One of the more well-known rules in physics is that light can only ever go one speed, so long as nothing stands in its way.
But new research has found there could be an interesting exception to this rule, where the mixing of light waves could bring them to a complete standstill.
The discovery hints at new ways of wrangling not just photons but nearly any kind of wave, which could be useful in technology that relies on information sent and stored using light.
Delaying light's journey isn't itself all that hard. Put a bunch of atoms in their way, and photons will take their time slipping in and out of the forest of particles.
More recently, it's been shown that light's pathway can be affected by changing its angular momentum, effectively twisting it so it takes longer than its usual 299,792,458 metres per second to get from A to B.
A small group of physicists from the Israel Institute of Technology and the Institute for Pure and Applied Mathematics (IMPA) in Brazil have now come up with another method, showing it's theoretically possible to weave waves of light together in such a way that they stop dead in their tracks.
The trick relies on tuning the light waves so they meet at what's called an exceptional point – mathematical jargon that describes how the features of different waves match one another at a given coordinate.
Exceptional points were little more than mathematical concepts until fairly recently, when researchers demonstrated experimentally that they could be created by confining microwaves within a narrow grid.
When we talk about light waves, most of us imagine ripples of varying heights and length.
Light is of course defined by qualities such as wavelength and frequency, but it also has numerous other properties that form repeating patterns as photons traverse space.
These patterns can be tweaked by constraining their properties using things called waveguides, so two light waves can coalesce within the same space. This combination of properties described as an exceptional point gives rise to some interesting behaviours.
Last year researchers applied these points to the development of sensors that could respond to the smallest disruptions.
Now physicists have shown using mathematical models that it's possible to use a kind of waveguide that balances the wave's energy to produce an exceptional point where light stands still.
By toggling the set-up in such a way that the waves can gain or lose energy, the light waves can be made to coalesce and freeze, or speed up and resume their journey out the other side.
If we're to be particularly pedantic, we shouldn't imagine it as photons standing still waiting for the go signal. No fundamental laws are being broken.
Just as light passing through a medium or being twisted is still technically moving at light speed, the photons in these waves are caught in a figurative electromagnetic whirlwind based on their interactions at the exceptional point.
For now, this achievement is still just by the numbers – a light-stopping device based on exceptional points hasn't been built.
But if it does, we'd have another way to manipulate waves of light. The researchers also speculate that the same concepts technically apply to any kind of wave, including sound.
Given photons are quickly becoming the new electrons in information technology, we need all the tools we can find to get a firm grip on these speedy little particles.
This research was published in Physical Review Letters.