theory and general ing he and Professor David Gross (Laureate of No- relativity.” The news media often describes him as the bel Physics Prize 2004) received Honorary Doctorate successor of Einstein. Even though he is very modest, from the University of Chinese Academy of Sciences, many fellow physicists would argue that his style also as conferred by the president of Chinese Academy of shares deep similarity with Newton because Newton Sciences, Chunli Bai. Professor Witten visited China is a truly unique master of modern science who is many times before, and on February 23, 2014, he was known as both a great physicist and a great mathe- invited to join the Panel Discussion Meeting on “Af- matician. ter the Higgs Boson Discovery: Where is Fundamental Physics Going” together with Nobel laureates David Hong-Jian He: Professor Witten, we just met you in Gross and Gerard ’t Hooft et al., held at Tsinghua August at String-2016 held at Tsinghua University. It University, Beijing. For the public in China, most peo- is our great pleasure to have this chance for an in- ple only know Professor Witten as the Fields Medal- terview with you. Our first question is to ask you for ist in mathematics and a leading string theorist. But, your comment on the vital importance of the inter- for the physics community, he is a prominent the- face between physics and mathematics, which had oretical physicist who made wide range of impor- revolutionized the progress of physics many times in tant contributions to fundamental physics. In fact, be- the past, including Newtonian Mechanics, Special and sides the Fields Medal (1990), he has won numerous General Relativity, Quantum Mechanics, and Quan- prestigious prizes in physics, including Dirac Medal tum Field Theory and Gauge Theory. Since you won (1985), Einstein Medal (1985), National Medal of Sci- both the Newton Medal and the Poincaré Prize, it is ence (Physics, 2002), Henri Poincaré Prize (2006), interesting to note that in the past of modern sci- Lorentz Medal (2010), Isaac Newton Medal (2010) ence, Newton is best known for discovering Newton’s Fundamental Physics Breakthrough Prize (2012), Al- Laws in physics, but he invented Calculus for math- ematics with the motivation of solving physics prob- * Hong-Jian He is a Professor of Physics at Tsinghua Uni- lem. On the other hand, Poincaré was a mathemati- versity (Beijing) working in particle physics, cosmology, and quantum gravity, and the interface between them. cian by birth, but he made fundamental contributions E-mail: [email protected] to physics. (David Hilbert is probably another such 64 NOTICES OF THE ICCM VOLUME 4,NUMBER 2 great figure after Poincaré.) Would you like to also far, we hope you can share your views with Chinese comment on both of them from your own experience? community regarding the lessons of the SSC and LHC. Edward Witten: It took a long time for people to un- Witten: It is a real shame that we in the United States derstand that to understand the natural world, one did not complete building the SSC, and if this had should aim for a precise, mathematical description occurred, our understanding of fundamental physics of basic phenomena. The ancient Greeks, for exam- might have been quite advanced compared to where ple, were very interested in mathematics and also in we are now. The United States would certainly have the natural world. But in their study of the natural maintained its leadership position in high energy world, they mainly aimed for qualitative descriptions physics if we had built the SSC. of everything, rather than looking for precise mathe- I think that one of the lessons from the failure of matical descriptions of selected things. the SSC project and the success of the LHC is that for My colleague Steve Weinberg recently wrote an a successful project of this kind, it is very valuable to outstanding book on the history of science explain- have the capability to make long term plans. The Eu- ing the long process by which people learned to look ropean countries are able to make multi-year commit- for precise mathematical explanations of simple phe- ments to CERN, and on the basis of this, it was possi- nomena and not just qualitative descriptions of ev- ble for the LHC to go ahead. Unfortunately, in the U.S., erything. Newton’s laws of motion were one of the Congress reconsiders the budget for a project every big milestones here. year and even if a project is approved and funded one Still, it is a bit of a mystery why mathematics year, or multiple years in a row, it might still eventu- is quite so powerful in understanding the physical ally be canceled. world. The best I can say is that physical laws, when I can also see one advantage of the way we do things in the U.S. In this country, the budget for a big we understand them better, turn out to be subtle and project would always have some sort of contingency elegant. Mathematics is the study of things that are allowance for unexpected costs. In Europe, there was subtle and elegant and can be described and studied a multiyear plan to build the LHC, but with no contin- without reference to any particular cultural context. gency plan for even a minor cost overrun. Hence when We might think that this explains, in part, the util- the LHC did run into a cost overrun, quite small in the ity of mathematics. Alternatively, but with tongue in scheme of things (less than 10% of the cost of project cheek, we might remark that perhaps the Universe cost if one takes into account the CERN laboratory re- was created by a mathematician—or at least, by a sources that were directed to the project), it caused lover of mathematics. political difficulties and led to a delay in the project He: Regarding the lessons of Superconducting of a couple of years. Super Collider (SSC) in USA, perhaps, may you have Based on this, my advice for a country that wants seen an article “The Crisis of Big Science” [1] written to think big is that it is important to make a multiyear by Steven Weinberg in 2012 for the New York Review plan with a realistic allowance for reasonable contin- of Books? Lately we recommended the media in gencies. China to publish its Chinese translation. The can- He: You have known the current Chinese plan of cellation of SSC by US congress in 1993 was a great the “Great Collider” project, whose first phase is loss for the high energy physics (HEP) community called CEPC, an electron-positron collider of energy in USA and worldwide; it seems to have made vital 250 GeV, running in a circular tunnel of circumfer- negative impacts on American HEP in particular and ence about 100 km long. It has a potential second in its whole fundamental science in general. On the phase for a proton-proton collider with energy up one hand, SSC was designed to collide proton-proton to 100 TeV. We are glad to tell you that this pro- beams at a center of mass energy of 40 TeV, which posal has been officially ranked as the “First Prior- is nearly a factor 3 larger than the current Run-2 ity HEP Project” at the recent “Strategic Plan Meeting colliding energy (13 TeV) of Large Hadron Collider for Future High Energy Physics” of the Chinese Par- (LHC) at CERN, Geneva (the European particle physics ticle Physics Association, held last month on August laboratory. The energy of the LHC (13 TeV, possibly 20–21, in Hefei. In fact, CERN is also taking very ac- eventually reaching 14 TeV) is just one-third the tive studies on a similar proposal, called FCC (Future energy that the SSC would have had. In designing Circular Colliders), despite that CERN will be mainly the LHC, the Europeans tried to compensate for occupied by the LHC Run-2 and the subsequent LHC the lower energy by designing a machine with high Upgrade over the next 15–20 years. Many colleagues luminosity (very intense particle beams) but there is worldwide think that this is a truly promising direc- only so far that one can go that way. tion for the next step forward of the high energy Since you have witnessed the termination of the physics. We recall that you and Professor David Gross SSC and the subsequent developments of the LHC so (the laureate of Nobel Physics Prize 2004) wrote a DECEMBER 2016 NOTICES OF THE ICCM 65 joint article “China’s Great Scientific Leap Forward” Witten: Ultimately, China will have to decide what [2] to support this project in last September (pub- sort of position in the world and in the world of sci- lished by The Wall Street Journal). Would you like to ence you aspire to.