When an object moves extremely fast – close to the speed of light – 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 when it is at rest, and time passes differently for the object than it does in the laboratory. All this has been repeatedly confirmed in experiments.
However, one interesting consequence of relativity has not yet been observed – the so-called Terrell-Penrose effect. In 1959, physicists James Terrell and Roger Penrose (Nobel laureate in 2020) independently concluded that fast-moving objects should appear rotated. However, this effect has never been demonstrated. Now, a collaboration between TU Wien (Vienna) and the University of Vienna has succeeded for the first time in reproducing the effect using laser pulses and precision cameras - at an effective speed of light of 2 metres per second.
The faster, the shorter: Einstein's length contraction
"Suppose a rocket whizzes past us at ninety per cent of the speed of light. For us, it no longer has the same length as before it took off, but is 2.3 times shorter," explains Prof. Peter Schattschneider from TU WIen. This is the relativistic length contraction, also known as the Lorentz contraction.
However, this contraction cannot be photographed. "If you wanted to take a picture of the rocket as it flew past, you would have to take into account that the light from different points took different lengths of time to reach the camera," explains Peter Schattschneider. The light coming from different parts of the object and arriving at the lens or our eye 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 that the super-fast object is a cube. Then the side facing away from us is further away than the side facing towards us. If two photons reach our eye at the same time, one from the front corner of the cube and one from the back corner, the photon from the back corner has travelled further. So it must have been emitted at an earlier time. And at that time, the cube was not at the same position as when the light was emitted from the front corner.
"This makes it look to us as if the cube had been rotated," says Peter Schattschneider. This is a combination of relativistic length contraction and the different travel times of light from different points. Together, this leads to an apparent rotation, as predicted by Terrell and Penrose.
Of course, this is irrelevant in everyday life, even when photographing an extremely fast car. Even the fastest Formula One car will only move a tiny fraction of the distance in the time difference between the light emitted by the side of the car facing away from us and the side facing towards us. But with a rocket travelling close to the speed of light, this effect would be clearly visible.
The effective speed of light trick
Technically, it is currently 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 around the lab and used the high-speed camera to record the laser flashes reflected from different points on these objects at different times," explain Victoria Helm and Dominik Hornof, the two students who carried out the experiment. "If you get the timing right, you can create a situation that produces the same results as if the speed of light were no more than 2 metres per second."
It is easy to combine images of different parts of a landscape into one large image. What has been done here for the first time is to include the time factor: the object is photographed at many different times. Then the areas illuminated by the laser flash at the moment when the light would have been emitted from that point if the speed of light was only 2 m/s are combined into one 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 circle each other
The demonstration of the Terrell-Penrose effect is not only a scientific success - it is also the result of an extraordinary symbiosis between art and science. The starting point was an art-science project by the artist Enar de Dios Rodriguez, who several years ago, in collaboration with the University of Vienna and the Vienna University of Technology, explored the possibilities of ultra-fast photography and the resulting 'slowness of light'.
The results have now been published in the journal Communications Physics - a step that may help us understand the intuitively elusive world of relativity a little better.
