The speed of light in a vacuum is the absolute speed limit of the universe. Nothing will go faster than 299,792 kilometers per second (186,000 miles per second), according to Einstein's work, as it would require an infinite amount of energy to do so.

However, that doesn't mean that light can't be beaten in terms of speed under the right set of circumstances. In water, for example, light is slowed down to 225,000 kilometers per second (139,800 miles per second), which is still pretty zippy, but can be beaten by particles (e.g. in a nuclear reactor) and result in Cherenkov light.

But 225,000 kilometers per second is far from the slowest that light has ever traveled. In 1998, scientists were able to slow it to just 17 meters per second, or an embarrassing 61.2 kilometers (38 miles) per hour.

Slowing light was not the ultimate aim of the experiment. The team were keen to study Bose-Einstein Condensate (BEC), a state of matter first hypothesized by Albert Einstein based on the work of theoretical physicist Satyendra Nath Bose. When a gas of bosons – subatomic force-carrying particles that have integer spin – are cooled to temperatures approaching absolute zero, they form a single quantum object, often compared to it acting as a single atom. 

"The wave function of a BEC corresponds to the ground state of a macroscopic quantum object," one paper explains. "In other words, a collection of atoms in a BEC behaves as a single quantum entity."

In this strange new state of matter, first created in the real world in 1995, you get a macroscopic look at quantum behavior.

It has plenty of weird properties, including zero viscosity. You get some of this stuff in a glass, it will crawl up the side of the glass. They can sustain vortices that can be used to create analog black holes, and explode in a way similar to a supernova, termed a bosenova. It's pretty clear why you'd want to study this stuff.

In 1998, scientists from the Rowland Institute for Science created a BEC by supercooling sodium atoms in a vacuum chamber. First they fired laser beams (moving at the usual speed of light) at the sodium, slowing down the particles as they absorb photons. Now slowed, they were put in another array of lasers, where the atoms were knocked back in whatever direction they came from, further slowing (and cooling) the cloud of atoms, kept in place by a powerful magnetic field.

Once this was done and a cloud of condensate was formed, the team fired one laser across the width of it to set up quantum interference, while a second laser was fired across its length. Under these conditions, the light was slowed dramatically.

"We obtain a light speed of 17 [meters per second] for pulse propagation in an atom cloud initially prepared as an almost pure Bose Einstein condensate (condensate fraction is ⩾90 percent)," the team wrote of their experiment. "Whether the cloud remains a condensate during and after pulse propagation is an issue that is beyond the scope of this Letter."

While satisfying, the team realized they could do better.

"Soon after, we succeeded in stopping a light pulse completely in an atomic cloud cooled to a temperature just above the transition temperature for BEC," the team explains on the Hau Lab website. "At the time when the light pulse is slowed, compressed, and contained within the atomic sample, we turn off the control laser field abruptly and then turn it back on at a later time. When the control laser is turned back on, the light pulse is regenerated: we can stop and controllably regenerate the light pulse."

The letter is published in Nature.