對於張首晟的離去，我感到非常痛心，就像一塊石頭狠狠地砸在我身上。 當我在網上看到他的死訊時，我簡直都不敢相信這是真的。十多年來，我一直知道首晟是世界一流的物理學家，自從他對拓撲絕緣體進行了令人難以置信的研究以來，他的名字就經常出現在我的認知雷達屏幕上。儘管如今他已離世，但他對物理和數學的深刻洞見是留給我們的寶貴財富。
Zhang Shouchang’s death hit me like a rock. When I saw his demise on the web, the cliché that 「I cannot believe」 was absolutely palpable.
I have known Shoucheng’s reputation as a world class physicist for more than a decade. Ever since his incredible research on topological insulators, his name had quite often appeared on my cognizant radar screen.
The first time I met him at Singapore’s conference on 「90 years of Quantum Mechanics」 in 2016. Purely by luck, I fond myself sitting next to him in one of the lectures. As such, we were able to strike-up a conversation.
Shoucheng Zhang delivering his talk at QM 90
It began by my asking him: 「Are you a Shanghainese?」
「Of course,」 was his answer. I could sense that he had the normal pride of a Shanghainese. I know this well because my mother, wife, daughter-in-law are all Shanghainese. Even my daughter, who is an BBC (British born Chinese), also speaks the language fluently. Therefore, I could sense a 「Shanghai pride」 a mile away!
Not only I could smell the 「Shanghai pride,」 I could also speak Shanghainese, probably as fluent as a native! That broke the ice of the awkwardness of a first meeting. Once we started to speak in Shanghainese, we literally became 「old friends,」 even though we had only met!
Throughout the conference, we had many occasions to chat. We talked about various issues, from physics to society to higher education. Among all the items we chat about, one seared into my mind deeply. That has to do with topological insulators.
Like Shoucheng, I was trained as a theoretical physicist. Mathematics to me is a tool to solve problems. I was indoctrinated that Newton 「discovered」 calculus in order to solve physical problems. Maxwell used partial differential equations to bring electromagnetic under the mathematical roof. One hears also often that 「a great physicist must have great intuition.
He/she would know the answer before solving the problem. Mathematics is merely to tool to prove his intuition is correct!」
As someone who spent a good part of my physics professional life utilizing a branch of mathematics known as Lie Algebra and Lie Groups to classify nuclear spectroscopy, to study quantum chaos and so on, mathematics is always secondary.
Except for a mathematics course I took on topology, to me it is a mysterious branch of mathematics. I simply cannot imagine how one could put topology and material physics in the same breath.
After the conference, I wrote an over report of it. In it, there is a short description about Shouchen’s talk:
「In the past several years, the term 「topological materials」 began to drain into my cognizant. However, not until I heard Shoucheng’s talk on the subject, I found it quite incompressible. But what was truly amazing to me was that Shoucheng, a traditionalist would call him a 「solid state physicist,」 is as comfortable in solid state physics technical jargon as he is in particle and field theory, even differential geometry jargon. What was truly remarkable was how he seamlessly used field theory language to deal with some of the most amazing materials behavior. 「
In one of our conversations, I asked Shoucheng the dumb question: 「How did you put topology and insulators together and come up with something so incredible?」
Shoucheng’s answer astounded me.
「I know for the culture of physics, mathematics is always treated something of a second-class citizen. I learned very early on that my love of mathematics is as much as my love of physics. That feeling was profoundly reinforced when I became a student of Professor Yang. When I was working on topological insulators, these two words, topological and insulators, to me, are of equal and fundamental importance. It is NOT insulators with topological as an adjective. It is also not topological with insulators as a appendix. The question I asked was 「what can topology tells me about insulators, just as much as I asked what topological nature does insulators exhibit. I approach the problem both as much as a physicist as I am a mathematician. When some good results emerged from the work, I simply cannot separate when is the end of mathematics and when is the end of physics. They were genuinely intertwined!」
These are truly immortal words from a great scientist which humanity had produced. Although Shoucheng is no longer with us, such profound way to think about mathematics and physics shall forever be one of his legacies.
Shoucheng Zhang was seated third on the fourth row
小編隊：王園琳、周灣、黃偉華
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