THE CLOCK STARTS TICKING I was born in 1975 in a small town in Zhejiang province. My father was a miner and my mother a farmer. My parents never had a chance to pur- sue higher education. They never had a chance to go to high school. It was a tough time for China. We didn't have electricity until I was about seven or eight. We were quite poor, but I didn't realise that for a long time, because everyone around us lived as we did and there wasn't so much access to the outside world. So I actually had a fun childhood. I have a son now and he has lots of computer games - we had our imaginations and the beautiful Chinese countryside.
POWER OF PHYSICS When I was six I told myself I would go to university. I was always a good student and even as a small child I loved reading and studying. It wasn't until I got to middle school that I touched on physics for the first time. Right away I was deeply moved. I saw that from a few simple principles, you could explain many things. I saw the power of physics and was impressed to learn that these principles had been discovered hundreds of years ago. Of course, at that time, physics classes in a school like mine were not very good. Resources were limited and there were about 60 kids in each class. Mostly, I would study on my own and read ahead in the textbooks, then ask the teacher questions after class. I was always top of my physics class and soon I was the best in the entire school. Like many Chinese of their generation, my parents understood that studying was one of the few ways we could change our lives for the better.
CHASING A DREAM By then the Chinese economy was developing rapidly and the country needed workers in the booming fields of electronics and computing. This was the hottest field in China and there were excellent opportunities for graduates. I decided to study engineering and for pure physics, I would just read by myself. But as I studied on my own, during my university life, I realised my real interest was simply to understand the universe. I continued my engineering degree, but also privately prepared to get into graduate school for pure physics. If I had failed, I would have gone into industry, but I was easily accepted into Peking University - China's best. Many of my friends and classmates were shocked when I turned down several good job offers to pursue a dream in physics that would never make me rich.
RELATIVELY SPEAKING I have an uncommon work ethic. I know I'm not the smartest individual. I have many friends who are much smarter than me. But I've always known that if I worked harder than everyone else, one day I might be among the best. Many people grow frustrated in graduate physics because, eventually, you begin to feel yourself reaching your natural limitations. I learned early to have faith in what I could achieve though self-study and hard work, but I did have doubts. Around that time I read a piece by [physicist Albert] Einstein in which he discusses the two types of people drawn to science. The first are the naturally gifted ones, who simply pursue physics because it gives them a chance to exercise their intellectual power and win acclaim. The second are different. They may not necessarily be the most gifted, but they don't care if they're not geniuses because they love physics with a passion. Einstein said if an angel had to choose who would remain in the field, she would choose the second group, because their devotion was most likely to advance science. This comforted me in moments of self-doubt - because I never doubted how much I enjoyed my work.
QUESTIONS AND ANSWERS When I was young, I had a firm faith in science and believed physics would ultimately answer all of the great questions. Later, when I continued my studies and research at the University of Colorado [in the United States] and then Stanford, I realised that the more I learned, the more I didn't understand. Every answer asks more questions and the universe is more open than I imagined. I came to believe the universe must have been built by something. I call it a god. Now I understand that the job of physics is just to try to understand, from our limited viewpoint, the shape of the picture. I became more and more interested in the fundamental elements. That's why I started with the most fundamental of all elements: the photon. Light.
LET THERE BE LIGHT My recent work attracted a lot of attention, which came as a surprise [Professor Du received more than 60 interview requests from local and international news media after his recent work was published]. My research demonstrates a simple thing: a single photon cannot move faster than the speed of light in a vacuum. Einstein's universal constant is the speed of light in a vacuum. In his theory of special relativity, this is a very important number: it defines the connection between time and space. If something could move faster than the speed of light in a vacuum, it might be possible to supersede causality - effectively go back in time. So the news connected my research with time travel, saying I've proved that it's impossible. That was not my focus. I was just trying to understand how a single photon moves. While I don't like to speculate about these things, it's true you could interpret my work to say this particular method of time travel - by moving faster than the speed of light in a vacuum - would now seem to be very difficult. But of course, there might be other ways.