### Interview: Edward Witten Discussing Future High Energy Colliders

##### -- Edward Witten、Hong-Jian He

[This interview will be published in ICCM Notices, and the Chinese translation had been published in Math. Sci. History & Culture (数理人文) magazine (Wechat version)].

Edward Witten is currently the Charles Simonyi Professor at Institute for Advanced Study, Princeton. He recently visited Beijing, where he attended the international conference of String-2016 hosted by Tsinghua University, and right before this meeting he and Professor David Gross (Laureate of Nobel Physics Prize 2004) received Honorary Doctorate from the University of Chinese Academy of Sciences, as conferred by the president of Chinese Academy of Sciences, Chunli Bai. Professor Witten visited China many times before, and on February 23, 2014, he was invited to join the Panel Discussion Meeting on “After the Higgs Boson Discovery: Where is Fundamental Physics Going” together with Nobel laureates David Gross and Gerard ‘t Hooft et al., held at Tsinghua University, Beijing.

For the public in China, most people only know Professor Witten as the Fields Medalist in mathematics and a leading string theorist. But, for the physics community, he is a prominent theoretical physicist who made wide range of important contributions to fundamental physics. In fact, besides the Fields Medal (1990), he also won numerous prestigious prizes in physics, including Dirac Medal (1985), Einstein Medal (1985), National Medal of Science (Physics, 2002), Henri Poincare Prize (2006), Lorentz Medal (2010), Isaac Newton Medal (2010) Fundamental Physics Breakthrough Prize (2012), Albert Einstein World Award of Science (2016), and APS Medal for Exceptional Achievement in Research (American Physics Society, 2016), to just name a few. The physics community has highly admired his exceptional creativities and contributions, just as the citation of Isaac Newton Medal stated, “for his many profound contributions that have transformed areas of particle theory, quantum field theory and general relativity.” The news media often describes him as the successor of Einstein. Even though he is very modest, many fellow physicists would argue that his style also shares deep similarity with Newton because Newton is a truly unique master of modern science who is known as both a great physicist and a great mathematician.

E. Witten at Tsinghua University in 2016

#### The Interview

Below are our interview questions (Q) and the answers of Professor Witten (W).

Q: Professor Witten, we just met you in August at String-2016 held at Tsinghua University. It is our great pleasure to have this chance for an interview with you. Our first question is to ask you for your comment on the vital importance of the interface between physics and mathematics, which had revolutionized the progress of physics many times in the past, including Newtonian Mechanics, Special and General Relativity, Quantum Mechanics, and Quantum Field Theory and Gauge Theory. Since you won both the Newton Medal and the Poincare Prize, it is interesting to note that in the past of modern science, Newton is best known for discovering Newton’s Laws in physics, but he invented Calculus for mathematics with the motivation of solving physics problem. On the other hand, Poincare was a mathematician by birth, but he made fundamental contributions to physics. (David Hilbert is probably another such great figure after Poincare.) Would you like to also comment on both of them from your own experience?

W: It took a long time for people to understand that to understand the natural world, one should aim for a precise, mathematical description of basic phenomena. The ancient Greeks, for example, were very interested in mathematics and also in the natural world. But in their study of the natural world, they mainly aimed for qualitative descriptions of everything, rather than looking for precise mathematical descriptions of selected things.

My colleague Steve Weinberg recently wrote an outstanding book on the history of science explaining the long process by which people learned to look for precise mathematical explanations of simple phenomena and not just qualitative descriptions of everything. Newton’s laws of motion were one of the big milestones here.

Still, it is a bit of a mystery why mathematics is quite so powerful in understanding the physical world. The best I can say is that physical laws, when we understand them better, turn out to be subtle and elegant. Mathematics is the study of things that are subtle and elegant and can be described and studied without reference to any particular cultural context. We might think that this explains, in part, the utility of mathematics. Alternatively, but with tongue in cheek, we might remark that perhaps the Universe was created by a mathematician – or at least, by a lover of mathematics.

Q: Regarding the lessons of Superconducting Super Collider (SSC) in USA, perhaps, may you have seen an article “The Crisis of Big Science” [1] written by Steven Weinberg in 2012 for the New York Review of Books? Lately we recommended the media in China to publish its Chinese translation. The cancellation of SSC by US congress in 1993 was a great loss for the high energy physics (HEP) community in USA and worldwide; it seems to have made vital negative impacts on American HEP in particular and in its whole fundamental science in general. On the one hand, SSC was designed to collide proton-proton beams at a center of mass energy of 40TeV, which is nearly a factor 3 larger than the current Run-2 colliding energy (13TeV) of Large Hadron Collider (LHC) at CERN, Geneva (the European particle physics laboratory. The energy of the LHC (13 TeV, possibly eventually reaching 14 TeV) is just one-third the energy that the SSC would have had. In designing the LHC, the Europeans tried to compensate for the lower energy by designing a machine with high luminosity (very intense particle beams) but there is only so far that one can go that way.

Since you have witnessed the termination of the SSC and the subsequent developments of the LHC so far, we hope you can share your views with Chinese community regarding the lessons of the SSC and LHC.

W: It is a real shame that we in the United States did not complete building the SSC, and if this had occurred, our understanding of fundamental physics might have been quite advanced compared to where we are now. The United States would certainly have maintained its leadership position in high energy physics if we had built the SSC.

I think that one of the lessons from the failure of the SSC project and the success of the LHC is that for a successful project of this kind, it is very valuable to have the capability to make long term plans. The European countries are able to make multi-year commitments to CERN, and on the basis of this, it was possible for the LHC to go ahead. Unfortunately, in the U.S., Congress reconsiders the budget for a project every year and even if a project is approved and funded one year, or multiple years in a row, it might still eventually be canceled.

I can also see one advantage of the way we do things in the U.S. In this country, the budget for a big project would always have some sort of contingency allowance for unexpected costs. In Europe, there was a multiyear plan to build the LHC, but with no contingency plan for even a minor cost overrun. Hence when the LHC did run into a cost overrun, quite small in the scheme of things (less than 10% of the cost of project cost if one takes into account the CERN laboratory resources that were directed to the project), it caused political difficulties and led to a delay in the project of a couple of years.

Based on this, my advice for a country that wants to think big is that it is important to make a multiyear plan with a realistic allowance for reasonable contingencies.

Q: You have known the current Chinese plan of the “Great Collider” project, whose first phase is called CEPC, an electron-positron collider of energy 250GeV, running in a circular tunnel of circumference about 100km long. It has a potential second phase for a proton-proton collider with energy up to 100TeV. We are glad to tell you that this proposal has been officially ranked as the “First Priority HEP Project” at the recent “Strategic Plan Meeting for Future High Energy Physics” of the Chinese Particle Physics Association, held last month on August 20-21, in Hefei. In fact, CERN is also taking very active studies on a similar proposal, called FCC (Future Circular Colliders), despite that CERN will be mainly occupied by the LHC Run-2 and the subsequent LHC Upgrade over the next 15-20 years. Many colleagues worldwide think that this is a truly promising direction for the next step forward of the high energy physics. We recall that you and Professor David Gross (the laureate of Nobel Physics Prize 2004) wrote a joint article “China’s Great Scientific Leap Forward” [2] to support this project in last September (published by The Wall Street Journal). Would you like to share your current views on this subject with Chinese publics? Also, please feel free to comment on the status and achievements of Chinese HEP community, including the past and on-going major experiments, such as the BEPC e+e- collider, the neutrino experiments Daya Bay and JUNO, and the dark matter experiment PandaX at Jinping deep underground lab, etc.

W: First I want to express my appreciation for the progress that China has made in fundamental physics. In particular, the pioneering measurement that was made at Daya Bay added an important ingredient in our understanding of neutrinos, which are mysterious elementary particles whose study has led to many surprises. The work at BEPC is also much appreciated and we look forward to results from other current and upcoming experiments.

During my visits to China --- I have been in the country five times, including three visits in the last several years --- I see that the country is advancing rapidly in many areas. Given the changes I see, it is not hard to believe that before too long, China might be the leading country in many areas of theoretical and experimental science.

The CEPC and the 100 TeV collider are very exciting projects and truly worthy of a country that aspires to leadership. For the future development of physics, it is very important to make a deep study of the Higgs particle, as CEPC can do, and to probe beyond the electroweak scale of energies, as can be done at the 100 TeV collider.

Q: You probably have heard about the on-going public debate in the Chinese community on whether such a Great Collider should be built in China at all. This debate was provoked early this month by the 94-year-old Chinese American theoretical physicist C. N. Yang (one of the Nobel laureates in 1957, retired in 1997), who has been strongly against any collider project in China, including the current CEPC-SPPC project led by IHEP director Yifang Wang. Attached below is the English translation of Yang’s recent public article and the Reply by Yifang Wang, which you may have heard earlier. It is clear that his major objection is that the project would cost too much for China, and he stressed the cost of the second phase of pp collision that would be built starting in the 2040’s. (As one may recall, the funds of the LEP and LHC at CERN were approved separately and in sequence.) It will be extremely helpful for the Chinese community to learn your independent viewpoint and advice from international side. We recall that you discussed this issue in your joint article with David Gross last fall and also in a recent press-interview at Tsinghua during String-2016. It is a pity that these were barely known to the Chinese publics and were largely forgotten so far.  Would you and your colleagues think that the funds invested for CEPC worthwhile? and what would this contribute to the world and to the society of China?

W: Ultimately, China will have to decide what sort of position in the world and in the world of science you aspire to.

Yes, China must continue economic development. China’s success in lifting hundreds of millions of people out of poverty is one of the inspiring changes that have occurred in the world in my lifetime. This process still has a long way to go.

Moreover, China’s continued economic development is really a precondition for being able to afford to build CEPC in the 2020’s and to build the 100 TeV collider starting in the 2040’s.

As it develops, China must decide its priorities and its ambitions. For what it is worth, I personally think that China is entirely capable of assuming leadership in many areas of science and that this is a worthy goal that befits your cultural traditions and can benefit your society. And I believe that leadership in high energy physics and multiple other areas can be entirely affordable as China continues to develop.

With that said, the scope of your ambitions is something that must ultimately be decided by the Chinese people.

#### References:

[1]. Steven Weinberg, “The Crisis of Big Science”, in The New York Review of Books, May 10, 2012. webpage For Chinese translation of this article, see: webpage

[2]. David Gross and Edward Witten, “China’s Great Scientific Leap Forward”, in The Wall Street Journal, September, 2015. webpage For Chinese translation of this article, see: webpage

#### China Should Not Build a Super-Collider Now

-- C. N. Yang

In an article published in WeChat public account “Mr. Gu on Geometry”, titled “S. T. Yau: A few comments on the construction of high energy collider in China, and answer to questions from the media”, he said he support the construction of super collider in China, but I (Yang) am against, he can’t believe this. This is specifically mention in the following paragraph:

“The theoretical basis of all these experiments used the theory invented by Mr. Yang. My respect for him grew after each breakthrough. Therefore, it is hard to understand why Mr. Yang opposes the idea that high energy physics needs further development”.

Professor Yau has misunderstood me! I am definitely not against the further development of high energy physics. Instead, I oppose we begin to construct super big collider in China now, for the following reasons:

1) Construction of big collider has been a painful experience for the US: In 1989, it started to build the world’s largest collider. The initial budget was estimated to be 3 billion dollars, but it was added upon for several times later, reaching 8 billion dollars, which triggered a lot of opposing voice. Eventually, the congress painfully cancelled this project in 1992, wasting about 3 billion dollars. This experience led many people to believe that the construction of large collider is a bottomless money sink.

Currently, the largest collider is the LHC at CERN. In 2012, a collaboration of about 6000 physicists discovered the Higgs boson at the LHC. This is a major achievement of particle physics, verified the Standard Model. It has taken many years to construct the LHC. The total cost is no less than 10 billion dollars, including the machine and the detectors. The budget of the super big collider proposed by IHEP is certainly more than 20 billion dollars.

2) IHEP proposed to construct the super big collider in China, sharing the cost among many countries. However, China must shoulder a significant portion of it. Today, to the amazement of the world, the GDP of China has jumped to the second place in the world. However, China is still a developing country, its GDP per capita is still less than that of Brazil, Mexico or Malaysia. There are still a couple hundred millions of farmers and migrant workers. There are urgent problems to deal with, including environment, education, health and medicine, etc. The construction of the super big collider will be very costly, and hamper the solution of those urgent problems. I think we should not consider it now.

3) The construction of the super big collider will significantly squeeze the funding for other basic science, including biological science, condensed matter physics, astrophysics, etc.

4) Why is the construction of super big collider enthusiastically supported by many high energy physicists? The reasons are:

a. High energy physics is a new area developed after the World War II. It has had spectacular success in the past 70 years, verified the Standard Model, enabling deep understanding of the three fundamental interactions in nature. However, there are still two unsolved big problems:

i. Deeper understanding of the remaining fourth fundamental interaction, gravity, have encountered fundamental difficulties.

ii. It has been not possible to unify force and mass. It is certain the hope of all physicists to solve these two problems.

b. Some high energy physicists hope that the super big collider can discover “supersymmetric particles”, and therefore point to the way of the solution for these two problems.

However, the search for supersymmetry has been going on for many years, with no discovery. The supersymmetric particles which they hope to find at the super big collider is only a hypothesis by a subset of high energy physicists. Most of the physicists, myself included, think the existence of supersymmetric particles is just a hypothesis without any experimental evidence. The hope to find them at the super big collider is only a hypothesis about a hypothesis.

5) What are tangible benefits to people’s life from the big achievement of the high energy physics in the past 70 years? Nothing. What are tangible benefits to people’s life if the super big collider proposed by IHEP can be built, and it can succeed in significantly advancing high energy physics? I think it is impossible in the short term, impossible in 30 or 50 years. Moreover, I know my point of view is agreed upon by the absolute majority of the physicists.

6) IHEP has been established for 30 years. How should we evaluate its achievements in this period? Among the important high energy physicists in the world, only less than 1 to 2 per cent are in China. For the super big collider, its design, construction, and the operation and data analysis after its completion, will definitely be led 90% by foreigners. If there will be Nobel prize, would it be awarded to Chinese?

7) Without the construction of the super collider, is there no future for high energy physics? No. I think there are at least two directions worth pursuing: A. Search for new accelerator design principles. B. Search for beautiful geometrical structure, such as in the research of string theory. The research in both of these directions do not cost as much money. It is suitable under the current trend of economic development of the world.

#### Attachement-2

Note: The English translation of Yifang Wang’s original Reply (“China Should Build a Super-Collider Now”) is not available yet. Below is a summary report of Wang’s refutation against Yang, by “China Daily”, which can be a useful reference.

[Published in China Daily, Updated: 2016-09-10 07:10]

#### Sufficient Reasons for Nation to Build a Supercollider

Editor's Note: Chen-Ning Yang, a Chinese-American Nobel Prize-winning physicist, recently advised China publically not to proceed to build a super collider. One major argument is that the project involves huge investment, which can be used for social programs and other research projects. Responding to this, two physicists expressed their views:

Progress will be worth the investment

Professor Yang's first argument in opposing the supercollider is the high cost. Citing the United States' Superconducting Super Collider, which had initially been budgeted at $3 billion in 1989, but was abandoned three years later because of rapidly rising cost. Yang said China should not repeat the mistake made by the US. But the SSC failed because of multiple reasons, among them are US federal government budget deficit, political struggles between the Democratic and Republican parties, as well as regional competition between Texas and other states. It would not be the case in China. The Institute of High Energy Physics of the Chinese Academy of Sciences has built a number of large scale scientific facilities, such as the Beijing Electron-Positron Colliders (BEPC, BEPCII) and the ADS injector. All the costs have been well controlled within budget range. There is no reason to expect China's supercollider program will exceed the budget as the case of SSC. Yang also said that the investment in such a huge project would mean less funding for programs to improve people's livelihoods and social welfare. Of course China needs to fund programs to improve social welfare, but it also has to spend on scientific research to facilitate technological breakthroughs. Furthermore, progresses made in high energy physics help improve people's livelihoods, too. Today we have Magnetic Resonance Imaging devices to diagnose diseases in hospitals, we have touch screens that allow us to conveniently use our smartphones, and we have the World Wide Web to share information. Without pursuing research in high energy physics for the past 70 years and the technology innovations associated with the research, all these would have been impossible, or significantly delayed in time. For China, a supercollider will allow its scientists to keep abreast of the latest developments in physics, even enabling the country to become a global research center. That is irreplaceable to its development in the long run. The budget for building a supercollider will not dry up funds for other research programs, either. Basic research accounts for only 5 percent of R&D funding in China, which only represents one third of developed countries. As the top Chinese leadership vows to increase investment in basic research, the fund for basic research is expected to increase by 100 billion yuan ($15 billion) annually; as a comparison, the planned supercollider would require 3 billion yuan a year from 2022 to 2032 in its first stage. Therefore, the supercollider will not squeeze the funds for basic research programs.

Yang was also pessimistic about building Chinese particle accelerators in the 1970s, yet the returns from the program were well worth the investment: the science came out of the BEPC, BEPCII colliders, the expertise acquired through the associated experimental programs was vital in building the successful Daya Bay experiment, and the synchrotron radiation facilities and the neutron spallation sources constructed and operated to serve the broad scientific community are all examples of good investment returns.

For the future we should listen more to the young scientists at the front line of research, who will carry China's science programs to the next level. We believe a supercollider in China will help in a big way.

Public discussion on program welcome

That Yang has used the mass media, instead of a professional journal, to disseminate his idea is welcome because the issue should be discussed not only by physicists, but also taxpayers, because taxpayers' money will fund the supercollider program.

Also, this will allow scientists to share their views with the public, which rarely happens. Physicists believe supercolliders are necessary for the advancement of physics but to build one, a country must have enough money, thoroughly discuss the issue, and cooperate with other countries in technology. China meets all these requirements.

There are fears that the supercollider program might take away a huge percentage of China's budget for basic research. But the cost of 3 billion yuan (\$449 million) a year from 2022 to 2032 is not very high considering the total research budget. Besides, a supercollider will help China catch up with the advanced world in science.

More importantly, particle physics advances on the basis of repeated experiments. Only when China has a supercollider to conduct experiments will its scientists be able to improve their research. Taxpayers should realize that the investment in a supercollider will yield promising returns because it will help upgrade technologies as a whole.