For the first time, physicists have observed “holes” in light moving faster than the light itself.
They’re called phase singularities or optical vortices, and scientists have predicted since the 1970s that just like eddies in a river can move faster than the surrounding flowing water, vortices in light waves can outpace the light they embed.
This does not break the theory of relativity, which states that nothing can move faster than the speed of light. This is because vortices carry no mass, energy or information, and their motion is based on the evolving geometry of the wave pattern rather than any physical movement in space.
However, capturing the reality of this phenomenon is difficult because it unfolds on extremely small spatial and temporal scales. This achievement was a triumph for electron microscopy.
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“Our findings reveal universal laws of nature shared by all types of waves, from sound waves and fluid flow to complex systems such as superconductors,” said physicist Ido Kaminer of the Technion-Israel Institute of Technology.
“This breakthrough gives us a powerful technical tool: the ability to map the motion of subtle nanoscale phenomena in materials, revealed by a new method (electron interferometry) that enhances image clarity.”
Although light appears uniform to us, there are many things about it that we cannot easily discern. Light can be subject to disturbances similar to those seen in other systems dominated by flow dynamics, including a type of phase singularity that scientists call optical vortices.
Light can appear as both particles and waves. Optical vortices form when waves twist like a spiral as they propagate. At the center of the twist, the light cancels itself out, leaving a point of zero intensity—a kind of dark “hole” in the light.
Mathematically speaking, two singularities in a reference frame will be drawn together, speeding up as they approach, reaching speeds that appear to exceed the speed of light in a vacuum.
“When oppositely charged singularities approach each other, their paths through spacetime must form a continuous curve at the annihilation point, forcing them to accelerate to unbounded velocities before annihilation,” the researchers explain in their paper.
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It has been observed in other systems, but studying how this occurs in light fields is a bit tricky. Much work has been done in physics labs to study it, but observations of optical vortices have been limited because the technology cannot keep up with the speed at which vortices form, move and collisions unfold.
To overcome these limitations, Kamina and his colleagues documented optical vortex behavior in a two-dimensional material called hexagonal boron nitride.
The material supports unusual light waves called phonon polaritons (a hybrid of light and atomic vibrations) that move much slower than light alone and can be tightly confined. This creates complex interference patterns filled with many vortices, allowing researchers to track their movements in detail.
The second key part is capturing these dynamics in real time. The team deployed a specialized high-speed electron microscope with unprecedented spatial and temporal resolution, which recorded events occurring in just 3 trillionths of a second.
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They ran the experiment multiple times, with each recording being slightly delayed from the previous run. By stacking together hundreds of images generated in this way, the researchers created time-lapse photography of the vortices as they hurtled toward each other and annihilated each other, in the process very briefly reaching superluminal speeds.
The experiment was conducted in a two-dimensional environment. The next step, the researchers say, is to try to extend their work to higher dimensions to observe more complex behavior. They also say the technique they developed could help address some of the current limitations of electron microscopy.
“We believe these innovative microscopy techniques will make it possible to study hidden processes in physics, chemistry and biology, revealing for the first time how nature behaves at its fastest and most elusive moments,” Kamina said.
The study was published in nature.
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