About the authors: Professor Stephen W. Hawking is probably the only living scientist today who needs no introduction to the publics in this world.
Professor Gordon Kane is Victor Weisskopf Distinguished University Professor at the University of Michigan and Director Emeritus at the Michigan Center for Theoretical Physics (MCTP). He received the Lilienfeld Prize from the American Physical Society in 2012, and the J. J. Sakurai Prize for Theoretical Particle Physics in 2017.
(Photo: Flickr / Lwp Kommunikáció and Wiki)
Throughout the history of human civilization, and especially for the past four centuries, understanding our physical universe has been a goal of many people. It is the focus of physics. By the end of the 20th century, we had arrived at a successful, but incomplete, description of our world: the Standard Models of particle physics and of cosmology. This description is valid to the highest energies and to the edges of the universe. It achieves the traditional goals of physics.
These Standard Models are descriptions, and we do not yet know why they are correct. In addition, the Standard Models do not include gravity, particularly a quantum theory of gravity. And they do not include an explanation of the dark matter of the universe, or why there were equal amounts of matter and antimatter at the big bang but today the amount of antimatter in the universe is only one billionth of the amount of matter, and much more.
The boundaries of physics have changed over the past few decades. Physicists have become more ambitious. Beginning in the 1970s, efforts were made to unify the forces into one underlying force rather than the several we apparently observe. Around the same time, the idea of supersymmetry was found to be a powerful ingredient in our potential understanding of such unification. That was reinforced in the 1980s with the discoveries of inflation and string theory. Back in the 1920s Ernest Rutherford said "Don't let me catch anyone talking about the universe in my department". Today it is different, as Steven Weinberg put it, "Scientists of the past were not just like scientists of today who didn't know as much as we do. They had completely different ideas of what there was to know or how you go about learning it".
Progress in physics can come from new concepts or new tools, such as new particle colliders or new detectors. Without the CERN Laboratory Large Hadron Collider (LHC) we would not know about the existence of the Higgs boson, which changes and sharpens in fundamental ways our understanding of the universe. For many people it is a source of awe and comfort to see that humans can understand our universe.
A historical guide as to how not to proceed comes from the U.S. cancellation of the Superconducting Super Collider (SSC) in 1993. That has led to the U.S. no longer being the world leader in basic particle physics, and created an opening for China to move toward that position. It is well documented that the SSC failed for several complicated reasons, political and accidental ones, mismanagement, demanding international participation, and more, with cost overruns not being a dominant one.
The discovery of the Higgs boson at CERN in 2012 was a wonderful and major step forward in understanding the universe. It taught us that the Standard Model of particle physics along with broken symmetries that allow mass lead to a successful description of our world. The role of the Higgs interaction is remarkable - if electrons could not get mass via interacting with the Higgs field then atoms would be the size of the universe and our world could not exist. Further, when electrons do get mass via the interaction with the Higgs field, quantum corrections make them so massive they turn into black holes unless some new physics yet to be discovered allows them to be stabilized at their actual mass. The proposed collider will search for clues to that new physics.
The proposed Chinese collider would have two phases. The first would be a Circular Electron Positron Collider (CEPC), and the second a Super Proton Proton Collider (SPPC). Both would be in a long tunnel, hopefully about 100 km around. The first phase would focus on learning what the Higgs physics is telling us about a deeper underlying theory. For example, the LHC data on Higgs boson (h) decays suggests that the observed Higgs is like a Standard Model Higgs, even though we know from quantum corrections that the Higgs cannot actually be a Standard Model one. The several Higgs boson decay branching ratios are all consistent with being equal to the Standard Model predictions, even though they could have been very different. But the LHC data still actually allows quite different outcomes. The most important decay is $h \to Z +Z$, where Z's are the bosons that mediate the weak neutral interactions. The ratio of its LHC value to the prediction is about $1.3\pm 0.3$. The LHC can only improve that uncertainty a little with further running, while CEPC could provide an order of magnitude better precision, and really tell us if the Higgs boson was Standard Model-like or not. The situation is similar for several other decays. Also, our present best understanding of the Higgs boson implies that it should be accompanied by partners. Finding them will require a higher energy new collider, and searching for them would be a major goal of a future collider. Better data about Higgs boson properties that could come from a new collider could lead to truly deeper understanding of the remarkable role of Higgs physics. There is an International Linear Collider program in Japan (ILC) whose goals overlap those of CEPC. There are also studies at CERN about future colliders. One, CLIC, is a linear electron-positron collider whose goals would overlap CEPC. In the past there have often been accelerators or colliders in different countries or regions with overlapping goals. Scientifically that can be valuable, and it is surely valuable for all the countries or regions that construct them, as we discuss below.
One great advantage of CEPC over other proposals, such as the ILC and CLIC, is that it can have a second phase, called SPPC, to collide protons at higher energies. The CEPC tunnel will be available for SPPC, for free. There are strong motivations for extending the total energy to at least two or three times the LHC energy, and perhaps ultimately about six or seven times the LHC energy could be feasible. That would require development of higher field superconducting magnets. With proton-proton collisions one can plan for the high luminosity needed to observe signals, and for a research program lasting decades. One major result to aim for at a higher energy collider is the data needed to understand how the Higgs boson itself gets its mass. The second main goal is to search at significantly higher energies to see what might be discovered.
While no one can be sure what might be discovered eventually at CEPC or SPPC beyond the guaranteed Higgs physics, one interesting possibility is the fundamental symmetry called supersymmetry. It might lead to observable partners of the Standard Model particles, just as the charge conjugation symmetry led to an antiparticle for every particle. If so, we know their properties are such that they might be observable at the higher energy SPPC.
Some people have said that the absence of superpartners or other phenomena at LHC so far makes discovery of superpartners unlikely. But history suggests otherwise. Once the b quark was found, in 1979, people argued that "naturally" the top quark would only be a few times heavier. In fact the top quark did exist, but was forty-one times heavier than the b quark, and was only found nearly twenty years later. If superpartners were forty-one times heavier than Z bosons they would be too heavy to detect at LHC and its upgrades, but could be detected at SPPC. In addition, a supersymmetric theory has the remarkable property that it can relate physics at our scale, where colliders take data, with the Planck scale, the natural scale for a fundamental physics theory, which may help in the efforts to find a deeper underlying theory. CERN is also studying building a higher energy proton-proton collider (FCC), with total energy eventually about six times that of LHC, perhaps initially only two-three times LHC. Most likely only one very high energy extension will be built since it will be fairly costly.
It would be of tremendous benefit to China to build CEPC and its future upgrades. An essential point to grasp is that when one is at the frontier of knowledge and understanding, progress requires new techniques and developments and insights. Otherwise discoveries would have already been made. Existing techniques and facilities cannot go further. This has shown up in the past from the LHC in a number of well documented areas, including inventing the World Wide Web with its huge impact on economies world-wide and then grid computing. Someone said imagine that CERN (where the World Wide Web was invented for particle physics) had one penny for each use, then particle physics would have all the funding it could use. More industries include magnet technology and superconducting wire technology, a multi-billion dollar accelerator industry, a multi-billion dollar imaging industry that owes its existence to the development of particle physics detectors, other billion dollar industries, and many tangible benefits. Such technologies generate revenues far exceeding the investment for collider construction.
Arguably the third industrial revolution was triggered by the invention of the World Wide Web at CERN. The requirements for data acquisition and storage and access, and the materials and technologies needed for CEPC and SPPC could help lead to the fourth industrial revolution. For the first decades of the third industrial revolution High Energy Physics led, and only in recent years industry has overtaken HEP. History may repeat itself for the fourth.
About half of all PhD's earned at CERN go to people who move into industries and areas outside of particle physics, and enrich those areas. That would happen with CEPC too. A major effect comes because innovations can lead to start-up companies, but start-ups can be risky. With LHC to provide an initial market for the products of the start-ups, they have been far more likely to succeed. That would be true for a Chinese collider too. New technologies emerge because particle physics necessarily is at the frontiers, and new approaches and techniques are needed to interrogate nature more deeply. China can accelerate the expansion of its economy by investing in a major collider.
Possibly the largest benefit would be attracting a large number of bright young Chinese to science and its goals. Those young people would get excited about many areas of science along the way, and decide to work in those areas, greatly strengthening the entire scientific enterprise in China. The Chinese educational system could handle the challenge of educating many more scientists and benefit greatly from it.
CEPC may make fundamental new discoveries. Even so, a proton-proton collider will be needed to discover more or explore properties of new particles, via a long circular ring with thousands of high field magnets. Again history provides a guide. The bosons (W and Z and gluons) that mediate the forces of the Standard Model were discovered at lower energy facilities. Then CERN built and ran the LEP electron-positron collider for two decades, studying the Standard Model and alternatives, and establishing the Standard Model. Then using the same tunnel, LHC colliding protons at higher energies was built, and discovered the Higgs boson.
Could there be any alternatives to a higher energy facility to discover or exclude new particles? People have invented clever methods to accelerate protons and/or electrons to higher energies, but unfortunately all approaches have led to luminosities far too small to discover new physics. At best they lead to a few events per decade, rather than the tens or hundreds of events a year needed. Seeing the Higgs boson signal at LHC above backgrounds that could fake it took over 200,000 events per detector. In the SSC era of the 1980s opponents of the SSC claimed that new magnet technologies would emerge that would replace the well-established superconducting magnets, but four decades later such new magnet technologies have still not arrived, and are unlikely to exist. A description of the scientific and cultural case for such a collider has been presented in "From the Great Wall to the Great Collider: China and the Quest to Uncover the Inner Workings of the Universe", by Steve Nadis and Shing-Tung Yau, published by the International Press of Boston in 2015.
China has several medium size scientific projects, such as the China Spallation Neutron Source that has just successfully turned on, operated by the Institute of High Energy Physics and the Institute of Physics, one of four such facilities in the world. CERN is unique in high energy physics, the world leader, and a world center for high energy physics with thousands of physicists from around the world working at CERN, and large numbers of visitors converging on CERN to see the laboratory and the detectors. If China built CEPC and then SPPC as a large science project it could become the international center of high energy physics, supplanting the role of CERN. CERN is also studying building such colliders, but only after a decade or more of upgrading and running the Large Hadron Collider.
The Chinese have so far taken a wise approach to financing a number of large science facilities, but mostly not at the leading position in the world, in terms of science, technology, investment scale, and cultural impact. It is important for China move ahead to take the leading role, at least in a few selected areas. The CEPC is a good choice for its scientific importance and technology impact, drawing on thirty years' experience with the BEPC. Nearly all the costs will be spent in China. Once China is proceeding, other countries will join in, stimulating great international collaboration centered around grand human ambitions, in a spirit of a peace and harmony.
Today collider construction is a mature technology. Cost and time estimates will be examined by experts, and are likely to be basically accurate. China's GDP per capita is not yet as high as that of wealthy nations. But that should not be a reason to back away from the collider. On the contrary, the collider will provide work and stimulate economic benefits for many more people. China's total GDP is now among the largest in the world, and can afford a future collider. It has been pointed out by Yifang Wang that the cost of CEPC (and even SPPC) as a fraction of GDP would not exceed that of the existing and scientifically very successful low energy Chinese collider, the Beijing Electron Positron Collider (BEPC) when it was built. Such investments stimulate the technological advances that raise developing nations to economic leaders. It is important for China to continue to show wisdom about supporting scientific research. Funds for a collider should not compete with nor adversely affect other science funding. Each area should have its funding at a level that is healthy for its development.
The Chinese particle physics community has matured. It mastered the low energy collider technology with the Beijing collider, BEPC. Many Chinese physicists have worked at collider laboratories such as CERN and Fermilab. If frontier activities are underway in China, foreign physicists will come to where the action is, and help make any effort maximally successful. When discoveries come, recognition is broadly spread. There is some tradition in particle physics for group leaders and for those whose efforts made the collider possible, to get Nobel Prizes. For the CERN collider the accelerator physicist Simon van der Meer and Carlo Rubbia were recipients, and for the earlier discovery of the charmed quark it was Samuel Ting and Burton Richter. We can expect Chinese Nobel Prizes.
Could new theoretical concepts or tools emerge that would move science forward without new collider facilities? Of course new ideas might lead to new insights. But no matter how elegant a theory might be, without data we will not know if it really describes or explains aspects of nature. Without the discovery of the Higgs boson, there would still be many doubters about the existence of the Higgs field describing our vacuum state. Results from astrophysics and cosmology and the cosmic microwave background provide information about important questions, but no amount of results from these areas could have told us about the top quark existence or mass, or about the Higgs physics, or the unification of forces and more. Data will be crucial to select theories about major issues such as what is the dark matter, or can we unify and simplify the theory of the forces and relate the forces to the Higgs mechanism that allows mass, or what causes the rapid inflation at the beginning of our universe, and more.
It is remarkable that human cultures could reach the level that provided data and ideas that have allowed us to take our understanding of our physical universe to the beginnings of time and to the edges of the universe. China could take us to the next deeper level via knowledge obtained from future collider data. The country that makes the greatest advances in discovering the workings of nature itself, via the sciences of particle physics and cosmology, will be permanently remembered in history for glorious achievements.
About the author: Professor Stephen W. Hawking is probably the only living scientist today who needs no introduction to the publics in this world.
Professor Stephen Hawking presented the following statements by the invitation of Professor Shing-Tung Yau.
(Photo: Flickr / Lwp Kommunikáció)
Particle physics is definitely not a dying field. It is however an entirely different enterprise than it was in 1980. Since then, the standard model looks to be essentially confirmed and this may give the impression that the field is complete. However, that is far from being true. There are phenomena that are just not included in the standard model. Some are CP violation, neutrino oscillations, dark matter. In theory, the problems are immense: how to include gravity, the recently discovered dualities of quantum field theories, quark confinement, dark energy, black holes, early-universe cosmology. It is a different world but one that offers huge challenges to ambitious young people interested in how our Universe works. China has an incredible opportunity to become the world leader here --- don’t waste it. A good example is to build the Great Collider that can lead high energy physics for the next fifty years.
【The Chinese translation (by Hong-Jian He & Zhongzhi Xianyu) had been published in Math. Sci. History & Culture magazine (Wechat version) web link】
粒子物理学绝对不是一个行将就木的领域，也与它在1980年代的面貌完全不同。从那以后，标准模型看起来基本上已被证实，这给人一种该领域已经完成的印象。然而，这绝不是真实情况。自然界还存在标准模型无法解释的许多现象，其中包括CP破坏，中微子振荡，和暗物质，等等。同时我们还有大量理论上的难题：如何包含引力、量子场论中新近发现的各种对偶、夸克禁闭、暗能量、黑洞、和早期宇宙学。这是一个非同寻常的领域，它对于有志向、有兴趣探索我们的宇宙如何运行的年轻人提出了巨大的挑战。在这方面，中国有成为世界领导者的绝佳机遇 —— 不要错过它。一个很好的范例就是建造巨型对撞机，它将在今后五十年中引领高能物理学。
霍金完成了许多意义深远的杰出研究工作，获奖无数。主要的贡献有他与罗杰·彭罗斯（Roger Penrose）共同合作提出的彭罗斯-霍金奇性定理，以及他关于黑洞辐射的理论预测（现称为霍金辐射）。他结合广义相对论与量子力学，对量子宇宙学进行了先驱性研究。他关于黑洞面积不减定理的理论预言得到最近 LIGO 引力波实验观测的支持。LIGO 是尖端大科学项目成功进行重大基础科学前沿探索的又一个范例。
Hawking and Yau (Photo: CMSA Blog)
About the Interviewee: Professor Gerard 't Hooft is a renowned theoretical physicist at Utrecht University, the Netherlands. He is among the founders of the standard model of particle physics, and was awarded Nobel Prize in Physics “for elucidating the quantum structure of electroweak interactions” (together with Martinus Veltman) in 1999. He has also received numerous other prestigious prizes and awards, including Heineman Prize of American Physical Society (1979), Wolf Prize (1981), Pius XI Medal (1983), Lorentz Medal (1986), Spinoza Prize (1995), Franklin Medal (1995), Gian Carlo Wick Commemorative Medal (1997), HEP Prize of European Physical Society (1999), Ettore Majorana Prize (2011), Lomonosov Gold Medal (2011), and 1st Prize of Gravity Research Foundation (2015). He is the author of many popular science books, including “In Search of the Ultimate Building Blocks”, and the recent books “Playing with Planets” and “Time in Powers of Ten”.
About the Interviewer: Hong-Jian He, Professor of Physics at Tsinghua University, working in particle physics, cosmology, quantum gravity and their interface.
by photographer Alex Kok
Below are our interview questions (Q) and the answers (A) of Professor ’t Hooft.
Q1: Professor 't Hooft, it is our great pleasure to have this interview with you. I newly read your very thoughtful article “Imagining the Future, or How the Standard Model May Survive the Attacks” . In particular, you discussed new thinking about the Higgs boson and hierarchy problem. You also commented on the possible hint from the LHC. The LHC Run-2 has been performing well to collide proton beams at 13TeV energy, and has collected about 10% of the planned full Run-2 data. Even though no new physics is announced at the ICHEP conference in this summer, would you like to comment on your expectation of possible new findings at the on-going LHC?
A1: In one way, LHC did what was expected: it found the Higgs particle, often regarded as the last missing link in the Standard Model, but then it did something unexpected as well: it showed that there seem to be no other particles with such properties, while most theoreticians did expect them, and so there was a surprise after all. Then, many of us expected the Standard Model soon to require modifications in the form of new particles. We had several kinds of theories for that, of which the supersymmetry theory was the most advanced and detailed of all. To the contrary, there seem to be no new particles at all.
Will this be the new world at the TeV scale? We did not expect that. LHC is like the Michelson-Morley experiment, which, by giving no result at all, led to Eistein’s relativity theory. Now, I am considering that possibility seriously again: new theories that should explain the non-existence of heavy particles. I hope that this will turn out to be wrong again, since new particles will be giving us much more information, information that may reveal new principles of nature.
Q2: Since you mentioned  that the Higgs boson (125GeV) is an important clue and given the fact that the LHC with pp collisions could not measure the Higgs boson precisely, would you feel crucial to build up an e+e- Higgs Factory such as the CEPC ? You visited China many times before, and on February 23, 2014, you joined the Panel Discussion Meeting on “After the Higgs Boson Discovery: Where is Fundamental Physics Going”, held at Tsinghua University, Beijing. What is your viewpoints on this subject now?
A2: My viewpoints have not changed much. The value of 125 GeV is special because it is close to what one could have expected from theories based on conformal invariance, a theory that might one day explain to us the absence of heavy fundamental particles. If they are indeed absent, we need other clues to find the truth, and one of these clues could be obtained from precision physics. An e+e- Higgs factory would be quite suitable for obtaining precision data that would be more difficult to produce in other machines.
Q3: Regarding the lessons of Superconducting Super Collider (SSC) in USA, perhaps, you may have seen an article “The Crisis of Big Science”  by Steven Weinberg in 2012? 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 made vital negative impacts on American HEP in particular and in its whole fundamental science in general. Would you like to share your views with the publics regarding the lessons of SSC and LHC?
A3: I do not quite share Weinberg’s interpretation of recent history of our science. His rather gloomy mood on how big science failed applies to some unfortunate events such as the cancellation of the American Superconducting Super Collider, which has turned out to be too large and too costly to be operated by a single nation. However, many other big projects were extremely successful. LIGO has spectacular successes, various space probes and telescopes found lots of exciting things in the universe, such as gigantic black holes colliding at cosmic distances, and less far away asteroids, dwarf planets, comets, and thousands of exoplanets. Of course I see the LHC as a great example of how big science can still be successful, and clearly nobody can be blamed for the nonexistence of particles at the TeV scale. We still do not understand why this should be, so we strongly applaud initiatives for the next, greater machine.
Q4: Perhaps, you already heard about 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. Many colleagues worldwide think that this is a truly promising direction for the next step forward in HEP . — Would you like to share your views on the CEPC Project with the Chinese community?
A4: We do have to live with the fact that science, no matter how big, evolves and its focus will change along with this evolution. If large particle accelerators and other large projects such as ITER will eventually not be further pursued, then this must be for sound scientific reasons. Perhaps we will find other ways to find answers to our questions. But today I do think we are not ready yet to give up hopes that higher energy machines will lead to important insights. It’s far too early to abandon that direction, but we do have to be united in our searches. The SSC might have been too ambitious at its time, and it might be too preposterous for us to ask China to succeed where the USA failed. But I would actually be pleased if China and Europe went into a friendly competition for building and operating the most powerful scientific instrument in the world – in that case, we scientists would all prosper from it. On the other hand, perhaps CERN’s present success is telling us that international collaboration, safeguarded by very strict regulations, is the way to go.
Q5: You probably have heard the on-going public debate in the Chinese community on whether this Collider should be built in China at all . This debate was provoked by the Chinese-American theoretical physicist C. N. Yang in this fall , who has been strongly against any collider project in China, including the current CEPC-SPPC project led by IHEP director Yifang Wang. It’s clear that Yang’s major objection is that this collider would cost too much for China, and a misconception of him was to stress the cost of the potential second phase SPPC. (As Yifang Wang showed in his refutation , the IHEP team estimated the CEPC cost to be about 6 billion US dollars invested over 10 years and its 25% will come from international collaboration. The SPPC would be built during 2040s if the required technologies become mature by then. As anyone may recall, the funds of the LEP and LHC at CERN were approved separately and in sequence.) — Would you like to share your opinion with the Chinese publics?
A5: It is important to have this discussion in China. I am sure that Prof. Yang understands China’s domestic and foreign political attitudes and problems, as well as its enormous potential as a world power, so he should be listened to. Yet I don’t quite follow his arguments. In planning the SSC, I suspect the scientists in the USA miscalculated the support they would receive from politicians, congress, and fellow scientists, at home as well as abroad. Maybe it was just a tiny miscalculation, but it was enough to topple the project. This does not have to mean that China will make the same mistakes. Instead, the Chinese should carefully study what went wrong with the SSC, and ensure a sufficiently stable political and financial basis for the realization of its ambitious plans. Then decide whether the plans can be realised. As for their benefit for humanity in general and China in particular, we should indeed not make too grand promises in that a giant new accelerator will bring many elementary breakthroughs, let alone new applications of big science that will boost China’s prosperity. That is not the main justification of these enterprises. What should be expected is that this accelerator, together with a number of other big science projects, will lead to joint investigations all over the world of humanity’s basic questions. Chinese scientists will take part in these discussions, bringing in their own observations and results. China will be part of a scientific intelligentsia discussing not only basic questions in physics, but in all sciences and problems faced by humanity.
Will it be worth-while to spend such amounts of money on a project whose purposes are obscure to a big majority of the population? This, the Chinese scientists and politicians must decide for themselves. I should warn the scientists in particular that, in my experience, this isn’t a zero-sum game. Money saved by cancelling this machine, will not be used for other branches of science, but most likely disappear into completely different activities, which you may or you may not agree about. Therefore, in my humble opinion all scientists should be in favor of reserving money for projects like this, just because it is money to be spent on fundamental science. If indeed China decides to go into this direction, other, totally different big science projects might follow.
I presume Prof. Yang observed that, while the LHC was built in a region that already had all the necessary infrastructure present, which will certainly have suppressed its costs, the new Chinese machine must be built from scratch. This will make it cost more, but then, such money is well-spent. A new city may arise, where scientists from all over the world pay frequent visits and discuss the world’s problems. If China could still be looked upon as a developing country now, it won’t be that anymore.
 Gerard ’t Hooft, “Imagining the Future, or How the Standard Model May Survive the Attacks”, Int. J. Mod. Phys. 31 (2016) 1630022.
 Circular Electron Positron Collider (CEPC) and Super pp Collider (SPPC), (http://cepc.ihep.ac.cn).
See the recent science book for introduction of this subject:
S. Nadis and S.-T. Yau, “From the Great Wall to the Great Collider --- China and the Quest to Uncover the Inner Workings of the Universe”, 2015, International Press of Boston, Inc., MA, USA (https://thegreatcollider.com).
 C. N. Yang, “China should not build a super-collider now”, September 4, 2016. Chinese version: web link.
 Yifang Wang, “It is Suitable Now for China to Build Large Collider”, September 5, 2016. English translation: web link
About the Interviewee: Steven Weinberg is a renowned theoretical physicist and a great master of modern physics. He is currently the Josey Regental Chair Professor in Science at the University of Texas at Austin, where he is on faculty of the Physics Department and Astronomy Department. He is a major founder of the Standard Model of particle physics, and was awarded the Nobel Prize in Physics in 1979 (together with Sheldon Glashow and Abdus Salam) “for their contributions to the theory of the unified weak and electromagnetic interaction between elementary particles, including, inter alia, the prediction of the weak neutral current”. His research on elementary particles and cosmology has been honored with numerous other prizes and awards, including National Medal of Science (1991), J. R. Oppenheimer Prize (1973), Heineman Prize of APS (1977), Elliott Cresson Medal of Franklin Institute (1979), James Madison Medal of Princeton University (1991), and Benjamin Franklin Medal of American Philosophical Society (2004). He has been elected to American National Academy of Sciences and Britain's Royal Society, as well as to the American Academy of Arts and Sciences. He has also served as consultant at the US Arms Control and Disarmament Agency, and the JASON group of defense consultants. He taught at Columbia, Berkeley, MIT, and Harvard where he was Higgins Professor of Physics, before coming to Texas in 1982.
He is the author of over 300 articles on elementary particle physics. His books include Gravitation and Cosmology (1972); The First Three Minutes (1977); The Discovery of Subatomic Particles (1983, 2003); Elementary Particles and The Laws of Physics (with R. P. Feynman) (1987); Dreams of a Final Theory --- The Search for the Fundamental Laws of Nature (1993); a trilogy, The Quantum Theory of Fields (1995, 1996, 2000); Facing Up --- Science and its Cultural Adversaries (2002); Glory and Terror --- The Growing Nuclear Danger (2004); Cosmology (2008); Lake Views: This World and the Universe (2010); and To Explain the World --- The Discovery of Modern Science (2015), etc. His book “Dreams of a Final Theory” was written for the support of building the Superconducting Super Collider (SSC) in USA. His article “Big Crisis of Big Science” was written in 2012 in which he discussed the importance of big projects for the sciences and high energy physics as well as the lessons of the SSC.
About the Interviewer: Hong-Jian He, Professor of Physics at Tsinghua University, working in particle physics, cosmology, quantum gravity and their interface.
Below are our interview questions (Q) and the answers of Professor Weinberg (A).
Q1: Professor Weinberg, it is our great pleasure to have this interview with you. I recently reread your article “Particle Physics, from Rutherford to the LHC”, first published in Physics Today , where you explained why the new physics is required to go beyond the Standard Model (SM) of particle physics for which you were a major founder, “It is clearly necessary to go beyond the standard model. There is a mysterious spectrum of quark and lepton masses and mixing angles that we have been staring at for decades, as if they were symbols in an unknown language, without our being able to interpret them. Also, something beyond the standard model is needed to account for cosmological dark matter.” These are indeed what the particle physics community has been striving for over the past thirty years, through the major high energy colliders including Tevatron in USA and LEP & LHC in Europe. The LHC Run-2 has been performing very well to collide proton-proton beams at an energy of 13TeV, which has collected about 10% of the planned full data sample of the Run-2 so far. Even though no new physics is announced at the ICHEP conference in August, would you like to comment on your expectation of possible new findings at the on-going LHC?
A1: It is impossible for anyone to know whether there are significant new discoveries that will be within the capabilities of the LHC. From the beginning, there had been strong reasons to anticipate that the LHC would be able to discover the mechanism by which the symmetry governing weak and electromagnetic forces is broken – either elementary scalar fields, as in the original electroweak theory, or new strong forces, as in technicolor theories. In either case, the observed strength of weak forces gave a powerful indication that new scalar particles or new strong forces would be observable at the LHC, as turned out to be the case. Indeed, this provided a guide in planning the LHC.
But, although there are several other phenomena of great importance that might be discovered at the LHC, including dark matter particles and superpartners of known particles, we have no strong reason to suppose, even if they exist, that they would be within the reach of the LHC. We will just have to wait and see.
Q2: We know you was the major supporter of the SSC . Early last month, we recommended the Chinese translation of your review article “The Crisis of Big Science” (2012)  to the Chinese publics. 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 a factor 3 larger than the current Run-2 colliding energy (13TeV) of Large Hadron Collider (LHC) at CERN, Geneva. Perhaps, it should not be so unexpected and disappointed that the LHC Run-2 has not found any new physics beyond the Standard Model (SM) so far, because we all know that the SSC with 40TeV colliding energy was designed to ensure a much more solid new physics discovery reach at TeV scales. As expected by many physicists, if the SSC had not been canceled in 1993, it would probably have already made revolutionary discovery of new physics beyond the SM and thus have pointed to a new direction for fundamental physics in 21st century. Since you have witnessed the full history of the SSC and the subsequent developments of the LHC so far, would you like to share your views with the Chinese community regarding the lessons of the SSC and LHC?
A2: Even after the SSC program had been approved by the US government, it continued to meet opposition from several directions. Part of the opposition came from those who generally prefer small government and low taxes, and therefore tend to oppose all large government projects, especially those projects for which there is no large number of immediate beneficiaries. The project would obviously provide economic benefits to people in its neighborhood, but these persons would be limited in number. One US senator commented to me that at that moment, before the site of the SSC had been decided, all 100 members of the Senate were in favor of it, but that once the site was chosen, the number of senators in favor would shrink to two, the two senators from the state of the chosen site. Even before the final site had been determined, one member of the House of Representatives who had favored the SSC turned against it once it was clear that the SSC would not go to his own district. All this was standard politics, perhaps of a sort that is not restricted to the US.
Much more disturbing was opposition from within the scientific community. No one argued that the SSC would not do important scientific research, but some urged that the money needed for the SSC would be better spent in other fields, such as their own. (It did not provide much consolation when the SSC was cancelled that the funds saved did not go into other areas of science.)
There was implicit opposition to the SSC from advocates of the LHC, who pointed to the financial savings from their use of an existing tunnel. The smaller circumference of this tunnel limited the LHC energy to only about one third of that possible for the SSC, but proponents of the LHC argued that the LHC could make up for its lower energy by operating at higher intensity, though this higher luminosity obviously carried its own difficulties, due to the several particle collisions in each intersection of bunches.
One explanation that is sometimes given for the cancellation of the SSC is that its projected costs kept increasing. This was certainly charged by some of the opponents of the SSC, but I don't believe it was a fair criticism. The only real increase in the cost of the project was approximately ten percent, made necessary by a calculation of the aperture needed for the SSC beam. Whatever increased cost there was beyond that came from the slowdown of funding from Congress, which required an extension of the time for construction, and hence an extended time in which construction personnel had to be employed.
The SSC project was killed chiefly by competition from a program that masqueraded as science, the International Space Station. This was to be administered at the Johnson Manned Space Flight Center, in Houston, Texas. It was not politically possible to support two large technological projects in Texas, and the Space Station was chosen. In the end, it cost ten times what the SSC would have cost, and has not led to any important scientific research. (The one possible exception, the Alpha Magnetic Spectrometer, could have been operated as well or better, and much more cheaply, on an unmanned satellite.)
The LHC has been a great success, with the discovery of the Higgs boson. Whatever the LHC’s chances for further important discoveries, it is clear that the much greater energy of the SSC would have provided a better chance for the future.
Q3: Perhaps, you already heard about 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 (SPPC). This proposal was officially ranked as the “First Priority HEP Project” at the recent “Strategy Plan Meeting for Future High Energy Physics” of the Chinese Particle Physics Association, held on August 20-21, 2016. This plan has received worldwide supports of the international HEP community since its inception . — You probably have heard about the on-going public debate in the Chinese community on whether this Collider should be built in China at all . This debate was provoked by the Chinese-American theoretical physicist C. N. Yang in September, 2016 , who has been strongly against any collider project in China, including the current CEPC-SPPC project led by IHEP director Yifang Wang. Attached below are English translations of Yang’s recent public article , and Yifang Wang’s refutation . It’s clear that Yang’s major objection is that this collider would cost too much for China, and a misconception of him was to stress the cost of the potential second phase SPPC. (The IHEP team estimated  the CEPC cost to be about 6 billion US dollars invested over 10 years and its 25% will come from international collaboration. The SPPC would be built during 2040s if the required technologies become mature by then.) As anyone 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 viewpoints and advice from international side. Would you think that the fund invested for CEPC worthwhile? and what would this contribute to the world and to the society of China?
A3: I have tremendous respect for the scientific research carried out by C. N. Yang, but I do not agree with his arguments against the proposed CEPC. Some of them are familiar, being used again and again against large scientific projects.
Yes, society has many other needs, including environment, health, education, and so on. It always does. But it also has needs for arts and sciences that make its civilization worthy of respect.
Yes, no immediate practical applications are likely to follow from discoveries made at particle accelerators. But the projects themselves have important practical consequences in the form of technological spin-offs. Frequently cited examples include synchrotron radiation, used to study the properties of materials, and the World Wide Web.
A less frequently cited spin-off is intellectual. The fundamental character of elementary particle physics makes it very attractive to bright young men and women, who then provide a technically sophisticated cadre available to deal with problems of society. In World War II microwave radar, cipher-breaking computers, and nuclear weapons were developed by scientists who before the war had been concerned with problems of fundamental scientific importance rather than of military value. One of the best graduate students who started work with me on elementary particle theory later became interested in more practical problems, and has developed what may become the leading approach to isotope separation. The country that pursues only research of direct practical importance is likely to become unable to make not only discoveries of fundamental importance but also those of practical value.
One of Professor Yang’s arguments is that progress can be made without new accelerators by the search for beautiful geometric structures. This reminds me of a position taken after World War II by another great theoretical physicist, Werner Heisenberg. He argued against German spending on particle accelerators, on the grounds that progress could be made through theoretical studies of certain field theories. It is true that without the impetus of new experimental results, in trying to understand the strong forces Yang and Mills did develop a field theory of a class that later turned out to be realized in nature. But the correct Yang-Mills theory of strong forces could not be guessed until accelerator experiments revealed a weakening of strong forces at high energy, and the relevance of Yang-Mills theories++ to the weak and electromagnetic interactions could not be confirmed without new accelerator experiments that discovered weak neutral currents. Theory can only go so far without experiment.
 Steven Weinberg, “Particle Physics, from Rutherford to the LHC”, Phys. Today 64N8 (2011) 29-33. See also, Int. J. Mod. Phys. A28 (2013) 1330055.
 Steven Weinberg, Dreams of a Final Theory --- The Search for the Fundamental Laws of Nature, published in New York, USA: Pantheon Books (1992).
 Circular Electron Positron Collider (CEPC) and Super pp Collider (SPPC), (http://cepc.ihep.ac.cn).
See the recent science book for introduction of this subject: S. Nadis and S.-T. Yau, “From the Great Wall to the Great Collider --- China and the Quest to Uncover the Inner Workings of the Universe”, 2015, International Press of Boston, Inc., MA, USA (https://thegreatcollider.com).
 C. N. Yang, “China should not build a super-collider now”, September 4, 2016. [Chinese version.]
 Yifang Wang, “It is Suitable Now for China to Build Large Collider”, September 5, 2016. [English translation].
 Note: This refers to, after the German atomic bomb project failed in World War II, Heisenberg's works on certain unified field theory of elementary particles since 1953 in which he tried to derive the so-called "world equation". As is well-known, his attempts ended up in vain and have been forgotten thereafter. See: Werner Heisenberg Biography (https://nobelprize.org); David C. Cassidy, “Uncertainty: The Life and Science of Werner Heisenberg”, Freeman (1992), cf. Appendix A.
如果你还有其他问题，我很乐意回答，也可以推荐你与一些著名的物理学家（如David Gross、Edward Witten和Nima Arkani-Hamed等）联系，你将会了解到他们与杨教授很不一样的看法。