Scientists slow down speed of light

23 January, 2015 12:45 PM

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A team of Scottish scientists has for the first time managed to slow down the speed of light as it travels through air, unimpeded by interactions with any other material, according to a new study.

Scientists have long known that the speed of light can be slowed slightly as it travels through materials such as water or glass.

However, it has generally been thought impossible for particles of light, known as photons, to be slowed as they travel through free space.

In a new paper titled ‘Spatially Structured Photons that Travel in Free Space Slower than the Speed of Light’ published in Science Express, researchers from the University of Glasgow and Heriot-Watt University described that they have managed to slow photons in free space for the first time.

The speed of light is regarded as an absolute and is about 186,000 miles per second in free space. The findings of the new experiment is likely to alter how science looks at light.

They sent photons through a special mask, which changed the photons’ shape, and slowed them to less than light speed.

The experiment was configured like a time trial race, with two photons released simultaneously across identical distances towards a defined finish line.

The researchers found that one photon reached the finish line as predicted, but the structured photon which had been reshaped by the mask arrived later, meaning it was travelling more slowly in free space.

Over a distance of 1 metre, the team measured a slowing of up to 20 wavelengths, many times greater than the measurement precision.

Crucially, this is very different to the slowing effect of passing light through a medium such as glass or water, where the light is only slowed during the time it is passing through the material -– it returns to the speed of light after it comes out the other side.

The experiment was carried out by a team from the University of Glasgow’s Optics Group, led by Professor Miles Padgett.

“The results give us a new way to think about the properties of light and we’re keen to continue exploring the potential of this discovery in future applications,” said Professor Padgett.

Jacquiline Romero, one of the lead authors of the paper, said: “This finding shows unambiguously that the propagation of light can be slowed below the commonly accepted figure of 299,792,458 metres per second, even when travelling in air or vacuum.”


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