For the first time, physicists have observed that 'holes' in light can move faster than the light itself.
They're known as phase singularities or optical vortices,
and since the 1970s, scientists have predicted that, just as eddies in a river
can move faster than the flowing water around them, so too can whirlpools in a
wave of light outrun the light they're embedded within.
This does not break relativity, which states that nothing
can travel faster than the speed of light. That's because the vortices carry no
mass, energy, or information, and their motion is based on the evolving
geometry of the wave pattern rather than any physical motion through space.
However, capturing this phenomenon in action has been
difficult to accomplish because it unfolds on extremely small scales of space
and time. The achievement is a triumph of electron microscopy.
"Our discovery reveals universal laws of nature shared
by all types of waves, from sound waves and fluid flows to complex systems such
as superconductors," says Ido Kaminer, physicist at the Technion Israel
Institute of Technology.
"This breakthrough provides us with a powerful
technological tool: the ability to map the motion of delicate nanoscale
phenomena in materials, revealed through a new method (electron interferometry)
that enhances image sharpness."
Although to our eyes light appears uniform, it has a lot
going on 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 scientists call optical vortices.
Light can behave both as a particle and a wave; an optical
vortex forms when the wave twists as it travels, like a corkscrew. At the very
center of that twist, the light cancels itself out, leaving a point of zero
intensity – a kind of dark "hole" in the light.
It's mathematically understood that two singularities in a
reference frame will be drawn together, gaining speed as they approach,
reaching velocities that appear to exceed the speed of light in a vacuum.
"As opposite-charged singularities approach each other,
their paths in spacetime must form a continuous curve at the annihilation
point, forcing their acceleration to unbounded velocities right before the
annihilation," the researchers explain in their paper.
It has been observed in other systems, but studying how this
scenario might play out in a light field is somewhat trickier. Much work has
been done in physics labs to study it, but observations of optical vortices
have been limited by the technology's inability to keep up with the speed at
which vortex formation, motion, and collision unfold.
To overcome these limitations, Kaminer and his colleagues
recorded the behavior of optical vortices in a two-dimensional material called
hexagonal boron nitride.
This material supports unusual light waves called phonon
polaritons – hybrids of light and atomic vibrations – that move much more
slowly than light alone and can be tightly confined. This creates intricate
interference patterns filled with many vortices, allowing the researchers to
track their motion in detail.
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The apparatus used to generate and record the optical
vortices. (Kaminer et al., Nature, 2026) |
The second, crucial part was capturing those dynamics in
real time. The team deployed a specialized high-speed electron microscope with
unprecedented spatial and temporal resolution, which recorded events unfolding
over just 3 quadrillionths of a second.
They ran the experiment many times, each time recording at a
slight delay compared to the previous run. By stacking together the hundreds of
images generated this way, the researchers created a timelapse of the vortices
as they hurtled towards and annihilated each other, their velocities very
briefly reaching superluminal speeds in the process.
The experiment took place in a two-dimensional context. The
next step, the researchers say, is to try to extend their work into higher
dimensions to observe more complicated behavior. They also say the techniques
they developed could help address some of the current limitations of electron
microscopy.
"We believe these innovative microscopy techniques will
enable the study of hidden processes in physics, chemistry, and biology,"
Kaminer says, "revealing for the first time how nature behaves in its
fastest and most elusive moments."
The research has been published in Nature.