Physicists have just found a way to slow light down without touching or intercepting it, simply by making it ‘twist’ as it moves through space.
That means they were able to make beams of light take longer to get from point A to point B, a skill that will be incredibly useful when it comes to controlling the next-generation of optical computers and telecommunication systems.But before you freak out, this research doesn’t change the fact that light is still travelling at, well, the speed of light.
Benefits of twisted light
Most people are familiar with the solid spot found in laser pointers created by Gaussian laser beams. In contrast, the corkscrew shape of twisted light creates a donut shape when shone on a surface. The light can carry an infinite number of twists over one wavelength.
The laws of physics state that light travels at a constant speed in a vacuum, wherever it is in the Universe. For those playing along at home, that speed is 299,792,458 m/s. And we haven’t broken the laws of physics just yet.This constant speed of light is simply referred to as ‘c’ in equations (like Einstein’s famous E = mc2), and as reported last year: “It’s a fundamental constant in physics: it defines the absolute speed limit for the transfer of energy, matter, and information.”
Of course, you might also be aware that we can already made light travel a lot slower than that – for example, when it moves through water, or any other medium with a greater refractive index than a vacuum, light’s speed drops off significantly.
That’s great, but those techniques don’t help us slow down light while still accurately transmitting information – something we’re doing more and more of over optical fibres. So in recent years, physicists have been looking into more subtle ways of slowing light down.By changing how some light beams rotate, the researchers from the National Institute of Physics were able to slow down light in a vacuum. The physicists used circularly symmetric light beams, known as Laguerre-Gauss beams, to change the way light twists around itself. Suddenly, the light beams were propagating more slowly.
The speed of light varies when it moves through different materials, and it does so at the expense of accuracy in transmitting information. For this reason, more and more people are interested in ways of manipulating the speed of light without affecting accuracy.
Last year, a team led by the University of Glasgow in Scotland took the first step and successfully managed to slow light down in a vacuum. They did this by sending photons through a ‘mask’, which altered the physical form the photons and slowed down their journey.Their experiment was configured like a race, with two photons released simultaneously across identical distances towards a defined finish line.
The team compare a beam of light, containing many photons, to a team of cyclists who share the work by taking it in turns to cycle at the front. Although the group travels along the road as a unit, the speed of individual cyclists can vary as they swap position.The group formation can make it difficult to define a single velocity for all cyclists, and the same applies to light. A single pulse of light contains many photons, and scientists know that light pulses are characterized by a number of different velocities.The researchers found that one photon reached the finish line as predicted, but the structured photon which had been reshaped by the mask arrived later, meaning it was traveling more slowly in free space. Over a distance of one meter, the team measured a slowing of up to 20 wavelengths, many times greater than the measurement precision.
The work demonstrates that, after passing the light beam through a mask, photons move more slowly through space. Crucially, this is very different to the slowing effect of passing light through a medium such as glass or water, where the light is only slowed during the time it is passing through the material – it returns to the speed of light after it comes out the other side. The effect of passing the light through the mask is to limit the top speed at which the photons can travel.
And now a team from the University of Philippines’ National Institute of Physics has taken things even further and demonstrated a new way that light can be ‘slowed down’, simply by changing the way it twists around itself.
In their experiment, the physicists used beams of light called Laguerre-Gauss (LG) beams, which are known to carry orbital angular momentum, or OAM.OAM is a type of angular momentum, which is a quantity that measures how much light is rotating. If you picture a beam of light twisting like a corkscrew through space, you’ll get a rough idea of what’s going on.Each beam of light has its own unique OAM, but the team was able to show that they could change the angular momentum of their LG beams without directly interfering with them, and demonstrated that this slowed the light down. Importantly, the team was able to calculate and predict the amount of time that each LG beam would be slowed down prior to the experiments, based on their OAM.
As we mentioned before, this doesn’t mean they actually changed the speed of light – they simply altered its path so it took a longer time to get where it’s going. But the potential applications are just as exciting. LG beams are already used in telecommunication systems, and early computer applications. So if the team can now predict how these beams will slow down – or even control the rate at which they travel – it will allow them to manipulate the flow of information, and hopefully make these systems more efficient.
It’s still early days, but this research is part of a growing body of work that shows light isn’t quite as well understood as we once thought. In fact, a few weeks ago, physicists discovered a brand new form of light, and earlier this year, a team discovered a fundamental new property of the electromagnetic radiation.
Importantly, all of this suggests that maybe light speed isn’t quite so constant after all. In fact, it might be more like a maximum limit for light travelling through a vacuum. And if that’s the case, then maybe we’re also not that far off finding a way to travel faster than the speed of light – if we haven’t already.