A Snapshot of Relativistic Motion: Special relativity made visible
A technical trick has been used to simulate a speed of light of only 2 m/s in the laboratory. This made it possible to reproduce the relativistic Terrell-Penrose effect for the first time
Vienna University of Technology
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The Terrell-Penrose-Effect: Fast objects appear rotated
view moreCredit: TU Wien
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.
Journal
Communications Physics
Method of Research
Experimental study
Subject of Research
Not applicable
Article Title
A snapshot of relativistic motion: visualizing the Terrell-Penrose effect
Article Publication Date
5-May-2025
New theory of gravity brings long-sought Theory of Everything a crucial step closer
A quantum theory of gravity would clear the path to answering some of the biggest questions in physics
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The gravity quantum field is calculated in flat spacetime. The curved classical metric is calculated using the expectation value of the gravity quantum field.
view moreCredit: Mikko Partanen and Jukka Tulkki / Aalto University.
At long last, a unified theory combining gravity with the other fundamental forces—electromagnetism and the strong and weak nuclear forces—is within reach. Bringing gravity into the fold has been the goal of generations of physicists, who have struggled to reconcile the incompatibility of two cornerstones of modern physics: quantum field theory and Einstein’s theory of gravity.
Researchers at Aalto University have developed a new quantum theory of gravity which describes gravity in a way that’s compatible with the Standard Model of particle physics, opening the door to an improved understanding of how the universe began. While the world of theoretical physics may seem remote from applicable tech, the findings are remarkable. Modern technology is built on such fundamental advances — for example, the GPS in your smartphone works thanks to Einstein’s theory of gravity.
Mikko Partanen and Jukka Tulkki describe their new theory in a paper just published in Reports on Progress in Physics. Lead author Partanen expects that within a few years, the findings will have unlocked critical understanding.
‘If this turns out to lead to a complete quantum field theory of gravity, then eventually it will give answers to the very difficult problems of understanding singularities in black holes and the Big Bang,’ he says.
‘A theory that coherently describes all fundamental forces of nature is often called the Theory of Everything,’ says Partanen, although he doesn’t like to use the term himself. ‘Some fundamental questions of physics still remain unanswered. For example, the present theories do not yet explain why there is more matter than antimatter in the observable universe.’
Reconciling the irreconcilable
The key was finding a way to describe gravity in a suitable gauge theory — a kind of theory in which particles interact with each other through a field. ‘The most familiar gauge field is the electromagnetic field. When electrically charged particles interact with each other, they interact through the electromagnetic field, which is the pertinent gauge field,’ explains Tulkki. ‘So when we have particles which have energy, the interactions they have just because they have energy would happen through the gravitational field.’
A challenge long facing physicists is finding a gauge theory of gravity that is compatible with the gauge theories of the other three fundamental forces — the electromagnetic force, the weak nuclear force and the strong nuclear force. The Standard Model of particle physics is a gauge theory which describes those three forces, and it has certain symmetries. ‘The main idea is to have a gravity gauge theory with a symmetry that is similar to the Standard Model symmetries, instead of basing the theory on the very different kind of spacetime symmetry of general relativity,’ says Partanen, the study’s lead author.
Without such a theory, physicists cannot reconcile our two most powerful theories, quantum field theory and general relativity. Quantum theory describes the world of the very small—tiny particles interacting in probabilistic ways—while general relativity describes the chunkier world of familiar objects and their gravitational interaction. They are descriptions of our universe from different perspectives, and both theories have been confirmed to extraordinary precision—yet they are incompatible with each other. Furthermore, because gravitational interactions are weak, more precision is needed to study true quantum gravity effects beyond general relativity, which is a classical theory.
‘A quantum theory of gravity is needed to understand what kind of phenomena there are in cases where there’s a gravitational field and high energies,’ says Partanen. Those are the conditions around black holes and in the very early universe, just after the Big Bang—areas where existing theories in physics stop working.
Always fascinated with the very big questions of physics, he discovered a new symmetry-based approach to the theory of gravity and began to develop the idea further with Tulkki. The resulting work has great potential to unlock a whole new era of scientific understanding, in much the same way as understanding gravity paved the way to eventually creating GPS.
Open invite to the scientific community
Although the theory is promising, the duo point out that they have not yet completed its proof. The theory uses a technical procedure known as renormalization, a mathematical way of dealing with infinities that show up in the calculations. So far Partanen and Tulkki have shown that this works up to a certain point—for so-called ‘first order’ terms—but they need to make sure the infinities can be eliminated throughout the entire calculation. ‘If renormalization doesn’t work for higher order terms, you’ll get infinite results. So it’s vital to show that this renormalization continues to work,’ explains Tulkki. ‘We still have to make a complete proof, but we believe it’s very likely we’ll succeed.’
Partanen concurs. There are still challenges ahead, he says, but with time and effort he expects they’ll be overcome. ‘I can’t say when, but I can say we’ll know much more about that in a few years.’
For now, they’ve published the theory as it stands, so that the rest of the scientific community can become familiar with it, check its results, help develop it further, and build on it.
‘Like quantum mechanics and the theory of relativity before it, we hope our theory will open countless avenues for scientists to explore,’ Partanen concludes.
Journal
Reports on Progress in Physics
