### Why China Should Build The Great Collider: A Response to C.N. Yang

##### -- David Gross

[This article 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)].

Professor C.N. Yang is one of the great figures in physics of the last century. However, I disagree with all the objections to building the "Great Collider" in China that he has recently voiced.

Before addressing Professor Yang's points, it is perhaps worth explaining why I feel compelled to voice my opinions on this matter.

First and foremost, I am very excited by the scientific potential of the Chinese collider project, and as a friend of Chinese science and a foreign member of the Chinese Academy of Science I am very excited about the many benefits that this project will produce for China.

Particle physics has entered a new epoch in the 21st century, driven by deep paradoxes that strike at the foundations of our understanding of the 20th century revolutions of dynamical space-time and quantum mechanics. Some of the deepest of these mysteries revolve around the Higgs boson, a particle unlike any we have discovered before. The answer to a very basic question about the Higgs --- is it point-like, or does it have substructure? --- will force fundamental physics down radically different paths in the coming decades. The LHC will not answer this question; a new particle accelerator is needed to decisively settle the issue. This is precisely what the Chinese collider project will do.

I am also moved to comment for a second reason, as an American physicist who witnessed the cancellation of the Superconducting Super Collider (SSC) project in the early 1990s. The most prominent detractors of the SSC put forward many of the same arguments made today by Professor Yang. But the cancellation of the SSC is now almost universally regarded, by supporters and detractors of the project alike, as a disaster for fundamental physics in the US, one with lasting negative effects that have proven difficult to recover from. The US had spent decades as the unquestioned world leader in particle physics, yet quickly ceded this mantle to Europe, and with it, a critical capacity to "think big" and pull off major, ambitious, long-term projects.

Today, China has a golden opportunity not only to do something great for physics, but also to catapult itself to world leadership in fundamental physics at an especially crucial juncture in the history of the subject. It would be a tragedy if the same calamitous errors made by the US in cancelling the SSC, fueled by similarly faulty arguments, were to derail the Chinese collider effort.

Before addressing Professor Yang’s remarks, it is important to clarify the collider projects under consideration. The machine currently being proposed by Chinese physicists is the "CEPC", a large electron-positron collider, between 50-100 km in circumference. The CEPC will function as a "Higgs factory" and settle outstanding questions about this deeply mysterious particle. This is the only machine under discussion for the next two decades.

Further into the future, the same 50-100 Km tunnel used for the CEPC could be used for a second machine, the "SPPC", that would collide protons at energies over 7 times higher than the LHC, securing the experimental future of fundamental particle physics on the 50 year timescale. Of course, the prospect of the SPPC following the CEPC adds significantly to the excitement and scientific potential of the CEPC project, but any concrete decisions about proceeding to the SPPC cannot be responsibly made till over a decade from now.

With these preliminaries aside, let us examine each of Professor Yang's criticisms in turn.

(1) Are accelerators a bottomless money sink?

No! The example of the SSC is way out of date and much has changed since. Indeed, the cancellation of the SSC forced the international particle physics community to learn some hard lessons, and subsequently every major accelerator project completed in the past twenty years, chief amongst them the Large Hadron Collider, has been completed essentially on time and on budget. Chinese particle physicists have made a detailed cost estimate for the CEPC project, which is on the order of $6 billion, not$20 billion. And they have an impressive record over the past decades, from BEPC to Daya Bay to the neutron spallation project.

Comparing to the cost of the LHC is not relevant, since electron-positron colliders like the CEPC are well-known to be far cheaper than proton-proton colliders like the LHC. As we have already stressed, beyond this it is not possible to make responsible estimates for the cost of the SPPC, which depend on the development of various new technologies in the coming 10-20 years.

(2) Can China, a developing country, afford to build the Collider?

Yes! Professor Yang argues that while China is wealthy in absolute terms, its low GDP per capita is not yet comparable to those of wealthy nations. But the size of the GDP per capita would only be relevant if China were planning to build a number of colliders in proportion to its population, whereas only a single facility is being discussed. Indeed, using the same logic one could argue that the cost of the collider per capita is significantly smaller in China than anywhere else in world!

China's ambitions on the world stage are high, more in line with its GDP than its GDP per capita. The GDP per capita has not prevented China from aiming to go to the moon, or completing outstanding engineering projects like the Three Gorges Dam. Pursuing bold initiatives in the basic sciences seems to be perfectly in line with these big ambitions.

But more importantly, I believe, and history has proved, that it is precisely such long-term investments at the frontiers of science that have stimulated the technological advances that lift developing countries to economic superpowers.

(3) Will funding for the CEPC hinder the development of other parts of Chinese science?

No! While I have no precise insight into how Chinese funding of the collider project will work, the scale of funds will obviously require new money to flow into the support of basic science, in accord with the stated goals of the Chinese government to rapidly increase the proportion of GDP spent on basic research. Other fields should not have to pay for the collider.

Furthermore, the SSC saga taught us that thinking in terms of a "zero-sum-game" for science funding is a losing proposition; other areas in physics did not get anticipated big extra funds after the SSC was cancelled. Instead, the overall ambition of the US to pursue big scientific goals diminished palpably, to the detriment of all.

The CEPC will stimulate the growth of science in another way; by becoming a magnet for international talent in physics and engineering, thus helping to create and sustain an intellectual infrastructure that will spur the development of many other technical fields in China.

(4) Is the purpose of the Collider to discover Supersymmetry, an unproven hypothesis?

No! Professor Yang argues that the chief scientific purpose of these machines is to discover supersymmetry, a new symmetry of space-time that is, at the moment, a hypothesis with no experimental support.

Alas it appears that Professor Yang has not read any of the scientific documents that have discussed the physics case for the CEPC/SPPC at some length. The central physics case for both the CEPC and the SPPC have little to do with speculations about supersymmetry and much to do with deeply understanding the mysteries of a particle we know exists --- the Higgs. The Higgs is the first seemingly elementary particle of “spin zero” we have ever seen, and is associated with deep theoretical mysteries --- in fact these mysteries are made deeper and more pressing by the absence of something like supersymmetry at the LHC.

The CEPC will put the Higgs under a powerful microscope, and probe its size to resolution 10-30 times better than the LHC. The CEPC has a rich and detailed experimental program that will either reveal substructure for the Higgs, or allow us to conclusively decide that the Higgs is an elementary particle on the same footing as quarks and leptons. The guaranteed physics of the SPPC is similarly centered on the Higgs: it will determine whether the Higgs looks point-like to itself. It will do this by establishing the existence of the most fundamental interactions elementary particles can have, where three identical particles meet at a common point in space-time. We have never seen this most basic of all possible interactions in Nature before, and the LHC will not be able to conclusively establish its presence. The SPPC will not only discover this self-interaction, but will measure it to an accuracy of a few percent!

Of course the SPPC will also explore much higher energies, and will have the power to produce new particles that are nearly ten times heavier than can be produced at the LHC. It will certainly continue the search for supersymmetry (amongst other things), precisely because supersymmetry is currently a hypothesis. Indeed, all the great ideas of physics, including the Yang-Mills idea of local gauge theories, as well the proposal of Quantum Chromodynamics (the theory of the nuclear force), were “just a hypothesis” until their predictions were tested by experiment.

But to repeat: the guaranteed physics of these machines is centered on revealing the deeper nature of the Higgs and discovering new interactions associated with it.

(5) High-energy physics produced any "tangible benefits" to society?

Yes! Even taking an extremely narrow view of this question, the technologies directly springing from particle physics have spawned huge industries that generate revenues far exceeding the magnitude of the investment in basic science. The multi-billion dollar accelerator industry, operating thousands of small-scale particle accelerators around the world ranging from light sources, to medical accelerators for cancer-fighting radiation therapies, owes its existence to particle physics. And the need for powerful magnets at proton colliders necessitated the development of superconducting magnet technology, itself a billion dollar industry, which are the critical component for MRI machines, a five billion dollar industry.

But of course, much more importantly, scientific research at the forefronts of knowledge is most powerfully driven by our fundamental curiosity to understand and master how Nature works. Time and again, this mastery has led to revolutionary technological developments that have transformed our lives. Often these have arisen as "spin-offs", not directly associated with the central thrust of the scientific questions, but arising inevitably in tackling and solving hard scientific problems. As one famous example, without the laws of quantum mechanics, none of the modern electronics industry would be possible; one can justifiably say that the understanding of quantum mechanics is responsible for 2/3 of the world’s GDP. Another famous example is the invention of the World-Wide-Web at CERN, which was developed to cope with the challenge of experimental particle physicists needing to share vast quantities of information with each other.

Why has the pursuit of science for its own sake had such a remarkable track record in generating transformative new technology? The reason must be that Nature poses deeper and more challenging questions than humans can do, and the struggle to understand Nature forces us to invent better and deeper ideas than we would if left to our devices.

(6) The Chinese particle physics community is not strong enough to undertake this project.

No! I strongly disagree with Professor Yang's assessment of the strength of the Chinese particle physics community. High-energy physics in China has a rich history going back to the construction of the Beijing Electron-Positron Collider (BEPC) in the 1980s. The BEPC put China on the world map in particle physics, and the ensuing decades have seen a continual rise in its strength.

A spectacular recent example was the Daya Bay experiment that beat many groups around the world who were chasing the most elusive of all neutrino-mixing phenomenon, making a beautiful and incisive measurement. The international community, through a number of prestigious prizes, has already recognized this achievement. Yifang Wang, who lead the Daya Bay effort, wants to build on this success by pursuing the much more ambitious goal of the CEPC/SPPC, a program that would immediately make China the world leader in high energy physics.

Professor Yang also worries that the project would be intellectually dominated by foreigners and that Chinese physicists won't get credit for its discoveries. I am surprised by his lack of confidence in the potential and brilliance of Chinese physicists! Of course the Chinese particle physics community will be stimulated to grow by these projects, tapping an ocean of talent and drawing many brilliant young minds into physics. Indeed, the engagement of this huge new pool of Chinese talent is one of the greatest contributions this project will make to physics itself! In 10-20 years, there is no doubt that Chinese physicists will be a major intellectual force leading the CEPC effort.

Finally, will Chinese physicists win a Nobel Prize for any discoveries made by these machines? Probably, maybe, who knows? And who cares! Grand scientific projects on this scale, now more than ever, transcend prizes and the pursuit of personal recognition and glory. What is certain is that with the CEPC, China will become the world center of activity in fundamental physics for the next twenty years, and extending to fifty years if the SPPC is built.

(7) There are other, cheaper avenues to pursue in fundamental physics.

No! On the experimental side, Professor Yang suggests developing novel accelerator technologies. This is certainly an important direction, and has been developing for a number of decades, with concrete ideas currently on the table for generating much larger particle accelerations, for instance using laser technology. But none of these methods can accelerate particles coherently enough; to produce the large number of collisions needed to study high-energy physics. Ways around these difficulties may be found, but it is impossible to predict the timescale for progress, which could well be several decades.

Amusingly, a similar argument was made by prominent opponents of the SSC, condensed-matter physicists who argued that the SSC should be delayed till high-temperature superconductors were developed to greatly reduce the cost of the magnets needed for the machine. Nearly three decades later we are still waiting for these practical high-temperature superconductors to materialize. In the meanwhile, the use of established superconducting magnets in particle colliders has continued to lead to major discoveries, from the top quark to the Higgs particle.

Professor Yang also suggests further theoretical investigations into beautiful geometric structures in physics. Obviously, as a theoretical physicist, I strongly believe in the power of theoretical physics to generate deep new hypotheses that might propel our understanding of the laws of Nature. The exploration of geometric structures is one of many avenues of this sort that have been and continue to be actively explored by theorists. But physics is most fundamentally an experimental science, and experiments have always played a critical role in the discovery of our deepest theories. The story of the Standard Model of particle physics perfectly illustrates this point. The leap from the profound classical Yang-Mills generalization of Maxwell’s equations to the powerful quantum theory that actually describes Nature required major new theoretical ideas. And the development of these crucial concepts was largely forced on theorists by a wide array of surprising experimental results.

The need for new experiments in fundamental physics is just as pressing today as it has always been. Indeed if anything, especially when confronting the mysteries associated with the Higgs particle, the lessons of the LHC have left us in a greater state of theoretical confusion than we have seen in decades. What we desperately need are incisive new inputs from experiment, of exactly the sort we will get from the CEPC.