Apparently the speed of light in our universe isn't quite as constant as science first thought.
The speed of light is considered one of the rare absolutes of the entire universe. When we're not calling light the fastest thing in the universe, we're using it to measure the distance between stars. What's more, light never slows down - even when it goes through something like a glass of water, the speed change is temporary and barely perceptible, returning to normal when it exits the other side. At least that's what we thought, until scientists found a way to slow light the rate of light's movement altogether.
The experiments behind this discovery were conducted in Glasgow university, where researchers constructed what basically amounts to a photon racetrack. This allowed the team to runs pairs of photons along the track while exposing one photon to a mask to change its speed. Normally, both photons should reach the end of the meter long track at the same time. But when using the mask one photon arrived at a close second, only by a few millionths of a meter, but still enough to tell that the speed had been reduced.
According to Glasgow professor Miles Padgett the crucial component is the mask itself, a software liquid crystal that looks like a bullseye target. "That mask patterns the light beam, and we show that it's the patterning of the light beam that slows it down," he explained. "But once that pattern has been imposed - even now the light is no longer in the mask, it's just propagating in free space - the speed is still slow."
There's a lot of super complicated quantum waveform data explaining what's happening here, and you can read the full study if you prefer the technical explanation. But it has some pretty big implications when considered fully - if something were to affect the light of distant stars to slow it down, then it means our measurements of the universe's size could be full of holes that need to be adjusted. We're probably okay when it comes to, say, the distance between the Earth and the Moon, but other short distances may need to be double-checked for slow-moving light photons, or taken into account so we don't misstep when traveling the stars.