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