First, a quick primer on light and refractivity. Light travels about 300,000 kilometers per second or 186,000 miles per second, but it can only reach this velocity in a vacuum. This maximum speed is known by the value ‘c’. When traveling through other substances, such as glass or water, light can be slowed down considerably. Under those conditions, the speed of light is referred to as ‘v’ for velocity. The ratio between c and v (in other words, how close to its maximum speed light can achieve when passing through a substance) is called the refractive index. The higher the refractive index, the slower light passes through that material.
One more thing: light is made of photons, which exhibit properties of both particles and waves. Don’t ask.
OK, on to the good stuff.
The team of scientists made a 30 layer thick stack of dielectric materials, alternating between high and low refractive index. Each layer was about 80 nm thick (about 1/4 of the wavelength of the light they were testing). They passed photons through this stack and measured the time it took for the photons to reach a detector on the other side. This turned out to be about 12.84 fs (femtosecond, or 10-15 of a second). They then added one more layer onto the end of the stack. For reference, based on the additional distance, adding a single layer should have increased the transit time by 0.58 fs.
Yet, when they added a low refractive index layer to the end of the stack, the transit time increased to 16.36 fs. Even more surprisingly, when they added a single high refractive index layer, the total transit time decreased to only 5.34 fs.
What’s going on?
I mentioned that light is made of both particles and waves. It is the quantum properties of the light waves interfering with each other at each boundary between the layers that creates the illusion of faster than light speed. If all the photons made it all the way through, the speed would be exactly as expected.
Got that? I’ll let Paul Lett, one of authors of this study, have the last word:
A key strangeness of quantum mechanics is that it lets a particle tunnel through a barrier and appear on the other side. Even stranger is that the time it takes to appear on the other side of the barrier can be independent of the thickness. This leads to apparent travel times that can be faster than the speed of light.
Diagram credit: JQI