If an object is moving extremely fast - on the order of the speed of light - then certain basic assumptions that we take for granted no longer apply - this is the central consequence of Albert Einstein's special theory of relativity. The object then has a different length than at rest, time passes differently for the object than in the laboratory. All of this has been confirmed time and again in experiments.

However, one interesting consequence of the theory of relativity has not yet been observed - the so-called Terrell-Penrose effect. In 1959, physicists James Terrell and Roger Penrose (Nobel Prize winner in 2020) independently came to the conclusion that fast-moving objects must appear rotated. This effect has not yet been proven. Now, for the first time, a collaboration between TU Wien and the University of Vienna, has succeeded in recreating the effect using laser pulses and precision cameras - at an effective speed of light of 2 meters per second.

The faster, the shorter: Einstein's length contraction

"Let's assume a rocket races past us at ninety percent of the speed of light. Then it no longer has the length it had before it was launched, but is a factor of 2.3 shorter," explains Prof. Peter Schattschneider from TU Wien. This is relativistic length contraction, also known as Lorentz contraction.

However, this shortening cannot be photographed. “If you were to take a photo of the rocket speeding past, you have to bear in mind that the light has traveled to the camera from different points for different lengths of time,” explains Peter Schattschneider. “Light that arrives in the lens or in our eye from different points on the object at the same time was not emitted at the same time - and this results in complicated optical effects.”

The racing cube: seemingly rotated

Let's imagine, for example, that the super-fast object is a cube. Then the side facing away from us is further away from us than the side facing us. If two photons reach our eye at the same time, one from the front corner of the cube and one from the rear corner of the cube, then the photon from the rear corner has traveled longer. It must therefore have been emitted at an earlier point in time. And at this point in time, the cube was in a different position than when the light was emitted from the front corner.

“As a result, it looks to us as if this cube has been rotated,” says Peter Schattschneider. "This is a combination of relativistic length contraction and the different light travel times from different points. Together, this leads to an apparent rotation, as Terrell and Penrose predicted."

Of course, this doesn't play a role in everyday life, not even when we photograph an extremely fast car. Even the fastest Formula 1 car only moves a tiny bit within the time difference between the light being emitted from the side of the car facing away from us and the side facing us. But with a rocket whose speed is close to the speed of light, this effect would be clearly visible.

The trick with the effective speed of light

Today, it is technically impossible to accelerate rockets to a speed at which this effect could be seen in a photograph. However, the group led by Peter Schattschneider from USTEM at TU Wien found another solution inspired by art: they used extremely short laser pulses and a high-speed camera to recreate the effect in the laboratory.

“We moved a cube and a sphere through the lab and recorded the laser flashes reflected from different points on these objects at different times with the high-speed camera,” explain Victoria Helm and Dominik Hornof, the two students who carried out the experiment. “If you time it right, you can create a situation that produces the same results as if the speed of light were no greater than 2 meters per second.”

Combining images that show different areas of a landscape into one large image is easy. What has been done here includes the time factor for the first time: The object is photographed at many different points in time. The areas that the laser flash illuminates at the point in time at which the light would have been emitted from this location if the speed of light were only 2 m/s are then combined to form a still image. This makes the Terrell-Penrose effect visible.

"We combined the still images into short video clips of the ultra-fast objects. The result was exactly what we expected," says Peter Schattschneider. “A cube appears twisted, a sphere remains a sphere, but the north pole is in a different place.”

When art and science revolve around each other

The demonstration of the Terrell Penrose effect that has now been realized is not only a scientific success - it is also the result of an extraordinary symbiosis between art and research. The starting point was an art-science project led by the artist Enar de Dios Rodriguez, who explored the possibilities of ultra-fast photography and the resulting “slowness of light” a few years ago in collaboration with the University of Vienna and Vienna University of Technology.

The results have now been published in the journal “communications physics” - a step that may help us to understand the intuitively elusive world of relativity a little better.

Original publication

D. Hornof et al., A snapshot of relativistic motion: visualizing the Terrell-Penrose effect, Communications Physics (2025)., opens an external URL in a new window
free version: https://arxiv.org/abs/2409.04296, opens an external URL in a new window

A popular scientific summary can also be found in Spektrum.de, opens an external URL in a new window (6.5.2025).
 

Requests

Prof. Peter Schattschneider
USTEM
Technische Universität Wien
+43 1 58801 13722
peter.schattschneider@tuwien.ac.at

Aussender:
Dr. Florian Aigner
Kommunikation
Technische Universität Wien
florian.aigner@tuwien.ac.at