He is noted for having led a team that performed an experiment showing individual photons cannot travel faster than the speed of light (c) in a vacuum, thus apparently removing one approach to time travel.
Du claims in a peer reviewed journal to have observed single photons' precursors, saying that they travel no faster than c in a vacuum. His experiment involved slow light as well as passing light through a vacuum. He generated two single photons, passing one through rubidium atoms that had been cooled with a laser (thus slowing the light) and passing one through a vacuum. Both times, apparently, the precursors preceded the photons' main bodies, and the precursor traveled at c in a vacuum. According to Du, this implies that there is no possibility of light traveling faster than c (and, thus, violating causality). (Some members of the media took this as an indication of proof that time travel was impossible.)
- Zhang, Shanchao; Chen, J.F.; Liu, Chang; Loy, M.M.T.; Wong, G.K.L.; Du, Shengwang (2011). "Optical Precursor of a Single Photon". Phys. Rev. Lett. (American Physical Society) 106 (24): 4. Bibcode:2011PhRvL.106x3602Z. doi:10.1103/PhysRevLett.106.243602. Retrieved 26 July 2011.
- It's official: Time machines won't work, Los Angeles Times July 25 2011
- HKUST Professors Prove Single Photons Do Not Exceed the Speed of Light
- "Technology". Los Angeles Times.
- "Why time travel will remain a sci-fi fantasy: Scientists prove nothing can travel faster than the speed of light". Daily Mail (London). 25 July 2011.
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