研究人员称，“降低光速”的方法可以被用于更多的物理学实验中，人类或能解开更多的自然界之谜。 图片来自赫瑞瓦特大学网站原标题：颠覆爱因斯坦理论？科学家成功降低真空中光速中新网1月27日电 据外媒26日报道，根据爱因斯坦的相对论，在任何参照系中，光在真空中的传播速度都是一个常数。但是科学家最近却成功让光在自由空间（free space，完美真空）中的速度降了下来。英国格拉斯哥大学和赫瑞瓦特大学的研究人员进行了一项实验。在实验中，科学家安装了一个特殊“隔层”，单个光子在通过这一装置时，形态会发生改变，而且速度出现了下降。奇妙的是，在通过这一特殊“隔层”之后，即便重新回到自由空间，光子仍会以较低的速度前行。这一实验说明，光的构造可能比人类知道的更为复杂。根据爱因斯坦的理论，光在自由空间中的速度约合每秒30万公里。在经过水、玻璃等介质时，光速会出现下降，但只要再次返回自由空间，光速就会回归正常。研究人员称，“降低光速”的方法可以被用于更多的物理学实验中，人类或能解开更多的自然界之谜 我很关心这件事情，希望得到数据，和详尽的报告，我们这里海内外一群朋友在联系着找，也希望大家有结果告诉一声，这也许是原文，还没顾得翻译，大家同享用吧 Structured photons slow down in a vacuum Jan 22, 2015 29 comments Easy does it: structured light slows in vacuum The speed of light in a vacuum is 299,792,458 m s –1 , right? Not necessarily, according to a team of physicists in the UK, which has found that the speed of an individual photon decreases by a tiny amount if it is initially sent through a patterned mask. The phenomenon – which is different to other observations of slow light – should also occur for sound waves, the researchers say. The speed of light has been measured since as far back as the 17th century, but it was not until the 1970s that physicists settled on a value that was accurate in a vacuum to just a few parts per billion. In 1983 that value became the official value, fixed against a new definition of the metre in the International System of Units. And an important value it is, for according to Einstein's special theory of relativity, the speed of light in a vacuum, c , is the maximum speed obtainable by any entity – no matter what inertial frame of reference it is measured in. Of course, light can appear to slow down if it travels through a dense medium – a result of the photons having to interact with the medium and take an indirect route through it. In water, the speed of light is roughly 225,000,000 m s –1 , while in glass it is roughly 200,000,000 m s –1 . The change can be even more drastic – particularly in highly nonlinear materials, in which light's speed can be reduced to just a few metres per second. Strange effects can also occur in a vacuum, including the Gouy phase shift, which happens when a beam of light is focused to a point and results in a tiny increase in its phase velocity. Structured photons Now it seems that physicists have come up with a new way of changing the speed of light in a vacuum. Over two years, Miles Padgett and colleagues at the University of Glasgow, together with Daniele Faccio of Heriot-Watt University in Edinburgh, designed an experiment that can determine whether light with a certain spatial structure travels substantially slower than regular light in a vacuum. The researchers created a source that emitted pairs of photons simultaneously. One of the photons went straight to a highly precise photon counter, while the other went via two liquid-crystal masks, which imparted their profile onto the passing particle of light. Across a propagation distance of 1 m, the team found that the spatially structured photon lagged behind its partner by between 10 and 20 wavelengths. That equated to a drop in speed of about 0.001%, says team member Jacquiline Romero . There are many ways of defining the speed of light: phase velocity, peak velocity, information velocity – definitions abound. Padgett and colleagues stick to the group velocity, which is a measure of how fast the envelope of an electromagnetic wave moves. When a beam of light passes through a mask, some of its constituent rays will continue to propagate at a slight angle to the beam's axis. These rays have to travel farther, therefore the group velocity of the entire envelope falls – and this is what the researchers observed. No ambiguities? The reliance on group velocity might seem like an important footnote, but the researchers believe that the use of single photons in their experiment should remove any ambiguities in interpretation. One of the nice things about our work is that we have taken the simple case of single photons, which when observed make a detector go 'click', says Padgett. Padgett does not know what, if any, applications could result from the findings. The effect is biggest, he explains, when the diameter of the optics used is large and the distances are short, so it is unlikely to have any impact in astronomy. Nonetheless, he believes the phenomenon should exist in any wave, including sound. We did this experiment really to satisfy our own curiosity, he explains. We have always been interested in structured beams. Gory Genty , a physicist at the Tampere University of Technology in Finland says that the experiment is interesting because it measures the group velocity of photons. In that sense, the results are not in contradiction with anything we know from textbooks, and certainly not with special relativity, he adds. There have been couple of experiments in the past to show this effect, but perhaps here the novelty lies in the fact that the researchers are dealing with single photons. The research is published in Science . Check out our free-to-read digital edition of Physics World magazine containing 10 of our best-ever features on the science and applications of light, which we have put together to mark the International Year of Light About the author Jon Cartwright is a freelance journalist based in Bristol, UK
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