Using ultra-fast laser pulses and special cameras, scientists have simulated an optical illusion that appears to defy Einstein’s. special theory of relativity.
One consequence of special relativity is that fast-moving objects should appear foreshortened in the direction of motion – a phenomenon known as Lorentz contraction. This effect was confirmed indirectly by experiments on particle accelerators.
Although previous models have worked with this illusion, now called the Terrell-Penrose effect, this is the first time it has been achieved in the laboratory. The team described their results in the journal Physics of communications.
“What I like most is simplicity” Dominique Hornofquantum 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 laboratory. This shows that even century-old predictions can come to life in a truly intuitive way.”
Recreate the illusion
In the new study, physicists used ultra-fast laser pulses and closed cameras to produce snapshots of a cube and a sphere “moving” at almost the speed of light. The results showed snapshots of rotated objects. This proved that the Terrell-Penrose effect was true.
But like any study, this one also had its difficulties. Moving any object at or near the speed of light is currently impossible. “In Einstein’s theory, the faster something moves, the more its effective mass increases. As we get closer to the speed of light, the energy we need increases dramatically,” Hornof said. We can’t generate enough energy to accelerate something like a cube, and “that’s why we need huge particle accelerators, even just to move electrons anywhere near that speed. That would take a huge amount of energy.”
So the team resorted to a smart replacement. “What we can do is mimic the visual effect,” Hornof said. They started with a cube about 1 meter on each side. Then they fired ultra-short laser pulses – each lasting just 300 picoseconds, or about a tenth of a billionth of a second – at the object. They captured the reflected light with a closed camera that only opened for that moment and produced a thin “slice” each time.
After each slice, they moved the cube forward about 1.9 inches (4.8 cm). This is the distance it would have traveled if it had moved at 80% the speed of light during the delay between pulses. Then the scientists put all of these slices together to get a snapshot of the moving cube.
“When you combine all the slices, the object appears to be running incredibly fast, even though it never moved at all,” Hornof said. “At the end of the day, it’s just a matter of geometry.”
They repeated the process with a sphere, moving it 2.4 inches (6 cm) per step to imitate a speed of light of 99.9%. When the slices were combined, the cube appeared rotated and the sphere appeared to be able to look around its sides.
“Rotation is not physical,” Hornof said. “It’s an optical illusion. The geometry of how the light arrives at the same time fools our eyes.”
This is why the Terrell-Penrose effect does not contradict Einstein’s special relativity. A fast-moving object is physically foreshortened in its direction of travel, but a camera does not capture it directly. Because light from behind takes longer to arrive than light from in front, the snapshot shifts in a way that makes it appear as if the object has rotated.
“When we did the calculations, we were surprised at how perfectly the geometry worked,” Hornof said. “Seeing him appear in the images was really exciting.”
