Their experimental set-up is remarkably similar to that used to slow light to a speed of just 17 metres per second last year. Wang and colleagues report evidence for a negative group velocity of -310 c, where c (=300 million metres per second) is the speed of light in vacuum. In a dispersive medium, however, they are different because the refractive index is a function of wavelength, which means that the different wavelengths travel at different speeds. In a vacuum all the phase velocities and the group velocity are the same. The individual waves travel at their own phase velocity, while the pulse itself travels with the group velocity. However, a pulse of light can have more than one speed because it is made up of light of different wavelengths. Special relativity prevents any object with mass travelling at the speed of light, and the principle of causality – the notion that the cause comes before the effect – is used to rule out the possibility of superluminal (faster-than-light) travel by light itself. However, the laws of physics remain intact because Lijun Wang and colleagues at the NEC Research Institute in Princeton in the US are able to explain the results of their experiment in terms of the classical theory of wave propagation. But this speed limit has been smashed in a recent experiment in which a laser pulse travels at more than 300 times the speed of light (L J Wang et al. One of the most sacred laws of physics is that nothing can travel faster than the speed of light in vacuum. Webinars Tune into online presentations that allow expert speakers to explain novel tools and applications.Video Watch our specially filmed videos to get a different slant on the latest science.Podcasts Our regular conversations with inspiring figures from the scientific community.Audio and video Explore the sights and sounds of the scientific world.Supercool physics Experiments that probe the exotic behaviour of matter at ultralow temperatures depend on the latest cryogenics technology.The science and business of space Explore the latest trends and opportunities associated with designing, building, launching and exploiting space-based technologies.Revolutions in computing Find out how scientists are exploiting digital technologies to understand online behaviour and drive research progress.Nanotechnology in action The challenges and opportunities of turning advances in nanotechnology into commercial products.#BlackInPhysics Celebrating Black physicists and revealing a more complete picture of what a physicist looks like.Artificial intelligence Explore the ways in which today’s world relies on AI, and ponder how this technology might shape the world of tomorrow.Collections Explore special collections that bring together our best content on trending topics.Innovation showcases A round-up of the latest innovation from our corporate partners.Projects and facilities Follow the latest progress at the world’s top scientific experiments.Policy and funding Understand how emerging policy changes could affect your work and career.Education and outreach Learn about novel approaches to educating and inspiring the scientists of the future.Business and innovation Find out how recent scientific breakthroughs are driving business innovation and commercial growth.Impact Explore the value of scientific research for industry, the economy and society.Personalities Find out who’s doing what in industry and academia.Interviews Discover the views of leading figures in the scientific community.Opinion and reviews Find out whether you agree with our expert commentators. ![]()
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