For the first time, physicists have simulated what objects moving near the speed of light would look like — an optical illusion called the Terrell-Penrose effect.
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The Vienna team stitched together slices of light to create
snapshots. At rest (left), the cube looks normal. But when simulated at 99.9%
of light speed (right), a sphere still looks round but reveals parts of its far
side. (Image credit: Hornof et al., 2025; CC BY 4.0) |
Using ultra-fast laser pulses and special cameras,
scientists have simulated an optical illusion that appears to defy Einstein's
theory of special relativity.
One consequence of special relativity is that fast-moving
objects should appear shortened in the direction of motion — a phenomenon known
as Lorentz contraction. This effect has been confirmed indirectly in particle
accelerator experiments.
But in 1959, mathematician Roger Penrose and physicist James
Terrell pointed out that an observer with a camera wouldn't actually see a
squashed object at all. Instead, because light from different parts of the
object takes different times to reach the camera, it would appear rotated.
Although previous models have worked with this illusion, now
called the Terrell-Penrose effect, this is the first time it has been done in a
lab setting. The team described their results in the journal CommunicationsPhysics.
"What I like most is the simplicity," Dominik Hornof, a quantum physicist at the Vienna University of Technology and first
author of the study, told Live Science. "With the right idea, you can
recreate relativistic effects in a small lab. It shows that even century-old
predictions can be brought to life in a really intuitive way."
Re-creating the illusion
In the new study, physicists used ultra-fast laser pulses
and gated cameras to produce snapshots of a cube and a sphere
"moving" at nearly the speed of light. The results showed snapshots
of rotated objects. This proved the Terrell-Penrose effect to be true.
But like every study, this one also had its difficulties.
Moving any object at or near the speed of light is currently impossible.
"As you get closer to the speed of light, the energy you need grows by a
lot," Hornof said. We cannot generate enough energy to accelerate
something like a cube, and "that's why we need huge particle accelerators,
even just to move electrons close to that speed. It would take a huge amount of
energy."
So the team used a clever substitute. "What we can do
is mimic the visual effect," Hornof said. They started with a cube of
about 3 feet (1 meter) on each side. Then, they fired ultra-short laser pulses
— each just 300 picoseconds long, or about a tenth of a billionth of a second —
at the object. They captured the reflected light with a gated camera that
opened only for that instant and produced a thin "slice" each time.
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The researchers fired ultra-short laser pulses at their test
object and then used a delay generator to tell the camera exactly when to open
its shutter (for just billionths of a second). This camera captured single
slices of light bouncing off the object. They repeated the process and shifted
the object between shots. The team built up the illusion of an object racing at
near light speed. (Image credit: Hornof et al., 2025; CC BY 4.0) |
After each slice, they moved the cube forward about 1.9
inches (4.8 cm). That is the distance it would have traveled if it were moving
at 80% the speed of light during the delay between pulses. Then, the scientists
put all of these slices together into a snapshot of the cube in motion.
"When you combine all the slices, the object looks like
it's racing incredibly fast, even though it never moved at all," Hornof
said. "At the end of the day, it's just geometry."
They repeated the process with a sphere, shifting it by 2.4
inches (6 cm) per step to mimic 99.9% light speed. When the slices were
combined, the cube appeared rotated and the sphere looked as if you could peek
around its sides.
"The rotation is not physical," Hornof said.
"It's an optical illusion. The geometry of how light arrives at the same
time tricks our eyes."
That is why the Terrell-Penrose effect does not contradict
Einstein's special relativity. A fast-moving object is physically shortened
along its direction of travel, but a camera doesn't capture that directly.
Because light from the back takes longer to arrive than light from the front,
the snapshot shifts in a way that makes the object appear rotated.
"When we did the calculations, we were surprised how
beautifully the geometry worked out," Hornof said. "Seeing it appear
in the images was really exciting."