WinSPIRE: Science in the NewsPicture a bank vault overflowing with money, green slips resting within. The money is tightly stored away, and security cameras monitor every happening, making it nearly impossible to break in. But is there a way around this? Is there a way to manipulate light so that an event could be seen as a blip in the dimension of time? And, if light carries information, could scientists manipulate it so that certain information would never reach an observer? While it may sound like science fiction, the answer might just as well lie in optics, the branch of physics that studies how light behaves. Let’s take a closer look at the physics of it all.
So, what exactly enables our sight? Simply put, objects are visible because they reflect light. For example, when you look at an apple, you see the light it reflects into your eyes. Light waves travel through space and hit the apple. The apple bounces some of that light back to your eyes (Figure 1), and your brain reconstructs an image. If light never touched the apple in the first place, it would be completely invisible, but it would still exist in space. Another way to think about this is that light carries information because its properties change after interacting with matter. Reflected light that reaches our eyes isn’t the same as the light that left the source. The change between the light source and the reflected light carries information about the apple’s appearance.
Since how we see objects is based on how light interacts with them, can we make objects invisible by making it so that light avoids them entirely? Imagine if every ray of light that would normally strike an object could instead be gently guided around it, almost like water flowing around a rock in a stream. The light would never interact with the object, and therefore none of it could reflect back to our eyes. Instead, the rays would reunite on the other side and continue along their original path (Figure 2). From the observer’s perspective, the object simply isn’t there. Physicists call this effect spatial cloaking, a concept scientists have been slowly developing over the years.
This naturally raises another question: if light usually travels in straight lines, how could it bend around an object in the first place? To answer this question, we need to understand refraction. This simply refers to the bending of light as it changes speed depending on the material it passes through. For example, what would happen if a line of people were marching shoulder-to-shoulder across a field at the same speed when, suddenly, the people on the left were slowed by thick mud? Since the people on the right keep moving at the original speed, the line will pivot, or rotate. Similarly, if one section of a wave of light enters a material and slows while the other continues moving faster, the wave will pivot, changing direction. This change in direction is what we call refraction.
Refraction explains how light bends, but that alone can’t make light follow a curved path around an object; there must be a continuously varying refractive index (how strongly materials slow light), and this is where metamaterials come in. Metamaterials engineered with microscopic structures often smaller than the wavelength of light present an elegant solution to this problem. While ordinary materials bend light uniformly because of their atomic structure, metamaterials are engineered with microscopic structures whose shape—rather than chemical composition—determines how light travels. Unlike ordinary materials, whose optical properties remain mostly uniform, metamaterials can be engineered so that different parts of the material bend light by different amounts. So, as light continually enters regions with slightly different refractive indices, it continuously refracts, causing it to curve smoothly around an object before returning to its original path.
While spatial cloaking hides an object in space, scientists later wondered whether a similar idea could hide an event in time. Let’s look at spatial cloaking in a different light. Suppose you have baggage on a conveyor belt in an airport. If the belt were split into sections with the first section slowed down and the second sped up, a gap would form, meaning that any event that happens in that interval, such as a bag being stolen, wouldn’t appear, creating a temporary gap in the light reaching the event. Physicists describe this phenomenon as temporal cloaking.
But how, you might ask, can we make some sections of light move faster than others? Light always travels at 3.00 × 10⁸ m/s in a vacuum, but it propagates more slowly through materials as a result of refraction. In relation, much like spatial cloaking, temporal cloaking relies on refraction, but instead of depending on light being redirected through space, it depends on redirections in time by carefully controlling when different portions of light arrive. Although light always travels at its fundamental speed in a vacuum, it moves more slowly through materials because it repeatedly interacts with the atoms inside them. This slowing allows scientists to carefully control the timing of different portions of a light pulse.
To induce temporal cloaking, scientists don’t simply put half of a laser beam in glass and the other half into air. Instead, they use optical devices to temporarily speed up one portion of a light pulse and slow down another relative to each other, creating a gap in the illumination where an event can occur unnoticed. Afterwards, they reverse the process so the observer sees what appears to be one continuous beam (Figure 3).
Whether guiding light around an object or creating a gap in the flow of information, both forms of cloaking rely on controlling how light carries information. By learning to control the paths that light travels, scientists are opening new doors that hold immense potential for innovation, from more secure communication systems to faster optical computers and improved technologies for exploring space. However, it’s important to emphasize that at this current stage, cloaking techniques only work under highly controlled laboratory conditions, often for specific wavelengths and directions and at extremely small scales. But, while spatial and temporal cloaking are still limited by complex engineering, expensive materials, and technical complications, each breakthrough deepens our understanding of physics and moves these once-fictional ideas closer to reality. The possibilities for future research are limitless! At this very moment, temporal cloaking was noted to have lasted only 20 trillionths of a second, but imagine if this could be increased to a larger scale. Who knows, perhaps one day invisibility will become a physics-verified reality—maybe even to the point of recreating an imperceptible bank robbery.
Written by: Shritha Mallaypalli and Sneha Pannala
Edited by: Katie Holmes, Hazel Milla, and Lauren Griffith
