24 July 2011

Light travels at the speed of light.

Hong Kong researchers have published in the prestigious Physical Review Letters from the American Physical Society that the photon precursor cannot travel faster than the speed of light. Abstract
Phys. Rev. Lett. 106, 243602 (2011)
Optical Precursor of a Single Photon

Shanchao Zhang, J. F. Chen, Chang Liu, M. M. T. Loy, G. K. L. Wong, and Shengwang Du*
Department of Physics, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China

Received 31 March 2011; published 16 June 2011

We report the direct observation of optical precursors of heralded single photons with step- and square-modulated wave packets passing through cold atoms. Using electromagnetically induced transparency and the slow-light effect, we separate the single-photon precursor, which always travels at the speed of light in vacuum, from its delayed main wave packet. In the two-level superluminal medium, our result suggests that the causality holds for a single photon.

URL: http://link.aps.org/doi/10.1103/PhysRevLett.106.243602
DOI: 10.1103/PhysRevLett.106.243602
PACS: 42.50.Gy, 32.80.Qk, 42.50.Dv

The press release from the Hong Kong University of Science and Technology of 19 July 2011 was titled "HKUST Professors Prove Single Photons Do Not Exceed the Speed of Light". Excerpt
A group of physicists at the Hong Kong University of Science and Technology (HKUST) led by Prof Shengwang Du reported the direct observation of optical precursor of a single photon and proved that single photons cannot travel faster than the speed of light in vacuum. HKUST's study reaffirms Einstein's theory that nothing travels faster than light and closes a decade-long debate about the speed of a single photon.

Prof Shengwang Du, Assistant Professor in HKUST's Department of Physics, and his research team have published their study in Physical Review Letters recently. Co-authors include three postgraduate students Shanchao Zhang, Jiefei Chen and Chang Liu, Chair Professors Michael M T Loy and George K L Wong. This is the fifth time in the last two years that Prof Du's team published their optics research in Physical Review Letters, one of the most prestigious scientific journals in physics, and the second time their papers were selected as editors' suggestion for reading. This research was also highlighted as a Physics Synopsis by American Physical Society with a title "Single photons obey the speed limits."

Prof Du's study demonstrates that a single photon, the fundamental quanta of light, also obeys the traffic law of the universe just like classical EM waves. Einstein claimed that the speed of light was the traffic law of the universe or in simple language, nothing can travel faster than light. HKUST's team is the first to experimentally show that optical precursors exist at the single-photon level, and that they are the fastest part of the single-photon wave packet even in a so called 'superluminal' medium.
Read more.  This was also reported by AFP via France 24, which made a rather assertive conclusion
Hong Kong physicists say they have proved that a single photon obeys Einstein's theory that nothing can travel faster than the speed of light -- demonstrating that outside science fiction, time travel is impossible.
Read more. Surely a single experiment proving that light (photon as a light particle) cannot travel faster than light would not rule out faster than light travel by other types of particles. In any case, there are other time travel theories that are not predicated on travelling at faster than the speed of light.

1 comment:

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