One night over drinks at a conference in San Jose, Miles Padgett, a physicist at Glasgow University in Scotland, was chatting with a colleague about whether or not they could make light go slower than its “lawful” speed in a vacuum. “It’s just one of those big, fundamental questions you may want to ask yourself at some point in the pub one night,” he told BBC News. Though light slows down when it passes through a medium, like water or air, the speed of light in a vacuum is usually regarded as an absolute.

This time, the pub talk proved to be a particularly fruitful exchange. Last month, Padgett and his collaborators made headlines when they revealed their surprising success : They raced two photons down a one-meter “track” and managed to slow one down just enough that it finished a few millionths of a meter behind its partner. The experiment showed that it is possible for light to travel at a slower speed even in free space—and Padgett and his colleagues did it at the scale of individual photons.

The notion that light has a particular speed, and that that speed is measurable, is relatively new. Prior to the 17th century, most natural philosophers assumed light traveled instantaneously. Galileo was one of the first to test this notion, which he did with the help of an assistant and two shuttered lanterns. First, Galileo would lift the shutter on his lantern. When his assistant, standing some distance away, saw that light, he would lift the shutter on his lantern in response. Galileo then timed how long it took for him to see the return signal from his assistant’s lantern, most likely using a water clock , or possibly his pulse. “If not instantaneous, it is extraordinarily rapid,” Galileo concluded, estimating that light travels at about ten times the speed of sound.

Over the ensuing centuries, many other scientists improved upon Galileo’s work by devising ingenious new methods for measuring the speed of light. Their results fell between 200,000 kilometers per second, recorded in 1675 by Ole Roemer, who made his measurement by studying eclipse patterns in Jupiter’s moons, and 313,000 kilometers per second, recorded in 1849 by Hippolyte Louis Fizeau, who sent light through a rotating tooth wheel and then reflected it back with a mirror. The current accepted value is 299,792.458 kilometers per second, or 669,600,000 miles per hour. Physicists represent this value with the constant c, and it is broadly understood to be the cosmic speed limit: all observers, no matter how fast they are going, will agree on it, and nothing can go faster.

This limit refers to the speed of light in a vacuum—empty space, with no “stuff” in it with which light can interact. Light traveling through air, water, or glass, for example, will move more slowly as it interacts with the atoms in that substance. In some cases, light will move so slowly that other particles shoot past it. This can create Cherenkov radiation, a “photonic boom” shockwave that can be seen as a flash of blue light. That telltale blue glow is common in certain types of nuclear reactors. (Doctor Manhattan, the ill-fated atomic scientist in Alan Moore’s classic “Watchmen” graphic novel, sports a Cherenkov-blue hue.) It is useful for radiation therapy and the detection of high-energy particles such as neutrinos and cosmic rays—and perhaps one day, dark matter particles—none of which would be possible without the ability of certain materials to slow down light.

But just how slow can light go? In his 1933 novel “Master of Light,” French science fiction writer Maurice Renard imagined a special kind of “slow glass” through which light would take 100 years to pass. Slow glass is very much the stuff of fiction, but it has an intriguing real-world parallel in an exotic form of matter known as a Bose-Einstein Condensate (BEC), which exploits the wave nature of matter to stop light completely. At normal temperatures atoms behave a lot like billiard balls, bouncing off one another and any containing walls. The lower the temperature, the slower they go. At billionths of a degree above absolute zero, if the atoms are densely packed enough, the matter waves associated with each atom will be able to “sense” one another and coordinate themselves as if they were one big “superatom.”

First predicted in the 1920s by Albert Einstein and the Indian physicist Satyendra Bose, BEC wasn’t achieved in the lab until 1995. The Nobel Prize winning research quickly launched an entirely new branch of physics, and in 1999, a group of Harvard physicists realized they could slow light all the way down to 17 miles per hour by passing it through a BEC made of ultracold sodium atoms. Within two years, the same group succeeded in stopping light completely in a BEC of rubidium atoms.

What was so special about the recent Glasgow experiments, then? Usually, once light exits a medium and enters a vacuum, it speeds right back up again, because the reduced velocity is due to changes in what’s known as phase velocity. Phase velocity tracks the motion of a particular point, like a peak or trough, in a light wave, and it is related to a material’s refractive index, which determines just how much that material will slow down light.

Padgett and his team found a way to keep the brakes on in their experiment by focusing on a property of light known as group velocity. Padgett likens the effect to a subatomic bicycle race , in which the photons are like riders grouped together in a peloton (light beam). As a group, they appear to be moving together at a constant speed. In reality, some individual riders slow down, while others speed up. The difference, he explained to BBC News, is that instead of using a light pulse made up of many photons, “We measure the speed of a single photon as it propagates, and we find it’s actually being slowed below the speed of light.”

The Glasgow researchers used a special liquid crystal mask to impose a pattern on one of two photons in a pair. Because light can act like both a particle and a wave—the famous wave-particle duality—the researchers could use the mask to reshape the wavefront of that photon, so instead of spreading out like an ocean wave traveling to the shore, it was focused onto a point. That change in shape corresponded to a slight decrease in speed. To the researchers’ surprise, the light continued to travel at the slightly slower speed even after leaving the confines of the mask. Because the two photons were produced simultaneously from the same light source, they should have crossed the finish line simultaneously; instead, the reshaped photon lagged just a few millionths of a meter behind its partner, evidence that it continued to travel at the slower speed even after passing through the medium of the mask.

Padgett and his colleagues are still pondering the next step in this intriguing line of research. One possibility is looking for a similar slow-down in light randomly scattered off a rough surface.

If so, it would be one more bit of evidence that the speed of light, so often touted as an unvarying fundamental constant, is more malleable than physicists previously thought. University of Rochester physicist Robert Boyd, while impressed with the group’s ingenuity and technical achievement, calmly took the news in stride. “I’m not surprised the effect exists,” he told Science News . “But it’s surprising that the effect is so large and robust.”

His nonchalance might strike non-physicists as strange: Shouldn’t this be momentous news poised to revolutionize physics? As always, there are caveats. When it comes to matters of light speed, it’s important to read the fine print. In this case, one must be careful not to confuse the speed at which light travels, which is just a feature of light, with its central role in special relativity, which holds that the speed of light is constant in all frames of reference. If Galileo measures the speed of light, he gets the same answer whether he is lounging at home in Pisa or cruising in a horse-drawn carriage. The same goes for his trusty assistant. This still holds true, centuries later, despite the exciting news out of Glasgow last month.

Go Deeper
Our picks for further reading

arXiv: Photons that travel in free space slower than the speed of light
Access a full preprint of Padgett and his colleagues’ paper, later published in Science.

Empire of Light: A History of Discovery in Science and Art
Sidney Perkowitz, a condensed matter physicist, looks at the history of our understanding of light in both art and science.

International Year of Light
The UN General Assembly has proclaimed 2015 as the International Year of Light (IYL 2015). Discover more about the science, technology, and social impact of light at the web site of the IYL.

NOVA: Ultracold Atoms
Take a closer look at Bose Einstein condensates in this Q&A with physicist Luis Orozco.

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