Behind the Light
Lights off – movie on! Not so with the documentary film “TRACING LIGHT – The Magic of Light,” which the filmmaker Thomas Riedelsheimer shot at the Max Planck Institute for the Science of Light, among other places. Light takes the leading role and guides the viewer through its complex world, in which space and time, high technology and nature, knowledge and beauty all converge on equal terms. Over two years, the director collected observations and impressions of how science and art deal with light. The result is a film that presents light in its many facets.
Playing kicker with laser light
The propagation of a light wave in space cannot be seen directly. The energy of such a light wave consists of small units, the photons. Only when a photon interacts with the eye does it become noticeable to humans. The artist duo Brunner/Ritz had a similar experience that when they played laser kicker together in the spacious atrium of the Max Planck Institute for the Science of Light (MPL). Instead of the ball that is usually shot into the goal by small footballers using mechanical forces, a laser beam is used here. This is passed on via miniature mirrors. As soon as a goal is scored, it triggers an acoustic signal. A special light form, laser light, is one of the most important tools for science at the MPL, used to research the basics of light and its interactions with matter.
Dr. Michael Frosz, head of the technology development and service unit fiber production and glass studio, uses a choir as a visual comparison to illustrate the difference between an harmonious wave of laser light and sunlight: “The sun is like a big party of people where everyone is talking at the same time, but each person is saying something different.” A laser is more like a choir in which the singers are encouraged to sing the same note at the same time, which then swells mightily.
Frosz´s team is working on the production of glasses and new types of special fibers, such as photonic crystal fibers, which are characterized by their extraordinary optical properties. Fine and highly precisely designed microstructures in the glass direct the light along the highly specific light guides. On the one hand, the scientists want to use this to advance the understanding of light and its interactions with matter. In addition, their findings have the potential to revolutionize industrial progress in new types of lasers, communication technology, sensor technology and medical technology.
The laser kicker players cannot see how the goal is scored – and that is not only because light travelling at a speed of 300,000 kilometers per second cannot be detected by the photoreceptors in the eye. “What we see is actually lost,” explains Prof. Birgit Stiller. “The fact that the light enters our eye means that it is no longer in the game.” The laser beam is reflected from one mirror to the next by light refraction, with the exit angle being the same as the entry angle. However, since mirror surfaces are not perfectly manufactured, the laser beam is partly scattered to the side and this part of this light falls on the receptors of the retina in the eye. “Light waves are everywhere in space at the same time, you cannot hold on to them,” says Del'Haye, trying to explain the properties of light as an electromagnetic wave. He and his “Microphotonics” research group precisely address this challenge.
Can one capture light?
The aim of “Integrated photonics” is to put circuits for light on computer chips. Light is coupled via waveguides in ring resonators, the diameter of which is smaller than that of a hair. They act like light traps: light completes several million revolutions in the ring resonators and is stored temporarily, so to speak. Light outputs of up to one megawatt circulate in the resonator – the stored energy would be enough to illuminate ten football stadiums for at least a few microseconds. In the dimensions of light, that is a very long time period. His fundamental findings should pave the way for new applications for optical sensors, quantum technologies and optical information processing. Photonic chips are a rapidly growing future technology that build on the success of electronic computer chips.
Light, space and time are all related to one another. As time passes at different speeds depending on gravity, these clocks can also be used to measure height differences in the Earth's gravitational field. You may also know this from the film “Interstellar”, in which time passes more slowly near a black hole. With the most precise optical clocks, height differences of just one centimeter can be detected. “It is through the propagation of light that we understand space and time,” says the physicist.
The interplay of sound and light
Birgit Stiller, on the other hand, is a cross-disciplinarian – she expands the world of light to include that of sound. The experimental physicist juggles the two vastly distinct types of waves. Light waves, for example, are 100,000 times faster than sound waves and, unlike sound, do not require matter to propagate in space. Stiller´s team generates sound waves, specifically hypersonic waves, using light and changes light waves through interaction with sound. The physicist's team takes advantage of the differences between the two types of waves in terms of frequency, speed or, for example, power loss to control optical signals without electronics and to temporarily store information. Her basic research in quantum signal manipulation and optical neural networks provides completely novel approaches for energy-efficient architectures for artificial intelligence or for applications in the field of secure quantum communication.
The scientists at the MPL cover a broad research spectrum on the science of light. The basic prerequisite for their research work is based on the primal behavior of light, combining linear and nonlinear properties at the same time. On the one hand, light at a macroscopic double slit appears to behave like a set of particles that pass through the slits and cast a corresponding shadow pattern on a detector. However, if you switch to sufficiently small dimensions, the wave character of the light reveals itself in the form of a specific interference pattern that is clearly different from a shadow pattern. If you now weaken the light beam to such an extent that only a few of the exceedingly small energy packets, the so-called photons, are on their way to the detector camera, then only a few camera pixels will respond here and there. This gives the impression that light consists of particles, i.e., individual quanta. However, if you register many of these events, you get the wave pattern again.
“Everyone thinks they know what light is,” says quantum physicist Prof. Daniele Faccio, who conducts research at the University of Glasgow. “But then you dig a little deeper and realize that you have no idea.” If, for example, you want to know which of the two slits the light has passed through and measure information such as the shape, amplitude or phase of the light wave, the experimenter inevitably becomes part of the experimental setup and thereby changes the state of the wave. It is therefore only possible to gain a single piece of information from one experimental procedure; in a second measurement, you find a completely different state of the wave. The information about all other properties of the wave is irretrievably lost. “The closer observation itself modifies the nature of light,” Faccio sums up, “and the way it behaves. The light notices that it is being observed and transforms. And that is really mysterious.” In quantum mechanics, it is impossible to predict what exactly will be measured next in an experiment. Only by preparing the same experimental setup repeatedly and repeating the measurements thousands of times can a picture of the natural phenomenon of light ultimately emerge – and this averaged picture is scientifically predictable.
Hot on the heels of photons
Together, the protagonists use different approaches to try to reverse the responsibilities: not to make light visible, but to illuminate the properties of light itself. Driven by the search for answers to what light actually is. “To be honest, the most exciting thing is when you reach the limit. Where the scientists at the Max Planck Institute say, `At this point, we ourselves don't know exactly how it behaves, but we have a theory of how it could be,´” says the artist Johannes Brunner. And in doing so, he hits the core of basic research at the Erlangen Institute. Physics tries to break down the proportionality of the nature of light into as few fundamental laws as possible – which physicists call the laws of nature – to explain it with these laws and to make predictions based on these laws. A fundamental law has been added through observation in microscopically small spatial dimensions, which also applies to light. This is about the quantum physical measurement process: during a measurement, a projection occurs on only one of several possible values, which, according to Heisenberg, leads to the blurring of measured values in quantum physics. In this context, too, the question of the essence of light remains the subject of research.
